Polyoxometalates in environmental remediation and energy storage

Abstract

Over recent decades, while environmental awareness and pollution control efforts have yielded localized improvements, ongoing industrial growth, rapid global population expansion, and escalating energy demands continue to drive significant global environmental pollution challenges. Polyoxometalates, a remarkable class of metal-oxide complexes, have recently emerged as promising compounds in the development of multifunctional materials for environmental pollutant removal, energy conversion and storage, and sensing. This review critically examines current research on their use for the removal of common toxic gases – such as H₂S, NO_x, and volatile organic compounds (VOCs) – from polluted air, as well as the elimination of various organic dyes, heavy metals, and pharmaceutical contaminants from wastewater. POMs have also gained recognition as adaptable redox-active materials suitable for next-generation energy storage systems. Their high electron-transfer capacity, structural flexibility, and remarkable chemical stability make them ideal candidates for various applications. POMs can facilitate multi-electron redox processes, allowing for their application in batteries, supercapacitors, and hybrid devices, which results in improved energy density and cycling performance. Recent developments in POM-based composites and electrode designs are further discussed for innovative, sustainable, and scalable energy storage solutions. Additionally, their tunable electrical and magnetic properties make them effective sensors for detecting various environmental pollutants.

---

Masoud Mirzaei

Masoud Mirzaei is the distinguished professor of Inorganic Chemistry at Ferdowsi University of Mashhad (FUM). His interdisciplinary research is at the interface of inorganic chemistry and materials science. His works focus on the fundamental science and applications of metal cluster-based complexes and materials such as polyoxometalates (POMs) and metal–organic frameworks (MOFs). He has published more than 250 peer-reviewed papers and books (H-index 40). He served as the President of Ferdowsi University of Mashhad (FUM), the Chancellor of Khorasan Science and Technology Park (KSTP), and the Chairman of Zeolite and Porous Materials Committee of the Iranian Chemical Society. He is also the Associate Editor of Inorganic Chemistry Research, a monthly open access journal published by Iranian Chemical Society, and a member of the Editorial Board of Polyhedron (Elsevier). He has received funds and awards for research and leadership from Iran National Science Foundation (INSF), Iran Science Elites Federation (ISEF), and the Academy of Sciences of the Islamic Republic of Iran. In 2021, 2023, 2024, and 2025, Professor Mirzaei was ranked among the top 1% International Scientists by ESI (Web of Science).

Sib Sankar Mal

Dr. Sib Sankar Mal received his Ph.D. degree in Chemistry from Jacobs University, Bremen, in 2008, under the supervision of Prof. Ulrich Kortz. He then completed postdoctoral work at the University of Ottawa, Canada, in 2011, and subsequently moved to Hamburg University, Germany, as an Alexander von Humboldt postdoctoral fellow. After completing his postdoctoral work, he joined the National Institute of Technology, Karnataka, India, as an Assistant Professor in 2013, where he currently serves as an Associate Professor in the Department of Chemistry. His primary research areas are energy storage, energy conversion, renewable energy, electrochemistry, catalysis, and polyoxometalates. He has published over 100 peer-reviewed papers (H-index: 32) and edited books. He has been awarded the Early Career Research Award by the Science and Engineering Research Board (SERB), India, and was recently honored with the Alexander von Humboldt Research Group Linkage Grant from Germany.

Manuel Aureliano

Manuel Aureliano is a Full Professor of Biochemistry, University of Algarve (UAlg), Portugal. Director of the Biochemistry degree (1998–2013; 2021–2025) and the PhD in Biological Sciences (2025). He investigates the role and applications of decavanadate and other polyoxometalates in the environment and biomedicine and has been an “Outstanding Reviewer” for Metallomics (2017, 2018, 2019). Reviewer of more than 950 papers from about 150 journals and Editor, Associate Editor, and/or Guest Editor, for several journals. From 2021 to 2025, he was included in the “World's Top 2% Scientists list” (impact-career and year). Recently, he was awarded the 3rd edition of the “UAlg Researcher Award”.

Annette Rompel

Annette Rompel studied Chemistry at the Westfälische Wilhelms University of Münster where she received her doctoral degree. Besides research at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, she was a visiting scientist at the RIKEN, Institute of Physical and Chemical Research, Sendai, Japan, and the University of Southern Denmark, Odense. Since 2008, she has been the Head of the Department of Biophysical Chemistry at the University of Vienna. Her main research interests are the structure/function elucidation of metalloenzymes and the synthesis and characterization of biologically active polyoxometalates.

---

Environmental significance

This comprehensive review covers remediation, sensing, and energy storage, inspiring sustainable polyoxometalate innovations. Polyoxometalates (POMs) are metal-oxide complexes with exceptional redox tunability, pseudocapacitive charge storage, and great structural versatility, making them ideal nanomaterials for environmental remediation. This review analyses the POM-based technologies for detection and treatment of air/water pollutants, surpassing conventional technologies that require harsh conditions for hard-to-remove contaminants such as refractory sulfur compounds. Global pollution includes refractory sulfur compounds from fossil fuels, toxic gases in air, and heavy metals, dyes, and emerging contaminants in water, driving acid rain, smog, antibiotic resistance and ecosystem toxicity. POMs provide efficient oxidative desulfurization, photocatalytic dye/heavy metal removal, and multipollutant adsorption in POM-based hybrid materials. POM structures enable visible-light mineralization in low-input environments with less energy; benefits include scalable low-toxicity remediation, while metal leaching risks under extreme pH are mitigated by heterogenization.

1 Introduction

In recent times, rapid industrial and technological development has caused a significant increase in energy demand and environmental pollution (EP).^1–3 The Encyclopaedia Britannica defines environmental pollution as the addition of any substance (solid, liquid, or gas) or any form of energy (such as heat, sound, or radioactivity) to the environment at a rate faster than it can be removed from the environment or stored in a harmless form. It further categorizes environmental pollution based on the affected medium into air, water, and land pollution.⁴ Increasing attention has been paid to the development of new methods for the removal of potential environmental pollutants^1–3 during industrial processes and clean energy production.⁵ Although industrial development has brought many positive aspects to everyday life (e.g. new technology, better food safety and supply, medicines, etc.), it has also increased consumption and pollution of natural resources (water, soil and air),^3,6 which has become both an environmental problem and a health threat for the entire human population.⁷

The global shortage of clean water and the pollution of water resources pose critical health, economic,⁸ and environmental challenges.^9–11 Especially in many underdeveloped and currently developing parts of the world, sewage wastewater and wastewater from different factories are discharged directly into the environment, causing catastrophic water pollution (section 2; Fig. 1) with hard-to-remove toxic chemicals – inorganic pollutants (section 2.2) such as heavy metals (section 2.2.1) and organic pollutants¹² (section 2.3) such as organic dyes¹³ and solvents.¹⁴

Fig. 1 Schematic illustration of the main roles of polyoxometalates (POMs) in environmental remediation and energy storage. The central part emphasizes POM-based environmental remediation, while the surrounding segments shows key applications, including pollutants detection, removal, sensing, treatment of (health) emerging pollutants, air pollutants and water contaminants, energy storage, energy conversion, and signalling.

The most prominent classes of health emerging pollutants (EPs)^15–17 (section 3; Fig. 1) are pharmaceuticals¹⁸ (section 3.1), pesticides and herbicides¹⁹ (section 3.2), cosmetics,^20,21 industrial and household products,²² metals¹³ (section 2.2), dyes¹³ and aromatic hydrocarbons (section 2.3.2).²³ The presence of EPs in wastewater has been associated with the development of bacterial resistance,^20,24 and mutagenicity and toxicity in aquatic organisms^21,25 and humans.^22,25

Pesticides and herbicides (for their removal, see section 3.2) are an inevitable part of the modern agricultural industry and food production.²⁶ However, in addition to ensuring yields and protecting crops from pests, the widespread use of these chemicals also affects soil enzymes and microorganisms²⁶ crucial for many essential biological processes, such as N₂-fixation in plants by rhizobacteria.²⁷ The excessive use of pesticides also impacts wildlife, with a scientific focus on bees, birds, fish and small mammals.^28–30 Human health is also affected by pesticide residues in the environment and food³¹ causing various health problems.^32–37 Therefore, many Western countries (e.g. EU, USA) have introduced stricter controls and limitations³⁸ on the use and allowable levels of pesticide residues in food, water and soil.³¹

Fossil fuels continue to be one of the primary energy sources in today's world.³⁹ Their combustion (section 4.1) produces various toxic refractory sulfur-containing compounds (dibenzothiophenes, DBTs)^40,41 and gases such as hydrogen sulfide⁴² (section 4.2.1), nitrogen oxides (section 4.2.2), and sulfur oxides (section 4.2.2),⁴³ which cause different severe environmental issues such as global warming,⁴⁰ smog⁴⁴ and acid rains.⁴⁵ Toxic gases generated from traffic and flue gases from the industry have made poor air quality an important factor in causing respiratory^46–48 and cardiovascular health⁴⁹ issues in urban areas.⁵⁰ Air purification (section 4; Fig. 1) using adsorption processes⁵¹ (section 4.2) and desulfurization of fossil fuels⁵² (section 4.1) is currently a logical approach to decreasing air pollution.

Global environmental pollution has escalated to crisis levels, driven significantly by fossil fuel combustion that releases refractory sulfur compounds such as dibenzothiophenes (DBTs), toxic gases including H₂S, NO_x, and SO₂, and emerging contaminants resistant to conventional treatment methods. These pollutants contribute directly to the formation of acid rain, smog, and severe health crises that impact billions worldwide.^52,53–59 Conventional technologies like hydrodesulfurization (HDS) are ineffective against sterically hindered DBTs and require extreme conditions (300–400 °C, 30–100 bar H₂), while amine scrubbing and selective catalytic reduction (SCR) systems⁶⁰ face limitations in capacity, cost-efficiency, and simultaneous multi-pollutant management for air purification.^53,61 Water faces persistent heavy metals, dyes, pharmaceuticals, and microplastics that evade standard filtration and oxidation.^{15–17,20,22,24,25} POMs offer a powerful, direct solution to these multifaceted challenges via mild-condition oxidative desulfurization achieving over 99% removal of refractory sulfur, versatile multi-pollutant adsorption and catalysis, and photocatalytic mineralization.^62,63 Their uniquely tunable redox properties and acidity provide sustainable remediation options precisely where traditional technologies are insufficient.^53,62

The first step in combating pollution is building a good system to monitor and detect various harmful compounds present in the environment. In this regard, various materials have been extensively researched and designed to develop new chemical,⁶⁴ electrochemical,⁶⁵ and biosensors⁶⁶ (section 5; Fig. 1) for environmental monitoring. For example, metal or metal oxide nanoparticles are widely used to develop various electrochemical sensors.^67–69

New efficient technologies for energy conversion and storage need to be developed (section 6; Fig. 1) because renewable energy sources such as wind, hydroelectric, and solar power alone cannot meet the world's current energy demands.⁷⁰ In addition, the growing popularity and use of various portable electronic devices in everyday life have led to intensive research and development of new efficient battery technologies such as lithium-ion,⁷¹ sodium-ion,⁷² and redox-flow batteries.⁷³ Rechargeable Li-ion batteries and supercapacitors have been commercially utilized due to their ability to hold high energy with power density for various applications (e.g., electric vehicles, power tools, or portable/wearable electronic devices).^74–76

1.1 Polyoxometalates

Polyoxometalates (POMs)⁷⁷ are a class of transition metal-oxide clusters, usually containing Mo or W ions in their highest oxidation states. They exhibit exciting and unique physical and chemical properties, such as controllable shape and size,⁷⁷ oxo-enriched surfaces, photoactivity,⁷⁸ molecular conductivity,⁷⁹ excellent chemical stability, and redox properties.⁸⁰ These properties have led to their increasing use in diverse fields, including catalysis,^81,82 magnetism,⁸³ medicine,^84,85 biotechnology,⁸⁶ protein crystallography,^87–89 and material science.⁹⁰

POMs are typically synthesized via controlled acidification and condensation of simple metal oxoanions such as Mo^VIO₄²⁻, W^VIO₄²⁻, or V^VO₄³⁻, which allows the precise formation of diverse structural archetypes, including some of the most common POM archetypes like Keggin (Fig. 2F), Wells–Dawson (Fig. 2H), and Anderson–Evans (Fig. 2I).^{76,77,91–93} Their functionality in pollutant removal is often enhanced by immobilization or hybridization,⁹⁴ where POMs are incorporated into different solid supports like metal–organic frameworks (MOFs),^95–105 porous silica,^106,107 graphene oxide (GO_x),^108,109 or polymeric supports,^94,110,111 improving POM stability and catalytic efficiency.^94,101,112 Ion exchange with organic or inorganic cations,^113–115 surface modifications,⁹⁴ or doping with lanthanide ions¹¹⁶ further tailor their physicochemical properties. Such synthetic versatility enables customization of POM-based materials to optimize catalytic, adsorptive, and photocatalytic performance in environmental remediation.^94,101,117 The structural characteristics of polyoxometalates can be divided into two main general subgroups, isopolyoxometalates and heteropolyoxometalates.^76,77 The isopolyoxometalates, with the general formula [M_xO_y]ⁿ⁻ (where M = Mo, W or V; Fig. 2A–D), contain only addenda metals and oxygen atoms in their structure, such as Lindqvist^118,119 ([M₆O₁₉]²⁻; Fig. 2A), heptamolybdate^120,121 ([Mo^VI₇O₂₄]⁶⁻; Fig. 2B), octamolybdate^122,123 ([Mo^VI₈O₂₆]⁴⁻; Fig. 2C), decatungstate^124,125 ([W^VI₁₀O₃₂]⁴⁻; Fig. 2D) and decavanadate^126,127 ([V^V₁₀O₂₈]⁶⁻; Fig. 2E). Heteropolyoxo species have the general formula [X_zM_xO_y]ⁿ⁻ (X = heteroion, M = Mo, W or V, z < x, y = number of oxygen atoms in the POM structure, n = overall anion charge), where different heteroions X are present alongside addenda ions M and oxygen atoms. This composition allows them to form a variety of structural types, including common ones such as Keggin^128,129 ([XM₁₂O₄₀]ⁿ⁻; Fig. 2F), lacunary Keggin¹³⁰ ([XM₁₁O₃₉]ⁿ⁻; Fig. 2G), Wells–Dawson^131,132 ([X₂M₁₈O₆₂]ⁿ⁻; Fig. 2H), Anderson–Evans^133,134 ([XM₆O₂₄]ⁿ⁻; Fig. 2I), Preyssler¹³⁵ ([MP₅M₃₀O₁₁₀]^(15−n)−; Fig. 2J), Strandberg^136,137 ([X₂Mo^VI₅O₂₃]ⁿ⁻, (X = P^V, S^VI, As^V, Se^VI); Fig. 2K), Weakley^138,139 ([M^III(M^VI₅O₁₈)₂]ⁿ⁻; Fig. 2L), among others. Moreover, if the POM solution is reduced, a unique class of giant molybdenum blue and molybdenum brown-type structures ({Mo₁₅₄} and {Mo₁₃₂}) are formed.¹⁴⁰ For more detailed information on POMs structures and general synthetic procedures, the reader is referred to the reviews in ref. 91–93, 141 and 142.

Fig. 2 Structures of isopolyoxometalates and heteropolyoxometalates: A) Lindqvist ([M^VI₆O₁₉]²⁻), B) heptamolybdate ([Mo^VI₇O₂₄]⁶⁻), C) β-octamolybdate (β-[Mo^VI₈O₂₆]⁴⁻), D) decatungstate ([W^VI₁₀O₃₂]⁴⁻), E) decavanadate ([V^V₁₀O₂₈]⁵⁻), F) Keggin ([XM^VI₁₂O₄₀]ⁿ⁻), G) monolacunary Keggin ([XM^VI₁₁O₃₉]ⁿ⁻), H) Wells–Dawson ([XM^VI₁₈O₆₂]ⁿ⁻), I) Anderson–Evans ([XM^VI₆O₂₄]ⁿ⁻), J) Preyssler ([MP₅M^VI₃₀O₁₁₀]^(15−n)−), K) Strandberg ([X₂Mo^VI₅O₂₃]ⁿ⁻), and L) Weakley ([M^III(M^VI₅O₁₈)₂]ⁿ⁻). Color legend: orange = M (either Mo^VI, W^VI or V^V), blue = Mo^VI, purple = W^VI, yellow = V^V, gray = X (heteroion), white = M^III, and red = oxygen.

Pure POMs exhibit different solution behaviors across the wide pH range; some, like Wells–Dawson-type structures, maintain their structural integrity, while others, such as Keggin-type POMs, undergo monolacunarization under acidic conditions relevant to environmental remediation.^143–145 Their high solubility in aqueous media presents significant challenges for their use in applications, including leaching during wastewater treatment and difficulties in catalyst recovery.¹¹³ While pure POMs often dissolve in aqueous media,¹¹³ strategic heterogenization approaches,⁹⁴ such as immobilization on mesoporous silica (SBA-15),^106,107,146 metal–organic frameworks (like UiO-66 and MIL-101),^95–101 and POM-supported ionic liquid phases (POM-SILPs),^112,147 address this issue. Such methods significantly reduce leaching to <1% after 10 cycles (Tables S1 and S2). These enable recyclability over 5–10 cycles with minimal activity loss (Table S1).^148–151 Nevertheless, challenges remain including potential metal cation leaching from POM-composites under prolonged extreme pH exposure and the need for long-term stability studies under real environmental conditions. These heterogenized systems demonstrate >95% POM retention after multiple uses.⁶³

Keggin-type POMs (Fig. 2F and G) are the most widely studied POM archetype, representing an average of 77.6% of all published articles, particularly in applications targeting environmental pollutant removal (approximately 69%). This predominance in environmental applications surpasses that of Wells–Dawson (Fig. 2H; ∼9%), Anderson–Evans (Fig. 2I; ∼9%), sandwich-type (Fig. 2L; ∼5%), isopolymolybdates (Fig. 2A–E; ∼5%), and other types of POMs (each ∼5%). In this review, Keggin-type POMs (Fig. 2F and G) are most frequently addressed in section 3 (wastewater treatment, 75%) and section 4 (air pollutant removal, 85%). Wells–Dawson type POMs (Fig. 2H) rank second in environmental pollutant removal (average 16.9%), with their primary use found in sensing (75%, section 5). Notably, section 4.1 showcases the broadest diversity of structural archetypes for POM-mediated fossil fuel desulfurization.

A literature search conducted on Web of Science in August 2025 (Fig. 3) revealed that approximately 12% (1928) of the published articles on POMs related to the keyword “environment”, out of a total of 15 830 articles. As of August 14, 2025, the number of articles varies by specific subject: the combination of “polyoxometalate” and “degradation” yielded 1306 articles, while “polyoxometalates” and “dyes” yielded 910 articles. These numbers exceed those for “polyoxometalate” combined with “pollutants” (353), “waste” (258), “industrial chemicals” (134), and “wastewater” (215). Fewer articles were found for combinations with “antibiotics” (98), “pesticides” (48), “fossil fuels” (40), and “air pollution” (26). The number of publications related to “antibiotics” and “wastewater” has more than doubled over the past 2 years, reflecting a marked increase in research interest in these areas.

Fig. 3 Number of articles containing the term “polyoxometalate” combined with keywords such as dyes, pollutants, industrial chemicals, wastewater, pesticides, and antibiotics, as of August 14, 2025.

In fact, the importance of POMs in environmental science and their relationship to sustainable development and green chemistry is clearly increasing. POMs are crucial in environmental science for their roles as catalysts and adsorbents, aiding in the degradation of emerging pollutants such as dyes, plastics, and antibiotics, in addition to well-known organic and inorganic contaminants.^152–160 Moreover, POMs can act as novel antibacterial agents for water purification.¹⁶¹ As described in the sections below, POMs are also fundamental for sustainable development by enabling energy applications such as solar hydrogen production and energy storage.^70–76 Recent studies further explore POMs as electrochemical sensors for the simultaneous detection of inorganic heavy metal ions and organic antibiotic contaminants in aquatic environments,¹⁶² and as triboelectric nanomaterials for gait monitoring.¹⁶³

2 Water decontamination by polyoxometalates

Inorganic contaminants (section 2.2) enter the environment as inorganic salts, mineral acids, sulfates, cyanides, and metal ions, including heavy and radioactive metals. These contaminants are generally more persistent and more difficult to eliminate than organic ones.^164,165 On the other hand, organic contaminants (section 2.3) represent a more diverse class, consisting of organic dyes, aromatic hydrocarbons, pesticides, and pharmaceuticals (see section 3 for pharmaceutical and pesticide removal). Due to rapid industrial development, large amounts of industrial, sewage, and agricultural waste discharged into water bodies cause organic pollutants to become pseudo-persistent in the ecosystem.^166,167 Therefore, the removal of this class of contaminants requires careful consideration to move toward a sustainable ecosystem.

As discussed in section 2.1, oxidation, catalysis, photocatalysis, ion-exchange, adsorption, and membranes are among the commonly used technologies for the removal of these pollutants due to their high efficiency, cleanliness, and simple operation. POMs have shown promise in mitigating the global water purification issue using the above-mentioned technologies. This section covers novel solutions by highlighting recent achievements in designing multi-component materials for use in water-purification systems.

2.1 Emerging pollution treatment technologies

Water treatment is a multi-stage process, comprising several stages with various technologies. Tertiary treatment is the final stage of the multi-stage wastewater treatment process. It is used after preliminary stages, and commonly used techniques utilized for the treatment include oxidation, photocatalysis, ion exchange, adsorption, and membranes technology (Fig. 4).¹⁶⁸

Fig. 4 Summary of tertiary treatment technologies used against inorganic and organic pollutants for water purification.

Chemical oxidation is a cost-effective and simple technology for the decontamination of both organic and inorganic pollutants using an oxidizing agent such as chlorine, hydrogen peroxide, ozone, and molecular oxygen. In advanced oxidation processes (AOPs), POMs (especially iron-containing POMs) have been used as efficient catalysts for the decomposition of the oxidizing agent (H₂O₂) and removal of organic pollutants.¹⁶⁹ In particular, POMs can initiate the activation through electron transfer to H₂O₂ (originating from the redox property of the addenda atoms) or via formation of peroxo complexes.^147,170 This method, however, may produce secondary pollutants that are formed after the initial oxidation. This may cause a decrease in the catalyst selectivity, while increasing the costs.

In photocatalysis, the ability of the catalyst to harvest photons from a light source and to generate free radicals to undergo photocatalytic oxidation or reduction reactions is crucial. In this regard, POMs have shown promise since i) their band gap value can be adjusted by changing the heteroatoms or adjusting the valence states of addenda atoms, and ii) they can store multiple electrons in one molecule; thus they exhibit fast charge transfer properties.^171,172 Due to some drawbacks associated with pure POMs (e.g., limited light absorption, high solubility), they are often employed in the form of hybrids or composites.¹¹² In these structures, the intermolecular interactions between two species can improve the stability and promote the lifetime of photogenerated charge carriers. In this regard, the incorporation of noble metals,^173,174 metals from the lanthanide series,¹⁷⁵ metal oxides,¹⁷⁶ metal–organic frameworks¹⁷⁷ and metal-free species¹¹⁰ have been reported to be effective. Ion exchange water purification technology relies on the availability of exchange surfaces with accessible specific surface area and the ability to reversibly uptake/release ions from water. POMs can fulfill some of these requirements. For example, their diverse topology, high negative charge, and redox properties of POMs have turned them into potential candidates for cation (heavy metal) uptake and exchange. However, POMs lack a high surface area that is problematic.¹¹³

Adsorption-based protocols have been extensively used for wastewater treatment on the account of cost, simplicity, and energy considerations. The concept of this approach is based on removing pollutants by promoting their adsorption on the adsorbent surface via physical or chemical interactions.¹⁷⁸ In this context, some intrinsic properties of POMs (e.g., high negative charge, strongly basic oxygen surfaces) are advantageous for the physi/chemisorption of adsorbate molecules. However, when considering POMs as water purifiers, some limitations such as their high solubility and the low surface area must be taken into consideration. The heterogenization of POMs by inorganic substrates^106,107,179 or organic matrices^111,179,180 is the common approach to solve their solubility issue and low specific surface area. In heterogenization with organic matrices, the surface chemistry of the matrix plays an important role. Along with the degree of POM dispersion and matrix morphology, it can enhance the physicochemical properties and improve the membrane's performance. Heterogenization by porous coordination polymers (MOFs) is another successful strategy that combines both the merits of POMs and MOFs (e.g. recyclability and porosity).^95–101 This strategy is commonly used for the adsorptive removal of cationic dyes.^102–105 However, the catalytic activity of POM composites greatly depends on their structural properties. In some cases, as for POM@MOF composites, the activity is mainly governed by pore-dependent diffusion limitation, where the match of pore aperture and POM diameter is essential.¹³⁹ Meanwhile, each individual structural component can also induce different electron transfer kinetics due to its unique electron-storage/transfer capacity.^181,182

Controlled deposition of POMs on substrates is another concept that enables the fabrication of POM-based functional devices for water purification.⁹⁴ Techniques such as layer-by-layer assembly, casting, and dip-coating have been recently reported.^183–185

Membrane filtration is a reliable, and environmentally friendly process with relatively low cost and simple operation, which has been widely used for water purification. Catalytic membranes represent a new generation of membranes created by incorporating inorganic particles, such as POMs, into a polymer matrix to enhance the membrane's (photo)catalytic properties.^186–189 As a convincing demonstration of this approach, Yao et al. designed and fabricated an amine-functionalized APTMS-treated PEI membrane for dye removal from wastewater. [PV^V₂Mo^VI₁₂O₄₀]⁵⁻ was incorporated into the matrix via a simple sol–gel protocol. The presence of [PV^V₂Mo^VI₁₂O₄₀]⁵⁻ in the membrane not only enhanced the mechanical strength of PEI but also catalyzed the degradation of RB5 in the presence of a diluted solution of an oxidant (Fig. 5).¹⁹⁰ The presence of different POM species was reported to be necessary for the self-cleaning property of the membrane.¹⁸³

Fig. 5 Illustration of a POM-integrated catalytic membrane for organic dye decontamination from water. Reproduced from ref. 190 with permission from Elsevier, copyright 2016.

2.2 Removal of inorganic pollutants

2.2.1 Removal of heavy metals. Catalysis and photocatalysis are appropriate strategies for removing reductive toxic metal ions from water.^108,191,192 Gong et al. demonstrated that different highly reduced molybdophosphate hybrid materials such as {Co^II[P^V₄Mo^V₆X₃₁]₂}ⁿ⁻ (X = O or OH)¹⁹³ or {Mn^II[P^V₄Mo^V₆O₃₁]₂}¹⁹⁴ clusters could act as efficient heterogeneous catalysts for the reduction of toxic Cr(VI) to nontoxic Cr(III) in the presence of formic acid as the reducing agent under mild conditions. These noble metal-free POM catalysts have great potential to replace high-priced Pt/Pd catalysts for the elimination of Cr(VI) from water.

POMs or their modified derivatives, acting as electron reservoirs, have demonstrated efficiency in photoactivity, especially in visible light photocatalysis. Therefore, there is continuous effort to design a POM-based photocatalyst that can utilize solar energy for the reduction of highly toxic Cr(VI). Due to the good photocatalytic response of Ag-based photocatalysts, Wang's group heterogenized H₃[PMo^VI₁₂O₄₀] with Ag⁺ counter cations.¹⁹⁵ Ag/Ag_xH_3−xPMo^VI₁₂O₄₀ nanowires were synthesized by a facile solid-state reaction route and in situ photodeposited method. The resulting Ag/Ag_x[H_3−xPMo^VI₁₂O₄₀] (Ag/AgHPMo₁₂) nanowires, where x denotes the irradiation time (x = 2, 4, 6, 8 h, respectively), showed higher stability and photocatalytic activity than traditional Ag-based photocatalysts (e.g. Ag/AgX (X = Cl, Br, I), AgPO₄ or AgVO₃)^196–199 for Cr(VI) reduction. This is attributed to their good visible-light absorption and reversible redox properties of the Keggin-type POM (Fig. 2F). In addition, a part of the Ag⁺ in the nanowires was in situ photoreduced to Ag NPs under visible light irradiation, and these Ag NPs enhanced visible-light absorption and the charge separation of photogenerated electrons (e⁻) and holes (h⁺) in Ag/[AgHPMo^VI₁₂]. In order to improve the catalytic efficiency of Ag/[Ag_xH_3−xPMo^VI₁₂O₄₀] nanowires, these Ag-loaded 1D silver POM nanowires were well dispersed on duplicated 2D graphite-like carbon nitride (g-C₃N₄) nanosheets.²⁰⁰ The obtained [Ag_xH_3−xPMo^VI₁₂O₄₀]/Ag/g-C₃N₄ (x represents the irradiation time; x = 2, 4, and 6 h, respectively) 1D/2D Z-scheme heterojunction photocatalyst exhibited excellent and durable photocatalytic performance towards the reduction of Cr(VI), methyl orange (MO) and tetracycline (TCY) under visible light.²⁰⁰

In attempts to obtain efficient photocatalysts based on inorganic–organic hybrid POMs, a series of [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)], [Zn(L)(H₂O)]₂[SiW^VI₁₂O₄₀]·3H₂O, [Cu(L)(H₂O)]₂[SiW^VI₁₂O₄₀], and [Cu₂(L)₂(HPW^VI₁₀W^V₂O₄₀)]·4H₂O (L = 1,4-bis(3-(2-pyridyl)pyrazole)butane), have been synthesized.²⁰¹ Interestingly, [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)] (1) hybrid was able to act as an efficient photocatalyst to reduce Cr(VI) using the scavenger isopropanol under visible light at ambient temperature. In comparison with [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)], the three other POM hybrids showed relatively weak photocatalytic activity. In a possible reduction mechanism of Cr(VI) to Cr(III), first, the [Ag₄(H₂O)(L)₃]⁴⁺ unit was excited under visible light, and the excited state electrons on the organic ligand were inclined to transfer to the [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)] POM. Simultaneously, the isopropanol on the surface of the hybrid yielded reducing radicals and captured the photoinduced holes produced by the hybrid photocatalyst. Finally, the isopropanol scavenged the photoinduced holes and formed CO₂, H₂O, and other products. This charge transfer maintains the recombination of holes and electrons. The electrons accumulated on [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)] were responsible for reducing Cr(VI). It was concluded that the much larger [Ag₄(H₂O)(L)₃]⁴⁺ metal–organic unit, in comparison to the other metal–organic units presented in other above-mentioned inorganic–organic hybrids, is probably responsible for the higher photocatalytic activity of the [Ag₄(H₂O)(L)₃(SiW^VI₁₂O₄₀)] compared to the other three compounds.²⁰¹ Adsorption is the other most used purification technique to remove heavy metals from wastewater. In order to prepare a multi-functional composite, Herrmann et al.⁶³ used a combination of lacunary Keggin anions [α-SiW^VI₁₁O₃₉]⁸⁻ and tetra-n-alkyl ammonium cations ((n-C₆H₁₃)₄N⁺ and (n-C₇H₁₅)₄N⁺) to prepare a highly viscous, lipophilic POM-IL complex, which was then immobilized on porous silica to give POM-SILP.⁶³ Each component of the POM-SILP composite contributed to the removal of a specific type of water contaminant. The lacunary Keggin tungstate anions (Fig. 2G) were responsible for metal–ion binding, whereas the long-chain quaternary organo-ammonium cations²⁰² acted as an antimicrobial. In addition, the POM-IL lipophilicity enabled the adsorption of organic contaminants, and the silica support bound radionuclides. Thus, using the water-insoluble POM-SILP composite in filtration columns allowed the simultaneous removal of toxic heavy metals (as Ni²⁺, Pb²⁺, Cu²⁺, Cr³⁺ and Co²⁺), microbes (E. coli), organic aromatics (trityl dye), and nuclear waste (UO₂⁺) from water (Fig. 6).⁶³

Fig. 6 Water purification using POM-SLIPs: the POM-SLIP column filter removes toxic heavy metals (e.g. Ni(II), Pb(II), UO₂(II)), microbes (E. coli), and aromatic organic pollutants (e.g. trityl dyes) due to the presence of lacunary polyoxometalate anions with specific metal-binding sites (yellow arrow) and antimicrobial tetra-alkyl ammonium cations. Reproduced from ref. 63 with permission form Wiley-VCH, copyright 2017.

The highly hydrophobic nature of POM-IL leads to surface heterogeneity and thus facilitates biphasic removal of metal ions from aqueous solutions. At the same time, the negative charge present on the POM units is the driving force for the removal of metal ions with a positive charge. In order to increase the removal of heavy metals from water by POM-IL, Shakeela and Rao synthesized a series of Keggin-based ionic liquids by reacting in situ generated first-row transition-metal ion (Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) substituted monolacunary Keggin with tetraoctylammonium (TOA) cations.²⁰³ Metal-substituted lacunary POMs carried a relatively higher negative charge which facilitated the absorption of metal cations. Thus, all these thermoreversible POM-ILs effectively removed Cd²⁺ and Pb²⁺ metal ions from the aqueous phase.²⁰³ Embedding POM-ILs with tri-lacunary Keggin [α-PW^VI₉O₃₄]⁹⁻ featuring coordinative binding of up to six metal cations into 3D printed organic polymers²⁰⁴ has been shown to produce a highly porous organic–inorganic composite for effective transition-metal removal (Fig. 7).²⁰⁵

Fig. 7 Schematic illustration of the POM-modified 3D-printed copolymer substrates used for transition-metal removal by the cation binding sites of the lacunary [α-PW^VI₉O₃₄]⁹⁻. Reproduced from ref. 205 with permission from The Royal Society of Chemistry, copyright 2018.

Cation exchange is another process for the removal of various metal cations from water. Synthetic inorganic ion exchangers with well-defined chemical and phase compositions appear to be the most suitable ones compared to organic ion exchangers due to higher thermal and chemical stability and higher exchange capacity and selectivity for a wide range of metal ions.¹¹³ For example, Cronin's group designed an inorganic open framework nanocube-based K₁₈Li₆[Mn^II₈(H₂O)₄₈P₈W^VI₄₈O₁₈₄]·108H₂O, from highly anionic crown-type POM ([P₈W^VI₄₈O₁₈₄]⁴⁰⁻) and Mn^II as linkers to accommodate Cu^II cations from a solution into the network of channels and cavities. The cation-exchange capacity and rate are controlled by oxidizing the Mn linkers from +II to +III.²⁰⁶ In some cases, POM-IL systems exhibited greater efficiency than conventional ion-exchange resins.²⁰⁷

2.2.2 Removal of radioactive metals. Although metal–organic frameworks (MOFs) initially exhibited a unique performance for the adsorptive removal of metal ions, most of these materials have low stability in aquatic media, which has limited their applications for water purification. To improve the stability of MOFs, Zou et al. functionalized HKUST-1 MOF with Keggin-type POM [H₃PW^VI₁₂O₄₀] POM (Fig. 2F) to form HKUST-1@[H₃PW^VI₁₂O₄₀] under microwave conditions. It was proposed that the improved water stability of HKUST-1@[H₃PW^VI₁₂O₄₀] was the result of POMs being encapsulated into HKUST-1 pores. The HKUST-1@[H₃PW^VI₁₂O₄₀] showed high adsorption affinity and capacity towards selective adsorption of heavy metal ions (highly selective for Pb²⁺ and Cd²⁺, but no adsorption of Hg²⁺) from contaminated water.²⁰⁸ Studies on HKUST-1@[H₃PW^VI₁₂O₄₀] adsorption ability to remove U(VI) from wastewater showed that it could selectively adsorb U(VI) from low concentration uranium solutions in the presence of other metal ions.²⁰⁹ The adsorption capacity of HKUST-1@[H₃PW^VI₁₂O₄₀] was strongly pH dependent and did not significantly decrease after three adsorption–desorption cycles. The presence of phosphate groups in the adsorbent structure has a great affinity for radioactive U(VI) ions in an aqueous solution.^210,211 In this regard, a ship-type nano-cage POM {[C₅NH₅]₉[H₃₁Mo^VIV^V₁₂O₂₄Co^II₁₂(PO₄)₂₃(H₂O)₄]}²⁻ (Co-POM) with 23 {PO₄} groups was designed and synthesized. The high adsorption capacity of this POM-based inorganic framework for U(VI) ions in aqueous solution was mainly ascribed to coordination interaction between U(VI) and O in the phosphate groups on Co-POM which was proved by FT-IR and XPS analyses.²¹² Composites of POMs (H₃PW^VI₁₂O₄₀) with graphene oxide also exhibited a significant potential for uranyl uptake from wastewater.¹¹³

The cation exchange studies by POMs have been widely used to separate radioactive metal ions from radioactive wastes.¹⁰⁹ Kortz's group worked on a cyclic K⁺-templated POM, [K⊂{(β-As^IIIW₈O₃₀)(W^VIO(H₂O))}₃]¹⁴⁻, which showed high selectivity to Rb⁺, due to the relatively large size of the central cavity for K⁺ (Fig. 8).¹¹⁴ Uchida's group combined the Keggin cluster [SiMo^VI₁₂O₄₀]⁴⁻ anions with a cationic oxo-centered trinuclear complex, to produce ionic crystals with isolated pores, (etpyH)₂[Cr₃O(OOCH)₆(etpy)₃]₂[SiMo^VI₁₂O₄₀]·3H₂O (etpy = 4-ethylpyridine), which selectively adsorbed Cs⁺ among alkali and alkaline earth metals via reduction of the Keggin [SiMo^VI₁₂O₄₀] with ascorbic acid.²¹³ The previously reported ionic crystal, (mepyH)[Cr₃O(OOCH)₆(mepy)₃]₂[PMo^VI₁₂O₄₀]·5H₂O (mepy = 4-methylpyridine, mepyH⁺ = 4-methylpyridinium ion), with 1D open channels, was able to incorporate Na⁺ as well as Cs⁺ by the reduction-induced cation exchange processes.¹¹⁵ The authors concluded that the high selectivity towards Cs⁺ is due to the existence of closed pores rather than open channels. Despite the high selectivity towards Cs⁺ however, several disadvantages such as the requirement of heating (343 K) and slow adsorption kinetics (12 h to reach equilibrium) limited the widespread application of (mepyH)[Cr₃O(OOCH)₆(mepy)₃]₂[PMo^VI₁₂O₄₀]·5H₂O. Later, this group overcame disadvantages by utilizing the large-molecular size and easily reducible Wells–Dawson-type of POMs [P₂Mo^VI₁₈O₆₂]⁶⁻ (M = Mo, W).²¹⁴ In comparison with the Keggin-type POM, the larger molecular size and higher reduction potential of Dawson-type POM increased the pore volume and facilitated the reduction-induced Cs⁺ exchange. As expected, the capacity and rate of Cs⁺ uptake increased significantly (with only 1 h to reach equilibrium at room temperature), demonstrating the potential application of these adsorbents for radioactive Cs⁺ (Cs-137) removal.²¹⁴

Fig. 8 Structure of [M⊂{(β-As^IIIW₈O₃₀)(WO(H₂O))}₃]¹⁴⁻ with the central guest being either K⁺ or Rb⁺. Color code: WO₆ (violet octahedra), As (orange), K/Rb (grey).¹¹⁴

2.3 Removal of organic pollutants

2.3.1 Removal of organic dyes. As shown in Table S2 (SI), the decontamination mechanisms, in the case of organic pollutants, are similar to previously discussed methods for inorganic ones. Adsorption of dye molecules, especially cationic ones, by POMs is strongly governed by solution pH. The selective adsorption of methylene blue (MB) in the presence of methyl orange (MO) over [P₂W^VI₁₈]/MOF-5 catalyst is spontaneous and endothermic. In addition, the pH value of dye solution should also be carefully controlled to obtain maximum adsorption capacity, because the surface charge of the adsorbent is strongly affected by the pH (pH_PZC; PZC = point-of-zero charge).²¹⁵ Furthermore, olation and oxolation processes are responsible for the high negative charge on the POM surface at lower pH values.²¹⁶

In a generally accepted approach, photooxidation of dye molecules occurs through generation of free OH˙. The proposed mechanism is based on the photoexcitation of Cs₄SiW^VI₁₂O₄₀ POM and a subsequent hydrogen abstraction reaction which results in the homolytic bond cleavage of H₂O. The photocatalytic activity of POMs, such as [SiW^VI₁₂O₄₀]⁴⁻, can be enhanced in the presence of semiconductors. In fact, in such heterojunction structures with suitable energy band alignment, photogenerated carriers could be separated more efficiently.²¹⁷ Dye sensitization is another mechanism that may contribute to dye degradation in photocatalytic reactions. Due to the visible-light absorption abilities of the sensitizers, dye-sensitized POM photocatalysts can be excited upon visible-light irradiations. In these cases, the oxidation of dye proceeds through electron transfer between the excited dye (e.g., thionine, phthalocyanine) and LUMO of Keggin ([PW^VI₁₂O₄₀]³⁻)^218,219 or Wells–Dawson-type ([α-P₂W^VI₁₈O₆₂]⁶⁻ (P₂W₁₈) and [α-P₂W^VI₁₇O₆₁]¹⁰⁻)²¹⁸ type POMs.^218,219

As an interesting example of membrane filtration technology, Yao et al.¹⁷⁹ incorporated surfactant-encapsulated POM microparticles into a PVDF matrix as a microfiltration membrane for the adsorptive removal of the anionic dye reactive black 5 (RB5). The authors prepared spherical microparticles through an ion exchange reaction between a cationic surfactant (DODA·Br) and [PV^V₂Mo^VI₁₀O₄₀]⁵⁻. This architecture enhanced the flow rate of the system and dye removal efficiency reached up to 97.5% within 120 min.¹⁷⁹ A similar concept has been applied in the case of surface-active ionic-liquid-encapsulated POMs.²²⁰ Ion exchange reaction has also been used to replace small anions in the structure of layered double hydroxides (LDHs) with large polyanions. By this method the surface area of the resulting composite can be enhanced, since the interlayer distances of LDH increase. These composites have been used for the removal of cationic dyes from water; however, the instability of LDH in acidic media may limit their application.^221,222 In 2005 Zhao and co-workers suggested that an active peroxo species is responsible in the photo-Fenton oxidation of Rhodamine B (RhB) under visible light irradiation. The authors proposed that the active species is formed upon the interaction of reduced POM with H₂O₂.²²³ Similarly, in Fenton systems the active species is formed by the coordination of iron to [PW^VI₁₂O₄₀]³⁻ POM.²²⁴ In Fenton-like systems the iron species is replaced with different POMs, like mentioned Keggin²²⁴ or [HPW^V₄W^VI₈O₄₀]⁶⁻ POMs.²²⁵ The radical-based pathways, however, can enhance apparent degradation rate if not properly identified or controlled.

Among different transition metals (Co, Ni, Cu), Co-substituted Wells–Dawson anions [α₂P₂W^VI₁₇CoO₆₁]⁸⁻ exhibited higher catalytic performance.²²⁶ Li's group prepared two POMCPs, [Ag₄(H₂pyttz-I)(H₂pyttz-II)(Hpyttz-II)][HSiW^VI₁₂O₄₀]·4H₂O (H₂pyttz-I = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl) and [Ag₄(H₂pyttz-II)(Hpyttz-II)₂][H₂SiW^VI₁₂O₄₀]·3H₂O (H₂pyttz-II = 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl) with similar structure and different tunnels (Fig. 9a). The photocatalytic degradation of methylene blue (MB) demonstrated that the structure of the hybrids influences the photocatalytic properties. The larger cavities in the compound and [Ag₄(H₂pyttz-II)(Hpyttz-II)₂][H₂SiW^VI₁₂O₄₀]·3H₂O increase the contact area between catalysts and crude materials and promote more active sites to participate in the reactions process. Thus, the photocatalytic properties of [Ag₄(H₂pyttz-II)(Hpyttz-II)₂][H₂SiW^VI₁₂O₄₀]·3H₂O were improved. The proposed mechanism for enhanced photocatalytic activity in these hybrids is shown in Fig. 9b. This mechanism includes LMCT from the HOMO to the LUMO, which was facilitated by Ag–O bridging units. In addition to this, Ag-pyttz acted as photosensitizers and promoted the transition of electrons onto [SiMo^VI₁₂O₄₀]⁴⁻ POMs. Therefore, the [SiMo^VI₁₂O₄₀]⁴⁻ POMs had a higher charge density and exhibited a considerable impact on the photocatalytic degradation of RhB.²²⁷

Fig. 9 a) Representation of the [Ag₄(H₂pyttz-I)(H₂pyttz-II)(Hpyttz-II)][HSiW₁₂O₄₀] and [Ag₄(H₂pyttz-II)(Hpyttz-II)₂][H₂SiW₁₂O₄₀] compounds ((H₂pyttz-I = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl) and H₂pyttz-II = 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl) with similar underlying frameworks and different tunnels. b) Representation of the photocatalytic mechanisms for POMCPs. Reproduced from ref. 227 with permission from The Royal Society of Chemistry, copyright 2015.

2.3.2 Removal of aromatic hydrocarbons. The oxidative potential of POMs has been broadly used in AOPs for phenol oxidation.²²⁸ For example, [PW₁₁O₃₉Fe^III(H₂O)]⁴⁻ can degrade chlorophenol (CP) compounds only if H₂O₂ is added to the solution. No photocatalytic activity was observed in aerated aqueous solution. In addition, the reaction rate was influenced by the initial concentration of the catalyst or H₂O₂ and the number of chlorines in the aromatic ring of CP.²²⁸ Iron-containing POMs have also been used to construct heterojunction photocatalysts by grafting Fe-POM nanoclusters onto oxygen-deficient TiO₂. The synergistic effect between photocatalysis and Fenton-like reactions resulted in efficient degradation of sulfosalicylic acid (SSA).²²⁹ Deposition of Au NPs on the surface of POM/TiO₂ is another strategy to improve light absorption and activity of the catalyst. A 4.6-fold increase was observed in photocatalytic degradation of nitrobenzene (NBZ).²³⁰ Zhang et al. prepared a ferrocene-containing silicotungstate catalyst via a co-precipitation method for the photocatalytic oxidation of 4-chlorophenol (4-CP). It was suggested that the synergism between ferrocene and silicotungstate leads to the charge-transition from ferrocene to the POM unit, which ultimately contributes to the oxidation of the organic pollutant through a Fenton-like mechanism.²³¹ In another study, [Cs₃PMo^VI₁₂O₄₀] was used as a modifier of the semiconductor Bi₂O₃. The experimental results indicated that the [Cs₃PMo^VI₁₂O₄₀] generated on the surface of the semiconductor creates a P–N heterojunction photocatalyst with visible-light activity in the degradation of phenol. The best photocatalytic performance was observed when 2.5% (mol) of [Cs₃PMo^VI₁₂O₄₀] was added to the semiconductor. Also, trapping experiments showed that the major active species involved in the degradation process are superoxide and hydroxyl radicals.²³² Heterogenization of POMs with graphene aerogels (GA) has also shown promise in the adsorptive removal of a series of organic compounds from water.²³³ A more comprehensive analysis of the studies from the past 5 years is provided in Table S2 in SI.

2.4 Summary of water treatment technologies by polyoxometalates

Although the literature review shows promising evidence on how POM-based materials have attracted considerable attention for water treatment, like any emerging technology, they also have their own set of challenges and limitations. As tabulated in Table S2, POM-based materials have often been utilized as photocatalysts with high removal efficiencies. A key negative result that is rarely reported, but likely exists, is the structural instability of POM-based photocatalysts under realistic water matrices (containing chloride, carbonate, or natural organic matter). Such components can significantly suppress the photocatalytic activity or even partially decompose the structure, yet these effects are often not disclosed. Acknowledging this limitation is important for assessing the practical applicability of POM materials. For their broad implementation, they must also maintain the cost of processed water as low as possible. In this regard, substantial costs associated with synthesizing POMs and their composites remain as a significant challenge. In terms of the technology itself, other economically beneficial methods such as adsorption and ion exchange should also be considered, as they tend to provide more affordable solutions for water purification.

3 Removal of emerging health pollutants

Some of the most prominent classes of emerging health pollutants (EPs) are pharmaceuticals (antibiotics, antifungals, antidepressants, synthetic hormones)^{12,13,18,28,234} plant protection products (pesticides, biocides),^31,235 and microplastics.^235–237 Excessive use of antibiotics and cosmetic products, e.g., disinfectants and cleaning products, has led to the development of bacterial resistance through DNA mutations of bacterial cells, which have resulted in the adaptation and resistance of bacteria to these products.^24,25,238 In addition, bacterial resistance also occurs through the horizontal gene transfer mechanism from resistant bacteria to non-resistant bacteria through transformation, transduction, or conjugation.²⁵ Moreover, water bodies containing EPs play an essential role in this horizontal gene transfer mechanism by facilitating the horizontal gene transfer from pathogenic to non-pathogenic microorganisms. In addition to contributing to the development of antibiotic resistance, pollutants such as UV filters from sunscreens have been shown to harm marine life significantly. These compounds accumulate in aquatic environments and negatively affect organisms, including phytoplankton, corals, microalgae, and sea urchins, by disrupting their physiology and ecosystem functions.^24,239

A study, conducted over two consecutive years (2015 and 2016), on the final effluents from wastewater treatment plants in Europe, revealed high average concentrations of antibiotics in wastewater, especially in countries such as Portugal, Spain, and Ireland. The study identified that the most commonly found antibiotics, ciprofloxacin, azithromycin, and cephalexin, have a potentially significant impact on aquatic systems and the development of antibiotic resistance.^24,240

Ciprofloxacin, a fluoroquinolone antibiotic, and erythromycin have also been detected in effluents and surface waters in other studies,²⁴ and are included, along with the macrolides azithromycin and clarithromycin, as well as the penicillin-type antibiotic amoxicillin, in the surface water Watch List under the European Water Framework Directive.^17,240,241 More recently, this report has been updated to include other pharmaceutical products such as the antibacterials sulfamethoxazole and trimethoprim, the antifungal clotrimazole, fluconazole, and miconazole, the antidepressant venlafaxine, and the synthetic hormone norethisterone (Fig. 10).^240,241 In addition to the aforementioned pharmaceuticals, such as proton pump inhibitors (PPIs), lansoprazole and omeprazole,^242,243 have been proposed as potential Watch List candidates due to their recently discovered possible mutagenic and toxic effects on aquatic organisms.^17,25,240

Fig. 10 Emergent pharmaceuticals pollutants included in the updated 4th water watch list under the European Water Framework Directive: the antibacterial sulfamethaxazole and trimethoprim; the anti-fungus clotrimazole, fluconazole and miconazole; the antidepressant venlafaxine and the synthetic hormone norethisterone.¹⁷

Herein, we focus on the POMs' ability to degrade priority pharmaceuticals, mainly antibiotics, pesticides, microplastics, and dyes, to identify POMs with higher removal efficiency and kinetics, thus facilitating the development of more environmentally friendly POM materials.^244,245

3.1 Removal of pharmaceutical pollutants

Every day, humans release pharmaceutical products into the environment in different forms and under different circumstances. This behavior of humanity has a major impact on health and economy and has a profound effect on our lives. It is therefore of great importance to conduct environmental protection in an effective and inexpensive manner to combat emerging health pollutants. Some of the most prominent classes of emerging pharmaceutical pollutants are the antimicrobial pharmaceuticals (antibiotics, antifungals) and other pharmaceuticals (antidepressants, synthetic hormones). It has been described that contamination of the environment with these pharmaceutical products can lead to bacterial resistance, which is an emerging and growing phenomenon worldwide in the 21st century.^{10,20,22,24,246} Nonconventional low-cost adsorbents for pharmaceutical removal from wastewater, pollutant removal mechanisms, and detection using nanodevices and polymer-based adsorbents, as well as using fungal treatments, were recently summarized.^12,13,18 POMs have also been used for the detection of several pharmaceuticals, such as drugs of abuse²⁴⁷ and triclosan (TCS),²⁴⁸ as well for the selective extraction of antidepressants in undiluted urine.²⁴⁹ TCS, a diphenyl ether with antibacterial properties, is used as a disinfectant in antiseptic creams, toothpaste, hand soaps, deodorants, and even in plastics.^21,22 In Europe, TCS is one of the most frequently detected contaminants in wastewater. However, studies from the United States have reported that its concentration in wastewater can be up to five times higher.²² TCS has already been detected in surface waters in several regions of the world, including in fish tissues. In fact, the methylated form of TCS (M-TCS) is bioaccumulative in tissues, due to its lipophilic properties and stability. Moreover, it has been described that contamination of the environment with TCS can lead to bacterial resistance to four antibiotics: chloramphenicol, tetracycline, ciprofloxacin, and colistin. This resistance poses potential risks to human health as well as aquaculture.^21,22

Of the seventeen pharmaceutical pollutants mentioned above, only one study has referred to the removal of ciprofloxacin by POMs. He et al. immobilized three Keggin-type POMs [H₃PMo^VI₁₂O₄₀]·nH₂O, [H₃PW^VI₁₂O₄₀]·nH₂O, and [H₃PW^VI₁₂O₄₀]·nH₂O onto nitrogen-deficient carbon nitride nanosheets (g-C₃N₄) and successfully utilized all three POM-based composites (Fig. 11A) for the removal of ciprofloxacin within only five minutes under visible light irradiation with 93.1%, 97.4% and 95.6% efficiency, respectively.²⁵⁰ This type of POM-based hybrid material was further explored on g-C₃N₄/PW₁₂/TiO₂ composites (PW₁₂ = [H₃PW^VI₁₂O₄₀]) (Fig. 11A and B),^250,251 which showed remarkable and stable photocatalytic performance under visible light irradiation, not only for the removal of TC but also for bisphenol A and Cr(VI).²⁵¹ Their removal properties and stability without any observed structural changes in the photocatalyst were attributed to the enhanced adsorption under visible light irradiation, a high specific surface area, effective separation, and photoinduced charge transfer via g-C₃N₄ and PW₁₂.²⁵¹

Fig. 11 A) Nitrogen-deficient g-C₃N_x/POMs porous nanosheets (where x denotes N-deficiency) with P–N heterojunctions capable of photocatalytic degradation of drugs; recreated from ref. 250 with permission from The Royal Society of Chemistry. B) Fabrication of g-C₃N₄/PW₁₂/TiO₂ composite with enhanced photocatalytic performance under visible light; reproduced from ref. 251 with permission form Elsevier, copyright 2021.

Moreover, Cheng et al.²⁵² have utilized the isopolyoxotungstate, decatungstate [W^VI₁₀O₃₂]⁴⁻ (Fig. 2D) as a photocatalyst for the oxidation of sulfasalazine (SZZ),²⁵³ an antibiotic commonly found in wastewater, and its human metabolite sulfapyridine (SPD). After 120 min in the presence of H₂O₂ and under UV irradiation, the metabolite SPD was more efficiently removed (75%) by decatungstate than was the SZZ antibiotic (25%). The proposed photocatalytic mechanism (Fig. 12), which involves the generation and utilization of hydroxyl radicals (˙OH) in the photocatalytic degradation of sulfasalazine,²⁵² has attracted increasing attention over the past decades. This mechanism has been extensively studied in the ongoing research and development of novel pollution removal technologies.^254,255 Therefore, a similar strategy has been employed for the photodegradation of antibiotics such as nitrofurazone, tetracyclines and berberine under UV or visible light irradiation. This process utilizes H₂O₂ and the photoactive POM-based composite [H₃PW^VI₁₂O₄₀]@β-EDA-CD, as shown in Fig. 13A.²⁵⁶

Fig. 12 Cycle of photocatalysis and degradation of antibiotics (left) through the isopolyoxometalate decatungstate. Reproduced from ref. 252 with permission from Elsevier, copyright 2002.

Fig. 13 A) Multivalent supramolecular self-assembly between β-cyclodextrin derivatives and polyoxometalate for photodegradation of dyes and antibiotics; reproduced from ref. 256 with permission from The American Chemical Society, copyright 2019. B) Encapsulate polyoxometalate into metal–organic frameworks as efficient and recyclable photocatalyst for drugs degradation; reproduced from ref. 257 with permission from Elsevier, copyright 2019.

Li et al. prepared a POM-based photocatalyst, PW₁₂@MFM-300(In) (Fig. 13B), by using an environmentally friendly solvent-free method for the encapsulation of the POM [H₃PW^VI₁₂O₄₀] into the metal–organic framework MFM-300(In). The PW₁₂@MFM-300(In) composite displayed its activity for room temperature visible-light-driven catalytic degradation of the pharmaceutically active compound SMT with a 98% removal efficiency within 2 h.²⁵⁷

3.2 Removal of pesticides, microbes and microplastic

POM-based catalysts have been used for decades in pesticide degradation. The decatungstate [W^VI₁₀O₃₂]⁴⁻, mentioned in the context of the removal of pharmaceutical pollutants (section 3.1), also showed photocatalytic activity in the degradation of two common pesticides, 2-(1-naphthyl)acetamide (NAD) and 2-mercaptobenzothiazole (MBT). In the study of da Silva et al., it was shown that [W^VI₁₀O₃₂]⁴⁻ could promote UV-light-driven degradation of NAD with an efficiency of 89% within 8 h.²⁵⁸ Additionally, Allaoui et al. described the photodegradation of the pesticide MBT using Na₄W^VI₁₀O₃₂ as a catalyst with an efficiency of 90% within 8 h.²⁵⁹ It has been proposed that the photodegradation of MBT occurs via e⁻ transfer and H-atom abstraction processes with W^VI₁₀O₃₂⁴⁻* excited species. The main products of such photodegradation when using decatungstate as a catalyst are monohydroxylated products, sulfoxide derivatives, and dimers of MBT. The whole process was shown to be O₂ dependent because photodegradation was restricted by W^VI₁₀O₃₂⁵⁻ reoxidation.²⁵⁹ The Keggin-type POM [PW^VI₁₂O₄₀]³⁻ showed activity for the complete photocatalytic degradation of the pesticide lindane to CO₂, H₂O, and Cl⁻ in an aqueous solution.²⁶⁰ Photocatalysis of lindane by [PW^VI₁₂O₄₀]³⁻ follows the same principle as that of TiO₂ catalysis, i.e. processes involving both oxidation and reduction pathways such as chlorination, dechlorination, hydroxylation, hydrogenation, dehydrogenation, which lead to the C–C bond cleavage and complete mineralization to the final products.²⁶⁰ Recently, a POM-IL²⁶¹ has also been used for the extraction of triazole pesticides (e.g., penconazole, hexaconazole, diniconazole, tebuconazole, triticonazole, and difenconazole) from aqueous samples.²⁶² In that article, the prepared POM-IL nanomaterial ([3-(1-methylimidazolium-3-yl)propane-1-sulfonate]₃PW^VI₁₂O₄₀) was utilized as a coating for a new solid-phase microextraction (SPME) device that was then successfully applied for the extraction of the six triazole pesticides from real aqueous samples. The longevity experiments (at least 50 extractions) of POM-IL coated SPME devices compared with commercially available PDMS-coated SPME devices (PDMS = polydimethylsiloxane) showed that the newly prepared device offers higher extraction efficiency and better longevity.²⁶² Moreover, the type of POM-IL material (Fig. 6), already described in section 2.2, was shown to efficiently remove previously mentioned inorganic and organic contaminants from wastewater, as well as various microbial pollutants, E. coli and B. subtilis.⁶³ Recent developments in these organic/inorganic hybrid materials, POM-based ionic liquid crystals and POM-ILs, and their applications, mainly in pollutants degradation, including microplastics, have been reported.²⁶³

Microplastics (MPs) are among the newly emergent health pollutants of worldwide concern, and their impact on human health and the environment is not yet completely understood.²⁶⁴ The first reported example of magnetic polyoxometalate-based ionic liquid phases (magPOM-SILPs) for the removal of MPs was designed by anchoring a POM-IL composite (POM = [α-SiW^VI₁₁O₃₉]⁸⁻ (Fig. 2F); IL = (n-C₇H₁₅)₄N⁺) to an Si-enclosed Fe₂O₃ supermagnetic core, Fe₂O₃@SiO₂ (Fig. 14). The magPOM-SILPs composite showed remarkable effectiveness (90%) for removing microplastic by binding MPs particles via the formation of hydrophobic interactions with the MPs surface and then removing MPs pollutants from water samples by magnetic recovery (Fig. 14).²⁶³

Fig. 14 Magnetic polyoxometalate-supported ionic liquid (magPOM-SILPs) for heavy metals, organic dyes, microbes and microplastics water removal.²⁶³

Cobalt-based POMs, Na₁₀[Co₄(H₂O)₂(V^VW^VI₉O₃₄)₂]·34H₂O were also examined for dye degradation. MB and RhB dyes were chosen as the subject dyes for the degradation test because of their carcinogenic properties and wide use in the textile industry. A 10 mg L⁻¹ dosage of this POM removed 87.8% of MB in 30 min. The time required for the complete decomposition of RhB was almost twice as long as that of MB. In this study, in addition to the excellent dye catalytic activity, these CoV-POMs also showed anticancer activities.²⁶⁵ However, POMs anticancer, antibacterial studies, and other biomedical studies are described elsewhere.^85,266–268 Another recent study, described the synthesis of two Keggin-type polyoxometalates ammonium phosphomolybdate (NH₄)₃PMo^VI₁₂O₄₀ (PMo) and ammonium phosphotungstate (NH₄)₃PW^VI₁₂O₄₀ (PW) that were used as adsorbents for the removal of various antibiotics and heavy metals from water systems. The adsorption efficiency of PMo for dyes and heavy metals was higher than that of PW for various antibiotics such as tetracycline. It was suggested that the more negative surface charges induced by Mo atoms with more electronegativity and higher specific surface area contributed to the superior adsorption efficiency of PMo for antibiotics and heavy metals.²⁶⁹

Table 1 summarizes the recent examples of POMs applications in removal of EPs covered in section 3.

Table 1 Examples of recent polyoxometalates studies in pollutants degradation: antibiotics (A), dyes (D), plastics (P), industrial chemicals (IC) and pesticides (Pest)

Formula	POM archetype	Pollutant	Conditions	Efficiency	Number of cycles	Ref.
Na4W^VI10O32	Decatungstate	Sulfasalazine	c(catalyst) = 40 μM; under UV irradiation	25% removal within 120 min	1	252
		(A) sulfapyridine		75% removal within 120 min
g-C3N4-POMs	Keggin	(A) ciprofloxacin	m(catalyst) = 0.01–0.1 g; under visible light	93% removal within 5 min	1	250
POMs: [PMo^VI12O40]^3−, [PW^VI12O40]^3−, [SiW^VI12O40]^4−
g-C3N4/H3PW^VI12O40/TiO2	Keggin	(A) tetracycline	m(catalyst) = 20 mg	>70% removal within 50 min (k = 0.03443 min^−1)	1	251
		(P) bisphenol A	m(catalyst) = 20 mg	>38% removal within 3 hours (k = 0.00712 min^−1)	1
		(IC) Cr(VI)	m(catalyst) = 20 mg	>65% removal within 60 min (k = 0.025 min^−1)	1
POM-IL, [3-(1-methylimidazolium-3-yl) propane-1-sulfonate]3PW^VI12O40	Keggin	(Pest) diniconazole	nsp	nsp	1	262
		(Pest) hexaconazole	nsp	nsp	1
		(Pest) tebuconazole	nsp	nsp	1
		(Pest) penconazole	nsp	nsp	1
		(Pest) diniconazole	nsp	nsp	1
		(Pest) triticonazole	nsp	nsp	1
Biochar-doped g-C3N4-Co2PMo11VO40	Keggin	(A) sulfamethoxazole	m(catalyst) = 0.2 g L^−1; under visible light	98.5% within 20 min (k = 0.215 min^−1)	1	273
Ag-L-SiW12@BiVO4 (L = thiacalix[4]arene)	Keggin	(A) ciprofloxacin	pH = 4; v(catalyst) = 30 μL; under simulated solar light	95% within 240 min (k = 0.0118 min^−1)	1	274
H3PW12O40–Fe3O4-biocar	Keggin	(A) metronidazole	pH = 1; c(catalyst) = 0.6 g L^−1	>94% removal within 60 min	1	275
α-K8SiW11O39-MIL-101(Cr)-CoFe2O4	Lacunary Keggin	(D) methylene blue	m(catalyst) = 30 mg	Methylene blue = 100% within 25 min	1	276
		(D) rhodamine B		Rhodamine B = 84% within 50 min
		(D) methyl orange		Methyl orange = 37% within 20 min
		(A) ciprofloxacin		Ciprofloxacin = 100% within 15 min
EDA-CD-[H3PW^VI12O40], (EDA-CD = per-6-deoxy-6-ethylenediamine-β-cyclodextrine)	Keggin	(A) nitrofurazone	c(catalyst) = 0.055 mM; under UV irradiation or sunlight; H2O2	k = 0.163 min^−1	1	256
		(A) tetracyclines	With H2O2	k = 0.152 min^−1	1
		(A) berberine	With H2O2	k = 0.115 min^−1	1
		(D) rhodamine B	With H2O2	k = 0.868 min^−1	1
		(D) xylenol Orange	With H2O2	k = 0.214 min^−1	1
		(D) methyl Orange	With H2O2	k = 0.164 min^−1	1
		(D) methylene blue	With H2O2	k = 0.119 min^−1	1
		(D) crystal violet	With H2O2	k = 0.084 min^−1	1
[H3PW^VI12O40],@MFM-300(In)	Keggin	(A) sulfamethazine (SMT)	nsp	98% removal within 60 min	1	257
MFM-300(In) = indium-based metal–organic framework
LnTiO2/P2W^VI18Sn3	Keggin	(D) methyl orange	nsp	100% removal within 5 min	1	270
Na4W^VI10O32	Decatungstate	(Pest) 2-(1-naphthyl)acetamide (NAD)	c(catalyst) = 300 μM	89% removal within 8 hours (k = 0.032 min^−1)	1	258
K2[V^V10O16(OH)6(CH3CH2CO2)6]	Decavanadate	(D) methylene blue	m(catalyst) = 5 mg	93% removal within 45 min	1	271
[Cu(OH2)3(2-amp)]2(trisH)2[V^V10O28]	Decavanadate	(D) methylene blue	m(catalyst) = 2–10 mg; with H2O2	93% removal within 2 min	1	272
2-amp = 2-aminopyridine
Tris = tris(hydroxymethyl)aminomethane
Na10[Co4(H2O)2(V^VW^VI9O34)2]·34H2O	Keggin	(D) methylene blue	c(catalyst) = 10 mg L^−1	88% removal within 30 min	1	265
		(D) rhodamine B		88% removal within 60 min
NH4PW^VI12O40 (PW)	Keggin	(IC) Ni^2+	m(catalyst) = 30 mg	72% removal within 1 min (PW)	1	269
NH4PMo^VI12O40 (PMo)				90% removal within 1 min (PMo)
		(D) tetracycline	m(catalyst) = 30 mg	71% removal within 30 min (PW)
				92% removal within 30 min (PMo)
α-H3PW^VI12O40·6H2O	Keggin	(D) methylene blue	m(catalyst) = 5 mg	>90% removal for all dyes within 30 min	1	277
α-H3PMo^VI12O40·14H2O		(D) rhodamine B
		(D) crystal violet
		(D) methyl orange
		(D) sunset yellow

---

3.3 Summary of POM-based technologies in removal of emerging health pollutants

Section 3 highlights emerging pollutants in the 21st century environment, such as drugs, pesticides, and microplastics, and emphasizes their dangers and consequences for human health. Several examples illustrate the use of pure POMs, nanoparticles, composites, or MOFs for removing organic and inorganic pollutants. The processes involving POMs in pollutant degradation are also discussed, many of which employ photocatalysis by UV and/or visible irradiation, in addition to adsorption or magnetic removal. In short, the different types of POMs mentioned in this section reveal their essential role in removing emerging pollutants from the environment, proving to be efficient and selective.

4 Polyoxometalates in air pollution

Various POMs alone and in combination with other compounds,^112,278 such as MOFs, CNTs and mesoporous silica supports, have shown promising results in the removal of air pollutants, such as refractory sulfur compounds²⁷⁹ from fossil fuels (section 4.1), toxic gases such as hydrogen sulfide¹¹⁶ (section 4.2.1), nitrogen oxides and sulfur dioxide²⁸⁰ (section 4.2.2) and carcinogenic volatile organic compounds (VOCs; section 4.3) present in indoor and outdoor air.^281,282

Among POM archetypes, Keggin-type structures dominate air purification applications due to their high catalytic activity, particularly in the oxidative desulfurization of refractory sulfur compounds from fossil fuels under mild conditions⁶¹ (∼85% of the reported literature; Table S1). Anderson–Evans POMs also contribute effectively to the desulfurization of fossil fuels by showing promising desulfurization performance through alkyl peroxide formation mechanisms with extended catalyst lifetimes.^283,284 Wells–Dawson-type POMs, especially when doped with lanthanide ions, exhibit enhanced regeneration and stability, making them effective for toxic gas removal (section 4.2; Table 2), such as H₂S, NO_x, and SO₂. Their tunable redox states and structural differences tailor their catalytic behavior, with rare-earth-doped Wells–Dawson POMs¹¹⁶ showing superior H₂S oxidation and the photocatalytic activity of Keggin/g-C₃N₄ composites enabling efficient VOC removal under visible light.^285,286 These reported examples of using different POM structures highlight the unique functions and advantages that structural diversity in POM chemistry provides for air pollutant remediation.^{116,248,282,283,285}

Table 2 List of polyoxometalates and POM-based materials utilized in air purification. All POMs are ordered chronologically from the most recent to the oldest published paper

Formula	POM archetype	Conditions	Efficiency	Number of cycles	Ref.
a nsp – not specified by authors.
Removal of H2S
PMo12@RH-MCM-14	Keggin (Fig. 2F)	T = rt; t = 120 min; m(catalyst) = 0.3 g; c0(H2S) = 1000 mg m^−3; flow rate = 100 mL min^−1 (N2/H2S gas mixture)	61.3% yield of H2S transformation to S	More than 8	327
PMo12 = [H3PMo^VI12O40]
(Himi)2[S^VIMo^VI12O40]·(imi)2·H2O	Keggin (Fig. 2F)	T = 0–50 °C; pH = 4–9; c(POM) = 1 mmol L^−1; c(H2S) = 2 g m^−3; flow rate = 100 mL min^−1 (N2/H2S gas mixture)	H2S capacity in water: 627 mg g^−1; after electro treatment up to 2174 mg g^−1	4 cycles	328
imi = imidazole
(n-Bu4N)3[VMo^VI12O40]/[Bmim]Oac	Keggin (Fig. 2F)	T = 150 °C; c(POM) = 0.005 mol L^−1; flow rate = 100 mL min^−1 (N2/H2S gas mixture); t = 10 h	98.6% within 10 h	At least 4 cycles	329
[Bmim] = 1-butyl-3-methylimidazolium
(NH4)11[Ln^III(PMo^VI11O39)2]	Lacunary Keggin (Fig. 2G)	T = rt; pH = 5; t = 360 min; c(catalyst) = 0.002 M; c0(H2S) = 2900 mg m^−3	94.8% within 360 min	At least 4	330
Ln = Sm, Ce, Dy and Gd
K17[Pr^III(P2Mo^VI17O61)2]	Wells–Dawson (Fig. 2H)	T = 25 °C; pH = 6.8; t = 400 min; c(catalyst) = 0.015 M; c0(H2S) = 2200 mg m^−3	90% within 400 min	nsp^a	116
K17[Gd^III(P2Mo^VI17O61)2]
K17[Sm^III(P2Mo^VI17O61)2]
K17[Eu^III(P2Mo^VI17O61)2]
[C4mim]3[PMo^VI12O40]-[C4mim]Cl	Keggin (Fig. 2F)	T = 80–180 °C; t = 60 min; H2S flow rate 100 mL min^−1; c(catalyst) = 0.001 M	100% within 60 min	More than 6	299
[C4mim] = 1-butyl-3-methylimidazolium
TM-salts of [H4PMo^VI11V^VO40],	Keggin (Fig. 2F)	T = 25 °C; t = 300 min; H2S gas flow = 200 mL min^−1; c0(H2S) = 1241 mg m^−3; c(catalyst) = 0.01 M, H2O2 – oxidant	98% within 300 min	nsp^a	54
(TM = Cu^II, Fe^III, Zn^II, Mn^IV and Cr^VI)
PyBs-PW, PhPyBs-PW and QBs-PW	Keggin (Fig. 2F)	T = 70 °C; t = 10 min; n(H2S)0 = 1 mmol, 30% H2O2 (n = 1 mmol); solvent mixture H2O/EtOH (v : v = 7 : 3); m(catalyst) = 80 mg	98% within 10 min	At least 5	331
PW = H3PW^VI12O40
[{(CH3)4N}4Cu^IIPW^VI11O39H]	Lacunary Keggin (Fig. 2G)	T = rt.; t = 20 h; m(cat.) = 10 mg; c(H2S)0 = 0.1 M	95.0% within 20 h	At least 2	300
[Na2HPMo^VI12O40]	Keggin (Fig. 2F)	T = 20 °C; c(catalyst) = 1.25 × 10^−2 M; c0(H2S) = 240.72 mg m^−3; H2S gas flow = 0.5 L min^−1	Sulfur loading capacity of 1.14 mol of H2S per mol of POM	nsp^a	332
[Na3PMo^VI12O40]	Keggin (Fig. 2F)	T = rt; t = 47 min; c(adsorbent) = 5 × 10^−3 M; c0(H2S) = 500.863 mg m^−3; H2S gas flow = 3.931 L min^−1	Up to 99.67% within 35–50 min	nsp^a	333
([Na3PMo^VI12O40] : NaVO3 : Na2CO3 : NaCl = 1 : 1 : 0.377 : 5.472)
PCDES@3C14-2Im	Keggin (Fig. 2F)	T = 25–200 °C; c(adsorbent) = 0.01 mol L^−1; t = 150 min, H2O2; 12.93 mg H2S per g adsorbent	Up to 100% within 150 min	At least 5	334
PCDES = long-chain ionic liquid hybrid POM deep eutectic solvent, POM present as [C14mim]3PMo^VI12O40
[C14mim] = 1-tetradecyl-3-methylimidazolium
PPILs@IBuPN-9	Keggin (Fig. 2F)	T = 100–200 °C; t = 2 h; c(PPILs@IBuPN-9) = 0.015 mol L^−1; 21.88 mg H2S per g PPILs@IBuPN-9	Up to 100% fpr 120 min	At least 4 cycles	335
PPILs = phosphazene POM ionic liquid, 1-bityl-3-methylimidazolium chloride with phosphazenes and H3PMo^VI12O40
PMo12@UiO-66@H2S-MIP-β-CDs	Keggin (Fig. 2F)	T = room temperature; 31.67 mg H2S per g; m(adsorbent) = 0.3 g, H2S 1000 mg m^−3	Up to 31.67 mg g^−1 H2S within 150 min	5 cycles	336
CD = β-cyclodextrin, MIP = molecular imprinted polymers; UiO-66 = metal–organic framework
PMo12-BmimCl@SiO2-0.05%	Keggin (Fig. 2F)	m(PMo12-BmimCl@SiO2–0.05%) = 5 g; H2S 1000 mg m^−3, flow rate = 100 mL min^−1; T = 100–200 °C	97% desulfurization for 480 min	3 cycles	337
BmimCl = 1-butyl-3-methylimidazolium chloride
Removal of NOx and SO2
PW12@Bi2O3−x/Bi	Keggin (Fig. 2F)	LED lamp (λ > 420 nm); m(catalyst) = 0.3 mg; c(NO) = 600 ppb (in air mixture), flow rate (NO) = 500 mL min^−1	83.3% within 30 min (in gas phase)	nsp^a	338
PW12 = H3PW^VI12O40
x = nsp
[H4GeW^VI12O40](HGeW), [H5GeW^VI11V^VO40] (HGeWV)	Keggin (Fig. 2F)	T = 100–350 °C; rate = 4 °C min^−1; t = 90 min; c(NOx) = 1696 mg m^−3; c(O2) = 8 vol%; c(H2O vapor) = 5 vol%	81.5% NOx removal with N2 selectivity of 68.3% within 90 min	At least 3	314
[H5GeMo^VI11V^VO40] (HGeMoV)
[H5GeW^VI9Mo^VI2V^VO40] (HGeWMoV)
H6P2W^VI18O62·28H2O	Wells–Dawson (Fig. 2H)	T = 50–200 °C; t = 60 min; c0(NOx) = 1696 mg m^−3; c(O2) = 8 vol%; c(vapor) = 4.5 vol%	Up to 90% of NOx adsorption within 60 min	At least 2	339
[Fe^III(C4H5NO4)]3[PW^VI12O40]·14H2O	Keggin (Fig. 2F)	T = 50 °C; t = 15 min; c(H2O2) = 4 mol L^−1; pH = 5.5; c0(NO) = 603 mg m^−3	94.6% within 15 min	3	312
(Fe^IIIAspPW)
Ce^IVO2/H3PW^VI12O40	Keggin (Fig. 2F)	T = 160–220 °C; t = 30 min; c0(NO) = 600 mg m^−3; c(NH3) = 600 mg m^−3	90% NO removal within 30 min	nsp^a	340
H4[(Cu4Cl)3(BTC)8]2[SiW^VI12O40]·(C4H12N)6·3H2O (NENU-15)	Keggin (Fig. 2F)	T = 20–300 °C; c(NO) = 1.74 mmol g^−1; m(cat.) = 0.2 g; gas mixture NO (5%) and He (95%), gas flow rate = 30 mL min^−1	NO adsorption efficiency of 1.74 mmol g^−1 of NO at rt, and 64% efficiency at 300 °C	nsp^a	341
[Fe^III(C4H5NO4)]3[PW^VI12O40]·14H2O	Keggin (Fig. 2F)	T = 65–80 °C; t = 15 min; c(NO)inlet = 614 mg m^−3; c(SO2)inlet = 2094 mg m^−3; c(catalyst) = 0.5 g L^−1	84.27% (NO) and 100% (SO2) within 15 min	3	280
(Fe^IIIAspPW)
HPW^VI–M/Ce^IVxZr^IV4−xO8 and HPW^VI–M/Ti^IVxZr^IV1−xO4	Keggin (Fig. 2F)	T = 170–250 °C; t = 31–32 min; m(catalyst) = 300 mg, gas mixture: NO = NO2 = 500 ppm, O2 = 10%, CO2 = 5%, H2O = 5%	48% NOx reduction efficiency and 84% NOx storage efficiency within 31–32 min	12	342
(M = Pt^IV, Pd^II or Rh^III (1 wt%); Zr^IV/Ce^IV = 0.5; Zr^IV/Ti^IV = 0.5)
H3PW^VI12O40·6H2O (HPW)	Keggin (Fig. 2F)	T = 80–170 °C; m(HPW) = 330 mg; gas mixture: NO = NO2 = 500 ppm, O2 = 10%, CO2 = 5%, H2O = 5%	NOx adsorption amount is equal to 38 mg g^−1 of HPW	6	343
[(NH4)3PW^VI12O40]	Keggin (Fig. 2F)	T = 150 °C; t = 60 min; He gas flow = 15 mL min^−1; n(NO2) = 17.0 μmol	68% NO2 removal within 60 min	3	344
MnCeOx–SiW, where SiW = H4[SiW^VI12O40]	Keggin (Fig. 2F)	Gas mixture: 100 ppm chlorobenzene, 500 ppm NO and 500 ppm NH3, 11 vol% O2; T = 120–180 °C; t = 30 min; m(catalyst) = 200 mg	100% NO and chlorobenzene conversion at 180 °C	nsp^a	345
10HPW-CS-Ce0.3–TiO2,	Keggin (Fig. 2F)	Gas mixture: 50 ppm chlorobenzene, 500 ppm NO, 500 ppm NH3, 5 vol% O2, and N2 as balance gas; m(catalyst) = 100 mg; T = 167–291 °C	100% conversion of NO at 167–288 °C, 90% conversion of chlorobenzene at 291 °C	nsp^a	346
HPW = H3PW^VI12O40, CS = chitosan
Removal of aldehydes
[SiW^VI9O37Ru^III3(H2O)3Cl3]^7−/CSH	Keggin (Fig. 2F)	T = rt; c(CH2O) = 833 ppm ± 10%; CH2O gas flow rate = 0.25 dm^3 min^−1; m(catalyst) = 110 mg	44% for 1st cycle	5	327
CSH = cellulose propylamine-modified silica
[n-Bu4N]4H5PW^VI6V^V6O40·20H2O (PW6V6)	Keggin (Fig. 2F)	T = rt; t = 144 h; c(CH2O) = 0.52 mol L^−1; P(air) = 1 atm; c(catalyst) = 3.8 mmol L^−1; solvent–DMA : H2O (v/v = 20/1); v(solvent) = 2 mL	Up to 42% of CH2O conversion within 144 h	At least 3	347
[n-Bu4N]6[PW^VI9V^V3O40] (PW9V3)
[n-Bu4N]5H2PW^VI8V^V4O40 (PW8V4)
H5PMo^VI10V^V2O40/APTS/SBA-15	Keggin (Fig. 2F)	T = 20 °C; t = 24 h; m(catalyst) = 0.1 g; v(O2) = 500 mL; O2 – oxidant	Up to 73% acetaldehyde conversion after 24 h	5	348
H6PMo^VI9V^V3O40/APTS/SBA-15
H4PMo^VI11V^VO40/APTS/SBA-15
APTS = γ-aminopropyltriethoxysilane
SBA-15 = aminosilylated silica
NaH3[SiW^VI11Ce^IVO39]	Keggin (Fig. 2F)	T = 20–60 °C; t = 5 h; P = 1 atm; c(CH2O) = 4 mM; c(catalyst) = 5.2 mM; solvent H2O	85% CH2O conversion within 5 h	30	278
TBA4HPW^VI11Co^IIIO39	Keggin (Fig. 2F)	T = 20–40 °C; t = 6 h; P = 1 atm; m(catalyst) = 100 mg; solvents: MeCN or H2O	92% conversion of isobutyraldehyde	At least 3	282

---

4.1 Removal of refractory sulfur compounds from fossil fuels

The governments worldwide have introduced stricter regulations and restrictions on the amount of sulfur in fuels to ultra-low levels (<10 ppm).⁵³ Therefore, the main goal of industry and science is to find a way to make the fuel desulfurization method efficient, inexpensive, clean, and safe.^52,53 Currently, the established industrial standard for fossil fuel desulfurization is hydrodesulfurization (HDS). The HDS method has proven itself to be very effective in removing thiols, inorganic sulfides, and disulfides. However, due to new regulations requiring ultra-low sulfur fuels,⁵³ HDS is insufficiently effective for removing the more difficult-to-remove refractory sulfur compounds. Moreover, HDS is a very expensive method and operates under harsh reaction conditions of 300–400 °C and 30–100 bar H₂ pressure. In contrast, POM-based oxidative desulfurization (ODS) operates under mild conditions (rt −100 °C, atmospheric pressure, H₂O₂/O₂ (Table S1)). POMs provide competitive advantages for the needed ultra-low sulfur fuels (<10 ppm)⁵³ through their reversible multi-electron redox capability, oxygen-rich surfaces, and high catalytic stability. This eliminates high-pressure H₂ handling and reduces energy demands for heating and compression.^52,61 ODS-based systems achieve 84–98% sulfur conversion from 3.5 wt% to <0.5 wt% with 55.57% energy efficiency, demonstrating superior energy utilization for refractory sulfur compounds like DBTs.²⁸⁷ Electrochemical regeneration (H₂O₂/O₂) further enhances POM recyclability (in most reported literature: >95% recovery, and 10+ cycles; Table S1). These data show that the ODS system is more energy cost-efficient for deep desulfurization than HDS.^287,288

He et al. reported a series of Keggin-type K_x[PMo^VI₁₂O₄₀] (K_xPMo, x = 1, 2, 3, 4) polyoxometalate salts prepared by hydrothermal synthesis using commercial F127 templates (Pluronic F127). The prepared K_xPMo salts (Fig. 15A) were mesoporous with a high surface area (>40 m² g⁻¹) and could be successfully utilized for complete ODS of model oil in 1 h. By comparing the catalytic activity of the prepared POM salts, K₄PMo showed the highest activity in the ODS process with a DBT removal rate of 99.5% within 60 minutes (Table S1 in SI, k = 0.076 min⁻¹). A reaction mechanism of DBT oxidation by the K₄PMo/H₂O₂ catalytic system has been proposed (Fig. 15B).²⁸⁹ In addition, the K₄PMo catalyst also showed activity for the removal of other refractory sulfur compounds, DMDBT and BT, with removal efficiencies of 99.0% and 60.3%, respectively. The authors concluded that the ODS activity of K_xPMo catalysts has a linear correlation with their electrochemically active surface area (ECSA). The higher activity of the K₄Mo catalyst can therefore be attributed to its largest ECSA value, which shows that K₄PMo exposes the largest number of anions [PMo^VI₁₂O₄₀]³⁻ among all prepared catalysts. XRD structural analysis confirmed the good structural stability and successful recovery of the K₄PMo catalyst that was used.²⁸⁹

Fig. 15 A) Illustration of one-pot hydrothermal synthesis of mesoporous K_xPMo material. The resulting K_xPMo material was highly crystalline with uniform and spherical morphology. It is denoted as K_xPMo, where x denotes the amount of HPMo added to the initial mixture. B) A schematic representation of the DBT oxidation mechanism in the presence of H₂O₂ catalysed by K_xPMo. DBT preferentially resides in the biphasic system's oil phase (n-octane), whereas the H₂O₂ oxidant and K_xPMo catalyst primarily reside in the extractant phase (methanol). Therefore, the first step is to extract into the extractant phase to react with H₂O₂ in the presence of K_xPMo.²⁸⁹

Besides commonly utilized Keggin-type POMs, other archetypes, especially Anderson–Evans and Wells–Dawson, have also been used in the ODS process. Eseva et al. prepared a series of Anderson-type polyoxometalates (Fig. 16), (NR₄)₃[X^IIIMo^VI₆O₂₄H₆] (X^III = Cr, Fe, Co; R = H or alkyl), and tested their catalytic properties in the ODS process of model fuel. The Co(III)-based Anderson type POM exhibited the highest catalytic activity in the desulfurization of model diesel with a 100% conversion rate of DBT within 60 minutes with a molar ratio of n(S) : n(cat.) = 50 : 1 (Table S1 in SI). By prolonging the reaction time to 120 min, 100% conversion was also achieved for BT. However, for 3-methylbenzene, only 59% conversion was achieved in 4 h.²⁸³ A reaction mechanism for DBT oxidation by the Co(III)-POM has been proposed (Fig. 16). The crucial oxidation step in the catalytic system is based on the oxidation of a solvent (decalin), with the formation of an alkyl peroxide as the active species. Alkyl peroxide formation occurs by the reaction with an O₂ molecule from the air in the presence of a Co(III)-POM to form alkyl peroxides and the subsequent formation of the polyoxometalate's metal-dioxo species, as the source of active oxygen in the further oxidation of DBT. The quaternary ammonium cation in the (NR₄)₃[X^IIIMo^VI₆O₂₄H₆] catalyst structure allows the catalyst to adsorb the substrate molecules (DBT) and coordinate with the sulfur atom, after which the coordinated DBT is oxidized to a sulfone, thus simultaneously reducing (NR₄)₃[Co^IIIMo^VI₆O₂₄H₆] POM. The reduced form of (NR₄)₃[Co^IIIMo^VI₆O₂₄H₆] POM is re-oxidized with a new peroxide molecule, and a new catalytic cycle is started.²⁸³

Fig. 16 A schematic representation of DBT oxidation mechanism catalyzed by Anderson-type polyoxometalates ((NR₄)₃[X^IIIMo^VI₆O₂₄H₆] (X = Cr, Fe, Co; R = H or alkyl)) in the presence of O₂ from air.²⁸³

Hybrid POM-based materials have also been researched and have shown promising results as catalysts in ODS processes. Chi et al. reported the preparation of a new biomimetic catalytic system consisting of an Anderson-type POM ([Na₃H₆Cr^IIIMo^VI₆O₂₄]) and deep eutectic solvents (DESs) and its successful application as a catalyst for the removal of sulfur compounds from both model and commercial diesel.²⁸⁴ Six different DESs (PEG/PAS, PEG/SSA, PEG/SA, PEG/DHBA, PEG/PXA and PEG/DL-MA) were combined with CrMo₆ (Fig. 17), and their activity was tested. Only the addition of PEG/SSA, DES, containing an –SO₃H group, resulted in 100% sulfur removal, while utilizing other DESs resulted in no higher than 30% sulfur removal.²⁸⁴ The desulfurization process followed the extraction–oxidation mechanism in which the POM and the DES acted as the electron transfer mediators and were both crucial for the process (Fig. 17).²⁸⁴

Fig. 17 Schematic representation of the reaction mechanism for the oxidation desulfurization of DBT catalysed by coupling CrMo₆ polyoxometalate with DESs under mild conditions (T = 60 °C).²⁸⁴

Ye et al. designed a new porous POM-based hybrid material by encapsulating a Keggin-type polyoxometalate [H₃PW^VI₁₂O₄₀] (PW) in the metal–organic framework UiO-66(Zr) and employed it as a catalyst in the ODS reaction of BT, DBT, and DMDBT at room temperature, with 98.2% DBT removal efficiency.²⁹⁰ A proposed reaction mechanism includes the extraction of DBT molecules from the model oil into the acetonitrile phase by the POM catalyst and H₂O₂. After extraction, DBT and H₂O₂ can be adsorbed into the catalyst pores, leading to the formation of ˙OH radicals via electron transfer from Zr–OH₂ active centers in UiO-66 (Zr). Another H₂O₂ molecule can react with a W(VI) metal ion in the [H₃PW^VI₁₂O₄₀] POM to form the W(VI)-peroxo species that lead to the formation of O₂˙⁻ radicals. Both O₂˙⁻ and ˙OH radicals can oxidize DBT to DBTO₂. The existence of two types of active centers in the catalyst, W(VI) in [H₃PW^VI₁₂O₄₀] and Zr–OH₂ in UiO-66 (Zr), which forms two different active species, is probably responsible for the high efficiency of the catalyst in the ODS process.²⁹⁰

For the desulfurization of fossil fuels, Gao et al. prepared a series of Wells–Dawson-type POMs [H_6+nP₂Mo^VI_18−nV^V_nO₆₂·mH₂O] (n = 1–5; Mo₁₇V₁, Mo₁₆V₂, Mo₁₅V₃, Mo₁₄V₄, and Mo₁₃V₅), immobilized them on CNT carriers, and thereby prepared two different types of catalysts, CNT@PDDA@POM and POM@CNT.²⁹¹ All prepared POM-based materials have shown to be catalytically active in the ODS process. CNT@PDDA@Mo₁₆V₂ showed the highest catalytic activity with 99.4% desulfurization efficiency. The better efficiency of this type of catalyst was due to a different POM position in CNT@PDDA@POM (on the surface of CNT@PDDA) compared to POM@CNT (deep in the CNTs' channel). Moreover, it was observed that the number of Mo centers replaced with V centers affects the efficiency, with a 16 : 2 ratio being the optimal Mo : V ratio for obtaining a high desulfurization activity of both catalysts. By combining CNT carriers with high mechanical properties, high thermal stability, and a high specific surface area, Gao et al. overcame disadvantages such as a low specific surface area and the difficulty of reclamation for pure POMs.²⁹¹ More literature-known POM-based catalysts and their efficiency in the removal of refractory compounds from fossil fuels are summarized in Table S1 in the SI.

4.2 Removal of toxic gases – H₂S, NO_x and SO₂

4.2.1 Hydrogen sulfide (H₂S) in air pollution. Hydrogen sulfide is naturally present in crude petroleum, natural gas, volcanic gases, and geothermal sources. It is also a common by-product of many human activities, such as wastewater treatment,²⁹² fossil fuel combustion,⁵⁴ sewage treatment facilities,⁵⁵ paper factories,⁵⁶ food processing factories, and agriculture.⁵⁷ Hydrogen sulfide is an odorous toxic gas with a corrosive nature and an adverse effect on human health and directly affects industrial production by reducing industrial catalysts' efficiency and causing equipment failure. It can also easily oxidize and form SO₂ gas (eqn (1)), one of the leading causes of acid rain:⁵⁸

H₂S + 3/2O₂ → SO₂ + H₂O (1)

Furthermore, hydrogen sulfide readily reacts with metals, such as copper, and forms the corresponding sulfides (Cu₂S) on the surface of electrical devices, causing electrical failures. H₂S can also cause corrosion on surfaces, which can cause damage to buildings, for example, sewage plant facilities.²⁹³ In addition to SO₂ (section 4.2.2), H₂S can react with different compounds present in the atmosphere and form many other toxic by-products, such as carbonyl sulfides (eqn (2)), carbon disulfides (eqn (3)), sulfurous acid (eqn (4)), and PMs, that have been linked to ozone layer depletion:²⁹⁴

H₂S + CO₂ → COS + H₂O (2)

2H₂S + CH₄ → CS₂ + 4H₂ (3)

2H₂S + 3O₂ → 2H₂SO₃ (4)

Scientists and engineers have developed different methods for removing H₂S from the environment, such as metal oxide oxidation,²⁹⁵ adsorption using different adsorbents (activated carbon or wet scrubbing),²⁹⁶ the Claus process,²⁹⁷ biofiltration, oxidative desulfurization, and the LRSR process.²⁹⁸ The latter two methods are recently the most commonly used methods with a very high desulfurization capacity and efficient production of elemental sulfur using various redox mediators (e.g., Fe(III)/Fe(II)).²⁹⁸ Such mediators have shown outstanding results, but they are still mostly chemically unstable and require low pH, which is unfavorable for H₂S removal processes.

POMs and different POM-based hybrid materials have shown high efficiency in H₂S removal due to their redox properties and structural stability. For the regeneration of these POM-based catalysts, a redox-mediated electrochemical regeneration method using oxidants such as H₂O₂ or O₂ has recently been shown to be effective.²⁸⁰

A purely inorganic POM was applied by Pei et al. who successfully synthesized a set of rare-earth Dawson-type polyoxometalates (K₁₇[Pr^III(P₂Mo^VI₁₇O₆₁)₂] (PrPMo), K₁₇[Gd^III(P₂Mo^VI₁₇O₆₁)₂] (GdPMo), K₁₇[Sm^III(P₂Mo^VI₁₇O₆₁)₂] (SmPMo) and K₁₇[Eu^III(P₂Mo^VI₁₇O₆₁)₂] (EuPMo)) and utilized them in the removal of H₂S. Due to the excellent redox properties of Ln(III)-doped POMs, the influence of different Ln(III) species on H₂S removal was investigated. From the experimental results, the prepared compounds were ranked according to their efficiency for the removal of H₂S in the following order: PrPMo (90%) > EuPMo (88%) > SmPMo (87%) > GdPMo (85%). The PrPMo polyoxometalate showed the best desulfurization and regeneration properties with 90% efficiency at 25 °C within 400 min. The XPS spectral analysis showed that H₂S is first oxidized to S by a redox reaction with PrPMo, in which Mo(VI) is simultaneously reduced to Mo(IV). During the electrochemical regeneration of PrPMo, S is further oxidized to SO₄²⁻ as the main desulfurization product, and Pr(IV) is reduced to Pr(III) during the regeneration process. The results of repeated XPS measurements confirmed the successful regeneration of PrPMo.¹¹⁶

Ma et al. described a new approach for an H₂S oxidation and sulfur recovery system using the hybrid POM-based hybrid materials, [C₄mim]₃PMo^VI₁₂O₄₀-ILs ([C₄mim]⁺ = 1-butyl-3-methylimidazolium cation), where they investigated the influence of several different [C₄mim]⁺-based ionic liquids (ILs), [C₄mim]Cl, [C₄mim]BF₄, [C₄mim]PF₆ and [C₄mim]NTf₂. Of all the POM-IL systems tested, the [C₄mim]₃PMo^VI₁₂O₄₀-[C₄mim]Cl system has shown to be the most effective for removing H₂S, with 100% efficiency. The adsorption mechanism of H₂S desulfurization is explained by the theory of cavities and the strong interaction between H₂S and Cl⁻. Additionally, they confirmed that the POM-IL material could be successfully recovered more than six times without losing its efficiency.²⁹⁹

Song et al. prepared a POM-based metal–organic framework [{(CH₃)₄N}₄CuPW^VI₁₁O₃₉H] (POM–MOF) hybrid material (Fig. 18) by combining a Keggin-type polyoxometalate [CuPW^VI₁₁O₃₉]⁵⁻ and MOF-199.³⁰⁰ The POM–MOF/O₂ catalytic system effectively oxidizes H₂S to solid S₈ with up to 95% H₂S removal efficiency. Additionally, it has been shown that the POM–MOF system can successfully oxidize mercaptans to disulfides. The POM–MOF catalyst can be successfully reused in the oxidation process after simple filtration, washing, and drying. The UV-vis and FT-IR spectra showed that the [CuPW^VI₁₁O₃₉]⁵⁻ structure was preserved in the POM–MOF catalyst at pH 11 for at least 12 h. The POM–MOF hybrid material showed better stability and pH resistance than the [CuPW^VI₁₁O₃₉]⁵⁻ POM alone.³⁰⁰

Fig. 18 Crystal structure of POM–MOF ([{(CH₃)₄N}₄CuPW₁₁O₃₉H]) material. The [CuPW₁₁O₃₉]⁵⁻ polyhedra are orientationally disordered into the pores. It was concluded that the catalytic decomposition of H₂S was taking place inside the pores.³⁰⁰

A summary of literature-reported POMs and POM-based hybrid materials and their efficiencies in H₂S removal are given in Table 2 at the end of section 4.

4.2.2 Nitrogen oxides (NO_x) and sulfur dioxide SO₂ in air pollution. Interest in NO_x emissions and their regulation began after 1952 with the confirmation of their role in the formation of photochemical smog.³⁰¹ Several different nitrogen oxides are present in the atmosphere, e.g., N₂O, NO, NO₂, N₂O₃, N₂O₄, NO₃, and N₂O₅. However, NO_x mainly refers to NO and NO₂ oxides because nitrogen oxides are primarily released into the environment in these forms, and NOx emissions contain 95% NO and 5% NO₂.³⁰² NO is considered less toxic than NO₂ and can cause eye irritation, but NO₂, even at low concentrations, can cause acute lung injury with pneumonitis³⁰³ and fulminant pulmonary edema.³⁰⁴ In urban areas where a higher concentration of NO₂ gas present, many respiratory and cardiovascular diseases and even increased mortality among the exposed population have been observed.^303,305

Moreover, H₂S and NO_x gases are considered to be among the major air pollutants because they are thought to be responsible for various environmental issues, such as photochemical smog, acid rain,³⁰⁶ tropospheric ozone,³⁰⁷ ozone layer depletion, and even global warming, as a result of N₂O.^308,309 NO_x gases are also associated with the greenhouse effect, and in the higher layers of the atmosphere, they can react with various compounds present there (O₃, VOCs, etc.), leading to ozone depletion. Most air pollution occurs and remains within the lowest layer of the atmosphere, the troposphere. NO_x gases can lead to the formation of tropospheric ozone after photochemical degradation to NO (eqn (5)):

NO₂ + hv (λ < 440 nm) → NO + O (5)

O + O₂ → O₃ (troposphere) (6)

With NO not absorbing radiation above 230 nm and thus not acting as an inhibitor in the lower atmosphere, the resulting atomic oxygen reacts with O₂ in the troposphere to form ozone (eqn (6)), leading to the tropospheric ozone formation.^301,307 Great efforts have been made to develop methods for removing NO_x from the atmosphere in the last few decades.^306,310,311 Adsorptive–desorption methods^307,309 and Fenton-like reactions,²⁸⁵ as examples of AOPs, have been extensively studied for the removal of NO_x and SO₂ gases. The Fenton-like oxidation process consists of oxidation and degradation of different pollutants in the presence of a catalyst and H₂O₂ as an oxidant activated by UV-light irradiation.^280,311

In the oxidation process, the generated reactive ˙OH radicals (eqn (7)) oxidize a wide range of different substrates. Such radical-assisted oxidation processes have been shown to be particularly effective in removing organic dyes, phenols, antibiotics, and insecticides from wastewater and are a popular research topic for pollution removal applications.²¹²

H₂O₂ + hv(cat.) → 2˙OH (7)

Zhao et al.³¹² reported the synthesis of an iron-substituted Keggin-type polyoxometalate-based catalyst Fe^IIIAspPW from ferric chloride (FeCl₃), aspartic acid (Asp), and phosphotungstic acid ([H₃PW^VI₁₂O₄₀]). The Fe^IIIAspPW was used to activate H₂O₂ to form active ˙OH species, which are crucial for the removal of NO from flue gas. The proposed catalytic mechanism consists of two redox cycles that occur on the surface of the Fe^IIIAspPW catalyst: the redox cycles of Fe^III ↔ Fe^II and POM ↔ POM⁻. In the Fenton-like process, first, in the redox cycle of Fe^III ↔ Fe^II, Fe³⁺ reacts with H₂O₂ to first form HOO˙ (eqn (8)) and then ˙OH (eqn (9)) active species:

Fe³⁺ + H₂O₂ → Fe²⁺ + H⁺ + HOO˙ (8)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH (9)

Fe³⁺ + HOO˙ → Fe²⁺ + H⁺ + O₂ (10)

Fe²⁺ + HOO˙ + H⁺ → Fe³⁺ + H₂O₂ (closing the cycle) (11)

In the POM ↔ POM⁻ redox cycle, the POM component is firstly reduced to the POM⁻ form in a reversible reaction, and then the reduced POM⁻ form further reacts with H₂O₂ to form active ˙OH species (eqn (13)). In addition, to close the redox cycle, POM⁻ is oxidized by O₂ or O₂˙:

Fe²⁺ + POM ⇄ POM⁻ + Fe³⁺ (12)

POM⁻ + H₂O₂ + H⁺ → POM + H₂O + ˙OH (13)

POM⁻ + O₂ → POM + O₂˙⁻ (14)

POM⁻ + O₂˙⁻ + 2H⁺ → POM + H₂O₂ (15)

This catalytic system showed great activity for removing NO with 94.6% efficiency.³¹² Moreover, Liu et al. showed that the Fe^IIIAspPW/H₂O₂ catalytic system could also be used to simultaneously remove SO₂ and NO from flue gas in a UV-Fenton-like process with efficiencies of the Fe^IIIAspPW catalyst of 100% for SO₂ removal and 84.27% for NO removal.³¹³ Wang et al. presented a series of Ge(IV)-based Keggin-type polyoxometalates (([H₄GeW^VI₁₂O₄₀] (HGeW), [H₅GeW^VI₁₁V^VO₄₀] (HGeWV), [H₅GeMo^VI₁₁V^VO₄₀] (HGeMoV), [H₅GeW^VI₉Mo^VI₂V^VO₄₀] (HGeWMoV)) and utilized them in the removal of NO_x pollutants.³¹⁴ The adsorption–desorption experiments showed the following adsorption efficiencies for the removal of NO_x gases: HGeW 81.5%) > HGeWV (74%) > HGeWMoV (67%) > HGeMoV (52%). The Keggin-type polyoxometalate HGeW (Fig. 2E) showed the highest NO_x removal activity with 81.5% removal and 68.3% N₂ selectivity, of which 65% was from fractionated NO and 35% NO₂ gas. Additionally, the H₂S removal efficiency of HGeW was compared with that of the parent Keggin [H₃PW^VI₁₂O₄₀] (HPW) polyoxometalate (54.1% efficiency). The FT-IR studies revealed that NO_x is adsorbed on HGeW mainly in the form of NOH⁺ and NO˙ species, but on the HPW, only NOH⁺ is observed as the main form during adsorption. Moreover, TPD-MS experiments were carried out to investigate the further decomposition mechanism of NO_x over HGeW and HPW. The TPD-MS analysis showed that while the decomposition products (NO, N₂O, N₂, and O₂) appear in the same order for both HPW and HGeW, they appear at different temperatures, lower in the case of HPW. The NO species appeared at the lowest temperature for both NO_x decomposition experiments. It is believed that a significant part of the NO_x is physically adsorbed onto HPW and HGeW in the form of NO at a lower temperature. Meanwhile, the later appearing N₂O could be a product of the disproportionation reaction of NO in which N₂ is formed because of the bonding effect of N-atom, which comes from N–O bond breakage. The difference in NO_x removal efficiency and N₂ selectivity between HPW and HGeW could be due to the HGeW's ability to intensively loosen the N–O bond, resulting in easier NO_x decomposition, and by better NO_x adsorption for HGeW in the form of both NO˙ and NOH⁺. It is believed that the presence of the Ge(IV) atom instead of P as the central atom plays a significant role in the processes described above.³¹⁴

4.3 Volatile organic compounds in air pollution (VOCs)

4.3.1 Removal of volatile organic compounds – refractory BETX compounds (benzene, ethylbenzene, toluene, and xylenes). VOCs are a group of liquid organic compounds that can easily evaporate at room temperature. In addition to their volatility, this group of compounds has variable lipophilicity, small molecular size, and are uncharged, resulting in inhalation as the primary route of human exposure.³¹⁵ VOCs are classified according to molecular structure and functional groups and include aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, ethers, esters, aldehydes, etc. Due to their properties and wide application in different areas of everyday life, they are common indoor and outdoor air pollutants.^285,315 As outdoor pollutants, they result from the development of industry and urbanization, which involves the increased use of fossil fuels in transport, industrial production, and wastewater treatment plants. As indoor air pollutants, VOCs are found in tobacco smoke, various air fresheners and perfumes, paints and coatings, cleaning products, etc., and can be harmful to human health at excessive concentrations.^285,286,315 Especially, the group of so-called refractory BETX compounds, which stands for benzene, ethylbenzene, toluene, and xylenes, is problematic due to their high toxicity and confirmed carcinogenic nature.^285,315 Besides being confirmed carcinogens, depending on the concentration and length of exposure, various consequences of VOCs exposure have been reported: eye and respiratory tract irritation, headache, dizziness, allergic skin reaction, fatigue, memory impairment, loss of consciousness, and even death.^286,315,316

Various methods²⁸⁵ have been studied in search of an efficient and affordable method for removing volatile organic compounds (VOCs) from the air, such as condensation, adsorption,^317,318 and (photo)catalytic oxidation.³¹⁶ Photocatalytic oxidation (PCO) is a promising method for removing VOCs from the air, and so far, TiO₂-based photocatalytic oxidation³¹⁹ has mainly been investigated. Due to the tendency to develop a sunlight/visible-light-driven method, TiO₂ has been shown to be a non-ideal photocatalyst due to its poor solar energy utilization.³²⁰ Therefore, there is a need to design new materials that could be successfully applied as photocatalysts for VOCs' photocatalytic oxidation.^315,321

Meng et al. have shown that photoactive PW₁₂/g-C₃N₄ optical films (Fig. 19B) can be obtained by combining the Keggin-type POM, [H₃PW^VI₁₂O₄₀], with polymeric graphitic carbon nitride (g-C₃N₄) and then successfully utilized them as photocatalysts for the efficient removal of benzene, toluene, and m-xylene. The PW₁₂/g-C₃N₄ optical films showed excellent removal efficiencies for benzene (90.3%), toluene (100%) and m-xylene (97.5%). They also demonstrated excellent stability and reusability for up to 30 cycles without signs of activity loss. The results of DMPO spin-trapping ESR measurements indicated that the PW₁₂/g-C₃N₄ films follow a simulated sunlight-driven direct Z-scheme-dictated charge carrier transformation mechanism that accelerates interfacial charge carrier separation and the formation of O₂⁻˙ and HO˙ radicals that are involved in VOCs oxidation. In the suggested mechanism (Fig. 19A), charge separation and formation of e⁻_CB–h⁺_VB pair occur (photocurrent), resulting in the formation of O₂⁻˙ and HO˙ active species that directly participate in the complete mineralization of VOCs to CO₂ and H₂O (Fig. 19A).²⁸⁶ Also, Gamelas et al. presented a series of new cellulose/silica hybrid composites functionalized with different Keggin-type POMs ([PV^V₂Mo^VI₁₀O₄₀]⁵⁻, [PV^VMo^VI₁₁O₄₀]⁴⁻, [PMo^VI₁₂O₄₀]³⁻ and [PW^VI₁₂O₄₀]³⁻) and investigated their potential application in the catalytic oxidation of VOCs present in urban air.²⁸¹ The new cellulose/silica hybrid materials were composed of approximately 56 wt% of polysaccharides, ca. 37 wt% of propylamine-modified silica, 2 wt% of POM, and 5 wt% of hydration water. Catalytic activity experiments were performed by pumping polluted air through Teflon tubes filled with the catalysts and then analyzing the treated air by GC-chromatography.

Fig. 19 A) Reaction mechanism of photocatalytic oxidation of VOCs catalysed by PW₁₂/g-C₃N₄ films. B) Schematic representation of the preparation of PW₁₂/g-C₃N₄ catalyst and its framework structure.²⁸⁶

The catalytic activity of the new POM-based hybrid material for VOCs oxidation was visible as a change in the color of the material from yellow to green, indicating the occurrence of V(V) → V(IV) reduction in the POM. The GC-chromatography of a real air sample treated with the new hybrid material indicated complete oxidation of most C₅–C₁₁ volatile organic compounds. The successful recovery of the used catalyst was achieved by passing purified air through the Teflon tubes filled with used catalyst, which was noticeable by the color change of the material from green to yellow.²⁸¹

POMs have also proven as suitable adsorbents for adsorption techniques to remove VOCs from the air. Ma et al. reported a newly synthesized POM/MOF hybrid material, K₂[Cu₁₂(BTC)₈·12H₂O][HPW^VI₁₂O₄₀]·28H₂O or NENU-28 and its possible application as an adsorbent for the adsorption of VOCs, including short-chain alcohols (MeOH and EtOH), cyclohexane, benzene, and toluene.³²² The adsorption capacity of NENU-28 for methanol, ethanol, 1-propanol, 2-propanol, cyclohexane, benzene and toluene was tested in VOCs adsorption experiments. The adsorption amount of MeOH for NENU-28 is 6.70 mmol g⁻¹ which corresponds to the adsorption of 37.52 molecules of MeOH per catalyst formula unit. Comparison with the initial MOF (Cu₃(BTC)₂), which can adsorb 5.14 mmol g⁻¹ methanol (14.36 MeOH molecules per formula unit), shows that POM-functionalized MOFs bring a significant improvement in the adsorption capacity for MeOH. The NENU-28 hybrid material also showed an increase in the amount of adsorbed EtOH (4.78 mmol g⁻¹ or 26.77 molecules of EtOH per formula unit) compared to Cu₃(BTC)₂ (3.54 mmol g⁻¹ or 9.89 molecules of EtOH per formula unit). Although the mechanistic details are not fully understood yet, the results indicate that the presence of the Keggin-type POM [HPW₁₂O₄₀] in the NENU-28 has a favorable effect on the adsorption properties of the POM–MOF material.³²²

4.3.2 Removal of aldehydes. Aldehydes, especially formaldehyde and acetaldehyde, are the most common VOCs present in the air as indoor air pollutants.³²³ The primary sources of these air pollutants come from building materials, varnishes, and paints, flooring, and furniture materials. Formaldehyde and acetaldehyde are classified as group 1 carcinogens and are therefore proven harmful to human health.^323,324 Several approaches have been developed to reduce their concentration. They can be divided into passive (e.g., better ventilation, using formaldehyde-free materials) and active (e.g., removal techniques – adsorption and catalytic oxidation) approaches.^323,324 In this section, the focus will be on the development of different active approaches for the removal of aldehydes.

[H₄SiW^VI₁₂O₄₀] and [K₈SiW^VI₁₁O₃₉] (0% efficiency). Kholdeeva et al. developed a new Ce-containing polyoxometalate NaH₃[SiW^VI₁₁Ce^IVO₃₉] (Ce-POM; Fig. 20)²⁷⁸ and its dimer in the solid-state, and tested their promising efficiency in the removal of formaldehyde (CH₂O) under mild conditions (20–40 °C). Although the reaction mechanism itself is complex and involves CH₂O autooxidation, the Haber–Weiss radical-chain process,³²⁵ and product formation inhibition, the reaction stoichiometry itself satisfies the equation in Fig. 20. The efficiency of an unoptimized oxidation process of CH₂O in the presence of Ce-POM/O₂ (efficiency 25%) was compared to the oxidation of CH₂O in the presence of Ce(SO₄)₂ (efficiency 9%) and in the presence of two POMs without Ce(IV) metal atom. The results of these efficiency comparisons suggested that the activity of the Ce-POM catalyst could be attributed to the synergistic action of the POM and Ce(IV). By optimizing the reaction conditions (adding a small amount of H₂O₂), the conversion efficiency of CH₂O increased from 25% to 85% with a yield of 66% HCOOH in the presence of NaH₃[SiW^VI₁₁Ce^IVO₃₉].²⁷⁸

Fig. 20 Aerobic oxidation of formaldehyde to formic acid catalysed by Ce-containing Keggin-type POM (NaH₃[SiW₁₁Ce^IVO₃₉]) under mild conditions (air, T = 25 °C).²⁷⁸

Gamelas et al. successfully immobilized the α-isomer of the polyoxometalate [SiW^VI₉O₃₇Ru^III₃(H₂O)₃Cl₃]⁷⁻ (Ru-POM) onto a CSH support, obtaining a heterogeneous catalyst Ru-POM-CSH that was active in formaldehyde oxidation.³²⁶ Oxidation of CH₂O was performed at room temperature by flushing an air/formaldehyde gas mixture through a Teflon tube filled with Ru-POM-CSH catalyst or only the CSH carrier without POM. Initially, the CH₂O degradation results for the first two cycles did not differ significantly between CSH and Ru-POM-CSH. This lack of degradation increase could be explained by chemisorption and the reaction between the amino groups of the CSH carrier and CH₂O:

Si(O)₃–(CH₂)₃–NH₂ + CH₂O → Si(O)₃–(CH₂)₃–N=CH₂ + H₂O (16)

After the second cycle, the efficiency of CSH in the removal of CH₂O dropped sharply. By the 4th cycle, it was 0%, which indicates the simple saturation of the CSH carrier. When Ru-POM-CSH was used as a catalyst, efficiency decreased more slowly, with about an 8% decrease between cycles after the 5th cycle. No catalyst saturation was observed, which can be attributed to the oxidation of CH₂O catalyzed by Ru-POM. After passing purified air through a Teflon tube containing Ru-POM-CSH material, unlike CSH alone, the material was successfully regenerated. Product analysis revealed that CO₂ and H₂O were the main reaction products formed by catalytic oxidation of CH₂O in the presence of Ru-POM-CSH. These results indicate that the reaction undergoes a predominantly non-radical mechanism because the final product would be formic acid and carbon monoxide in the case of a radical mechanism.³²⁶

The following mechanism of a CH₂O oxidation reaction in the presence of Ru-POM-CSH was proposed:

POM–Ru^III₃(H₂O) + CH₂O ⇄ POM–Ru^III₃(CH₂O) + H₂O (17)

POM–Ru^III₃(CH₂O) → HPOM–Ru^III₂Ru^II + ˙CHO (18)

The initial step probably involves oxidation of the substrate (CH₂O) by a catalyst through ligand replacement, binding of O₂ to the partially reduced catalyst (eqn (19)), and its activation and further reaction with ˙CHO:

HPOM–Ru^III₂Ru^II + O₂ ↔ HPOM–Ru^III₂Ru^II–O₂ ↔ HPOM–Ru^III₂Ru^III–O₂˙ (19)

HPOM–Ru^III₂Ru^III–O₂˙ + ˙CHO → POM–Ru^III₃ + CO₂ + H₂O (20)

The oxidation reaction of CH₂O with Ru-POM-CSH can be summarized as follows:³²⁵

CH₂O + O₂ → CO₂ + H₂O (21)

Kholdeeva et al. also synthesized tetra-n-butylammonium (TBA) salts of Co-substituted Keggin-type polyoxometalates [TBA₄HPW^VI₁₁CoO₃₉] (I) and [TBA₅PW^VI₁₁CoO₃₉] (II) (Co-POM) and immobilized them onto both NH₂⁻ and NH₃⁺ modified mesoporous silica surfaces.²⁸²

The catalytic activity of the solid Co-POM materials (I) and (II) was tested for the oxidation of isobutyraldehyde (IBA) and compared with the activity of the homogeneous Co-POM salts (I) and (II). The results showed that the IBA conversion rate in MeCN under mild conditions (1 atm of air, T = 20–40 °C) without a catalyst was 28%. In the presence of only the NH₂-modified mesoporous silica support, the IBA conversion rate was only 6%, indicating that the NH₂⁻ silica support is an inhibitor of the IBA oxidation. When one of the solid Co-POM catalysts, [TBA₄HPW^VI₁₁CoO₃₉] (I) or the non-protonated [TBA₅PW^VI₁₁CoO₃₉] (II), (immobilized on NH₂⁻ or NH₃⁺-silica support) was added to the reaction mixture, the IBA oxidation to IBAc continued at room temperature. The protonated salt [TBA₄HPW^VI₁₁CoO₃₉] (I) had a higher redox potential and better catalytic activity for IBA oxidation than the non-protonated salt (II). The catalytic activity of the immobilized Co-POM (I) and the homogeneous salt (I) exhibited similar catalytic performance (92% IBA conversion) for the first two cycles. However, after the third cycle, the immobilized Co-POM (I) catalyst lost up to 15% of its activity due to Co-POM leaching, showing that the homogeneous Co-POM (I) salt had better long-term stability.²⁸²

All literature-known polyoxometalates and their applications in removing aldehydes are summarized in Table 2.

4.4 Summary of POM-based technologies in air purification

Various POMs alone and combined with MOFs, CNTs, and mesoporous silica supports show promising results for removing air pollutants including refractory sulfur compounds from fossil fuels (section 4.1), toxic gases like H₂S (section 4.2.1), NO_x/SO₂ (section 4.2.2), and carcinogenic VOCs (section 4.3) in indoor/outdoor air. Keggin-type POM structures dominate oxidative desulfurization of fossil fuels under mild conditions (∼85% of literature; Table S1), outperforming traditional HDS processes and avoiding high pressures/temperatures while meeting ultra-low sulfur regulations. Anderson–Evans POMs enable efficient desulfurization through alkyl peroxide mechanisms with extended lifetimes, while lanthanide-doped Wells–Dawson POMs exhibit superior H₂S oxidation and stability for NO_x/SO₂ removal (Table 2).

POM-based hybrid materials further enhance performance, such as K₄PMo mesoporous salts for rapid DBT removal (Table S1), PW₁₂/g-C₃N₄ films mineralizing BETX VOCs under visible light via Z-scheme mechanism, and POM–MOFs like NENU-28 boosting VOC adsorption (section 4.3). Ce- and Ru-containing Keggin POMs catalyze aldehyde oxidation to CO₂/H₂O at room temperature, with Ru-POM-CSH showing sustained activity over cycles without saturation (Table 2). Structural diversity tailors redox properties and active oxygen species (˙OH, O₂˙⁻), addressing key air pollutants effectively.

5 Polyoxometalates in sensor applications

Immobilization of POMs on the different supporting surfaces facilitates their electrochemical properties for sensor applications.³⁴⁹ Numerous methods, such as chemical adsorption,^350,351 electrodeposition,^352,353 encapsulation,³⁵⁴ the Langmuir–Blodgett process,^355,356 and layer-by-layer deposition,^357,358 have been used to deposit POMs on electrodes to form monolayer or multilayer structures.³⁵¹ As can be seen in Fig. 21, POM-based sensors are used as the analytical unit, in which the POM is immobilized onto a solid substrate utilized as a transducer. If the POM has been successfully immobilized onto the transducer while preserving its structural integrity, the POM part of the sensor should be able to recognize and catalyze the analyte via an induced chemical reaction followed by the transformation of the chemical reaction energy into an electrical signal. The electrical signal is later amplified and converted by signal processing equipment into a display.³⁴⁹ The POM-based sensors, like other sensors, show all main characteristics such as sensitivity, selectivity, linear range, response time, detection limit, and stability.³⁵⁹ The most critical properties of most POM-based sensors are selectivity and response rate, and often, they are not addressed by authors. For sensors to have high selectivity, the sensor should have a heightened response to a substrate but an inadequate response to interferences. Recently, it has been shown that these issues could be solved by combining the POMs with organic moieties or CNTs with the addition of noble metal NPs. Generally, the POM-based sensors showed good selectivity and low response time while being stable and active at neutral pH.^360,361

Fig. 21 Schematic representation of POM-based electrochemical sensors.³⁴⁹

POM-based sensors operate through a synergistic mechanism that involves redox-driven signal transduction, coordination-induced structural alterations, and catalytic amplification processes. This enables the highly sensitive detection of various chemical and biological analytes. The multi-electron redox functionality of POM clusters allows them to undergo reversible changes in oxidation state upon interaction with target species, resulting in measurable outputs that can be electrochemical, optical, or conductometric. In the realm of electrochemical sensing, POMs facilitate rapid electron transfer at the electrode–analyte interface, a process that can be enhanced through their incorporation into conductive matrices or nanostructured supports, thereby optimizing charge-transfer kinetics and reducing detection limits.³⁶² Optical sensors utilize intervalence charge-transfer transitions or ligand-to-metal charge-transfer phenomena that occur when analytes interact with or reduce the POM framework, resulting in observable shifts in absorbance or luminescence.³⁶³ Furthermore, the catalytic sensing mechanisms exploit the inherent oxidative or reductive catalytic properties of POMs, where reactions initiated by the analyte generate amplified signals under controlled conditions such as specific pH levels, ionic strength adjustments, or the presence of co-substrates.³⁶⁴ The overall performance of these sensors is heavily influenced by various experimental factors, including the speciation of POMs, electrode modification strategies, solvent polarity, and the stability range of the POM in the working environment. Consequently, methodological optimization becomes vital for achieving selectivity, reproducibility, and reliability in practical applications.³⁶⁵

5.1 POM-based sensors in the detection of water pollution

The POM-based sensors have already explored various analyte classes dispersed in either the gas or liquid phase. The electrocatalytic reduction of nitrate, iodate, bromate, nitrite, and hydrogen peroxide by POMs immobilized on a substrate was carried out for sensing applications. Starting with stable Keggin and Dawson type POMs ([H₃PMo^VI₁₂O₄₀], [H₆P₂Mo^VI₁₈O₆₂·nH₂O], [H₃PW^VI₁₂O₄₀], [P₂W^VI₁₈O₆₂]⁶⁻, and α-[H₄SiMo^VI₁₂O₄₀]) are extensively explored as electrochemical sensors.^{361,366–369} Though the sensors showed prominent sensitivity and wide linear range, they operated at a low pH (pH < 2) to stabilize the POM architecture.³⁷⁰ In 2012, Ma et al.³⁷¹ synthesized a layer-by-layer composite film using palladium nanoparticles and a Dawson-type POM ([K₇P₂W^VI₁₇O₆₁(FeOH₂)·8H₂O], (P₂W^VI₁₇Fe)) to determine the electrolytic behavior towards the oxidation of hydrazine sulfate (N₂H₄SO₄) and reduction of hydrogen peroxide. The H₂O₂ exhibits sensitivity, detection limit, and linear concentration in the range of 66.7 μA mM⁻¹, 1 μM (S/N = 3), 1.5 μM to 3.9 mM, respectively. Likewise, N₂H₄SO₄ displays the same parameter in the range of 0.2 μA mM⁻¹, 1.5 μM (S/N = 3), 2 μM to 3.4 mM, respectively, with sensing response time around 4 s.³⁷¹ Furthermore, Zhu et al.³⁷² synthesized four Preyssler-type POM-based organic–inorganic crystals to effectively detect non-enzymatic H₂O₂. The compounds exhibit the lowest detection limit of 0.13 mM with a high sensitivity of 4.35 μA mM⁻¹ and a response time of 1 s.³⁷² Ag-doped MoO₃ immobilized on the graphene-like carbon nitride (C₃N₄) was first prepared and employed as an electrochemical sensor by Zhao et al.³⁷³ to detect H₂O₂. Herein, [Ag₆Mo^VI₇O₂₄]/Ag-MOF precursor was used to synthesize the nanoporous structure resulting in a linear detection range of 0.25 μM–0.43 mM towards H₂O₂ owing to its efficient electrocatalytic property.³⁷³ Additionally, isopolymolybdate-based compounds are explored as photoelectric sensors for detecting inorganic ions (e.g., Cr(VI), Hg²⁺, NO₂⁻).³⁷⁴ Additionally, complex POM structures (e.g., pyrazole derivative Keggin ions,³⁷⁵ 3D coordination polymers doped with Keggin POM³⁷⁶ or hourglass-type POM crystals³⁷⁷) have been explored as the active electrode for the acute and faster sensing of bromate, nitrate, and heavy, metal ions.

5.2 POM-based sensors in the detection of air pollution

Krutovertsev et al. first addressed POM-based gas sensors by employing various Wells–Dawson type POMs doped with polyaniline to detect ammonia gas.³⁷⁸ POM-doped conducting polymer film is ideal for gas sensing as POMs react with the gas, and conducting polymer substrate converts that into an electrical signal. The recognition of other hazardous gases, such as NO_x, CO, and the vapors of organic solvents, can also be determined because the proton-conducting POMs enhance the material's selectivity and sensitivity.^379,380 Ammam et al.³⁸¹ recently reported a sensitive and selective NO_x gas sensor using the [K₆P₂Mo^VI₁₈O₆₂·H₂O] POM and polypyrrole (PPy), exhibiting extended linearities (up to 5500 ppm NO_x). Although all so far mentioned POM-modified electrodes shows catalytic properties and can recognize the analyte, not all can be employed as sensors. In order to achieve a high-performance sensor, the modified electrode should fulfill the conditions of molecular recognition between POMs and specific analytes.³⁸¹

A high-performance gas sensor was developed by Wang et al.³⁸² by using heteropolytungstate (HPT) doped SnO₂ nanorods [HPT abbreviation as (C₄H₁₀ON)₂₃[HN(CH₂CH₂OH)₃]₁₀H₂[Fe^III(CN)₆(α₂-P₂W^VI₁₇O₆₁Co^II)₄]·27H₂O]·SnO₂/HPT composite film, which demonstrated higher photoconductivity than pristine SnO₂ and revealed improved gas sensing for the methylbenzene and formaldehyde at room temperature (25 °C). Electron–hole recombination in the composite was retarded due to the photo-induced transfer of an electron from SnO₂ to HPT. An n-type semiconductor material BiVO₄ loaded with different POMs, was exploited as a photo-anode for photoelectrochemical gas sensing capability for NO₂.³⁸² Among different Keggin type POMs ([Na₇PW^VI₁₁O₃₉], [H₃PW^VI₁₂O₄₀], [H₃PMo^VI₁₂O₄₀], [Na₁₀SiW^VI₉O₃₄]), [H₃PW^VI₁₂O₄₀] displayed the highest photocurrent response intensity. In addition, BiVO₄/[H₃PMo^VI₁₂O₄₀] demonstrates an enhanced response of 32.8% toward 50 ppm of NO₂.³⁸³ In similarity with the previous discussion, herein, the electron–hole recombination was slowed down as the POM facilitates charge separation and photogenerated electron transfer to the semiconductor. Shi et al.³⁸⁴ made an interface modification on the grain boundary by integrating TiO₂, and Ti^IV substituted POMs (K₅[PW^VI₁₁Ti^IVO₄₀] and K₅[PW^VI₁₀Ti^IV₂O₄₀]). The resultant nanocomposite exhibited improved photoconductivity and elevated gas sensing properties towards acetone gas.³⁸⁴ Tian et al.³⁸⁵ investigated the effect of [H₃PW^VI₁₂O₄₀] doped In₂O₃ compound for gas sensing at room temperature toward formaldehyde. The doping of the POM successfully suppressed the recombination of photo-induced carriers in the system resulting in a 35% enhancement in photoconductivity alongside a 26% gas sensing response compared with pristine In₂O₃.³⁸⁵ Similarly, Wang et al.³⁸⁶ also incorporated [PW^VI₁₂O₄₀]³⁻ with Cu₂ZnSnS₄ for high-performance NO₂ gas sensors. The composite exhibited 88.83% enhanced gas sensing properties compared with pristine Cu₂ZnSnS₄ due to the restriction of electron–hole recombination and effective charge transfer through the POM.³⁸⁶ Furthermore, Sun et al.³⁸⁷ developed dye-sensitized TiO₂–PW₁₂ using a simple, economical sol–gel method followed by a screen-printing technique for faster NO₂ gas sensing at room temperature under visible light irradiation. The heterostructure enabled faster separation and transportation of the photogenerated carriers as the POM acted as the electron acceptors. The effective increase in sensitivity (233.1–1 ppm) over a wide range of NO₂ concentration (50 ppb–5 ppm) for POM decorated dye/TiO₂ film occurred due to the expansion of the narrow bandgap of the POM doped dye under visible light without loss in thermal energy.³⁸⁷ An inorganic–organic hybrid film was fabricated by Kida et al. for selective H₂ (50–500 ppm) and NH₃ (10–100 ppm) sensing using yttrium-stabilized zirconia with Mo^VI₇O₂₄⁶⁻/hexylamine hybrid film. Calcination of the POM alkylamine hybrid film resulted in porous MoO₃ particles, making them an effective precursor for synthesizing nanosized metal oxide.³⁸⁸ POM-based supramolecular chemosensors were developed for the acute gas sensing of toxic gases. Wei et al. demonstrated a CO₂ sensor using Na₉DyW^VI₁₀O₃₆ and block copolymer poly(ethylene oxide-b-N,N-dimethyl aminoethyl methacrylate).³⁸⁹ Likewise, Guo et al. developed POM-based supramolecular chemosensors for H₂S detection (detection limit 1.25 μM) with dual signals (via absorption spectra and fluorescence).³⁹⁰ In the field, rapid detection of acutely corrosive and toxic gases like H₂S at room temperature is important. Bezdek et al. developed enhanced chemiresistive gas sensors to detect H₂S using highly oxidized Pt-doped POM with single-walled CNT. They have also demonstrated ppb level detection with high stability and a wide range of selectivity.³⁹¹ Furthermore, Liu et al.³⁹² immobilized POMs on a polyelectrolyte matrix and then used them for the sensitive detection of NO. The ability to electrocatalyze the reduction of NO resulted in a wide range of selectivity (1 nM to 10 μM).³⁹²

Triethylamine gas sensors developed by Cai et al.³⁹³ exhibited ultra-sensitive selectivity and stability over repeated use. One-dimensional heterostructure nanofibers of ZnO and ZnWO₄ were synthesized via POM (varying the molar ratio of H₃PW^VI₁₂O₄₀) assisted electrospinning methods. The highly porous structure of the nanofibers and the synergistic effect between the ZnO and ZnWO₄ resulted in an enhanced relative response of 108.5 for 50 ppm triethylamine. The barrier-control electron transfer at the interface was attributed to remarkable selectivity with a low detection level of 150 ppb.³⁹³ The recent advances led Tian et al.³⁹⁴ to fabricate POM–semiconductor heterojunctions via a one-step coaxial electrospinning technique for the effective sensing of ethanol gas. One-dimensional tandem heterojunctions SnO₂/POM/WO₃ significantly increased the sensing characteristics compared with the SnO₂/WO₃ nanofibers. The sensitivity was optimized to 100 ppm of ethanol. The construction of the interface allowed the POM to act as the electron acceptor, promoting faster carrier separation and exhibiting enhanced sensing behavior.³⁹⁴ Next, a bottom-up POM-assisted in situ growth of 1D nanofilament architecture was achieved by electrospinning, followed by the thermal oxidation method for the detection of acetone. A broad range of concentration, i.e., 50 ppb–50 ppm, was detected with enhanced selectivity and sensitivity owing to the charge transfer to the interface of the ZnO–ZnMoO₄ nanofilament.³⁹⁵ A unique nanostructure was developed by Ren et al.³⁹⁶ using Pt-draped Si-doped WO₃ nanowires interwoven into a three-dimensional mesoporous superstructure for low-temperature ethanol gas sensing (with a detection limit of 0.5 ppm).³⁹⁶ Selective and ultrasensitive dual detection (Raman and photochromic) of ethylenediamine gas was demonstrated by Zhang et al. using POM/viologen hybrid crystal. It exhibits a very low detection limit of 0.1 ppb via Raman signal output.³⁹⁷

5.3 POM-based sensors in the detection of emerging health pollutants

Very recently, Wang et al.³⁹⁸ synthesized isostructural Anderson-type POM-based compounds and fabricated photoelectric sensors to detect inorganic ions. Three different transition metal ions (M^II = Co^II, Cd^II, Zn^II) were incorporated for the preparation of the [M^II₂(H₃bdpm)₂TeMo^VI₆O₂₄·6H₂O] (H₃bdpm = 1,1′-bis(3,5-dimethyl-1H-pyrazolatemethane)) compounds which contain a 2D supramolecular layer and 1D chain structures. All prepared [M^II₂(H₃bdpm)₂TeMo^VI₆O₂₄·6H₂O] compounds have been successfully utilized as fluorescence sensors toward Cr₂O₇²⁻ at different concentrations. Furthermore, the compounds with Co^II and Cd^II also exhibited electrochemical sensing behavior for detecting NO₂⁻ (Cd-containing compound possesses a response time of 2.16 s at a detection limit of 5.11 × 10⁻⁵ M alongside a sensitivity of 43.10 μA mM⁻¹).³⁹⁸

POM and Zn-based complexes derived from pyrazole were reported by Tian et al. for photocatalysis and electrochemical sensors to detect hydrogen peroxide, bromate, and nitrite by tuning pH.³⁹⁹ Likewise, Zhang et al. tuned the N and O coordination donors in morpholine and piperazine derivatives to derive various POM-based compounds for photocatalysis, electrochemical, and fluorescent sensor applications (towards Hg²⁺).⁴⁰⁰ Furthermore, researchers explored POM-modified MOFs for various sensing applications, e.g., photocatalytic, electrochemical (towards the detection of inorganic ions, H₂O₂, Cr(VI), bromate, etc.).^401–405

All literature known polyoxometalates and their applications in sensing are summarized in Table 3.

Table 3 Summarization of the reported POM-based sensors

POM-based composite	POM archetype	Type of sensor	Significant results	Ref.
(P2W^VI17Fe) and palladium NPs; NPs = nanoparticles	Wells–Dawson (Fig. 2H)	Electrochemical sensor towards H2O2 and N2H4SO4	The H2O2 and N2H4SO4 exhibit sensitivity, detection limit, and linear concentration in the range of 66.7 μA mM^−1, 1 μM (S/N = 3), 1.5 μM to 3.9 mM, and 0.2 μA mM^−1, 1.5 μM (S/N = 3), 2 μM to 3.4 mM, respectively	371
[M^n+(H2O)P5W30O110]^(15−n)−	Preyssler-type	Electrochemical sensor towards H2O2	Exhibit the lowest detection limit of 0.13 mM with a high sensitivity of 4.35 μA mM^−1 and response time of 1 s	372
K6P2Mo^VI18O62·H2O with polypyrrole	Wells–Dawson (Fig. 2H)	NOx gas sensor	Exhibits extended linearities up to 5500 ppm NOx	381
SnO2/HPT composite film	Keggin (Fig. 2F)	Gas sensor for the formaldehyde and methylbenzene	Higher photoconductivity compared with pristine SnO2	382
BiVO4/H3PW^VI12O40	Keggin (Fig. 2F)	NO2 gas sensor	Enhanced response of 32.8% towards the 50 ppm of NO2	383
[M^II2(H3bdpm)2TeMo^VI6O24.6H2O]; H3bdpm = 1,1′-bis(3,5-dimethyl-1H-pyrazolatemethane)	Anderson–Evans (Fig. 2I)	Photoelectric sensors for the detection of inorganic ions	Cd-based compound possesses a response time of 2.16 s at a detection limit of 5.11 × 10^−5 M with a sensitivity of 43.10 μA mM^−1	398
BiVO4/(H3PW^VI12O40 or H3PMo^VI12O40 or Na7PW^VI11O39 or Na10SiW^VI9O34)	Keggin (Fig. 2F)	NO2 gas sensor	BiVO4/PW12 exhibits highest response of 32.8% towards 50 ppm of NO2	383
TiO2/[PW^VI11TiO40]^5− and TiO2/[PW^VI10Ti2O40]^7−	Keggin (Fig. 2F)	Acetone gas sensor	Low detection concentration level of acetone is 50 and 80 ppm forTiO2/[PW^VI11TiO40]^5− and TiO2/[PW^VI10Ti2O40]^7−, respectively	384
H3PW^VI12O40 doped In2O3 compound	Keggin (Fig. 2F)	Gas sensor for the formaldehyde at room temperature	35% enhancement in photoconductivity alongside a 26% of gas sensing response compared with pristine In2O3	385
H3PW^VI12O40 with Cu2ZnSnS4	Keggin (Fig. 2F)	NO2 gas sensor	Exhibits 88.83% enhanced gas sensing property compared with pristine Cu2ZnSnS4	386

---

5.4 Summary of POM-based sensors

POM-based sensors for water pollution, air pollution, and emerging health pollutants are discussed thoroughly. In aqueous sensing, Keggin, Dawson, Preyssler, and isopolymolybdate POMs exhibit strong electrocatalytic activity toward species such as hydrogen peroxide, nitrate, bromate, nitrite, and heavy metal ions, often achieving low detection limits and quick response times. For gas sensing, POM–polymer, POM–metal oxide, and POM–semiconductor heterostructures enable the sensitive and selective detection of gases, including NO₂, NH₃, H₂S, formaldehyde, acetone, ethanol, and volatile amines, mainly by promoting charge separation and reducing electron–hole recombination. Lastly, emerging health-related pollutants are addressed through advanced POM-based supramolecular systems, MOFs, and hybrid complexes that offer electrochemical, photoelectrochemical, and fluorescent sensing modes. Overall, the manuscript highlights the versatility of POMs as functional building blocks for high-performance, multifunctional sensors that operate under mild and environmentally friendly conditions.

6 Polyoxometalate based battery and supercapacitors

POMs emerge as an exceptional electrode component for supercapacitors (SCs) or batteries due to their high proton mobility and extraordinary redox chemistry.^406–408 POM's variable redox activities and outstanding electron/proton transport capacities apply POM-based composite materials in electrochemical fields. As a powerful electron reservoir in the multi-electron reduction process, POM enables high proton conductivity even in the composite. This interesting behavior has led to various applications of POM-based composites such as green catalysis, sensors, and electrochemical energy storage devices (batteries and SCs). However, POMs are pH-sensitive; therefore, a well-known strategy of coordination chemistry has been used to enhance the mechanical and electrochemical properties of the electrode material for better performance.^407–411

6.1 POM-based battery electrodes

6.1.1 POM as the electrode for lithium-ion batteries (LiBs). Transition metal oxides are used as the cathode/anode material for LiBs as they are oxidized to their highest oxidation state when the Li has been released.⁴¹² The first reported POMs for LiB are focused on polyoxomolybdates.⁴¹³ Further improvements of the electrode material have been made by modifying the structural and electronic states of POMs, altering the reversible faradaic reaction associated with them. Vanadium-based POMs are being explored as cathode materials for rechargeable batteries to achieve high energy and power density by multi-electron redox processes via fast transfer of Li ions. Chen et al.⁴¹⁴ reported Li₇[V^V₁₅O₃₆(CO₃)] as a cathode material with a specific capacity of 250 mA h g⁻¹ alongside energy and power densities of 1.5 kW h L⁻¹ and 55 kW L⁻¹, respectively. Additionally, Li₇[V^V₁₅O₃₆(CO₃)] exhibits a very high potential window (1.9 to 4.0 V) for reversible redox reactions. The theoretical calculation for the specific capacity for the oxometalate mentioned above at the same potential window (by considering n is 14, which is the next nearest integer no. of electrons) shows the specific capacity of 259 mA h g⁻¹, which is in corroboration with the experimental data.⁴¹⁴ Further, the vanadium-based K₇[NiV^V₁₃O₃₈] structure is explored by Ni et al.⁴¹⁵ The maximum discharge capacity of 218.2 mA h g⁻¹ was recorded at a discharge current density of 17 mA g⁻¹ with 93.2% coulombic efficiency.⁴¹⁵ Thus, the nano-sized polyoxovanadates can be utilized as cathode materials for LiBs for moderate capacity and rate capability.

Furthermore, POMs are combined with carbonaceous nanostructures for better cycle and rate performance. Ma et al.⁴¹⁶ synthesized covalent functional pyrene (Py) with [H₄SiW^VI₁₂O₄₀] (SiW₁₂) and attached it to the surface of SWCNTs via spontaneous adsorption. SWCNT/Py-SiW₁₁ exhibited an initial discharge capacity of 1569.8 mA h g⁻¹ at a current density of 0.5 mA cm⁻². However, the capacity decreased to 580 mA h g⁻¹ after 100 cycles at the same current density.⁴¹⁶ Graphene sheets are represented by single-layer two-dimensional sp²-bonded carbon atoms, having a high affinity towards POMs. Wang et al.⁴¹⁷ synthesized environmentally friendly nanomaterials by incorporating reduced graphene oxide (rGO) with Keggin type [H₄SiW^VI₁₂O₄₀] (SiW₁₂) clusters. rGO/SiW₁₂ exhibits a discharge capacity of 275 mA h g⁻¹ with an increased potential of 4 V at a current density of 50 mA g⁻¹. The nanocomposite can hold a capacity of 120 mA h g⁻¹ at 1.5 V operating potential even at a high current density of 2000 mA g⁻¹.⁴¹⁷ Besides carbonaceous nanostructures, POMs are often synthesized with silver nanoparticles due to their chemical structure, elevated surface area, and high electrical conductivity.^418,419

In recent years, the POM-based composite structure has been further modified by including MXenes, e.g., i) POM@PANI/Mo₂TiC₂T_xMXene/CNTs delivers lithium storage capacity of 621 mA h g⁻¹ at 0.1 A g⁻¹ and promising cyclic stability (445 mA h g⁻¹ after 1000 periods at 1.0 A g⁻¹);³⁹⁰ and ii) PMo₁₂@PPy/Ti₃C₂T_x delivers high capacity of 764 mA h g⁻¹ at 0.1 Ag⁻¹ with long cycling stability of 2000 cycles at 3 A g⁻¹.⁴²⁰ Additionally, the hybridization of various POMs with different supports such as porphyrins,⁴²⁰ CoS₂/MoS₂/functionalized rGO,⁴²¹ and various MOFs^422–425 results in enhanced lithium capacity and overall stability as an anode.

6.1.2 POM as the electrode for sodium-ion batteries. Besides LiBs, POM-based composites are applied as cathode/anode material for Na-ion batteries. Liu et al.⁴²⁶ prepared a robust composite by coating Na₂H₈[MnV^V₁₃O₃₈] (POM) clusters on the graphene nanoflakes. The discharge process of the composite demonstrates a two-phase reaction due to the presence of V(V)/V(IV) redox couple related to Na-ion insertion, and a high capacity of 202 mA h g⁻¹ is recorded at 1.5 V (at the end of the discharge). Furthermore, the composite can retain 81% of its initial capacity over 100 cycles at 0.2 C with 95% coulombic efficiency.⁴²⁶ Hartung et al.⁴²⁷ reported that the sodium salt of decavanadate, Na₆[V^V₁₀O₂₈], acts as a high-performance cathode material for rechargeable Na-ion batteries. The potential discharge range observed from the CV graph is within the range of 0.01–3.0 V. The capacitive process associated with the Na₆[V^V₁₀O₂₈] ion is completed by the insertion of the Na ion in the voids of [V^V₁₀O₂₈]⁶⁻ cluster.⁴²⁷

Additionally, MOFs are proven to be effective supporting materials for POMs. Using a simple impregnation strategy, Cao et al.⁴²⁸ demonstrated that PMo₁₂/MIL-88B/GO composite delivers an excellent specific capacity of 214.2 mA h g⁻¹ for 600 cycles at 2 A g⁻¹. Another example is a layer-by-layer arrangement of vanadium-based POM immobilized on Co-based MOF resulted in a capacity of 413 mA h g⁻¹ due to accommodating the larger Na⁺ ions efficiently.⁴²⁸

6.2 POM-based supercapacitor electrodes

Electrochemical capacitors or SCs, on the other hand, are promising energy storage devices that meet a significant performance gap between batteries and electrostatic capacitors. They supply high-power electric pulses over a short time scale, exhibiting a high dynamic of charge propagation with elevated charge and discharge rates.⁴²⁹ In the maximum reported SC, high capacitance and energy are achieved by incorporating a pseudocapacitive or faradaic type of active material with a double-layer capacitive component. Mostly, metal oxides and sulfides show promising results for SC electrodes as they generate a large number of charges at the electrode interface via multi-step reversible redox reactions.

6.2.1 Composite-type hybrid electrode. Early in 2005, Gómez-Romero et al.⁴⁰⁷ established the POM-based composite hybrid electrode for SC as they dispersed three different POMs, namely, [H₃PW^VI₁₂O₄₀], [H₄SiW^VI₁₂O₄₀], and [H₃PMo^VI₁₂O₄₀], in the conducting polymer PANI. The highest specific capacitance of 120 F g⁻¹ with cycle stability over 1000 cycles was observed for PANI/[H₃PMo^VI₁₂O₄₀], which is higher than the other two POM ([H₃PW^VI₁₂O₄₀], [H₄SiW^VI₁₂O₄₀]) composite, due to the higher proton conductivity of the [H₃PMo^VI₁₂O₄₀] in 1 M HClO₄ electrolyte.⁴⁰⁷ In the later years, the same group deposited [H₃PMo^VI₁₂O₄₀] on different conducting polymers (e.g., poly(3,4-ethylenedioxythiophene) (PEDOT)) with an external oxidizing agent (H₂O₂) for further electrochemical improvement (Fig. 22).⁴⁰⁸ Later, the Freund's⁴³⁰ group used the same Keggin POM, [H₃PMo^VI₁₂O₄₀], incorporated into the porous PPy, exhibiting a specific capacitance of 210 F g⁻¹ in 0.5 M H₂SO₄ electrolyte in three-electrode configuration.⁴³⁰ Recently, Vannathan et al.⁴³¹ reported high-performance pseudocapacitors of vanadium substituted Keggin POMs and combined with a conducting polymer for enhancement of electrochemical activity.⁴³¹

Fig. 22 Schematic illustration of steps involved in synthesizing polypyrrole nanopipes and polyoxometalates (POMs, PMo₁₂, or PW₁₂) hybrid material with the simple chemical method.⁴⁰⁸

Carbonaceous nanostructures (e.g., CNT, GO/rGO, AC) come into play as the supporting elements to the POMs as they provide better mechanical and electrochemical stability.⁴²⁹ To replace the conducting polymer as a supporting element for POM, inventors need a high electrical conducting substrate like the former. CNTs exhibit higher electrical conductivity due to their hierarchical architecture among all the carbonaceous nanostructures. At first, Cuentas-Gallegos et al.⁴³² prepared a single-wall CNT and POM composite using Cs substituted phosphomolybdate (Cs-[PMo^VI₁₂O₄₀]³⁻). The composite material presented a specific capacitance of 285 F g⁻¹ and an energy density of 57 W h kg⁻¹.⁴³² Later Skunik et al.⁴³³ further developed this concept using multi-walled CNT instead of a single wall. Phosphomolybdic acid-modified multi-walled CNT revealed a specific capacitance of 40 F g⁻¹ at a discharged current of 7 mA.⁴³³ Furthermore, to achieve a higher surface area substrate without compromising electrical conductivity, the researchers employed AC as a supporting material because it possesses a larger surface area (up to 3000 m² g⁻¹) with different pore distribution (micro, meso, or macropores). Ruiz et al.⁴³⁴ prepared a hybrid electrode by integrating activated carbon with Keggin-type phosphomolybdate [H₃PMo^VI₁₂O₄₀] (PMo₁₂). The highest specific capacitance was generated due to the faradaic component, around 183 F g⁻¹ at 2 A g⁻¹ current density.⁴³⁴ In 2014, the same group used molybdenum-based POMs instead of phosphotungstate [H₃PW^VI₁₂O₄₀] for an electrochemical study and observed an enhancement of the capacitance to 254 F g⁻¹ in an operating potential of 1.6 V. Moreover, the composite can possess 98% capacitance over 30 000 cycles.⁴³⁵ Besides Keggin-type POMs, Mu et al.⁴³⁶ for the first time embedded a Dawson-type POM, (NH₄)₆[P₂Mo^VI₁₈O₆₂] on AC and achieved the highest capacitance of 308 F g⁻¹ at 2 A g⁻¹ current density due to the high proton conductivity and unique redox behavior of the faradaic component.⁴³⁶ Besides commercially available activated carbon, Lian et al. used biomass-derived pinecone activated carbon, in which POMs (PMo^VI₁₂O₄₀³⁻) contributed to a high specific capacitance of 361 F g⁻¹, showing the trend of proton-coupled electron transfer (Fig. 23).⁴³⁷ Recently, Maity et al.⁴³⁸ developed vanadium-substituted Keggin structures (PMo^VI₁₁VO₄₀ and PMo^VI₁₀V^V₂O₄₀) impregnated into the surface of AC. The vanadium concentration in the polyanion plays a vital role as it decides the morphology and microstructure of the nanocomposite.⁴³⁸

Fig. 23 Synthesis schematic for porous pinecone biomass carbon and fabrication of pinecone – polyoxometalate hybrid material.⁴³⁷

Graphene or its oxide derivatives (GO and rGO) are used mainly as substrate components other than CNTs and AC because of their high surface area with sizeable electrical conductivity. Additionally, the presence of oxygen-containing functional groups in GO and rGO enables many active sites for the physisorption of a faradaic component. Gómez-Romero and his team did permutation and combined possible routes to achieve high-performance SC using POM and graphene offshoots.^439,440 In this course, they have found a new route to synthesize the hybrid PMo₁₂–rGO nanoelectrode with a hydroquinone-doped hybrid gel hybrid electrolyte. The double hybridization enhances cell potential (1.6 V) and electrochemical properties by increasing the volumetric capacitance to 3.18 F cm⁻³. Similarly, for the phosphotungstate composite (rGO–PW₁₂), the areal capacitance is calculated as 2.95 F cm⁻³.^439,440

Instead of a single supporting medium for POMs, Qin et al.⁴⁴¹ (Fig. 24) prepared a new type of composite by anchoring PMo₁₂ to PPy/rGO by layer-by-layer deposition for high-performance micro-SC in solid gel electrolyte medium (PVA/H₂SO₄; PVA = polyvinyl alcohol). The resultant composite exhibited high energy and power densities of 4.8 mW h cc⁻¹ and 645.1 mW cc⁻¹, respectively. Also, due to the presence of a solid electrolyte, it presents excellent mechanical flexibility (96% capacitance retention at a highly bending angle of 180°).⁴⁴¹ Furthermore, surface modifications of graphene derivatives were made using various POM structures, demonstrating enhanced electrochemical performances.^442–445 To achieve seamless ion transportation to the electrode/electrolyte interface Maity et al.⁴⁴⁶ designed and tailored a facile bottom-up approach in which vanadium-substituted Keggin POMs (PMo₁₁VO₄₀) were used to oxidize pyrrole monomer followed by the deposition on the GO surface. The resultant nanohybrid not only exhibits unique architecture but displays high-performance supercapacitive behavior.⁴⁴⁶ The designing and construction of polyoxometalates-based metal–organic frameworks composites further expands the search for promising high-performance electrode materials for SCs. A Dawson type⁴⁴⁷ the basket-shaped heteropoly blue,⁴⁴⁸ Keggin type,⁴⁴⁹ and Anderson type^450,451 POMs hybridized in metal or covalent organic frameworks overcome the limitations of POMs, e.g., high solubility in common electrolytes and results in better stability over longer cycles with improved capacitance.

Fig. 24 Scheme illustration of fabrication procedure of mPPy@rGO-POM nanosheets.⁴⁴¹

6.2.2 Asymmetric type hybrid electrode. Asymmetric type hybrid enhances electrochemical performances in two ways; for instance, incorporating two types of material in a single device enables different charge storage mechanisms simultaneously. Secondly, the cell voltage is tuneable (mainly can be enhanced) due to the presence of various active materials in electrodes. Chen et al.⁴⁵² studied the electrochemical properties of vanadium-based iso-polyanion, sodium decavanadate ([Na₆V^V₁₀O₂₈]) in 1 M LiClO₄ organic solution, exhibiting an excellent electrochemical behavior in a 3-electrode configuration. Furthermore, an asymmetric SC configuration was developed using activated carbon as the positive and [Na₆V^V₁₀O₂₈] as the negative electrode, exhibiting a maximum specific capacitance of 269 F g⁻¹, with energy and power densities of 73 W h kg⁻¹ and 312 W kg⁻¹, respectively, in a 2.8 V operating potential.⁴⁵² Hu et al.⁴⁵³ studied a composite type of electrode using regular PMo₁₂ anchored on AC in a protic ionic liquid electrolyte. Later, the nanocomposite was assembled as an asymmetric SC device with commercially available AC. The asymmetric cell operates in an elevated potential window of 0–0.85 V, even at a high current density (10 A g⁻¹).⁴⁵³

Dubal et al.⁴⁵⁴ developed a high-performance symmetric SC based on PMo^VI₁₂ and PMo^VI₁₂–rGO. They assembled an asymmetrical SC device using rGO–PMo^VI₁₂ and rGO–PW^VI₁₂ electrodes for higher energy density. The SC cell also operates at 1.6 V potential and elevated energy density of 39 Wh kg⁻¹ at a power density of 658 W kg⁻¹.⁴⁵⁴ Maity et al.⁴⁵⁵ optimized the effective loading of POM (NiV^V₁₄O₄₀)⁷⁻ on the AC surface for the first time and employed the nanocomposite as the cathode in an asymmetric configuration with AC as the anode. The resultant device exhibited an enhanced specific energy of 90 W h kg⁻¹ and specific power of 2400 W kg⁻¹. Moreover, the nanocomposite-based asymmetric configuration with pristine POM as the positive electrode showed supercapattery behavior.⁴⁵⁵

All literature-known POM-based batteries and supercapacitors are summarized in Table 4.

Table 4 Summarization of the reported POM-based battery and supercapacitors

POM-based composite	POM archetype	Type of energy storage	Significant results	Ref.
Li7[V^V15O36(CO3)]	Spherical isopolyvanadate	Li-ion battery	Specific capacity of 250 mA h g^−1 alongside energy and power density of 1.5 kW h L^−1 and 55 kW L^−1, respectively	414
SWCNT/Py-SiW^VI11; SWCNT = single-walled carbon nanotubes	Lacunary Keggin	Li-ion battery	Exhibits an initial discharge capacity of 1569.8 mA h g^−1 at a current density of 0.5 mA cm^−2	416
Na2H8[MnV^V13O38] cluster on the graphene nanoflakes	Trimeric polyoxovanadate	Na ion battery	High capacity of 202 mA h g^−1 is recorded at 1.5 V with 81% of its initial capacity retention over 100 cycles	426
PANI/H3PMo^VI12O40; PANI = polyaniline	Keggin	Composite type SC	Highest specific capacitance of 120 F g^−1 with cycle stability over 1000 cycles	407
([PV^VMo^VI11O40]^4−, [PV^V2Mo^VI10O40]^5−) with AC	Keggin	Composite type SC	AC–VMo11 composite displayed an enhanced capacitance of 450 F g^−1 with an improved energy density of 59.7 W h kg^−1 alongside 99.99% capacitance retention of over 5000 cycles	438
PMo^VI12 to PPy/rGO by layer-by-layer deposition; PPy = polypyrrole; rGO = reduced graphene oxide	Keggin	Composite type SC	Composite possesses high energy and power densities of 4.8 mW h cc^−1 and 645.1 mW cc^−1, respectively	441
[MnV^V14O40]^6− on the AC and GO; AC = activated carbon; GO = graphene oxide	Lindqvist	Composite type SC	AC/MnV14 nanohybrid exhibits a specific capacitance of 547 F g^−1 with specific energy and power of 76 W h kg^−1 and 1600 W kg^−1, respectively, at 0.8 Ag^−1 current density. GO/MnV14 shows a specific capacitance of 330 F g^−1 with specific energy and power of 30 W h kg^−1 and 1276 W kg^−1, respectively, at the same current density	445
PMo^VI12 anchored on AC in a protic ionic liquid; AC = activated carbon	Keggin	Asymmetric SC	Asymmetric cell operates in a potential window of 0–0.85 V at 10 A g^−1 of current density	453
rGO–PMo^VI12 and rGO–PW^VI12; rGO = reduced graphene oxide	Keggin	Asymmetric SC	The cell operates at 1.6 V potential and elevated energy density to 39 W h kg^−1 with a power density of 658 W kg^−1	454
AC//AC-K2H5[NiV^V14O40]; AC = activated carbon	Lindqvist	Asymmetric SC	Increased the potential window up to 1.5 V and enhanced the specific energy and power values (90.1 W h kg^−1 and 2400 W kg^−1, respectively), with 98% coulombic efficiency	455

---

6.3 Summary of POM-based batteries and supercapacitors

The use of polyoxometalates (POMs) as advanced electrode materials for electrochemical energy storage highlights their remarkable redox activity, high proton mobility, and fast electron/proton transport. These inherent qualities make POMs appealing for use in batteries and supercapacitors, although their sensitivity to pH and solubility issues necessitate structural modifications and hybridization via coordination chemistry to develop mechanically durable and electrochemically stable electrodes. In batteries, especially those based on vanadium- and molybdenum-based clusters, POMs serve as active materials in lithium- and sodium-ion batteries. Their multi-electron redox processes allow for moderate to high specific capacities and a wide range of operating potentials. Hybridizing POMs with conductive supports, such as carbon nanotubes, graphene, MXenes, metal nanoparticles, MOFs, and polymer matrices, significantly improves capacity retention, rate performance, and long-term cycling stability. These approaches effectively overcome the limitations of pure POMs and facilitate efficient ion accommodation. In supercapacitors, POM-based composite and asymmetric electrodes bridge the performance gap with batteries by combining faradaic pseudocapacitance and electric double-layer storage. Key supports such as conducting polymers, carbon materials, graphene derivatives, and porous carbons enhance electrical conductivity, surface area, and mechanical strength. Advanced hybrid structures—including layer-by-layer assemblies, POM–graphene gels, MOF-supported POMs, and asymmetric devices—offer high specific capacitance, broader voltage ranges, excellent energy and power densities, and long cycle life. Overall, this manuscript presents POM-based composites as versatile, high-performance electrode platforms for future energy storage solutions.

7 Conclusions and outlook

It is almost impossible to overemphasize the applications of POMs in environmental remediation. By looking at the number of environmental studies mentioning POMs in the removal of various pollutants from water, soil or air, it seems that POMs are involved everywhere. This increasing number of environmental degradation studies (Fig. 3) involving POMs could be mostly explained by the versatility of the structural chemistry of POMs (Fig. 2) and the catalytic features specific to transition metals.

POMs in water column filters and/or in porous organic–inorganic composites proved to be effective in the removal of toxic heavy metals, aromatic organic pollutants, and bacteria (Fig. 4, 5 and 7). POMs in porous nanosheets are capable of the photocatalytic degradation of emergent pollutants, particularly antibiotics (Fig. 8, Tables 1 and S2), with enhanced photocatalytic performance under visible light (Fig. 9), but also dyes, plastics, industrial chemicals, and pesticides (Tables 1 and S2). Moreover, a magnetic core enclosed by polyoxometalate-based ionic liquid phases (Fig. 12) was used to remove dyes, heavy metals, microbes, and microplastics (MPs). MPs are not only one of the new emergent health pollutants but also a major one of worldwide concern, in addition to being associated with joint contamination with heavy metals.

POMs, alone and/or in combination with other compounds, such as metal–organic frameworks (MOFs), carbon nanotubes (CNTs) and mesoporous silica supports, have shown promising results in the removal of air pollutants from fossil fuels due to their selective catalytic properties for the oxidation of sulfur compounds (Fig. 13–15, Table S1). In addition, toxic gases such as hydrogen sulfide, nitrogen oxides and sulfur dioxide are efficiently removed by POMs (Fig. 16, Table 2), whereas the volatile organic compounds' reaction mechanism involves a photocatalytic oxidation catalyzed by the PW₁₂/g-C₃N₄ hybrid material (Fig. 17).

The immobilization of POMs on different supporting surfaces facilitates their electrochemical properties for sensor application (Fig. 19, Table 3). Conversely, their variable redox activities and outstanding electron/proton transport capacities make POM-based composite materials suitable for use in electrochemical fields as an exceptional electrode component for supercapacitors and batteries (Table 4). A high-performance pseudocapacitor was obtained by replacing multiple Mo centers in [H₃PMo^VI₁₂O₄₀] with vanadium and incorporating modified a phosphomolybdate with a conducting polymer for improved electrochemical activity (Fig. 20), whereas a biomass-derived pinecone activated carbon, that includes POMs contributed to a high specific capacitance (Fig. 21). Carbon nanostructures, graphene oxide/reduced graphene oxide, and activated carbon composites come into play as supporting elements for the POMs as they provide better mechanical and electrochemical stability for broader electrochemical applications (Fig. 22 and 23). Although this review does not reveal everything, it may help to get closer to viable solutions for the effective use of the POM-based materials for the removal of the environmental pollutants. The future is bright for POM applications in environmental treatments!

Conflicts of interest

There are no conflicts to declare.

Abbreviations

Ac	Acetic acid
AC	Activated carbon
AOP	Advanced oxidation process
APTMS	3-Aminopropyltrimethoxysilane
APTS	γ-Aminopropyltriethoxysilane
Asp	Aspartic acid
Bbi	1,1′-(1,4-Butanediyl)bis(imidazole)
BE	Berberine
bimb	1,4-Bis(1-imidazolyl)benzene
bipy	Bipyridine
BMIM or bmim	1-Butyl-3-methylimidazolium
BPA	Bisphenol A
BPA-Br	Bromobisphenol-A
BPy	1-Butylpyridinium or N-butylpyridinium
BR46	Basic red 46
BT	Benzothiophene
BTC	1,3,5-Benzenetricarboxylate
CCNF	Carbonized cellulose nanofiber
CNTs	Carbon nanotubes
CP	Chlorphenole
4-CP	4-Chlorphenole
CPF	Ciprofloxacin
CPBPY	N-(3-Carboxyphenyl)-4,4′-bipyridinium
cpt	4-(4′-Carboxyphenyl)-1,2,4-triazolate
CSH	Cellulose propylamine-modified silica
CTS	Chitosan
CV	Crystal violet
DBP	Di-n-butyl phthalate
DBT	Dibenzothiophene
DESs	Deep eutectic solvents
DMDBT	4,6-Dimethyldibenzotiophene
DODA·Br	Dimethyldioctadecylammonium bromide
DODMAC	Dimethyldioctadecylammonium chloride
ECSA	Electrochemically active surface area
EDA-CD	Per-(6-deoxy-6-iodo)-β-cyclodextrin
ELSA	Electrochemically active surface area
en	Ethylenediamine
EPs	Emergent pollutants
EtOH	Ethanol
etpy	4-Ethylpyridine
EY	Eosin Y
g-BN	Graphene-like hexagonal boron nitride
GA	Graphene aerogel
GO	Graphene oxide
Gr	Graphene
HOMO	Highest occupied molecular orbital
HPW or PW12	[H3PW^VI12O40·6H2O]
H2pyttz-I	3-(Pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl
H2pyttz-II	3-(Pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl
H3bdpm	1,1′-Bis(3,5-dimethyl-1H-pyrazolate)methane
IBA	Isobutyraldehyde
IBAc	Isobutyric acid
IBP	Ibuprofen
IL	Ionic liquid
imi	Imidazole
iPAF-1	Porous aromatic framework
LiBs	Lithium-ion batteries
LDH	Layered double hydroxide
LMCT	Ligand to metal charge transfer
LPMS	Large-pore mesoporous silica
LRSR	Liquid-redox sulfur recovery
LUMO	Lowest unoccupied molecular orbital
MB	Methylene blue
MBT	2-Mercaptobenzothiazole
MCM-41	Conventional molecular sieve MCM-41
MeCN	Acetonitrile
MeOH	Methanol
mepy	4-Methylpyridine
MO	Methyl orange
MOFs	Metal–organic frameworks
MOG	Metal–organic gel
MPs	Microplastics
MR	Methyl red
M-TCS	Methyl triclosan
NAD	2-(1-Naphthyl)acetamide
NBZ	Nitrobenzene
NFZ	Nitrofurazone
NPs	(Metal) nanoparticles
ODS	Oxidative desulfurization
PANI	Polyaniline
PBV	Patent blue V
pca	Pyridine-2-carboxylic acid
PDDA	Poly(diallyldimethylammonium chloride)
PEI	Polyetherimide
phen	1,10-Phenanthroline
PIL	Protic ionic liquid
PMIn	Polyionene
PMOE	(Ethylene-bridged) periodic mesoporous organosilica
PMs	Particulate matters
POM	Polyoxometalate
POMCP	POM-based coordination polymer
POM-IL	Polyoxometalate-based ionic liquid
POMos	Polyoxomolybdates
POM-SILP	Polyoxometalate-supported ionic liquid phase
POT	Polyoxotungstate
PPI	Proton pump inhibitor
PPy	Polypyrrole
PS	Ponceau S
PTMS	3-Aminopropyl trimethoxysilane
PVA	Polyvinyl alcohol
PVDF	Polyvinylidene fluoride
py	Pyrene
PyPS	3-(Pyridine-1-ium-1-yl)propane-1-sulfonate
PZC	Point-of-zero charge
RB	Rose bengal
RB5	Reactive black 5
RhB	Rhodamine B
RH	Rice husk
rGO	Reduced graphene oxide
SAB	Sodium-activated bentonite
SBA-15	Aminosilylated silica
SC	Supercapacitor
SCR	Selective catalytic reduction
SDV	Sodium decavanadate
SMT	Sulfamethazine
SPD	Sulfapyridine
SPME	Solid-phase microextraction
SSA	5-Sulfosalicylic acid
SSZ	Sulfasalazine
SWCNTs	Single-walled carbon nanotubes
TB	Toluidine blue
TBA	Tetra-n-butylammonium ion
TBBA	Tetrabromobisphenol-A
TC	Tetracycline
TCS	Triclosan
TCY	Tetracycline
TMA	N-Trimethoxysilypropyl-N,N,N-trimethylammonium
TMR4A	Resorcin[4]arene-based ligand
TOA	Tetraoctylammonium
TPD-MS	Temperature-programmed desorption-mass spectroscopy
VOCs	Volatile organic compounds
4,6-DMDBT	4,6-Dimethyl dibenzothiophene
[mim(CH2)3COO]	1-Carboxypropyl-3-methyl imidazole
[C4mim]^+	1-Butyl-3-methylimidazolium ion
β-EDA-CD	Per-6-deoxy-6-ethylenediamine-β-cyclodextrin

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Supplementary information (SI): the SI includes summary tables of recently published studies on POM-based water treatment technologies (section 2) and POM-based catalysts for the removal of refractory sulfur compounds from fossil fuels (section 4.1). See DOI: https://doi.org/10.1039/d5en00964b.

Acknowledgements

M. M. is grateful to Ferdowsi University of Mashhad and RADA Think Tank (Research for Academic Development & Advancement) for financial support. This work is based upon research founded by Iranian National Science Foundation (INSF) under project No. 40405103. This study received Portuguese national funds from FCT – Foundation for Science and Technology through contracts UID/04326/2025 (DOI: https://doi.org/10.54499/UID/04326/2025), UID/PRR/04326/2025 (DOI: https://doi.org/10.54499/UID/PRR/04326/2025) and LA/P/0101/2020 (DOI: https://doi.org/10.54499/LA/P/0101/2020) (M. A.).

This research was also funded in whole or in part by the Austrian Science Fund (FWF) (Grant DOI: https://doi.org/10.55776/PAT4299925 (A. R.)). For open access purposes, the author has applied a CC BY public copyright license to any author accepted manuscript version arising from this submission.

The authors would like to acknowledge Dr João Mateus for providing the professional illustration of Fig. 1 and also for the Graphical abstract illustration.

Notes and references

1. L. Rizzo, S. Malato, D. Antakyali, V. G. Beretsou, M. B. Đolić and W. Gernjak, et al., Consolidated vs new advanced treatment methods for the removal of contaminants of emerging concern from urban wastewater, Sci. Total Environ., 2019, 655, 986–1008,  DOI:10.1016/j.scitotenv.2018.11.265.
2. M. A. Mustapha, Z. A. Manan and S. R. Wan Alwi, A New Quantitative Overall Environmental Performance Indicator for a Wastewater Treatment Plant, J. Cleaner Prod., 2017, 167, 815–823,  DOI:10.1016/j.jclepro.2017.08.169.
3. D. Y. C. Leung and E. Drakaki, Outdoor-Indoor Air Pollution in Urban Environment: Challenges and Opportunity, Front. Environ. Sci., 2015, 2(69), 1–7,  DOI:10.3389/fenvs.2014.00069.
4. J. A. Nathanson, Pollution [Internet], in Encyclopedia Britannica, Encyclopædia Britannica, Inc., Chicago, 2025, [cited 2025 Sep 17], Available from: https://www.britannica.com/science/pollution-environment.
5. Directorate-General for Energy, European Commission, In focus: renewable energy in Europe [Internet], European Commission, Brussels, 2020, Mar [cited 2025 Sep 17], Available from: https://ec.europa.eu/info/news/focus-renewable-energy-europe-2020-mar-18_en.
6. P. Klepac, I. Locatelli, S. Korošec, N. Künzli and A. Kukec, Ambient air pollution and pregnancy outcomes: a comprehensive review and identification of environmental public health challenges, Environ. Res., 2018, 167, 144–159,  DOI:10.1016/j.envres.2018.07.008.
7. D. Briggs, Environmental Pollution and the Global Burden of Disease, Br. Med. Bull., 2003, 68(1), 1–24,  DOI:10.1093/bmb/ldg019.
8. L. Parker, What You Need to Know About the World's Water Wars, National Geographic, 2020, https://www.nationalgeographic.com/science/article/world-aquifers-water-wars, Accessed Sep 17, 2025.
9. P. H. Gleick, Water In Crisis: Paths To Sustainable Water Use, Ecol. Appl., 1998, 8(3), 571–579,  DOI:10.1890/1051-0761(1998)008[0571:WICPTS]2.0.CO;2.
10. V. Geissen, H. Mol, E. Klumpp, G. Umlauf, M. Nadal and M. van der Ploeg, et al., Emerging pollutants in the environment: a challenge for water resource management, Int. Soil Water Conserv. Res., 2015, 3(1), 57–65,  DOI:10.1016/j.iswcr.2015.03.002.
11. United Nations Environment Programme and World Meteorological Organization, International Conference on Water and the Environment: Development Issues for the 21st Century [internet], UNEP, Nairobi, 1992, [cited 2025 Sep 1], Available from: https://wedocs.unep.org/20.500.11822/30961.
12. R. Zhuo and F. Fan, A Comprehensive Insight into the Application of White Rot Fungi and Their Lignocellulolytic Enzymes in the Removal of Organic Pollutants, Sci. Total Environ., 2021, 778, 146132,  DOI:10.1016/j.scitotenv.2021.146132.
13. S. Alipoori, H. Rouhi, E. Linn, H. Stumpfl, H. Mokarizadeh and M. R. Esfahani, et al., Polymer-based devices and remediation strategies for emerging contaminants in water, ACS Appl. Polym. Mater., 2021, 3(2), 549–577,  DOI:10.1021/acsapm.0c0117.
14. K. M. Witkowski and N. E. Johnson, Organic-solvent water pollution and low birth weight in Michigan, Soc. Biol., 1992, 39(1–2), 45–54,  DOI:10.1080/19485565.1992.9988803.
15. United Nations World Water Assessment Programme, World Water Development Report: Water for people, water for life (WWDR1) [Internet], UN-Water, Paris, 2003, Mar [cited 2025 Sep 1], Available from: https://www.unwater.org/publications/un-world-water-development-report-2003.
16. S. D. Richardson and T. A. Ternes, Water Analysis: Emerging Contaminants and Current Issues, Anal. Chem., 2011, 83(12), 4614–4648,  DOI:10.1021/ac200915r.
17. P. C. von der Ohe, V. Dulio, J. Slobodnik, E. De Deckere, R. Kühne and R.-U. Ebert, et al., A new risk assessment approach for the prioritization of 500 classical and emerging organic microcontaminants as potential river basin specific pollutants under the European Water Framework Directive, Sci. Total Environ., 2011, 409(11), 2064–2077,  DOI:10.1016/j.scitotenv.2011.01.054.
18. J. R. de Andrade, M. F. Oliveira, M. G. C. da Silva and M. G. A. Vieira, Adsorption of Pharmaceuticals from Water and Wastewater Using Nonconventional Low-Cost Materials: A Review, Ind. Eng. Chem. Res., 2018, 57(9), 3103–3127,  DOI:10.1021/acs.iecr.7b05137.
19. A. Marican and E. F. Durán-Lara, A Review on Pesticide Removal through Different Processes, Environ. Sci. Pollut. Res., 2018, 25(3), 2051–2064,  DOI:10.1007/s11356-017-0796-2.
20. C. Juliano and G. A. Magrini, Cosmetic Ingredients as Emerging Pollutants of Environmental and Health Concern, A Mini-Review, Cosmetics, 2017, 4, 11–29,  DOI:10.3390/cosmetics4020011.
21. M. Bilal, S. Mehmood and H. M. N. Iqbal, The Beast of Beauty: Environmental and Health Concerns of Toxic Components in Cosmetics, Cosmetics, 2020, 7(1), 13–31,  DOI:10.3390/cosmetics7010013.
22. J. M. Brausch and G. M. Rand, A Review of Personal Care Products in the Aquatic Environment: Environmental Concentrations and Toxicity, Chemosphere, 2011, 82(11), 1518–1532,  DOI:10.1016/j.chemosphere.2010.11.018.
23. M. Oliveira, K. Slezakova, C. Delerue-Matos, M. C. Pereira and S. Morais, Children Environmental Exposure to Particulate Matter and Polycyclic Aromatic Hydrocarbons and Biomonitoring in School Environments: A Review on Indoor and Outdoor Exposure Levels, Major Sources and Health Impacts, Environ. Int., 2019, 124, 180–204,  DOI:10.1016/j.envint.2018.12.052.
24. I. Sanseverino, R. Loos, A. Navarro Cuenca, D. Marinov and T. Lettieri, State of the art on the contribution of water to antimicrobial resistance, EUR 29592 EN, Publications Office of the European Union, Luxembourg, 2018,  DOI:10.2760/771124.
25. S. Rodriguez-Mozaz, I. Vaz-Moreira, S. Varela Della Giustina, M. Llorca, D. Barceló and S. Schubert, et al., Antibiotic residues in final effluents of European wastewater treatment plants and their impact on the aquatic environment, Environ. Int., 2020, 140, 105733,  DOI:10.1016/j.envint.2020.105733.
26. E. Wołejko, A. Jabłońska-Trypuć, U. Wydro, A. Butarewicz and B. Łozowicka, Soil Biological Activity as an Indicator of Soil Pollution with Pesticides – A Review, Appl. Soil Ecol., 2020, 147, 103356,  DOI:10.1016/j.apsoil.2019.09.006.
27. S. Hussain, T. Siddique, M. Saleem, M. Arshad and A. Khalid, Impact of pesticides on soil microbial diversity, enzymes, and biochemical reactions, in Advances in agronomy, Academic Press, San Diego, 2009, vol. 102, pp. 159–200,  DOI:10.1016/S0065-2113(09)01005-0.
28. H.-R. Köhler and R. Triebskorn, Wildlife Ecotoxicology of Pesticides: Can We Track Effects to the Population Level and Beyond?, Science, 2013, 341(6147), 759–765,  DOI:10.1126/science.1237591.
29. B. A. Rattner, History of wildlife toxicology, Ecotoxicology, 2009, 18, 773–783,  DOI:10.1007/s10646-009-0354-x.
30. P. Berny, Pesticides and the intoxication of wild animals, J. Vet. Pharmacol. Ther., 2007, 30, 93–100,  DOI:10.1111/j.1365-2885.2007.00836.x.
31. F. P. Carvalho, Pesticides, environment, and food safety, Food Energy Secur., 2017, 6(2), 48–60,  DOI:10.1002/fes3.108.
32. J. George and Y. Shukla, Pesticides and cancer: Insights into toxicoproteomic-based findings, J. Proteomics, 2011, 74(12), 2713–2722,  DOI:10.1016/j.jprot.2011.09.024.
33. K. L. Bassil, C. Vakil, M. Sanborn, D. C. Cole, J. S. Kaur and J. K. Kerr, Cancer health effects of pesticides, Can. Fam. Physician, 2007, 53(10), 1704–1711 , Available from: https://www.cfp.ca/content/53/10/1704.
34. M. Kumar, D. K. Sarma, S. Shubham, M. Kumawat, V. Verma, A. Prakash and R. Tiwari, Environmental Endocrine-Disrupting Chemical Exposure: Role in Non-Communicable Diseases, Front. Public Health, 2020, 8, 553850,  DOI:10.3389/fpubh.2020.553850.
35. F. Jiang, Y. Peng and Q. Sun, Pesticides exposure induced obesity and its associated diseases: recent progress and challenges, J. Future Foods, 2022, 2(2), 119–124,  DOI:10.1016/j.jfutfo.2022.03.005.
36. Y. A. Kim, J. B. Park, M. S. Woo, S. Y. Lee, H. Y. Kim and Y. H. Yoo, Persistent Organic Pollutant-Mediated Insulin Resistance, Int. J. Environ. Res. Public Health, 2019, 16, 448–462,  DOI:10.3390/ijerph16030448.
37. J. S. A. de Araujo, I. F. Delgado and F. J. R. Paumgartten, Glyphosate and Adverse Pregnancy Outcomes, a Systematic Review of Observational Studies, BMC Public Health, 2016, 16, 472–485,  DOI:10.1186/s12889-016-3153-3.
38. L. Carrasco Cabrera and P. P. Medina, The 2019 European Union Report on Pesticide Residues in Food, EFSA J., 2021, 19(4), 6491,  DOI:10.2903/j.efsa.2021.6491.
39. V. S. Arutyunov and G. V. Lisichkin, Energy Resources of the 21st Century: Problems and Forecasts. Can Renewable Energy Sources Replace Fossil Fuels?, Russ. Chem. Rev., 2017, 86(8), 777–804,  DOI:10.1070/rcr4723.
40. F. Barbir, T. N. Veziroǧlu and H. J. Plass, Environmental damage due to fossil fuels use, Int. J. Hydrogen Energy, 1990, 15(10), 739–749,  DOI:10.1016/0360-3199(90)90005-J.
41. K. Mukhopadhyay and O. Forssell, An empirical investigation of air pollution from fossil fuel combustion and its impact on health in India during 1973-1974 to 1996-1997, Ecol. Econ., 2005, 55(2), 235–250,  DOI:10.1016/j.ecolecon.2004.09.022.
42. S. Loppi and J. Nascimbene, Monitoring H₂S air pollution caused by the industrial exploitation of geothermal energy: The pitfall of using lichens as bioindicators, Environ. Pollut., 2010, 158(8), 2635–2639,  DOI:10.1016/j.envpol.2010.05.002.
43. J. M. Ellison and R. E. Waller, A review of sulphur oxides and particulate matter as air pollutants with particular reference to effects on health in the United Kingdom, Environ. Res., 1978, 16(1–3), 302–325,  DOI:10.1016/0013-9351(78)90164-0.
44. T.-Y. Wong, Smog induces oxidative stress and microbiota disruption, J. Food Drug Anal., 2017, 25(2), 235–244,  DOI:10.1016/j.jfda.2017.02.003.
45. A. Singh and M. Agrawal, Acid rain and its ecological consequences, J. Environ. Biol., 2008, 29(1), 15–24,  DOI:10.4103/0254-8704.44191 , https://pubmed.ncbi.nlm.nih.gov/18831326/.
46. I. Galán, A. Tobías, J. R. Banegas and E. Aránguez, Short-term effects of air pollution on daily asthma emergency room admissions, Eur. Respir. J., 2003, 22, 802–808,  DOI:10.1183/09031936.03.00013003.
47. C. H. Linaker, D. Coggon, S. T. Holgate, J. Clough, L. Josephs and A. J. Chauhan, et al., Personal exposure to nitrogen dioxide and risk of airflow obstruction in asthmatic children with upper respiratory infection, Thorax, 2000, 55(11), 930–933,  DOI:10.1136/thorax.55.11.930.
48. O. Kar Kurt, J. Zhang and K. E. Pinkerton, Pulmonary Health Effects of Air Pollution, Curr. Opin. Pulm. Med., 2016, 22(2), 138–143,  DOI:10.1097/MCP.0000000000000248.
49. K. A. Miller, D. S. Siscovick, L. Sheppard, K. Shepherd, J. H. Sullivan, G. L. Anderson and J. D. Kaufman, Long-Term Exposure to Air Pollution and Incidence of Cardiovascular Events in Women, N. Engl. J. Med., 2007, 356, 447–458,  DOI:10.1056/NEJMoa054409.
50. D. Schwela, Air Pollution and Health in Urban Areas, Rev. Environ. Health, 2000, 15(1–2), 13–42,  DOI:10.1515/REVEH.2000.15.1-2.13.
51. D. S. Aribike, M. A. Usman and M. M. Oloruntoba, Adsorptive desulfurization of diesel using activated sewage sludge: kinetics, equilibrium and thermodynamics studies, Appl. Petrochem. Res., 2020, 10, 1–12,  DOI:10.1007/s13203-019-00239-2.
52. C. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catal. Today, 2003, 86(1–4), 211–263,  DOI:10.1016/S0920-5861(03)00412-7.
53. H. Yang, B. Jiang, Y. Sun, L. Zhang, Z. Huang and Z. Sun, et al., Heterogeneous oxidative desulfurization of diesel fuel catalyzed by mesoporous polyoxometallate-based polymeric hybrid, J. Hazard. Mater., 2017, 333, 63–72,  DOI:10.1016/j.jhazmat.2017.03.017.
54. Y.-Q. Ma, X.-P. Liu, J.-P. Li, R. Wang and M.-Q. Yu, Transition Metal Salts of H4PMo11VO40 for Efficient H₂S Removal in the Liquid Redox Process, Chem. Pap., 2017, 71, 647–652,  DOI:10.1007/s11696-016-0064-9.
55. J. Zhang, W. Zuo, Y. Tian, L. Yin, Z. Gong and J. Zhang, Release of Hydrogen Sulfide during Microwave Pyrolysis of Sewage Sludge: Effect of Operating Parameters and Mechanism, J. Hazard. Mater., 2017, 331, 117–122,  DOI:10.1016/j.jhazmat.2017.02.040.
56. J. J. Jaakkola, V. Vilkka, O. Marttila, P. Jäppinen and T. Haahtela, The South Karelia Air Pollution Study: The effects of malodorous sulfur compounds from pulp mills on respiratory and other symptoms, Am. Rev. Respir. Dis., 1990, 142(6 Pt 1), 1344–1350,  DOI:10.1164/ajrccm/142.6_Pt_1.1344.
57. N. S. Chaudhari, A. P. Bhirud, R. S. Sonawane, L. K. Nikam, S. S. Warule and V. H. Rane, et al., Ecofriendly hydrogen production from abundant hydrogen sulfide using solar light-driven hierarchical nanostructured ZnIn₂S₄ photocatalyst, Green Chem., 2011, 13(9), 2500–2506,  10.1039/C1GC15515F.
58. E. Lim, O. Mbowe, A. S. W. Lee and J. Davis, Effect of Environmental Exposure to Hydrogen Sulfide on Central Nervous System and Respiratory Function: A Systematic Review of Human Studies, Int. J. Occup. Environ. Health, 2016, 22(1), 80–90,  DOI:10.1080/10773525.2016.1145881.
59. European Environment Agency, State of Europe's environment not good: threats to nature and impacts of climate change top challenges, EEA, Copenhagen, 2025,  DOI:10.2800/3817344.
60. X. Cheng and X. T. Bi, A review of recent advances in selective catalytic NO_x reduction reactor technologies, Particuology, 2014, 16, 1–18,  DOI:10.1016/j.partic.2014.01.006.
61. C. Song, An Overview of New Approaches to Deep Desulfurization for Ultra-Clean Gasoline, Diesel Fuel and Jet Fuel, Catal. Today, 2003, 86(1–4), 211–263,  DOI:10.1016/S0920-5861(03)00412-7.
62. S. J. C. Ng, A. E. S. Choi, N. P. Dugos and M.-W. Wan, Driving sustainable solutions: exploring supported-polyoxometalate catalysts for enhanced oxidative desulfurization, Chem. Eng. Trans., 2024, 113, 79–84,  DOI:10.3303/CET24113014.
63. S. Herrmann, L. de Matteis, J. M. de la Fuente, S. G. Mitchell and C. Streb, Removal of Multiple Contaminants from Water by Polyoxometalate Supported Ionic Liquid Phases (POM-SILPs), Angew. Chem., Int. Ed., 2017, 56(6), 1667–1670,  DOI:10.1002/anie.201611072.
64. M. A. A. Mamun and M. R. Yuce, Recent Progress in Nanomaterial Enabled Chemical Sensors for Wearable Environmental Monitoring Applications, Adv. Funct. Mater., 2020, 30, 2005703,  DOI:10.1002/adfm.202005703.
65. G. Hanrahan, D. G. Patila and J. Wang, Electrochemical sensors for environmental monitoring: design, development and applications, J. Environ. Monit., 2004, 6, 657–664,  10.1039/B403975K.
66. C. I. L. L. Justino, A. C. Duarte and T. A. P. Rocha-Santos, Recent Progress in Biosensors for Environmental Monitoring: A Review, Sensors, 2017, 17(12), 2918–2943,  DOI:10.3390/s17122918.
67. S. Tajik, H. Beitollahi, F. G. Nejad, I. Sheikhshoaie, A. S. Nugraha and H. W. Jang, et al., Performance of metal-organic frameworks in the electrochemical sensing of environmental pollutants, J. Mater. Chem. A, 2021, 9, 8195–8220,  10.1039/D0TA08344E.
68. L. Rassaei, F. Marken, M. Sillanpää, M. Amiri, C. M. Cirtiu and M. Sillanpää, Nanoparticles in Electrochemical Sensors for Environmental Monitoring, TrAC, Trends Anal. Chem., 2011, 30(11), 1704–1715,  DOI:10.1016/j.trac.2011.05.009.
69. E. Priyadarshini and N. Pradhan, Gold nanoparticles as efficient sensors in colorimetric detection of toxic metal ions: A review, Sens. Actuators, B, 2017, 238, 888–902,  DOI:10.1016/j.snb.2016.06.081.
70. M. R. Horn, A. Singh, S. Alomari, S. Goberna-Ferrón, R. Benages-Vilau and N. Chodankar, et al., Polyoxometalates (POMs): from electroactive clusters to energy materials, Energy Environ. Sci., 2021, 14(4), 1652–1700,  10.1039/D0EE03407J.
71. M. S. Whittingham, Lithium Batteries: 50 Years of Advances to Address the Next 20 Years of Climate Issues, Nano Lett., 2020, 20(12), 8435–8437,  DOI:10.1021/acs.nanolett.0c04347.
72. C. Delmas, Sodium and Sodium-Ion Batteries: 50 Years of Research, Adv. Energy Mater., 2018, 8(17), 1703137,  DOI:10.1002/aenm.201703137.
73. A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick and Q. Liu, Redox Flow Batteries: A Review, J. Appl. Electrochem., 2011, 41(10), 1137–1164,  DOI:10.1007/s10800-011-0348-2.
74. A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon and W. van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, in Materials for sustainable energy, Nature Publishing Group, London, 2010, pp. 148–159,  DOI:10.1142/9789814317665_0022.
75. A. A. Vannathan, T. Kella, D. Shee and S. S. Mal, One-Pot Synthesis of Polyoxometalate Decorated Polyindole for Energy Storage Supercapacitors, ACS Omega, 2021, 6(17), 11199–11208,  DOI:10.1021/acsomega.0c05967.
76. S. Herrmann, N. Aydemir, F. Nägele, D. Fantauzzi, T. Jacob and J. Travas-Sejdic, et al., Enhanced capacitive energy storage in polyoxometalate-doped polypyrrole, Adv. Funct. Mater., 2017, 27(25), 1700881,  DOI:10.1002/adfm.201700881.
77. M. T. Pope and A. Müller, Polyoxometalate Chemistry From Topology via Self-Assembly to Applications, Springer, Dordrecht, 2001,  DOI:10.1007/0-306-47625-8.
78. C. Streb, New Trends in Polyoxometalate Photoredox Chemistry: From Photosensitisation to Water Oxidation Catalysis, Dalton Trans., 2012, 41, 1651–1659,  10.1039/C1DT11220A.
79. E. Coronado, C. Giménez-Saiz and C. J. Gómez-García, Recent Advances in Polyoxometalate-Containing Molecular Conductors, Coord. Chem. Rev., 2005, 249(17), 1776–1796,  DOI:10.1016/j.ccr.2005.02.017.
80. X. López, J. A. Fernández and J. M. Poblet, Redox Properties of Polyoxometalates: New Insights on the Anion Charge Effect, Dalton Trans., 2006, 1162–1167,  10.1039/B507599H.
81. S.-S. Wang and G.-Y. Yang, Recent Advances in Polyoxometalate-Catalyzed Reactions, Chem. Rev., 2015, 115(11), 4893–4962,  DOI:10.1021/cr500390v.
82. M. Lotfian, M. M. Heravi, M. Mirzaei and B. Heidari, Applications of inorganic-organic hybrid architectures based on polyoxometalates in catalyzed and photocatalyzed chemical transformations, Appl. Organomet. Chem., 2019, 33(4), e4808,  DOI:10.1002/aoc.4808.
83. J. M. Clemente-Juan, E. Coronado and A. Gaita-Ariño, Magnetic Polyoxometalates: From Molecular Magnetism to Molecular Spintronics and Quantum Computing, Chem. Soc. Rev., 2012, 41(22), 7464–7478,  10.1039/C2CS35205B.
84. S. Wang, W. Sun, Q. Hu, H. Yan and Y. Zeng, Synthesis and Evaluation of Pyridinium Polyoxometalates as Anti-HIV-1 Agents, Bioorg. Med. Chem. Lett., 2017, 27(11), 2357–2359,  DOI:10.1016/j.bmcl.2017.04.025.
85. A. Bijelic, M. Aureliano and A. Rompel, Polyoxometalates as Potential Next-Generation Metallodrugs in the Combat Against Cancer, Angew. Chem., Int. Ed., 2019, 58(10), 2980–2999,  DOI:10.1002/anie.201803868.
86. L. S. van Rompu and T. N. Parac-Vogt, Interactions between Polyoxometalates and Biological Systems: From Drug Design to Artificial Enzymes, Curr. Opin. Biotechnol., 2019, 58, 92–99,  DOI:10.1016/j.copbio.2018.11.013.
87. A. Bijelic and A. Rompel, The Use of Polyoxometalates in Protein Crystallography – An Attempt to Widen a Well-Known Bottleneck, Coord. Chem. Rev., 2015, 299, 22–38,  DOI:10.1016/j.ccr.2015.03.018.
88. A. Bijelic and A. Rompel, Polyoxometalates: More than a Phasing Tool in Protein Crystallography, ChemTexts, 2018, 4(10), 1–28,  DOI:10.1007/s40828-018-0064-1.
89. A. Blazevic, E. Al-Sayed, A. Roller, G. Giester and A. Rompel, Tris-Functionalized Hybrid Anderson Polyoxometalates: Synthesis, Characterization, Hydrolytic Stability and Inversion of Protein Surface Charge, Chem. – Eur. J., 2015, 21(12), 4762–4771,  DOI:10.1002/chem.201405644.
90. Y. Ji, L. Huang, J. Hu, C. Streb and Y.-F. Song, Polyoxometalate-Functionalized Nanocarbon Materials for Energy Conversion, Energy Storage and Sensor Systems, Energy Environ. Sci., 2015, 8, 776–789,  10.1039/C4EE03749A.
91. D.-L. Long, R. Tsunashima and L. Cronin, Polyoxometalates: Building Blocks for Functional Nanoscale Systems, Angew. Chem., Int. Ed., 2010, 49(10), 1736–1758,  DOI:10.1002/anie.200902483.
92. M. T. Pope and U. Kortz, Polyoxometalates, in Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons Ltd, Hoboken (NJ), 2012,  DOI:10.1002/9781119951438.eibc0185.pub2.
93. E. Al-Sayed and A. Rompel, Lanthanides Singing the Blues: Their Fascinating Role in the Assembly of Gigantic Molybdenum Blue Wheels, ACS Nanosci. Au, 2022, 2(3), 179–197,  DOI:10.1021/acsnanoscienceau.1c00036.
94. A. S. Cherevan, S. P. Nandan, I. Roger, R. Liu and C. Streb, Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives, Adv. Sci., 2020, 7, 1903511,  DOI:10.1002/advs.201903511.
95. L. Zeng, L. Xiao, Y. Long and X. Shi, Trichloroacetic acid-modulated synthesis of polyoxometalate@UiO-66 for selective adsorption of cationic dyes, J. Colloid Interface Sci., 2018, 516, 274–283,  DOI:10.1016/j.jcis.2018.01.070.
96. T.-T. Zhu, Z.-M. Zhang, W.-L. Chen, Z.-J. Liu and E.-B. Wang, Encapsulation of tungstophosphoric acid into harmless MIL-101(Fe) for effectively removing cationic dye from aqueous solution, RSC Adv., 2016, 6, 81622–81630,  10.1039/C6RA16716K.
97. D.-F. Chai, M. Wang, C. Zhang, F. Ning, W. Xu and H. Pang, et al., A novel 3D POMOF based on dinuclear copper (II)-oxalate complexes and Keggin polyoxoanions with excellent photocatalytic activity, Inorg. Chem. Commun., 2017, 83, 16–19,  DOI:10.1016/j.inoche.2017.05.028.
98. B.-W. Cong, Z.-H. Su, Z.-F. Zhao, W.-Q. Zhao, X.-J. Ma and Q. Xu, et al., A new 3D POMOF with two channels consisting of Wells–Dawson arsenotungstate and {Cl₄Cu₁₀(pz)₁₁} complexes: synthesis, crystal structure, and properties, New J. Chem., 2018, 42, 4596–4602,  10.1039/C7NJ04854H.
99. M. Huo, W. Yang, H. Zhang, L. Zhang, J. Liao and L. Lin, et al., A new POM–MOF hybrid microporous material with ultrahigh thermal stability and selective adsorption of organic dyes, RSC Adv., 2016, 6, 111549–111555,  10.1039/C6RA10422C.
100. P. Mialane, C. Mellot-Draznieks, P. Gairola, M. Duguet, Y. Benseghir and O. Oms, et al., Heterogenisation of polyoxometalates and other metal-based complexes in metal–organic frameworks: from synthesis to characterisation and applications in catalysis, Chem. Soc. Rev., 2021, 50, 6152–6220,  10.1039/D0CS00323A.
101. M. Samaniyan, M. Mirzaei, R. Khajavian, H. Eshtiagh-Hosseini and C. Streb, Heterogeneous Catalysis by Polyoxometalates in Metal–Organic Frameworks, ACS Catal., 2019, 9, 10174–10191,  DOI:10.1021/acscatal.9b03439.
102. A.-A. Hoseini, S. Farhadi and A. Zabardasti, Yolk–shell microspheres assembled from Preyssler-type NaP₅W₃₀O₁₁₀¹⁴⁻ polyoxometalate and MIL-101(Cr) metal–organic framework: A new inorganic–organic nanohybrid for fast and selective removal of cationic organic dyes from aqueous media, Appl. Organomet. Chem., 2019, 33, e4656,  DOI:10.1002/aoc.4656.
103. A. Jarrah and S. Farhadi, Preparation and characterization of novel polyoxometalate/CoFe₂O₄/metal-organic framework magnetic core-shell nanocomposites for the rapid removal of organic dyes from water, RSC Adv., 2020, 10, 39881–39893,  10.1039/D0RA04603E.
104. X. Liu, J. Luo, Y. Zhu, Y. Yang and S. Yang, Removal of methylene blue from aqueous solutions by an adsorbent based on metal-organic framework and polyoxometalate, J. Alloys Compd., 2015, 648, 986–993,  DOI:10.1016/j.jallcom.2015.07.065.
105. J.-W. Sun, P.-F. Yan, G.-H. An, J.-Q. Sha, G.-M. Li and G.-Y. Yang, Immobilization of Polyoxometalate in the Metal-Organic Framework rht-MOF-1: Towards a Highly Effective Heterogeneous Catalyst and Dye Scavenger, Sci. Rep., 2016, 6, 25595,  DOI:10.1038/srep25595.
106. D. Li, Y. Guo, C. Hu, L. Mao and E. Wang, Photocatalytic degradation of aqueous formic acid over the silica composite films based on lacunary Keggin-type polyoxometalates, Appl. Catal., A, 2002, 235, 11–20,  DOI:10.1016/S0926-860X(02)00238-7.
107. A. Maldotti, A. Molinari, G. Varani, M. Lenarda, L. Storaro and F. Bigi, et al., Immobilization of (n-Bu₄N)₄W₁₀O₃₂ on Mesoporous MCM-41 and Amorphous Silicas for Photocatalytic Oxidation of Cycloalkanes with Molecular Oxygen, J. Catal., 2002, 209, 210–216,  DOI:10.1006/jcat.2002.3618.
108. H. Zhang, X. Wang, N. Li, J. Xia, Q. Meng and J. Ding, et al., Synthesis and characterization of TiO₂/graphene oxide nanocomposites for photoreduction of heavy metal ions in reverse osmosis concentrate, RSC Adv., 2018, 8(60), 34241–34251,  10.1039/C8RA06681G.
109. X. Zhang, L. Li and D. Shao, Uptake of uranium from wastewater by polyoxometalate modified graphene oxide, Sep. Purif. Technol., 2022, 302, 122154,  DOI:10.1016/j.seppur.2022.122154.
110. J. He, H. Sun, S. Indrawirawan, X. Duan, M. O. Tade and S. Wang, Novel polyoxometalate@g-C₃N₄ hybrid photocatalysts for degradation of dyes and phenolics, J. Colloid Interface Sci., 2015, 456, 15–21,  DOI:10.1016/j.jcis.2015.06.003.
111. M. Ghali, C. Brahmi, M. Benltifa, F. Dumur, S. Duval and C. Simonnet-Jégat, et al., New hybrid polyoxometalate/polymer composites for photodegradation of eosin dye, J. Polym. Sci., Part A: Polym. Chem., 2019, 57, 1538–1549,  DOI:10.1002/pola.29416.
112. M. M. Heravi and M. Mirzaei, Polyoxometalate-Based Hybrids and their Applications, 1st edn, Elsevier, 2023, Paperback ISBN: 9780323917315, eBook ISBN: 9780323983464.
113. S. Uchida, Frontiers and progress in cation-uptake and exchange chemistry of polyoxometalate-based compounds, Chem. Sci., 2019, 10, 7670–7679,  10.1039/C9SC02823D.
114. B. Kandasamy, T. Sudmeier, W. W. Ayass, Z. Lin, Q. Feng, B. S. Bassil and U. Kortz, Selective Rb⁺ vs. K⁺ Guest Incorporation in Wheel-Shaped 27-Tungsto-3-Arsenate(III) Host, [M⊂(β-As^IIIW₈O₃₀)(WO(H₂O))₃]¹⁴⁻ (M= K, Rb), Eur. J. Inorg. Chem., 2019,(3–4), 502–505,  DOI:10.1002/ejic.201800788.
115. R. Kawahara, S. Uchida and N. Mizuno, Redox-Induced Reversible Uptake-Release of Cations in Porous Ionic Crystals Based on Polyoxometalate: Cooperative Migration of Electrons with Alkali Metal Ions, Chem. Mater., 2015, 27(6), 2092–2099,  DOI:10.1021/cm504526z.
116. X. Pei and R. Wang, Desulfurization Performance of Rare Earth Mono-Substituted Heteropoly Compounds, Aerosol Air Qual. Res., 2019, 19(12), 2888–2898,  DOI:10.4209/aaqr.2019.10.0540.
117. R. Dehghani, S. Aber and F. Mahdizadeh, Polyoxometalates and Their Composites as Photocatalysts for Organic Pollutants Degradation in Aqueous Media-A Review, Clean: Soil, Air, Water, 2018, 46(12), 1800413,  DOI:10.1002/clen.201800413.
118. I. Lindqvist, The Structure of the Hexaniobate Ion in 7Na₂O·6Nb₂O₅·32H₂O, Ark. Kemi, 1953, 5, 247–250.
119. K. F. Jahr, J. Fuchs and R. Oberhauser, Zur Hydrolyse amphoterer Metallalkoxide, IX. Die Verseifung des Wolfram(VI)-säure-tetramethylesters in Gegenwart von Tetraalkylammoniumbasen, Chem. Ber., 1968, 101, 477–481,  DOI:10.1002/cber.19681010214.
120. E. Shimao, Structure of the Mo₇O₂₄⁶⁻ Ion in a Crystal of Ammonium Heptamolybdate Tetrahydrate, Nature, 1967, 214, 170–171,  DOI:10.1038/214170a0.
121. J. H. Sturdivant, The Formula of Ammonium Paramolybdate, J. Am. Chem. Soc., 1937, 59(4), 630–631,  DOI:10.1021/ja01283a010.
122. J. Fuchs and H. Hartl, Anion Structure of Tetrabutylammonium Octamolybdate [N(C₄H₉)₄]₄Mo₈O₂₆, Angew. Chem., Int. Ed. Engl., 1976, 15, 375–376,  DOI:10.1002/anie.197603751.
123. A. J. Bridgeman, The Electronic Structure and Stability of the Isomers of Octamolybdate, J. Phys. Chem. A, 2002, 106(50), 12151–12160,  DOI:10.1021/jp027037l.
124. J. Fuchs, H. Hartl, W. Schiller and U. Gerlach, Die Kristallstruktur des Tributylammoniumdekawolframats [(C₄H₉)₂NH]₄W₁₀O₂₃, Acta Crystallogr., Sect. B, 1976, 32, 740–749,  DOI:10.1107/S0567740876003907.
125. A. Chemseddine, C. Sanchez, J. Livage, J. P. Launay and M. Fournier, Electrochemical and photochemical reduction of decatungstate: a reinvestigation, Inorg. Chem., 1984, 23(17), 2609–2613,  DOI:10.1021/ic00185a014.
126. H. T. Evans Jr., The Molecular Structure of the Isopoly Complex Ion, Decavanadate (V₁₀O₂₈⁶⁻), Inorg. Chem., 1966, 5(6), 967–977,  DOI:10.1021/ic50040a004.
127. F. J. C. Rossotti and H. Rossotti, Equilibrium Studies of Polyanions, Acta Chem. Scand., 1956, 10(6), 957–984 , http://actachemscand.org/pdf/acta_vol_10_p0957-0984.pdf.
128. J. F. Keggin, Structure of the Molecule of 12-Phosphotungstic Acid, Nature, 1933, 131, 908–909,  DOI:10.1038/131908b0.
129. A. Linz, Preparation of Phosphomolybdic Acid from Phosphoric Acid and Pure Molybdic Acid, Ind. Eng. Chem., Anal. Ed., 1943, 15(7), 459,  DOI:10.1021/i560119a015.
130. R. Khajavian, V. Jodaian, F. Taghipour, J. T. Mague and M. Mirzaei, Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic-Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates, Molecules, 2021, 26, 5994–6020,  DOI:10.3390/molecules26195994.
131. B. Dawson, The Structure of the 9(18)-Heteropoly Anion in Potassium 9(18)-Tungstophosphate, K₆(P₂W₁₈O₆₂)x14H₂O, Acta Crystallogr., 1953, 6, 113–126,  DOI:10.1107/S0365110X53000466.
132. F. Khermann, Zur Kenntnis der komplexen anorganischen Säuren, III. Abhandlung, Z. Anorg. Allg. Chem., 1892, 1, 423–441,  DOI:10.1002/zaac.18920010139.
133. H. T. Evans, The Crystal Structures Of Ammonium And Potassium Molybdotellurates, J. Am. Chem. Soc., 1948, 70(3), 1291–1292,  DOI:10.1021/ja01183a521.
134. V. W. Meloche and W. Woodstock, The Preparation And Study Of Two Ammonium Molybdotellurates, J. Am. Chem. Soc., 1929, 51(1), 171–174,  DOI:10.1021/ja01376a020.
135. M. H. Alizadeh, S. P. Harmalker, Y. Jeannin, J. Martin-Frere and M. T. Pope, A heteropolyanion with fivefold molecular symmetry that contains a nonlabile encapsulated sodium ion. The structure and chemistry of [NaP₅W₃₀O₁₁₀]¹⁴⁻, J. Am. Chem. Soc., 1985, 107, 2662–2669,  DOI:10.1021/ja00295a019.
136. B. Hedman and R. Strandberg, Multicomponent polyanions. 19. The molecular and crystal structure of Na₅HMo₅P₂O₂₃(H₂O)₁₁, a superstructure with sodium-coordinated monohydrogenpentamolybdodiphosphate anions, Acta Crystallogr., Sect. B, 1979, 35, 278–284,  DOI:10.1107/S0567740879003356.
137. P. L. Multicomponent and I. Polyanions, On Yellow and Colorless Molybdophosphates in 3 M Na(ClO₄). A Determination of Formation Constants for Three Colourless pentamolybdodiphosphates in the pH-range 3-9, Acta Chem. Scand., 1971, 25, 1959–1974,  DOI:10.3891/acta.chem.scand.25-1959.
138. T. J. R. Weakley, H. T. Evans, J. S. Showell, G. F. Tourné and C. M. Tourné, 18-Tungstotetracobalto(II)Diphosphate and Related Anions: A Novel Structural Class of Heteropolyanions, J. Chem. Soc., Chem. Commun., 1973, 139–140,  10.1039/C39730000139.
139. R. D. Peacock and T. J. R. Weakley, Heteropolytungstate complexes of the lanthanide elements. Part I. Preparation and reactions, J. Chem. Soc. A, 1971, 1836–1839,  10.1039/J19710001836.
140. A. Müller and S. Roy, En route from the mystery of molybdenum blue via related manipulatable building blocks to aspects of materials science, Coord. Chem. Rev., 2003, 245(1–2), 153–166,  DOI:10.1016/S0010-8545(03)00110-3.
141. A. Müller, F. Peters, M. T. Pope and D. Gatteschi, Polyoxometalates: Very Large Clusters-Nanoscale Magnets, Chem. Rev., 1998, 98(1), 239–272,  DOI:10.1021/cr9603946.
142. Y. P. Jeannin, The Nomenclature of Polyoxometalates: How To Connect a Name and a Structure, Chem. Rev., 1998, 98(1), 51–76,  DOI:10.1021/cr960397i.
143. N. I. Gumerova and A. Rompel, Speciation atlas of polyoxometalates in aqueous solutions, Sci. Adv., 2023, 9(25), eadi0814,  DOI:10.1126/sciadv.adi0814.
144. M. Zdrnja, N. I. Gumerova and A. Rompel, Exploring polyoxometalate speciation: the interplay of concentration, ionic strength, and buffer composition, Front. Chem. Biol., 2024, 3, 1444359,  DOI:10.3389/fchbi.2024.1444359.
145. I. Gregorovic, N. I. Gumerova and A. Rompel, Speciation atlas of polyoxometalates in aqueous solution (Part II): molybdenum browns, Sci. Adv., 2025, 11(44), eaea1910,  DOI:10.1126/sciadv.aea1910.
146. Y. Guo, C. Hu, S. Jiang, C. Guo, Y. Yang and E. Wang, Heterogeneous photodegradation of aqueous hydroxy butanedioic acid by microporous polyoxometalates, Appl. Catal., B, 2002, 36, 9–17,  DOI:10.1016/S0926-3373(01)00260-0.
147. X. Chen, B. Souvanhthong, H. Wang, H. Zheng, X. Wang and M. Huo, Polyoxometalate-based Ionic liquid as thermoregulated and environmentally friendly catalyst for starch oxidation, Appl. Catal., B, 2013, 138, 161–166,  DOI:10.1016/j.apcatb.2013.02.028.
148. M. A. Rezvani and M. Aghmasheh, Synthesis of t-B.PWFe/NiO nanocomposite as an efficient and heterogeneous green nanocatalyst for catalytic oxidative-extractive desulfurization of gasoline, Environ. Prog. Sustainable Energy, 2021, 40, e13616,  DOI:10.1002/ep.13616.
149. S. Xun, Q. Ti, L. Wu, M. He, C. Wang and L. Chen, et al., Few layer g-C₃N₄ dispersed quaternary phosphonium ionic liquid for highly efficient catalytic oxidative desulfurization of fuel, Energy Fuels, 2020, 34(10), 12379–12387,  DOI:10.1021/acs.energyfuels.0c02357.
150. P. Wang, L. Jiang, X. Zou, H. Tan, P. Zhang and J. Li, et al., Confining polyoxometalate clusters into porous aromatic framework materials for catalytic desulfurization of dibenzothiophene, ACS Appl. Mater. Interfaces, 2020, 12(23), 25910–25919,  DOI:10.1021/acsami.0c05392.
151. J. Li, Z. Yang, G. Hu and J. Zhao, Heteropolyacid supported MOF fibers for oxidative desulfurization of fuel, Chem. Eng. J., 2020, 388, 124325,  DOI:10.1016/j.cej.2020.124325.
152. X. Y. Zhao, X. Wang, Y. Zhao, H. Sun, H. Tan and T. Qiu, et al., Polyoxometalates-doped TiO₂/Ag hybrid heterojunction: removal of multiple pollutants and mechanism investigation, Environ. Sci.: Nano, 2021, 8, 3855–3864,  10.1039/D1EN00827G.
153. K. Chen, S. She, J. Zhang, A. Bayaguud and Y. Wei, Label-free colorimetric detection of mercury via Hg²⁺ ions-accelerated structural transformation of nanoscale metal-oxo clusters, Sci. Rep., 2015, 5, 16316,  DOI:10.1038/srep16316.
154. J.-W. Sun, P.-F. Yan, G.-H. An, J.-Q. Sha, G.-M. Li and G.-Y. Yang, Immobilization of polyoxometalate in the metal-organic framework rht-MOF-1: towards a highly effective heterogeneous catalyst and dye scavenger, Sci. Rep., 2016, 6, 25595,  DOI:10.1038/srep25595.
155. H. Song, M.-S. Guo, J.-F. Wang, Y.-Q. Liu, H.-X. Bi and J. Du, et al., Reduced phosphomolybdate as photoassisted electrochemical crystalline sensor for trace Cr(VI) detection, Polyoxometalates, 2024, 3(4), 9140065,  DOI:10.26599/POM.2024.9140065.
156. S. Li, Y. Zheng, G.-C. Liu, X.-H. Li, Z. Zhang and X.-L. Wang, New two-fold interpenetrating 3D polyoxovanadate-based metal–organic framework as bifunctional catalyst for the removal of 2-chloroethyl ethyl sulfide and phenolic compounds, Polyoxometalates, 2024, 3(3), 9140061,  DOI:10.26599/POM.2024.9140061.
157. Y.-C. Dai, S.-Y. Zhang, X.-X. Xiao, M.-J. Li, J.-C. Liu and L.-J. Chen, et al., A double-tartrate-bridged deca-nuclearity europium–tungsten cluster embedded selenotungstate and its selective optical sensing of o-nitrophenol, Polyoxometalates, 2023, 2(4), 9140041,  DOI:10.26599/POM.2023.9140041.
158. Z. Xia, L. Wang, Q. Zhang, F. Li and L. Xu, Fast degradation of phenol over porphyrin–polyoxometalate composite photocatalysts under visible light, Polyoxometalates, 2022, 1(1), 9140001,  DOI:10.26599/POM.2022.9140001.
159. R. Liang, R. Huang, S. Ying, X. Wang, G. Yan and L. Wu, Facile in situ growth of highly dispersed palladium on phosphotungstic-acid-encapsulated MIL-100(Fe) for the degradation of pharmaceuticals and personal care products under visible light, Nano Res., 2018, 11, 1109–1123,  DOI:10.1007/s12274-017-1730-0.
160. K. Chen, A. Bayaguud, H. Li, Y. Chu, H. Zhang and H. Jia, et al., Improved peroxidase-mimic property: sustainable, high-efficiency interfacial catalysis with H₂O₂ on the surface of vesicles of hexavanadate–organic hybrid surfactants, Nano Res., 2018, 11, 1313–1321,  DOI:10.1007/s12274-017-1746-5.
161. A. M. Mohamed, W. A. Abbas, G. E. Khedr, W. Abass and N. K. Allam, Computational and experimental elucidation of the boosted stability and antibacterial activity of ZIF-67 upon optimized encapsulation with polyoxometalates, Sci. Rep., 2022, 12, 15989,  DOI:10.1038/s41598-022-20392-4.
162. X.-Y. Ma, H.-X. Bi, X.-J. Zhang, J. Du, Y.-Y. Ma and Z.-G. Han, Effect of bridging units on the detection performance of Cd{P₄Mo₆}₂-based electrochemical sensors for trace chromium(VI) and tetracycline, Polyoxometalates, 2025, 4(2), 9140090,  DOI:10.26599/POM.2025.9140090.
163. Y. Hao, T. Ji, J. Zhang and W. Chen, Triboelectric nanogenerator based on changing the nanomorphology of polyoxometalates for gait monitoring of teenagers, Nano Res., 2025, 18(2), 94907192,  DOI:10.26599/NR.2025.94907192.
164. A. L. Srivastav and M. Ranjan, Chapter 1 - Inorganic Water Pollutants, in Inorganic Pollutants in Water, Elsevier, 1st edn, 2020, pp. 1–15,  DOI:10.1016/B978-0-12-818965-8.00001-9.
165. K. L. Wasewar, S. Singh and S. K. Kansal, Chapter 13 - Process Intensification of Treatment of Inorganic Water Pollutants, in Inorganic Pollutants in Water, Elsevier, 1st edn, 2020, pp. 245–271,  DOI:10.1016/B978-0-12-818965-8.00013-5.
166. B. D'Cruz, M. O. Amin and E. Al-Hetlani, Polyoxometalate-Based Materials for the Removal of Contaminants from Wastewater: A Review, Ind. Eng. Chem. Res., 2021, 60(30), 10960–10977,  DOI:10.1021/acs.iecr.1c02007.
167. F.-L. Xu, S. E. Jorgensen, Y. Shimizu and E. Silow, Editorial Persistent Organic Pollutants in Fresh Water Ecosystems, Sci. World J., 2013, 2013, 303815,  DOI:10.1155/2013/303815.
168. S. Bolisetty, M. Peydayesh and R. Mezzenga, Sustainable technologies for water purification from heavy metals: review and analysis, Chem. Soc. Rev., 2019, 48, 463–487,  10.1039/C8CS00493E.
169. M. Hu and Y. Xu, Visible light induced degradation of chlorophenols in the presence of H₂O₂ and iron substituted polyoxotungstate, Chem. Eng. J., 2014, 246, 299–305,  DOI:10.1016/j.cej.2014.02.072.
170. M. Taghdiri, N. Saadatjou, N. Zamani and R. Farrokhi, Heterogeneous degradation of precipitated hexamine from wastewater by catalytic function of silicotungstic acid in the presence of H₂O₂ and H₂O₂/Fe²⁺, J. Hazard. Mater., 2013, 246, 206–212,  DOI:10.1016/j.jhazmat.2012.12.029.
171. J. Lan, Y. Wang, B. Huang, Z. Xiao and P. Wu, Application of polyoxometalates in photocatalytic degradation of organic pollutants, Nanoscale Adv., 2021, 3, 4646–4658,  10.1039/D1NA00408E.
172. Y. Wang, G. Fang, V. V. Ordomsky and A. Y. Khodakov, Polyoxometalate photocatalysts: solar-driven activation of small molecules for energy conversion and greenhouse gas valorization, Chem. Commun., 2025, 61, 10630–10642,  10.1039/D5CC01494H.
173. N. Lu, Y. Wang, S. Ning, W. Zhao, M. Qian and Y. Ma, et al., Design of plasmonic Ag-TiO₂/H₃PW₁₂O₄₀ composite film with enhanced sunlight photocatalytic activity towards o-chlorophenol degradation, Sci. Rep., 2017, 7, 17298,  DOI:10.1038/s41598-017-17221-4.
174. M. Tahmasebi, M. Mirzaei and A. Frontera, Noble metals in polyoxometalates, Inorg. Chim. Acta, 2021, 523, 120410,  DOI:10.1016/j.ica.2021.120410.
175. M. A. Fashapoyeh, M. Mirzaei, H. Eshtiagh-Hosseini, A. Rajagopal, M. Lechner and R. Liu, et al., Photochemical and electrochemical hydrogen evolution reactivity of lanthanide-functionalized polyoxotungstates, Chem. Commun., 2018, 54, 10427–10430,  10.1039/C8CC06334F.
176. K. Li, Y. Guo, F. Ma, H. Li, L. Chen and Y. Guo, Design of ordered mesoporous H₃PW₁₂O₄₀-titania materials and their photocatalytic activity to dye methyl orange degradation, Catal. Commun., 2010, 11, 839–843,  DOI:10.1016/j.catcom.2010.03.004.
177. Y. Liu, C. Tang, M. Cheng, M. Chen, S. Chen and L. Lei, et al., Polyoxometalate@Metal-Organic Framework Composites as Effective Photocatalysts, ACS Catal., 2021, 11(21), 13374–13396,  DOI:10.1021/acscatal.1c03866.
178. R. Liang, A. Hu, M. Hatat-Fraile and N. Zhou, Fundamentals on Adsorption, Membrane Filtration, and Advanced Oxidation Processes for Water Treatment, in Nanotechnology for Water Treatment and Purification, Springer, Cham, 2014, pp. 1–45,  DOI:10.1007/978-3-319-06578-6_1.
179. L. Yao, Z. Long, Z. Chen, Q. Cheng, Y. Liao and M. Tian, Property Characterization and Mechanism Analysis of Polyoxometalates-Functionalized PVDF Membranes by Electrochemical Impedance Spectroscopy, Membranes, 2020, 10, 214,  DOI:10.3390/membranes10090214.
180. L. Yao, S. K. Lua, L. Zhang, R. Wang and Z. Dong, Dye removal by surfactant encapsulated polyoxometalates, J. Hazard. Mater., 2014, 280, 428–435,  DOI:10.1016/j.jhazmat.2014.08.026.
181. X. Lu, T. Cheng, Y. V. Geletii, J. Bacsa and C. L. Hill, Reactivity and stability synergism directed by the electron transfer between polyoxometalates and metal-organic frameworks, Catal. Sci. Technol., 2023, 13(17), 5094–5103,  10.1039/D3CY00569K.
182. J. Du, Y. Y. Ma, W. J. Cui, S. M. Zhang, Z. G. Han, R. H. Li, X. Q. Han, W. Guan, Y. H. Wang, Y. Q. Li, Y. Liu, F. Y. Yu, K. Q. Wei, H. Q. Tan, Z. H. Kang and Y. G. Li, Unraveling photocatalytic electron transfer mechanism in polyoxometalate-encapsulated metal-organic frameworks for high-efficient CO₂ reduction reaction, Appl. Catal., B, 2022, 318, 121812,  DOI:10.1016/j.apcatb.2022.121812.
183. F. Galiano, R. Mancuso, M. Carraro, J. Bundschuh, J. Hoinkis and M. Bonchio, et al., Apolyoxometalate-based self-cleaning smart material with oxygenic activity for water remediation with membrane technology, Appl. Mater. Today, 2021, 23, 101002,  DOI:10.1016/j.apmt.2021.101002.
184. M.-M. Zhang, A.-K. Li, M.-J. Tang, Q.-Y. He, Y.-H. Peng and R.-J. Fan, et al., Constructing polyoxometalates-based electrocatalytic nanofiltration membranes for nitrite removal, J. Membr. Sci., 2024, 699, 122668,  DOI:10.1016/j.memsci.2024.122668.
185. Z. Liao, T. Gao, J. Zhang, Q. Wu, J. Shi and Z. Yang, Polyoxometalates decoration combining with solvent activation for enhanced separation performance of nanofiltration membrane, J. Membr. Sci., 2024, 706, 122964,  DOI:10.1016/j.memsci.2024.122964.
186. M. Bonchio, M. Carraro, G. Scorrano, E. Fontananova and E. Drioli, Heterogeneous Photooxidation of Alcohols in Water by Photocatalytic Membranes Incorporating Decatungstate, Adv. Synth. Catal., 2003, 345, 1119–1126,  DOI:10.1002/adsc.200303076.
187. E. Fontananova, L. Donato, E. Drioli, L. C. Lopez, P. Favia and R. d'Agostino, Heterogenization of Polyoxometalates on the Surface of Plasma-Modified Polymeric Membranes, Chem. Mater., 2006, 18, 1561–1568,  DOI:10.1021/cm051739g.
188. I. Romanenko, M. Lechner, F. Wendler, C. Hörenz, C. Streb and F. H. Schacher, POMbranes: polyoxometalate-functionalized block copolymer membranes for oxidation catalysis, J. Mater. Chem. A, 2017, 5, 15789–15796,  10.1039/C7TA03220J.
189. L. Yao, L.-Z. Zhang, R. Wang, C. H. Loh and Z.-L. Dong, Fabrication of catalytic membrane contactors based on polyoxometalates and polyvinylidene fluoride intended for degrading phenol in wastewater under mild conditions, Sep. Purif. Technol., 2013, 118, 162–169,  DOI:10.1016/j.seppur.2013.06.029.
190. L. Yao, L. Zhang, R. Wang, S. Chou and Z. Dong, A new integrated approach for dye removal from wastewater by polyoxometalates functionalized membranes, J. Hazard. Mater., 2016, 301, 462–470,  DOI:10.1016/j.jhazmat.2015.09.027.
191. A. E. Giannakas, M. Antonopoulou, C. Daikopoulos, Y. Deligiannakis and I. Konstantinou, Characterization and Catalytic Performance of B-Doped, B-N Co-Doped and B-N-F Tri-Doped TiO₂ towards Simultaneous Cr(VI) Reduction and Benzoic Acid Oxidation, Appl. Catal., B, 2016, 184, 44–54,  DOI:10.1016/j.apcatb.2015.11.009.
192. J. G. McEvoy and Z. Zhang, Synthesis and Characterization of Magnetically Separable Ag/AgCl-Magnetic Activated Carbon Composites for Visible Light Induced Photocatalytic Detoxification and Disinfection, Appl. Catal., B, 2014, 160, 267–278,  DOI:10.1016/j.apcatb.2014.04.043.
193. K. Gong, W. Wang, J. Yan and Z. Han, Highly Reduced Molybdophosphate as a Noble-Metal-Free Catalyst for the Reduction of Chromium Using Formic Acid as a Reducing Agent, J. Mater. Chem. A, 2015, 3(11), 6019–6027,  10.1039/C4TA06830K.
194. K. Gong, Y. Liu, W. Wang, T. Fang, C. Zhao, Z. Han and X. Zhai, Reduced Phosphomolybdates as Molecular Catalysts for Hexavalent Chromium Reduction, Eur. J. Inorg. Chem., 2015, 32, 5351–5356,  DOI:10.1002/EJIC.201500883.
195. H.-F. Shi, G. Yan, Y. Zhang, H.-Q. Tan, W.-Z. Zhou, Y.-Y. Ma, Y.-G. Li, W. Chen and E.-B. Wang, Ag/Ag_xH_3-xPMo₁₂O₄₀ nanowires with enhanced visible-light-driven photocatalytic performance, ACS Appl. Mater. Interfaces, 2017, 9(1), 422–430,  DOI:10.1021/acsami.6b13009.
196. H. Daupor and S. Wongnawa, Urchinlike Ag/AgCl Photocatalyst: Synthesis, Characterization, and Activity, Appl. Catal., A, 2014, 473, 59–69,  DOI:10.1016/J.APCATA.2013.12.036.
197. D. Wang, Y. Duan, Q. Luo, X. Li and L. Bao, Visible Light Photocatalytic Activities of Plasmonic Ag/AgBr Particles Synthesized by a Double Jet Method, Desalination, 2011, 270(1–3), 174–180,  DOI:10.1016/J.DESAL.2010.11.042.
198. J. Wan, E. Liu, J. Fan, X. Hu, L. Sun and C. Tang, et al., In-situ synthesis of plasmonic Ag/Ag₃PO₄ tetrahedron with exposed {111} facets for high visible-light photocatalytic activity and stability, Ceram. Int., 2015, 41(5), 6933–6940,  DOI:10.1016/j.ceramint.2015.01.148.
199. W. Zhao, Y. Guo, Y. Faiz, W. T. Yuan, C. Sun and S. M. Wang, et al., Facile in-suit synthesis of Ag/AgVO₃ one-dimensional hybrid nanoribbons with enhanced performance of plasmonic visible-light photocatalysis, Appl. Catal., B, 2015, 163, 288–297,  DOI:10.1016/j.apcatb.2014.08.015.
200. X. Zhao, Y. Zhang, Y. Zhao, H. Tan, Z. Zhao and H. Shi, et al., Ag_xH_3-xPMo₁₂O₄₀/Ag nanorods/g-C₃N₄ 1D/2D Z-scheme heterojunction for highly efficient visible-light photocatalysis, Dalton Trans., 2019, 48(19), 6484–6491,  10.1039/C9DT00744J.
201. H. Zhang, J. Yang, Y.-Y. Liu, S.-Y. Song, X.-L. Liu and J.-F. Ma, Visible Light Photodegradation of Organic Dyes, Reduction of CrVI and Catalytic Oxidative Desulfurization by a Class of Polyoxometalate-Based Inorganic-Organic Hybrid Compounds, Dyes Pigm., 2016, 133, 189–200,  DOI:10.1016/j.dyepig.2016.05.051S.
202. S. Buffet-Bataillon, P. Tattevin, M. Bonnaure-Mallet and A. Jolivet-Gougeon, Emergence of Resistance to Antibacterial Agents: The Role of Quaternary Ammonium Compounds-a Critical Review, Int. J. Antimicrob. Agents, 2012, 39(5), 381–389,  DOI:10.1016/j.ijantimicag.2012.01.011.
203. K. Shakeela and G. R. Rao, Thermoreversible, Hydrophobic Ionic Liquids of Keggin-Type Polyanions and Their Application for the Removal of Metal Ions from Water, ACS Appl. Nano Mater., 2018, 1(9), 4642–4651,  DOI:10.1021/acsanm.8b00920.
204. P. J. Kitson, M. D. Symes, V. Dragone and L. Cronin, Combining 3D Printing and Liquid Handling to Produce User-Friendly Reactionware for Chemical Synthesis and Purification, Chem. Sci., 2013, 4(8), 3099–3103,  10.1039/C3SC51253C.
205. Y. Ji, Y. Ma, Y. Ma, J. Asenbauer, S. Passerini and C. Streb, Water Decontamination by Polyoxometalate-Functionalized 3D-Printed Hierarchical Porous Devices, Chem. Commun., 2018, 54(24), 3018–3021,  10.1039/C8CC00821C.
206. S. G. Mitchell, C. Streb, H. N. Miras, T. Boyd, D.-L. Long and L. Cronin, Face-Directed Self-Assembly of an Electronically Active Archimedean Polyoxometalate Architecture, Nat. Chem., 2010, 2, 308–312,  DOI:10.1038/nchem.581.
207. S. Wang, X. Geng, Z. Zhao, M. Zhang, Y. Song and K. Sun, et al., Ammoniated-driven green synthesis of charged polyoxometalate supported ionic liquids for exceptional heavy metal remediation in actual industrial wastewater, Water Res., 2025, 272, 122939,  DOI:10.1016/j.watres.2024.122939.
208. F. Zou, R. Yu, R. Li and W. Li, Microwave-assisted Synthesis of HKUST-1 and Functionalized HKUST-1-@ H₃PW₁₂O₄₀: Selective Adsorption of Heavy Metal Ions in Water Analyzed with Synchrotron Radiation, ChemPhysChem, 2013, 14(12), 2825–2832,  DOI:10.1002/cphc.201300215.
209. H. Zhang, J. Xue, N. Hu, J. Sun, D. Ding and Y. Wang, et al., Selective removal of U(VI) from low concentration wastewater by functionalized HKUST-1@H₃PW₁₂O₄₀, J. Radioanal. Nucl. Chem., 2016, 308, 865–875,  DOI:10.1007/s10967-015-4603-6.
210. R. L. Frost, J. Čejka and M. J. Dickfos, Raman Spectroscopic Study of the Uranyl Minerals Vanmeersscheite U(OH)₄[(UO₂)₃(PO₄)₂(OH)₂]·4H₂O and Arsenouranylite Ca(UO₂)[(UO₂)₃(AsO₄)₂(OH)₂]·(OH)₂·6H₂O, Spectrochim. Acta, Part A, 2009, 71(5), 1799–1803,  DOI:10.1016/j.saa.2008.06.041.
211. D. Shao, Y. Li, X. Wang, S. Hu, J. Wen and J. Xiong, et al., Phosphate-functionalized polyethylene with high adsorption of uranium(VI), ACS Omega, 2017, 2(7), 3267–3275,  DOI:10.1021/acsomega.7b00375.
212. X. Han, Y. Wang, X. Cao, Y. Dai, Y. Liu and Z. Dong, et al., Adsorptive performance of ship-type nano-cage polyoxometalates for U(VI) in aqueous solution, Appl. Surf. Sci., 2019, 484, 1035–1040,  DOI:10.1016/j.apsusc.2019.04.121.
213. S. Seino, R. Kawahara, Y. Ogasawara, N. Mizuno and S. Uchida, Reduction-Induced Highly Selective Uptake of Cesium Ions by an Ionic Crystal Based on Silicododecamolybdate, Angew. Chem., Int. Ed., 2016, 55(12), 4055–4059,  DOI:10.1002/ange.201511633.
214. S. Hitose and S. Uchida, Rapid Uptake/Release of Cs⁺ in Isostructural Redox-Active Porous Ionic Crystals with Large-Molecular-Size and Easily Reducible Dawson-Type Polyoxometalates as Building Blocks, Inorg. Chem., 2018, 57(9), 4833–4836,  DOI:10.1021/acs.inorgchem.8b00801.
215. X. Liu, W. Gong, J. Luo, C. Zou, Y. Yang and S. Yang, Selective adsorption of cationic dyes from aqueous solution by polyoxometalate-based metal-organic framework composite, Appl. Surf. Sci., 2016, 362, 517–524,  DOI:10.1016/j.apsusc.2015.11.151.
216. C. Sabarinathan, P. Karuppasamy, C. Vijayakumar and T. Arumuganathan, Development of methylene blue removal methodology by adsorption using molecular polyoxometalate: Kinetics, Thermodynamics and Mechanistic Study, Microchem. J., 2019, 146, 315–326,  DOI:10.1016/j.microc.2019.01.015.
217. L. Bai, X. Pan, R. Guo, X. Linghu, Y. Shu and Y. Wu, et al., Sunlight-driven photocatalytic degradation of organic dyes in wastewater by chemically fabricated ZnO/Cs₄SiW₁₂O₄₀ nanoheterojunction, Appl. Surf. Sci., 2022, 599, 153912,  DOI:10.1016/j.apsusc.2022.153912.
218. R. Wang, Y. Liu, P. Zuo, Z. Zhang, N. Lei and Y. Liu, Phthalocyanine-sensitized evolution of hydrogen and degradation of organic pollutants using polyoxometalate photocatalysts, Environ. Sci. Pollut. Res., 2020, 27, 18831–18842,  DOI:10.1007/s11356-020-08425-9.
219. Z. Yang, S. Gao, H. Li and R. Cao, Synthesis and visible light photocatalytic properties of polyoxometalate-thionine composite films immobilized on porous TiO₂ microspheres, J. Colloid Interface Sci., 2012, 375(1), 172–179,  DOI:10.1016/j.jcis.2012.02.043.
220. L. Qi, Y. Gong, M. Fang, Z. Jia, N. Cheng and L. Yu, Surface-Active Ionic-Liquid-Encapsulated Polyoxometalate Nanospheres: Construction, Self-Assembly, Adsorption Behavior, and Application for Dye Removal, ACS Appl. Nano Mater., 2020, 3(1), 375–383,  DOI:10.1021/acsanm.9b02012.
221. B. Bi, L. Xu, B. Xu and X. Liu, Heteropoly blue-intercalated layered double hydroxides for cationic dye removal from aqueous media, Appl. Clay Sci., 2011, 54, 242–247,  DOI:10.1016/j.clay.2011.09.003.
222. A. Lesbani, T. Taher, N. Rahayu Palapa, R. Mohadi, A. Rachmat and R. Mardiyanto, Preparation and utilization of Keggin-type polyoxometalate intercalated Ni-Fe layered double hydroxides for enhanced adsorptive removal of cationic dye, SN Appl. Sci., 2020, 2, 470,  DOI:10.1007/s42452-020-2300-8.
223. P. Lei, C. Chen, J. Yang, W. Ma, J. Zhao and L. Zang, Degradation of Dye Pollutants by Immobilized Polyoxometalate with H₂O₂ under Visible-Light Irradiation, Environ. Sci. Technol., 2005, 39(21), 8466–8474,  DOI:10.1021/es050321g.
224. C. Lee, C. R. Keenan and D. L. Sedlak, Polyoxometalate-Enhanced Oxidation of Organic Compounds by Nanoparticulate Zero-Valent Iron and Ferrous Ion in the Presence of Oxygen, Environ. Sci. Technol., 2008, 42(13), 4921–4926,  DOI:10.1021/es800317j.
225. B.-L. Fei, J.-K. Zhong, N.-P. Deng, J.-H. Wang, Q.-B. Liu and Y.-G. Li, et al., A novel 3D heteropoly blue type photo-Fenton-like catalyst and its ability to remove dye pollution, Chemosphere, 2018, 197, 241–250,  DOI:10.1016/j.chemosphere.2018.01.053.
226. M. Grabsi, N. Zabat, N. Khellaf and F. Ismail, Synthesis of an environmental nano-polyoxometalate (α₂P₂W₁₇CoO₆₁)⁸⁻ as catalyst for dyes degradation: A comparative study oxidation of indigoid and azo dyes, Environ. Nanotechnol., Monit. Manage., 2019, 12, 100269,  DOI:10.1016/j.enmm.2019.100269.
227. L. Li, J.-W. Sun, J.-Q. Sha, G.-M. Li, P.-F. Yan, C. Wang and L. Yu, Structure Refinement and Photocatalytic Properties of Porous POMCPs by Selecting the Isomerous PYTTZ, Dalton Trans., 2015, 44(4), 1948–1954,  10.1039/C4DT02960G.
228. A. Shokri and M. S. Fard, A critical review in Fenton-like approach for the removal of pollutants in the aqueous environment, Environ. Challenges, 2022, 7, 100534,  DOI:10.1016/j.envc.2022.100534.
229. X. An, Q. Tang, H. Lan, H. Liu and J. Qu, Polyoxometalates/TiO₂ Fenton-like photocatalysts with rearranged oxygen vacancies for enhanced synergetic degradation, Appl. Catal., B, 2019, 244, 407–413,  DOI:10.1016/j.apcatb.2018.11.063.
230. Y. Orooji, B. Tanhaei, A. Ayati, S. H. Tabrizi, M. Alizadeh, F. F. Bamoharram, F. Karimi, S. Salmanpour, J. Rouhi, S. Afshar, M. Sillanpää, R. Darabi and H. Karimi-Maleh, Heterogeneous UV-switchable Au nanoparticles decorated tungstophosphoric acid/TiO₂ for efficient photocatalytic degradation proces, Chemosphere, 2021, 281, 130795,  DOI:10.1016/j.chemosphere.2021.130795.
231. L. Zhang, H. Chen, X. Zhao, Q. Zhai, D. Yin, Y. Sun and J. Li, The Marriage of Ferrocene and Silicotungstate: An Ingenious Heterogeneous Fenton-like Synergistic Photocatalyst, Appl. Catal., B, 2016, 193, 47–57,  DOI:10.1016/j.apcatb.2016.04.019.
232. Q. Wang, E. Liu, C. Zhang, S. Huang, Y. Cong and Y. Zhang, Synthesis of Cs₃PMo₁₂O₄₀/Bi₂O₃ Composite with Highly Enhanced Photocatalytic Activity under Visible-Light Irradiation, J. Colloid Interface Sci., 2018, 516, 304–311,  DOI:10.1016/j.jcis.2018.01.065.
233. S. Wang, K. Wang, X.-Y. Shi, C.-X. Meng, C.-L. Sun and Z.-S. Wu, A three-dimensional polyoxometalate/graphene aerogel as a highly efficient and recyclable absorbent for oil/water separation, New Carbon Mater., 2021, 36(1), 189–197,  DOI:10.1016/S1872-5805(21)60013-6.
234. K. Samal, S. Mahapatra and M. H. Ali, Pharmaceutical wastewater as Emerging Contaminants (EC): Treatment technologies, impact on environment and human health, Energy Nexus, 2022, 6, 100076,  DOI:10.1016/j.nexus.2022.100076.
235. R. K. Mishra, S. S. Mentha, Y. Mirsa and N. Dwivedi, Emerging pollutants of severe environmental concern in water and wastewater: A comprehensive review on current developments and future research, Water-Energy Nexus, 2023, 6, 74–95,  DOI:10.1016/j.wen.2023.08.002.
236. S. Wang, X. Wang, X.-Y. Shi, C.-X. Meng, C.-L. Sun and Z.-S. Wu, et al., Emerging contaminants: A One Health perspective, Innovation, 2024, 4, 100612,  DOI:10.1016/j.xinn.2024.100612.
237. C. Rubio-Armendáriz, S. Alejandro-Vega, S. Paz-Montelongo, Á. J. Gutiérrez-Fernández, C. J. Carrascosa-Iruzubieta and A. Hardisson-de la Torre, Microplastics as Emerging Food Contaminants: A Challenge for Food Safety, Int. J. Environ. Res. Public Health, 2022, 19(3), 1174,  DOI:10.3390/ijerph19031174.
238. M. Sousa, I. Machado, L. C. Simões and M. Simões, Biocides as drivers of antibiotic resistance: A critical review of environmental implications and public health risks, Environ. Sci. Ecotechnology, 2025, 25, 100557,  DOI:10.1016/j.ese.2025.100557.
239. S. Caloni, T. Durazzano, G. Franci and L. Marsili, Sunscreens' UV Filters Risk for Coastal Marine Environment Biodiversity: A Review, Diversity, 2021, 13, 374,  DOI:10.3390/d13080374.
240. European Commission, Joint Research Centre, L. Gomez Cortes, D. Marinov, I. Sanseverino and A. Navarro Cuenca, et al., Selection of substances for the 4th Watch List under the Water Framework Directive, EUR 31148 EN, Publications Office of the European Union, Luxembourg, 2022, ISBN 978-92-76-55020-4,  DOI:10.2760/01939.
241. European Chemicals Bureau, Institute for Health and Consumer Protection (Joint Research Centre). Technical Guidance Document on Risk Assessment in Support of Commission Directive 93/67/EEC on Risk Assessment for New Notified Substances, Commission Regulation (EC) No 1488/94 on Risk Assessment for Existing Substances, Directive 98/8/EC of the European Parliament and of the Council Concerning the Placing of Biocidal Products on the Market. Part III, European Commission – Joint Research Centre, Luxembourg, 2008, https://op.europa.eu/en/publication-detail/-/publication/212940b8-3e55-43f8-8448-ba258d0374bb.
242. C. Boix, M. Ibáñez, T. Zamora, J. V. Sancho, W. M. A. Niessen and F. Hernández, Identification of New Omeprazole Metabolites in Wastewaters and Surface Waters, Sci. Total Environ., 2014, 468, 706–714,  DOI:10.1016/j.scitotenv.2013.08.095.
243. R. Wielens Becker, M. Ibáñez, E. Cuervo Lumbaque, M. L. Wilde, T. Flores da Rosa and F. Hernández, et al., Investigation of pharmaceuticals and their metabolites in Brazilian hospital wastewater by LC-QTOF MS screening combined with a preliminary exposure and in silico risk assessment, Sci. Total Environ., 2020, 699, 134218,  DOI:10.1016/j.scitotenv.2019.134218.
244. X. Qiu and R. Wang, Polyoxometalate-Based Photocatalytic New Materials for the Treatment of Water Pollutants: Mechanism, Advances, and Challenges, Catalysts, 2025, 15, 613,  DOI:10.3390/catal15070613.
245. G. Murmu, T. H. Panigrahi and S. Saha, Recent advances in the development of polyoxometalates and their composites for the degradation of toxic chemical dyes, Prog. Solid State Chem., 2024, 76, 100489,  DOI:10.1016/j.progsolidstchem.2024.100489.
246. M. Oliveira, W. Antunes, S. Mota, A. Madureira-Carvalho, R. J. Dinis-Oliveira and D. Dias da Silva, An Overview of the Recent Advances in Antimicrobial Resistance, Microorganisms, 2024, 12, 1920,  DOI:10.3390/microorganisms12091920.
247. S. Derakhshanrad, M. Mirzaei, C. Streb, A. Amiri and C. Ritchie, Polyoxometalate-Based Frameworks as Adsorbents for Drug of Abuse Extraction from Hair Samples, Inorg. Chem., 2021, 60(3), 1472–1479,  DOI:10.1021/acs.inorgchem.0c02769.
248. M. L. Yola, N. Atar, T. Eren, H. Karimi-Maleh and S. Wang, Sensitive and selective determination of aqueous triclosan based on gold nanoparticles on polyoxometalate/reduced graphene oxide nanohybrid, RSC Adv., 2015, 5, 65953–65962,  10.1039/C5RA07443F.
249. J. Cai, G.-T. Zhu, X.-M. He, Z. Zhang, R.-Q. Wang and Y.-Q. Feng, Polyoxometalate incorporated polymer monolith microextraction for highly selective extraction of antidepressants in undiluted urine, Talanta, 2017, 170, 252–259,  DOI:10.1016/j.talanta.2017.04.020.
250. R. He, K. Xue, J. Wang, Y. Yan, Y. Peng and T. Yang, et al., Nitrogen-Deficient g-C₃N_x/POMs Porous Nanosheets with P-N Heterojunctions Capable of the Efficient Photocatalytic Degradation of Ciprofloxacin, Chemosphere, 2020, 259, 127465,  DOI:10.1016/j.chemosphere.2020.127465.
251. H. Shi, T. Zhao, J. Wang, Y. Wang, Z. Chen and B. Liu, et al., Fabrication of g-C₃N₄/PW₁₂/TiO₂ Composite with Significantly Enhanced Photocatalytic Performance under Visible Light, J. Alloys Compd., 2021, 860, 157924,  DOI:10.1016/j.jallcom.2020.157924.
252. P. Cheng, Y. Wang, M. Sarakha and G. Mailhot, Enhancement of the Photocatalytic Activity of Decatungstate, W₁₀O₃₂⁴⁻, for the Oxidation of Sulfasalazie/Sulfapyridine in the Presence of Hydrogen Peroxide, J. Photochem. Photobiol., 2021, 404, 112890,  DOI:10.1016/j.jphotochem.2020.112890.
253. M. V. Volin, P. L. Campbell, M. A. Connors, D. C. Woodruff and A. E. Koch, The Effect of Sulfasalazine on Rheumatoid Arthritic Synovial Tissue Chemokine Production, Exp. Mol. Pathol., 2002, 73(2), 84–92,  DOI:10.1006/exmp.2002.2460.
254. J. L. Wang and L. E. Xu, Advanced Oxidation Processes for Wastewater Treatment: Formation of Hydroxyl Radical and Application, Crit. Rev. Environ. Sci. Technol., 2012, 42(3), 251–325,  DOI:10.1080/10643389.2010.507698.
255. S. Khan, M. Sohail, C. Han, J. A. Khan, H. M. Khan and D. D. Dionysiou, Degradation of Highly Chlorinated Pesticide, Lindane, in Water Using UV/Persulfate: Kinetics and Mechanism, Toxicity Evaluation, and Synergism by H₂O₂, J. Hazard. Mater., 2021, 402, 123558,  DOI:10.1016/j.jhazmat.2020.123558.
256. J. Wang, Y. Chen, N. Cheng, L. Feng, B.-H. Gu and Y. Liu, Multivalent Supramolecular Self-Assembly between β-Cyclodextrin Derivatives and Polyoxometalate for Photodegradation of Dyes and Antibiotics, ACS Appl. Bio Mater., 2019, 2(12), 5898–5904,  DOI:10.1021/acsabm.9b00845.
257. G. Li, K. Zhang, C. Li, R. Gao, Y. Cheng and L. Hou, et al., Solvent-free method to encapsulate polyoxometalate into metal-organic frameworks as efficient and recyclable photocatalyst for harmful sulfamethazine degrading in water, Appl. Catal., B, 2019, 245, 753–759,  DOI:10.1016/j.apcatb.2019.01.012.
258. E. S. da Silva, M. Sarakha, H. D. Burrows and P. Wong-Wah-Chung, Decatungstate Anion as an Efficient Photocatalytic Species for the Transformation of the Pesticide 2-(1-Naphthyl) Acetamide in Aqueous Solution, J. Photochem. Photobiol., A, 2017, 334, 61–73,  DOI:10.1016/j.jphotochem.2016.10.036.
259. A. Allaoui, M. A. Malouki and P. Wong-Wah-Chung, Homogeneous Photodegradation Study of 2-Mercaptobenzothiazole Photocatalysed by Sodium Decatungstate Salts: Kinetics and Mechanistic Pathways, J. Photochem. Photobiol., A, 2010, 212(2–3), 153–160,  DOI:10.1016/j.jphotochem.2010.04.010.
260. S. Antonaraki, T. M. Triantis, E. Papaconstantinou and A. Hiskia, Photocatalytic Degradation of Lindane by Polyoxometalates: Intermediates and Mechanistic Aspects, Catal. Today, 2010, 151(1–2), 119–124,  DOI:10.1016/j.cattod.2010.02.017.
261. Y. Martinetto, B. Pégot, C. Roch-Marchal, B. Cottyn-Boitte and S. Floquet, Designing Functional Polyoxometalate-Based Ionic Liquid Crystals and Ionic Liquids, Eur. J. Inorg. Chem., 2020, 3, 228–247,  DOI:10.1002/ejic.201900990.
262. M. Majdafshar, M. Piryaei, M. M. Abolghasemi and E. Rafiee, Polyoxometalate-Based Ionic Liquid Coating for Solid Phase Microextraction of Triazole Pesticides in Water Samples, Sep. Sci. Technol., 2019, 54(10), 1553–1559,  DOI:10.1080/01496395.2019.1572625.
263. A. Misra, C. Zambrzycki, G. Kloker, A. Kotyrba, M. H. Anjass and I. Franco Castillo, et al., Water purification and microplastics removal using magnetic polyoxometalate-supported ionic liquid phases (MagPOM-SILPs), Angew. Chem., Int. Ed., 2020, 59(4), 1601–1605,  DOI:10.1002/anie.201912111.
264. The Royal Society, Microplastics in Freshwater and Soil, The Royal Society, London, 2019, ISBN: 978-1-78252-434-2, https://royalsociety.org/-/media/policy/projects/microplastics/microplastics-evidence-synthesis-report.pdf, Accessed September 1, 2025.
265. B. C. Ong, H. K. Lim, C. Y. Tay, T.-T. Lim and Z. Dong, Polyoxometalates for bifunctional applications: Catalytic dye degradation and anticancer activity, Chemosphere, 2022, 286, 131869,  DOI:10.1016/j.chemosphere.2021.131869.
266. A. Bijelic, M. Aureliano and A. Rompel, The antibacterial activity of polyoxometalates: structures, antibiotic effects and future perspectives, Chem. Commun., 2018, 54, 1153–1169,  10.1039/C7CC07549A.
267. M. Aureliano, N. I. Gumerova, G. Sciortino, E. Garribba, A. Rompel and D. C. Crans, Polyoxovanadates with emerging biomedical activities, Coord. Chem. Rev., 2021, 447, 214143,  DOI:10.1016/j.ccr.2021.214143.
268. M. Moghadasi, M. Abbasi, M. Mousavi and M. Mirzaei, Polyoxometalate-based materials in therapeutic and biomedical applications: current status and perspective, Dalton Trans., 2025, 54, 6333–6345,  10.1039/D4DT03428G.
269. H. Zhang, M. Li, Z. Liu, X. Zhang and C. Du, Two Keggin-type polyoxometalates used as adsorbents with high efficiency and selectivity toward antibiotics and heavy metals, J. Mol. Struct., 2022, 1267, 133604,  DOI:10.1016/j.molstruc.2022.133604.
270. P. Pazhooh, R. Khoshnavazi, L. Bahrami and E. Naseri, Synthesis and Photocatalytic Activity Assessing of the TiO2 Nanocomposites Modified by Some Lanthanide Ions and Tin-Derivative Sandwich-Type Polyoxometalates, J. Iran. Chem. Soc., 2018, 15(8), 1775–1783,  DOI:10.1007/s13738-018-1375-2.
271. K. Wang, Y. He, Y. Zhao, P. Ma and J. A. Wang, Propionate-Functionalized Polyoxovanadate K₂[V₁₀O₁₆(OH)₆(CH₃CH₂CO₂)₆]·20H₂O: As Catalyst for Degradation of Methylene Blue, J. Mol. Struct., 2019, 1195, 184–188,  DOI:10.1016/j.molstruc.2019.05.130.
272. J. M. Missina, L. B. P. Leme, K. Postal, F. S. Santana, D. L. Hughes and E. L. de Sá, et al., Accessing Decavanadate Chemistry with Tris(Hydroxymethyl)Aminomethane, and Evaluation of Methylene Blue Bleaching, Polyhedron, 2020, 180, 114414,  DOI:10.1016/j.poly.2020.114414.
273. F. Yang, L. Zhu, Z. Xu, Y. Han, X. Lin and J. Shi, et al., Multi-active photocatalysts of biochar-doped g-C₃N₄ incorporated with polyoxometalates for the high-efficient degradation of sulfamethoxazole, Environ. Pollut., 2024, 361, 124715,  DOI:10.1016/j.envpol.2024.124715.
274. Y. Song, T. Bo, J.-C. Ma and J.-F. Ma, Highly efficient photoelectrocatalytic degradation for ciprofloxacin with a new polyoxometalate-based metal-organic hybrid/BiVO₄ photoanode, Green Energy Environ., 2025, 10(7), 1531–1542,  DOI:10.1016/j.gee.2025.01.007.
275. N. Mohammadian, T. T. Firozjaee, J. Abdi, M. Moghadasi and M. Mirzaei, PW₁₂/Fe₃O₄/biochar nanocomposite as an efficient adsorbent for metronidazole removal from aqueous solution: Synthesis and optimization, Surf. Interfaces, 2024, 52, 104946,  DOI:10.1016/j.surfin.2024.104946.
276. H. Nourolahi, S. Farhadi, R. Malakooti, M. Maleki and F. Mahmoudi, Construction of lacunary α-K₈SiW₁₁O₃₉ polyoxometalate/MIL-101(Cr) MOF/CoFe₂O₄ magnetic nanocomposites for adsorptive removal of toxic azo dyes and antibiotics from wastewater, CrystEngComm, 2025, 27, 1185–1205,  10.1039/D5CE00013K.
277. J. Li, Z. Yu, J. Zhang, C. Liu, Q. Zhang and H. Shi, et al., Rapid, massive, and green synthesis of polyoxometalate-based metal-organic frameworks to fabricate POMOF/PAN nanofiber membranes for selective filtration of cationic dyes, Molecules, 2024, 29, 1493,  DOI:10.3390/molecules29071493.
278. O. A. Kholdeeva, M. N. Timofeeva, G. M. Maksimov, R. I. Maksimovskaya, W. A. Neiwert and C. L. Hill, Aerobic Oxidation of Formaldehyde Mediated by a Ce-Containing Polyoxometalate under Mild Conditions, Inorg. Chem., 2005, 44(3), 666–672,  DOI:10.1021/ic049109o.
279. M. Te, C. Fairbridge and Z. Ring, Oxidation Reactivities of Dibenzothiophenes in Polyoxometalate/H2O2 and Formic Acid/H₂O₂ Systems, Appl. Catal., A, 2001, 219(1–2), 267–280,  DOI:10.1016/S0926-860X(01)00699-8.
280. Y. Liu, J. Zhang, C. Sheng, Y. Zhang and L. Zhao, Simultaneous Removal of NO and SO₂ from Coal-Fired Flue Gas by UV/H₂O₂ Advanced Oxidation Process, Chem. Eng. J., 2010, 162(3), 1006–1011,  DOI:10.1016/j.cej.2010.07.009.
281. J. A. F. Gamelas, M. G. Evtyugina, I. Portugal and D. V. Evtuguin, New Polyoxometalate-Functionalized Cellulosic Fibre/Silica Hybrids for Environmental Applications, RSC Adv., 2012, 2(3), 831–839,  10.1039/C1RA00371B.
282. O. A. Kholdeeva, M. P. Vanina, M. N. Timofeeva, R. I. Maksimovskaya, T. A. Trubitsina and M. S. Melgunov, et al., Co-containing polyoxometalate-based heterogeneous catalysts for the selective aerobic oxidation of aldehydes under ambient conditions, J. Catal., 2004, 226(2), 363–371,  DOI:10.1016/j.jcat.2004.05.032.
283. E. Eseva, A. Akopyan, A. Schepina, A. Anisimov and A. Maximov, Deep Aerobic Oxidative Desulfurization of Model Fuel by Anderson-Type Polyoxometalate Catalysts, Catal. Commun., 2021, 149, 106256,  DOI:10.1016/j.catcom.2020.106256.
284. M. Chi, Z. Zhu, L. Sun, T. Su, W. Liao and C. Deng, et al., Construction of biomimetic catalysis system coupling polyoxometalates with deep eutectic solvents for selective aerobic oxidation desulfurization, Appl. Catal., B, 2019, 259, 118089,  DOI:10.1016/j.apcatb.2019.118089.
285. F. I. Khan and A. K. Ghoshal, Removal of Volatile Organic Compounds from Polluted Air, J. Loss Prev. Process Ind., 2000, 13(6), 527–545,  DOI:10.1016/S0950-4230(00)00007-3.
286. J. Meng, X. Wang, X. Yang, A. Hu, Y. Guo and Y. Yang, Enhanced Gas-Phase Photocatalytic Removal of Aromatics over Direct Z-Scheme-Dictated H₃PW₁₂O₄₀/g-C₃N₄ Film-Coated Optical Fibers, Appl. Catal., B, 2019, 251, 168–180,  DOI:10.1016/j.apcatb.2019.03.063.
287. I. Ullah, Industrial-scale oxidative desulfurization: comprehensive process design and economic study for maritime oil production, Appl. Eng., 2025, 9(2), 64–87,  DOI:10.11648/j.ae.20250902.12.
288. A. Kanu, A. Kuye and J. Idikwu, Comparative study on technologies for crude oil desulfurization: Hydrodesulfurization vs oxidative desulfurization, Int. J. Innov. Sci. Res. Technol., 2025, 4(6), 1–15,  DOI:10.5281/zenodo.8108205.
289. J. He, L. Guan, Y. Zhou, P. Shao, Y. Yao and S. Lu, et al., One-pot preparation of mesoporous K_xPMo₁₂O₄₀ (x = 1, 2, 3, 4) materials for oxidative desulfurization: electrochemically-active surface area (ECSA) determines their activity, React. Chem. Eng., 2020, 5(9), 1776–1782,  10.1039/D0RE00213E.
290. G. Ye, L. Hu, Y. Gu, C. Lancelot, A. Rives and C. Lamonier, et al., Synthesis of polyoxometalate encapsulated in UiO-66(Zr) with hierarchical porosity and double active sites for oxidation desulfurization of fuel oil at room temperature, J. Mater. Chem. A, 2020, 8(37), 19396–19404,  10.1039/D0TA04337K.
291. Y. Gao, R. Gao, G. Zhang, Y. Zheng and J. Zhao, Oxidative Desulfurization of Model Fuel in the Presence of Molecular Oxygen over Polyoxometalate Based Catalysts Supported on Carbon Nanotubes, Fuel, 2018, 224, 261–270,  DOI:10.1016/j.fuel.2018.03.034.
292. Å. D. Austigard, K. Svendsen and K. K. Heldal, Hydrogen Sulphide Exposure in Waste Water Treatment, J. Occup. Med. Toxicol., 2018, 13, 10–20,  DOI:10.1186/s12995-018-0191-z.
293. G. L. Badilla, B. Valdez and M. Schorr, Air Quality - New Perspective, IntechOpen, London, 2012, ISBN: 978-953-51-0674-6,  DOI:10.5772/2561.
294. O. A. Habeeb, R. Kanthasamy, G. A. M. Ali, S. Sethupathi and R. B. M. Yunus, Hydrogen Sulfide Emission Sources, Regulations, and Removal Techniques: A Review, Rev. Chem. Eng., 2018, 34(6), 837–854,  DOI:10.1515/revce-2017-0004.
295. C. Yang, Y. Wang, H. Fan, G. de Falco, S. Yang and J. Shangguan, et al., Bifunctional ZnO-MgO/activated carbon adsorbents boost H₂S room temperature adsorption and catalytic oxidation, Appl. Catal., B, 2020, 266, 118674,  DOI:10.1016/j.apcatb.2020.118674.
296. F. Adib, A. Bagreev and T. J. Bandosz, Analysis of the relationship between H₂S removal capacity and surface properties of unimpregnated activated carbons, Environ. Sci. Technol., 2000, 34(4), 686–692,  DOI:10.1021/es990341g.
297. K. Huang, X. Feng, X.-M. Zhang, Y.-T. Wu and X.-B. Hu, The ionic liquid-mediated Claus reaction: a highly efficient capture and conversion of hydrogen sulfide, Green Chem., 2016, 18(7), 1859–1863,  10.1039/C5GC03016A.
298. S. Holz, P. Köster, H. Thielert, Z. Guetta and J.-U. Repke, Investigation of the degradation of chelate complexes in liquid redox desulfurization processes, Chem. Eng. Technol., 2020, 43(3), 476–483,  DOI:10.1002/ceat.201900420.
299. Y. Ma, X. Liu and R. Wang, Efficient removal of H2S at high temperature using the ionic liquid solutions of [C₄mim]₃PMo₁₂O₄₀—an organic polyoxometalate, J. Hazard. Mater., 2017, 331, 109–116,  DOI:10.1016/j.jhazmat.2017.02.036.
300. J. Song, Z. Luo, D. K. Britt, H. Furukawa, O. M. Yaghi and K. I. Hardcastle, et al., A multiunit catalyst with synergistic stability and reactivity: a polyoxometalate-metal organic framework for aerobic decontamination, J. Am. Chem. Soc., 2011, 133(42), 16839–16846,  DOI:10.1021/ja203695h.
301. L. J. Muzio and G. C. Quartucy, Implementing NOx control: research to application, Prog. Energy Combust. Sci., 1997, 23(3), 233–266,  DOI:10.1016/S0360-1285(97)00002-6.
302. K. Skalska, J. S. Miller and S. Ledakowicz, Trends in NO_x abatement: a review, Sci. Total Environ., 2010, 408(19), 3976–3989,  DOI:10.1016/j.scitotenv.2010.06.001.
303. P. Woodrow, Nitric oxide: some nursing implications, Intensive Crit. Care Nurs., 1997, 13(2), 87–92,  DOI:10.1016/S0964-3397(97)80186-3.
304. U. Scherrer, L. Vollenweider, A. Delabays, M. Savcic, U. Eichenberger and G. R. Kleger, et al., Inhaled nitric oxide for high-altitude pulmonary edema, N. Engl. J. Med., 1996, 334(10), 624–629,  DOI:10.1056/NEJM199603073341003.
305. A. Chaloulakou, I. Mavroidis and I. Gavriil, Compliance with the annual NO₂ air quality standard in Athens. Required NOx levels and expected health implications, Atmos. Environ., 2008, 42(3), 454–465,  DOI:10.1016/j.atmosenv.2007.09.067.
306. M. A. Gómez-García, V. Pitchon and A. Kiennemann, Pollution by nitrogen oxides: an approach to NO_x abatement by using sorbing catalytic materials, Environ. Int., 2005, 31(3), 445–467,  DOI:10.1016/j.envint.2004.09.006.
307. G. W. van Loon and S. J. Duffy, Environmental chemistry: a global perspective, Oxford University Press, Oxford, 2nd edn, 2005, pp. 421–424, ISBN:0199274991.
308. Y. Qu, J. An, Y. He and J. Zheng, An overview of emissions of SO2 and NOx and the long-range transport of oxidized sulfur and nitrogen pollutants in East Asia, J. Environ. Sci., 2016, 44, 13–25,  DOI:10.1016/j.jes.2015.08.028.
309. F. Lin, Z. Wang, Z. Zhang, Y. He, Y. Zhu and J. Shao, et al., Flue gas treatment with ozone oxidation: an overview on NO_x, organic pollutants, and mercury, Chem. Eng. J., 2020, 382, 123030,  DOI:10.1016/j.cej.2019.123030.
310. Z. Fan, J.-W. Shi, C. Gao, G. Gao, B. Wang, Y. Wang, C. He and C. Niu, Gd-modified MnOx for the selective catalytic reduction of NO by NH₃: the promoting effect of Gd on the catalytic performance and sulfur resistance, Chem. Eng. J., 2018, 348, 820–830,  DOI:10.1016/j.cej.2018.05.038.
311. R. Hao, Y. Zhang, Z. Wang, Y. Li, B. Yuan, X. Mao and Y. Zhao, An advanced wet method for simultaneous removal of SO2 and NO from coal-fired flue gas by utilizing a complex absorbent, Chem. Eng. J., 2017, 307, 562–571,  DOI:10.1016/j.cej.2016.08.103.
312. Y. Zhao, Y. Han and Z. Zhao, Removal of NO from flue gas by a heterogeneous Fenton-like process, Chem. Eng. Technol., 2018, 41(11), 2203–2211,  DOI:10.1002/ceat.201700717.
313. Y. Liu, J. Zhang, J. Pan and A. Tang, Investigation on the removal of NO from SO₂-containing simulated flue gas by an ultraviolet/Fenton-like reaction, Energy Fuels, 2012, 26(9), 5430–5436,  DOI:10.1021/ef3008568.
314. R. Wang, X. Zhang and Z. Ren, Germanium-based polyoxometalates for the adsorption-decomposition of NO_x, J. Hazard. Mater., 2021, 402, 123494,  DOI:10.1016/j.jhazmat.2020.123494.
315. R. Montero-Montoya, R. López-Vargas and O. Arellano-Aguilar, Volatile organic compounds in air: sources, distribution, exposure and associated illnesses in children, Ann. Glob. Health, 2018, 84(2), 225–238,  DOI:10.29024/aogh.910.
316. S. Weon, E. Choi, H. Kim, J. Y. Kim, H.-J. Park and S. Kim, et al., Active 001 facet exposed TiO₂ nanotubes photocatalyst filter for volatile organic compounds removal: from material development to commercial indoor air cleaner application, Environ. Sci. Technol., 2018, 52(16), 9330–9340,  DOI:10.1021/acs.est.8b02282.
317. L. Mølhave, Volatile organic compounds, indoor air quality and health, Indoor Air, 1991, 1, 357–376,  DOI:10.1111/j.1600-0668.1991.00001.x.
318. M. Harper, Sorbent trapping of volatile organic compounds from air, J. Chromatogr. A, 2000, 885(1–2), 129–151,  DOI:10.1016/S0021-9673(00)00363-0.
319. C. Raillard, V. Héquet, P. Le Cloirec and J. Legrand, Comparison of different TiO₂ photocatalysts for the gas phase oxidation of volatile organic compounds, Water Sci. Technol., 2004, 50(4), 241–250,  DOI:10.2166/wst.2004.0274.
320. C. Raillard, V. Héquet, P. Le Cloirec and J. Legrand, Photocatalytic oxidation of volatile organic compounds present in airborne environment adjacent to sewage treatment plants, Water Sci. Technol., 2004, 49(1), 111–114,  DOI:10.2166/wst.2004.0032.
321. P. Dong, X. Xi and G. Hou, Typical non–TiO₂-based visible-light photocatalysts, in Semiconductor Photocatalysis, IntechOpen, London, 2016,  DOI:10.5772/62889.
322. F.-J. Ma, S.-X. Liu, D.-D. Liang, G.-J. Ren, F. Wei, Y.-G. Chen and Z.-M. Su, Adsorption of volatile organic compounds in porous metal–organic frameworks functionalized by polyoxometalates, J. Solid State Chem., 2011, 184(11), 3034–3039,  DOI:10.1016/j.jssc.2011.09.002.
323. T. Salthammer, S. Mentese and R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev., 2010, 110(4), 2536–2572,  DOI:10.1021/cr800399g.
324. T. Salthammer, Formaldehyde in the ambient atmosphere: from an indoor pollutant to an outdoor pollutant?, Angew. Chem., Int. Ed., 2013, 52(12), 3320–3327,  DOI:10.1002/anie.201205984.
325. J. P. Kehrer, The Haber–Weiss reaction and mechanisms of toxicity, Toxicology, 2000, 149(1), 43–50,  DOI:10.1016/S0300-483X(00)00231-6.
326. J. A. F. Gamelas, F. Oliveira, M. G. Evtyugina, I. Portugal and D. V. Evtuguin, Catalytic oxidation of formaldehyde by ruthenium multisubstituted tungstosilicic polyoxometalate supported on cellulose/silica hybrid, Appl. Catal., A, 2016, 509, 8–16,  DOI:10.1016/j.apcata.2015.10.003.
327. Y. Huang, J. Wang, S. Ma, R. Wang and Y. Wang, Enhanced adsorption-oxidation performance of PMo₁₂ immobilized onto porous MCM-41 derived from rice husk for H2S at room temperature, Fuel, 2023, 333, 126448,  DOI:10.1016/j.fuel.2022.126448.
328. R. Wang, L. Zhang and X. Wang, Tuning the redox activity of polyoxometalate by central atom for high-efficient desulfurization, J. Hazard. Mater., 2022, 440, 129710,  DOI:10.1016/j.jhazmat.2022.129710.
329. J. Li and R. Wang, Polyoxometalate/ionic liquid desulfurization system for hydrogen sulfide removal from high-temperature gas stream, Molecules, 2022, 27, 6723,  DOI:10.3390/molecules27196723.
330. J. Li, R. Wang and S. Dou, Electrolytic cell–assisted polyoxometalate based redox mediator for H₂S conversion to elemental sulphur and hydrogen, Chem. Eng. J., 2021, 404, 127090,  DOI:10.1016/j.cej.2020.127090.
331. E. Rafiee and F. Mirnezami, Keggin-structured polyoxometalate-based ionic liquid salts: thermoregulated catalysts for rapid oxidation of sulfur-based compounds using H₂O₂ and extractive oxidation desulfurization of sulfur-containing model oil, J. Mol. Liq., 2014, 199, 156–161,  DOI:10.1016/j.molliq.2014.08.036.
332. R. Wang, Performance of new liquid redox desulfurization system of heteropoly compound in comparison with that of iron chelate, Korean J. Chem. Eng., 2003, 20, 659–663,  DOI:10.1007/BF02706904.
333. R. Wang, Investigation on a new liquid redox method for H₂S removal and sulfur recovery with heteropoly compound, Sep. Purif. Technol., 2003, 31(1), 111–121,  DOI:10.1016/S1383-5866(02)00153-3.
334. B. Wang and R. Wang, Highly-efficient H₂S capture by deep eutectic solvents based on ionic liquid hybridized polyoxometalate: insights into conformational transitions and structure-activity relationships, J. Hazard. Mater., 2025, 495, 139037,  DOI:10.1016/j.jhazmat.2025.139037.
335. B. Wang and R. Wang, Medium and high temperature H2S removal via phosphazene polyoxometalate ionic liquids: performance evaluation and mechanism exploration, Sep. Purif. Technol., 2025, 357(Part B), 130196,  DOI:10.1016/j.seppur.2024.130196.
336. Y. Huang, Q. Zuo, M. Yin, J. Wang, B. Gao and J. Song, et al., Cyclodextrin-assisted high selectivity of imprinted adsorbents loaded on polyoxometalate@UiO-66 for H₂S removal at ambient temperature, Sep. Purif. Technol., 2025, 356(Part B), 129932,  DOI:10.1016/j.seppur.2024.129932.
337. F. Liu, Y. Deng, L. Niu, S. Wang, B. Wang and M. Li, et al., Study on desulfurization and regeneration performance of nanofluid system based on heteropoly compound/ionic liquid solutions, Ind. Eng. Chem. Res., 2024, 63(27), 12155–12165,  DOI:10.1021/acs.iecr.4c01242.
338. Y. Zhao, X. Qin, X. Zhao, X. Wang, H. Tan and H. Sun, et al., Polyoxometalates-doped Bi₂O_3–x/Bi photocatalyst for highly efficient visible-light photodegradation of tetrabromobisphenol A and removal of NO, Chin. J. Catal., 2022, 43(3), 771–781,  DOI:10.1016/S1872-2067(21)63843-3.
339. X. Zhang, R. Wang, H. Zhu and Y. Chen, Performance of NO_x capture with Dawson polyoxometalate H₆P₂W₁₈O₆₂·28H₂O, Chem. Eng. J., 2020, 400, 125880,  DOI:10.1016/j.cej.2020.125880.
340. X. Weng, X. Dai, Q. Zeng, Y. Liu and Z. Wu, DRIFT studies on promotion mechanism of H₃PW₁₂O₄₀ in selective catalytic reduction of NO with NH₃, J. Colloid Interface Sci., 2016, 461, 9–14,  DOI:10.1016/j.jcis.2015.09.004.
341. F.-J. Ma, S.-X. Liu, G.-J. Ren, D.-D. Liang and S. Sha, A hybrid compound based on porous metal–organic frameworks and polyoxometalates: NO adsorption and decomposition, Inorg. Chem. Commun., 2012, 22, 174–177,  DOI:10.1016/j.inoche.2012.05.055.
342. M. A. Gómez-García, V. Pitchon and A. Kiennemann, Multifunctional catalysts for de-NOx processes: the case of H₃PW₁₂O₄₀·6H₂O–metal supported on mixed oxides, Appl. Catal., B, 2007, 70(1–4), 151–159,  DOI:10.1016/j.apcatb.2005.12.029.
343. S. Hodjati, C. Petit, V. Pitchon and A. Kiennemann, Removal of NO_x from a lean exhaust gas by absorption on heteropolyacids: reversible sorption of nitrogen oxides in H₃PW₁₂O₄₀·6H₂O, J. Catal., 2001, 197(2), 324–334,  DOI:10.1006/jcat.2000.3108.
344. R. Bélanger and J. B. Moffat, Sorption and reduction of NO2 on microporous ammonium 12-tungstophosphate, Langmuir, 1996, 12(9), 2230–2238,  DOI:10.1021/la950952w.
345. S. Lu, X. Guo, X. Xu, Z. Han, M. Chen and B. Lin, et al., POM-promoted synergistic catalysis of NO and chlorobenzene over amorphous MnCeO_x catalysts: activation of lattice oxygen, role of acid site, catalytic mechanism, J. Hazard. Mater., 2025, 495, 138873,  DOI:10.1016/j.jhazmat.2025.138873.
346. M. Ma, R. Zhang, Y. Shen, X. Zhou, Y. Zhai and Y. Han, et al., Mesoporous Ce–Ti catalysts modified by phosphotungstic acid and chitosan for the synergistic catalysis of CVOCs and NO_x, Catalysts, 2025, 15, 119,  DOI:10.3390/catal15020119.
347. W. Guo, Z. Luo, H. Lv and C. L. Hill, Aerobic oxidation of formaldehyde catalyzed by polyvanadotungstates, ACS Catal., 2014, 4(4), 1154–1161,  DOI:10.1021/cs5000763.
348. Y. Zhou, B. Yue, R.-L. Bao, S.-X. Liu and H.-Y. He, Catalytic aerobic oxidation of acetaldehyde over Keggin-type molybdovanadophosphoric acid/SBA-15 under ambient condition, Chin. J. Chem., 2006, 24(8), 1001–1005,  DOI:10.1002/cjoc.200690187.
349. M. Ammam, Polyoxometalates: formation, structures, principal properties, main deposition methods and application in sensing, J. Mater. Chem. A, 2013, 1(21), 6291–6312,  10.1039/C3TA01663C.
350. A. Khodadadi Dizaji, H. R. Mortaheb and B. Mokhtarani, Preparation of supported catalyst by adsorption of polyoxometalate on graphene oxide/reduced graphene oxide, Mater. Chem. Phys., 2017, 199, 424–434,  DOI:10.1016/j.matchemphys.2017.07.016.
351. W. Putzbach and N. Ronkainen, Immobilization techniques in the fabrication of nanomaterial-based electrochemical biosensors: a review, Sensors, 2013, 13(4), 4811–4840,  DOI:10.3390/s130404811.
352. K. W. Johnson, Reproducible electrodeposition of biomolecules for the fabrication of miniature electroenzymatic biosensors, Sens. Actuators, B, 1991, 5(1–4), 85–89,  DOI:10.1016/0925-4005(91)80225-9.
353. M. Yang, D. S. Kim, J. H. Yoon, S. B. Hong, S. W. Jeong and D. E. Yoo, et al., Nanopillar films with polyoxometalate-doped polyaniline for electrochemical detection of hydrogen peroxide, Analyst, 2016, 141(4), 1319–1324,  10.1039/C5AN02134K.
354. S. Pourbeyram, M. Moosavifar and V. Hasanzadeh, Electrochemical characterization of the encapsulated polyoxometalates (POMs) into the zeolite, J. Electroanal. Chem., 2014, 714–715, 19–24,  DOI:10.1016/j.jelechem.2013.12.014.
355. J. A. Zasadzinski, R. Viswanathan, L. Madsen, J. Garnaes and D. K. Schwartz, Langmuir–Blodgett films, Science, 1994, 263(5154), 1726–1733,  DOI:10.1126/science.8134836.
356. T. Ito, H. Yashiro and T. Yamase, Regular two-dimensional molecular array of photoluminescent Anderson-type polyoxometalate constructed by Langmuir–Blodgett technique, Langmuir, 2006, 22(6), 2806–2810,  DOI:10.1021/la052972w.
357. W. Chen and T. J. McCarthy, Layer-by-layer deposition: a tool for polymer surface modification, Macromolecules, 1997, 30(1), 78–86,  DOI:10.1021/ma961096d.
358. H. Ma, J. Peng, Z. Han, Y. Feng and E. Wang, Preparation and characterization of luminescent nanocomposite film containing polyoxometalate, Thin Solid Films, 2004, 446(2), 161–166,  DOI:10.1016/j.tsf.2003.09.040.
359. M. Ammam and E. B. Easton, Novel organic–inorganic hybrid material based on tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate and Dawson-type tungstophosphate K₇[H₄PW₁₈O₆₂]·18H₂O as a bifunctional hydrogen peroxide electrocatalyst for biosensors, Sens. Actuators, B, 2012, 161(1), 520–527,  DOI:10.1016/j.snb.2011.10.070.
360. M. Ammam and J. Fransaer, Ionic liquid–heteropolyacid: Synthesis, characterization, and supercapacitor study of films deposited by electrophoresis, J. Electrochem. Soc., 2011, 158(1), A14–A21,  DOI:10.1149/1.3507254.
361. L. Zhang, C. Li, F. Li, S. Li, H. Ma and F. Gu, A sensing platform based on Cu-MOF encapsulated Dawson-type polyoxometalate crystal material for electrochemical detection of xanthine, Microchim. Acta, 2023, 190, 24,  DOI:10.1007/s00604-022-05601-1.
362. S. S. Shetty, B. Moosa, L. Zhang, B. Alshankiti, W. Baslyman and V. Mani, et al., Polyoxometalate–cyclodextrin supramolecular entities for real-time in situ monitoring of dopamine released from neuroblastoma cells, Biosens. Bioelectron., 2023, 229, 115240,  DOI:10.1016/j.bios.2023.115240.
363. R. Tian, B. Zhang, M. Zhao, Q. Ma and Y. Qi, Polyoxometalates as promising enzyme mimics for the sensitive detection of hydrogen peroxide by fluorometric method, Talanta, 2018, 188, 332–338,  DOI:10.1016/j.talanta.2018.05.085.
364. M. I. S. Veríssimo, D. V. Evtuguin and M. T. S. R. Gomes, Polyoxometalate functionalized sensors: a review, Front. Chem., 2022, 10, 840657,  DOI:10.3389/fchem.2022.840657.
365. M. Ammam and E. B. Easton, Selective determination of ascorbic acid with a novel hybrid material based 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and the Dawson type ion [P₂Mo₁₈O₆₂]⁶⁻ immobilized on glassy carbon, Electrochim. Acta, 2011, 56(7), 2847–2855,  DOI:10.1016/j.electacta.2010.12.072.
366. R. Wang, D. Jia and Y. Cao, Facile synthesis and enhanced electrocatalytic activities of organic–inorganic hybrid ionic liquid polyoxometalate nanomaterials by solid-state chemical reaction, Electrochim. Acta, 2012, 72, 101–107,  DOI:10.1016/j.electacta.2012.04.011.
367. S. Liu, Z. Tang, Z. Wang, Z. Peng, E. Wang and S. Dong, Oriented polyoxometalate–polycation multilayers on a carbon substrate, J. Mater. Chem., 2000, 10(12), 2727–2733,  10.1039/B004142O.
368. A. Salimi, A. Korani, R. Hallaj, R. Khoshnavazi and H. Hadadzadeh, Immobilization of [Cu(bpy)₂]Br₂ complex onto a glassy carbon electrode modified with α-SiMo₁₂O₄₀⁴⁻ and single walled carbon nanotubes: application to nanomolar detection of hydrogen peroxide and bromate, Anal. Chim. Acta, 2009, 635(1), 63–70,  DOI:10.1016/j.aca.2009.01.007.
369. B. Wang, L. Cheng and S. Dong, Construction of a heteropolyanion-modified electrode by a two-step sol–gel method and its electrocatalytic applications, J. Electroanal. Chem., 2001, 516(1–2), 17–22,  DOI:10.1016/S0022-0728(01)00677-5.
370. R. Thangamuthu, Y.-C. Pan and S.-M. Chen, Iodate sensing electrodes based on phosphotungstate-doped-glutaraldehyde-cross-linked poly-L-lysine coatings, Electroanalysis, 2010, 22(16), 1812–1816,  DOI:10.1002/elan.201000024.
371. H. Ma, Z. Zhang, H. Pang, S. Li, Y. Chen and W. Zhang, Fabrication and electrochemical sensing property of a composite film based on a polyoxometalate and palladium nanoparticles, Electrochim. Acta, 2012, 69, 379–383,  DOI:10.1016/j.electacta.2012.03.017.
372. H. Zhu, W. Tang, Y. Ma, Y. Wang, H. Tan and Y. Li, Preyssler-type polyoxometalate-based crystalline materials for the electrochemical detection of H₂O₂, CrystEngComm, 2021, 23(10), 2071–2080,  10.1039/D1CE00059D.
373. Y. Y. Hu, R. X. Han, L. Mei, J. L. Liu, J. C. Sun, K. Yang and J. W. Zhao, Design principles of MOF-related materials for highly stable metal anodes in secondary metal-based batteries, Mater. Today Energy, 2021, 19, 100608,  DOI:10.1016/j.mtener.2020.100608.
374. C. Wang, J. Ying, H.-C. Mou, A.-X. Tian and X.-I. Wang, Multi-functional photoelectric sensors based on a series of isopolymolybdate-based compounds for detecting different ions, Inorg. Chem. Front., 2020, 7(20), 3882–3894,  10.1039/D0QI00505C.
375. A. Tian, M. Yang, H. Ni, N. Sun, Y. Yang, Y. Fu and J. Ying, Use of symmetrical and pendant pyrazole derivatives for the construction of two polyoxometalate-based complexes as electrochemical sensors, Transition Met. Chem., 2018, 43, 621–633,  DOI:10.1007/s11243-018-0250-4.
376. M. Yang, S. Rong, X. Wang, H. Ma, H. Pang and L. Tan, et al., Preparation and application of Keggin polyoxometalate-based 3D coordination polymer materials as supercapacitors and amperometric sensors, ChemNanoMat, 2021, 7(3), 299–306,  DOI:10.1002/cnma.202000654.
377. X. Xin, N. Hu, Y. Ma, Y. Wang, L. Hou and H. Zhang, et al., Polyoxometalate-based crystalline materials as a highly sensitive electrochemical sensor for detecting trace Cr(VI), Dalton Trans., 2020, 49, 4570–4577,  10.1039/D0DT00446D.
378. S. A. Krutovertsev, O. M. Ivanova and S. I. Sorokin, Sensing properties of polyaniline films doped with Dawson heteropoly compounds, J. Anal. Chem., 2001, 56, 1057–1060,  DOI:10.1023/A:1012569127685.
379. M. I. Khan, K. Aydemir, M. R. H. Siddiqui, A. A. Alwarthan and C. L. Marshall, Oxidative dehydrogenation properties of novel nanostructured polyoxovanadate based materials, Catal. Lett., 2011, 141, 538–543,  DOI:10.1007/s10562-011-0547-9.
380. M. E. Tess and J. A. Cox, Humidity-independent solid-state amperometric sensor for carbon monoxide based on an electrolyte prepared by sol–gel chemistry, Anal. Chem., 1998, 70(1), 187–190,  DOI:10.1021/ac9708396.
381. M. Ammam and E. B. Easton, Advanced NOx gas sensing based on novel hybrid organic–inorganic semiconducting nanomaterial formed between pyrrole and Dawson type polyoxoanion [P₂Mo₁₈O₆₂]⁶⁻, J. Mater. Chem., 2011, 21(22), 7886–7891,  10.1039/C1JM11244A.
382. T. Wang, Z. Sun, Y. Wang, R. Liu, M. Sun and L. Xu, Enhanced photoelectric gas sensing performance of SnO₂ flower-like nanorods modified with polyoxometalate for detection of volatile organic compound at room temperature, Sens. Actuators, B, 2017, 246, 769–775,  DOI:10.1016/j.snb.2017.02.108.
383. Q. Zhang, T. Wang, Z. Sun, L. Xi and L. Xu, Performance improvement of photoelectrochemical NO₂ gas sensing at room temperature by BiVO₄–polyoxometalate nanocomposite photoanode, Sens. Actuators, B, 2018, 272, 289–295,  DOI:10.1016/j.snb.2018.05.169.
384. H. Shi, N. Li, Z. Sun, T. Wang and L. Xu, Interface modification of titanium dioxide nanoparticles by titanium-substituted polyoxometalate doping for improvement of photoconductivity and gas sensing applications, J. Phys. Chem. Solids, 2018, 120, 57–63,  DOI:10.1016/j.jpcs.2018.04.014.
385. J. Tian, X. Chen, T. Wang, W. Pei, F. Li and D. Li, et al., Modification of indium oxide nanofibers by polyoxometalate electron acceptor doping for enhancement of gas sensing at room temperature, Sens. Actuators, B, 2021, 344, 130227,  DOI:10.1016/j.snb.2021.130227.
386. Y. Wang, X. Fu, T. Wang, F. Li, D. Li and Y. Yang, et al., Polyoxometalate electron acceptor incorporated improved properties of Cu₂ZnSnS₄-based room temperature NO₂ gas sensor, Sens. Actuators, B, 2021, 348, 130683,  DOI:10.1016/j.snb.2021.130683.
387. X. Sun, Q. Lan, J. Geng, M. Yu, Y. Li and X. Li, et al., Polyoxometalate as electron acceptor in dye/TiO₂ films to accelerate room-temperature NO₂ gas sensing, Sens. Actuators, B, 2023, 374, 132795,  DOI:10.1016/j.snb.2022.132795.
388. T. Kida, K. Kawasaki, K. Lemura, K. Teshima and M. Nagano, Gas sensing properties of a stabilized zirconia-based sensor with a porous MoO₃ electrode prepared from a molybdenum polyoxometallate–alkylamine hybrid film, Sens. Actuators, B, 2006, 119(2), 562–569,  DOI:10.1016/j.snb.2006.01.025.
389. H. Wei, J. Zhang, N. Shi, Y. Liu, B. Zhang and J. Zhang, et al., A recyclable polyoxometalate-based supramolecular chemosensor for efficient detection of carbon dioxide, Chem. Sci., 2015, 6, 7201–7205,  10.1039/C5SC02020D.
390. Y. Guo, Y. Gong, L. Qi, Y. Gao and L. Yu, A polyoxometalate-based supramolecular chemosensor for rapid detection of hydrogen sulfide with dual signals, J. Colloid Interface Sci., 2017, 485, 280–287,  DOI:10.1016/j.jcis.2016.09.047.
391. M. J. Bezdek, S.-X. L. Luo, R. Y. Liu, Q. He and T. M. Swager, Trace hydrogen sulfide sensing inspired by polyoxometalate-mediated aerobic oxidation, ACS Cent. Sci., 2021, 7(9), 1572–1580,  DOI:10.1021/acscentsci.1c00746.
392. S. Liu, D. Volkmer and D. G. Kurth, Smart polyoxometalate-based nitrogen monoxide sensors, Anal. Chem., 2004, 76(15), 4579–4582,  DOI:10.1021/ac0495283.
393. L.-X. Cai, L. Chen, X.-Q. Sun, J. Geng, C.-C. Liu and Y. Wang, et al., Ultra-sensitive triethylamine gas sensors based on polyoxometalate-assisted synthesis of ZnWO₄/ZnO hetero-structured nanofibers, Sens. Actuators, B, 2022, 370, 132422,  DOI:10.1016/j.snb.2022.132422.
394. J. Tian, B. Jiang, H. Shao, Y. Wang, T. Wang and F. Li, et al., A new strategy to one-step construct polyoxometalate/semiconductor one-dimensional tandem heterojunctions toward optimized conductometric sensing performances of ethanol gas, Sens. Actuators, B, 2023, 374, 132797,  DOI:10.1016/j.snb.2022.132797.
395. L. Chen, L.-X. Cai, J. Geng, C.-C. Liu, Y. Wang and Z. Guo, Polyoxometalate-assisted in situ growth of ZnMoO₄ on ZnO nanofibers for the selective detection of ppb-level acetone, Sens. Actuators, B, 2022, 369, 132354,  DOI:10.1016/j.snb.2022.132354.
396. Y. Ren, W. Xie, Y. Li, J. Ma, J. Li and Y. Liu, et al., Noble metal nanoparticles decorated metal oxide semiconducting nanowire arrays interwoven into 3D mesoporous superstructures for low-temperature gas sensing, ACS Cent. Sci., 2021, 7(11), 1885–1897,  DOI:10.1021/acscentsci.1c00912.
397. X.-Z. Zhang, W.-J. Zhu, Z.-X. Yang, Y. Feng, L.-L. Fan and G.-G. Gao, et al., Ultrasensitive photochromic and Raman dual response to ethylenediamine gas through polyoxometalate–viologen crystalline hybrid, J. Mater. Chem. C, 2022, 10(41), 15451–15457,  10.1039/D2TC03053E.
398. C. Wang, J. Ying, X. Zhang, B. Zhang, A. Tian and Y. Zhang, POM-based compounds as capacitor materials and their photoelectric-sensing properties toward inorganic ions, J. Coord. Chem., 2021, 74(14), 2315–2326,  DOI:10.1080/00958972.2021.1952998.
399. A.-X. Tian, M.-L. Yang, N. Sun, Y.-B. Fu and J. Ying, A series of pH-dependent POM-based compounds as photocatalysts and electrochemical sensors, Polyhedron, 2018, 155, 337–350,  DOI:10.1016/j.poly.2018.08.065.
400. X. Zhang, Y. Zhang, J. Ying, B. Zhang, C. Wang and A. Tian, A series of POM-based compounds constructed by piperazine and morpholine derivatives: characterization, selective photocatalytic and electrochemical/fluorescence sensing properties, J. Solid State Chem., 2021, 295, 121888,  DOI:10.1016/j.jssc.2020.121888.
401. C. Wang, M. Zhou, Y. Ma, H. Tan, Y. Wang and Y. Li, Hybridized polyoxometalate-based metal–organic framework with ketjenblack for the nonenzymatic detection of H₂O₂, Asian J. Chem., 2018, 13(16), 2054–2059,  DOI:10.1002/asia.201800758.
402. X. Wang, L. Li, X. Wang and Y. Zhang, Various amide-derived ligands induced five octamolybdate-based metal–organic complexes: synthesis, structure, electrochemical sensing and photocatalytic properties, CrystEngComm, 2021, 23(30), 5176–5183,  10.1039/D1CE00266J.
403. C. Liang, X. Wang, D. Yu, W. Guo, F. Zhang and F. Qu, In-situ immobilization of a polyoxometalate metal-organic framework (NENU-3) on functionalized reduced graphene oxide for hydrazine sensing, Chin. J. Chem., 2021, 39(10), 2889–2897,  DOI:10.1002/cjoc.202100314.
404. Q.-Q. Liu, X.-L. Wang, H.-Y. Lin, Z.-H. Chang, Y.-C. Zhang and Y. Tian, et al., Two new polyoxometalate-based metal–organic complexes for the detection of trace Cr(VI) and their capacitor performance, Dalton Trans., 2021, 50(27), 9450–9456,  10.1039/D1DT01247A.
405. Y. Zhang, Y. Zhang, L. Li, J. Chen, P. Li and W. Huang, One-step in situ growth of high-density POMOFs films on carbon cloth for the electrochemical detection of bromate, J. Electroanal. Chem., 2020, 861, 113939,  DOI:10.1016/j.jelechem.2020.113939.
406. C. Largeot, C. Portet, J. Chmiola, P.-L. Taberna, Y. Gogotsi and P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor, J. Am. Chem. Soc., 2008, 130(9), 2730–2731,  DOI:10.1021/ja7106178.
407. A. K. Cuentas-Gallegos, M. Lira-Cantú, N. Casañ-Pastor and P. Gómez-Romero, Nanocomposite hybrid molecular materials for application in solid-state electrochemical supercapacitors, Adv. Funct. Mater., 2005, 15(7), 1125–1133,  DOI:10.1002/adfm.200400326.
408. J. Vaillant, M. Lira-Cantu, K. Cuentas-Gallegos, N. Casañ-Pastor and P. Gómez-Romero, Chemical synthesis of hybrid materials based on PAni and PEDOT with polyoxometalates for electrochemical supercapacitors, Prog. Solid State Chem., 2006, 34(2), 147–159,  DOI:10.1016/j.progsolidstchem.2005.11.015.
409. M. T. Pope and A. Müller, Polyoxometalate chemistry: an old field with new dimensions in several disciplines, Angew. Chem., Int. Ed. Engl., 1991, 30(1), 34–48,  DOI:10.1002/anie.199100341.
410. M. T. Pope, Heteropoly and isopoly oxometalates, Inorganic Chemistry Concepts, Springer-Verlag, New York, 1983.
411. Polyoxometalates: from platonic solids to anti-retroviral activity, ed. M. T. Pope and A. Müller, Springer, Dordrecht, 1994,  DOI:10.1007/978-94-011-0920-8.
412. Y.-G. Guo, J.-S. Hu and L.-J. Wan, Nanostructured materials for electrochemical energy conversion and storage devices, Adv. Mater., 2008, 20(15), 2878–2887,  DOI:10.1002/adma.200800627.
413. N. Sonoyama, Y. Suganuma, T. Kume and Z. Quan, Lithium intercalation reaction into the Keggin type polyoxomolybdates, J. Power Sources, 2011, 196(16), 6822–6827,  DOI:10.1016/j.jpowsour.2010.09.107.
414. J.-J. Chen, M. D. Symes, S.-C. Fan, M.-S. Zheng, H. N. Miras and Q.-F. Dong, et al., High-performance polyoxometalate-based cathode materials for rechargeable lithium-ion batteries, Adv. Mater., 2015, 27(31), 4649–4654,  DOI:10.1002/adma.201501088.
415. E. Ni, S. Uematsu, Z. Quan and N. Sonoyama, Improved electrochemical property of nanoparticle polyoxovanadate K₇NiV₁₃O₃₈ as cathode material for lithium battery, J. Nanopart. Res., 2013, 15(6), 1732,  DOI:10.1007/s11051-013-1732-0.
416. D. Ma, L. Liang, W. Chen, H. Liu and Y.-F. Song, Covalently tethered polyoxometalate–pyrene hybrids for noncovalent sidewall functionalization of single-walled carbon nanotubes as high-performance anode material, Adv. Funct. Mater., 2013, 23(48), 6100–6105,  DOI:10.1002/adfm.201301624.
417. S. Wang, H. Li, S. Li, F. Liu, D. Wu and X. Feng, et al., Electrochemical-reduction-assisted assembly of a polyoxometalate/graphene nanocomposite and its enhanced lithium-storage performance, Chem. – Eur. J., 2013, 19(33), 10895–10902,  DOI:10.1002/chem.201300319.
418. C. Xu, X. Yang, C. Hu, J. Zhang, L. Yang and S. Yin, One pot synthesis of polyoxometalates@polyaniline/MXene/CNTs quaternary composites with a 3D structure as efficient electrode materials, Compos. Commun., 2024, 34, 101814,  DOI:10.1016/j.coco.2024.101814.
419. M. Mahajan, G. Sigla and S. Ogale, Polypyrrole-encapsulates polyoxomolybdate decorated MXene as a functional 2D/3D nanohybrid for a robust and high performance Li-ion battery, ACS Appl. Energy Mater., 2021, 4(5), 4541–4550,  DOI:10.1021/acsaem.1c00175.
420. Y. Liu, X. Zhou, T. Qiu, R. Yao, F. Yu and T. Song, et al., Co-assembly of polyoxometalates and porphyrins as anode for high-performance lithium-ion batteries, Adv. Mater., 2024, 36(35), 2407705,  DOI:10.1002/adma.202407705.
421. Q. Li, M. Xu, T. Wang, H. Wang, J. Sun and J. Sha, Nanohybridization of CoS₂/MoS₂ heterostructure with polyoxometalate on functionalized reduced graphene oxide for high-performance LIBs, Chem. – Eur. J., 2022, 28(20), e202200207,  DOI:10.1002/chem.202200207.
422. M.-T. Li, X.-H. Zhou, Q.-M. Liang, J. Chen, J.-W. Sun and Y. Yu, et al., POMs-based metal-organic frameworks with interpenetrating structures and their carbon nanotube-coated materials for lithium-ion anode applications, J. Solid State Chem., 2024, 337, 124787,  DOI:10.1016/j.jssc.2024.124787.
423. S. Sun, L. Cui, K. Yu, M. Wang, J. Lv and S. Ge, et al., 3D porous metal–organic skeleton based on polyoxometalate nanoclusters as an anode in a lithium-ion battery, ACS Appl. Nano Mater., 2024, 7(1), 1310–1318,  DOI:10.1021/acsanm.3c05315.
424. J.-H. Liu, M.-Y. Yu, W.-Y. Pei, T. Wang and J.-F. Ma, Self-assembly of polyoxometalate-resorcinarene-based inorganic-organic complexes: metal ion effects on the electrochemical performance of lithium ion batteries, Chem. – Eur. J., 2021, 27(39), 10123–10133,  DOI:10.1002/chem.202100780.
425. I. Ullah, T. R. Aldhafeeri, A. Haider, X. Wu, Z. Ullah and S. Chang, et al., Layered arrangement of polyoxometalate on a metal–organic framework as a high-capacity anode material for sodium-ion batteries, ACS Appl. Energy Mater., 2025, 8(3), 1743–1751,  DOI:10.1021/acsaem.4c02904.
426. J. Liu, Z. Chen, S. Chen, B. Zhang, J. Wang and H. Wang, et al., “Electron/ion sponge”-like V-based polyoxometalate: toward high-performance cathode for rechargeable sodium ion batteries, ACS Nano, 2017, 11(7), 6911–6920,  DOI:10.1021/acsnano.7b02062.
427. S. Hartung, N. Bucher, H.-Y. Chen, R. Al-Oweini, S. Sreejith and P. Borah, et al., Vanadium-based polyoxometalate as new material for sodium-ion battery anodes, J. Power Sources, 2015, 288, 270–277,  DOI:10.1016/j.jpowsour.2015.04.009.
428. D. Cao, Q. Sha, J. Wang, J. Li, J. Ren and T. Shen, et al., Advanced anode materials for sodium-ion batteries: confining polyoxometalates in flexible metal–organic frameworks by the “breathing effect”, ACS Appl. Mater. Interfaces, 2022, 14(19), 22186–22196,  DOI:10.1021/acsami.2c04077.
429. L. L. Zhang and X. S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev., 2009, 38(9), 2520–2531,  10.1039/B813846J.
430. G. M. Suppes, B. A. Deore and M. S. Freund, Porous conducting polymer/heteropolyoxometalate hybrid material for electrochemical supercapacitor applications, Langmuir, 2008, 24(3), 1064–1069,  DOI:10.1021/la702837j.
431. A. A. Vannathan, S. Maity, T. Kella, D. Shee, P. P. Das and S. S. Mal, In situ vanadophosphomolybdate impregnated into conducting polypyrrole for supercapacitor, Electrochim. Acta, 2020, 364, 137286,  DOI:10.1016/j.electacta.2020.137286.
432. A. K. Cuentas-Gallegos, R. Martínez-Rosales, M. Baibarac, P. Gómez-Romero and M. E. Rincón, Electrochemical supercapacitors based on novel hybrid materials made of carbon nanotubes and polyoxometalates, Electrochem. Commun., 2007, 9(8), 2088–2092,  DOI:10.1016/j.elecom.2007.06.003.
433. M. Skunik, M. Chojak, I. A. Rutkowska and P. J. Kulesza, Improved capacitance characteristics during electrochemical charging of carbon nanotubes modified with polyoxometallate monolayers, Electrochim. Acta, 2008, 53(11), 3862–3869,  DOI:10.1016/j.electacta.2007.11.049.
434. V. Ruiz, J. Suárez-Guevara and P. Gomez-Romero, Hybrid electrodes based on polyoxometalate–carbon materials for electrochemical supercapacitors, Electrochem. Commun., 2012, 24, 35–38,  DOI:10.1016/j.elecom.2012.08.003.
435. J. Suárez-Guevara, V. Ruiz and P. Gomez-Romero, Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon–phosphotungstate hybrid materials, J. Mater. Chem. A, 2014, 2(4), 1014–1021,  10.1039/C3TA14455K.
436. A. Mu, J. Li, W. Chen, X. Sang, Z. Su and E. Wang, The composite material based on Dawson-type polyoxometalate and activated carbon as the supercapacitor electrode, Inorg. Chem. Commun., 2015, 55, 149–152,  DOI:10.1016/j.inoche.2015.03.032.
437. M. Genovese and K. Lian, Polyoxometalate modified pine cone biochar carbon for supercapacitor electrodes, J. Mater. Chem. A, 2017, 5(8), 3939–3947,  10.1039/C6TA10382K.
438. S. Maity, A. A. Vannathan, T. Kella, D. Shee, P. P. Das and S. S. Mal, Electrochemical performance of activated carbon-supported vanadomolybdates electrodes for energy conversion, Ceram. Int., 2021, 47(19), 27132–27141,  DOI:10.1016/j.ceramint.2021.06.128.
439. D. P. Dubal, B. Nagar, J. Suarez-Guevara, D. Tonti, E. Enciso and P. Palomino, et al., Ultrahigh energy density supercapacitors through a double hybrid strategy, Mater. Today Energy, 2017, 5, 58–65,  DOI:10.1016/j.mtener.2017.05.001.
440. J. Suárez-Guevara, V. Ruiz and P. Gómez-Romero, Stable graphene–polyoxometalate nanomaterials for application in hybrid supercapacitors, Phys. Chem. Chem. Phys., 2014, 16(38), 20411–20414,  10.1039/C4CP03321C.
441. J. Qin, F. Zhou, H. Xiao, R. Ren and Z.-S. Wu, Mesoporous polypyrrole-based graphene nanosheets anchoring redox polyoxometalate for all-solid-state micro-supercapacitors with enhanced volumetric capacitance, Sci. China Mater., 2018, 61(2), 233–242,  DOI:10.1007/s40843-017-9132-8.
442. S. Maity, P. P. Das and S. S. Mal, Decavanadate-graphene oxide nanocomposite as an electrode material for electrochemical capacitor, Mater. Technol., 2022, 37, 1129–1139,  DOI:10.1080/10667857.2021.1924439.
443. S. Maity, A. A. Vannathan, K. Kumar, P. P. Das and S. S. Mal, Enhanced power density of graphene oxide–phosphotetradecavanadate nanohybrid for supercapacitor electrode, J. Mater. Eng. Perform., 2021, 30, 1371–1377,  DOI:10.1007/s11665-020-05349-w.
444. S. Kumari, S. Maity, A. A. Vannathan, D. Shee, P. P. Das and S. S. Mal, Improved electrochemical performance of graphene oxide supported vanadomanganate (IV) nanohybrid electrode material for supercapacitors, Ceram. Int., 2020, 46, 3028–3035,  DOI:10.1016/j.ceramint.2019.10.002.
445. S. Maity, A. A. Vannathan, P. R. Chandewar, D. Shee, P. P. Das and S. S. Mal, Vanadomanganate as a synergistic component in high-performance symmetric supercapacitor, J. Alloys Compd., 2022, 899, 163239,  DOI:10.1016/j.jallcom.2021.163239.
446. S. Maity, M. JE, B. R. Biradar, P. R. Chandewar, D. Shee and P. P. Das, et al., Polyoxomolybdate–polypyrrole–graphene oxide nanohybrid electrode for high-power symmetric supercapacitors, Energy Fuels, 2021, 35(22), 18824–18832,  DOI:10.1021/acs.energyfuels.1c03300.
447. B.-Y. Zhang, X.-S. Wu, N.-H. Wang, X.-L. Wang, X.-Q. Han and Z.-M. Su, Polyoxometalates-based metal-organic frameworks with conjugated acid-base pairs for proton supercapacitors, Chem. Eng. J., 2024, 500, 157502,  DOI:10.1016/j.cej.2024.157502.
448. L. Cui, M. Wang, K. Yu, J. Lv, X. Zheng and B. Zhou, The phosphomolybdate hybrids based on nanoscale heteropoly blue and metal-organic chain for supercapacitor and dual-functional electrochemical biosensor, J. Energy Storage, 2023, 60, 106592,  DOI:10.1016/j.est.2022.106592.
449. T. Li, P. He, Y.-N. Dong, W. Chen, T. Wang and J. Gong, et al., Polyoxometalate-based metal-organic framework/polypyrrole composites toward enhanced supercapacitor performance, Eur. J. Inorg. Chem., 2021, 2021(21), 2063–2069,  DOI:10.1002/ejic.202100202.
450. D. Pakulski, A. Gorczyński, D. Brykczyńska, V. Montes-García and W. Czepa, et al., New Anderson-based polyoxometalate covalent organic frameworks as electrodes for energy storage boosted through keto-enol tautomerization, Angew. Chem., Int. Ed., 2023, 62(32), e202305239,  DOI:10.1002/anie.202305239.
451. L. Zhang, H. Jiang, C. Wang, K. Yu, J. Lv and C. Wang, et al., Improved supercapacitors and water splitting performances of Anderson-type manganese(III)-polyoxomolybdate through assembly with Zn-MOF in a host–guest structure, J. Colloid Interface Sci., 2024, 654, 1393–1404,  DOI:10.1016/j.jcis.2023.10.136.
452. H.-Y. Chen, G. Wee, R. Al-Oweini, J. Friedl, K. S. Tan and Y. Wang, et al., Polyoxovanadate as an advanced electrode material for supercapacitors, ChemPhysChem, 2014, 15(10), 2162–2169,  DOI:10.1002/cphc.201400091.
453. C. Hu, E. Zhao, N. Nitta, A. Magasinski, G. Berdichevsky and G. Yushin, Aqueous solutions of acidic ionic liquids for enhanced stability of polyoxometalate-carbon supercapacitor electrodes, J. Power Sources, 2016, 326, 569–574,  DOI:10.1016/j.jpowsour.2016.04.036.
454. D. P. Dubal, N. R. Chodankar, A. Vinu, D.-H. Kim and P. Gomez-Romero, Asymmetric supercapacitors based on reduced graphene oxide with different polyoxometalates as positive and negative electrodes, ChemSusChem, 2017, 10(13), 2742–2750,  DOI:10.1002/cssc.201700792.
455. S. Maity, B. Neethu, T. Kella, D. Shee, P. P. Das and S. S. Mal, Activated carbon-supported vanado-nickelate (IV) based hybrid materials for energy application, J. Energy Storage, 2021, 40, 102727,  DOI:10.1016/j.est.2021.102727.

---