Green-synthesized nanoparticles: the next frontier in the bioelectrochemical mitigation of pesticides

Abstract

Universal and equitable accessibility to clean and affordable drinking water is one of the sustainable goals established by the United Nations General Assembly to achieve the millennium development goals. However, the contamination of natural freshwater reservoirs by toxic agrochemicals like pesticides has reduced the availability of safe drinking water, necessitating the development of innovative mitigation approaches. Recently, bioinspired green NPs synthesized using biological entities have evolved as a sustainable choice for the catalytic degradation of a broad spectrum of recalcitrant emerging pollutants due to their conducive properties and cost-effectiveness. In this regard, the present review comprehensively examines the potential application of green nanoparticles (NPs) in (bio)electrochemical systems for the effective mineralisation of pesticides. Pesticide removal in the range of 79.3% to 100.0% has been reported using green NPs, while a power density up to 4.7 W m⁻³ has been attained in (bio)electrochemical systems. This study further highlights the antibacterial properties of green NPs, offering potential applications in the agricultural, environmental and biomedical fields. This review also highlights the environmental impacts and sustainability of green NPs, along with their critical limitations, particularly in the context of (bio)electrochemical systems. Ultimately, plausible strategies to overcome the impending challenges in green synthesis techniques have been outlined as a future perspective that will aid in standardising and streamlining these novel synthesis procedures.

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Ritu Kshatriya

Ritu Kshatriya is a Research Scholar in the Department of Civil and Environmental Engineering at IIT Delhi, working under the supervision of Dr. Sovik Das. Her research focuses on the treatment of emerging contaminants using sustainable, nature-derived nanomaterials. Specifically, she investigates the integration of green nanoparticles with bioelectrochemical systems to enhance pollutant degradation efficiency and support eco-friendly wastewater treatment. Her research aims to develop low-energy, high-performance remediation technologies that address complex contaminants while minimizing the environmental impacts.

Yasser Bashir

Yasser Bashir is a Research Scholar in the Department of Civil and Environmental Engineering at IIT Delhi, working under the supervision of Dr. Sovik Das. He holds an MTech in Environmental Engineering and Management from IIT Kharagpur, India. His research focuses on the abatement of beta-blockers and other emerging contaminants through advanced electrochemical and bio-electrochemical wastewater treatment processes. He works on developing efficient electrocatalysts, metal–organic frameworks (MOFs), and Fenton-based systems for enhanced pollutant degradation. His interests also include resource recovery and circular economy approaches in water systems.

Divyanshu Sikarwar

Divyanshu Sikarwar is a Research Scholar in the Department of Civil and Environmental Engineering at IIT Delhi, working under the supervision of Dr. Sovik Das. He completed his MTech from the Indian Institute of Technology Kharagpur. His research focuses on advanced wastewater treatment and hydrogen recovery using innovative environmental engineering approaches. He is dedicated to developing sustainable and efficient treatment technologies that contribute to cleaner water systems and renewable energy generation.

Rishabh Raj

Dr. Rishabh Raj completed his PhD at IIT Kharagpur, specializing in emerging contaminant removal using (bio)electro-Fenton systems with repurposed cathodes and catalysts. He briefly served as a Postdoctoral Researcher at IIT Delhi, where he contributed to several real-site environmental remediation projects. He is currently a Postdoctoral Researcher at Luleå University of Technology (LTU), Sweden, focusing on microplastic mitigation through bioelectrochemical systems. His research integrates sustainable materials and advanced electrochemical processes for water and environmental protection.

Sovik Das

Dr. Sovik Das is an Assistant Professor in the Department of Civil and Environmental Engineering at the Indian Institute of Technology Delhi, India. He completed his PhD in Environmental Engineering from the Department of Civil Engineering at IIT Kharagpur. His research focuses on bioelectrochemical systems, including microbial electrosynthesis cells, microbial fuel cells, and microbial electrolysis cells for the treatment of diverse waste streams with simultaneous resource recovery. He is actively involved in developing sustainable technologies aimed at wastewater treatment, energy generation, and value-added product formation, contributing significantly to advancing environmental engineering solutions.

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Environmental significance

The contamination of natural water bodies with pesticide residues poses significant threats to the human health and water-based ecosystems. These refractory chemical contaminants are unamenable to conventional treatments, necessitating the application of innovative approaches for their elimination. In this regard, the integration of green nanoparticles with (bio)electrochemical systems can offer a synergistic approach for mitigating pesticide pollution with minimal environmental footprint. These green nanoparticles are synthesized by leveraging the reducing ability of naturally available biogenic compounds and have demonstrated removal efficiency comparable to their conventional counterparts, thereby offering an economical and environmentally friendly approach for wastewater treatment. Thus, the present review advocates for the adaptation of green nanoparticles in environmental application as it aligns with the United Nations (UN) sustainable development goals (SDGs).

1. Introduction

Pesticides are organic chemicals used extensively for the prevention of pest infestation on agricultural crops. They play significant roles in controlling insects, rodents, fungi, and unwanted plant growth to protect crops from diseases, thereby enhancing the agricultural yield.¹ However, the widespread application of pesticides has risen sharply and steadily since the mid-1940s, majorly due to commercial farming.² Despite being highly effective, only a small amount of the active pesticide ingredients reach the target crop, while the residual part percolates into the soil strata and contaminates the aquifer.³ Moreover, these deleterious chemicals ultimately find their way into different environmental compartments, such as soil, air, and water, through drift, leaching, and runoff, posing hazards to the ecosystem and human health. Owing to their toxic nature, pesticides potentially contaminate terrestrial and marine waterbodies, depending on their soil mobility, water solubility, and persistence.

Although the use of pesticides in agriculture is often considered necessary to protect crops from diseases, their excessive and frequent use adulterates water bodies, soil, fruits, vegetables, processed food, and air. Consequently, the incidents of acute and chronic health effects from pesticide exposure is rising alarmingly, especially in developing nations.⁴ Pesticide exposure can cause a variety of health issues due to its carcinogenic, cytotoxic, neurotoxic, teratogenic, endocrine-disrupting, and mutagenic nature.⁵ According to the United Nations Environment Programme report of 1990, more than three million people are exposed to pesticide contamination globally, with approximately 0.2 million succumbing to related diseases.² These fatalities usually results from the intake of pesticide-contaminated water, causing respiratory failure, organ damage, or neurotoxicity in exposed patients.² In view of this, the United Nations has adopted the sustainable development goals (SDGs), which aim to provide clean water and sanitation to all and safeguard the aquatic ecosystem (SDG 6 and 14). Therefore, nations worldwide are prioritizing the phasing-out of toxic chemicals and decontamination of polluted sites, particularly water bodies that are affected.

Conventional technologies such as the activated sludge process, trickling filter, coagulation and adsorption process are usually not efficient enough in tackling these recalcitrant micropollutants and can even cause secondary pollution issues.⁶ As an alternative, electrochemical and bioelectrochemical systems (BES), jointly termed (bio)electrochemical systems, are being increasingly investigated for remediating these refractory contaminants from wastewater. These systems have a low footprint, produce reactive species in situ using electrochemical concepts, and do not produce toxic by-products. Consequently, electrochemical technologies (ETs), like electrochemical oxidation, anodic oxidation, electrocoagulation, and electro-Fenton process, are extensively being used for pesticide degradation.⁷ However, a considerable amount of electricity is required for driving the electrochemical degradation process, making the operation of ETs energy-intensive.⁸ Thus, to overcome the dependency on external energy, BES provides a greener and cost-effective alternative by generating electricity within the system itself, which is then utilised for concomitant contaminant degradation.⁹

However, despite the self-sustaining abilities of BES, it faces some critical operational challenges, such as sluggish oxygen reduction reaction kinetics, meagre power production, and elevated fabrication and operational costs.¹⁰ The electrodes contribute significantly to the fabrication and operational cost of BES. Additionally, the overall efficacy and pragmatic application of ETs and BES are impeded by the rate-limiting steps involved in their redox cycles primarily due to the slow electron transfer and inefficient regeneration of the catalyst.¹¹ Generally, electrocatalysts like metallic nanoparticles (NPs), metal–carbon composites and heteroatom-doped nanocomposites are employed in BES to boost their anodic and cathodic activity.¹² However, these functional materials are frequently synthesized through chemical-intensive routes, incurring severe environmental consequences.¹³ Also, although the required quantity of catalyst is not a major concern, its associated cost and environmental impact inhibit its industrial adoption.

Therefore, green-synthesized NPs could provide a sustainable alternative to the conventionally used NPs by reducing their cost and environmental impact, thus serving as effective electrocatalysts in BES and ETs employed for contaminant degradation. To explicate this, Yuan et al.¹⁴ demonstrated the successful application of green-synthesized Fe NPs in a Fenton-like reaction to degrade methyl orange dye. This modified electro-Fenton achieved up to 98.8% decomposition of methyl orange, which can be attributed to the higher H₂O₂ generation leading to higher radical generation, thereby enhancing the removal rate.¹⁴ Similarly, in another investigation, green synthesized Fe₂O₃ NPs were employed for the photocatalytic degradation of methyl orange dye and achieved around 96.8% removal efficiency in 120 min. These investigations highlight the fact that green synthesized NPs could potentially improve the degradation kinetics of advanced technologies such as BES. In addition to enhanced degradation, green NPs also offer other desirable benefits such as low synthesis cost, superior biocompatibility, and limited environmental toxicity compared to conventional NPs. Therefore, the present review article critically explains the use of green synthesized NPs for the abatement of pesticides from wastewater, their sources, and impact and challenges associated with pesticide pollution, followed by various synthesis approaches and their respective properties. Moreover, the environmental impact and sustainability of green-synthesized NPs along with their shortcomings and future directions are also elucidated, which can be implemented for catalysing the BES to eradicate pesticides from wastewater in a green and sustainable way.

2. Pesticide pollution: sources, impact, and challenges

The presence of pesticides in the surrounding environment is a significant cause of concern worldwide; thus, identifying the sources responsible for the release of pesticides in the surrounding environment is essential for their monitoring and remediation. The different sources of pesticides primarily stem from anthropogenic activities across industrial, urban, and agricultural sectors, which are employed for crop protection, pest control, and as antifouling agents and vector control for malaria and dengue.^15–20 Agricultural practices are one of the largest contributors of synthetic pesticides such as atrazine and isoproturon to the environment through sources such as pesticide drift during field spray, surface runoff, leaching into the ground, spillage during filling and cleaning operations of spraying equipment.^16,21 Moreover, pesticides are classified based on numerous factors; however, their persistence, mobility and toxicity are mainly dependent on their active components and chemical makeup. Based on their chemical composition, pesticides are classified into four main categories, namely, organochlorines, organophosphorus, carbamates, and pyrethroids.²² Organochlorines are more persistent and chemically stable compared to organophosphorus and carbamates, which can degrade through hydrolysis and biodegradation, while pyrethroids are the least stable.^23,24 Furthermore, organophosphorus and carbamates are highly water soluble compared to organochlorines and pyrethroids, making them more mobile and susceptible to leaching into aquatic systems.²³ As organochlorines are chemically stable and the least water soluble, they bioaccumulate and also show chronic neurological toxicity compared to organophosphorus and carbamates, which are mainly responsible for acute neurological toxicity.^22,25,26 In addition, the extensive use of herbicides, insecticides, and rodenticides, mainly in urban areas, to control weeds, insects, and rodents further amplifies the distribution of pesticides.

The pesticide-laden water generated from these sources gets discharged into soil, septic tanks, sewerage, wastewater treatment plants, and ultimately into ground and surface water bodies, along with effluent. Further, the recalcitrant nature of many chemical pesticides intensifies their localization in soil sediments and water, augmenting the risk of bioaccumulation and biomagnification along the food chain. Additionally, humans are exposed to pesticides mainly through the ingestion and dermal pathways, which can cause health issues such as headaches, diarrhoea, nausea, vomiting, bronchitis, gastroenteritis, hormone disruption, immunosuppression, reproductive distortion, neurological problems, cardiovascular diseases, cognitive impairment, cancer, and even death based on the toxicity and degree of exposure.^15,27–32

Considering the severe health impacts caused by pesticide exposure, it is extremely crucial to detect these compounds to ensure strict monitoring and regulate their residual concentration in the surrounding environment. The detection of pesticides in environmental samples can be achieved through an array of analytical techniques capable of identifying and quantifying with variable selectivity and sensitivity, even at very low concentrations. In this context, chromatographic methods such as liquid chromatography (LC) and gas chromatography (GC) combined with mass spectrometry (MS) are ideal for pesticide detection due to their high precision and ability to resolve complex compounds.^33,34 In particular, LC–MS is more suitable for thermally unstable and polar pesticides, while GC–MS is effective for the detection of volatile and partially volatile pesticides, though the analysis is expensive, destructive, and provides complex data, which requires specialised technical expertise. In contrast, spectroscopy techniques such as Fourier transform infrared, ultraviolet-visible, near-infrared, and nuclear magnetic resonance spectroscopy, provide quick and non-destructive analysis of the molecular structure of pesticides; however, are less sensitive compared to chromatography techniques.^35–39 Furthermore, immunoassay techniques such as enzyme-linked immunosorbent and immunochromatographic assay, which work based on antigen–antibody interactions, have attracted notable attention due to their ability to produce quick results, low-cost instrument and ability to be automated, rendering higher analytical efficiency.^40–42 Nonetheless, immunoassay techniques are limited by specific antibodies for target pesticides and can give false positives due to the cross-reactivity with structurally identical compounds.

Post-detection, it is equally important to eliminate pesticides from water matrices and reduce their concentration below the regulatory limits to ensure water and food safety. In this case, an array of physical, chemical and biological techniques can be utilized for pesticide remediation from different water matrices. Physical treatment methods, such as membrane filtration and physical adsorption, can achieve 85% to 95% and 78% to 99% pesticide removal from complex water and wastewater matrices, respectively.^43–46 However, although physical treatment methods can attain high removal efficiency for pesticides, their large-scale application can be hindered by operational challenges like membrane fouling and high energy demand.^47,48 Regarding chemical treatment, the prevalent methods for pesticide removal include chlorination and advanced oxidation processes (AOPs), such as Fenton, ozonation, photo-Fenton, and similar radical oxidation-based techniques. These chemical treatments are capable of achieving high removal efficiencies ranging from 50% to 100%, with partial mineralization of the pesticides present in different water matrices.^49–52 However, chemical oxidation often generates toxic degradation by-products, requires strict pH control, and has high operational cost. Alternatively, most biological methods can achieve moderate pesticide removal efficiency of around 30% to 90% and have relatively lower operation cost compared to chemical treatment due to their reduced chemical and energy consumption.^53–55 However, the removal efficiency of biological methods can fluctuate substantially with variations in hydraulic retention time, physicochemical properties of the feed, and climatic conditions.^56,57 Thus, in recent times, innovative strategies like coupling biological treatment with AOPs and bioelectrochemically mediated AOPs have exhibited encouraging outcomes in terms of contaminant removal; however, these hybrid systems are still in their infancy and require technical and design refinements to economically compete with existing technologies.

3. Green nanoparticles: synthesis and properties

Nanotechnology is one of the most revolutionary scientific breakthroughs of the 21st century, pivoting research advancements in the domains of physics, chemistry, biology, environmental science, materials science, medicine, and pharmacy.⁵⁸ In essence, nanotechnology is the science of manipulation and control of matter at the nanoscale, specifically in the range of 1 to 100 nm.^59,60 At this minute scale, NPs exhibit distinctive properties due to their specific size, composition, shape, high surface area-to-volume ratio, and purity of their individual constituents.⁶¹ Owing to these unique characteristics, the use of nanomaterials has surged in a variety of applications, including environmental monitoring, wastewater treatment, agriculture, biotechnology, medicine, and antimicrobial agents.⁵⁹

The unique nature of NPs is influenced substantially by the distinctive method adopted for their synthesis. Usually, the synthesis of NPs is achieved by physical, chemical, and biological routes; however, physical and chemical methods are expensive and produce toxic by-products during the synthesis process.⁶¹ Particularly in chemical synthesis, hazardous chemicals, reductants, and stabilizers are utilized, which ultimately lead to environmental contamination.⁶¹ These constraints have driven the development of alternative biosynthesis routes, wherein sustainable bio-resources and green synthesis methodologies are employed.⁶² The biogenic synthesis of NPs through greener routes provides a cost-effective and environmentally sustainable alternative to conventional procedures (Fig. 1).⁶¹ The green synthesis technique involves the utilization of prokaryotic and eukaryotic cells as well as biomolecules such as proteins, enzymes, vitamins, and polysaccharides, which act as reducing, capping, and stabilizing agents during the synthesis of NPs.

Fig. 1 Schematic showcasing the mechanism of the green synthesis of NPs.

Alternatively, plant extracts and microbes like fungi and algae with natural anti-oxidant properties are used instead of chemicals to facilitate the formation of NPs.⁶³ Moreover, environmental factors such as pH, temperature, metal ion concentration, dosage of reducing agent, reaction time, and type of microorganism play a significant role in defining the size, shape, and texture of NPs.⁶¹ These parameters not only influence the characteristics of the NPs but are also closely linked to the eco-friendly nature of their synthesis. Consequently, the eco-friendly approach in the synthesis process is the result of the resistance mechanism developed by microorganisms against certain metals. The synthesis mechanism can be either intracellular or extracellular, which occurs via distinct biochemical pathways, depending on the microorganism. A more detailed comparison of the green and conventional synthesis of NPs is presented in Table 1.

Table 1 Comparative analysis of the green and conventional synthesis of NPs

Green synthesis of NPs		Conventional synthesis of NPs
Advantages	Disadvantages	Advantages	Disadvantages
Utilization of natural bioactive compounds such as phenols and terpenoids as reducing and stabilizing agents	Dependent on biological material, which may vary geographically and seasonally	Precise control over the NP size and shape	Often requires toxic chemicals such as hydrazine and sodium borohydride
Synthesis occurs under mild conditions	Comparatively longer synthesis time and less control over the reaction kinetics	Rapid synthesis and high yield of NPs	Fast synthesis may cause aggregation of NPs
Formation of eco-friendly, cost efficient and biocompatible NPs	Natural capping of NPs may limit certain catalytic applications	Formation of reproducible, scalable and stable NPs	Cost extensive and non-biocompatible NPs
Avoids the generation of toxic intermediates and low carbon footprint	Process is inconsistent, limiting its scalability	Consistent and standardized process of synthesis	Generation of hazardous by-products
Suitable for environmental and biomedical applications	May require purification of NPs	Suitable for industrial applications	High energy requirement increases environmental burden

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3.1. Classification of green synthesis of nanoparticles

3.1.1. Green synthesis of nanoparticles using bacteria. In the biogenic synthesis of NPs, the inherent bioactive compounds present in bacteria, such as enzymes, peptides, proteins, and metabolites like nicotinamide adenine dinucleotide (NADH), act as reducing, capping, and stabilizing agents, enabling the transformation of metal ions into metallic NPs.⁶³ However, the synthesis pathway is predominantly governed by biological determinants such as bacterial strain, cell wall structure, and enzyme secretion mechanism. In addition, external environmental conditions such as temperature, and pH of reaction mixture can also pivot synthesis mechanism.⁶³ In bacteria, NP synthesis can occur either through an intracellular or extracellular mechanism. In extracellular synthesis, enzymes secreted by bacteria such as nitrate reductase and hydrogenase facilitate the reduction of metal ions to zero-valent form, leading to the formation of NPs.⁶⁴ Furthermore, biomolecules like proteins and extracellular polymeric substance stabilize the NPs and prevent their aggregation.⁶⁵ Alternatively, in the intracellular mechanism, metal ions are transported into the cell cytoplasm through ion channels located in the membrane. After entering the cell, enzymatic reduction of metal ions occurs via intracellular reductases, resulting in the formation of stable NPs.⁶⁶

In this regard, bacterial cells are considered a potential bio-factory for synthesizing gold, silver, and cadmium sulfide NPs as it is well established that bacteria can generate inorganic compounds such as silver, gold, cadmium sulfide, and zinc oxide NPs both intracellularly and extracellularly (Table 2).⁵⁹ Therefore, utilizing bacterial culture as a source of reducing and stabilizing agents limits the usage of toxic chemicals during the synthesis process.⁶⁷ Metallic NPs synthesized using Desulforibrio caledoiensis, Enterococcus sp., Escherichia coli VM1, and Ochrobactrum anthropi have been previously documented for their notable photocatalytic properties and antimicrobial efficacy.⁵⁹ In another investigation, Zamanpour et al.⁶⁸ reported an extracellular method for fabricating spheroidal AgNPs from a Vibrio strain with a size in the range of 33 to 107 nm, exhibiting antibacterial activity against E. coli and Staphylococcus aureus. Similarly, Pseudomonas aeruginosa, Rhodopseudomonas capsulata, Streptomyces, Rhodococcus, and Nocardia are widely utilized for the bio-reduction of gold NPs with a size ranging from 10 to 20 nm.⁶³ However, the bacterial synthesis of NPs is associated with a few challenges, particularly particle size regulation, limited or no morphological maneuvering, and purification of the NPs, which limit their large-scale adoption.⁶³ In this regard, genetic engineering and metabolic pathway modulation can significantly enhance the bacterial NP yield, shape control and reproducibility. The genetic and metabolic modulation pathways facilitate the overexpression of reductase enzyme and the activation of certain metabolic pathways, which subsequently improve the fabrication and yield of NPs. For example, a genetically modified strain of E. coli produced an enhanced level of glutathione and phytochelatins, which resulted in 2.5-times higher yield of CdS NPs compared to the wild-type strain.⁶⁹

Table 2 Biogenic synthesis of NPs and their applications

Source	Nanoparticle	Particle size (nm)	Mode of synthesis	Application	Ref.
NA: not applicable and NR: not reported.
Bacteria
Bacillus subtilis	Fe3O4	60.0–80.0	Extracellular	Antimicrobial agent	70
Desulfovibrio vulgaris	Pt	NR	Extracellular	Catalyst for the removal of pharmaceutical compounds	71
Escherichia coli	Ag	5.0–50.0	Extracellular	Antimicrobial agent	72
Lactobacillus kimchicus	Au	5.0–30.0	Intracellular	Antioxidant	73
Staphylococcus aureus	ZnO	10.0–50.0	Intracellular	Antibacterial agent	74
Fungi
Penicillium tardochrysogenum	Se	60.2–104.5	Extracellular	Anticancer, antimicrobial, and antioxidant	75
Fusarium oxysporum	Ag	20.0–25.0	Extracellular	Antibacterial agent	76
Fusarium chlamydosporum	ZnO	19.3	Extracellular	Anticancer and antibacterial	77
Rhizopus oryzae	Au	10.0	Intracellular	Pesticide degradation	78
Algae
Oscillatoria limnetica	Ag	3.3–18.0	Extracellular	Antibacterial activity	79
Spirulina platensis	CuO	1.7–13.5	Extracellular	Antibacterial and antifungal activity	80
Chlorella vulgaris	Pd	50.0–90.0	Extracellular	Adsorbent	81
Spirulina platensis	Ag	5.0–50.0	Extracellular	Antiviral and antibacterial agents	82
Ulva flexuosa	Fe3O4	12.3	Extracellular	Antimicrobial agent	83
Plants
Terminalia arjuna	Fe2O3	43.0	NA	Degradation of methylene blue	84
Azadirachta indica	Ag	206.4	NA	Degradation of dye malachite green and as an antimicrobial agent	85
Withania somnifera	Cu	6.2	NA	Antibacterial, antioxidants	86
Orthosiphon stamineus	Ag	8.0–25.0	NA	Degradation of 2,4-dichlorophenoxyacetic acid	87
Croton sparsiflorus	Au	16.0–17.0	NA	Ultraviolet ray protection, antibacterial, and anticancer agent	88

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3.1.2. Green synthesis of nanoparticles by fungi. Fungi are regarded as a promising source for synthesizing metal and metal sulfide NPs due to the presence of cellular enzymes and proteins that possess the ability to convert metal ions into their zero-valent state.⁸⁹ Fungal enzymes such as nitrate reductase and peroxidases play a crucial role in the formation of metal NPs, with reductase enzymes in particular facilitating electron transfer to metal ions, and thereby driving the biogenic reduction process.⁹⁰ Alternatively, the protein-based functional groups in fungi, including thiols, amines, carboxyl, and sulfhydryl, serve as natural capping agents.⁹¹ Additionally, these bioactive compounds also prevent the agglomeration of NPs, enhancing their stability during the synthesis process.⁶¹ Similar to bacteria, the extracellular mechanism is dominant in fungal-based synthesis due to the secretion of large quantities of extracellular enzymes and metabolites, although intracellular synthesis may also occur.⁹² In extracellular synthesis, metal ions interact with the negatively charged fungal cell wall, which is rich in chitin, proteins, and polysaccharides.⁹³ These bioactive compounds assist in the adsorption and subsequent bio-reduction of metal ions into NPs. The reaction is catalysed by the enzymes NADH-dependent reductases, which play an important role in the transfer of electrons from enzymes to metal species during the synthesis process. Later, biomolecules aid in the nucleation, growth, and stabilization of NPs.⁵⁹ In this context, polydispersed spherical silver NPs ranging between 1 to 20 nm were synthesized via an extracellular mechanism from Aspergillus terreus, showing wide applications such as antioxidant, antimicrobial, and anticancer agents.^59,63 Alternatively, in the case of intracellular synthesis, metal ions penetrate the fungal cell through membrane channels and are reduced into NPs via intracellular reductases. For example, Verticillium luteoalbum and Candida parapsilosis ATCC 7330 have been reported to synthesize spherical gold NPs with a size between 20 to 40 nm through intracellular mycogenic synthesis.^59,94 Additionally, fungal genera including Aspergillus flavus, Fusarium oxysporum OSF18, Penicillium aurantiogriseum, and Trichoderma koningiopsis have been employed to synthesize antimicrobial copper NPs with a size in the range of 2 to 295 nm.⁹⁵ The fungal-based biosynthesis procedure possesses significant advantages over bacterial methods, including high biomass availability, ease of handling, and natural resistance against toxicity caused due to diverse metal ions.⁶³ However, despite its potential, fungi-mediated synthesis still faces notable challenges like the presence of impurities in the NPs, limited control of the size of NPs, and extraction of NPs from fungal biomass.⁹⁶ However, fungal co-cultures improve the overall production of NPs by activating the silent genes and metabolic pathways within the cellular environment.^97,98 In addition, fungal co-cultures induce the overexpression of certain key enzymes such as peroxidases and laccase, which play crucial roles in the reduction and stabilization of biogenic NPs.⁹⁸

3.1.3. Green synthesis of nanoparticles by algae. Algae are regarded as model organisms for the bio-fabrication of nanomaterials, owing to their low toxicity as well as exceptional metal bio-accumulating and reducing capabilities.^61,63 Algal extracts are rich in pigments, carbohydrates, proteins, minerals, polyunsaturated fatty acids, and other bioactive compounds such as terpenoids, flavonoids, and polyphenolic compounds, serving as reducing and stabilizing agents during the fabrication of NPs.^59,99 Both live and dead algal cells facilitate the green synthesis of NPs due to the presence of functional biomolecules in their cell wall and cytoplasm, through mechanisms similar to those occurring during bacterial and fungal synthesis. In brief, the synthesis mechanism involves metal ion uptake, enzymatic reduction, nucleation, growth, capping, and stabilization.¹⁰⁰ Additionally, algal synthesis also follows intracellular and extracellular synthesis pathways. In extracellular synthesis, metal ions are adsorbed onto the negatively charged cell walls, followed by surface enzymatic reduction. To illustrate this, Thangaraju et al.¹⁰¹ fabricated spherical silver NPs with a size in the range of 5 to 7 nm using Sargassum polycystum, demonstrating antibacterial properties against a broad range of pathogenic bacteria such as Klebsiella sp., Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus. Similarly, Chlorella vulgaris is the most extensively studied green microalgae for the extracellular synthesis of different metallic NPs, including gold NPs (40–69 nm), silver NPs (8–90.6 nm), and palladium NPs (2–15 nm).¹⁰²

In contrast, intracellular biosynthesis involves the internalization of metal ions into the cytoplasm, where they undergo bioreduction through intracellular enzymatic processes.⁶³ For instance, Muthusamy et al.⁸² reported the intracellular synthesis of silver NPs from an extract of Spirulina platensis, with an average particle size in the range of 5 to 50 nm. The synthesized silver NPs showcased remarkable antibacterial properties against Staphylococcus and Klebsiella species. Moreover, Rajeshkumar et al.¹⁰³ successfully synthesized spherical silver nanoparticles with a diameter of 14 nm, exhibiting antibacterial properties against Bacillus sp., Pseudomonas sp., Bacillus subtilis, and Klebsiella planticola. Likewise, Galdieria sp. have been utilized for the fabrication of silver, zinc, and iron NPs, while species like Dunaliella tertiolecta, Tetraselmis suecica, and Chlorella kessleri have been explored for the production of copper NPs.¹⁰⁴ Thus, algae-mediated NP synthesis has emerged as a powerful and eco-friendly strategy for producing multifunctional NPs with significant potential in antibacterial applications and environmental remediation.

3.1.4. Green synthesis of nanoparticles by plants. The plant-mediated synthesis of NPs, known as phytonanotechnology, is a sustainable and eco-friendly approach, as it utilizes phytoactive compounds, avoids the use of toxic chemicals, and operates under mild conditions, which align with the concepts of green chemistry. In plant-mediated NP synthesis, metal ions adsorb onto the plant extract and interact with phytoactive compounds such as terpenoids, flavonoids, polyphenols, and alkaloids. These compounds act as reducing agents, transforming metal ions into their elemental forms.⁶³ Furthermore, the nucleation process occurs, where the reduced ions aggregate to form stable NPs, which are further stabilized by the bioactive compounds. Moreover, these biomolecules play a significant role in preventing agglomeration, and hence controlling the particle size.⁵⁹ The use of the plant extract-based green synthesis process has several advantages, including non-toxicity, availability, biocompatibility, cost-effectiveness, and reduced toxicity of the by-products.¹⁰⁵

Plant-synthesized NPs are frequently employed in contaminant removal; for instance, tea extract-based Fe NPs and guar gum-derived CdMgFe₂O₄@TiO₂ nanocomposites have attained degradation efficiencies of 88% and 94% for ametryn with an initial concentration of 100 mg L⁻¹ and endosulfan with an initial concentration of 20 mg L⁻¹, respectively.^106,107 Similarly, Sengupta et al.⁸⁵ documented the synthesis of silver NPs using the leaf extract of Azadirachta indica, which exhibited 99% photocatalytic degradation of malachite green (initial concentration of 30 mg L⁻¹), and also possessed antimicrobial properties against Escherichia coli, Bacillus subtilis, Staphylococcus, and Klebsiella sp. Furthermore, iron oxide synthesized from Ceratonia siliqua with a particle size of 7 nm demonstrated 99% removal efficiency for amoxicillin for an initial concentration of 100 mg L⁻¹ from simulated wastewater.¹⁰⁸ In a nutshell, plant-based synthesis of NPs offers a sustainable and eco-friendly approach with multifaceted potential across biomedical, environmental, and industrial applications.

4. Comparative analysis of pesticide removal by conventional and green nanoparticles

Water pollution caused by residual pesticides in the agricultural field is a significant environmental concern. Conventional methods, including physicochemical and biological approaches such as filtration, coagulation, ozonation, photocatalysis, and microbial degradation, are commonly employed to eliminate pesticides from aquatic environments.¹⁰⁹ However, despite their widespread use, these methods often encounter limitations related to removal efficiency, process reliability, high operational costs, and environmental contamination.¹¹⁰ Thus, to overcome the limitations of conventional treatments, an array of NP-based pesticide mitigation techniques has been utilized in water and wastewater systems. In water matrices, NPs interact with pesticides through several mechanisms including adsorption, photocatalytic degradation, and chemical transformation.^109,111 For instance, in adsorption, pesticides adhere onto the surface of NPs through electrostatic forces, hydrogen bonding, van der Waal forces, etc. Alternatively, in photocatalytic degradation, pollutant oxidation is facilitated by the generation of reactive oxygen species. Furthermore, chemical transformation entails redox-mediated reactions, which convert pesticides into less toxic and stable molecules.¹⁰⁹

NPs synthesized through physical and chemical methods demonstrate a high adsorption efficiency for pesticides due to their large specific surface area as well as their structural and chemical stability. However, despite their effectiveness, they face economical constraints and synthesis process-related environmental toxicities. To mitigate environmental concerns, greener routes have emerged as a sustainable alternative for the synthesis of NPs (Fig. 2). The green synthesis approach avoids the use of expensive and hazardous chemicals, enhancing the economic as well as environmental viability of the synthesized NPs. The application of these green NPs in pesticide removal presents a sustainable and cost-effective solution for complex wastewater remediation with potential to offset the environmental damage associated with the application of traditionally synthesized NPs.

Fig. 2 Comparative analysis of the synthesis and fate of NPs.

4.1. Role of NPs morphology in pesticide degradation

The morphology of nanoparticles plays a pivotal role in determining the efficiency of nanocatalysts. Over the past few decades, extensive research has been conducted on improving the catalytic performance of nanoparticles by precisely modulating their size and shape.¹¹² However, altering the physiochemical parameters such as size, shape and bulk composition of nanoparticles directly influences the surface chemistry and structure of the nanocatalysts.¹¹³ Moreover, the morphology of nanoparticles is closely linked to the structural attributes and surface chemistry of nanoparticle catalysts, which determines their catalytic performance in water matrices.¹¹³ For instance, the size-dependent factor plays crucial role in the degradation efficiency of catalysts, given that as the size of nanoparticles decreases, the atoms located on their edges and corners become increasingly pronounced, which directly enhances the catalytic behaviour of the nanoparticles.¹¹² Also, nanoparticles with a smaller particle size provide more accessible surface area for adsorption and catalysis, thereby enhancing the interaction and degradation rate of the contaminant.^112,113 For example, Jagpreet Singh et al.¹¹² synthesized AgNPs using the leaf extract of tulsi with a particle size in the range of 5–10 nm, which were subsequently utilized for the degradation of 4-nitrophenol. The results illustrated that, 10 μL of catalyst achieved 100% conversion of 4-introphenol to 4-aminophenol in just 30 min of treatment, which is attributed to the greater number of active sites for interaction. Hence, the catalytic performance of nanoparticles is strongly dependent on their size and characteristics.¹¹³ Similarly, nanoparticles with different shapes strongly influence the interaction with contaminants, which is linked to their varying atomic density and electronic structure, holistically determining the catalytic efficiency of nanoparticles.^112,113 Consequently, non-spherical morphologies such as cuboidal, rods, sheets, plates and octahedral, provide a larger surface-to-volume ratio and elevated contact time with the contaminant, thereby enhancing the removal efficiency of the nanocatalyst. For instance, GG–CdMgFe₂O₄@TiO₂ nanosheets with a particle size of less than 100 nm prepared by guar gum-mediated green synthesis showcased a high removal efficiency of 94% for endosulfan and 88% of DDE at a catalyst dose of 20 mg L⁻¹ under neutral pH, where the ameliorated removal is attributed to the increased contact area and more reactive facets of the nanocatalyst.¹⁰⁶

4.2. Conventional approaches

The conventional methods for the synthesis of NPs are broadly categorized into physical and chemical approaches. These conventional methods are extensively utilized for the fabrication of NPs due to their precise control over the size, morphology, and composition of the NPs.¹¹⁴ The properties of NPs can be fine-tuned during their chemical synthesis to suit remediation applications (Table 3). For example, El-Temsah et al.¹¹⁵ synthesized zero-valent iron with a particle size ranging from 20 to 100 nm, which was subsequently used for the degradation of 500 mg L⁻¹ of dichlorodiphenyltrichloroethane (DDT). The results demonstrated that zero-valent iron at a concentration of 1000 mg L⁻¹ can effectively degrade approximately 92.0% and 22.4% of DDT present in water and soil, respectively. Similarly, Tian et al.¹¹⁶ observed that bimetallic Ni/Fe NPs, with size ranging from 80 to 140 nm, can effectively degrade DDT under both weak acidic and alkaline conditions. To explicate, on adding 5000 mg L⁻¹ of Ni/Fe in aqueous solution, around 85% degradation of DDT was achieved in just eight hours. This enhanced degradation of DDT can be attributed to the favourable weakly acidic and alkaline conditions, whereas an extreme pH level retards the degradation process due to the precipitation of ferrous hydroxide. In another study performed by Stavrinou et al.,¹¹⁷ they fabricated a hybrid adsorbent-photo catalyst TiO₂-activated carbon via the sol–gel method for the synergistic elimination of lindane. This degradation process achieved 90% total organic carbon removal in three hours, revealing the effective generation of in situ reactive oxygen species such as superoxide radical (·O₂⁻), hydroxyl radical (·OH), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂) facilitated by light irradiation. In a related investigation, Sumantrao et al.¹¹⁸ employed a GO/ZnO nanocomposite with a size ranging from 50 to 100 nm for the degradation of chlorpyrifos with an initial concentration of 30 mg L⁻¹ at pH 7.25 and a catalyst dose of 15 mg L⁻¹. The nanocomposite achieved 93.58% degradation under 90 min of sunlight irradiation, owing to its enhanced surface area and photocatalytic activity.

Table 3 List of various pesticides degraded using conventionally synthesized NPs

Nanoparticles	Characteristics	Pesticide	Degradation mechanism	Degradation efficiency (%)	Ref.
BET: Brunauer–Emmett–Teller and DDT: dichlorodiphenyltrichloroethane.
C/ZnO/CdS	Mixture of C/ZnO flakes and spherical CdS with average particle diameter of 50 nm	4-Chlorophenol	Photocatalytic degradation	98.0	122
TiO2	Aggregates with an average radius of 94.7 nm and zeta potential of −48.4 ± 1.4 mV	Chlorpyrifos	Photocatalysis	80.0	123
Fe^0 and Fe3O4	Particle size of 70 nm and surface area of 9.07 m^2 g^−1	Lindane	Redox degradation	100.0	124
		DDT		81.0
		Aldrin		79.0
Ag-reduced graphene oxide	Silver dots deposited on graphene sheets with thin wrinkles with average particle size of 5 nm	Lindane	Degradation	99.9	125
TiO2–zeolite	Particles BET surface area of 510 m^2 g^−1 and a crystallite size of 12.9 nm	Monocrotophos	Photocatalysis	100.0	126
		Dichlorvos
ZnO–chitosan	Particles are globular and porous, with a size of 58 nm	Permethrin	Adsorption	99.0	127
CuO–chitosan	Rectangular flakes with 700 to 750 nm size, BET surface area of 20 m^2 g^−1, and total pore volume of 0.11 cc g^−1	Malathion	Adsorption	99.9	128
Coating of Fe–granular activated carbon	Surface area of 745 m^2 g^−1 and micropore area of 729 m^2 g^−1	Chlorpyrifos	Oxidation	100.0	129
		Cypermethrin		100.0
		Chlorothalonil		100.0
N-Bent-NAl2O3–NZnO	Semi-spherical particles with wrinkled surface with BET surface area of 7.6 m^2 g^−1 and average pore diameter of 11.7 nm	DDT	Oxidation	94.8	130
		Malathion		97.4

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However, despite their high removal efficiency, NPs synthesized via physical and chemical approaches pose a serious environmental concern due to the involvement of hazardous reducing and stabilizing agents during the synthesis process.¹¹⁹ In addition to the generation of toxic by-products, conventional synthesis processes possess notable challenges, including the requirement of high-purity precursors, substantial energy consumption, and high synthesis cost.¹²⁰ Moreover, Pourzahedi et al.¹¹⁹ reported that for a life cycle assessment (LCA) of silver NPs synthesized through physical, chemical, and biological processes, the physical and chemical methods incurred higher environmental impacts compared to biological methods. These impacts were linked to the substantial energy requirements and use of toxic chemicals, which led to ozone depletion and ecotoxicity.¹²¹ Hence, it is evident that conventional synthesis techniques are environmentally unfriendly and necessitate the exploration of alternative approaches to engineer sustainable and eco-friendly NPs for environmental applications.

4.3. Biogenic approaches

In the last few decades, the synthesis of biogenic NPs using plants and microorganisms has emerged as a state-of-the-art approach for mitigating emerging contaminants. This sustainable strategy leverages environmentally friendly nanomaterial fabrication strategies and their subsequent application to degrade pollutants into benign by-products. Consequently, green NPs have been extensively used to degrade pesticides from different water matrices. For instance, Manviri Rani et al.¹³¹ fabricated metal hexacyanoferrate (HCF) nanotubes using Sapindus mukorossi to assess the removal efficiency of hazardous pesticides, such as chlorpyrifos, thiamethoxam, and tebuconazole. Biogenic NPs, namely ZnHCF and CuHCF (∼100 nm) as well as CoHCF and NiHCF nanospheres (<10 nm), were synthesized and utilized for pesticide degradation with an initial concentration of 50 mg L⁻¹ under the conditions of neutral pH, solar irradiation, and 15 mg L⁻¹ catalyst dosage. Among the catalysts, ZnHCF showcased a superior performance of 91–98% degradation, which is attributed to its higher zeta potential of −40.5 mV and BET surface area of 108.70 m² g⁻¹. In this study, ZnHCF showed the highest degradation of chlorpyrifos (83%), thiamethoxam (76%), and tebuconazole (70%) due to the presence of unbound electrons in their structures, which aided in higher removal.

In another investigation, Ningthoujam et al.¹³² fabricated magnetic NPs (FeNPs) using pomegranate peel extract, with an average particle size of 25.1 nm, which were employed to degrade lindane from wastewater using the green synthesis approach. The results illustrated that 0.1 g L⁻¹ of FeNPs can efficiently degrade 99% of 50 mg L⁻¹ of lindane within 24 h. Furthermore, the mineralization of lindane through biogenic FeNPs notably reduced the cytotoxicity by transforming it into less harmful by-products. Similarly, Manviri Rani et al.¹¹³ utilized a straightforward and rapid method for the photocatalytic degradation of endosulfan and ethion. For this, nanohybrid mesoporous iron NPs (Fe₂O₃) were synthesized from green tea extract supported on biochar (BC) from Citrus limetta peels, which possessed a particle size of 8 nm, zeta potential of −22.5 mV, and a specific surface area of 74 m² g⁻¹. The mesoporous BC@Fe₂O₃ facilitated the photocatalysis of 94% of endosulfan for an initial concentration of 50 mg L⁻¹ and 91% of ethion with an initial concentration of 50 mg L⁻¹ employing a catalyst dose of 25 mg L⁻¹ under sunlight. In contrast, bare Fe₂O₃ with a specific surface area of 38 m² g⁻¹ achieved only 50% degradation of endosulfan and 45% degradation of ethion, respectively. The elevated performance of BC@Fe₂O₃ was attributed to its higher zeta potential, increased specific surface area, porosity, BC support, and lower energy band gap of 1.8 eV, which augmented its catalytic and semiconducting properties. Synergistically, these properties improved the adsorption, charge transfer, and catalytic activity of BC@Fe₂O₃, thereby boosting its contaminant degradation efficiency. To highlight further results, green-synthesized TiO₂ demonstrated a higher yield (92% vs. 74%), shorter reaction time for its synthesis (3 h vs. 3.25 h), and 1.06-times lower cost compared to the conventionally fabricated NPs, leading to lower energy consumption and reduced greenhouse gas emissions. Additionally, unlike the conventional approach, which generates hazardous by-products, green synthesis is environmentally safer, producing less toxic by-products.^133,134 Hence, while green NPs may not always outperform the conventional NPs in terms of contaminant removal, they offer an economically viable and comparable removal efficiency with much lower environmental impacts (Table 4). In this context, hybridization of biogenic NPs with ligands such as amines, thiols and other biomolecules dramatically enhanced the removal of contaminants by improving the selectivity towards heavy metals, dyes and other organic contaminants present in wastewater.^135,136 Additionally, post functionalization of green NPs with biomacromolecules can remarkably improve their reusability and the stability of biogenic NPs.¹³⁵

Table 4 List of pesticides degraded using green NPs

Plant	Nanoparticle	Size (nm)	Pesticide	Degradation method	Degradation efficiency (%)	Ref.
NR: not reported and NPs: nanoparticles.
Rice husk	Magnetic mesoporous silica	NR	2,4-Dichlorophenoxyacetic acid	Adsorption	83.4	137
			Glyphosate		79.3
Black tea leaf extract	FeNPs	40–50	Cyanazine	Adsorption	95.8	138
Java tea leaf extract	AgNPs	8–30	2,4-Dichlorophenoxyacetic acid	Photodegradation	97.8	87
Tulsi leaf extract	Carbon dots	35–45	2,6-Dichloro-4-nitroaniline	Photocatalysis	86.0	139
	NiFe2O4	∼200
Tulsi leaf extract	AgNPs	5–10	4-Nitrophenol	Catalysis	100.0	112
Mint leaf extract	AgNPs	15–20	Cartap	Adsorption	90.0	140
			Cypermethrin		90.0
Caper berry leaf extract	CuO NPs	900	Lambda	Photocatalysis	99.0	141
	NiO NPs		Cyhalothrin		89.0
Green tea leaf extract	FeNPs	40–50	Ametryn	Adsorption	88.0	107
Mango peel	CuO@NiO	64	Glyphosate	Photocatalysis	71.2	142
Radish leaf extract	Cr2O3	48	2,4-Dichlorophenoxyacetic acid	Photodegradation	73.0	143
Olive pomace	GQDs@Fe3O4	NR	Malathion	Adsorption	97.2	144

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5. Role of nanoparticles in improving the performance of bioelectrochemical systems

5.1. Microbial fuel cell

A microbial fuel cell (MFC) is a BES that converts chemical energy from organic molecules into electrical energy through the metabolic activity of electroactive microorganisms.¹⁴⁵ Incorporating MFC technology to treat wastewater provides a sustainable approach for the generation of renewable energy and environmental restoration. However, despite being green technology, the impending challenges associated with the practical implementation of MFC include low power generation, high installation cost, biofouling of the electrode, and bacterial instability.¹⁴⁶ In this regard, low-cost green synthesized NPs have emerged as a promising tool for enhancing the overall performance of MFC, mitigating the environmental impacts associated with the use of conventional NPs.

For instance, Tesfaye et al.¹⁴⁷ fabricated Fe₃O₄/polyaniline nanocomposites using Moringa oleifera to modify the anode, which significantly enhanced the surface area of the electrode, as demonstrated by the high peak current due to the increase in charge transfer and surface area of the electrode. Additionally, the nanocomposite promoted the formation of an E. coli biofilm, owing to the improved bacterial adhesion to the anode surface. Similarly, Abbas and colleagues conducted an investigation on the treatment of hospital wastewater through MFC, utilizing ground flaxseed nanopowder to coat multiwalled carbon nanotubes up to 10% (w/v).¹⁴⁸ The incorporation of this green catalyst in MFC achieved 91.06% chemical oxygen demand (COD) removal efficiency with an initial COD of 650 ± 50 mg L⁻¹ and power output of 1072.65 mW m⁻³. In another study, Cu-doped FeO NPs synthesized using the leaf extract of Amaranthus blitum were employed to modify the anode surface of an MFC, which was subsequently utilized to treat dairy effluent. It demonstrated 75% COD removal efficiency, which was higher compared to the uncoated anode (64.2%), and also substantially elevated the power density to 161.5 mW m⁻².¹⁴⁹

5.2. Microbial electrolysis cell

The microbial electrolysis cell (MEC) is neoteric technology prominently employed for biohydrogen production from wastewater. Similar to MFC, an MEC oxidizes organic substrates via electroactive microbes in the anodic chamber; however, the cathodic chamber is hermetically sealed, and an external bias is applied to produce hydrogen gas at the cathode.¹⁵⁰ A number of investigations have showcased the application of MEC in wastewater treatment, simultaneously demonstrating its potential for energy production and resource recovery.^150,151 Nonetheless, this technology faces operational challenges, such as activation overpotential, concentration overpotential, ohmic losses, and poor microbial activity.¹⁵² Thus, to address these challenges, the incorporation of green NPs into MECs is an innovative approach for improving the overall performance of the system.

In this case, Murugaiyan et al.¹⁵² modified the cathode with a zinc ferrite/green graphene oxide (ZnFe₂O₄/GGO) nanocomposite to improve the electrical conductivity, redox potential, and surface area of the electrode. As a result, the nanocomposite-modified electrode exhibited a superior performance with a maximum hydrogen yield of 2.279 ± 0.05 mmol L⁻¹ day compared to ZnFe₂O₄ (1.966 ± 0.02 mmol L⁻¹ day), GGO (1.564 ± 0.03 mmol L⁻¹ day), and plain nickel foam (0.938 ± 0.02 mmol L⁻¹ day). In the same study, a peak current density of 38 A m⁻² and a low Warburg resistance of 0.479 kΩ were reported, owing to the enhanced ion transport and hydrogen evolution reaction kinetics. A similar investigation by Gini Rani et al.¹⁵¹ demonstrated that Fe₃O₄ NP-coated carbon cloth and graphite electrodes possesses a high conductivity of 58 S m⁻¹ and low resistivity of 0.4 kΩ, thereby improving the MEC performance. The current and power densities were enhanced nearly 10-fold compared to the control, reaching 15.2 mA cm⁻² and 10.6 mW cm⁻², respectively for the MEC with the Fe₃O₄-coated electrode. Moreover, Hu et al.¹⁵⁰ studied the effect of Fe₃O₄ NPs on the performance of the sulphate-reducing biocathode in MEC. In this regard, the sulfate reduction was enhanced by 122%, the electron recovery efficiency by 56.1%, and the peak current was doubled due to the improvement in the electrochemical activity of the biocathode. Moreover, magnetite NPs also accelerated the formation of a Desulfovibrio sp. biofilm, which directly increase the reduction of sulfate. Hence, these findings clearly indicate that the integration of green NPs with BESs can improve the electrochemical performance of the reactor.

5.3. Plant microbial fuel cell

A plant microbial fuel cell (PMFC) is a modified biosystem that converts solar energy into bioelectricity by leveraging the symbiotic association between plant roots and electroactive microorganisms present in the rhizosphere.¹⁵³ In PMFC, electroactive microbes oxidize the root exudates secreted by plant roots in the rhizosphere to generate electrons, which are later transported to the cathode through an external circuit, thereby generating bioelectricity.¹⁵⁴ However, despite the enormous potential of PMFC, this technology still faces notable challenges such as scarcity of compatible plant species, quantity of root exudates, and efficient electrodes, which considerably hinder its performance.¹⁵⁵ Thus, to resolve these issues, green NPs play a significant role in overcoming the key limitations in PMFCs. The application of biogenic NPs in PMFCs substantially enhances the surface area and conductivity of the electrode due to the inherent properties of NPs, leading to efficient electron transfer to the electrode. In this regard, many studies demonstrated that the utilization of biogenic NPs in BESs resulted in an increase in the surface area of the electrode, owing to the enhancement in proton transfer, power density, current density, and oxygen reduction reaction (ORR).^149,156,157 Hence, green-synthesized NPs can assist in overcoming these challenges, thereby improving the performance and sustainability of PMFCs.

5.4. Bioelectro-Fenton system

The bio-electro-Fenton system (BEF) is hybrid technology that integrates the principles of BESs and Fenton reaction, which is increasingly being employed for wastewater treatment and concomitant bioelectricity generation.¹⁵⁸ In this process, the two-electron ORR is facilitated at the cathode to generate hydrogen peroxide (H₂O₂), which is subsequently catalysed into ·OH using a suitable catalyst.¹⁵⁹ Importantly, ·OH, being a strong oxidizing and non-selective species, effectively degrades a wide range of contaminants in water matrices.¹⁶⁰ However, although it is energy-efficient technology, it still faces critical challenges, including the generation of iron sludge, low current density, pH imbalance, and limited ORR kinetics.¹⁵⁸ Although conventional-synthesized catalysts have been integrated into BEF to enhance the production of H₂O₂ and ·OH, they often involve high production costs and environmental toxicity.¹⁵⁹

In this context, the synthesis of biogenic NPs presents a promising solution for the development of sustainable, low-cost catalysts, thereby mitigating the adverse implications associated with hazardous by-products formation resulting from the conventional synthesis of nanocatalysts.^158,160 For example, Raj et al.¹⁵⁹ synthesized iron-activated charcoal (Gt–Fe/AC) to serve as a cathode catalyst using waste derived from green tea extract. This green catalyst achieved a maximum operating voltage of 108 ± 3 mV and a maximum power density of 111.7 ± 3.1 mW m⁻², while simultaneously degrading 96.8% of Congo red dye, 100.0% of Coomassie brilliant blue, and 90.9% of methylparaben, each with an initial concentration of 20 mg L⁻¹. In a similar manner, Shahwan et al.¹⁶¹ demonstrated the complete decolorization of methylene blue and methyl orange dyes with initial concentrations ranging from 10 to 200 mg L⁻¹, utilizing iron NPs synthesized from green tea leaves. In another investigation, Weng et al.¹⁶⁰ analyzed the degradation of ofloxacin and enrofloxacin antibiotics in contaminated water. The iron NPs synthesized via Euphorbia cochinchinensis leaf extracts exhibited removal efficiencies of 91.8% for ofloxacin and 90.7% for enrofloxacin. Thus, the integration of green nanotechnology with BEF systems facilitates a transformative leap to realize an improved performance and sustainable wastewater remediation.

5.5. Microbial desalination cell

The microbial desalination cell (MDC) is an emerging BES that combines an MFC with electrodialysis to treat wastewater and desalinate brine water, while simultaneously generating bioelectricity.¹⁶² However, the performance of MDC is governed by many elements, including its internal resistance, electrolyte conductivity, usage of appropriate catalyst, electrode material, and effectiveness of its membrane.^163,164 In this context, the application of green catalysts can significantly boost the performance of MDCs by lowering their internal resistance, elevating their power density, and increasing their desalination rate. For instance, Elawwad et al.¹⁶³ successfully developed MnO₂/graphene nanosheets as cathode catalysts to improve the performance of MDC. The modified cathode catalyst achieved an average desalination efficiency of 15.67 ± 3.32%, COD removal efficiency of 86.20 ± 4.85%, and lower internal resistance of 430 Ω with a maximum power density of 12.5 mW m⁻².¹⁶³

Similarly, Shruti Singh et al.¹⁶⁵ modified the anode using Fe₃O₄ NPs, attaining a COD removal of 74%, coulombic efficiency of10.3%, and power density of 4.3 W m⁻³, indicating the improved performance of the NP-incorporated MDC. Further, Kumari et al.¹⁶⁶ synthesized CuO and AgO NPs using wood extract of Cedrus deodara, with particle sizes of 65 nm and 55 nm, respectively. The resultant AgO-modified cathode exhibited superior electrocatalytic activity, achieving a desalination efficiency of 92.02% and power density of 4.9 W m⁻³, while the CuO-coated cathode showcased a power density of 4.6 W m⁻³ and a coulombic efficiency of 9.19% over an operational cycle of 60 days. These findings clearly suggest that utilizing green NPs in MDC could be a viable alternative to conventional catalysts, catapulting the upscaling of MDC technology.

5.6. Electrochemical oxidation

Electrochemical oxidation is an advanced eco-friendly treatment process, which is extensively utilized for the degradation of persistent organic pollutants in water or wastewater via the in situ generation of ·OH. This process involves the generation of ·OH via the electrooxidation of water molecules at the anode surface.¹⁶⁷ Depending on the nature of the anode material, physisorbed or chemisorbed ·OH is formed, which interacts with pollutants and initiates their degradation.¹⁶⁷ The efficiency of degradation or mineralization of contaminants relies on many factors, such as the anode material, current density, solution pH, supporting electrolyte, and initial concentration of the contaminant.¹⁶⁸

Recent research has demonstrated the high mineralization efficiency of the electrochemical oxidation process, both independently and in conjunction with other technologies for the treatment of pesticide-contaminated water.¹⁶⁷ For instance, Barbari et al.¹⁶⁸ successfully developed a bifunctional electrode, PbO₂/SnO₂–Sb₂O₃/Ti//Ti/TiO₂, for the photo-assisted electrochemical oxidation of fenuron. This bifunctional electrode resulted in the degradation of 97.5% and mineralization of 97.4% of fenuron, with an initial concentration of 17.72 mg L⁻¹. Similarly, Govindaraj et al.¹⁶⁹ synthesized ZnO NPs with a size in the range of 25 to 50 nm using Rubus fairholmianus as a reducing agent, which were subsequently employed to investigate the degradation of 20 mg L⁻¹ bisphenol-A (BPA). The results demonstrated the complete mineralization of BPA in 110 min employing a conjugated electrocoagulation and photoelectrocatalytic oxidation process. The ameliorated removal of BPA could be attributed to the generation of ·OH, facilitated by ZnO-mediated photoelectrocatalysis. Consequently, combining green-synthesized NPs with the electrooxidation process significantly enhanced the degradation efficiency of emerging agrochemicals, thus offering a method for the sustainable treatment of contaminated water.

6. Sustainability and environmental impact of green nanoparticles

6.1. Life cycle assessment of green nanoparticles in pesticide removal

The LCA is an exceptional tool to assess the environmental impacts of the synthesis, use and end-of-life disposal of NPs after their utilization for the removal of pesticides from wastewater. In principle, green synthesis should exert much less environmental impact than conventional approaches as it utilizes plant- and microbe-based bioactive compounds instead of harsh chemicals for reducing and stabilizing metal ions. For instance, Patiño-Ruiz et al.¹²¹ compared the environmental impact of 1 g iron oxide NPs (IONP) synthesized through coprecipitation (CP) and green synthesis methods (Table 5) and found that the environmental impact of IONP synthesized through green synthesis was 70% to 90% lower across different environmental impact categories compared to that of IONP synthesized through CP.¹²¹ Further, the major environmental impact of IONP synthesized through CP was attributed to the marine aquatic ecotoxicity (MAE) impact category, which was almost 16-times higher than the environmental impact of green synthesis, primarily resulting from the indirect involvement of fossil fuel in the production of electricity and generation of wastewater from upstream activities such as mining operation. Likewise, Rodríguez-Rojas et al.¹⁷⁰ estimated that the plant extract-mediated green synthesis of TiO₂ can offset acidification, eutrophication, carcinogenic, respiratory effects, and other related environmental impacts by 75% to 100% compared to the NPs synthesized by chemical synthesis. However, one investigation reported that TiO₂ NPs synthesized via green synthesis demonstrated around 200-times higher environmental ecotoxicity than chemically synthesized NPs.¹⁷⁰ This elevated ecotoxicity of the green TiO₂ NPs is possibly due to the improper disposal of the plant residue generated during leaching of the bioactive compounds from lemongrass plant leaves.¹⁷⁰

Table 5 Life cycle assessment of different green and non-green NP synthesis methods

Nanoparticles	Synthesis method		Impact category	Quantity		Functional unit	Ref.
				Green synthesis	Non-green synthesis
CTUh: comparative toxic units for human toxicity; CTUe: comparative toxic units for ecosystems; NMVOC: non-methane volatile organic compounds; MJ: megajoule; DB: dichlorobenzene; and CFC: chlorofluorocarbon.
Iron oxide	Green synthesis	Cymbopogon citratus and sodium carbonate	Abiotic depletion (kg Sb eq.)	5.8 × 10^−12	1.8 × 10^−11	1 g of NPs	121
			Abiotic or fossil fuels depletion (MJ)	7.5 × 10^−11	3.1 × 10^−10
			Global warming potential (kg CO2 eq.)	2.5 × 10^−11	1.6 × 10^−10
			Ozone layer depletion (kg CFC-11 eq.)	6.6 × 10^−14	4.1 × 10^−12
			Human toxicity (kg 1,4-DB eq.)	1.5 × 10^−11	7.1 × 10^−11
			Freshwater aquatic ecotoxicity (kg 1,4-DB eq.)	8.5 × 10^−11	1.1 × 10^−9
	Non-green synthesis	Co-precipitation	Marine aquatic ecotoxicity (kg 1,4-DB eq.)	7.6 × 10^−10	1.2 × 10^−8
			Terrestrial ecotoxicity (kg 1,4-DB eq.)	6.7 × 10^−12	3.6 × 10^−11
			Photochemical oxidation (kg C2H4 eq.)	9.3 × 10^−12	1.9 × 10^−11
			Terrestrial acidification (kg SO2 eq.)	1.6 × 10^−11	1.2 × 10^−10
			Eutrophication (kg PO4 eq.)	1.4 × 10^−11	2.8 × 10^−10
Titanium dioxide	Green synthesis	Cymbopogon citratus	Ecotoxicity (kg 1,4-DB eq.)	9.3 × 10^−6	4.7 × 10^−8	1 kg of NPs	170
			Carcinogenic (CTUh)	0	3.9 ×10^−10
			Climate change (kg CO2 eq.)	5.2 × 10^−8	7.6 × 10^−7
	Non-green synthesis	Chloride route	Ozone layer depletion (kg CFC-11 eq.)	0	1.5 × 10^−11
			Respiratory effects (inorganic) (kg PM2.5 eq.)	1.3 × 10^−7	1.2 × 10^−6
			Respiratory effects (organic) (kg PM2.5 eq.)	1.6 × 10^−7	8.2 × 10^−7
			Minerals (kg Sb eq.)	1.7 × 10^−3	1.1 × 10^−3
Lanthanum iron oxide	Green synthesis	Terminalia arjuna leaf	Ozone depletion (kg CFC-11 eq.)	4.5 × 10^−6	3.5 × 10^−6	1 kg of NPs	173
			Global warming potential (kg CO2 eq.)	33.5	19.7
			Smog (kg O3 eq.)	1.2	1.1
			Acidification (kg SO2 eq.)	8.3 × 10^−3	7.0 × 10^−3
			Eutrophication (kg N eq.)	3.9 × 10^−2	2.9 × 10^−2
			Human health cancer (CTUh)	6.3 × 10^−7	5.1 × 10^−7
	Non-green synthesis	Citric acid sol–gel method	Human health noncancer (CTUh)	6.2 × 10^−6	2.4 × 10^−6
			Respiratory effects (kg PM2.5 eq.)	8.0 × 10^−2	2.8 × 10^−2
			Ecotoxicity (CTUe)	30.3	18.8
			Fossil fuel depletion (MJ)	40.2	30.6
Magnetite	Green synthesis	L-Glutathione	Climate change (kg CO2 eq.)	9.9 × 10^−3	3.8 × 10^−2	1 g of NPs	171
			Global warming potential (kg CO2 eq.)	6.1 × 10^−3	3.8 × 10^−2
			Terrestrial acidification (kg SO2 eq.)	1.9 × 10^−5	2.9 × 10^−5
			Ozone depletion (kg CFC-11 eq.)	3.5 × 10^−13	4.5 × 10–13
			Fossil depletion (kg oil eq.)	3.3 × 10^−5	1.4 × 10^−2
			Human toxicity (CTUh)	3.5 × 10^−12	4.1 × 10^−12
	Non-green synthesis	Chemical co-precipitation	Marine eutrophication (kg N eq.)	1.3 × 10^−5	2.3 × 10^−5
			Photochemical oxidant (kg of NMVOC)	1.6 × 10^−5	2.3 × 10^−5
			Particulate matter (kg PM10 eq.)	5.6 × 10^−6	7.6 × 10^−6
			Water depletion (m^3)	8.6 × 10^−3	2.5 × 10^−2
			Eutrophication (mole of N eq.)	9.1 × 10^−5	1.1 × 10^−4
			Acidification (mole of H^+ eq.)	2.5 × 10^−5	3.8 × 10^−5
Silver	Green synthesis	Soluble starch	Ozone depletion (kg CFC-11 eq.)	3.6 × 10^−5	3.9 × 10^−5	1 kg of NPs	119
			Global warming potential (kg CO2 eq.)	4.3 × 10^2	5.4 × 10^2
			Smog formation (kg O3 eq.)	3.3 × 10^1	9.9 × 10^1
			Acidification (kg SO2 eq.)	4.7	9.8
			Eutrophication (kg N eq.)	8.1	26.0
	Non-green synthesis	Flame spray pyrolysis	Human health cancer (CTUh)	5.9 × 10^−5	1.9 × 10^−4
			Human health noncancer (CTUh)	2.1 × 10^−4	6.0 × 10^−4
			Respiratory effects (kg PM2.5 eq.)	3.9 × 10^−1	9.1 × 10^−1
			Ecotoxicity (CTUe)	5.3 × 10^3	7.9 × 10^3
			Fossil fuel depletion (MJ)	5.9 × 10^2	6.4 × 10^2

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Intuitively, green-synthesized NPs should exert significantly less environmental burden than conventional-synthesized NPs, as they utilise plant and microbe-based bioactive compounds instead of harsh chemicals for reducing and stabilising metal ions. For instance, Marimón-Bolívar et al.¹⁷¹ compared the environmental impact of 1 g of magnetite synthesized through chemical reduction (CR) and green synthesis and found that the global warming potential and fossil depletion of NPs synthesized through green synthesis were six and 400 times higher compared to NPs synthesized through CR, respectively. However, other mid-point indicators such as climate change, acidification, human toxicity, particulate matter formation, and eutrophication were comparable, indicating no distinct advantage of green-synthesized NPs over NPs synthesized via CR.¹⁷¹ Further, according to Martins and coworkers, the environmental impact of 1 g of nano zero valent iron (nZVI) synthesized through a green route was 50% lower compared to that prepared by CR synthesis.⁷¹ Moreover, findings on green synthesis from previous LCA investigations can further assist in identifying adverse environmental hotspots within different stages of green NP synthesis and can be further utilised to compare different synthesis methods, aiding in the optimisation of the synthesis process.

Moreover, LCA studies are focused on cradle-to-gate analysis, which only considers the synthesis of green NPs, overlooking the application aspect of green NPs for water and wastewater treatment. Similarly, the sustainability of the application of NPs in pesticide remediation through bioelectrochemical systems also needs to be validated. This is because the application of NPs in BES can potentially lower the environmental impact of wastewater treatment. For instance, the use of green NPs/non-platinum-based electrodes in BES can significantly reduce their human carcinogenic toxicity compared to platinum-based electrodes.¹⁷² However, to validate the sustainability of green NPs, the scope of the LCA should be extended to a cradle-to-grave system boundary to include final disposal as well, which could be instrumental in compensating for the adverse environmental hotspots such as ecotoxicity. Additionally, these explorations can also aid in gauging the life cycle impacts of green NP-based water and wastewater treatment, providing insights for further process improvement.

However, the observations regarding ecotoxicity were consistent for other green-synthesized NPs such as Ag, LaFeO₃, and Au, where their higher ecotoxicity was attributed to their relatively higher mining emissions, toxic by-product formation, metal leaching, etc.^173–175 Nevertheless, many LCA-based investigations have reported substantially higher environmental burdens for conventionally prepared NPs in comparison to their greener counterparts, barring the ecotoxicological category.^{119,171,176,177} Moreover, the environmental impact of green-synthesized NPs can be further reduced by switching to efficient heating methods such as microwave heating, reclamation and reuse of washing reagents. To further validate the sustainability of green NPs, the LCA should be extended to remediation application as well, which could be instrumental in compensating for the adverse environmental hotspots such as ecotoxicity. Additionally, these explorations can aid in gauging the life cycle impacts of green NP-based water and wastewater treatments, providing insights for further process improvements.

6.2. Strengths, weaknesses, opportunities and threats analysis of green nanoparticles in pesticide removal

The strength, weakness, opportunity, and threat (SWOT) analysis for pesticide removal from water matrices through green-synthesized NPs highlights the advantages, limitations, and scope for further improvements in the remediation process (Fig. 3). According to the SWOT analysis, the key strength of greener synthesis approaches lies in the use of plant extract as a reducing and stabilising agent, which curtails the need for hazardous chemicals, limits the generation of post-process toxic waste, and reduces the overall production cost.^105,178 Moreover, the ancillary benefits of the application of biocompatible NPs include reduction in the negative impacts on the surrounding ecosystem, such as marine environment and offsetting other environmental hotspots.¹⁷⁹

Fig. 3 SWOT analysis of green NP-based pesticide removal.

On the downside, the preparation of NPs through green synthesis results in a lower yield, relatively higher ecotoxicity, and compromised morphological stability than conventional NPs. These weaknesses primarily arise from the inconsistent composition and weaker electrosteric stability of biomolecules responsible for the reduction and stabilisation of green-synthesized NPs.^105,180,181 Nevertheless, green synthesis presents a spectrum of opportunities such as recovery and reuse of washing reagents, replacement of metal salt with alternative waste-based precursors, and use of more efficient and environment friendly heating sources to reduce the capital and environmental costs. Currently, the green synthesis of NPs has limited scalability, and its long-term environmental impacts are yet to be explored thoroughly. However, further exploration can provide an avenue to address the current limitations of green-synthesized NPs employed for pesticide removal and streamline their application in wastewater treatment.

7. Challenges and future perspectives

BESs have demonstrated outstanding efficiency in the removal of emerging contaminants from water matrices, such as pesticides. The present article highlights the superior efficiency of BESs in the removal of emerging contaminants due to the addition of NPs such as silver and iron-based NPs, which act as electron shuttles to ameliorate the performance of BESs. A plethora of NP-based catalysts, including green-synthesized catalysts, have been employed in BESs to lower their overall operational cost by replacing noble metal-based catalysts.¹⁸² Besides, the green synthesis of NPs is a promising strategy for offsetting the environmental footprint of conventional methods; however, these neoteric synthesis techniques have challenges and are yet to be standardised.^105,183 One of the major hindrances is the inconsistency in source precursors and unreliable supply of raw materials. For example, various plant species used for synthesising green NPs are region specific or available based on season, which can break the supply and production chain.^183,184 Additionally, some raw materials require complex preprocessing or chemical modification prior to their use, which results in elevated costs and can potentially undermine the environmental benefits of the process.¹⁸⁵ Furthermore, the requirement of high temperatures, extended reaction times, or energy-intensive preservation methods for plant extracts also contributes to higher energy requirements, leading to hefty operational expenses.¹⁸⁶

Moreover, the lack of a detailed mechanistic understanding of the green synthesis process makes the process optimisation even more erratic, especially in the case of mass production. Green-synthesized NPs often exhibit considerable variability in size and shape, resulting in heterogeneous particle formation, which reduces their suitability for applications requiring uniform particles.¹⁸⁶ Moreover, the conversion rates from metal ions to NPs also tend to be low, which reduces the overall efficiency and economic viability of this synthesis method compared to the prevailing physical or chemical methods. In terms of application, the practical performance of green-synthesized NPs is inconsistent, and their efficiency in contaminant degradation, especially in complex mixtures, is often lower than that of their chemically synthesized counterparts.^105,186 The operational inconsistencies of green NPs are further aggravated by their aggregation, limited reusability, low selectivity, and high cost, restricting their pragmatic application in the wastewater sector.¹⁴⁰ Therefore, future research should focus on the identification of abundant, fast-growing, and easily processed plant sources, as well as the development of synthesis protocols requiring low-energy, short production time, and are amenable to room temperature.

Furthermore, theoretical models such as density functional theory calculations could aid in the optimization of the synthesis and reaction mechanism of the catalyst. A deeper mechanistic understanding of the synthesis process will be crucial for improving the control and reproducibility of NPs, along with scaling up strategies. For instance, agricultural waste, especially lignin and cellulose, could be used for the synthesis of NPs, which can reduce the precursor cost from 40 to 60%.¹⁸⁷ Additionally, LCA and techno-economic analysis should be employed to evaluate the environmental impact and cost associated with these NP-mediated BESs. Addressing these challenges will be essential for realising the full potential of green synthesis in sustainable nanotechnology.

8. Conclusion

The green synthesis of NPs is garnering wide attention due to its environmental and economic benefits. Moreover, the incorporation of these NPs as catalysts in BESs presents an alternative approach for mitigating persistent organic pollutants sustainably from different environmental compartments. The NPs synthesized via greener pathways exhibit excellent physicochemical properties, including high surface area, improved extracellular electron transfer, redox potential, and biocompatibility, thus proving a viable substitute to conventional NPs for application in catalytic environmental remediation. However, despite their enhanced electrochemical performance, green NPs face significant roadblocks such as aggregation, leaching, and minimal morphological maneuvering. These challenges open the scope for future research on efficient catalyst design with an impetus on optimizing the synthesis methodology to attain prolonged stability and robustness for field-scale applications. In the future, the green production of nanomaterials must also explore novel sources of biomolecules such as genetically engineered microorganisms, optimizing the dosage of bioactive compounds like plant extract and microbial inoculum, standardizing the parameters for the synthesis of NPs, use of solar or microwave assisted synthesis, and shifting towards non-toxic renewable solvents such as deionised water for the purification of NPs. These technological innovations will assist in streamlining the production of green NPs and pave the way for the commercial application of NPs in BES-based technologies.

Author contributions

Ritu Kshatriya: conceptualisation, literature review, writing – original draft, data curation; Yasser Bashir: literature review, writing – original draft, validation; Divyanshu Sikarwar: literature review, writing original draft; Rishabh Raj: review, validation & editing; Sovik Das: funding acquisition, project administration, validation, supervision, review & editing.

Conflicts of interest

Authors declare that they have no known conflict of interest.

Data availability

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.

Acknowledgements

The corresponding author would like to thank the Department of Civil & Environmental Engineering, IIT Delhi, for providing the infrastructure required for this research.

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