MXene based nanoarchitectures for organic contaminants degradation under sonophotocatalytic environment: eco-friendly synthesis, catalytic attributes and recent advancements

Abstract

The 6^th and 12^th Sustainable Development Goals (SDGs) focus on reducing environmental pollution and improving sanitation through responsible production and consumption. However, current wastewater treatment technologies fall short, with projections indicating that up to 4.8 billion people may face health issues due to inadequate water purification by 2030. Preventing environmental problems is more effective than addressing their consequences. Hybrid advanced oxidation processes (h-AOPs), such as sonophotocatalysis, offer a sustainable and efficient method for catalytic conversion, decomposing environmental toxins in water without secondary pollution. Recently, MXene-based nanocatalysts have attracted attention due to their unique properties, including high surface area, excellent adsorption capabilities, and internal electric fields, making them ideal for AOP-related applications. This review comprehensively discusses the synergistic benefits of sonophotocatalysis for the degradation of various organic pollutants, sustainable synthesis routes for MXene and its precursor (MAX) and their alignment with SDG targets. We have reviewed various reports on MXene-based nanocatalysts for contaminant mineralization, systematically tabulated recent advancements. The optical properties and heterojunction-driven charge carrier migration were discussed with respect to novel MXene-based nanocomposites. Finally, future perspectives on MXene-based nanoarchitectures, synergistic ultrasonic approaches and greener approaches to minimize secondary pollution are outlined to support effective AOP-assisted wastewater treatment.

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S. K. Sheik Moideen Thaha

Sheik Moideen Thaha S. K. is a doctoral research scholar at Vellore Institute of Technology (VIT) Vellore, India. His area of research interest includes 2D materials, surface-engineered nanomaterials, ultrasonics, and reactor systems. He is currently working in the Ultrasonics and Advanced Photochemistry Laboratory under the supervision of Dr Sathishkumar P., focusing on the development of custom-designed synergy reactors integrating ultrasound and visible light for the degradation of environmental toxins. His research emphasizes the synthesis and application of novel nanocomposites for advanced oxidation processes, aiming to advance efficient and sustainable methods for water purification and environmental remediation.

Panneerselvam Sathishkumar

Sathishkumar Panneerselvam began his research in nanomaterials and photocatalysis in the year 2007 and he earned his PhD from the National Institute of Technology (NIT), Tiruchirappalli, India in 2010 and later pursued post-doctoral research at the University of Concepción, Chile. With over 15 years of experience in sonochemistry, photocatalysis and nanomaterials, he has published more than 60 original research articles, ten book chapters and edited two books in Springer nature and IOP publications. He has held various research positions in the international level. After joining Vellore Institute of Technology (VIT), Vellore, India as an Assistant Professor, he has led the Ultrasonics and Advanced Photochemistry Laboratory since 2020.

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1. Introduction

Alarming projections reveal that unless urgent action is taken, 4.8 billion people will be exposed to hazardous health risks and economic instability by 2030, stemming from the world's current wastewater treatment and water quality shortcomings.¹ The urgency of this forecast has sparked a rise in research focused on sustainable, eco-friendly strategies for eliminating aquatic toxins. As the global community strives to achieve the (Sustainable Development Goals) SDGs, particularly in wastewater treatment, there is a heightened focus on promoting eco-innovations that prioritize scalability, sustainability, and pollution prevention. Conventional wastewater treatment technologies, including ozone treatment, direct-radical promoter treatment and other oxidant-based methods, often generate secondary pollution through oxidant leaching.^2–4 These homogeneous catalytic treatments pose significant challenges in achieving the Sustainable Development Goals (SDGs) while also meeting cost-effectiveness requirements.⁵ With the world's population projected to reach 8.2 billion and beyond, there is an urgent need for a robust system that can neutralize deadly toxins without causing secondary pollutionIn response, researchers have increasingly turned to hybrid advanced oxidation processes (h-AOPs) and novel nanocatalysts like MXene, which boasts outstanding catalytic capabilities.⁶

The success of h-AOP technology is primarily attributed to the synergistic effects, which enhance efficiency, reduce treatment time, and improve cost-effectiveness.⁷ The reactive oxygen species (ROS) are generated in situ during treatment and are highly reactive in oxidizing the toxic organic and inorganic residues present in the aquatic ecosystem. There are various individual AOP technologies, each producing ROS through distinct processes. In brief, photocatalytic processes generate free radicals by separating electron–hole pairs at the semiconductor, which causes a redox reaction and the production of reactive species. On the other hand, physical processes like sonocatalysis generates free radicals through the acoustic cavitation which involves the collapse of unstable bubbles. Alternatively, chemical processes like Fenton or Fenton-like processes primarily produce free radicals through the exchange of variable redox potentials of metals and H₂O₂. The ozone process is characterised by the production of reactive free radicals by the electrophilic O₃. The h-AOPs is achieved by combining two or more treatment methods. When analyzing the potential h-AOPs opportunities in light of the SDGs and cost-effectiveness, the ultrasound-assisted photocatalysis (sonophotocatalysis) is considered to be the most promising technology.⁸ The combined irradiation of the ultrasound and light energy offers the prospect of sustainability and an eco-friendly nature. Moreover, the irradiation of ultrasound (US) and light energy enables scalable opportunities for industrial-level wastewater treatment, with reduced time and economic aspects. These remediation strategies are particularly vital due to the increased presence of persistent organic pollutants (POPs) such as antibiotics, dyes, pesticides and pharmaceutical residues in wastewater.⁹ These POPs often originate from domestic discharge, textile industries, and agricultural runoff, and are known for their lethal toxicity, bioaccumulation and long-term ecological harm.¹⁰ Their resistance to conventional treatment poses a serious risk to aquatic ecosystem. Therefore, understanding the degradation mechanisms of such pollutants under hybrid AOPs, especially sonophotocatalysis, is essential for designing targeted and efficient detoxification systems.

The mechanism of US-assisted photocatalysis involves the acoustic cavitation of bubbles and photoinduced excitation of electrons within the semiconductor nanocatalyst. Notably, materials with superior surface features tend to exhibit significant enhancements in the US-assisted catalytic process (sonocatalysis). Furthermore, nanomaterials with broad light absorption and distinct semiconductor properties demonstrate superior photocatalytic activity due to their efficient light-harvesting capabilities and ROS generation potential such as TiO₂, ZnO, r-GO and g-C₃N₄.^11,12 However, in the context of US-assisted sonophotocatalysis, the properties of the nanocatalyst are more critical than those of individual AOPs as the nanocatalyst must actively participate in the generation of ROS while also facilitating synergistic interactions to effectively decompose toxic substances in water environments. In this regard, various nanocatalysts supported by carbon-based materials such as carbon nanotubes, graphitic nanodiamonds, and carbon nanorods have recently been employed to enhance catalytic efficiency.¹³ However, these materials face significant challenges including low mass transfer, leaching from the aqueous medium, photocorrosion, and difficulties in selective synthesis.¹⁴ The leaching of supported metal ions presents an additional challenge contributing to secondary pollution during the oxidation process and complicating their removal.¹⁵ Although some traditional nanocatalysts such as TiO₂ and ZnO have demonstrated improved US-assisted photocatalytic efficiency against various environmental contaminants in recent years, they have exhibited persistent issues such as photocorrosion, low surface area, poor responsiveness to direct solar light, and rapid charge recombination.^16–19 These challenges require focused attention to advance research and develop superior nanocatalysts suitable for industrial-scale applications more preferably h-AOP.

In the 21^st century, often regarded as the era of nanoscience and nanotechnology, metal carbides and nitrides have attracted significant interest from a wide range of research communities.²⁰ The primary foundation of this attraction lies in their potential for wide combinations and tunability to suit specific applications.²¹ Numerous nanostructures have been developed and studied to enhance their characteristic features, such as surface area and surface terminal properties.^22,23 Among these, the 1st breakthrough occurred in 2011 when Naguib et al. discovered 2D layered metal carbides by etching the MAX phase of titanium-based ceramic to derive 2D MXenes (Ti₃C₂T_x).²⁴ Since then, there has been an exponential increase in research interest in MXenes in various dimensions of research, as illustrated in Fig. 1a. The distinctive properties observed in MXenes, such as high surface area, excellent conductivity, layered structure, unique electronic features, heterogeneity, and remarkable mechanical qualities, are the paramount rationale behind the exponential growth in research interest. WOS category analysis demonstrated significant attention towards multidisciplinary and fundamental research with a dominant focus on environmental and energy-related applications (Fig. 1b). MXenes are synthesized by etching the “A” layer from 3D MAX phases to produce 2D layered MX phases. Consequently, chemical routes are preferentially utilized to remove the A layer and prepare MXenes. The interlayer distance plays a critical role in tuning the resulting MXenes into various forms, including lamellar nanosheets, accordion-like multilayers, stacked layers, and monolayers. As a result, MXenes hold significant potential for applications in h-AOPs as well as in single-handed AOPs, owing to their exceptional attributes.

Fig. 1 (a) Statistics on number of research publications related to MXene since its discovery (2011) (b) web of science publications categories available on MXene for selected keywords {keywords; (a) MXene (b) MXene and photocatalysis}. Obtained from the Web of science.

The novelty of the current study lies in consolidating a comprehensive understanding of the ultrasound (US) mechanism, the synergistic effects derived from the sonophotocatalytic process, and perspectives on greener approaches for synthesizing MAX phases and MXenes, along with their concurrent applications in degrading various hazardous water contaminants. Currently, no literature provides such an in-depth and comprehensive analysis of the US mechanism and its synergism along in the context of SDG-oriented synthesis of MAX and MX phases. However, few review articles/chapters have focused on MXenes as emerging sonocatalysts for the removal of pharmaceutical pollutants.^25,26 The motivation behind this study is to present a complete review that demonstrates how US-based processes, particularly sonophotocatalysis, can effectively activate MXene-based nanoarchitectures and to explore their future prospects through greener synthetic strategies. The primary databases used to collect relevant literature include Web of Science, Scopus, and SciFinder. The selected papers were arranged and tabulated chronologically under three categories: sonophotocatalysis, sonocatalysis, and photocatalysis. A targeted search was conducted for MXene-based nanocatalysts in environmental remediation using keywords such as MXene, photocatalysis, sonocatalysis, dyes, antibiotics, pesticides, and effluents (2015 to 2025). Each collected study was interpreted based on the process type, pollutant, and the mechanism followed by the nanocatalyst. Papers were then selected for discussion based on their relevance to the study objectives. Notably, many MXene based nanocatalysts were reported with Z-scheme and S-scheme mechanisms, therefore, some results were excluded if already covered in existing reviews. Instead, few novel and interesting nanostructures were discussed to maintain the comprehensiveness of this review. The greener synthesis approaches for MAX and MX phases were compiled through manual searches using broader terms. These findings were interpreted and summarized to align with the study's focus on methods for MAX and MX synthesis, use of greener chemicals, alternative etching approaches, etching time, and their respective outcomes.

In recent high-impact studies of MXenes and MXene-based nanocatalysts have attracted significant attention for environmental applications, particularly in the removal of organic and inorganic pollutants. Consequently, only a few reviews have explored the catalytic efficiency of MXenes for environmental remediation. However, these reviews have largely overlooked the potential of sonocatalysis and hybrid advanced oxidation processes (h-AOPs), particularly in the context of US-assisted photocatalysis.^27,28 Notably, research on h-AOPs, specifically sonophotocatalysis, remains in its infancy. As a consequence, updates on the latest advancements related to MXene-based nanocatalysts for sonophotocatalysis are indispensable. These updates reflects the foundational understanding of the role of US and the underlying mechanisms driving these synergism. Therefore, the motivation of our current study is to provide comprehensive insights into sonocatalytic processes, the synergism of h-AOPs (US and light energy), SDGs intented synthesis routes for MAX and MXene phases, MXene-based heterojunction engineering for organic contaminants removal, synergistic effects and future perspectives.

2. Role of ultrasound in environmental cleaning

The importance of soundwaves in science and engineering has grown substantially as a result of multitude use cases. As a result, sonochemistry plays a vital part in the disintegration of pollutant, from the pretreatment of sludges from industrial waste to tertiary treatment.^29,30 Sonocatalysis differs from conventional oxidation processes such as Fenton or ozonation by generating enormous quantities of energy in the aquatic environment without developing any secondary pollution.³¹ Sonocatalysis is considered relatively more sustainable than conventional oxidation processes and could serve as a promising green alternative, particularly if the required ultrasound energy is generated through renewable sources such as solar cells, fuel cells, or other renewable energies. In contrast, visible-light-driven photocatalysis may be constrained by seasonal variations and geographic location.^32,33 Consequently, it makes sonocatalysis a superlative technology among other oxidation process.

Sonochemical effects are the result of sound energy traveling through an aqueous media. The process known as cavitation is one of the sonochemical effects and produces the extreme energy required to destroy or disrupt the contaminant. In general, soundwaves are classified based on its frequency range as shown in Fig. 2a. A sonochemical effect causes unique cavitation events (acoustic, hydrodynamic, optic and particle cavitation) depending on the frequency range, method of generation, and sound parameters (power, intensity, solvent, shape of reactor).^34,35 In contrast to optic and particle cavitations, hydrodynamic and acoustic cavitations are more ideally suited to oxidize contaminants present within the medium. Acoustic cavitations produces more violent energy since it is induced by pressure flocculation in the passageway of sound. The accumulation of energy in a narrow area creates massive physiochemical conditions. Sonocatalysis has been explained by a few mechanisms, including heterogeneous nucleation, photocatalytic excitation, and thermal excitation.

Fig. 2 (a) Pictorial illustration of frequency ranges of soundwaves, (b) mechanism of sonocatalytic process involving heterogenous nucleation, sonoluminescence, and thermal excitation and (c) implosive collapsed bubble's regional physical property (adopted from ref. 125).

2.1 Mechanism of cavitational bubble generation and sonocatalysis

Sound is made up of pressure waves in the form of longitudinal that are transmitted as a cycle of compression and rarefaction. In a sonochemical or sonocatalytic method, the ultrasonic waves are passed through the solution in the range of 20–1000 kHz using an ultrasonicator.³⁶ The transmitted sound waves give rise to the formation of cavity bubbles. These bubbles are extremely small, confined, and serve as micro-reactors in the aqueous system. The ultrasonic waves outstrips the attractive forces in the water molecules, which leads to a event referred as cavitation event that include the nucleation, formation, growth, and collapse of the bubbles. The implosive collapse of bubbles generates an extremely localised temperature of about 5000 K, pressure of about 500 atm and exhibit radiation in the range of 200–800 nm which is known as sonoluminescence (SL).³⁷ The cavitational events are divided into two category; stable and transient. Stable events result in less implosive collapse than transient cavitation, thus making transient cavitation more effective for sonocatalysis. The mechanism of acoustic cavitation is shown Fig. 2b.

2.1.1 Mechanisms of sonocatalysis. To understand the phenomenon of sonocatalysis, we need to understand the region and surroundings of bubble cavity (hotspot). The region of bubble cavity is shown in Fig. 2c.

(i) The region inside the bubble cavity also known as supreme reactor region. Here most of the pyrolytic reactions (water splitting) as well as hydroxyl radical reactions occurs at small extent. This region is known as pyrolytic reactions zone and the process is known as sonolysis. Superior physical conditions are observed in this region (i.e.) 5000 K, 500 atm and sonoluminescence.

(ii) The region at bubble-liquid layer or interface is known as hydroxyl radical reactions zone since predominant reactions are occurs through OH˙ radical pathway. Here the physical conditions are lighter than in (i) but the formation and reaction flow of OH˙ radicals and H₂O₂ was predominant.

(iii) The region outside (i) and (ii) is bulk liquid which is also known as secondary sonochemical reaction zone. The radicals that jump from the (i) and (ii) undergoes secondary reactions here. Hydrogen, hydroxyl, peroxyl radicals, molecular oxygen and hydrogen peroxide are generated in this region.

Till today, three mechanisms have been proposed for semiconductor-assisted sonocatalysis and further research in this direction may leads to find new insights in sonochemistry.

2.1.1.1 Sonoluminescence mechanism. As mentioned earlier, sonoluminescence is one of the phenomena exhibited by acoustic cavitation. The luminescence encompasses a extent wavelength range (200–800 nm) and relatively high intensity.³⁸ When a sonocatalyst (semiconductor) is energized by the ultrasound irradiation to produce sonoluminesence (SL), the energy of the light emitted during SL is equal to or greater than the semiconductor bandgap, the electrons from (Valence Band) VB could be excited to (Conduction Band) CB. As a result, charge carriers (holes h⁺ and electrons e⁻) will be generated.³⁹
2.1.1.2 Heterogenous nucleation mechanism. Unlike the sonoluminescene- and thermal-excitation mechanisms, the elucidation of the heterogeneous mechanism of sonocatalysis remains inadequate; only a few literature reviews addressed the heterogeneous process with limited context.^36,40,41 The significant stage in sonocatalysis is acoustic cavitation which is a non-linear process, and it can be breakdown into 3 stages; formation, growth, implosive collapse of bubble.⁴² In this mechanism, the formation of bubbles (i.e. nucleation) occurred on the surface of the solid catalyst (sonocatalyst). In contrary to homogenous nucleation, the presence of heterogenous catalysts in the ultrasonic medium induces preferential nucleus development at the catalyst surface, which enhances bubbles birth or generation rate. Cavitation events occur in homogeneous nucleation via the temporary gas void (i.e. the combustion process), whereas heterogeneous nucleation events occur via crevices on the surface of the sonocatalyst as illustrated in Fig. 3a. The crevices and topological features of solid catalyst provides active sites which is advantageous for the cavity bubble nucleation and free radical formation.⁴¹ Consequently, the excitation of electrons can be occurred through heat and light whereas in homogenous cavitation only pyrolysis and OH˙ radical reaction occurs.⁴⁰ The growth rate of bubbles at the solid surface significantly influenced by three additional factors, (i) physicochemical properties of the sonocatalyst (morphology, particle size, topology, roughness and porosity) (ii) state of sonication (frequency, power and amount of liquid bulk) and (iii) environment (pH, temperature, pressure and type of adsorbed gas).

Fig. 3 (a) Heterogenous nucleation in solid surfaces of catalyst during sonocatalytic processes and (b) possible synergistic implications of sonophotocatalysis (sorption mechanics, internal turbulence, improved radical production and enhanced lifetime of carriers).

2.1.1.3 Thermal catalytic mechanism. The presence of localized high temperature caused by the implosive cavitation can possibly stimulate the thermal excitation of the semiconductor, resulting in the formation of electron–hole pairs. The radical generation and the degradation of the target pollutants occurred in the similar pathways as discussed in the previous sections. There are numerous studies on thermally assisted semiconductor catalysis are available in the literature and redirected for further studies.^43,44 The catalysts' improved efficiency could be attributable by the thermal catalytic activation.

2.2 Sonophotocatalysis (SPC)

Photocatalysis is a light-driven single-handed AOP where a semiconductor catalyst absorbs photons of suitable energy and generates electron–hole pairs on its fermi levels. These charge carriers migrate to the catalyst surface and initiate redox reactions that produce ROS (HO˙, and HOO˙) capable of oxidize POPs.^45,46 This AOP technique is widely valued for utilizing light energy, avoiding harmful energy sources, and supporting complete degradation of POPs. The principles of photocatalysis form the foundation of sonophotocatalysis, which combines ultrasound and light energy to enhance pollutant elimination through synergistic effects. As far as hybrid oxidation processes are concerned, SPC is more efficient and convincing than sonocatalytic and photocatalytic processes for the generation of radicals. In typical sonophotocatalytic process, a nanocatalyst in the presence of ultrasound energy and light irradiation is used to destruct the pollutant. As a consequence of sonophotocatalysis, six AOP processes namely, sonolysis, photolysis, photocatalysis, sonocatalysis, sonophotolysis and sonophotocatalysis can occur simultaneously. In comparison to individual AOP technologies, h-AOP can be an effective tool for generating ROS at substantially higher yields.^31,47 The combination of sono- and photo-catalysis processes together demonstrate enhanced efficiency which may be due to several synergy effects as shown in Fig. 3b. The main aim of the h-AOPs is to reduce the operational cost and shorten the treatment duration.

The combination of various AOPs are responsible for the observed synergistic effect during the sonophotocatalytic mitigation of environmental contaminants. The following advantages are considered for the enhanced performance of the sonocatalytic and photocatalytic processes.

(i) The availability of light increases the production of oxidising species.

(ii) Sonolytic cleavage of water give rise to increase in oxidizing species.

(iii) Deaggregation induced by ultrasonic irradiation accelerates the mass transfer of the organic pollutant to the catalyst surface.

(iv) Limiting the recombination of electron–hole pairs.

(v) Turbulence caused by the US restrict the catalyst from aggregation.

(vi) Ultrasonic waves constantly clean the surface of the catalyst to stop pollutants and their intermediates from accumulation.

The adsorbed organic pollutant on the surface of sonophotocatalyst reacts with the ROS effected through photocatalytic process. Subsequently the transient implosion of cavity bubbles improvises the adsorption–desorption process, pyrolysis of water molecules and mass transfer. However, these h-AOPs sometimes can exhibit negative synergy effects.^48,49 S. Chakma and V. S. Moholkar studied the mechanistic issue of h-AOPs using pure and doped catalysts.⁵⁰ The authors discovered that shock waves decrease the possibility of dye–catalyst interaction, and the sonoluminescence light emitted during cavitation is also unable to activate the catalyst in the absence of external light. Thus, a negative synergy effect results; this demonstrates to achieve the synergy, the targeted design of the nanocatalyst facilitate to the nature of the pollutant and AOP processes are crucial.

3. SDGs intented synthesis routes of MAX phases and MXene

When it comes to the synthesis routes for MXenes, several challenges arise in both the synthesis of their precursor (MAX) and the subsequent etching process. Among the available methods, the hydrofluoric acid (HF)-assisted route is widely regarded as the most efficient and straightforward approach to remove the A layer from 3D MAX phases. This is because HF can effectively oxidizes the A layer in MAX precursors, facilitating its disintegration and forming A fluoride (AF₃) {A = Al, Ga, P, Sn etc.,} as a by-product. However, the reagents involved in this process, as well as the resulting byproducts, exhibit highly hazardous properties to the environment. HF is a well-known strong acid with the ability to disintegrate most materials; it can permeate the skin and underlying tissues, posing severe risks to human health and safety.⁵¹ Consequently, the traditional large-scale synthetic approach using HF could lead to significant environmental imbalances such as soil contamination, workplace safety, damage to biodiversity and bioaccumulation. To address these concerns, it is indispensable to explore strategies aimed at minimizing the harmful effects associated with HF-based synthesis. The adoption of safer chemicals and alternative routes for synthesizing MXenes from MAX phases is not only necessary but also highly desirable towards SDG.⁵² In light of these challenges, this section discusses potential synthesis strategies to mitigate these adverse effects.

3.1 Optimizing MAX phase synthesis under reduced hazardous conditions

While analyzing the strategic routes for preparing MXenes, the top-down approach has proven to be more prominent which utilizes MAX phase precursors to produce MXenes.^53,54 Consequently, industries have adopted methods to synthesize MAX phases. However, concerns regarding environmental impact persist, as these procedures consume significant energy, with most methods requiring temperatures exceeding 1400 °C for the solid-state reaction to proceed.⁵⁵ To address these concerns, one viable solution is to harness renewable energy sources, such as solar energy, to power the reaction. This approach promotes sustainability and reduces environmental implications. By exploiting renewable energy, large-scale production of MAX phases can be achieved with a smaller carbon footprint and lower CO₂ emissions compared to processes reliant on fossil fuels. However, we believe that relying solely on renewable energy for MAX phase synthesis does not fully align with the SDGs.⁵⁶ A more sustainable method would involve synthesizing MAX phases at lower temperatures. This approach would not only minimize environmental pollution but also open up various pathways for broader research applications.

The preparation of MAX phases at lower temperatures can significantly reduce both economic and environmental impacts. For example, Ti₃Al(Sn)C₂ synthesized at low temperatures has introduced promising prospects for further research. Yang et al. discovered the inclusion of suitable “A” materials can effectively enhance oxidation resistance, as Sn can occupy Al sites, enabling successful synthesis at reduced temperatures, such as 900 °C.⁵⁷ Moreover, Z. Li et al. reported the synthesis of high-purity Ti₃AlC₂ MAX phases annealed at a low temperature of 700 °C using a high-power impulse magnetron sputtering method within just 90 minutes. The TEM analysis of the synthesized Ti₃AlC₂ MAX phases revealed superior growth with an intact morphology, as shown in Fig. 4a–d. Notably, the sequential stacking of Ti–C layers sandwiched between Al atom layers was precisely demonstrated by the atomic arrangement STEM micrograph of Ti₃AlC₂ grains (Fig. 4e). These novel process intensification studies in MAX phases have stimulated broader implications for other MAX phases.⁵⁸ Each synthesis route may vary depending on the objectives of the study, the precursors used and the conditions proposed for synthesis. However, some general aspects of adopting low-temperature MAX phase synthesis include economic feasibility, energy savings, reduced carbon emissions and more efficient utilization of available resources.

Fig. 4 (a and b) TEM micrographs of HiPIMS induced synthesis of Ti₃AlC₂ MAX phase, (c) grain size distributions calculated for (b) and (d) HRTEM micrographs of Ti₃AlC₂, (e) atomic arrangement STEM micrographs of Ti₃AlC₂ (reprinted with permission from Elsevier⁵⁸).

On the other hand, adopting simpler strategies for preparing MAX phases such as utilizing milder reaction conditions and selective chemical condition, may significantly reduce the complexity of MAX synthesis while also lowering costs. Direct sol–gel chemical reactions (wet synthesis) are considered among the most effective and straightforward synthesis routes for various chemicals, as they offer tunability and size controllability by optimizing reaction conditions (i.e. solvent, reagents, templates and temperature). Additionally, adopting wet synthesis would result in MAX precursors to engage in atomic and molecular–level reactions, facilitating faster and more direct diffusion channels for the high-purity synthesis of MAX phases. In light of this, Sinclair et al. prepared a phosphorus-containing MAX phase, V₂PC, using a direct sol–gel method. The stoichiometrically optimized quantities of vanadium, phosphorus, and carbon sources were carefully mixed to form the V–P–C ternary phases treated at 140 °C. The resulting gel was then subjected to calcination to promote complete phase transformation and to eliminate impure secondary phases.⁵⁹ This direct strategy for preparing MAX phases via the sol–gel method enables safe and large-scale synthesis, reducing reliance on advanced/complex synthesis facilities such as CVD, milling, or plasma-based equipment. Beyond high-purity synthesis, the sol–gel method also offers chemical versatility that direct solid-state reactions cannot promote, such as size controllability and shape tunability. For example, Siebert et al. reported the straightforward synthesis of Cr₂GaC MAX phases through a sol–gel method utilizing nitrate salts of chromium and gallium, with chitosan as the carbon source.⁶⁰ The results from sol–gel-assisted MAX phases highlight the chemical versatility of sol–gel chemistry, with proposed reactions utilizing suitable carbon sources enabling various shape control possibilities, such as microspheres, hollow microspheres, and thin films.⁶¹

3.2 Etching of MAX phases using chemicals safer than HF

HF is a highly dangerous and toxic chemical commonly used during the etching of the “A” layer from MAX phases to prepare 2D MXene layers. However, exposure to HF poses significant health risks, including cardiovascular, pulmonary, and neuromuscular issues, especially when it penetrates the skin and tissues through dermal contact. Even at a concentration of 0.04 ppm, HF can produce a pungent odor and fuming gas, while the PEL (Permissible Exposure Limit) is set at 3 ppm highlighting the HF potency even at such lower concentrations.^62,63 Consequently, exposure to HF fumes can lead to the deposition of highly toxic fluoride (F⁻) ions in the lungs and internal organs, resulting in tissue and muscle damage/corrosion. As a result, the synthesis of MXenes should prioritize the use of less harmful chemicals compared to HF, thereby advancing this area of research toward achieving the SDGs.

Nevertheless, efforts have been made to synthesize MXenes from MAX phases without relying on hypertoxic HF acids or similar methods. For example, fluorine-free synthesis routes have emerged as a less-harmful pathway for etching the “A” layer from MAX phases, utilizing high concentrations of alkali hydroxides. T. Li et al. reported the high-purity synthesis of Ti₃C₂T_x MXene with controllable terminal groups (T = OH and O) through a fluorine-free route. Interestingly, this alkali-assisted hydrothermal synthesis method was inspired by the Bayer process used for bauxite refining. The authors achieved up to 92% purity of Ti₃C₂T_x MXene with excellent gravimetric capacitance.⁶⁴ The characteristic properties of fluorine-free synthesized MXene were analyzed using XPS and various micrographic tools, as shown in Fig. 5a–h. The survey XPS spectra of unetched MAX (Ti₃AlC₂) and etched MXene demonstrated a decline in Al orbitals indicating successful etching which was further confirmed by high-resolution XPS scans at the Al 2p orbital (Fig. 5e). SEM, TEM, and HAADF-STEM micrographs of Ti₃C₂T_x flakes revealed characteristic accordion-like etched 2D MXene layers (Fig. 5f and g), and HAADF-STEM analysis showed the perfect alignment of Ti atomic positions, confirmed by non-uniform interlayer spacing (Fig. 5h). Alternatively, fluorine-free molten salts of Lewis acids with higher electrochemical redox potentials have been considered a more sustainable and effective method compared to the previous NaOH treatment.⁶⁵ These Lewis acid salts are regenerated upon the completion of the etching process, offering lower corrosivity towards the environment compared to HF.⁶⁶ Moreover, this synthesis method is adaptable to a broader range of MAX phases, whereas HF etching is predominantly selective for Al-based MAX precursors. By tweaking the electrochemical potential of the A-site in MAX phases and the corresponding Lewis acid etchant, targeted MXenes can be efficiently achieved with reduced treatment time and minimal temperature requirements, as shown in Fig. 6a. The use of various Lewis acid chloride salts directly influence the A-site in MAX phases. Fig. 6b–g depicts the book-like structure of etched MXenes derived from various MAX phases. Alternatively, Jiamin Zhu et al. developed a one-step A-etching method for MAX phases using the gas phase selective etching (GPSE) technique without relying on Lewis acids or molten salts. Pure 2D MXene phases were synthesized by etching various “A” elements such as Al, Si, and Sn through gas-induced functionalization in a sealed ampoule. Selective metal halides like CuBr₂ were heated above their decomposition temperature to produce etching gases that reacted with the ‘A’ layers in the MAX phases. The ampoule contained two temperature zones where by-products were separated in the lower zone and etched MXene formed in the higher zone as scheme mentioned eqn (1) and (2).⁶⁷ This method offers a cleaner pathway and better economic feasibility for scaling up MXene synthesis from MAX phases. Few other methods have been proposed focusing on the use of less harmful chemicals than HF for MXene etching. However, there remains a gap in fully utilizing sustainable resources such as US energy or solar energy to their full potential in the synthesis process of MXene.

2M₃AX₂ + 3I_2(g) → 2M₃X₂ + 2AI_3(g) (1)

2M₃X₂ + xI_2(g) → 2M₃X₂I_x (2)

Fig. 5 (a) Digital photograph of Ti₃C₂T_x powder (b) survey XPS scan of Ti₃AlC₂ MAX and Ti₃AlC₂ MX (c–e) high-resolution orbital XPS scan of Ti₃AlC₂ MAX and Ti₃AlC₂ MX (f–h) SEM, TEM and HAADF-STEM micrographs of Ti₃AlC₂ MX (reprinted with permission from John Wiley and Sons¹²⁶).

Fig. 6 (a) Redox potentials of A-sites in various MAX phases compared to molten salt (Cl⁻) etchants of different cations, indicated based on Gibbs free energy. (b–g) SEM micrographs of various MXenes etched from MAX phases with different A-sites and corresponding Lewis acids (reprinted with permission from Nature Materials¹²⁷).

3.3 Biocompatibility and toxicity of MXene in aquatic environment

The environmental biocompatibility and toxicity of MXenes, particularly titanium-based MXenes are increasingly recognized as a critical consideration for their catalytic use in wastewater treatment, especially in AOPs. Comparative studies on other MXenes such as Nb₂C, Mo₂C and V₂C have shown that toxicity levels vary significantly depending on the transition metal composition, metal ion leaching, M–X layer counts and surface terminals. For instance, Nb₂C exhibits relatively lower cytotoxicity compared to Ti₃C₂T_x, while Mo₂C and V₂C demonstrate higher oxidative stress potential rates and ROS generation.^68,69 Nevertheless, Ti₃C₂T_x remains one of the more extensively studied MXene materials and shows moderate toxicity under controlled conditions, primarily due to its atomically thin edges and surface terminations (–OH, –F, –O) that can cause cell membrane damage at higher doses.⁷⁰ However, biocompatibility assessments disclose that when used at environmentally relevant concentrations or in immobilized forms (e.g., lower concentrations, nanocomposites or supported membranes), MXene generally exhibits minimal cytotoxicity and is well tolerated.⁷¹ For instance, in vitro toxicity analysis of Ti₃C₂T_x nanosheets was performed on neural stem cells (NSCs) to evaluate their cytotoxicity.⁷² Results indicated that Ti₃C₂T_x nanosheets exhibited no adverse effects at a concentration of 12.5 μg mL⁻¹ on NSCs and NSC-derived cells. However, increasing the dosage to 25 μg mL⁻¹ led to significant cytotoxicity, primarily due to an elevated rate of apoptosis in the NSCs. Additionally, in vivo studies using the zebrafish (Danio rerio) model were conducted by Nasrallah et al. to assess the acute toxicity of Ti₃C₂T_x nanosheets.⁷³ No mortality was observed up to a dosage of 50 μg mL⁻¹. Upon increasing the dosage to 100 μg mL⁻¹ of nanosheets, the cumulative mortality rate of embryo model reached 21%. Based on in vitro and in vivo studies, the biocompatibility of MXenes is largely dose-dependent. However, standardized bio-dosage thresholds and regulatory guidelines are currently lacking due to the diverse composition and terminal functionalization of the MXene family (e.g., Ti, Mo, V, W, Nb, and others). Establishing uniform toxicity profiles and dosage limits is essential for advancing their safe and responsible application. Since MXene are more susceptible to environmental oxidation (oxide formation) and can decomposed into transition metal oxides, using immobilized forms instead of colloidal mixtures helps limit direct environmental leaching into aquatic systems.⁷⁴ Furthermore, lifecycle assessments and comprehensive ecotoxicological evaluation are essential to ensure long-term safety and regulatory compliance for their use in AOP-driven wastewater remediation.⁷⁵

4. MXene for sonophotocatalytic degradation of various environmental toxins

4.1 Optical characteristics of MXene towards environmental application

For a material to be active in catalysis, especially in AOP-based catalysis, it should possess suitable electronic characteristics to participate in and promote the generation of ROS.⁷⁶ When it comes to MXenes, the pure M_n+1X_n phases are inherently metallic in nature making them more effective as cocatalysts rather than as sole catalysts for heterogeneous catalysis reactions.⁷⁷ However, it is important to note that it is nearly impossible to prepare an ultra-pure phase of M_n+1X_n. During the etching process from MAX to MXene, surface functional groups such as F, OH, and O are introduced.⁷⁸ These functional groups and their concentrations, significantly alter the electronic properties of the resulting MXene, leading to a shift from metallic to semiconducting behavior. One of the main objectives in photocatalysis or sonocatalysis is the efficient utilization of energy for degradation. As a result of the functionalization of MXenes, they can be photoexcited when suitable energy is irradiated on their surface. However, as mentioned earlier even after surface functionalization, their semiconducting properties are not as strong as those of other materials like metal oxides. Many metal carbides possess bandgaps ranging from 0.9 to 2.1 eV, which are relatively low for effective visible light harvesting, making them less suitable as sole catalysts for light absorption and ROS generation. Consequently, a suitable material is required to enhance the visible light harvesting capability and improve the overall catalytic efficiency. For instance, Janani et al. reported a quaternary nanocomposite comprising Ti₃C₂/g-C₃N₄ with Fe₂WO₆/BiIO₄ for multifunctional applications.⁷⁹ Optical characterizations revealed that the bandgap of the resulting nanocomposite could be effectively optimized to the narrow visible light region. As shown in Fig. 7A and B, the inclusion of MXene in the quaternary nanocomposite shifted the band edges towards the 400–700 nm range. Consequently, the optical bandgap of pristine BiIO₄ shifted from 2.98 to 1.9 eV, resulting in improved visible-light harvesting. The steady-state PL analysis of the nanocomposites exhibited reduced luminescence, indicating a significant enhancement in the separation of charge carriers (Fig. 7C), which was further confirmed by EIS analysis. The quaternary nanocomposites demonstrated a notable reduction in electron mobility, directly illustrating the effective separation of electronic charge carriers (Fig. 7D). Moreover, similar results were reported by D. A. Upar et al., fabricated a rational Z-scheme nanocomposite of g-C₃N₄–CdS–MXene for photo/electro/photoelectrocatalytic applications.⁸⁰ Due to the incomplete semiconducting properties and limited light absorption, MXene was employed as an electronic charge migrator. The charge migration was expected to occur from CdS to g-C₃N₄ via MXene, thereby providing an enhanced lifetime for photogenerated e⁻ and h⁺ in an aqueous medium. Consequently, the production of ROS was increased, enhancing the kinetic rate to k = 7.65 × 10⁻³ s⁻¹, which led to superior photocatalytic efficiency in the degradation of methylene blue (MB) compared to pure CdS (1.6 × 10⁻⁴ s⁻¹). Various reports have suggested that MXene possesses excellent electronic charge migrator properties, indicating its potential in various heterojunction applications.

Fig. 7 Optical properties of synthesized nanocomposites along with its pure and binary counterparts. (A) UV-vis DRS specta (B) Tauc plot (C) steady state PL spectra (D) EIS spectra (reprinted with permission from Elsevier⁷⁹).

4.2 Surface features of MXene for enhanced cavitational events

Sonocatalysis prominently differs from photocatalysis in terms of stability and surface properties. In sonocatalysis, the primary mechanism of ROS production stems from the sonoluminescence excitation of the catalyst, as highlighted in numerous reviews over the years. However, when considering MXene, its 2D stacked interlayers and crevices within the accordion-like structure of M_n+1X_n layers enhance acoustic cavitation, leading to increased intensity and the formation of transient bubbles as discussed in Section 2.1. The surface charges of MXene are also critical when employing it for environmental applications. In this context, B. M. Jun et al. studied the sonocatalytic degradation of two industrial dyes using Ti₃C₂T_x as a sole sonocatalyst.⁸¹ The results revealed that the MXene surfaces are negatively charged due to the inherent nature of metallic carbides, which facilitated better adsorption of methylene blue (MB) compared to acid blue (AB). In addition, some studies have conducted comprehensive analyses of the microstructure of MXene. Notably, S. Wang et al. discovered potential ionic tunnels within the MXene layers and the presence of extended interlayer spacings between neighboring Ti₃C₂T_x sheets. As depicted in Fig. 8, cracks and crevices in the MXene nanostructure were clearly evidenced in atomically resolved annular dark field (ADF) micrographs.⁸² This suggests that MXene-based materials are not only suitable for absorption-related applications but also for acoustic-related applications. The crevices, acting as neutral gas pockets within the nanostructure, facilitate heterogeneous nucleation, which further enhances the production of transient cavitation events.⁸³ However, the focused study on the interlayer distances of MXene and their effect on sonocatalytic performance remains unexplored to date.

Fig. 8 (A and B) Scheme of Ti₃C₂T_x synthesis (C–H) representation ADF micrographs of Ti₃C₂T_x (I and J) MD simulation of Ti₃C₂T_x.

4.3 Heterojunctional structures of MXene-based nanocatalysts

A sustainable and scalable approach in AOPs that fulfills the primary objective of catalytic decomposition of environmental contaminants involves the effective separation of charge carriers within the nanocatalyst's energy levels. The separation of charge carriers is directly proportional to the generation of ROS; the greater the separation, the longer the lifetime of the charge carriers, led to an enhanced generation of redox reactions and, consequently, more ROS. Therefore, it is essential for a supporting nanomaterial to form various heterojunctions within the nanostructures. Regarding MXene, it has been integrated into various heterojunctional structures, predominantly Z-scheme and S-scheme based. This section explores the potential heterojunctional structures derived from MXene in AOPs, with a particular focus on sonocatalysis and photocatalysis.

4.3.1 Z-scheme derived MXene nanocatalyst. The transfer of e⁻ from one band to another is considered a feasible and spontaneous method to enhance the separation time of h⁺ and e⁻.^84,85 Therefore, the Z-scheme is a significant type of heterojunction where electrons can migrate via an electron mediator, such as MXene. For example, Gao et al. reported a BiOBr/MXene/g-C₃N₄ Z-scheme for the photocatalytic removal of tetracycline (TC). The engineered heterojunction was characterized using various analytical tools to evaluate its extended lifetime and significance. As shown in Fig. 9a, steady-state photoluminescence (PL) demonstrated a notable decline in luminescence intensity, indicating that recombination is significantly restricted in the presence of Z-scheme migration. Moreover, the extended lifetime of charge carriers was confirmed by transient PL through fluorescence lifetime measurements (Fig. 9b). Additionally, the photocurrent response and Nyquist plots provided additional evidence of reduced mobility within individual bands and restricted recombination (Fig. 9c and d). The enhancement in TC degradation was attributed to the presence of oxygen vacancies and MXene quantum dots. However, a detailed analysis of the transpired changes occurring within the reused nanocomposite (8th cycle) in this context was lacking. Lastly, the authors illustrated the mechanism of TC photodegradation in the presence of the Z-scheme BiOBr/MXene/g-C₃N₄ composite, as shown in Fig. 9e.⁸⁶ The NHE potentials and bandgaps of each component in the composite support the formation of Z-scheme electron migration towards g-C₃N₄, facilitating the redox reaction of H₂O. Similarly, S. Li et al. fabricated a Z-scheme nanostructure of ZnO/Nb₂CT_x/g-C₃N₄ for the photocatalytic degradation of enrofloxacin (EFX). The catalyst achieved up to 98.2% elimination of EFX within 40 minutes of light irradiation, attributed to the efficient charge separation mechanism facilitated by Nb₂CT_x MXene, which minimized recombination. The enhanced separation efficiency was supported by photocurrent studies and the Nyquist plot of the synthesized Z-scheme nanocatalyst.⁸⁷ Notably, both steady-state PL and transient PL demonstrated that the Z-scheme effectively enhanced the lifetime of charge carriers, as shown in Fig. 9h and i. Ultimately, the degradation mechanism comprising charge migration pathway was illustrated in Fig. 9j.

Fig. 9 (a and h) Steady state PL of BiOBr/MXene/g-C₃N₄, ZnO/Nb₂CT_x/g-C₃N₄ and its pristine counterparts (b and i) transient PL (c and f) transient photocurrent response (d and g) EIS Nyquist plot of the Z-scheme nanocomposites (e) photocatalytic mechanism of TC degradation in the presence of BiOBr/MXene/g-C₃N₄ (j) photocatalytic mechanism of EFX degradation in the presence of ZnO/Nb₂CT_x/g-C₃N₄ (reprinted with permission from Elsevier^86,87).

4.3.2 S-scheme derived MXene nanocatalyst. The Schottky junction-based charge migration involves separating electrons from the CB of one semiconductor to another, thereby limiting recombination.^88–90 Given MXene's optical properties, which include limited light absorption and excellent electrical conductivity, electron migration from the CB is easily facilitated. Due to these properties, various nanocatalyst combinations have been reported as suitable S-scheme-based nanocatalysts for environmental applications (Table 1). For example, D. E. Lee et al. synthesized a novel CoAl layered double hydroxide supported by TiO₂/Ti₃C₂ MXene for the sustainable elimination of Diclofenac (DFC), RhB, and Indigo Carmine (IDC). The nanocomposites feature 2D/0D/3D contacts with TiO₂ engineered for maximum light absorption. Consequently, e⁻ excited by light energy were migrated to the Ti₃C₂ MXene CB by restricting recombination. Additionally, photocatalytic degradation experiments revealed that the nanocomposites achieved 75%, 69%, and 78% removal of DFC, RhB, and IDC, respectively.⁹¹ However, the efficiency was higher under mild conditions suggesting a narrow pH activity range, which may be attributed to the oxidation of layered double hydroxides under strongly acidic or basic environments. The coupling Ti₃C₂ MXene in the nanocomposite structure enabled the Schottky junction, thereby separating electrons and reducing recombination, as illustrated in Fig. 10a. Similarly, C. Yao et al. fabricated a 2D/2D Schottky nanocatalyst using α-Fe₂O₃ supported with single-layered Ti₃C₂ MXene (SL-TIC) for PMS-activated degradation of tetracycline (TC). It was revealed that the α-Fe₂O₃/SL-TIC/PMS system achieved up to 17.3- and 5.6-fold enhanced degradation of TC compared to α-Fe₂O₃ and α-Fe₂O₃/PMS, respectively. This drastic enhancement is attributed to the photoexcited electrons migrating to the CB of MXene, thereby allowing the h⁺ in the VB of α-Fe₂O₃ to favorably activate PMS, generating SO₄˙²⁻ radicals. Consequently, the backflow of e⁻ to the VB was restricted, and the photoreduction of H₂O subsequently resulted in the generation of ROS to degrade TC, as illustrated in Fig. 10b.⁹²

Table 1 List of MXene based nanoarchitectures for the degradation of various organic pollutants

S. no.	Catalyst	Pollutant	Degradation (%)	Catalytic process	Reactor	Intermediate analysis	Ref.
1	Ni2Mg4−xS4@Ti3C2Tx	BPA	92%	Sonophotocatalysis	970 kHz, 400 W US and 250 W LED lamp	Yes	128
2	Ti3C2Tx@TiO2/WS2	TC	98.9%	Sonophotocatalysis	—	No	98
3	N–TiO2/Ti3C2Tx	Rhodamine B	98.1%	Sonophotocatalysis	53 kHz, 35 kHz US and xenon lamp 500 W	No	129
		Methylene blue	88.4%
		Levofloxacin	65.9%
		p-Nitrophenol	44.6%
4	Ti3C2Tx/MIL-101 (Cr)	Sulfadiazine	96.1%	Sonocatalysis	40 kHz, 180 W US	No	105
		Acetaminophen	99.3%
5	Ti3C2Tx/MIL-125 (Ti)	Rhodamine B	∼90%	Sonocatalysis	35 kHz, 150 W US	No	130
		Microplastics	∼75%
6	Single layered and multilayered MXene	Diclofenac	85.1% and 81.8%	Sonocatalysis	570 kHz, 180 W	No	102
		Verapamil	100%
7	CoTiO3 –Ti3C2Tx	Bisphenol A	96.9%	Sonocatalysis	970 kHz, 300 W	No	131
8	Ti3C2Tx	Methylene blue	90%	Sonocatalysis	970 kHz, 180 W	No	81
9	Bi2O2CO3/Ti3C2/g-C3N4	BPA	80.2%	Multi-frequency sonophotocatalysis	20 + 40 + 80 kHz US, blue LED (457 nm, 12 W)	Yes	99
10	ZnO/Nb2CTx/g-C3N4	Enrofloxacin	98.2%	Photocatalysis	Xenon lamp	Yes	87
11	Boron nitride/Ti3C2/ZnIn2S4	4-Chlorophenol	90%	Photocatalysis	Xenon lamp, 300 W	No	132
12	CoAl (LDH)/TiO2/Ti3C2	Diclofenac	75%	Photocatalysis	Xenon lamp, 150 W	Yes (investigated for DFC)	91
		Rhodamine B	69%
		Indigo carmine	78%
13	a-Fe2O3/single-layered Ti3C2	Tetracycline	99%	Photocatalysis	Sunlight	No	92
14	Ti3C2/MoS2	Methyl orange	97%	Photocatalysis	Xenon lamp	No	95
15	TiO2/Ti3C2Tx	Carbamazepine	98.67%	Photocatalysis	Solar simulator light & UV light	Yes	134
16	CuFe2O4/Ti3C2Tx	Sulfamethazine	60%	Photocatalysis	Xenon lamp, 300 W	Yes	135
17	Gd, Sn doped BiFeO3/Ti3C2Tx	Congo red	100%	Photocatalysis	400–700 nm, Xe lamp, 300 W	No	136
18	g-C3N4/Ti3C2Tx/black phosphorus	Ciprofloxacin	99% within 60 min	Photocatalysis	420 nm cut off filter, Xe lamp, 300 W	Yes	137
19	Ti3C2Tx/MoS2	Ranitidine	88.4%	Photocatalysis	LED lamp, 25 W	No	138
20	Ti3C2Tx/BiFeO3	Congo red & acetophenone	100% & 100%	Photocatalysis	Xenon lamp, 300 W	No	139
21	Ti3C2Tx – oxides	Methylene blue	89% on UV & 95% on vis	Photocatalysis	—	No	140
22	Gd doped V2O5/Ti3C2Tx	Methylene blue	92%	Photocatalysis	—	No	141
23	Bi2WO6/Nb2CTx	Methylene blue	99.8%	Photocatalysis	(λ > 420 nm), xenon lamp, 500 W	No	96
		Rhodamine B	92.7%
		Tetracycline hydrochloride	83.1%
24	BiOIO3/Ti3C2	Tetracycline hydrochloride	93.4%	Photocatalysis	Xe lamp, 500 W	No	142
		Diclofenac sodium	87.6%
		Levofloxacin hydrochloride	77.4%
		4-Nitrophenol	43.3%
25	Fe2O3/Ti3C2Tx	Rhodamine B	91%	Photocatalysis	(λ > 420 nm), Xe lamp, 500 W	No	143
26	Holey-Ti3C2/Ag/AgBr	Methyl orange	99%	Photocatalysis	(λ > 420 nm)	No	144
27	N–Ti3C2	Methyl orange	98.73% within 20 min	Photocatalysis	Mercury lamp, 1000 W	No	145
28	Defect Ti3C2/BiOIO3	Methyl orange	95.2% in 30 min	Photocatalysis	Xenon lamp, 300 W	No	146
29	g-C3N4/TiO2/Ti3C2	Methyl orange	93.1%	Photocatalysis	Xenon lamp, 300 W	No	147
30	BiOI/Ti3C2	Rhodamine B	99.8%	Photocatalysis	Xenon lamp, 300 W	No	148
31	Urchin-like TiO2/Ti3C2Ox	Levofloxacin	7.7 folds higher than pure TiO2	Photocatalysis	8 W LED lamp	Yes	149
32	g-C3N4/BiOBr/MXene	Tetracycline hydrochloride	99%	Photocatalysis	Xe lamp, 300 W	Yes	86
33	ZnO/Ti3C2Tx	Formaldehyde	83%	Photocatalysis	125 W mercury lamp or 300 xenon lamp	No	97
		Methanol	84%
		Acetone	80%
34	BiOCl/Ti3C2Tx	Ciprofloxacin	90% in 30 min	Photocatalysis	Not mentioned	No	150
35	Bi2WO6–Ti3C2–SnNb2O6	Tetracycline hydrochloride	84%	Photocatalysis	Xenon lamp, 300 W	Yes	151
36	ZnS/Ti3C2Tx	Tetracycline hydrochloride	96%	Photocatalysis	Metal halide lamp, 500 W	No	152
37	C/Ti3C2/(001)TiO2	Tetracycline	92.3%	Photocatalysis	Xenon lamp, 350 W	Yes	153
38	TiO2/Ti3C2Tx/AgI	Tetracycline hydrochloride	97%	Photocatalysis	Xenon lamp, 350 W	Yes	154
39	Bi2O2CO3/Ti3C2	Tetracycline	81%	Photocatalysis	Xenon lamp, 300 W (420–780 nm)	Yes	155
40	Ti3C2–SO3H/g-C3N4	Tetracycline	75%	Photocatalysis	Xenon lamp, 35 W	Yes	156
41	V2C/g-C3N4	Methyl orange	94%	Photocatalysis	Xenon lamp, 300 W	No	157
42	CdTiO3/Ti3C2	Rhodamine B	85.8% in 180 min	Photocatalysis	Xenon lamp, 200 W	No	158
43	BiOBr/Bi2MoO6/Ti3C2/MMTex (MMTex = exfoliated montmorillonite)	Norfloxacin	91%	Photocatalysis	Xenon lamp, 500 W	Yes (investigated for levofloxacin)	93
		Ciprofloxacin	90%
		Ofloxacin	97%
		Levofloxacin	99%
44	g-C3N4/Ti2C3 derived	Rhodamine B	97.2%	Photocatalysis	Xenon lamp, 270 W	Yes	159
		Tetracycline	86%

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Fig. 10 Schematic representation of plausible S-scheme mechanism of photocatalytic degradation of targeted pollutants using (a) CoAl-LDH/TiO₂/Ti₃C₂ (b) α-Fe₂O₃/Ti₃C₂ (c) BiOBr/Bi₂MoO₆/Ti₃C₂/MMT_ex (reprinted with permission from Elsevier^91–93).

The utilization of MXene in multifaceted nanocomposites, such as quaternary composites, has increased in recent years due to its electron sink properties, which effectively enhance the lifetime of charge carriers through migration across multiple counterparts. These advanced catalysts are expected to deliver improved degradation efficiency and higher pollutant removal rates. In this context, Liu et al. developed a quaternary nanocomposite comprising BiOBr/Bi₂MoO₆ supported by oxygen-functionalized Ti₃C₂ with montmorillonite (BBTM) for the decomposition of levofloxacin under visible-light photocatalysis. The BBTM photocatalyst demonstrated exceptional photoactivity, achieving 99% removal of levofloxacin (LEV) within 120 minutes. This remarkable performance was attributed to the restricted backflow of electrons from the CB of BiOBr/Bi₂MoO₆ to its VB. Furthermore, the work function (E_f) of transpired nanocomposites supported the favorable bending of band edges, facilitating electron migration to Ti₃C₂/montmorillonite.⁹³ The schematic illustration of the photocatalytic mechanism of the quaternary nanocomposite for LEV degradation is depicted in Fig. 10c. This work underscores the versatility of MXene in nanocomposites and its feasibility in S-scheme-based photocatalysis.

4.3.3 Other unique heterojunctions. Among the heterojunctions, Type II and Type I junctions are quite common and were therefore intentionally excluded from our focus. However, in recent years, several new types of heterojunctions have been proposed for MXene-derived nanocatalysts, specifically for environmental remediation and related applications. Y. Wu et al. fabricated a unique heterojunction photocatalyst based on MXene and termed it as “M-scheme,” for the photoreduction of CO₂. The photocatalyst consisted of two metal oxides (CeO₂ and TiO₂) supported by Ti₃C₂ to facilitate efficient charge separation. Consequently, the nanocatalyst was synthesized through chemical etching of the MAX phase (Ti₃AlC₂), followed by loading the metal oxides onto the surface of Ti₃C₂ using a hydrothermal route. The results from the photoreduction of CO₂ showed a selectivity of up to 70.6% with 0.22 μmol h⁻¹ upon using 50 mg of the fabricated nanocatalyst.⁹⁴ The superior photocatalytic activity was attributed to the photogenerated charge migration of e⁻ from the metal oxides to the CB of MXene, and the corresponding mechanism was labeled as the M-scheme, as shown in Fig. 11a. In a similar context, Z. Yao et al. proposed a new scheme called the “r-scheme” and used Ti₃C₂ with MoS₂ as an effective photocatalyst for energy and environmental applications. The fabricated Ti₃C₂/MoS₂ nanocatalyst demonstrated 97.4% removal of methyl orange and 380.2 μmol h⁻¹ g⁻¹ of H₂ evolution under visible light. The enhanced degradation and H₂ evolution were ascribed to the electron sink properties of Ti₃C₂, which acted as a Schottky junction to migrate electrons from MoS₂, thereby reducing spatial recombination during photocatalysis. However, the authors described the mechanism as the r-scheme, which should be more appropriately labelled as the S-scheme based on the experimental results, as shown in Fig. 11b.⁹⁵

Fig. 11 Schematic representation of photocatalytic mechanism of (a) CeO₂/Ti₃C₂/TiO₂ – M-scheme (b) Ti₃C₂/MoS₂ – r-scheme (reprinted with permission from Elsevier⁹⁴).

4.4 Recent advancements on MXene assisted sonophotocatalysis

The key characteristics of MXenes are beneficial to h-AOPs which includes (i) terminal functional groups that can result in strong connectivity to various counterparts; (ii) uniform porosity distribution and large surface area for active redox chemical reactions; (iii) metallic conductivity of MXene that enhances charge migration throughout extended structure; (iv) acts as Schottky junction to recombinations; and (v) hydrophilicity of MXene facilitates the adsorption of aqueous pollutants with suitable surface charges. Over the past few years, researchers have shown more interests in preparing new class of MXenes with alternative metal centres to explore the role of catalysts in the AOP.^96,97 The present section summarizes the recent findings and advancements in the MXene-based catalysts used in sono–photo-catalytic degradation technologies. Most of the research works are focused on photocatalysis only, however other AOPs also explored as years progressing. Nevertheless, we believe that previous reviews related to the context of our study have lacked in presenting comprehensive data on MXene-assisted h-AOP. Therefore, Table 1 summarises recent works and findings from the MXene-based catalysts in the field of AOPs (sono–photo-catalysis).

In this regard, Vigneshwaran et al. reported the sonophotocatalytic removal of BPA using a novel bimetallic nanocomposite tethered with MXene (Ti₃C₂T_x).¹²⁸ They fabricated MXene as a co-catalyst with Ni₂Mg_4−xS₄ (NMS), and the nanocomposite was synthesized via a simple hydrothermal method, as shown in Fig. 12a. The NMS@Ti₃C₂T_x exhibited superior catalytic activity under various AOP conditions (Fig. 12b). Notably, the highest degradation efficiency was achieved with h-AOP (sonophotocatalysis), resulting in a BPA degradation efficacy of approximately 91%. Furthermore, the authors employed the MXene nanocomposite to evaluate its catalytic potential against various major environmental pollutants. Interestingly, the nanocomposite demonstrated significant catalytic potential for environmental applications, including the removal of Cr(IV), methyl orange, chlorophenols, and azo dyes (Fig. 12c). Consequently, this study highlighted the broad applicability of MXene-based nanocatalysts in addressing a wide range of environmental toxins. In alignment with the MXene-based catalyst for h-AOP, Ranjith et al. investigated the decomposition efficiency of tetracycline (TC) using WS₂-intercalated Ti₃C₂T_x@TiO₂ nanostructure under sonophotocatalytic conditions. The nanostructure was fabricated through HF etching followed by hydrothermal treatment to produce Ti₃C₂T_x@TiO₂-WS₂, as illustrated in Fig. 13a. Although the authors did not disclose specific working conditions for the sonophotocatalytic environment, such as ultrasound frequency and light intensity, their experimental results demonstrated that the Ti₃C₂T_x@TiO₂-WS₂ nanostructure significantly enhanced TC decomposition efficiency under sonophotocatalysis.⁹⁸ The synergistic effect of h-AOP resulted in a decomposition rate of 0.0341 min⁻¹ for sonophotocatalysis, compared to 0.0078 min⁻¹ for sonocatalysis and 0.0171 min⁻¹ for photocatalysis, as shown in Fig. 13b–d. This study further highlights the versatility of MXene-based nanostructures for h-AOP applications in water treatment. Moreover, similar results were reported by Ding et al., utilized nitrogen-doped TiO₂ supported by Ti₃C₂ MXene for the removal of a wide range of environmental contaminants. They demonstrated enhanced removal efficiencies of various pollutants, including Rhodamine B, methylene blue, levofloxacin, and p-nitrophenol, under sonophotocatalytic (h-AOP) conditions.

Fig. 12 (a) Representative scheme of Ni₂Mg_4−xS₄/Ti₃C₂ bimetallic nanocomposite. (b) Time-dependent degradation efficiency of various AOP/h-AOP processes, including sonolysis, photolysis, sonocatalysis, photocatalysis, and sonophotocatalysis. (c) Degradation efficiency of Ni₂Mg_4−xS₄/Ti₃C₂ bimetallic nanocomposite against various environmental contaminants (reprinted with permission from Elsevier¹²⁸).

Fig. 13 (a) Schematic illustration of Ti₃C₂T_x@TiO₂-WS₂ nanostructure synthesis. (b) UV-vis spectra of TC degradation under various AOP and h-AOP conditions. (c & d) Degradation efficiency and rate comparison of different AOPs using Ti₃C₂T_x@TiO₂-WS₂.

The toxicity of intermediates formed during the degradation process was analysed since these by-products may sometimes be more harmful than the parent compounds and persist in the environment for long time. Therefore, the intermediates identified in the degradation pathway of organic pollutants should be carefully assess for their toxicity and biosafety. In this regard, Sheik Moideen Thaha and Sathishkumar reported the fragmentation of intermediates during the degradation of BPA using Bi₂O₂CO₃/Ti₃C₂/g-C₃N₄ nanocomposites under the sonophotocatalytic environment.⁹⁹ The toxicity of these intermediates were evaluated using Toxicity Estimation Software Tool (T.E.S.T v.5.1.2), and tested against models like fathead minnow, T. pyriformis, and D. magna. The inhibitory growth concentration (IGC50) and lethal concentration (LC50) values of the intermediates were higher than those of BPA, demonstrating that the degradation products were less toxic. Similar findings have also been reported by Qiang Li et al. and S. Birudukota et al.^100,101

Although numerous efforts have been initiated in exploring MXene as a co-catalyst, only a few studies have thoroughly examined the effects of various parameters, such as the number of layers, interlayer distances and terminal functionalizations, in light to environmental applications. In this context, Sewoon Kim et al. studied the degradation of diclofenac and verapamil using single and multi-layered MXene as a sonocatalyst. The authors discovered that MXene solid surfaces improved OH˙ radical production by 48.8% for single-layered MX and 59.8% for multi-layered MX.¹⁰² The observed results indicating the remarkable hydrophilicity property possessed by the MXene surface. In addition, MXene has a high electron content (sink) and a negative charge in its natural state and verapamil has a positive charge, which resulted 100% degradation, whereas diclofenac has shown only to a lesser extent. This is due to the pollutant's electrostatic attraction to the surface of MXene, which is extended by the terminal functional groups (F, O, –OH, –COOH) presents in the MXene.

Despite adsorption-based AOPs are not the primary focus of our current review, it is important to note that nearly all AOPs include an initial rest/dark/adsorption–desorption time without energy irradiation to establish equilibrium between oxidant and contaminant.¹⁰³ Therefore, the adsorption properties of MXene are also a significant parameter to investigate in order to optimize the nanocatalyst. S. Kim et al. investigated the adsorption performance of sonicated MXene (Ti₃C₂T_x) with various pharmaceutical compounds, and their findings confirmed that positively charged pollutants such as amitriptyline and verapamil showed good adsorption to MX.¹⁰⁴ The kinetics of adsorption were studied using two different sonication frequencies (28 and 570 kHz). Low frequency sonication improved the degradation performance by producing larger cavitations when compared to the high frequency ultrasound. Due to the unique surface morphological properties of MXene, numerous combinations have been reported to date (Table 1). However, MXene-based nanocatalysts for environmental applications have predominantly focused on metal oxide-derived MXene composites. While Metal Organic Framework (MOF)-based nanocatalysts have gained significant attention in recent years, the potential of MXene/MOF-based nanocatalysts for environmental applications remains relatively unexplored compared to their metal oxide counterparts. S. Ranjith et al. prepared an MOF-supported MXene-based sonocatalyst for the elimination of sulfadiazine (SD) and acetaminophen (AAP) in aquatic conditions.¹⁰⁵ The MXene was synthesized via chemical etching of the MAX phase and MX@MOF combination were prepared using H₂BDC and Cr salt along with the layered MXene under a hydrothermal route, as illustrated in Fig. 14a. The concentrations of the synthesized MX@MIL-101 were optimized and employed to degrade SD and AAP under various AOPs (Fig. 14b, c, e, and f). The MX@MIL-101 nanocatalyst demonstrated promising removal efficiencies of up to ∼96% for SD and ∼99% for AAP, indicating that MXene is highly suitable for heterogeneous nucleation and for reducing recombination with MOF derived nanostructures, as shown in Fig. 14d and g. Yang et al. synthesized MIL-125 (Ti) supported with Ti₃C₂T_x MXene using hydrothermal treatment and investigated its efficacy in degrading Rhodamine B (RhB) and microplastics (MP) under sonocatalytic conditions.¹³⁰ The synthesized MX@MOF nanocatalyst demonstrated the ability to degrade RhB up to 93.1% and decompose MP up to 78% without the use of any radical promoters. Optimization of catalyst dosage and ultrasonic (US) power was conducted to enhance the degradation efficiency. The authors described MXene as an electron trap, where the sonoluminescence-excited electrons of MIL-125 can migrate to the MXene band, thereby reducing the recombination effect.

Fig. 14 (a) Schematic representation of MX@MIL-101 (Cr) preparation (b & e) degradation efficiency of various AOPs with radical promoters against sulfadiazine and acetaminophen (c & f) Effect of MXene loading on degradation efficiency against sulfadiazine and acetaminophen (d & g) representative bar graphs illustrating the synergistic effect of the sonocatalytic process.

Unlike sonocatalysis or sonophotocatalysis, the exploration of MXene in photocatalysis is more extensive and well-developed. Various combinations of nanocatalysts have been employed with MXene for multifaceted applications.^106–108 In the context of photocatalytic wastewater treatment, notable work was initiated by Chao Peng et al., synthesized Ti₃C₂T_x-supported TiO₂ nanocatalyst for the removal of Methyl Orange under photocatalytic conditions.¹⁰⁹ The authors described MXene as forming a “Schottky junction with 2D Ti₃C₂ acting as a reservoir of holes”. Since this work was reported in 2016, research on MXene-based photocatalysis has grown exponentially. Numerous contaminants and catalyst combinations have been proposed; however, addressing all of them is beyond the scope of this review and many review have been already established in terms of photocatalysis and MXene.^110–113 Therefore, we have summarized selected works that align with our current review's objectives, focusing on value addition by heterojunctions.

4.5 Limitations and trade-offs of MXene based nanoarchitectures

Evaluation of compromising trade-offs is essential to translating MXene-based nanoarchitectures to real-world wastewater applications such as h-AOP technologies. Upon careful analysis on various MXene-based nanocomposites for organic contaminants degradation, several limitations from the materials side were observed in related to sonocatalysis, photocatalysis and sonophotocatalysis. The major limitations and trade-offs of MXene are; (1) prone to surface oxidation of Ti₃C₂ to metal oxides (2) structural stability of MXene layers under strong acidic or basic conditions (3) hydrophobicity of M–X layers that limit the scope of contaminants (4) scalability and reproducibility of highly surface engineered MXenes (5) lack of regulatory terms for environmental and biological impacts of MXenes. In context to the trade-offs, Lukatskaya et al. demonstrated the potential of MXenes for energy storage applications, highlighting the high volumetric capacitance of Ti₃C₂ MXene enabled by reversible cation intercalation. However, their study also revealed a key limitation: MXenes are prone to oxidation in aqueous environments, which can degrade their conductivity and long-term performance.¹¹⁴ Additionally, the hydrophobic nature of MXene surfaces can lead to pH-dependent surface charge variations which may restrict the electrostatic attraction and adsorption of a broad range of organic contaminants in water.⁸¹ Under sonocatalytic conditions, although MXenes can promote heterogeneous nucleation of cavitation bubbles, the intense US energy can lead to partial surface oxidation, forming TiO₂, which alters the material's original catalytic characteristics.⁹⁸ Indeed, the use of MXenes in nanocomposites typically involves low concentrations and recent advancements in near eco-friendly synthesis methods suggest that these trade-offs may be addressed through future process intensification studies.

5. Synergy effect of h-AOP processes

The sonophotocatalytic processes provide a better platform for the degradation of toxic contaminants through six combined processes (sonolysis, photolysis, photocatalysis, sonocatalysis, sonophotolysis and sonophotocatalysis),³¹ compared to the sono and photo-alone processes. The combination of six processes at once would result in increased efficiency and enhanced mineralization. The enhancement is considered to be an outcome of the synergistic effects of sono and photocatalysis as discussed in 2.2. Various researchers investigated about the synergistic effect of their catalysts' performances in sonophotocatalysis. The synergistic effect of the sonophotocatalytic study can be evaluated by the rate constants obtained from the individual degradation study such as sonocatalysis and photocatalysis along with sonophotocatalysis. The precise method to assess the synergistic effect of sonophotocatalysis is to find the synergy index or percentage of synergism. The eqn (3) is the synergy index formula for the evaluation of synergy effect resulted from the h-AOPs.whereas k_{sonocatalysis}, k_{photocatalysis}, and k_{sonophotocatalysis} are the rate constants of sonocatalysis, photocatalysis and sonophotocatalysis, respectively. The evaluation of synergy index varies based on the involvement of the distinct processes. In addition, synergy index and % are expressed mathematically in eqn (3) and (4), respectively. Using eqn (3), the corresponding value demonstrates the involvement of sonophotolysis and sonophotocatalysis. However, to examine the true synergy of h-AOP processes, eqn (5) must be taken into account in which the rate constants of sonolysis and photolysis are also adopted. The synergy index is completely dependent on the type of degradation system (light source, power, frequency). Nonetheless, various synergy index expression has also been proposed over the years, some of them are represented in the following eqn (6) and (7).¹¹⁵

Various reports were discussed about the synergism in sonophotocatalytic processes and other h-AOPs.^115–121 For instance, K. Yun et al. evaluated the synergistic effect of ultrasound irradiation and light energy in the sonophotocatalytic degradation of ciprofloxacin. A synergy up to 68% between ultrasound and light was achieved, based on the comparison of single handed AOPs.¹²² While some studies report synergy as a percentage to indicate the enhancement, the use of a synergy index (S. I.) is a more appropriate and standardized approach. Typically, the S. I. value may be greater than, equal to, or less than unity, for synergistic, additive, or antagonistic effects, respectively. The effective surface cleaning of nanocatalysts by US and the enhanced light penetration due to US-induced turbulence can complement each other during the degradation process resulting in a S. I. greater than unity. In most cases, the S. I. values for sonophotocatalytic systems range between 0.7 to 2.0. This variation is attributed to several factors influencing the h-AOP, including the efficient properties of the nanocatalyst, the wavelength of the light source, nature of the contaminant, and frequency/power of US irradiation. An S. I. value less than one may indicate limitations such as poor light penetration, shielding or scattering effects, mismatched US energy, catalyst deactivation, or even catalyst leaching. Notably few studies on sonophotocatalytic degradation of organic pollutants have evaluated the synergy index using eqn (4).^123,124 However, the reports of MXene in sonophotocatalytic environmental remediation is still limited and requires more extensive studies in the future. Additionally, we observed that the existing reports on MXene in sonophotocatalysis have not adequately addressed the evaluation of synergistic effects. Therefore, future studies should incorporate such evaluations to enhance the understanding of h-AOPs.

6. Perspectives of the research in future

6.1 Degradation pathway

Although significant advancements have been made in the hazardous pollutant degradation using AOP technologies like sonocatalysis or photocatalysis, a major gap remains in understanding the precise degradation mechanisms and pathways. Future studies should prioritize: quantitative identification of reactive species generated during the degradation processes, Toxicological evaluation of intermediate compounds formed during reactions, and mechanistic analysis to determine the individual and hybrid contributions of photo- and ultrasonic-effects to ROS formation and pollutant breakdown. These aspects are often overlooked or not presented alongside degradation results, yet limiting the assessment of secondary pollution risks. Developing standardized protocols for degradation pathway analysis and integrating them into routine evaluations will enable a more comprehensive and environmentally responsible application of h-AOPs in wastewater treatment. Furthermore, extensive studies on the degradation pathway and intermediate analysis should be the primary focus of future research in this area.

6.2 Multiple pollutants removal

Many studies have concentrated on eliminating a single contaminant class. In real-time scenario, wastewater effluents may contain multiple contaminants.¹³³ In light of this, multiple pollutant removal must be investigated in order to mimic natural system, commercialise the process and advance the practical applications. Future research should address: synergistic or antagonistic interactions between different classes of organic contaminants, efficacy of AOPs in mixed pollutant systems, and design of hybrid or modular AOP systems that can adaptively eliminate a range of co-existing pollutants. Such direction of investigations are essential to move from laboratory-scale models toward commercial viability and real-world deployment of AOP technologies.

6.3 Operational conditions and advanced materials

Despite diverse reports on sonocatalytic, photocatalytic, and sonophotocatalytic degradation using various instruments and conditions, there remain a need for: systematic comparisons of operation modes such as simultaneous, sequential, and interval based treatment conditions to better understand their effects on degradation efficiency and energy consumption, development of frequency varied sonication probes and multi-frequency ultrasound systems to enhance efficiency intensification and higher reactivity and emphasis on eco-friendly/non-toxic catalyst materials capable of large-scale usage without generating harmful residues. Special attention should be given to green-synthesized MXenes in these applications which could provide valuable insights and direct future research efforts.

7. Conclusion

The current wastewater treatment technologies often fail to reduce secondary pollution and are inefficient in terms of catalytic conversion, especially in the current population growth and industrialization. The 6^th and 12^th SDGs call for the development of sustainable technologies and materials to eliminate toxic contaminants from water environments. Recent studies highlight the positive synergy effect of sonophotocatalysis, addressing many of the practical limitations associated with traditional methods. These studies demonstrate that sonophotocatalysis can reduce treatment times and achieve more effective degradation, owing to the cooperative interaction between sonocatalysis and photocatalysis. On the materials side, the rise of MXene-based nanocatalysts for the degradation of various environmental pollutants presents promising alternatives for water remediation. The growing attention towards MXene synthesis in green and environmentally safer routes were summarised with their prospects and future directions (Table 2). The potential industrial-scale synthesis routes for MXene and its precursors (MAX) in catalyst production have been critically examined in relation to SDG targets. Moreover, the flexibility of MXene-assisted sonophotocatalysis has been thoroughly explored, and the available findings have been summarized in a comprehensive table to guide future research. The functional properties of MXene in relation to catalytic activation under visible-light irradiation and US energy have been comprehensively discussed. However, the simultaneous removal of multiple pollutants using sono, photo, and sonophotocatalytic processes, along with a deeper analysis of the synergistic effects, remains an area that requires further attention. Future studies should also focus on investigating advanced intermediates, quantifying ROS, and exploring the commercialization potential of MXene-based h-AOPs. The overall summary of this review is presented in Fig. 15. These aspects will be crucial in advancing the field and realizing the full potential of MXene in wastewater remediation applications, particularly in AOPs.

Table 2 Overview of merits and demerits in various MXene synthesis techniques

S. no.	Synthesis methodology	Merits	Demerits	Ref.
1	HF etching (direct etching)	• Simple and straightforward	• Hazardous HF handling	160
		• Widely used	• Environment and safety concerns
		• High yield of MXene nanosheets
2	In situ HF etching (e.g., LiF + HCl)	• Safer than direct HF	• Longer reaction time	161
		• Tunable surface terminations	• Incomplete etching of MAX phases
3	Molten salt etching	• HF-free	• Requires high temperature	162
		• Potential for large scale and clean synthesis	• Could alter resulting MXene composition
4	Electrochemical etching	• Cleaner method	• Toxicity depends on electrolyte; some may be corrosive or hazardous	163
		• Controllable reaction conditions	• Limited to conductive MAX phases
		• HF-free
5	Alkali-assisted etching	• Safer than HF method	• Low etching efficiency	164
		• It can introduce oxygen functional groups	• Stronger concentration required
			• Structural degradation possible
6	Hydrothermal etching	• Possible HF-free approach	• Often requires high temperature and longer reaction time	165
		• Enables tunable surface terminations	• Safety still depends on etchant used
		• Suitable eco-friendly specific functionalization
7	Sonochemical etching	• Rapid exfoliation	• Not a standalone etching methodology	166
		• Prevents restacking of layers	• Safety depends on chemicals used
		• Aids surface activation and cleaning
8	Organic salts assisted etching	• Safer alternative to HF etching	• Reagent cost may be higher	167
		• Enables various surface terminations	• Safety and eco-friendliness still depend on the etchant used
9	Halogen etching	• HF-free approach	• Involves toxic and volatile halogens	168
		• Can be done in room or low temperature conditions	• Requires inert atmosphere and anhydrous solvents
		• Enables surface termination control	• Need cryogenic or heated conditions
		• Avoids harsh acid/base treatments	• Scalability limited

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Fig. 15 Schematic roadmap summarizing the key themes and future directions discussed in this review.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could appear to influence the work reported in this paper.

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.

Acknowledgements

The authors thank VIT for providing ‘VIT SEED GRANT (RGEMS) – Sanctioned Order No. SG20230012’ for carrying out this research work.

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