Ali Nikkhah,Hasan Nikkhah,Hadis langari,Alireza Nouri,Abdul Wahab Mohammad,5,Ang Wei Lun,Ng law Yong,Rosiah Rohani,Ebrahim Mahmoudi,

1 Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Malaysia

2 Department of Mechanical and Aerospace Engineering, University at Buffalo, Buffalo, USA

3 Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, USA

4 Department of Chemistry and Medicinal Chemistry, University at Buffalo, Buffalo, USA

5 Chemical and Water Desalination Engineering Program, College of Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates

6 Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Sungai Long, Bandar Sungai Long, Kajang, Selangor, Malaysia

Keywords:MXene 2D materials Wastewater treatment CO2 capture Adsorption Photodegradation

ABSTRACT A new burgeoning family of two-dimensional (2D) transition metal carbides/nitrides,better known as MXenes,have received extensive attention because of their distinct properties,such as metallic conductivity,good hydrophilicity,large surface area,good mechanical stability,and biodegradability.About 40 different MXenes have been synthesized,and dozens more structures and properties have been theoretically predicted.However,the recent progress in MXenes development is not well covered in chronological order based on different applications.This review article focuses on emerging synthesis methods,the properties of MXenes,and mainly the applications of MXenes and MXene-based material family in environmental remediation,a comprehensive review of gaseous and aqueous pollutants treatment.

1.Introduction

Today,we live in a cutting-edge world;however,it is irrefutable that this economic and industrial growth has led to the degradation of the ecological environment,and the water supply has been severely polluted[1].Environmental contaminants such as organic compounds,heavy metals,and radionuclides could have a detrimental effect on humans due to their chemical toxicity and carcinogenic effects [2,3].A trace concentration of them can cause irreparable harm to organisms.Techniques such as ion exchange,adsorption,sedimentation,and electrochemical approaches have been developed to remove these environmental pollutants to overcome severe environmental challenges [1,4-7].There is a growing consensus among these methods that adsorption is the most costeffective and environmentally friendly option for wastewater treatment.However,developing effective adsorbents to remove pollutants such as radionuclides from demanding environmental conditions remains an important task,and it is vital to formulate appropriate materials with high adsorption capacity [1,8-11].

The discovery of graphene in 2004 caused significant attention toward novel two-dimensional(2D)materials,which has led to the fast development of numerous synthesized compositions [12-17].In the past decade,a wide range of 2D sheets has been created,from clays to boron nitride (BN) and transition metal dichalcogenides.Many of the 2D materials’exciting features cannot be seen in their bulk counterparts,and reducing dimensions and sizes clarifies immense potential for numerous applications,from electrical and optoelectronic devices to electrochemical catalysis[1,18].Also,unique combinations of qualities that are not present in any bulk material can be achieved by combining several 2D materials.

MXenes,one of these 2D particles and a new 2D early transition metal carbides and/or nitrides family,is swiftly becoming popular in various applications [18-22].The first discovered MXene was titanium carbide (Ti3C2) by Naguibet al.in 2011 [23].Since then,more than 40 types of MXenes have been successfully fabricated[23,24].Generally,MXenes are represented by the general formula Mn+1XnTx(n=1,2 or 3),derived from selective extraction of Aelement from precursor MAX (Mn+1AXn) [25].Fig.1 shows the MXenes and MAX phases that make up the periodic table elements.Here,M is an early transition metal like Ti,Cr,or W (indicated by the blue balls in Fig.1) [26].A represents A-group elements containing XII to XV elements in the periodic table,such as Al,Ga,Ge,and Si (indicated by green balls in Fig.1).X is carbon (C)and/or nitrogen(N)(indicated by the red balls in Fig.1),T denotes the surface termination groups such as F,O,Cl,and OH (indicated by the yellow balls in Fig.1),andxrepresents the number of surface functionalities [26].As shown in Fig.2(a),MXenes possess a hexagonal structure [12] with an interlayer spacing of about 1 nm,depending on the value ofnin Mn+1XnTx[27,28].MXenes typically have three formulas: M2X,M3X2,and M4X3,as shown in Fig.2(b) [13].

Fig.1.Elements in the periodic table as known for the formation of Mn+1AXn and Mn+1XnTx.Adapted from Ref.[26].

Fig.2.(a)MAX phase P63/mmc symmetry.Blue balls denote the A layer atoms,red balls denote the M layer atoms,and the black balls occupying the octahedral site denote the X atoms.Adapted from Ref.[12],(b)The early reported three different structures of 2D transition metal carbide/nitride(MXenes)(non-terminated):M2X,M3X2 and M4X3.Adapted from Ref.[13].

The flexible chemistry of the MXene family makes it possible to tailor their properties for a wide range of applications.Oxygen-or hydroxyl-terminated MXenes,such as Ti3C2O2,have been found to have redox-capable transition metals layers on their surfaces and to combine high electronic conductivity with hydrophilicity and rapid ionic transport compared to most 2D materials [29].Therefore,they have aroused significant consideration from scientists due to their unique properties and promising performance in various applications [30].

These 2D materials,known as ‘‘conductive clays”,usually combine the properties of metals and ceramics,such as high chemical stability and electrical conductivity[13].MXenes have a wide,versatile chemical composition and structural arrangement at the atomic scale that endows them with some exceptional characteristics,including hydrophilicity,ease of functionalization,high electrical conductivity (reaching～20000 S·cm-1),excellent chemical stability,plastic layer structure,small bandgap,and large redoxactive surface area [24,25,31].Because of their exceptional features,MXenes have found countless applications in energy storage,water purification,environmental remediation,chemical sensors,and photo or electrocatalysis[14,29].In addition,significant efforts have been made to employ MXenes in the biomedical field(including imaging,biosensing,and drug delivery)owing to their high surface area,hydrophilicity and biocompatibility [15,16,32].

Due to abundant research about MXenes(more than 7000 articles since 2011),it is crucial to provide a comprehensive update of research efforts in this area.Hence,in this review paper,the authors discussed the different synthesis methods of MXene and thoroughly covered the MXene applications and performance for removing gaseous pollutants such as NOxand carbon capture,heavy toxic metals,dyes and pharmaceutical compounds from water and wastewater.Fig.3 shows the outline of this review paper.

Fig.3.Outline for the current review paper.

2.Synthetic Methods

As explained,MXenes are created mainly by etching the A layers from the MAX phases.During the etching procedure,functional groups such as oxygen (—O),hydroxyl (—OH),or fluorine (—F) are always applied to the Mn+1Xnunits’surfaces.So,the MXenes chemical formula is summarized as Mn+1XnTx,where Txindicates the functional groups on the surface [18].Firstly,hydrofluoric acid was used to synthesize the first Ti3C2Txlayer,but alternative methods were devised and used to produce other MXenes with different compositions.

The synthetic techniques to produce 2D layered nanomaterials can be divided into two distinct routes: (i) top-down and (ii)bottom-up approaches.Both strategies have been used to produce a single-,few-layer or multilayer nanostructure of MXenes [29].The main difference between these approaches is that the MAX phase can be separated into layers using the top-down method.In contrast,the bottom-up method uses MXene deposition technology to produce the material from its base to the surface [17].Fig.4 depicts the synthesis methods based on top-down and bottom-up methods [17,33].

Fig.4.Various synthesis methods of MXene based on top-down and bottom-up methods.

Most of the studies in MXene synthesis have focused on topdown methods,the process in which a bulk material or powder is reduced,made smaller,or part of the bulk structure is removed,leaving micro-to nano-sized structures (Fig.5) [34,35].Top-down approaches are cheap and have the advantage of scalability in currently existing technologies [35].The produced materials are mostly large irregular MXene sheets,functionalized with —OH and —O groups and a thickness between 10 and 200 nm [16].Top-down methods include selective etching in a mixture of fluoride salts and various acids [24];and alternative synthesis routes such as using molten salts,alkaline solutions,electrochemical etching methods,and hydrothermal treatments [34,35].Experiments have demonstrated that wet chemical etching is only helpful for carbon-based MXenes as it fails to remove the A-layer from nitride-based MAX phases [23].

Fig.5.Schematic of the top-down method of synthesis Ti3C2Tx.Adapted from Ref.[35].

Bottom-up approaches are less used and employ small organic/inorganic molecules/atoms as precursors to form 2D-ordered layers following crystal growth.The produced materials are mostly defect-free films of multilayers with a lateral size between 10 and 100 μm without functional groups [16].The two principal approaches for bottom-up synthesis are physical vapour deposition and chemical vapour deposition [34].

2.1.Top-down strategy

The most common approach for attaining 2D materials is to separate a single or few atomic layers from the layered structure.As mentioned before,the precursors for MXenes belong to the family of layered ternary carbides and nitrides called MAX phases[36].MXenes can hardly be produced by mechanical exfoliation of MAX phases due to relatively strong metallic M-A bonds.However,M-A bonds are relatively weaker and easier to break than M-X bonds.It is possible to select etching‘‘A”layers with various etchants,employing chemical etching or heating to destroy the M-A bond.As heating with a high temperature may demolish the layered structure,MXene is mainly prepared by wet etching [37,38].

One of the most commonly used etchants is hydrofluoric acid(HF) [39].Fig.6 shows the synthesis process of MXene using HF.Fig.6(a) shows that the top-down approach generally has two steps: etching the MAX phase and delamination to separate the MXene sheets[16].After etching,the resulting multilayers are typically washed multiple times with water to remove the byproducts of the synthesis,such as aluminium fluoride (AlF3).An acid-pre wash with HCl or H2SO4can also help dissolve salts such as AlF3or LiF[40].Delamination in MXene usually happens through intercalation by inserting cations,organic molecules,or large organic bases before an optional sonication step [41].Intercalating agents are often used to increase the c-lattice parameter (the distance between two consecutive MXene sheets),making it easier for MXene sheets to separate into single nanosheets.Intercalants that have been commonly used during the synthesis of MXenes include polar organic solvents(such as dimethyl sulfoxide(DMSO)and isopropylamine [42]) and green metal cations (such as Li+,Na+,Ca2+,and Al3+[41]).

Fig.6.Schematic of (a) synthesis of MXene by HF,(b) exfoliation and etching by HF.Adapted from Ref.[16].

A mild sonication after an intercalation process can typically lead to delining multilayered structures into a few ultrathin layers[43].Ultrasonication contributes to the final size of MXene flakes[44].However,additional sonication or intercalation is not required during an etching process when fluoride salts such as KF,NaF,or LiF dissolved in HCl are used.Delamination occurs with etching simultaneously as the cations in the fluoride salts intercalate into the layers of MXene [45].The obtained MXene sheets are more stable and oxidation resistant as they usually have larger sizes with few defects,which are essential features for some applications requiring high electrical conductivity [35,44].

MXenes synthesized with the chemical etching method usually possess numerous surface functional groups,including oxygen (-O),hydroxyl (-OH),or fluorine (-F).For example,Ti3C2MXene can at least have the following three formulae:Ti3C2(OH)2,Ti3C2O2,and Ti3C2F2.The quantities of each terminal group depend highly on the synthesis process [1].Studies have demonstrated that several parameters in the etching process,such as the initial particle size of MAX phases,intercalation agent,etching solution,temperature,and time may influence the final MXene [30,38].

As mentioned,the HF etching method is one of the most used chemical liquid-phase methods due to its convenience and low reaction temperature (less than 55 °C) [25,46].Fig.6(b) gives the details of exfoliation and etching by HF.In 2011,Naguibet al.[23] took the lead and immersed Ti3AlC2MAX Phase in 50% concentrated HF at room temperature for two hours to prepare Ti3C2-TxMXene.However,HF is corrosive and harmful to the human body and environment,and it is necessary to develop alternative milder etching processes[18].To avoid the direct usage of concentrated HF,in situHF-forming methods are preferred over conventional HF methods [35,47].

Mixing the fluoride salts with acid for thein situformation of HF has become a mature etching method to convert the MAX phase to MXene[47].In 2014,Ghidiuet al.[48]used a mixed solution of LiF and HCl to etch the Ti3AlC2MAX phase.In this method,the spontaneous insertion of cations such as Li+between MXene layers expands the interlayer spacing.Further hydration of the cations facilitates the weakening of the interaction between MXene layers,resulting in the delamination of MXene in water.Therefore,no exfoliation agent is needed in this method.This method’s many advantages include using mild reactants compared to HF,eliminating the extra intercalation steps,high exfoliation yield,approximately 70% of the flakes with one or two layers,and achieving flexible clay-like MXenes [23,30,37].

Different characteristic analyses can illustrate the difference in features of the initial MAX and produced MXene.Royet al.[49]showed the comparison result of X-ray powder diffraction (XRD)of the Ti2AlC MAX and Ti2CTxMXene phases.Fig.7(a),the vital strength peaks at 12.58° (002) and 39.02° (104) illustrate the formation of a highly crystalline Ti2AlC MAX phase.However,after the treatment of MAX with LiF and HCl,Al particles were etched,and characteristic peaks of Ti2AlC diminished.As illustrated in Fig.7(b),the corresponding peak of Ti2CTxMXene appears at 7.47°(002)with a small amount of LiF and unreacted Ti2AlC,which proves the MXene synthesis.Micusiket al.[50] investigated the chemical and morphological stability of MXene during aging in the air.As shown in Fig.7(c),the etching process of Ti3AlC2MAX disappeared the Al2p peak in the X-ray photoelectron spectroscopy(XPS) survey at approximately 72-74 eV.The XPS results explain that different factors,such as storage conditions and air contact duration,can change the chemical compositions of C,O and Ti in the produced MXenes.

Fig.7.XRD characterization of(a)Ti2AlC MAX phase and,(b)Ti2CTx MXene before washing with 0.5 mol·L-1 HCl.Adapted with permission from Ref.[49],(c)XPS survey of the Ti3AlC2 MAX phase and MXene samples.Adapted with permission from Ref.[50].

To further enhance the operating safety,milder bifluoride etchants that are solids at room temperature (such as NH4HF2,NaHF2,and KHF2) can be used as an alternative to HF [51,52] and can be integrated with various fluoride salts (such as LiF,NaF,KF,NH4F)to etch Al from Ti3AlC2[40].Other combinations of acids (H2SO4)and fluorine salts have also been used to synthesize MXenes.If the etchant becomes less acidic,the reaction should be associated with a much higher etching temperature [18].Another drawback of this type of etching is the abundance of fluoride ions on the surface of MXene,which decreases the amount of other functional groups,such as hydroxyl and oxygen ions.In addition,considering the difficulty in post-treatment of the fluoride-containing waste solution,fluorine-free etching methods have also raised great attention in recent years to produce MXenes with controllable functional surface termination [16,51].The quality of produced MXene nanosheets depends on many parameters,such as etching time and temperature.Khoslaet al.[53] investigated the effect of Ti3C2TxMXenes etched at elevated temperatures,and they found that higher temperatures led to higher surface area and specific capacitance.However,a further increase in temperature reduced the specific capacitance due to degradation.Tanget al.[54] also studied the effect of etching time on Ti3C2and found that a longer etching time can result in better electrochemical properties,as more carbon atoms are exposed when the etching time increases.Table 1 briefly summarises the crucial parameters of produced MXene nanosheets in two previous studies.

Table 1 Effect of different parameters on the quality of MXene

2.2.Bottom-up approach

Numerous bottom-up synthesis techniques have been developed,including chemical vapor deposition (CVD),the template approach,and plasma-enhanced pulsed laser deposition (PEPLD).Small organic or inorganic molecules or atoms are the usual starting point for bottom-up synthesis,after which crystal growth can be organized to build a 2D-ordered layer.Materials made using bottom-up techniques,especially chemical vapor deposition(CVD),have a superior crystalline quality to those made by selective etching [16,40].CVD is a conventional method to obtain 2D structures such as graphene.However,it is not popular in the production of MXenes because instead of producing single-layer MXenes,it only creates very thin films that are often multilayered(at least six layers) [16,56].MXenes without surface functional groups,such as Mo2C,WC,and TaC,have been successfully synthesized using this method.As the unterminated metal atoms are highly reactive,these MXenes are liable to combine with other substances and thus are great candidates to be used as adsorbents[13].The other method is the template method.The yields from the template method are significantly greater than those from CVD techniques.All of the template approaches begin with 2D transition metal oxide (TMO) nanosheets,which are then carbonized or nitrided to produce 2D transition metal carbides(TMCs)or transition metal nitrides (TMNs).There was also some research into using plasma-enhanced pulsed laser deposition (PELPD) to create MXene [57].Methane plasma was the carbon source for the first ultrathin Mo2C films produced by pulsed laser deposition (PELPD)[16].

In both methods,the safe procedure for large-scale production of MXene should be further investigated.Table 2 brings the advantages and disadvantages of different synthesis methods of MXene[33].

Table 2 Advantages and disadvantages of different synthesis methods

3.Environmental Application of MXene

Environmental contamination is burgeoning and becoming a severe problem in different parts of the world;thus,various treatment methods have been used to tackle the different environmental pollution [2-5].Adsorption is one of the most important methods which has gained attention.It is simple,cost-effective,and easy to operate.For better adsorption,adsorbents must have a large specific surface area.Plenty of porous materials have been developed as adsorbents for environmental pollutants,such as activated carbon and kaolinite.MXene,a new type of adsorbent regarding 2D transition metal carbide/nitride materials,has been developed due to its specific structures,high mechanical flexibility,selective permeability(for instance,MXene-polymer hybrid membrane) [53] and unique properties for removing environmental pollutants [3,28,58].

3.1.Gaseous pollutants removal

One of the most significant environmental issues is air pollution which is becoming more critical [18].Gaseous contamination mainly consists of toxic compounds such as (NOx,SOx,H2S,NH3,and CO)and volatile organic compounds(VOCs)that can cause various human diseases.Therefore,researchers have investigated removing gaseous contamination and greenhouse gases and CO2capturing by adsorption using MXenes [58].

3.1.1.CO2 capture

One of the leading causes of the greenhouse effect and climate change is high carbon dioxide emissions[59].Scientists use carbon capture utilization (CCU) technology to deal with this issue because it is easy to convert it into high-value chemical products[60].Several materials have been used for this purpose;however,in recent years,many efforts have been made to use MXenes and MXene-based materials for CO2capture,reduction and conversion[60-70].For instance,Wanget al.[62]worked on the adsorption of CO2on Ti3C2Txand V2CTxMXene.The results showed that both Ti3-C2Txand V2CTxMXene were intercalated with dimethylsulfoxide,increasing specific surface area (SSA).It was shown that intercalated Ti3C2Txwith a SSA of 66 m2·g-1had a capacity of 5.79 mmol·g-1,almost equal to many common sorbents.The volume capacity for intercalated Ti3C2Txwas 502 ml·ml-1,which is almost higher than many other sorbents.

Morales-Garciaet al.[70]studied the effect of the MXene thickness on CO2adsorption.They investigated a group of MXenes with M2C,M3C2,and M4C3.It was reported that carbide MXenes composed of d2metals(M=Ti,Zr,and Hf)have the most considerable adsorption energy,which is less than 3.0 eV and followed by d3(M=V,Nb,and Ta) with adsorption energies around 2.5 eV,and lastly followed by d4(M=Cr and Mo) metal MXenes,which has the lowest adsorption with values smaller than 1.7 eV.

3.1.2.CO2 reduction and conversion

Another benefit of MXenes material is their ability for CO2reduction and conversion into valuable chemicals [67,69,71-80].Handokoet al.[71] worked on CO2reduction reaction (CO2RR) on O-terminated M2XO2type MXenes,where M was one of Sc,Ti,Zr,Hf,V,Nb,Ta,Cr,Mo,W;and X was C or N.They found that two MXenes,W2CO2and Ti2CO2,are suitable electrocatalysts for reducing CO2into CH4.These two MXenes are promising due to their low over-potential and good selectivity.This reaction can represent the CO2reduction:CO2+8H+8e-→CH4+2H2O.To put it differently,this conversion requires eight electrons/protons to be transferred.Fig.8 illustrates a possible pathway for converting CO2to CH4on the Ti2CO2surface.For instance,at first,when CO2is adsorbed on the surface,the incoming H is attached to the O terminal of CO2,producing an intermediate component called*COOH,which is bonded to Ti2CO2through the C atom [72].Two options were investigated for the second hydrogenation stage.The first iscommon in transition metal catalysts,where entering hydrogen binds to the-OH side of the*COOH to give*CO while also releasing water.The high reaction energy (G=1.73 eV) indicates that this route is unfavourable on the surface of Ti2CO2and is unlikely to succeed.A second alternative,in which*HCOOH is generated by attaching incoming hydrogen to the C atom of*COOH,was discovered to be substantially more favourable.Following the*HCOOH pathway,the lowest energy path leading to CH4synthesis on Ti2-CO2is:CO2(g)→*COOH →*HCOOH →*CHO →*H2CO →*CH2OH→*HOCH3→*CH3→CH4(g).

Fig.8.A possible pathway for converting CO2 to CH4 on the Ti2CO2 surface: (a) energy diagram,(b) ball and stick model.Adapted from Ref.[72].

The binding of the intermediates on the O-terminated MXene surface alternates between C and H coordination:*COOH,*CHO,*CH2OH,and*CH3bind to the MXene surface through the C atom,whereas*HCOOH,*H2CO,and*HOCH3connect to the MXene surface through the H atom.

Liet al.[69] investigated on MXenes with formulas M3C2as a CO2conversion catalyst.It was shown that MXenes from groups IV to VI could capture CO2.It was also indicated that Cr3C2and Mo3C2MXenes are suitable for converting CO2to CH4.It has been shown that 1.05 and 1.31 eV is required to convert CO2into CH4using Cr3C2and Mo3C2MXenes,respectively.It is also possible to reduce the energy consumption to 0.35 and 0.54 eV when the surface of Mxenes is terminated with —O or —OH,respectively.For instance,Fig.9 shows the minimum energy required for converting CO2to CH4and H2O on the Cr3C2surface,along with the intermediate product and transition states (TS).Gibbs free energies for reaction (red line) and activation (orange line) are expressed in eV.Different shades represent spontaneous(blue)and nonspontaneous (red) reactions and the activation barrier for each reaction(orange).The primary energy demand is related to the regeneration of a bare surface from OH termination.

Fig.9.Minimum energy path for the CO2 conversion into *CH4 and **H2O catalyzed by Cr3C2.Adapted from Ref.[69].

For the first hydrogenation of CO2to generate the**OCHO·radical,much energy is required in a single process.When it comes to Cr3C2,the spontaneous reaction energy of 0.014 eV in the case of MXene is clear.Their Grimme’s popular DFT-D3 computations have also found the TSs and their structures and energy.Fig.9 shows that the first transition state (TS1),which connects CO2and**OCHO·,has an activation barrier of 0.38 eV.Cr3C2also has a smooth reaction profile throughout the CO2electrochemical conversion process.Although the activation barriers for TS1 and TS2 are low at 0.38 and 0.83,these transition states can lead to intermediate species such as OCHO and OCH2O.As an example,the electrochemical reduction of**·OCH2O· to obtain**HOCH2O· has a higher energy barrier in the case of TS3 (1.04 eV).As a result,H2O is released without an energy barrier from the hydrocarbon molecule,resulting in**H2CO.**H2CO is reduced electrochemically in TS4 to**CH3O·,which has similar behaviour to TS3 but is far more difficult to accomplish(1.01 eV).After the seventh h+/e-pair gain,the chemisorbed CH4must be released by injecting 1.05 eV.A flowing CO2atmosphere removes the CH4product through continual equilibration with the gas phase,which removes CH4over time.

3.1.3.Photocatalytic CO2 reduction

One other way for CO2reduction is to use MXenes for photocatalytic CO2reduction [67,74,75,77,79-83].Fig.10 illustrates the mechanism for solar energy conversion over 2D/2D MXenesbased photocatalysts under visible light irradiation.The 2D MXene platform functions as an electron mediator to develop the interlayer interaction of face-to-face heterostructures,accelerating the electron extraction from the 2D photocatalysts for photocatalytic reactions [84].

Fig.10.The mechanism of 2D/2D MXene-based photocatalyst for production of solar fuel.Adapted from Ref.[84].

The utilization of MXenes as a co-catalyst has been another research area in the past decade[75,85,86].Yeet al.[75]used Ti3-C2MXene to increase titania’s photocatalytic activity (P25) for CO2reduction in solar hydrocarbon fuels.Also,it was found that surface alkalinization of Ti3C2could augment the selectivity for CH4.Surface-alkalinization of Ti3C2to substitute—F with—OH also boosted the photocatalytic activity.The result shows that the evolution rates of CO (11.74 μmol·g-1·h-1) and CH4(16.61 μmol·g-1-·h-1) are three and 277 times higher than those of bare P25,respectively.High electrical conductivity,a high Ti3C2thermodynamic energy level,and abundant CO2adsorption can cause this remarkable increase.Furthermore,the surface -OH groups can act as active sites for strongly adsorbing and activating CO2molecules.

Another study was done to increase the photocatalytic CO2reduction and photoelectric detection of MXene nanosheets.Panet al.[74] grow CsPbBr3perovskite nanocrystals (NCs) on MXene nanosheets (CsPbBr3/MXene).This combination can be used as an active photocatalyst to reduce CO2to CO and CH4.In this study,they used various amounts of MXene for synthesis,named CsPbBr3/MXene-n(n=10,20,30,50),andnshows the composition of MXene.In this study,to test the feasibility of photocatalytic CO2reduction of CsPbBr3/MXene-20,the experiment was done under solar light illumination (300 W Xe lamp with a cut-off filter＞420 nm).In addition,ethyl acetate was used as the solvent due to the solubility of CO2in it.The results showed that CH4and CO were the primary products,and no H2was produced.Also,the findings show that CsPbBr3/MXene-20 nanocomposites can have high CO and CH4formation rates of 26.32 and 7.25 μmol·g-1·h-1,respectively.The conduction band offset,about 1.5 eV between the two components,moves the photogenerated excitons from CsPbBr3NCs and transfers the electron to the MXene surface,which in turn causes the conversion of CO2to CO and CH4.

Many other studies have been done on CO2adsorption,reduction,and conversion into valuable chemical and photocatalyst reduction of CO2using MXene and MXene-based material,summarized in Table 3.

Table 3 Progress on CO2 adsorption,reduction,conversion,and photocatalyst reduction of CO2,using MXenes and MXenes-based material in chronological order

3.1.4.Detection and removal of VOCs

VOCs are indoor air contaminants,and their crucial role in producing ground-level ozone and carcinogens can put human life at risk.Many VOCs are toxic and odorous;therefore,it is essential to develop a way to remove them[1,91,92].It was proved that photocatalysis could degrade VOCs such as toluene into other compounds such as CO2and H2O [93].MXene shows excellent properties in adsorbing gaseous contaminants [94] and VOC photodegradation [95].Many different materials with various sensing mechanisms are used as VOC sensors.The metal dichalcogenides use charge transfer to detect their surroundings,but functional groups also play a significant part in the detection process.Chemical or physical adsorption of gas molecules on sensing materials can affect the concentration of charge carriers in the local area,resulting in an increase or decrease in the charge transfer mechanism [96].

There are several studies on the removal and detection of VOCs by MXene-based materials.Guoet al.[97] usedab initiosimulations based on density functional theory to uncover an underlying sensing mechanism between MoC2Txand VOC.Selective detection for toluene is based on the interaction between the benzene rings in toluene molecules and MoC2Tx,which has a high affinity for the toluene ring.The benzene ring interacts more powerfully with MoC2Txand significantly reduces the number of charge carriers because of its better activity.Toluene has a more robust response than benzene because the methyl group enhances the activity of the benzene ring.Anab initiosimulation demonstrating the maximum adsorption energy between toluene and MoC2Txshows their strongest interaction.

Zhouet al.[92]investigated the effect of Z-scheme g-CN/Zr2CO2on VOC photocatalytic degradation.It illustrated that the photocatalytic mechanism of VOC degradation on g-CN/Zr2CO2is based on the migration of electrons from the valence band (VB) of the g-CN layer to the conduction band (CB) location when g-CN/Zr2CO2is exposed to sunshine.As illustrated in Fig.11,when g-CN/Zr2CO2is exposed to sunlight radiation,the electrons migrate from the VB of g-CN to the CB.Meanwhile,the electrons in the VB of Zr2CO2move to CB.Therefore,the accumulation of electrons at CB can generate radical oxygen,whereas,at the VB,the absence of electrons can induce OH to its radical forms.These radicals can attach VOC molecules to produce other intermediates,reactive oxygen species that could take part in converting VOCs to products such as CO2or H2O.A Z-scheme pathway was then used to combine electrons in the CB of the g-CN layers with VB holes in the Zr2CO2layer,resulting in an improved extraction and utilization of photogenerated electrons.

Fig.11.Schematic of the mechanism of VOC photocatalytic degradation on g-CN/Zr2CO2.Adapted from Ref.[92].

Since formaldehyde is a gas that can cause severe damage to humans,Zhanget al.[94] studied reducing indoor formaldehyde pollution.It was shown that O-terminated titanium carbide(Ti3C2-O2) nanosheets could adsorb formaldehyde with an adsorption capacity of more than 6 mmol·g-1.They found that weak interactions could raise optimal adsorption energy.Table 4 shows the progress in removing and detecting VOCs using MXenes-based material in chronological order.

Table 4 Progress on MXene application for removal of VOCs and its detections in chronological order

3.1.5.Removal of NOx

Nitrogen oxide (NOx) is one of the primary gaseous pollutants caused by burning fossil fuels,which can seriously affect human health and cause many environmental problems [102,103],such as causing acid rain and photochemical smog [104].Different biological,chemical,and physical methods have been proposed to overcome the issue of NOx.Amongst these,photocatalysis techniques are deemed adequate to purify NOx[105].Serval studies have been done on removing NOxusing MXene as a photocatalyst[102-106].

Wanget al.[102] used photocatalytic techniques to remove low-concentration NO in the ambient atmosphere.In this study,Nb2C MXene was used to prepare oxygen vacancy-rich Nb2O5nanorods/Nb2C visible-light photocatalysts (Ov-Nb2O5/Nb2C).It was shown that the optimized photocatalyst could remove NO,producing a low amount of toxic NO2.Several theories about the Ov-Nb2O5/Nb2C photocatalytic mechanism and energy band have been mentioned to explain the NO elimination.When exposed to visible light,the synthesized (0 0 1) facet-dominant pseudohexagonal (TT-Nb2O5) nanorods are stimulated to create electrons and holes.Oxygen-vacancy-rich TT-Nb2O5have increased light absorption in the visible spectrum.As shown in Fig.12,the photogene-rated electrons of Ov-Nb2O5/Nb2C move from the VB to the CB when irradiated.Their tight integration can transfer electrons more easily between the MXene and Nb2O5nanorods.Recombination of photogene-rated carriers can be further restricted by oxygen vacancies acting as electron traps and providing additional paths for electron transfer.Also,water molecules inthe feed gas cannot be converted to ·OH effectively;hence they only had a minimal impact on the NO conversion.Ov-Nb2O5/Nb2C has excellent NO removal properties because of the abundance of ·O2and the tremendous reaction interface and oxygen vacancies [102].

Fig.12.Schematic of the mechanism of NO photocatalytic removal by Nb2O5/Nb2C.Adapted from Ref.[102].

In another study for NO removal by Wanget al.[102],Ti3C2quantum dots (QDs) were incorporated into SiC to create a new heterojunction catalyst (TQDs/SiC).As the MXene QDs can serve as a channel for transporting electrons and holes,after exposure to visible light,the TQDs/SiC composite could remove 74.6% of the NO pollutant,which is 3.1 and 3.7 times greater than the pure Ti3C2QDs and SiC,respectively.Compared to theEVBof/NO(0.96 eV),theEVBof TQDs/SiC was 1.21 eV,showing that the photogenerated holes in TQDs/SiC can function efficiently in oxidizing NO.Also,because the VB level was lower than the OH-/OH redox potential (1.99 eVvsNHE),holes could not directly oxidize OHto OH free radicals.The photocatalytic mechanism of the composite(Fig.13)shows that photoelectrons can be transported from the VB of SiC to activate O2to form,which assists in eliminating NO fromand directly oxidizes NO.This radical could likewise be converted into an OH radical that might be involved in removing NO [103].Table 5 reveals various MXene utilised in the removal of NOx.

Table 5 Progress on removing NOx using MXene in chronological order

Fig.13.Schematic of the mechanism of NO photocatalytic degradation by TQDs/SiC.Adapted from Ref.[103].

3.2.Heavy metals removal

Water scarcity is becoming a prevalent issue worldwide.The situation worsens as industrial toxic waste containing heavy metals,such as As,Zn,Cu,Pb,Hg,Cr,Ni,and Cd,is released into the surrounding environment.These poisonous heavy metals are non-degradable,can easily leak into the soil and fresh water,and are finally absorbed by humans,resulting in chronic health problems such as cancer [107].Several efforts have been made to remove heavy metal ions from water.Various techniques,such as electrochemical,membrane,bioremediation,and adsorption,have been developed to remove these serious containments.However,in recent years,adsorption treatment gained much attention due to its effectiveness and affordability.MXene has captured scientists’ attention as an ideal adsorbent due to its large surface area,abundant active surface,increased hydrophilicity,high conductivity,changing band gaps,and robust electrochemistry [108-110].2D MXenes have an enormous surface area and abundant fluorine(-F),hydroxyl (OH),and other oxygen-containing groups.These groups provide numerous reaction sites on the MXene surface,which enable the interaction and binding of metal cations on the surface [111].

3.2.1.Removal of Cr(VI)

Hexavalent chromium (Cr (VI)) pollution poses severe risks to human beings and the ecosystem due to its high toxicity;therefore,it is critical to treat Cr(VI)contamination[112].The negative surface charge of MXenes restricts their application in the adsorption of the anionic pollutant;however,several studies were done by modifying MXenes to remove Cr (VI).Konget al.[113] developed amino-functionalized MXenes (NH2-Ti3C2Tx) for the efficient removal of Cr (VI) ions in an aqueous solution.This work showed that the modified MXenes have exceptional removal capacity under an acidic environment due to the introduction of the positively charged group (-NH2) and proper surface area.Therefore,the NH2-Ti3C2Txnanosheets have a maximum adsorption capacity of 107.4 mg·g-1,calculated by the Langmuir model.However,a further increase in the content of (3-aminopropyl)triethoxysilane(APTES),as the positively charged group,leads to lower adsorption capacity.This is primarily due to a dramatic fall in the specific surface area of NH2-Ti3C2Tx(8.6 m2·g-1).The adsorption mechanism of NH2-Ti3C2Txnanosheets for Cr (VI) is in three stages: first,Cr (VI) primarily binds to amino groups (——/——NH2).Then adjacent electron donors (Ti and N species) rapidly reduced Cr(VI) to Cr (III),whereas Ti (II) and -species oxidized to Ti(IV) and NO3,respectively.Finally,Cr (VI) and Cr (III) ions adsorb on the surface of NH2-Ti3C2Txnanosheets.

In another study by Fenget al.[114],MXenes/PEI-modified sodium alginate aerogel (MPA) was prepared by introducing polyethyleneimine(PEI)and amino-functionalized Ti3C2Txinto sodium alginate through cross-linking reactions.Due to the abundant active groups of PEI,MXenes/PEI has a dramatic removal capacity of Cr(VI)ions.The adsorption mechanism of MXenes/PET is utterly similar to Konget al.[113] study.However,there is a significant difference in adsorption capacity,which can be attributed to the MPA’s three-dimensional network structure,with interconnected skeleton structures forming numerous pores ranging in size from nanometers to micrometers.Such physically and chemically cross-linked networks benefit the exposure of various binding sites and the mass transfer of Cr (VI),enhancing the pollutant removal process.

On the other hand,the effect of pH on adsorption was investigated because it is an essential factor in the Cr(VI)removal process based on its influences on the surface electric charge of MPA and the existing forms of Cr (VI) ions.Multiple Cr (VI) forms generally exist based on the pH range in an aqueous solution.andexist primarily in the pH range of 2.0-6.0,whereas H2CrO4exists at pH ＜1.0 andpredominates when pH exceeds 6.0.In the study,MPA was positively charged as a dipolar molecule at a wide pH range because its numerous amine groups were rapidly protonated.The Cr(VI)adsorption capacity increased gradually as the pH values decreased from 8.0 to 2.0.It might be rationally stated that as the number of protonated amino groups increased,negatively charged Cr (VI) species binding to positively charged MPA got easier,owing to the stronger electrostatic attraction [114].

Although MXenes have a sufficient adsorption capacity and stability for heavy metal ions,the lack of active sites on the surface or interlayer of MXenes limits their application in heavy metal removal.In addition,the short interlayer spacing prevents heavy metal ion diffusion and lowers the adsorptive capacity.Heet al.[115] overcame these issues by introducing nanoscale zerovalent iron (nZVI) into the interlayer structure of alkaline intercalated Ti3C2(Alk-Ti3C2).Fig.14 illustrates the preparation steps and configuration of modified MXenes with and without alkalization by adding KOH.It describes the insertion of OH groups in MXenes,which increased the interlayer gap of MXenes and provided active sites for heavy metal adsorption.However,these OH groups are exclusively active for metal cations that form metal-O-Ti bonds(coordination bonds).They are inactive with metal-oxo anions (e.g.,,) and even have a negative effect on the adsorption.This study found that the nZVI-Alk-Ti3C2has the highest adsorption capacity compared to Alk-Ti3C2,nZVI-Ti3C2and nZVI mixed with Alk-Ti3C2mainly because the treated nanocomposite poses more interlayer space and better dispersion of nZVI,resulting in improved uptake capacity of Cr (VI).The impact of pH on Cr (VI) adsorption revealed that since the nZVI oxides on the surface are eroded by protons and reveal more active Fe0species,the nZVI-Alk-Ti3C2system provides the most efficient Cr (VI)adsorption performance,particularly in acidic conditions.Therefore,the highest adsorption capacity can be 194.87 mg·g-1,which is the top among nZVI-based adsorbents.

Fig.14.Steps to synthesize Alk-Ti3C2,nZVI-Ti3C2,and nZVI-Alk-Ti3C2.Adapted from Ref.[115].

The mechanism of Cr (VI) uptake by nZVI-Alk-Ti3C2is rationalized in two ways:the Fe0species in the nZVI converts Cr(VI)to Cr(III),while the Fe0species are oxidized,and the Fe-O-Cr(III)species are produced.At the same time,positively charged H+or Fen+species coordinated to the OH group readily adsorbed negatively charged Cr(VI) on the Ti3C2surface.The H+species are produced by the acid solution,while the Fen+species are produced and released by the oxidation of nZVI.Following that,the synergistic effects of nZVI and Alk-Ti3C2nanosheets in nZVI-Alk-Ti3C2composites for Cr (VI) adsorption have been extensively established.On the one hand,adding nZVI to the Alk-Ti3C2increases interlayer distance,exposing additional active sites for Cr(VI)adsorption.On the other hand,Alk-Ti3C2is an excellent support that prevents nZVI aggregation [115].

3.2.2.Removal of Pb(II)

With the growth of printed circuit board manufacturers,metal plating plants and battery storage industries worldwide,lead (Pb(II)) contamination is becoming prevalent in the environment as most industries release vast amounts of toxic wastewater into the river.Pb(II)can be absorbed and accumulated in living organisms and livestock,posing a real threat to the ecosystem and food chain.Pb(II)contamination can cause fatal dames to the brain,kidneys,circulatory and nervous systems.Hence,treating contaminated water with Pb (II) ions is indispensable [116-118].Among several strategies adapted to remove Pb (II) from wastewater,MXene has been widely used in numerous studies with modifications to improve its adsorption capacity for Pb (II) removal.

Junet al.[119] compared Ti3C2Txand powder-activated carbon(PAC) in the adsorption treatment of solution contaminated by some heavy metals.The MXene surface area was 50 times less than PAC;however,Ti3C2Txshows relatively good adsorption capacity mainly because of the high negative surface charge of the MXene.It was found that Ti3C2Txadsorb heavy metals in the order of Pb(II) ＞Cu(II) ＞Zn (II) ＞Cd (II),which is attributed to the difference in metallic electronegativities.The adsorbent reached equilibrium after 30 min,and the adsorption capacity for Pb (II) was 36.6 mg·g-1.Donget al.[120] modified Ti3C2Txby cross-linking with alginate for Pb(II)and Cu(II)ions removal from wastewater.The Ti3C2Tx/alginate composite had the maximum adsorption capacity of 382.7 mg·g-1for Pb(II)and 87.7 mg·g-1for Cu(II),with an adsorption equilibrium time of 15 minutes.Suitable adsorption capacity,short transportation time and independency of temperature made this adsorbent more preferable.Based on the authors’findings,ion exchange and chemical coordination are possible adsorption mechanisms.

Zhanget al.[121] studied a different type of MXene modification by alkalization-grafting and designing a high-quality aminofunctional Ti3C2Txnanosheet (Alk-MXene-NH2).In this study,surface modification can increase the uptake capacity to 384.63 mg·g-1with an equilibrium time of 20 min.The suitable adsorption was explained by rising specific surface area and interlayer spacing due to alkalization-grafting modification,which led to van der Waals and electrostatic interaction with pollutants.Furthermore,as illustrated in Fig.15,Alk-MXene-NH2contained an enormous number of functional groups (—OH,—ONa and —NH2),which through complexing,have high adsorption capacity for Pb(II).

Fig.15.Possible adsorption mechanism for Alk-Mxene-NH2.Adapted from Ref.[121].

3.2.3.Removal of Cu(II)

Copper ion(Cu(II))is another toxic metal that must be removed from wastewater and potable water.This contaminant results from toxic wastewater from electronic manufacturers,pharmaceutical production,paper plant,fertilizer producers,and mining industries[122] and has detrimental effects on human organs.Excessive intake of Cu(II)has been linked to severe diseases such as gastrointestinal disorders,liver and kidney damage,Alzheimer’s,Menkes’s,and Wilson’s disease,jeopardizing the quality of life [123].Hence,treating water contaminated with Cu (II) is indispensable.The allowable limit of Cu (II) ions in drinkable water is 1.3 mg·L-1[124].Several techniques have been used to remove copper ions,but in recent years,MXene-based adsorbent is becoming ubiquitous in removing Cu (II) ions.

In 2017,Shahzadet al.[125]investigated the performance of 2D Ti3C2Txnanosheets in removing Cu (II) ions.Researchers used delaminated Ti3C2Tx(DL-Ti3C2Tx) to remove Cu (II) ions as this novel MXene poses large surface areas,a unique surface function group and high hydrophilicity.These properties enabled DLTi3C2Txto remove approximately 80% total content of the metal ions within 1 min and achieve a maximum adsorption capacity of 78.45 mg·g-1within 3 min.Ion exchange reactions between positively charged Cu (II) ions and negatively charged oxygenated surface functional groups (O and OH) on the DL-Ti3C2Txsurface could lead to Cu adsorption on the Ti3C2Txsurface,producing Cu2O and CuO species.

MXene-based polymeric component (Ti3C2TX-PDOPA) can be fabricated to enhance the adsorption capacity for Cu (II) ion removal [126].As illustrated in Fig.16,MXene powders (Ti3C2TX)and levodopa(DOPA)solution was mixed at pH=8.5 to synthesize Ti3C2TX-PDOPA.Through this reaction,DOPA is attached to the surface layer of MXeneviaself-polymerization.As a result,numerous carboxyl groups can be introduced during the polymerization,which aids the adsorption process through electrostatic attraction.This study found that the adsorption of Cu(II)on Ti3C2TX-PDOPA is highly contingent on solution pH.The carboxyl and amine groups on the surface layer of modified MXene may be protonated at low pH,forming electrostatic repulsion to Cu(II)ions and reducing the adsorption capacity.

Fig.16.The synthesis of Ti3C2Tx-PDOPA.Adapted from Ref.[126].

3.2.4.Removal of Hg(II)

Mercury(Hg(II))is another toxic environmental metal that can be released from various sources such as gold industries,coal combustion,chlorine manufacturing,cement production,and waste disposal[127-128].Since Hg(II)ions do not degrade into harmless products,they can be accumulated in the ecosystem.Once it enters the food chain,it accumulates in humans and animals,leading to devastating health effects [129].Studies show that changes in the nervous system,headaches,hearing and cognitive loss,and dysarthria can be attributed to exposure to a high dosage of mercury.Even at low doses,it can affect endothelial and cardiovascular function [130].Several methods have been adapted to remove Hg(II) ions from aqueous solutions while using MXene as an adsorbent has gained attention in recent years.

In 2018,Shahzadet al.[131]worked on capturing Hg(II)ions by producing recoverable titanium carbide magnetic nanocomposite(MGMX).The adsorbents’ magnetic characteristics make separating from the solution easy after the adsorption,which means that nanomaterials will be kept out of the environment and is accessible for regeneration and repurposing.In addition,because MXenes tend to oxidize in an oxic environment,modification with magnetic properties can aid in stabilizing the composite oxidation when the substance is exposed to oxygenated conditions.This study’s maximum adsorption capacity was 1128.41 mg·g-1with an equilibrium time of 300 min.This high adsorption capacity is attributed to oxygen-containing groups on the composite surface,as denoted in Fig.17.High hydrophilicity,large specific surface area,excellent recyclability and ease of interaction with pollutant ions make MGMX nanocomposite highly favourable for environmental application.

Fig.17.MGMX structure.Adapted from Ref.[131].

In 2019,the same team produced Ti3C2Txcore-shell aerogel spheres (MX-SA) to remove Hg (II) ions [132].Due to its oxygenated functional groups,active binding site and the sizeable specific area,the MX-SA sphere shows a remarkable adsorption capacity.The equilibrium time for MX-SA is around 120 min,nearly three times less than their previous nanocomposite(MGMX).On the other hand,MGMX had low removal efficiency(18.70%) at pH=2,while MX-SA spheres showed almost high removal efficiency over all tested pH ranges.The solid ionic repulsion could explain the high removal efficiency over all tested pH ranges between MGMX nanocomposite and cationic Mg (II) ions and the pH-independency of MX-SA.Several adsorption mechanisms were involved in the Hg(II)removal by MX-SA.The sphere’s porous shape of the adsorbent was the only essential physical characteristic in capturing the metal ions during adsorption.An ion exchange mechanism by Ca (II) and Hg (II),[Ti-O]-H+groups inner-surface complexation with Hg (II),as well as their metal-ligand solid interaction,are the other main uptake mechanisms.In addition,alginate oxygen-containing groups and alkanes had electrostatic binding with Hg (II).

In 2021,low-dimensional Ti3C2Txand Ti3CNTxwere designed with an exceptional adsorption capacity of 5473 mg·g-1and 4606 mg·g-1,respectively [133].Ti3C2Txnanosheets had a higher adsorption capacity because of many active sites on Ti3C2Txand a well-2D layered structure.The HF-etching of parent Ti3AlC2and Ti3AlCN MAX phases was used to prepare Ti3C2Txand Ti3CNTx.Noteworthy is the fact that this novel MXene is pH-independent and can work well in acidic and alkaline conditions.

Organic and inorganic pollutants can be found in wastewater from hydraulic fracking.Inorganic pollutants can be radioactive nucleoids such as barium Ba (II) and strontium Sr (II).These two ions have a devastating effect on human well-being.Ba (II) may cause a heart attack,and Sr (II) might lead to oxygen shortage[134,135].MXene can also be used for these contaminants [136].More details about MXene for heavy metal removal containing kinetic and adsorption conditions such as pH and temperature are mentioned in Table 6.

Table 6 Progress on heavy metal removal in chronological order based on types of heavy metal

3.3.Application of MXene for dye removal

Another practical application of MXene is treating water contaminated with dye and pharmaceutical compounds from industrial and agricultural activities such as textile plants.Effluents released from the textile industry are coloured with high pH,COD,and BOD values can result in the death of aquatic life[149].Due to the importance of removing dyes,several strategies such as coagulation-flocculation,biological,adsorption,membrane,ozonation,and advanced oxidation processes have been adapted to remove dyes from aqueous solution [150].MXene-based composites can be a valuable material for removing these pollutants because of their high porosity,metallic conductivity,hydrophilicity,and large specific area [151].

There are many researchers on the usage of MXene in dye adsorption [152-157].For instance,Nb2CTxwas fabricated by Yanet al.[156] to remove the cationic dye,methylene blue (MB),and the anionic dye,methyl orange (MO).The result showed that the Nb2CTxcould remove MO dye up to 99%in less than 30 minutes for the initial concentration of 100 and 200 mg·L-1.

MPA composite,mentioned in Cr (VI) section,was used to absorb Congo red (CR) from an aqueous solution [114].Fenget al.found that MPA has an adsorption capacity of 3568 mg·g-1toward CR.pH is an essential parameter for efficient removal.Under acidic conditions,CR exists in cationic form,whereas,under basic conditions,it is in aniconic form.It was found that pH=3 was the optimum pH for CR removal.As for the adsorption mechanism of CR,there were two driving forces for the CR adsorption on MPA:firstly,CR,an anion dye withpolar groups,was quickly attracted by the positively charged MPA through electrostatic interaction.Besides,many hydrogen bonds would develop between the MPA and CR,favouring removing CR.This process might be further verified by the fact that MPA showed strong adsorption ability towards another anionic dye (MO).

Another viable solution for removing dye is utilizing a photocatalyst.Photocatalytic degradation is affordable and highly efficient,and some researchers investigated it [158-162].Many semiconductor nanomaterials can be used as photocatalysts to degrade organic contaminants in wastewater.These nanomaterials (such as TiO2) can be coupled with carbon-based materials (such as MXene)to ameliorate photo activity performance[159].For example,Ti3C2Txdecorated with silver and palladium nanoparticles has been successfully utilized in the photocatalytic degradation of MB and rhodamine B (RhB).This study found that AgNPs/TiO2/Ti3C2Txpossessed higher degradation efficiencies toward MB and RhB.It can also reduce 23% of total organic carbon in the effluent [158].

The typical photodegradation mechanism of organic molecules involves redox reactions with the formation of free radicals,primarily by scavenging photoinduced electrons by O2molecules to form O2·-anion radicals or by oxidizing hydroxyl groups and water molecules with photogenerated holes to form OH· radicals.The suggested degradation process incorporates charge transfer methods.As shown in Fig.18,Photogenerated electrons travel from TiO′2s CB to its VB.Photoinduced electrons can migrate to Ti3C2Txsheets over the TiO2/Ti3C2TxSchottky barrier,leaving holes in TiO′2s VB.This method increases photogenerated electrons’ lifespan by minimizing recombination.Photogenerated electrons travel from TiO2to MXene,an electron reservoir.· radicals occur ons electron-rich reactive sites.Photogenerated holes can oxidize TiO2hydroxyl groups,surrounding water molecules,possibly MXene,and OH groups to create OH·radicals[156].Table 7 shows the progress on dye removal using MXene in chronological order.

Table 7 Progress on dye removal in chronological order based on the types of dyes.

Fig.18.Potential mechanisms of the photoinduced charge-transfer process of Ti3C2Tx MXene modified with metal oxides.Adapted from Ref.[156].

4.Challenges of Using MXene

Despite lots of benefits that MXene offers,they have some limitations.Transparent,flexible,compact,and low-cost devices with increased electrochemical performance and mechanical stability still require further development.In addition,a great deal of work has been done on Ti-based MXenes,but less work has been done on non-Ti-based,double-M,and double-X MXenes for supercapacitor applications.Compared to mono-Ti-based MXenes,it is possible that their properties may improve and be better.

In addition,one main disadvantage is associated with the MXenes synthesis method.Even though many synthetic techniques have been established,most MXenes are still prepared using hazardous strong acids like HF,impairing electrochemical performance due to fluorine termination.As a result,researchers are looking for green etchants that may produce MXene at a lower cost and in a shorter period without sacrificing their quality.Molten salt etching and electrochemical etching can circumvent toxicity issues and provide the opportunity for commercial-scale production compared to more hazardous HF or fluoric salt etching procedures[169,170].It is crucial for researchers to investigate further the toxicity,biosafety,cytotoxicity,and biocompatibility of MXenes;their storage conditions,life cycle evaluations,and assessments of possible hazardous impacts of these materials are still required [171].

Low MXenes production yields and the associated high cost in production have significantly restricted the development and application of MXenes.Small-scale production of MXenes is currently only possible in the laboratory [172].We can increase our understanding of MXenes’ commercial potential if we can design a more efficient,environmentally friendly,and cost-effective system for large-scale manufacturing.For large-scale production,it is envisaged that the cost will be cheap.As an adsorbent,MXenes must be tested in continuous-running systems [173].Life cycle studies and toxicological assessments of MXene-based nanomaterials are also critical challenges.A recent study indicated the biocompatibility of Ti3C2Txpossible toxicity [173-175].

Another issue about MXene in environmental remediation technologies,MXene stability,is a stumbling block.The oxidation processes in various solvents and the resultant MXene lattice shift must be thoroughly investigated to be understood.In addition,the atomic-level approach to research will help examine how doping MXenes with various metals and metal oxides affects their electronic structure,which will lead to more efficient photocatalysts [176].

Effective removal of pollutants from water by MXenes may be hampered by its volatility in more complicated environments.Examining their behaviour in complicated multicomponent waters is essential to evaluate the environmental performance of MXenes fully.In order to better understand how MXenes can be used in wastewater and freshwater treatment,a thorough investigation of the roles of pH and dissolved organic matter is necessary.Aside from that,an experiment involving the continual removal of MXenes from columns must be evaluated.In the case of MXenes,recycling and regeneration must be examined in light of their ability to be reduced [172].

In the end,it is worthwhile to mention that machine learning will be adapted to help predict the characteristic of the MXenebased composite and can significantly assist in having better performance MXene material.Future research papers on MXene can circle using machine learning for MXene characteristics and performance prediction and maybe optimization of its structure [18].

5.Conclusions

This study outlines current advancements in the synthesis,characteristics,and environmental applications of MXenes and MXene-based composites,from removing gaseous contaminants to removing heavy metal and toxic pharmaceutical substances from water and wastewater.This review showed that MXenes and related composites are well suited for water and wastewater remediation,such as removing heavy metals and using them as photocatalysts for dye removal.In addition,due to their particular morphologies and unique characteristics,they can be used for the photocatalytic reduction of CO2and the conversion of CO2.Due to their wide environmental application,MXene could replace standard precious metal catalysts in the future.In addition,the findings show that MXene is gaining more interest from researchers and are now used in the membrane process to enhance separation performance.To date,titanium carbide/carbonitride MXenes or Ti3C2Txis a popular MXene suitable for various environmental applications.More MXene could be explored in the future to enhance its performance in environmental applications while having lower production costs and shorter production duration.

Data Availability

No data was used for the research described in the article.

Declaration of Competing Interest

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

Acknowledgements

This paper was made possible by FRGS/1/2021/TK0/UKM/02/41 grant from Universiti Kebangsaan Malaysia.The statements made herein are solely the responsibility of the authors.