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A Review on MXene and Its Applications in Environmental Remediation

Yanan Han , Jiahui Hong , Anrui Zhang , Ruoxuan Guo , Kexin Lin , Yuejie Ai

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1229 -1244.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1229-1244 DOI: 10.7536/PC210614
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A Review on MXene and Its Applications in Environmental Remediation
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Abstract

MXenes is advanced two-dimensional inorganic compound materials that consist of transition metal carbides, nitrides or carbonitrides at a few atomic thicknesses, which are widely used in energy, optics, catalysis, adsorption and other fields. Due to the high hydrophilicity, large specific surface area, negative surface charge and strong ion exchange capability, it is considered an excellent adsorption material. Based on the excellent adsorption performance of MXene, this material has significant potential for environmental remediation. MXene is expected to become an ideal carrier for heavy metal ions and radionuclides through electrostatic attraction and ligand chelation. The full text mainly reviews the application of MXene as an adsorbent in the field of environmental remediation. The structure and synthesis method of MXene, and the application of MXene for extracting heavy metal ions (such as chromium (Cr), mercury (Hg), lead (Pb), nickel (Ni)) and radionuclides (such as uranium (U), cesium (Cs), europium (Eu), barium (Ba) and strontium (Sr)), etc. are reviewed systematically. In particular, the interactive mechanism of different MXene systems is discussed and highlighted. In addition, the existing challenges of MXene materials for environmental remediation and its future development are presented for a prospect.

Contents

1 Introduction

2 Structure and synthesis method of MXenes

2.1 Structure of MXenes

2.2 Synthesis of MXenes

3 Removal of heavy metal ions with MXenes

3.1 Pb(Ⅱ)

3.2 Hg(Ⅱ)

3.3 Cr(Ⅵ)

3.4 Ni(Ⅱ)

4 Radionuclide removal with MXenes

4.1 U(Ⅵ)

4.2 Eu(Ⅲ)

4.3 Cs(Ⅰ)

4.4 Ba(Ⅱ)and Sr(Ⅱ)

5 Conclusion and outlook

Graphical abstract

Keywords

MXene / two-dimensional materials / heavy metals / radionuclides / adsorbent

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Yanan Han, Jiahui Hong, Anrui Zhang, Ruoxuan Guo, Kexin Lin, Yuejie Ai. A Review on MXene and Its Applications in Environmental Remediation. Progress in Chemistry, 2022, 34(5): 1229-1244 DOI:10.7536/PC210614

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1 Introduction

In recent years, industrialization has accelerated mining, metallurgy and other industrial activities and produced a large amount of industrial wastewater, which contains a large number of metal ions (such as Pb2+, Cr2+, Hg2+, Ni2+, etc.) That can lead to water or soil deterioration. In addition, with the rapid development of modern industry, human demand for energy continues to grow, and the over-exploitation of traditional fuels such as coal and oil makes people's demand for new energy more urgent[1]. Although nuclear energy is a kind of clean energy with mature technology, the treatment and disposal of nuclear waste and the leakage of radionuclides cause the risk of radioactive pollution, such as the increase of nuclear leakage accidents, nuclear weapon tests, improper disposal of nuclear waste and so on[2][3]. Therefore, it is very important to efficiently remove heavy metal ions and radioactive elements from the environment, which has become a hot issue in the field of environmental remediation.
Many methods, such as membrane treatment, biological treatment, electrochemical treatment and oxygen chemical treatment, have been rapidly developed and applied to the removal of heavy metals or radioactive elements[4][5][6][7]. Among them, the adsorption method is favored by scientists because of its high efficiency, simple operation, low cost, and green safety[8]. Organic materials, such as functionalized graphene, nanoporous polymers and metal-organic frameworks, have the advantages of good selectivity and high adsorption capacity, but the chemical structure stability of these materials is poor[9][10][11]. Inorganic materials, such as natural clay minerals, zeolites and metal oxide/hydroxide nanomaterials, have the characteristics of simple preparation, low cost and good stability, but their efficient enrichment performance is deficient due to the lack of active sites[12][13][14][2]. Therefore, it is a great challenge to develop new inorganic nanomaterials with high adsorption capacity, good selectivity and good stability to remove toxic heavy metal ions or radionuclides.
MXene materials are a new class of graphene-like two-dimensional materials developed in recent years, which are composed of layered transition metal carbides, nitrides or carbonitrides. MXene materials are etched from MAX (M n + 1 (AXn) materials, and are named MXene because of their similar lamellar structure to Graphene. The chemical formula of MXene is expressed as M n + 1 XnTx, where M represents a transition metal element (such as Ti, Cr, Sc, V, Y, Nb, Zr, Mo, Hf, etc., Fig. 1), X represents a carbon or nitrogen element, n is 1, 2, 3, and represents three common structures :M2XTx, M3X2Tx, and M4X3Tx. T represents the surface groups (such as — O, — F, — OH, etc.) introduced after etching, and X in the Tx represents the number of surface groups[15].
MXene materials are usually prepared by selective etching of the "A" layer in ternary layered cermet materials (MAX, M n + 1 AXn), where A represents a main group III or IV element (Fig. 1). The MAX space group is P63/mmc, which belongs to the hexagonal crystal structure (Fig. 2), and covalent, ionic and metallic bonds coexist in the structure. Among them, the bond with high bonding strength is M — X bond, mainly covalent bond and ionic bond. However, M-A bond and A-A bond have more metallic bond components, and the bond strength is relatively weak[16]. Thus, MXene materials can be prepared by removing the A-group atoms from MAX[17]. There are more than 70 kinds of MAX phases, and they have many advantages of both ceramics and metals, such as low density, high toughness, high oxidation resistance and good heat resistance, so the MXene materials prepared by MAX phases not only have a variety of quantities and types, but also have excellent properties[18]. Since the first MXene member titanium carbide (Ti3C2Tx) was prepared by Yury Gogotsi and others in 2011, it has attracted wide attention of many scientists[19⇓~21]. The unique physical and chemical properties of MXene have been used in many fields such as energy storage and conversion, sensors, catalytic materials, multifunctional polymer composites and so on in recent years[22,23][24][25,26][27⇓~29]. Due to its large specific surface area, good hydrophilicity, abundant active sites, controllable interlayer spacing and high ion exchange capacity, MXene has become the most promising nanomaterial for wastewater treatment and environmental remediation, and is gradually being developed as an efficient adsorbent for the treatment of various pollutants such as heavy metals and radionuclides in the environment[30⇓⇓~33][34⇓⇓~37]. At present, there are more than 20 types of MXene materials that can be successfully synthesized. With different elemental compositions and structures, the synthesized MXene materials show different properties. Many researchers are committed to improving the structural stability of MXene materials and studying the surface functionalization of MXene, so as to develop more efficient, excellent, green and environmentally friendly adsorbent materials. Based on this, this paper mainly summarizes MXene materials and synthesis methods, and discusses the application of MXene materials in heavy metal and radionuclide adsorption.

2 Structure and Synthesis of MXenes

2.1 Structure of MXenes.

The MAX phase is the precursor of MXene, and the A atomic layers in the MAX phase structure exist in the alternating lamellae formed by M and X through a similar intercalation, that is, the transition metal carbides and A atomic lamellae are alternately arranged in the following order: A/ [ M n + 1 Xn]/A/ [ M n + 1 Xn]… (Fig. 3)[38].
The structure of MXene is similar to that of the MAX precursor, and there are mainly three structures of MXene, :M2X, M3X2 and M4X3 (Fig. 3)[20]. The arrangement order of the M atoms differs among the three structures. In M2X, the M atoms follow the ABABAB arrangement order (hexagonal close packing), while in M3X2 and M4X3, the M atoms follow the ABCABC arrangement order (face-centered cubic packing)[39]. Urbankowski et al. Found that the order of atoms is an important factor affecting the stability of MXene, and the Mo2CTx configuration is stable, while the Mo3C2Tx and Mo4C3Tx configurations are unstable[40].
So far, MXenes materials synthesized from solutions containing acidic fluorine have functional groups such as OH, O, and F attached to the surface of the material, which is represented as T in the structural formula of MXene materials[39]. The formation of surface functional groups is due to the solution existing between the layers of MXene materials, which have high surface activity and react with H+, F-, H2O in the solution to obtain surface groups. These functional groups have a great influence on the physicochemical properties of MXene. Therefore, it is crucial to investigate the surface groups and the role they play in MXene materials[41⇓⇓~44]. Xie et al. Used theoretical simulation calculation combined with experiments to prove that there are functional groups OH and a small amount of O and F functional groups on the surface of MXene[45]. Tang et al. Used density functional theory (DFT) to study the lattice orientation of surface functional groups on Ti3C2T2, and obtained three possible configurations, as shown in Fig. 4 (a – C)[46]. In configuration I, the T group is located above the hollow site between three adjacent carbon atoms, and the T group is directed to the Ti atom. In configuration II, the T group is located above the carbon atom. Configuration III is a mixed structure of configuration I and configuration II, one side of configuration III is configuration I, the other side is configuration II, and the T group follows the arrangement of configuration I and II. The structural stability of different configurations was estimated by comparing their relative DFT energies, and for Ti3C2F2 and Ti3C2(OH)2, the most stable configuration was configuration Ⅰ, while configuration Ⅱ was the least stable.

2.2 Synthesis of MXenes

Since Gogotsi et al. First successfully prepared MXene materials by etching MAX phase materials with hydrofluoric acid in 2011, more and more methods for synthesizing MXene have emerged[107]. HF, HCl + LiF and (NH4)HF2 etching methods are collectively referred to as chemical liquid phase etching, or solution method for short, which is the most widely used method to prepare MXene. In addition, there are methods such as high temperature fluoride melting method and hydrothermal method.

2.2.1 Chemical liquid etching (HF, HCl + LiF and (NH4)HF2 etching)

The solution method preparation of MXene is mainly by fluoride ion containing solution such as hydrofluoric acid aqueous solution (HF), mixture of lithium fluoride and hydrochloric acid (LiF + HCl) or ammonium bifluoride ( (NH4)HF2) reagent, which selectively etches the A layer of MAX phase. During the etching process, the A layer of the MAX phase reacts with HF (or fluoride ions) to form AF and gaseous hydrogen, resulting in the stripping of the MXene layer, as shown in chemical reaction equation (1):
M n + 1 A X n ( s ) + 3 H F ( a q ) = M n + 1 X n ( s ) + A F 3 ( s ) + 3 / 2 H 2 ( g )
In the above process, the A atom is replaced by O, OH or F hydrophilic groups, which leads to the weakening of the interaction between the M n + 1 Xn bonds, and finally forms a loose, exfoliated MXene graphite layered structure, see chemical reaction equations (2) and (3):
M n + 1 X n ( s ) + 2 H 2 O ( l ) = M n + 1 X n ( O H ) 2 ( s ) + H 2 ( g )
M n + 1 X n ( s ) + 2 H F ( a q ) = M n + 1 X n F 2 ( s ) + H 2 ( g )
The formation of surface groups during the preparation of MXene will make the surface of the material negatively charged. These surface functional groups favor the formation of a more stable MXene configuration. The results show that MXene with these surface groups has higher thermodynamic stability than the original MXene[47,48].
HF etching is the earliest and most widely used method to prepare MXene. In this method, the MAX precursor powder is first added to a certain concentration of HF acid or a mixed solution of fluoride and strong acid.Stir at the specified temperature, then separate the precipitate by centrifugation or filtration, and wash and separate the precipitate repeatedly with ultra-clean water or deionized water until the pH value of the solution is between 4 and 6[20,49,50]. After the above treatment, the MAX bulk material is transformed into an accordion-like graphene-like MXene structure (fig. 5) by ultrasonic layering, and the obtained MXene has a large number of F end groups[51]. Among the various MXene configurations prepared using MAX phase-selective etching, Ti3C2Tx is the most widely studied configuration. In addition, HF etching can also synthesize many new MXene structures, such as Ti2C, Mo2C, Cr2TiC2, Ti3SiC2, etc[49][52][53][54].
However, because HF is a strong acid, it is dangerous to use. An existing alternative to HF etch is to use a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF) to produce HF to produce MXene. For example, Ghidiu et al. Used HCl and LiF to gently etch the aluminum layer in the Ti3AlC2 to prepare a large amount of MXene[55]. The flake area of MXene synthesized by this method is larger and the defects are smaller. In addition, there is another less harmful and milder method, ammonium difluoride (NH4HF2) reagent, to prepare MXene. Halim et al. Used NH4HF2 to corrode Ti3AlC2, so that —NH3 and — N H 4 + were inserted into the interlayer of Ti3C2Tx at the same time, which made the material configuration more stable[56].

2.2.2 High temperature fluoride fusion method

The high temperature fluoride melting method is to mix fluorine-containing metal salts with MAX and introduce inert gas to heat and react at high temperature to produce MXene. Compared with the liquid phase etching method which uses strong acid for etching, the steps are cumbersome, the time consumption is long, and the environmental pollution is serious, while the melting method can greatly shorten the preparation time, has less environmental pollution, and can control the reaction temperature and different surface groups by selecting different molten salts[57]. Li et al. Used Zn element in molten ZnCl2 and MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN and V2AlC) to heat treat at 550 ℃ for 5 H under the protection of argon.Element replacement with Al element in the precursor to prepare new MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN and V2ZnC),And then MXenes(Ti3C2Cl2 and U Ti2CCl2 are formed by stripping Zn[58]. The results show that the high temperature fluoride melting method is an efficient and controllable method for the synthesis of new nano-MAX phases, and MXenes with halide functional groups are derived from the MAX phases, which is the first time that MXene with full Cl surface groups has been prepared. Urbankowski et al. Used the molten salt method to prepare Ti4N3Tx, and successfully prepared two-dimensional nitride-based MXene——Ti4N3 by treating the mixture of Ti4AlN3 powder and fluoride salts (such as KF, LiF, and NaF) with the same quality under argon gas[59]. Compared with the F-containing solvent etching method, the high temperature fluoride melting method is more efficient, green and safe, which provides a new way for the large-scale production of MXene materials.

2.2.3 Hydrothermal method

Hydrothermal method belongs to the category of liquid phase reaction, which refers to the use of aqueous solution as a reaction system in a specific closed reactor, and the corresponding synthesis reaction is carried out by heating and pressurizing the reaction system. Compared with chemical etching method and melting method, hydrothermal method has the advantages of simple equipment, simple operation, high efficiency and safety, and shows good diversity in the synthesis of nanomaterials[54]. Xie et Al. Soaked Ti3AlC2 in NaOH (1 mol/L) solution and then hydrothermally treated at H2SO4(1 mol/L)480℃ for 2 H to obtain surface Al-leached Ti3AlC2 particles (e-TAC) (Fig. 6)[60]. This study highlights the use of optimized hydrothermal etching to develop Ti3AlC2 support materials with good structure and function. Gu et al. Put the Ti3AlC2 into different concentrations of NaOH solution for continuous magnetic stirring for 1 H, and then heat treated at 200 ℃ for 12 H. After centrifugal dispersion treatment, MXenes with different morphologies (e-TACFs and e-TACSs) were obtained[61]. In addition, Peng et al. Used two reagents of NaBF4 and HCl to gradually generate HF in the hydrothermal process, and the generated HF was used as an etching source to etch the Ti3AlC2 and the Nb2AlC[62].
And yields the corresponding MXene(Ti3C2 and Nb2C). Compared with the traditional HF liquid phase etching method, the hydrothermal method can remove the aluminum layer to a better extent, the MXene sheet is easier to be peeled off by ultrasonic, and the produced MXene has a larger interlayer spacing and a larger BET specific surface area.
In the process of using the above three methods, more or less F-containing waste gas and liquid will be produced, which will cause certain harm to human body and the environment. In the future, we need to continue to explore more mild, harmless and environmentally friendly preparation methods[63].

3 Removal of Heavy Metal Ions by MXenes

Heavy metal ions Pb (Ⅱ), Hg (Ⅱ), Cr (Ⅱ) and Ni (Ⅱ) are common pollutants in soil and water because of their high toxicity to human beings, animals and plants. At present, the most effective method to remove these heavy metal ions is adsorption. Heavy metal ions can be gathered in the adsorption material by adsorption method to achieve the removal effect[64]. The surface of 2D layered MXene material has abundant active sites, high hydrophilicity, high ion exchange capacity, high stability and excellent acid resistance[32,65,66]. There are many adsorption mechanisms, such as ion exchange, internal sphere complexation and electrostatic interaction[67][66][68]. In recent years, researchers have done a lot of research on the adsorption of heavy metal ions using MXene.

3.1 Removal of Pb (Ⅱ)

Among all kinds of toxic heavy metal ions, Pb (Ⅱ) is one of the most common pollutants, and is listed as a key heavy metal pollutant by most national environmental protection agencies. In 2014, Peng et al. First reported the adsorption of heavy metal ion Pb (Ⅱ) by two-dimensional titanium carbide MXene[34]. They used chemical exfoliation and alkaline intercalation to prepare Ti3C2MXene, and attached -OH and -ONa groups on the surface, and then studied the adsorption capacity of Ti3C2(OH/ONa)xF2-xMXene materials for Pb (Ⅱ). The results showed that the two-dimensional Alk-MXene(Ti3C2(OH/ONa)xF2-x) material had high adsorption capacity for Pb (Ⅱ), and the adsorption equilibrium was reached within 120 s, and the maximum theoretical adsorption capacity was about 2 800 mg/G. Fig. 7 shows the electron local density (ELF) map of Ti3C2(OH)2 and Ti3C2(O2 H 2 - 2 m Pbm), which can provide bonding information for atoms[69]. It can be seen from fig. 7 that the hydroxyl groups around the lead atom and the oxygen atoms under the lead atom jointly constrain the adsorbed lead atom, forming a hexagonal hydroxyl potential well. In Ti3C2(O2 H 2 - 2 m Pbm), the bond formed by Pb under the action of Pb atom with the hydroxyl group around Pb atom (the hydroxyl group losing H atom) is stronger than that with oxygen atom. In summary, the adsorption mechanism is that Pb (II) is attracted by the hexagonal hydroxyl potential well to promote the ion exchange of Pb (II). It is worth noting that another particularity of this MXene material is its excellent selectivity for specific pollutants. Alk-MXene selectively adsorbs Pb (Ⅱ) in the presence of competing ions such as Mg (Ⅱ) and Ca (Ⅱ).
DFT predicts that Alk-MXene can be a good candidate for the adsorption of toxic heavy metals[70]. Gu et al. Studied the effective adsorption of Pb (II) on corroded Ti3AlC2 nanofiber materials (named E-TACFs and E-TACSs) by experiments combined with theoretical methods[61]. The maximum adsorption capacities of the E-TACFs and E-TACSs for Pb2+ were 285.9 and 218.3 mg/G, respectively. Cation exchange and surface complexation are interaction mechanisms that promote high affinity for Pb2+. They applied DFT for geometry optimization of the adsorption process as shown in Fig. 8. Pb (Ⅱ) forms a strong Pb — O bond with O atom on the etched Ti3AlC2 surface, and the bond length is 2.550 550 Å. The binding energy is 30.90 kcal/mol, which proves that the process has strong chemisorption behavior.
Wang et al. Added biosurfactants such as Enzymatic hydrolysis lignin (EHL, nonionic surfactant), Lignosulfonate (LS) and Chitosan (CS) to Enzymatic hydrolysis lignin nanosheets to increase the adsorption performance for Pb (II)[72]. The functionalization treatment of EHL is considered to be effective in improving the ion exchange efficiency due to the increase of active adsorption sites. The adsorption mechanism of Pb (Ⅱ) ion by Ti2CTx-EHL is shown in Fig. 9, and the maximum adsorption capacity is 232.9 mg/G. Pb (Ⅱ) was adsorbed on the surface of the Ti2CTx-EHL through the ion exchange interaction between the Ti2CTx nanomaterials and the groups on the EHL and the chelation of the polycyclic structure. The surface groups (-OH, -COOH) of Ti2CTx-EHL and Pb (Ⅱ) enhance the adsorption ability by chemical adsorption[64].

3.2 Removal of Hg (Ⅱ)

Mercury (Hg) is one of the most toxic heavy metals in the environment[73]. Generally, mercury exists in nature in three forms: metallic mercury (Hg0), inorganic mercury (Hg (I) and Hg (II)), and organic mercury (such as dimethylmercury, phenylmercury, and methylmercury). The metallic mercury Hg0 is harmful to the environment and human body, and cannot be decomposed or degraded into harmless substances; Inorganic mercury (Hg () and Hg (Ⅱ)) mostly exist in the environment in the form of compounds or inorganic salts. Organic forms of mercury are the most toxic and can be enriched in fish and shellfish along the food chain[74]. Mercury entering soil and water directly or indirectly affects and damages the lives of microorganisms, plants and aquatic organisms in soil, and causes serious damage to the ecological environment[75]. Mercury in water mainly exists in the form of divalent mercury ion, which can be converted into methylmercury by microorganisms in water. Therefore, the treatment of Hg (Ⅱ) is a continuous concern of scientists[76⇓⇓~79].
Shahzad et al. Used a simple hydrothermal method to add Fe2O3 and Fe3O4 materials to synthesize a highly stable magnetic titanium carbide (Ti3C2Tx)MXene nanocomposite (Magnetic Ti3C2TxMXene,MGMX) for the removal of Hg (II) from water[75]. Vibrating sample magnetometer (VSM) analysis by Magnetic property measurement system (MPMS) shows (fig. 10a) that the synthesized MGMX nanocomposite is supermagnetic. This characteristic provides a simple and effective separation method for its selective adsorption of pollutants. In the presence of interfering ions such as Ca (Ⅱ), K (I) and Mg (Ⅱ), the adsorption effect on Hg (Ⅱ) is still significant.
The specific surface area of MGMX is larger than that of Ti3C2TxMXene, which increases from 56.51 m2/g to 68.39 m2/g. The maximum adsorption capacity of MGMX for Hg (Ⅱ) is 1128.41 mg/G. Considering the charge, Fe and Hg (Ⅱ) are positively charged, while O is negatively charged.Hg (II) is adsorbed by MGMX and converted into the form of HgO and Hg(OH)2, which is also confirmed by the Powdered X-ray diffraction (PXRD) spectrum (some peaks circled in purple in Fig. 10B are HgO and Hg(OH)2). The synergistic effect of Ti3C2Tx and Fe2O3 in the composite is an important reason for the high adsorption of Hg (Ⅱ). It is generally believed that particles with Zeta potential in the range of -30 to + 30 mV are stable. In the experiment, the potential was -47.8 mV at pH 10.5, and the zero point charge was found at pH 3.2. The stability of MGMX was verified by Zeta potential test in the pH range of 3.2 – 10.5. The material can carry out multiple adsorption-desorption cycles, has good renewable capacity, is green and environment-friendly, and can play an important role in wastewater treatment.
In 2019, Shahzad et al. Used different concentrations of Ti3C2TxMXene and Sodium alginate (SA) as raw materials to prepare core-shell aerogel spheres with titanium carbide MXene (MX-SA) to remove Hg (Ⅱ)[68]. In this study, sodium alginate was deposited into calcium chloride aqueous solution at different mass ratios, and Ca2+ was used as crosslinking agent to prepare MX-SA filled spheres. They prepared gel spheres with different concentrations,Such as MX-SA1∶20, MX-SA2∶20, MX-SA3∶20, MX-SA6∶20, MX-SA4∶20,GO-SA4∶20 (subscripts denote the concentrations of Ti3C2Tx and SA in the mixed solution, respectively),According to the adsorption capacity and material characteristics, the adsorption performance of MX-SA4∶20 was the best, and the maximum adsorption capacity was 932. 84 mg/G. The ability of MX-SA4∶20 microspheres to remove Hg2+ from the aqueous phase is driven by multiple mechanisms, including complex formation, ion exchange, and electrostatic interactions. The adsorption of Hg2+ is mainly caused by the inner surface complexation between [Ti-O]∶H+ and Hg2+ and the ion exchange reaction between Ca2+ and Hg2+. From the X-ray photo electron spectroscopy (XPS) before and after adsorption (Fig. 11A, B), it is observed that the Ti 2p peak shifts to a higher binding energy after adsorption, confirming the existence of a strong interaction between ray photo electron spectroscopy and Ti — O. In addition, the hydroxyl group (-OH), alkane (-CH) and carboxyl group (-COO) of alginate biopolymer also play an important role in the adsorption mechanism.

3.3 Removal of Cr (Ⅵ)

Chromium has many valence States — divalent, trivalent, and hexavalent. The natural source is mainly rock weathering, mostly trivalent; The main source of man-made pollution is the discharge of industrial chromium-containing waste gas and wastewater. Chromium in industrial wastewater mainly exists as hexavalent compounds, often in the form of chromate ions. Both trivalent chromium and hexavalent chromium are harmful to human health, but the toxicity of hexavalent chromium is about 100 times higher than that of trivalent chromium, which is a strong mutator and can induce lung cancer and nasopharyngeal cancer[80]. In order to eliminate the toxicity of Cr (Ⅵ) in industry, various chemical reagents, such as sulfur dioxide gas, iron salt and sodium bisulfite, are used to reduce Cr (Ⅵ) to Cr (Ⅲ)[81]. However, this method produces a large amount of secondary waste and requires the use of acid or alkali reagents to adjust the pH of the solution again. Therefore, from the perspective of practical application, it is necessary to find a friendly material for rapid and effective removal of Cr (Ⅵ)[82⇓⇓~85].
Ying et al. Reported that the new Ti3C2TxMXene nanomaterials prepared by intercalation exfoliation method have the advantages of large specific surface area, good dispersion and reducibility, and have good removal ability of Cr (Ⅵ) in water[82]. The maximum adsorption capacity of the novel Ti3C2TxMXene nanosheets for Cr (Ⅵ) was 250 mg/G, and the residual Cr (Ⅵ) in the treated water was far below the WHO standard (0.05 ppm). The most important thing is that the Ti3C2TxMXene material can not only reduce Cr (Ⅵ) in aqueous solution to Cr (Ⅲ) which is less toxic, but also adsorb the reduced Cr (Ⅲ).
Fig. 12 is a mechanism diagram of simultaneous reduction and removal of Cr (Ⅵ) by Ti3C2TxMXene nanomaterials. The initial adsorption of Cr (Ⅵ) on Ti3C2Tx is due to the fact that the surface of MXene is positively charged at low pH, and there are a large number of protonated hydroxyl groups on the surface, which attract the negatively charged Cr2 O 7 2 - through electrostatic attraction. When the Cr2 O 7 2 - is adsorbed on the surface of the material, the electrons on the Ti3C2Tx are transferred to the Cr2 O 7 2 - with the assistance of hydroxyl groups, causing the reduction of Cr (Ⅵ) to form Cr (Ⅲ). Finally, the formed Cr (Ⅲ) reacts with the [Ti — O] bond on MXene to form a Ti-O-Cr (Ⅲ) structure, so that Cr (Ⅲ] is adsorbed on the surface of MXene.
Wan et al. Added chitosan (CS) to MXene by solution method to form a new type of MXene-CS composite[86]. This is the first attempt to synthesize MXene-CS composite and use it to remove Cr (Ⅵ) from aqueous solution. Fig. 13 is a Scanning electron microscopy (SEM) image of MXene-CS. It can be observed that the layered structure of MXene does not completely disappear, and irregular dense particles are formed. The uneven surface and pore structure of MXene-CS increase its specific surface area, which is beneficial to the contact between the active groups on the surface of MXene-CS and Cr (Ⅵ) pollutants in aqueous solution. Fig. 13b shows the XPS spectra of the chemical States of Ti, C, Cr, and O before and after the adsorption of MXene-CS. A Cr 2p peak is observed near 577.4 eV, indicating that the adsorption of Cr (Ⅵ) has occurred at this time. Fig. 13C shows the XPS spectrum of the Cr 2p core layer, and it can be observed that the Cr 2p1/2 and Cr 2p3/2 peaks are detected near 586.68 and 577.4 eV, respectively, indicating that Cr (Ⅵ) is in contact with the surface of the MXene-CS composite and is removed.
Zou et al. Prepared urchin-like rutile TiO2-C nanomaterials (u-RTC) containing a large number of (110) crystal planes by in-situ phase transformation of Ti3C2(OH)0.8F1.2MXene in the presence of FeCl3 (Fig. 14 a), and showed high adsorption capacity for Cr (Ⅵ)[83]. In the mixed solution containing ethylene glycol (EG), FeCl3 and isopropyl alcohol (IPA), MXene decomposed in situ to obtain a new acidified structure (Fig. 14 B). Nanoscale rectangular flakes (A-TiO2) and rod-like precipitates (R-TiO2) exist on the surface of MXene. With the increase of time, the number of thin rectangular nanosheets decreased, while the number and length of rod-like precipitates increased, and finally gradually changed into needle-like precipitates. These needle-like precipitates are TiO2-C/TiC, which can produce strong internal sphere complexation with different forms of Cr (Ⅵ) such as CrO 4 2 -, HCrO 4 -, Cr2 O 7 2 -, thus achieving the adsorption effect. The material is prepared by adopting a solvothermal phase in-situ conversion method, compared with the traditional method, the thermal decomposition reaction is milder and more controllable, the separation condition is more convenient and purer, and more importantly, the surface of MXene after removing Al has F groups,These F groups can be used to synthesize TiO2-C composites with different orientations in situ by adjusting the growth of crystal planes, presenting some regularly arranged TiO2 carbon-based composites. The addition of FeCl3 hinders the growth of the diameter of the material and avoids the formation of large sheets by the coalescence of the side surface of the material. Through the preparation method, the controllable in-situ phase change self-assembled two-dimensional material has unique properties, and a new way is provided for the subsequent preparation of the material.

3.4 Removal of Ni (Ⅱ)

With the development of nickel ore and the rapid development of electroplating industry in China, there are also corresponding pollution problems[87]. Nickel in electroplating wastewater exists in the form of nickel ion. However, the nickel ions in the electroless nickel plating wastewater are difficult to remove because of the formation of complex bonds with complexing agents. The research on the adsorption of heavy metal nickel by MXene is still in the preliminary stage. In 2020, Feng et al. Constructed a two-dimensional Alk-MXene/LDH material to remove Ni2+ by using two-dimensional Layered double metal hydroxide and Layered double metal hydroxide (LDH) as raw materials, combined with in-situ mechanical self-assembly and alkalization intercalation[88]. The results showed that the maximum adsorption capacity of Alk-MXene/LDH for Ni2+ was 222. 717 mg/G, and the removal rate reached 97. 35% under the optimal adsorption conditions (pH = 7, 25 ℃, t = 120 min). The introduction of LDH in the material makes it uniformly dispersed on the surface of MXene, reducing the possibility of agglomeration. The heterojunction formed by Alk-MXene and LDH improves the dispersion of the material, thereby improving the adsorption capacity of heavy metals. The adsorption mechanism of Alk-MXene/LDH material for Ni2+ is shown in Fig. 15: firstly, a part of Ni2+ will generate Ni(OH)2 with hydroxide on the MXene layer, and a part of Ni2+ will form external surface complexes O … Ni with some deprotonated hydroxyl groups (O-)[89]. At the same time, some Ni2+ ions enter the intercalation and react with CO 3 2 - to form NiCO3. Through the synergistic effect of the above adsorption effects, the effect of efficient removal of Ni2+ is finally achieved.

4 Removal of radioactive elements by MXenes

Nuclear energy, as a carbon-free energy source, plays an important role in meeting the world's energy needs due to its high energy density and non-production of harmful substances. However, a large amount of radioactive waste is the main by-product of nuclear power plants. The radioactive nuclides involved in the disposal of nuclear waste are uranium (U (Ⅵ)), europium (Eu (Ⅲ)), cesium (Cs (Ⅰ)), barium (Ba (Ⅱ)) and strontium (Sr (Ⅱ)). These radionuclides are classified as high-risk materials due to their long half-life, strong chemical toxicity and high mobility[90]. Therefore, the safe disposal of these radionuclides becomes an important issue for the sustainable development of nuclear energy.

4.1 Removal of U (Ⅵ)

As one of the core components of the traditional nuclear fuel cycle, uranium is a potential environmental pollutant with chemical and radiological toxicity. Mining, reprocessing of nuclear fuel and improper disposal of nuclear waste have resulted in the release of uranium into the soil and aquifers[91⇓~93]. Uranium in the environment mainly exists as the uranyl ion ( UO 2 2 +) of U (Ⅵ). Because of its high water solubility and fluidity, U (Ⅵ) is easy to migrate in the environment, so the treatment of U (Ⅵ) radioactive pollution in the environment is very important.
In 2016, Wang et al. First reported the use of multilayer V2CTxMXene materials to capture U (Ⅵ) from aqueous solution[94]. The maximum absorption capacity of the V2CTxMXene material for U (Ⅵ) reached 174 mg/G. The removal mechanism of U (Ⅵ) by V2CTxMXene materials is mainly coordination and ion exchange. DFT calculations and Extended X-ray absorption fine structure (EXAFS) results show that U (Ⅵ) binds to the heterogeneous adsorption sites on the nanosheets to form a bidentate inner-sphere complex. The deprotonation of the hydroxyl groups on the surface of the material after the bonding between V2CTx and U (Ⅵ) indicates that the adsorption process follows the ion-exchange mechanism.
The original MXene multilayer structure has limited interlayer space, which limits its ability in scientific applications. Based on this, Wang et al. Expanded the interlayer spacing of Ti3C2TxMXene by using the hydration intercalation method to form a unique multilayer structure (Fig. 16)[65]. The hydration interlayer method enhances the interaction of the anchoring functional groups on the surface of MXene, thereby improving the adsorption capacity for U (Ⅵ). The experimental results show that the maximum adsorption capacity of U (Ⅵ) is 214 mg/G at pH 5, and the adsorption equilibrium time is 4. 5 H. They also explored the strategy of rapid low-temperature calcination after adsorption, that is, the adsorbed U (Ⅵ) was sealed in the interlayer of Ti3C2Tx, which realized the immobilization of U (Ⅵ) (Fig. 16). Desorption experiments showed that the dissolution rate of U (Ⅵ) in water was inhibited to much less than 1%. The dissolution rate in 0.5 mol/L nitric acid is less than 6%. These results demonstrate the potential of Ti3C2TxMXene for radionuclide encapsulation under the environmental conditions of HF strong acid.
Using ab initio molecular dynamics, electronic density of States, differential charge density and other methods, Zhang et al. Found that the formation of hydrogen bonds and chemical bonding are the main effects of stable adsorption of U (Ⅵ) on V2C(OH)2MXene materials[95]. U (Ⅵ) can be stably adsorbed on V2C materials mainly through the chemical bonding between uranium atoms and surface groups and the weak interaction between oxygen on U (Ⅵ) groups and surface groups. At the same time, the effect of other functionalized surface groups on the adsorption of U (Ⅵ) on V2CTxMXene materials was also studied, and it was predicted that the F surface group would inhibit the adsorption of U (Ⅵ). Wang et al. Also applied DFT calculations to study the adsorption properties of uranium on two-dimensional MXene materials, including Ti2C, V2C, and Cr2C, and the effect of surface functional groups on the adsorption properties[96]. The calculation results show that these materials have good radiation resistance.
Δ E b = ( E m - 2 A d s o r b + 2 E S i n g l e U - E A d s o r b m ) / 2 , m > 1
Equation (4) defines the binding energy difference (ΔEb) to estimate the maximum adsorption capacity of various MXenes.
E b = ( E P r i s t i n e + m E S i n g l e U - E m A d s o r b ) / m
In formula (4), ESingleU is the energy of a single U atom; E m A d s o r b 和 E m - 2 A d s o r b are the total energies of MXenes adsorbed with m and m − 2 atoms, respectively. Here, even numbers such as 2, 4, 6 … and 18 are chosen in the corresponding calculations since the adsorbed atom pairs are added on both sides of the MXene monolayer. In equation (5), EPristine is the total energy of the original MXene, Eb is the average binding energy, and E m A d s o r b 和 E m - 2 A d s o r b can be obtained by equation (5).
According to the above formula, the maximum adsorption of U (Ⅵ) by Ti2C, V2C and Cr2C materials using different types of surface decoration is shown in Fig. 17, and it is obvious that the surface decoration with O has the highest theoretical value. The adsorption capacity of Ti2CO2, V2CO2 and Cr2CO2 for uranium is 1890, 1090 and 1070 mg/G, respectively, which is 2.0 – 2.5 times higher than that of the bare surface, proving that the O functional group can significantly enhance the interaction with uranium atoms. At the same time, the theoretical adsorption capacity of MXene decorated with H and OH groups decreased sharply compared with that of O groups, indicating that the adsorption of U (Ⅵ) on materials with these end groups was unstable. This also illustrates that the type of functional group largely affects uranyl adsorption.
In addition to the main form of U (Ⅵ), there is also the form of U (Ⅳ) in the environment. Because U (Ⅵ) ion has a greater impact on the environment than U (Ⅳ), the reduction of highly mobile U (Ⅵ) to less soluble U (Ⅳ) has been considered as another effective method for the environmental remediation of uranium. In 2018, Wang et al. Used highly reactive Ti2CTxMXene to reduce poorly soluble U (Ⅵ) to U (Ⅳ) with low solubility, and proved that Ti2CTxMXene could effectively remove uranium by adsorption-reduction strategy[36]. The results show that Ti2CTxMXene can adsorb U (Ⅳ) effectively in a wide pH range, and is reduced to mononuclear U (Ⅳ) binding to MXene substrate at low pH, while UO2+x nanoparticles adsorb on Ti2CTxMXene at nearly neutral pH, and part of the Ti2CTx is converted to amorphous TiO2. This study also emphasizes that the removal of U (Ⅵ) is due to the reductive induction of U (Ⅵ) by Ti2CTxMXene, and part of the reduction depends on the pH of the solution.

4.2 Removal of Eu (Ⅲ)

As one of the fission products of nuclear reactor, trivalent lanthanide, europium (Eu) has strong toxicity, which is harmful to the environment and human health[97,98]. Zhang et al. Fabricated Hierarchical titanate nanostructures (HTNs) using two-dimensional MXene crystal precursors using a mild in situ chemical conversion strategy[99]. The material has high stability and high adsorption capacity for Eu (Ⅲ). The results show that the adsorption of Eu (Ⅲ) on HTNs is carried out by ion exchange process. In this process, Eu (Ⅲ) replaces the H+/Na+/K+ ion on Ti2CTxMXene by stronger electrostatic attraction and hydrogen bonding. When Eu (Ⅲ) ions are exchanged into the HTNs layer, Eu (Ⅲ) is confined in the nanoscale intermediate layer of HTNs in the form of intra-sphere complexation to achieve the effect of adsorption removal (Fig. 18). The two HTNs synthesized in the study were Na-HTNs and K-HTNs. Among them, Na-HTNs show higher adsorption capacity, which is due to the higher ion hydration energy of Na and the larger hydration radius, so that the interlayer space of Na-HTNs is larger. A large number of interlayer water molecules can promote the rapid diffusion and migration of exchangeable cations between the layers of Na-HTNs, so that Na-HTNs has good ion exchange properties.
Zhang et al. Grafted carboxyl functional groups onto the surface of MXene by introducing diazonium salt, which improved its stability in water, antioxidant capacity, and enhanced its chelating ability for radionuclides[100]. The material is prepared by providing electrons to the diazonium salt by MXene, so that the diazonium salt is cracked to release nitrogen and generate new aromatic free radicals. The new radical combines with the H atom on the surface of MXene, making Ti — OH become Ti — O radical, and finally Ti — O combines with another newly generated aromatic radical to form Ti — O — C. The optimal modified MXene is determined by controlling the reaction ratio of MXene and diazonium salt. Carboxyl functionalized Ti3C2TxMXene(TCCH) has good adsorption ability for radioactive elements Eu (Ⅲ) and U (Ⅵ), and can reach the adsorption effect in a short time (3 min) (the maximum adsorption capacity of U is 344. 8 mg/G, and the maximum adsorption capacity of Eu is 97. 1 mg/G). The adsorption mechanism of Eu (Ⅲ) and U (Ⅵ) on TCCH was determined by XPS and EXAFS. The results showed that the :Eu3+ and UO 2 2 + of Eu (Ⅲ) and U (Ⅵ) on TCCH were mainly coordinated with carboxyl groups on the surface of TCCH to form inner-sphere surface complexes. At the same time, electrostatic interaction and ion exchange interaction also have a certain contribution to the adsorption of Eu (Ⅲ) on TCCH.

4.3 Removal of Cs (Ⅰ)

The radioactive element cesium (Cs+) is one of the most biologically dangerous radionuclides, with a half-life of 30.17 years, and its β-decay is accompanied by medium-energy γ-rays[101]. Radioactive Cs in soil, as a chemical analogue of K, is easily absorbed by plants. Entering the food chain will lead to deformity and cancer, and human health will be threatened. Therefore, it is necessary to minimize the harm of radioactive Cs to human beings[102,103].
Khan et al. Used Ti3C2TxMXene as an adsorbent for the radioactive contaminant Cs+ in water[103]. The Cs 3D doublet at around 725 and 738 eV can be observed according to the XPS spectrum of Fig. 19, confirming the adsorption of Cs+ on Ti3C2Tx. The adsorption potential of Ti3C2Tx for Cs+ was evaluated by adsorption isotherm experiments, and the experimental data were fitted with Langmuir and Freundlich isotherm models, and it was concluded that the Freundlich adsorption isotherm was consistent with the adsorption data, and the maximum adsorption capacity was 25. 4 mg/G at room temperature. This reveals the heterogeneity of the sites present on Ti3C2Tx and the multilayer adsorption behavior of this adsorbent. The heterogeneity of the sites indicates the presence of several functional groups (— O, — OH) with different binding energies on the surface of MXene. Heterogeneous sites with smaller binding energy are occupied first, and this trend decreases exponentially until complete adsorption of Cs+ on Ti3C2Tx is achieved. The study shows that the removal of Cs+ ions in polluted water by Ti3C2TxMXene is due to its pores and interlayer space, which can accommodate Cs+ ions, as well as the influence of multi-functional phase points such as hydroxyl, fluorine and oxygen groups. In the presence of a plurality of interfering ions (Li+, Na+, K+, Mg2+, Sr2+),It still has high selectivity for Cs+. After that, Jun et al. Carried out adsorption experiments on the removal of radioactive Cs+ in model nuclear wastewater by using Porous activated carbon, and compared the results with those of Porous activated carbon (PAC)[102]. Although the specific surface area of PAC is larger than that of MXene, the adsorption capacity of Ti3C2TxMXene for Cs+ is 148 mg/G, which is higher than that of PAC, because the adsorption of Cs+ is mainly electrostatic interaction, while MXene has a large number of negative charges on its surface.
Recently, Hassan et al. Used Hydroxyapatite (HAp) as a matrix for green immobilization of MXene(Ti3C2Tx) adsorbed by radioactive cesium (Cs+)[104]. The immobilization process was carried out by cold sintering at low temperature (200 ℃), which prevented the possible oxidation and chemical degradation of the spent adsorbent. The thermal stability of the MXene-HAp (MX-HAp) composite was significantly improved compared to pure MXene for Cs adsorption, and non-volatile immobilization of the dried MX-HAp composite was achieved by cold sintering. In this study, MXene materials and HAp were combined to develop different types of composite materials, which made the green immobilization process of waste adsorbents to a higher level.

4.4 Removal of Ba (Ⅱ) and Sr (Ⅱ)

Barium (Ba (Ⅱ)) and strontium (Sr (Ⅱ)) are usually the most toxic fission products released during nuclear accidents. However, due to the improper management and disposal of radioactive waste, they exist in large quantities in radioactive liquid waste[67,105]. Isotopes of radioactive barium (such as 133-Ba and 140-Ba) are important fission products, causing respiratory distress and liver, heart, and kidney damage[67]. Radioactive strontium may cause anemia and hypoxia[106]. Fard et al. First used Ti3C2TxMXene nanomaterials as a new adsorbent for water treatment to adsorb barium, and its removal efficiency reached 100% under optimized conditions[67]. They also studied the adsorption properties of MXene for Ba (II) in the presence of arsenic (III), lead (II), chromium (IV), strontium (II), barium (II) and other ions. The results show that the removal rate of Ba (Ⅱ) can reach 98% despite the presence of different ions in the solution. This indicates that Ba2+ can adsorb pollutant ions on the surface of MXene by physical adsorption, in addition to chemical adsorption by the functional groups — OH, — F and — O on the surface of the material through chemical bonds. The adsorption process of Ba2+ is shown in Fig. 20. Ba2+ combines with the — OH and — F functional groups on the surface of Ti3C2TxMXene to generate Ba(OH)2 and Ba(F)2, respectively, to achieve the removal effect[1]. Although it has been proved that MXene material can remove barium from water, its maximum adsorption capacity is only 9. 3 mg/G, and its low adsorption capacity limits its application in nuclear wastewater treatment. Subsequently, in 2018, Mu et al. Improved the adsorption capacity of Ti3C2Tx for Ba by directly activating its surface and using NaOH for metal alkaline intercalation to generate Alk-Ti3C2Tx[105]. The study showed that the Zata potential of Ti3C2Tx decreased obviously with the increase of pH value after activation with NaOH solution, which indicated that the number of OH groups on the surface of Alk-Ti3C2Tx increased and the negative charge on its surface was improved, thus providing more adsorption sites for capturing Ba2+ (Fig. 21). The results showed that the maximum adsorption capacity of Alk-Ti3C2Tx for Ba2+ was 46.46 mg/G, which was almost three times higher than that of unmodified Ti3C2Tx. Therefore, NaOH activation treatment is a promising method to improve the adsorption capacity of MXene materials.
Recently, Jun et al. Used MXene(Ti3C2Tx) to adsorb two radionuclides, Ba2+ and Sr2+, from hydraulic fracturing wastewater[106]. The study shows that Ti3C2TxMXene has a high negative surface charge and adsorbs Ba2+ and Sr2+ by electrostatic attraction, and the adsorption capacity for Ba2+ and Sr2+ is 180 and 225 mg/G, respectively (Fig. 22). In addition, they also confirmed that the adsorption of Ti3C2TxMXene on Ba2+ and Sr2+ occurred through chemisorption, ion exchange, and intrasphere complexation mechanisms by kinetic, XPS, and FTIR studies, respectively. More importantly, the Ti3C2TxMXene can be reused for 4 adsorption/desorption cycles, which is of great significance for the removal of radionuclides from hydraulic fracturing wastewater.
It was found that the adsorption mechanisms of MXene for removing most heavy metal ions were mainly ion exchange and complexation, while for radionuclides, the main adsorption mechanisms were electrostatic interaction and complexation. However, these interactions are not absolute and often exist simultaneously. For example, when Ti3C2TxMXene materials remove Cr (Ⅵ), negatively charged 2$Cr_{2}O_{7}^{2-}$ are adsorbed on the surface of materials through electrostatic interaction[82]. However, the removal of Hg2+ from aqueous phase by MX-SA4∶20 microspheres is driven by multiple mechanisms, including ion exchange, complex formation and electrostatic interaction[68]. Therefore, the removal of pollutant ions by MXene materials is a collaborative process of multiple interactions.

5 Conclusion and prospect

In the past 10 years, MXene and its nanocomposites have developed rapidly and achieved many results in the field of environmental applications. MXene material is a very promising material in two-dimensional layered nanomaterials. Compared with traditional adsorbents, MXene material has the advantages of environmental friendliness, large adsorption capacity, high chemical and thermal stability, and good hydrophilicity, which can be used as an efficient adsorbent to remove harmful substances. In this paper, the structure of MXene materials and its different synthesis methods are summarized. The application part mainly introduces the latest progress of MXene materials in the removal of heavy metal ions and radionuclides. However, there are still many questions to be answered and several challenges to be overcome before these materials can be used in practice.
First, while theoretical simulations predict a large number of MXene materials, the number of experimentally synthesized MXene is still limited. One of the reasons for the difficulty in the preparation of MXene material is that the synthesis conditions of this material are relatively harsh, which requires a long time at a specific temperature. Another reason is that this material is not easy to preserve. The stability of pure MXene material is poor, and it is very easy to oxidize in air. Therefore, it needs to be stripped in water phase during preparation, and it is best to use it now to avoid oxidation of the material for a long time. For example, ultrathin MXene materials have a large number of active sites and larger specific surface area, which can better achieve the enrichment and separation of toxic heavy metals and radionuclides, but ultrathin MXene materials are not easy to prepare and preserve. Therefore, obtaining an environmentally friendly and simple method for preparing MXene materials and improving its stability in the environment are the problems to be solved in this field. In addition, in addition to flat MXene films, it is worth investigating different morphologies of MXene, such as nanotubes or nanocages, for environmental remediation applications.
Secondly, many studies have shown that the functional groups on the surface of MXene are closely related to its adsorption capacity. In general, MXene obtained by conventional synthetic routes has multiple surface groups. Therefore, in the future, it is necessary to develop a new synthesis approach and treatment method under the existing conditions of less MXene, and to modify the synthesized MXene materials or their surface groups (such as — OH, — F, — O) to improve their adsorption capacity. Unlike laboratory synthesis, most theoretical calculations and modeling are controllable and designable. Therefore, theoretical simulation, classical molecular dynamics and finite element modeling are very important for the prediction of experimental conditions and the design of MXene with new functional groups.
Finally, with the rapid utilization of MXene materials in different fields and their growing number, it is necessary to conduct more in-depth research on the toxicity of MXene and its potential harm to the environment. In addition, the potential hazards of MXene entering the environment should be assessed. We are committed to developing green and environmentally friendly MXene to expand its application in environmental remediation.

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