Cleaning the environment with MXenes

Rapid global industrialization constantly impacts the environment by discharging pollutants. Therefore, various materials are currently being investigated for environmental applications, including two-dimensional (2D) MXenes. Herein, we focus on MXene-enabled technologies for removing inorganic and organic contaminants present in gaseous and liquid forms, especially organic dyes, pharmaceuticals, and solid pollutants. We foresee a considerable potential for MXene-enabled technologies to remove heavy ions and radionuclides and recover precious elements. We show that MXenes could efficiently inactivate microorganisms without harming the environment. Finally, we discuss the associated opportunities and challenges in MXenes’ surface chemistry, semiconducting activity, interfacial effects, adsorption, and photocatalysis. Altogether, this article showcases outstanding opportunities for MXenes in the rapidly growing field of environmental applications.


Introduction
The natural environment must be protected to enable its availability to future generations. However, global industrialization dramatically impacts the environment by discharging various pollutants into ecosystems. The US Environmental Protection Agency (EPA) states that water contamination has become a global threat. Due to industrial production, about 300-400 million tons of pollutants are released globally into water supplies. 1 These pollutants can be organic and inorganic, and their persistence urges to develop efficient remediation technologies. Therefore, various materials are currently being investigated for environmental applications, including twodimensional (2D) nanomaterials.
Two-dimensional nanomaterials, including graphene, boron nitride, graphene oxide, metal-organic frameworks (MOFs), and layered double hydroxides, have been extensively researched as prospective candidates in applied environmental cleaning, owing to their planar structure, large surface area, surface activity, and mechanical flexibility. Recently, a new member in the 2D family appeared-MXenes, which received considerable attention for their potential use in gas separation, water desalination, antibacterial protection, and antifouling. 2,3 MXenes are transition-metal carbides, nitrides, and carbonitrides, 4 introduced in 2011 by Naguib, Barsoum, and Gogotsi. Their chemical formula is M n+1 X n T x , where M is an early transition metal, X is carbon and nitrogen, T x relates to surface functional groups, and n spans from 1 to 4. In the most classical approach, MXenes are obtained by selectively etching out Al atomic layers from ternary MAX phases using aqueous HF acid at room temperature. Apart from HF-etching, novel approaches adopted for their synthesis, including sustainable and green ones, can significantly advance the current thinking on 2D materials for environmental applications. 5,6 Native MXenes have outstanding variability of inherent properties, including high electrical conductivity, redox activity of transition metal, tunable surface chemistry, and excellent water dispersibility. When adding oxidation capability, the set of properties expands into optical action and 2D confinement of excitons. Altogether, this makes MXenes attractive active agents for environmental remediation technologies such as adsorption, photocatalysis, or separation. 7 In this article, we focus on MXenes' multifunctionality by addressing the variety of pollutants that MXenes can remove from the air, water, and soil environments, as presented in Figure 1. Catch and release or redox approaches were verified for MXenes to remove Cr 6+ , Cd 2+ , and Pb 2+ , among many other heavy-metal ions, radionuclides, and recovery of precious and rare earth elements. MXene-based hybrid nanocomposites can further target various unspecific combinations, such as inorganic and organic pollutants. The proven efficiency accounts for decomposing organic dyes, phenolic compounds, antibiotics, perfluorinated compounds, and microplastics. However, what needs to be explained are cooperative mechanisms between involved counterparts in adsorptive activity and catalytic behavior.
The main goal of this work is to showcase the excellent properties of MXenes and discuss opportunities and challenges for their utilization in decomposing organic and inorganic pollutants. In particular, we analyze MXenes' efficiency in reducing the amount of gaseous contaminants, metal ions and radionuclides. Simultaneously, we indicate that MXenes can be antibacterial while maintaining safety for environmentally viable organisms such as green microalgae. Therefore, this article inspires researchers to focus their studies on MXenebased technologies for environmental applications.
To guide the new approaches, we focus on MXenes' functional characteristics and associated interfacial interactions. The variable surface terminations open countless possibilities for controlling the adsorptive and reactive characteristics of MXenes. 8 However, the main challenge in developing MXene-based adsorbents is the application in demanding settings. Moreover, a wide range of physicochemical conditions of natural waters spanning from clean freshwater to polluted waters such as saline mines or seawater may hardly influence MXene parameters. Research on MXenes' selectivity in complicated matrices, effluents toxicity, and MXenes' potential ecotoxicity is necessary to satisfy industrially viable applications.

Sustainable synthesis methods
MXenes have been tested in a wide range of environmental applications. However, their sustainability is still an issue, mainly due to synthesis approaches that need to ensure removing the A element layer from the MAX phase structure. In the first experiment leading to obtaining a MXene, 9 the starting Ti 3 AlC 2 phase was transformed into Ti 3 C 2 T x (T x = -OH, -F, and =O) using HF etching.
Notably, reactions led to achieving -OH and -F terminal functional groups on the MXene surface. The HF-based approach was confirmed to work for etching almost all MAX phases with Al in their structure. Variables between techniques include HF concentration, temperature, and time, which should be optimized based on MAX chemical composition, and their particle size. The etched MXene is in the form of multilayered harmony-like structures and can be further intercalated and delaminated with large organic cations such as tetramethylammonium hydroxide (TMAOH), tetrabutylammonium hydroxide (TBAOH), or tetrabutylammonium fluoride (TBAF). Notably, few-layered (FL) and single-layered (SL) MXene flakes are obtained in this step. 10 Because the first key step in MXene synthesis is based on Al etching, it became critical to avoid harmful and unsustainable HF acid and exchange it with milder ammonium bifluoride (NH 4 HF 2 ), which could satisfy simultaneous etching and delamination. 11 However, the intralayer intercalation of NH 3 and NH 4 + is less efficient, and Ti 3 C 2 T x is more restacked instead of separated into individual flakes. Nevertheless, a new approach appeared, based on mixing LiF and HCl and forming an in situ generated HF at the MXene/solution interface. The new approach, based on 12 M LiF/9 M HCl was called minimally intensive layer delamination (MILD), as confirmed by a reduced number of defects in the MXene, compared to HF. Following this approach, other fluoride salts such as NaF, KF, CaF 2 , and CsF were also studied. 12,13 Nevertheless, more green techniques for MXene delamination are underway. Energy-supported fragmentation techniques are promising since defragmentation is more robust and eliminates harsh chemicals. High-energy microfluidic devices are supported by the cavitation energy generated by acoustic waves or ultrasounds. In particular, mechanical forces are highly localized and can defragment MXene multilayers similarly to chemical delamination.
Moreover, a technique of surface acoustic waves (SAWs) was recently investigated for the Ti 3 C 2 T x MXene. 14 The SAW process involves LiF solution and is based on high localization of mechanical vibrations that enable rapid water dissociation and proton evolution under acoustic wave impact. The Ti 3 C 2 T x MXene was also recently delaminated with a vortex fluidic device (VFD). 15 The VDF inclines at a 45° horizontal-relative tilt angle and can rotate at 4000 rpm. The delamination of 0.5 mg/ml MXene solution (1:1 mixture of isopropanol and water) with VDF requires a nitrogen atmosphere and solution flow rate of 0.5 ml/min.
Green approaches for MXene delamination were also recently developed. Albumin-assisted delamination with cascading centrifugation was applied to produce stable MXene colloids. 16 Other approaches involved green algae extract to delaminate the V 2 CT x MXene into single flakes in an aqueous solution with about 90% yield.
Besides sustainable high-energy MXene synthesis and delamination techniques, MAX phases can also become green when obtained from low-cost precursors. Instead of highquality elemental powders, the Ti 3 AlC 2 phase was recently synthesized from recycled aluminum scrap, waste tire-based carbon, and titanium dioxide at 1.9C:6Al:3TiO 2 molar ratios. 17 The resulting Ti 3 C 2 T x MXene becomes even more sustainable when MILD-delaminated under N 2 shielding. Obtained Ti 3 C 2 T x showed state-of-the-art parameters such as 5857 ± 680 S/cm conductivity and 285 F/g capacitance (1012 F/cm 3 ) at scan rates of 20 mV/s.

Exceptional surface activity
Exceptional surface activity of MXenes precedes them to hold the top position in applications based on surface physicochemical action. A MXene surface shows a highly negative charge with the point of zero charges (PZCs) ranging from 2 to 3. 18,19 For pH above PZC, the surface becomes negative and subsequently positive below this value. Therefore, a pH value above three gives an electrostatic capacity to adsorb cationic dyes and, reversibly, the anionic dyes below pH 2. In such conditions, only one type of ion can be adsorbed. Therefore, researchers prepared composites involving MXenes, or decorated their negative surface with positively charged nanoparticles 20 to satisfy both cases.
The adsorption efficiency can be additionally improved when the MXene's surface is treated with KOH, NaOH, and LiOH. 21 The anionic exchange mechanisms comes into play and roughly corresponds to the appearance of [M-O]-H + termination for metal cations adsorption, for which A is an alkali metal, and [M-O]-A + for metal ions and corresponds to the increase of interlayer spacing. In this way, MXenes become an excellent candidate for pollutant removal via adsorption.
Yet, MXenes also show excellent catalytic properties thanks to enhanced light adsorption capability and rapid charge-carrier transfer. Moreover, they have photothermal conversion properties, allowing reactive oxygen species to be generated. Simultaneously produced holes (h + ) and superoxide anion radicals (O 2 − ) boost their efficiency. 22 Thus, MXenes become the spotlight of scientists using them as active agents in photocatalysis, the Fenton process, and other catalytic reactions.

Removing gaseous contaminants
MXenes offer a set of promising properties for removing gaseous contaminants such as CO, CO 2 , CH 4 , NO x , and SO x , among others. These greenhouse gases come from urbanization because of fossil fuel burning for increased energy demand. Many materials, which base their activity on large specific surface areas and active surface sites, were analyzed for the adsorption of these gases. In this field, the MXene family has also secured their place.
Carbon capture was tested theoretically on M 2 C (M = V, Ti, Hf, Nb, Zr, W, Ta) MXenes using density functional theory (DFT) calculations. 23 Obtained results showed intense activation energy (3.69 eV) for CO 2 and extensive loading up to 8.25 mol CO 2 /kg. Further experimental studies verified these findings. For instance, Ti 3 C 2 T x showed high CO 2 adsorption under low (12 mol/kg) pressure. In the case of the H 2 and CO 2 mixture, the Ti 3 C 2 T x exhibited selectivity toward CO 2 instead of N 2 . Such results demonstrate the potential use of MXenes in carbon capture technologies. Surface defects in Mo 2 TiC 2 T x could support CO 2 adsorption because of strong interaction from enhanced and localized electronegativity. 24 Therefore, increasing the number of defects can boost MXenes' adsorption capacity.
Further studies focused on developing complex (threedimensional [3D]) forms of MXenes, porous aerogels with mean pore sizes spanning from 2.5 nm to 11.4 nm. 25 Surprisingly, 3D aerogels had lowered specific surface area, but showed selectivity toward CO 2 over N 2 . However, their mechanical properties are unsatisfactory for further use.
Mechanical properties of 3D MXene aerogels could be enhanced by including polymer binders such as Pebax, a polyether-block amide-based thermoplastic. Pebax/MXene composites showed good diffusion of CO 2 and its removal. 26 The porous framework works as a molecular sieve for only CO 2 diffusion, whereas diffusion of N 2 through the membrane is not allowed by 0.35-nm spacing between MXene nanosheets.
MXenes can also help to eliminate SO x , NO x , NH 3 , H 2 S, CH 4, and volatile organic compounds (VOCs). First-principles studies revealed the ability of a Ti 2 CO 2 MXene to chemisorb efficiently only NH 3 (0.174 e charge transfer) among other gases such as H 2 , N 2 , NH 3 , CO, CH 4 , CO 2 , NO 2, and O 2 . 27 Further development of Ti 3 C 2 T x /graphene composite fibers enabled using them as a binder-free wearable and flexible gas sensor. 28 The fibers showed NH 3 sensitivity at ambient conditions and excellent mechanical flexibility (±0.2% resistance fluctuation). After 2000 bending cycles, the composite had a low value of noise resistance.
MXenes can also be used as electrocatalytic nitrogen fixation. The N 2 reduction into NH 3 was realized by a MoS 2 / Ti 3 C 2 T x nanocomposite in which semiconducting MoS 2 nanospots were attached to a conductive Ti 3 C 2 T x MXene. 29 The composite demonstrated good N 2 reduction activity with faradic efficiency of 10.94% and 30.33 μg/h mg. The yield of NH 3 was −0.3 V compared to the reference electrode. Furthermore, a ternary composite made of Ti 3 C 2 T x MXene, g-C 3 N 4, and TiO 2 showed good 66.3% efficiency in NO x sequestration due to its superior light absorption and Z-scheme photocatalyst structure.
MXenes can also detect methane and VOC with 50-100 parts per billion (ppb) sensitivity at ambient conditions with a significant signal-to-noise ratio. 30 Detection of SO 2 by M 2 CO 2 (M = Hf, Sc, Ti, Zr) MXene was assessed with the first-principles approach. 31 The calculations revealed superior sensing, selectivity, and tuneability for Sc 2 CO 2 MXene. The catch and release approach could also be satisfied with external tensile strain.
All of these results suggest that MXenes have excellent properties in removing gaseous pollutants. In addition, their adsorption capacity is enhanced due to the simultaneous nonselective oxidation reactions of the adsorbed particles owing to their oxidizing properties. Thus, they can be used as a material for efficiently capturing exhaust gases in power plants or internal combustion engines, petrochemical or polymer industries (e.g., using a flowing bed). However, the application of MXenes in the industry is still challenging due to their poor thermal stability at high temperatures. Therefore, there is a need for further modification of MXene compounds with cocatalysts, which offers a chance to change their properties and would enable their applicability.

Organic dyes
The MXene family attracted substantial interest for many environmental applications based on adsorption and decomposition. Cationic dyes have been mainly studied as organic pollutants because of their wide use in printing, textiles, and the paper industry and their discharge to aquatic ecosystems. This problem can be solved by applying a state-of-the-art Ti 3 C 2 T x MXene, which efficiently removes cationic dyes. In this regard, a plethora of mechanisms can be efficiently employed during dye removal, as presented in Figure 2.
Approximately 209 mg/g of methylene blue (MB) can be removed by the Ti 3 C 2 T x MXene, pillared with terephthalate. 32 Other approaches were based on increasing the interlayer spacing between the layers. For instance, treatment with LiOH can expand the space between the Ti 3 C 2 T x interlayer by about 29% and exchange -F to -OH termination. Such LiOH and NaOHtreated Ti 3 C 2 T x show excellent adsorption of MB with 189 mg/g removal capacity. 21 Also, a largely F-terminated MXene can become attractive for dye removal from wastewater. The F-terminated MXene can adsorb about 92% of MB from 20 μM of MB solutions within only 5 min of the process. 33 Involving a composite structure can boost a MXene's adsorption efficiency. Magnetic MXene@Fe 3 O 4 systems have good adsorption of MB at various temperatures and even 55°C. 34 The MB removal (11.68 mg/g) was relevant to Langmuir isotherm mechanisms. 35 The adsorption is pH-sensitive with optimal conditions at pH 3 or 11. Apart from hydrogen bonding, electrostatic attraction is the primary mechanism of the adsorptive activity of the MXene. Furthermore, the Ti 3 C 2 T x -SO 3 H composite showed 111.11 mg/g capacity for MB adsorption via electrostatic attraction. 36 MXenes synthesized with hydrothermal etching generally have better cationic dye adsorption than those synthesized via standard HF etching due to having a higher BET-specific surface area. 37 In this regard, the Al-based MOF removes both MB and anionic dye (AB) from model wastewater. 38 Again, the PZC played the leading role. At pH 3, the MB removal capacity was 190 mg/g for MB. The AB removal capacity was 200 mg/g. However, the MXene showed low selectivity when two adsorbents were mixed.
The pressure-assisted membrane technology can be the solution for low selectivities, such as in the case of a MXene@CNT membrane, synthesized by a thermal crosslinking technique. 39 The 50-h operational experiments showed good adsorption behavior of the membranes toward methyl orange, Congo red, and rhodamine B, together with anti-swelling properties and high efficiency of the membranes. The p-MX/SWCNTs showed great capacity (1068.8 mg/g) for MB within 1.2 V of applied voltage. 40 The electrodes were also selective for cationic dyes over anionic ones, even at various pH ranges.
The MB removal efficiency by Ti 3 C 2 T x can be enhanced by pronounced heterojunction. A nanocomposite based on Ti 3 C 2 T x and rutile TiO 2 octahedrons doped with Ti 3+ exposed (111) facets boosted photocatalytic efficiency toward rhodamine B (RhB) dye compared to native Ti 3 C 2 T x and commercial TiO 2 41 (see Figure 3). Charge kinetics were tuned by interfacial interactions TiO 2 and -OH terminated Ti 3 C 2 T x supporting trapping of holes effect of 2D Ti 3 C 2 . Hydrazine hydrate reduction allowed doping of TiO 2 by Ti 3+ ions and visible-light-driven photocatalysis.
The essential element of water purification is the complete separation of the catalyst after the process. Moreover, the catalysts could be regenerated/recycled and reused, thus making the process environmentally friendly. The Ti 3 C 2 MXene can be attached to magnetite via self-polymerization of dopamine and subsequent mild pyrolysis. The catalyst showed 97% decomposition of methylene blue with the Fenton process and high efficiency after five continuous cycles. 43 Scientists also tested magnetic nanoscale zero-valent iron (nZVI)@Ti 3 C 2based MXene nanosheets, synthesized via an in situ reductive deposition method. Obtained results showed 91.1% of ranitidine removal and 81.8-84.8% over five reuse cycles. 44

Pharmaceuticals
Removing pharmaceuticals is an emerging field in which MXenes can showcase their efficiency. Especially wastewaters with active compounds are dangerous for the environment due to permanent changes that they cause in the organisms. Thus, using efficient catalysts, such as MXenes, greatly benefits pharmaceutical removal.  Previous research has shown that MXenes are excellent materials to decompose many pharmaceuticals such as amitriptyline (AMT), verapamil, carbamazepine, 17 α-ethinyl estradiol, ibuprofen, and diclofenac. The Ti 3 C 2 T x exhibited adsorption capacity (58.7 mg/g) for AMT due to electrostatic attraction between a negatively charged MXene and positively charged pharmaceutical molecules. 46 For instance, a 2D/2D Bi 2 WO 6 /Ti 3 C 2 MXene heterostructure was used to remove amoxicillin, the most widely used antibiotic. Interestingly, the removal of compounds was observed for less than 40 min of the photocatalysis, thanks to produced reactive oxygen species. 22 A nanocellulose-intercalated MXene membrane showed ∼99.0% of azithromycin decomposition. What's more, the membrane was characterized by anti-swelling properties in a water environment up to 76 h and pure water permeance (∼26.0 L m −2 h −1 bar −1 ). 47

Soil pollutants
Researchers could use the excellent properties of MXenes to remediate contaminated or degraded soil. For this purpose, they could apply MXenes' aqueous solution with excellent adsorption and nonselective oxidation. 48  Obtained results retained a sufficient adsorption capacity of ∼68 mg g −1 for methylene blue as a model pollutant. 49 MXenes in the aqueous phase would open up a wide range of remediation techniques involving all kinds of soil flushing, both in situ and ex situ. Using a material with adsorption-oxidation properties increases the attractiveness of remediation methods by partially eliminating the disadvantages associated with large quantities of contaminated washings.

Inactivating microorganisms
Recent developments in the antimicrobial applications of MXenes are further discussed. Pioneering studies on MXenes activity against bacteria have shown the physicochemical antimicrobial mode-of-action of colloidal Ti 3 C 2 T x 50 (Figure 4a). Further studies confirmed that MXene flakes couple the nanoblade effect with ROS generation 51 (Figure 4b). Selected MXene-based nanocomposites were inoculated with Gram-positive Escherichia coli bacteria and Gram-negative Bacillus sp., Sarcina lutea, and Staphylococcus aureus. Their efficiency was rated according to growth inhibition zones that surrounded nanocomposite samples. Researchers revealed that the MXene's antimicrobial efficiency could be tuned by adding ceramic oxide and noble metal nanoparticles such as Al 2 O 3 / Ag, SiO 2 /Ag, and SiO 2 /Pd. 52 In addition, partial oxidation of the Ti 3 C 2 T x surface to TiO 2 improved bactericidal activity. 53 However, the oxidation cannot be massive, transforming the Ti 3 C 2 T x MXene into only TiO 2 . Preventing MXene oxidation with antioxidants 6 and keeping it mild gives bandgap tunability, thus assuring efficient light activation. 51 More complex heterostructures can solve this issue and ensure near-infrared (NIR) activation. For instance, one-dimensional (1D)/2D heterostructure based on a combination of 1D cobalt nanowires (CoNWs) and MXene (Figure 4c). The 1D CoNWs trapped the electrons photogenerated from the 2D Ti 3 C 2 T x MXene upon 808-nm NIR illumination, which further prevented hot electron-hole recombination. The efficient transfer of charge carriers enhanced reactive oxygen species (ROS) production. Therefore, an antibacterial efficacy of over 90% within 20 min was supported by additional hyperthermia. 53 Altogether, we conclude that MXene-based nanocomposites are promising for developing efficient antimicrobial protection technologies.
Obtained results confirmed the possibility of utilizing MXenes in antimicrobial systems. For instance, the 2D Ti 3 C 2 / Al 2 O 3 /Ag/Cu nanocomposite showed promise in point-of-use water treatment systems, essential in the case of limited access to safe water resources. The material showed effective elimination of microorganisms (collecting 99.6% of bacteria in the filter) and self-disinfecting potential. 54

Removing heavy-metal ions
Heavy metals are predominant and dangerous contaminants because of their non-degradability and accumulation in the food chain. Adsorption is the most promising technology among others to remove inorganic contaminants from water because of its relative simplicity, affordability, and effectiveness. MXenes can advance the adsorption process ( Figure 5) by showing electrostatic attraction (Figure 5a), surface complexation (Figure 5b), or ion exchange (Figure 5c). 55 An alkalized (alk) MXene showed that surface -OH groups are active sites for the adsorption of lead (Pb 2+ ) ions by an ionic exchange mechanism. 56 The Pb 2+ adsorption was also influenced by pH changes, for which the optimum range was 5-7. Such Ti 2 C(OH) 2 showed 2560 mg/g adsorption capacity toward Pb 2+ due to large uptake kinetics per unit mass of material.
Further DFT calculations on Ti 3 C 2 (OH) x F 2−x confirmed that -OH groups are essential for heavy-metal ion removal, whereas -F does not contribute to Pb 2+ removal 57 due to different binding energies toward metal ions. 58 Reversibly, Zr 2 C(OH) 2 and Sc 2 C(OH) 2 are the only M 2 C(OH) 2 carbide MXenes, as well as nitride MXenes that do not show affinity to Pb 2+ because of their positive or low formation energies. 58 However, V 2 CT x has demonstrated a high affinity to Pb 2+ , but its lack of stability and formation of toxic vanadium oxides hinder its practical applications. 59 In general, MXenes' efficiency in adsorbing metal ions such as Pb 2+ , cadmium (Cd 2+ ), copper (Cu 2+ ), and zinc (Zn 2+ ) is better than for activated carbons due to their highly negative surface charge. 60 The XPS and FTIR studies revealed the mechanism of metal ion adsorption for electrostatic interactions, ion exchange, and inner-sphere complexes formation.
MXene-based magnetic nanocomposites, obtained via hydrothermal process, were further tested for the removal of mercury (Hg 2+ ) ions. The removal of Hg 2+ with 1128.41 mg/g absorption capacity was demonstrated in a wide pH range together with regeneration and reuse capability. 61 Also, core-shell aerogel spheres with a Ti 3 C 2 T x MXene showed a 932.84 mg/g absorption capacity of Hg 2+ . 62 Interestingly, 100% removal efficiency was found for Hg 2+ , and over 90% for chromium (Cr 3+ ), Cd 2+ , and Pb 2+ , even at very high pH values. The aerogel had spherical microsizes, thus allowing them to be applied in packed columns. Alk-Ti 2 C MXene-based nanofibers and sheets also showed efficient Cd 2+ removal 63 with 325.89 mg/g adsorption capacity, the a b c highest reported for other 2D materials (e.g., graphene oxide). Apart from Pb 2+ , Cd 2+ , and Hg 2+ , barium ions (Ba 2+ ) can also be adsorbed by MXenes. Barium is a byproduct of the gas and oil industries. However, its persistence in the aquatic environment allows it to travel long distances within various water-based ecosystems. 64 Fortunately, MXenes can help in removing Ba 2+ from water. The Ti 3 C 2 T x MXene demonstrates high selectivity toward Ba 2+ over other competitive metals and a promising removal capacity of 9.3 mg/g. 65 As expected, highly negative sights on the MXene surface promoted the adsorption of Ba 2+ ions via physisorption and chemisorption mechanisms. Further alkalization of the MXene's surface into high -OH loading on the surface can increase the adsorption capacity of Ba 2+ by three times (46.46 mg/g) compared to native Ti 3 C 2 T x . 66 MXenes are considered perfect carriers for heavy-metal ions due to electrostatic attraction and ion exchange mechanisms. MXenes can be further tailored to adsorb heavy metals via the engineering of surface terminating groups. Finally, to better understand the adsorptive behavior of MXene-based structures, the removal parameters such as pH, ionic strength, counterions, and impact of various conditions, such as the presence of organic matter, are strongly required.

Removing radionuclides
Nuclear energy has received attention because of its advantages over rapidly depleting conventional fossil fuel supplies. However, the nuclear industry generates radioactive pollutants having a half-life from months to millions of years and a high potential to disperse into the environment. Therefore, their complete elimination and disposal are critical in nuclear waste management.
The multilayer V 2 CT x MXene was used to extract U 6+ from wastewater. 67 DFT and EXAFS studies revealed that U(VI) adsorbs on V 2 CT x via MXene -OH termination. 68 A similar approach involved hydroxylated V 2 C nanosheets. 69 A strong association between uranyl ions and -OH ligands weakens U-O bonding at the adsorption sites. Furthermore, having -F terminations on the MXene surface is unfavorable because U-F bonds are weaker than U-O bonds.
In another report, researchers functionalized MXene with carboxylate-terminated aryl diazonium salt and increased its radionuclide chelating ability. 70 The carboxyl-terminated Ti 3 C 2 T x exhibits significant adsorption capacities such as 97.1 mg/g for Eu 3+ and 344.8 mg/g for U 6+ and greater than 90% radionuclide removal efficiency from simulated water.
Alongside uranium, thorium (Th 4+ ) is an alternative energy source for power plant operation in the nuclear energy sector, having weak but long-living radioactivity. 71 It was shown that Th 4+ adsorbs more efficiently to -OH saturated Ti 2 CT x surface than a dried one. A maximum removal capacity of hydrated Ti 2 CT x was 213.2 mg/g. Further XPS analysis revealed that the binding energy of Th 4+ to Ti 2 CT x is lower than for Th(NO 3 ) 4 , suggesting sorption and complexation mechanisms as a driving force for Th 4+ removal.
MXenes were also tested for cesium (Cs + ) removal sowing 25.4 mg/g capacity in neutral to slightly alkaline conditions. Interestingly, Cs + was adsorbed preferentially in the presence of competitive Li + , Na + , K + , Mg 2+ , and Sr 2+ cations. 72 MXene retained 91% of its performance for 5× sorption cycles and regeneration with 0.2 M HCl. The XPS combined with FTIR showed the leading ion exchange mechanism for Cs + removal from wastewater. Unfortunately, the selectivity was poor, with preferential binding to negatively charged citric acid rather than MXene.

Recovery of precious elements
Palladium (Pd 2+ ) is a precious element used in jewelry, electronics, and catalysis and needs to be recovered from processing wastes. For this purpose, MXene samples were synthesized with varying etching temperatures, such as 25, 35, and 45°C. 73 The 25, 35, and 45-MXenes showed 118.86, 163.82, and 184.56 mg/g adsorption capacities toward Pd 2+ , respectively. Higher temperature increases an interlayer d-spacing, thus providing more binding sites. 73 Apart from this study, other works confirmed MXene's affinity to precious metal ions.
The MXene's water dispersibility, large specific surface area, and presence of -OH surface groups allow delaminated Ti 3 C 2 T x to adsorb Cu 2+ ions up to 78.45 mg/g capacity, meaning 80% efficiency. A removal mechanism is based on Cu oxidation to CuO or Cu 2 O on the MXene's surface. 18 Notably, the removal of Cu 2+ was 2.7× higher than for activated carbon. However, the MXene's structure deteriorated while increasing adsorption-desorption cycles. This disadvantage resulted from copper oxide formation, chemically bonded and embedded in the MXene's surface TiO 2 oxidation layer.
MXene-derived titanate hierarchical nanostructures were synthesized by in situ oxidation to recover Eu 3+ by ion exchange mechanism. 74 First, the -OH groups present on the MXene surface become first saturated with Na + and K + ions. These active sites are further exchanged to hydrated Eu 3+ via strong electrostatic attraction. Altogether, reported results clearly show promising characteristics of MXenes in metal ion removal and recovery.
Yet, scientists still try to adjust the method of preparation materials to be used in industry. They are concentrated on achieving the state of equilibrium between the adsorbent and the adsorbate in a short time. The MXene is characterized by a large surface area and the functional groups on its surface, which depends on the synthesis process of materials. Thus, the adsorption properties are conditioned by their properties and preparation. However, the first MXene, which characterized excellent adsorption, was noted. Instead, the adsorption process with a 2D alk-MXene shows high selectivity with Pb 2+ effluent of about 2 µg/L for 4500 kg water/kg sorbent with a short equilibrium time of 2 min in the present study.

Application and environmental implications
The previously discussed examples of MXenes' ability to remove organic compounds suggest the potential to use them for pharmaceutical and cationic dyes using industries' wastewater treatment. MXene-based water treatment with membrane filtration also would be an exciting option. A flow bed enriched with MXene could be an effective treatment option for industrial wastewater or gaseous pollutants. However, using MXenes in the industry must involve safety analysis and an environmental impact.
Recent reports have shown that engineered nanomaterials could impact living species and that potential ecotoxicity can vary depending on dosage and time. However, a deep understanding of the MXene's impact on the environment is yet to come. In this field, few studies have revealed that MXenes are not ecotoxic. Green microalgae (Desmodesmus quadricauda) and higher plants such as sorghum (Sorghum saccharatum) and charlock (Sinapis alba) showed unique properties of the Ti 3 C 2 T x MXene. 52 At low concentrations, the MXene stimulated the growth of green microalgae. In addition, various MXene-based nanocomposites did not exhibit any toxic effects against studied organisms. For instance, the Ti 3 C 2 MXene surface-modified with Al 2 O 3 /Ag was not ecotoxic. Zebrafish embryos were also challenged with the Ti 3 C 2 T x MXene, which showed a lack of severe toxicity. 52 Notably, MXenes can be assumed to be inorganic crystalline contaminants that can be further bioremediated in the natural environment. This effect was revealed for green microalgae Raphidocelis subcapitata challenged by Nb-MXenes. 75 The microalgae could decompose Nb-MXenes via surfacerelated physicochemical interactions, oxidation into NbO and Nb 2 O 5 , and subsequent oxide consumption (Figure 6). An uptake-associated nutritional effect allowed any microalgae recovery and normal growth.
In general, it was revealed that the short-and long-term presence of MXenes in freshwater ecosystems causes mild environmental effects. However, lateral nanoflake size, thickness, chemical composition and functionalization, surface area, charge, and solution concentration are only a few examples of MXenes' parameters associated with synthesis conditions. These can further affect the environmental impact of MXenes, which needs to be researched.

Conclusion and outlook
This article discussed the development of MXene-based systems for environmental applications. We presented the most recent state of the art on sustainable approaches to MXene synthesis, its exceptional surface activity, and mechanisms underlying the removal of gaseous and organic contaminants, inactivating microorganisms, removal of heavy-metal ions, radionuclides, and also recovery of precious elements. The most viable methods are membrane technologies, primarily based on cheap and commonly used polymers or grainy bulk supports. Good compatibility of MXenes with polymers and other surfaces gives rise to extraordinary physicochemical properties in MXene-enriched hybrid membranes, including strong mechanical durability, excellent adsorptive behavior, high mechanical flexibility, and selective permeability.
Altogether, we presented the most concise overview of the potential application of MXenes for environmental remediation. We believe this article inspires and opens the door for many innovative studies on MXene-based technologies that will meet environmental safety requirements.
Scientists observed that MXenes are promising in eliminating SO x , NO x , NH 3 , H 2 S, and CH 4 . Moreover, they observed the activation energy (3.69 eV) for CO 2 and extensive loading up to 8.25 mol CO 2 for kg. Thus, the unusual properties of MXenes suggest that they are brilliant to use as an active part of a flowing bed. The materials could also remove organic contaminations such as organic dyes, pharmaceuticals, and soil pollutants with the adsorption and photocatalysis process. Scientists showed about 97% decomposition of methylene blue during the Fenton process with the MXene as a catalyst. The excellent results (91.1% of ranitidine removal and 81.8-84.8%) was also obtained during the in situ reductive deposition method with MXenes. The unusual decomposition results were the effect of the active surface chemistry of MXenes, their semiconductor properties, and interfacial effects. Similar excellent adsorption efficiency was also observed for heavy metals and radionuclides. In addition, MXenes could inactivate microorganisms and be safe for organisms and the environment, such as green microalgae.
MXenes present a wide range of promising features for environmental remediation. However, we see here challenging technological limitations that should be removed to broaden the application spectrum for MXenes and fully exploit their potential at a large scale. In particular, MXene synthesis still involves toxic compounds. Hence, developing ecologically benign methods to produce MXenes with green chemistry approaches is essential. MXene engineering needs to be extensively explored to enable its use in various environmental applications and long-term storage.
To date, reported adsorption experiments have been done mainly on a laboratory scale that does not correctly represent real contaminated sites. Furthermore, these investigations include higher pollutant concentrations that are not environmentally realistic. The same situation refers to adsorption studies based on the batch adsorption approach. For future research, column-based dynamic operations are required to achieve commercial viability. Also, a significant aspect to consider is MXenes' life-cycle assessment in the environment, which will help assess their pathway and fate within ecosystems.