1 Introduction

MXenes (pronounced “maxines”), a new family of 2D transition metal carbides, nitrides, and carbonitrides, were discovered by researchers at Drexel University in 2011 [1,2,3,4]. The first-ever MXene comprised of 2D titanium carbide (Ti3C2) was synthesized by selectively etching the “A” (Al atoms) in layered hexagonal ternary carbide, Ti3AlC2, with hydrofluoric acid (HF) at room temperature [1]. MXenes are represented by the general formula Mn+1XnTx (n = 1–3) and are derived from the precursor MAX phase (Mn+1AXn), where M is an early transition metal, X is carbon and/or nitrogen, A represents an element from groups 12 to 16, T denotes the surface termination groups such as fluorine (−F), oxygen (=O), chlorine (−Cl), and hydroxyl (–OH), and x represents the number of surface functionalities [2, 64]. Nitrogen do** significantly enhances the surface area of MXene to 368.8 m2 g−1 that is the highest value reported in the literature for any MXene-based electrode. N–Ti3C2Tx demonstrated an average salt adsorption capacity of 43.5  ±  1.7 mg g−1 under 1.2 V in 5000 mg L−1 NaCl solution. The electrode shows good stability over 24 CDI cycles. The etching process also influences the desalination characteristics of MXene electrode. Ma et al. [65] employed the LiF/HCL etching method to prepare a freestanding Ti3C2Tx MXene electrode without any binder and evaluated its desalination performance. The LiF/HCl etching resulted in the increased interlayer spacing of Ti3C2Tx and enhanced desalination capacity. The electrode exhibited a desalination capacity of 68 mg g−1 at 1.2 V for NaCl concentration of 585 mg L−1.

Ar plasma modification of MXene nanosheets resulted in the increased interlayer distance between the sheets and hence improved desalination performance [60]. The surface of Ti3C2Tx was modified to introduce amorphous carbon and anatase TiO2 layer using Ar plasma treatment. The desalination performance of the electrode was evaluated using 500 mg L−1 NaCl solution in the voltage range of 0.8–1.6 V, as shown in Fig. 11e. The maximum removal capacity of 26.8 mg g−1 was obtained at 1.2 V. The Ti3C2-based electrode showed good regeneration ability and reproducible results for several cycles of electrosorption and desorption.

Desalination performance of MXene electrode is influenced by the operating conditions such as flow rate, half-cycle length (HCL), and discharge potential [67]. Agartan et al. [67] reported that salt adsorption rate and capacity increased by 152% at lower discharge potentials and decreased at faster flow rates. Likewise, half-cycle length decreased salt adsorption rate by 54% and capacity by 32%. Preconditioned MXene electrodes exhibited better volumetric performance than activated carbon cloth electrodes owing to their hydrophilicity and high electrochemical activity.

2.3 Solar Desalination

MXenes have superb light-to-heat conversion efficiency that makes them an ideal applicant for application in solar-based desalination [71]. The photothermal water evaporation capability of MXenes is yet another energy-efficient characteristic of these fascinating 2D materials. Table 3 enlists the solar evaporation performance of MXene membranes.

Table 3 Solar evaporation performance of MXene membranes
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Zhao et al. [72] reported the synthesis and solar desalination potential of the hydrophobic MXene membrane. The delaminated Ti3C2 (d-Ti3C2) was obtained by HCl/LiF etching from the MAX phase, followed by vacuum deoxidation and ultrasonication, as shown in Fig. 13. The hydrophobic membranes were obtained by surface modification of the d-Ti3C2 with trimethoxy(1H,1H,2H,2H-perfluorodecyl)silane (PFDTMS) [72].

Fig. 13
figure 13

Copyright © (2019) Royal Society of Chemistry

Schematic illustration of a fabrication process of the hydrophobic d-Ti3C2 membrane, and b the hydrophobic d-Ti3C2 membrane-based solar desalination device. Reprinted with permission from Ref. [72].

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The hydrophobic MXene membrane obtained after PFDTMS modification was employed in a solar evaporation device that was self-floated on the seawater. The membrane achieved a solar steam conversion efficiency of 71%, the solar evaporation rate of 1.31 kg m2 h−1, and stability under high salinity conditions over 200 h under one sun. The rejection rate for the four primary ions (Ca2+, Mg2+, Mg2+, and Na+) was over 99.5%, while for organic dyes and heavy metals, nearly 100% rejection rate was attained, as shown in Fig. 14. These membranes are not appropriate for long-term solar desalination applications due to poor salt-blocking after an elongated period.

Fig. 14
figure 14

Reprinted with permission from Ref. [72]. Copyright © (2019) Royal Society of Chemistry

a Measured salinity of four primary ions before and after solar desalination. b Organic and heavy metal ion rejection performance.

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MXene coating improved the antifouling and photothermal characteristics of the PVDF (polyvinylidene difluoride) in a solar-assisted direct contact membrane distillation system [73]. MXene-coated membrane demonstrated around 56% reduction in flux decline and a 12% drop in heater energy input per unit volume of distillate. However, MXene-coated membranes exhibited lesser fluxes due to the presence of an additional coating layer.

Zhang et al. [74] reported the desalination performance of vertically aligned Janus MXene aerogel (VA-MXA) with two layers, i.e., hydrophilic (at the bottom) and hydrophobic (at the top). The process of VA-MXA synthesis is presented in Fig. 15. MXene obtained from the Ti3AlC2 phase is frozen by liquid nitrogen under Ar protection in a polytetrafluoroethylene (PTFE) mold with a Ti plate. The freezing process yields a black frozen material consisting of ice crystals surrounded by Ti3C2 nanosheets. A vertically aligned framework of the Ti3C2 nanosheets is obtained by removing ice crystals via vacuum freeze-drying. The freestanding VA-MXA was placed in a ring-shaped sponge mold, and the hydrophobic layer was formed by floating the sponge on fluorinated alkyl silane under vacuum conditions, followed by drying under Ar environment.

Fig. 15
figure 15

Reprinted with permission from Ref. [74]. Copyright © (2019) American Chemical Society

a Fabrication process of a Janus VA-MXA with vertically aligned channels. b Digital photograph of the top view of the as-prepared Janus VA-MXA. c Photograph of the side view. d Photograph of the fracture face. e SEM images of the upper layer of prepared Janus VA-MXA.

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The hydrophilic bottom layer of VA-MXA pumped water and the hydrophobic upper layer absorbed light. The salts crystallized on the hydrophilic bottom layer are quickly dissolved due to continuous pum** of water. The VA-MXA demonstrated an excellent water evaporation rate of 1.46 kg (m2 h) −1 and conversion efficiency of ~87%. The high water evaporation rate is attributed to the strong capillary pum** and fast water diffusion through the vertically aligned channels of the VA-MXA.

2.4 Pervaporation Desalination

Pervaporation desalination is a combination of water diffusion through a membrane followed by its evaporation into the vapor phase on the other side of the membrane. MXene/PAN composite and freestanding MXene membranes were prepared by vacuum filtration of the MXene suspension through the polymeric substrate, as shown in Fig. 16 [79]. It is essential to develop an effective technique for storing MXene solution for a long time without oxidizing.

The traditional method for the synthesis of MXenes using hazardous HF is associated with serious health and environmental concerns. The replacement of HF with green or less toxic chemicals can assure an environment-friendly technique for the synthesis of MXenes. There are some attempts in recent times to substitute HF with less toxic chemicals [8, 80, 81]. However, more attention is required to advance research in this direction. Furthermore, the difficulty in the synthesis of MXenes with uniform and pure surface termination is another obstacle in their practical applications [82].

Another major challenge is the high cost and low yield of MXenes production [17, 18]. Currently, MXenes are mainly produced at the laboratory scale with a small yield. The design of a cost-effective, efficient, and environment-friendly system for the large-scale production of MXenes will be helpful to further advance research in this field and will open a new door of possible applications of MXenes on a commercial scale. It is expected that the cost will be comparatively low for large-scale production.

One more crucial challenge is the need for life cycle analysis and assessment of potential toxic effects of MXenes and MXene-based nanomaterials [18, 83, 84]. Still, studies on the potentially toxic effects of MXenes are limited. With the rapid deployment of MXenes in various applications, it is necessary to fully investigate its lethal effects on the environment, human health, and other organisms. Surface modification of MXenes could be effective in improving its stability, biocompatibility, and recyclability and reducing cytotoxicity. The aggregation of MXenes is also an issue that reduces the adsorption capability and surface area of these 2D materials. The surface chemistry of MXenes and their influence on the removal of pollutants must be further explored to fully understand the removal mechanisms.

Until now, Ti3C2Tx is extensively employed in desalination and water treatment, and it is essential to develop a new MXene structure and discover other environmental remediation applications of various MXenes. Moreover, several theoretical studies such as DFT predicted superior characteristics of MXenes in desalination and environmental remediation applications [17, 85,86,87]. Proper experimentation and development of an efficient system are required to confirm the results of these theoretical studies [88,89,90]. There is no suspicion that commercial MXene-based product will be introduced in the market soon and MXenes will discover their role in the future direction of desalination technology. Based on the current promising results, it can be securely foreseen that MXenes could be the next-generation materials for water treatment and desalination.

4 Conclusion

MXenes and MXene-based nanomaterials have offered tremendous advantages, and they have emerged as ideal entrants for future desalination technology. Despite copious hurdles that need to be addressed, based on the promising results from the current research, a remarkable development in the synthesis techniques and applications of these exceptional nanomaterials is anticipated in the near future. For MXenes to be a forerunner in desalination, further research is vital to overawe the existing hurdles. There is no suspicion that MXenes has assured an era of the next-generation 2D nanomaterials and will have a bright future in water purification and environmental remediation.