Recent progresses in the modification strategies of MXene-based membranes for water and wastewater treatments

Abstract

Membrane technology stands as a leading method for water and wastewater treatments. MXene, a type of two-dimensional material, has garnered significant interest as a promising next-generation membrane material. Its customizable pore structure, uniform pore size, and hydrophilicity make it highly suitable for membrane separation technologies. This work elucidates the modification strategies employed by MXene-based membranes (MBMs) and evaluates their performances. Initially, the preparation of MXene nanosheets, pivotal to membrane fabrication, is detailed. Subsequently, the fabrication methods and existing problems of MBMs are presented. Furthermore, we emphasize the modification strategies employed to enhance the performance of MBMs. These encompass the regulation of MXene nanosheets in terms of lateral size, terminal functional groups, and in-plane pores. Furthermore, adjustments are made to the membrane assembly processes, focusing on controlling the interlayer spacing. This includes methods such as self-crosslinking, insertion, and the incorporation of hybrid functional layers. Additionally, surface modifications encompass the regulation of surface charge, surface wettability, and management of surface defects. Next, we delineate the key membrane applications, encompassing separation mechanisms and their promising utilities. Lastly, we present the challenges and opportunities that MBMs face in the field of water purification, with the hope of providing profound insights into the design and synthesis of advanced MBMs.

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Yingchao Du

Yingchao Du is a postdoctoral fellow in the College of Environmental and Resource Sciences at Zhejiang University under the guidance of Prof. Xiaoying Zhu. She received her Ph.D. from the Chinese Academy of Sciences in 2021. Her current research focuses on the preparation and applications of MXene-based membranes.

Jingyu Yu

Jingyu Yu is currently a master's degree student in the College of Environmental and Resource Sciences at Zhejiang University under the guidance of Prof Xiaoying Zhu. She received her bachelor's degree from the College of Life and Environmental Sciences, Wenzhou University in 2023. Her current research focuses on MXene-based membranes for water treatment applications.

Baoliang Chen

Baoliang Chen is currently a professor in the College of Environmental and Resource Sciences at Zhejiang University. He received his doctoral degree from the Zhejiang University in 2004. His research focuses on soil pollution control and remediation, and the design and application of environmentally friendly adsorption materials.

Xiaoying Zhu

Xiaoying Zhu is currently a professor in the College of Environmental and Resource Sciences at Zhejiang University. He received his doctoral degree in Civil and Environmental Engineering from the National University of Singapore in 2012. His research focuses on membrane functional materials, including the regulation of the interface properties of membrane functional materials and their microscopic characterizations.

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Environmental significance

Substantial progress has been achieved in the development of highly selective separation membranes for a range of water treatment processes, including desalination, water purification, pollution remediation, and resource recovery. Research has been intensively targeted on advanced materials to boost the separation efficiency. Notably, atomic-thin 2D materials are widely studied for their potential in fabricating high-performance membranes with nanoscale precision. The rise of MXene-based membranes, known for their selective ion transport and regulated molecular flow, has piqued significant scientific curiosity in water treatment advancements. This review summarizes the common modification strategies of MXene-based membranes in water and wastewater treatment, aiming to design advanced membranes. It is expected to provide valuable insights for implementing MXene-based membranes with excellent performances, thus contributing significantly to the field.

1. Introduction

Membrane separation is widely recognized as an effective water purification technology due to its numerous advantages, including low energy consumption, easy operation, and high efficiency.¹ The pressure-driven membranes including nanofiltration (NF), reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF) membranes are widely used in various water treatment processes, encompassing desalination, water purification, pollution remediation, and resource recovery.² In addition, 2D materials have gained significant attention in the field of environmental remediation and are often referred to as “wonder materials” due to their unique properties, such as high aspect ratio, excellent mechanical properties, and customizable and controllable structure, and potential applications in addressing environmental challenges.³ Additionally, the unique properties of 2D materials have made them attractive for the fabrication of high-performance membranes with both high separation and performance capabilities. Graphene-based membranes, in particular, have paved the way for the development of various 2D materials for membrane applications. Consequently, the development of 2D material-based membranes, including graphene, MoS₂, zeolites, covalent organic frameworks, and metal–organic frameworks, hold great promises in various membrane separation applications.^4–7

MXenes, a family of 2D transition metal carbides, nitrides, and carbonitrides, were introduced in 2011 by Yury Gogotsi's group at Drexel University and quickly drew attention due to their impressive properties. These materials, derived from the selective etching of A layers from MAX phases, had shown potential in areas such as catalysis, energy, and environmental applications.^8,9 The MAX phases can be expressed by the formula M_n+1AX_n, where M is an early transition metal, A is a IIIA or IVA element (Si, Al, Ga and Ge), X is a carbon or nitrogen atom, and n ranges from 1 to 3. As for the first artificially prepared MXene Ti₃C₂T_x, T stands for the terminal functional groups, such as oxygen, fluorine and hydroxyl, and X stands for the number of these functional groups. The unique properties of MXenes, including their customized pore structure, uniform pore size distribution, hydrophilicity, and strength, make them promising candidates for membrane fabrication. Over the past few decades, researchers have successfully synthesized over 30 different types of MXenes whose potentials for advanced membrane preparation have been explored on a laboratory scale, exploring their potential applications in membrane technology.¹⁰ As shown in Fig. 1, the MXene-based membranes have received widespread attention and have shown a fast growing tendency as compared to the membranes prepared from other 2D materials; in addition, the applications of the MXene-based membrane in water and wastewater treatments are rapidly increasing.

Fig. 1 Number of publications on MXenes, MOFs, COFs, MoS₂ and zeolite membranes: (a) publications from 2014 to 2024, and (b) publications on the applications of MXene-based membranes based on the web of science databases.

Given the promising, accessible, and versatile nature of MXenes as 2D materials, several reviews have covered the performance-enhancing modifications of MBMs in separation processes. For instance, Sun et al. provided a concise overview of the new designing approaches to enhance the performance of MXene-based membranes including physical intercalation, chemical cross-linking, and membrane surface modification.² Kwon et al. mentioned the membrane modification strategies of the interlayer control and surface modification.¹¹ In addition, Lin et al. described the preparation and physicochemical properties of MXene nanomaterials, as well as the construction and modification of MBMs.¹² Khosla et al. introduced the latest MXene-polymer hybrid membranes, focusing on advancement and tuning in attributes of MXene-polymer hybrid membranes.¹³ Nonetheless, it was frequently overlooked that the alteration of MXene nanosheets and the manipulation of MXene-based membranes (MBMs) were equally crucial to the separation efficiency of membrane materials. Consequently, there was a paucity of reports delving into the modification strategies that specifically enhanced the filtration performance of MBMs, particularly from the perspectives of regulating the MXene nanosheets and optimizing the assembly process of the membranes. Herein, it is vital to sum up a comprehensive review covering the modification strategies of the MXene-based membranes in the water and wastewater treatment for designing and exploiting advanced membranes. As shown in Fig. 2, the main contents of this review are divided into four parts: 1) preparation methods of MXene nanosheets and MBMs, respectively, 2) strategies for the performance improvement of MBMs, covering the regulations of MXene nanosheets and the adjustments of membrane assembly process, 3) applications of MBMs in water and wastewater treatment and separation mechanisms, and 4) challenges and opportunities for MBMs in water treatment. This review is expected to provide valuable insights into the design and implementation of MBMs with excellent performance.

Fig. 2 Main contents of this review.

2. A brief introduction to MXene nanosheets

Thus far, more than 30 types of MXenes have been synthesized and examined in laboratory settings. MXene nanosheets, crucial for MXene membrane fabrication, required specific methods for isolation due to their strong bonding within MAX phases. Generally, a three-step (etching-intercalation-delamination) procedure was put forward to obtain few-layer or single-layer MXene sheets.¹⁴ The properties of MXene nanosheets depended on the preparation conditions, etchants used, and their concentration. These resulting 2D MXenes possessed negative charges and hydrophilic surfaces, rendering them highly attractive for developing membranes with exceptional properties.¹⁵

2.1 Preparation of MXene nanosheets

Since 2011, MXene nanomaterial preparation techniques have advanced significantly, with synthesis methods represented in detail in Fig. 3, including representative preparation strategies. These strategies allowed for the isolation of MXene nanosheets from MAX or non-MAX phase precursors, facilitating the formation of MXene phases from diverse element combinations. The preparation methods of MXene nanosheets generally fall into two categories, namely chemical and physical methods.

Fig. 3 Timeline of synthesis development to fabricate novel MXene nanomaterials.^9,16–24 (Reproduced from ref. 9 with permission from Wiley Online Library, copyright 2011; reproduced from ref. 16 with permission from Springer Nature, copyright 2014; reproduced from ref. 17 with permission from Springer Nature, copyright 2015; reproduced from ref. 18 with permission from the Royal Society of Chemistry, copyright 2016; reproduced from ref. 19 with permission from the Royal Society of Chemistry, copyright 2017; reproduced from ref. 20 with permission from Elsevier, copyright 2018; reproduced from ref. 21 with permission from Springer Nature, copyright 2020; reproduced from ref. 22 with permission from Wiley Online Library, copyright 2020; reproduced from ref. 23 with permission from Elsevier, copyright 2022; reproduced from ref. 24 with permission from the American Chemical Society, copyright 2017).

2.1.1 Chemical methods for MXene nanosheets preparation. These chemical techniques leveraged the differences in bonding energy and chemical reactivity between the M-A bonds and the metallic bonding within the layers.^9,25 Wet chemical etching techniques indeed facilitated the surface functionalization of MXene layers, leading to the expansion of the lattice along the c-axis.¹⁴ During the etching process, the removal of A layers from MAX phases and the subsequent formation of MXene nanosheets could lead to a decrease in the chemical potential and an improvement in thermodynamic stability.²⁶ When exposed to highly concentrated HF solutions, A elements were prone to eliminate from the crystal structure:

M_n+1AlX_n + 3HF = AlF₃ + 1.5H₂ + M_n+1X_n (1)

The stacked MXene multilayers were retained by hydrogen bonding and van der Waals forces. Then, some organic molecules including tetramethylammonium hydroxide (TMAOH), tetrabutylammonium hydroxide (TBAOH) and dimethyl sulfoxide (DMSO) were applied to chemically intercalate into these produced MXenes, which were delaminated into single- or few-layer sheets by sonication.^27,28 As shown in Fig. 4(a), the HF-containing etchants such as ammonium bifluoride (NH₄HF₂) salts and in situ HF through addition of lithium fluoride (LiF) salts to hydrochloric acid (HCl) were applied to synthesize MXene sheets and Ti₃C₂T_x “clay”, respectively.²⁴ This method generated terminal groups (–F, –O, and –OH) that endowed Ti₃C₂T_x materials with intrinsic hydrophilicity, giving them better tunability than that of other 2D materials. In addition, the LiF/Ti₃AlC₂ molar ratio and sonication could have strong relation to the lateral size of Ti₃C₂T_x nanosheets. To reduce the layer interactions for MXene nanosheet preparation, organic solvents are employed, requiring solvent exchange pretreatment for water delamination, with limited reports of stable nanosheet dispersions in organic solvents.²⁹ For water-free synthesis, NH₄HF₂ and polar organic solvents are used to fabricate Ti₃C₂T_x MXenes, resulting in large interlayer spacings due to NH⁴⁺ and solvent intercalation.²² In addition, NH₄HF₂ can be dissolved in high-boiling-point dimethylsulfoxide for efficient, high-yield anhydrous synthesis.³⁰ Water-free etchants enhanced MXene chemical stability and reduced oxidation, aiding in defect removal. Recent methods using halogen compounds and iodine in anhydrous acetonitrile had been proposed to produce MXene derivatives with increased interlayer spacings and halogen homolysis, which is shown in Fig. 4(b).^31,32

Fig. 4 (a) General map for Ti₃C₂T_x synthesis from Ti₃AlC₂ (ref. 24) (reproduced from ref. 24 with permission from the American Chemical Society, copyright 2017), (b) iodine-assisted etching and delamination towards 2D MXene sheets³² (reproduced from ref. 32 with permission from Wiley-VCH GmbH, copyright 2021), (c) schematic of the experimental setup and underlying physiochemical mechanism responsible for the SAW-facilitated derivation of Ti₃C₂T_z MXene³³ (reproduced from ref. 33 with permission from the American Chemical Society, copyright 2021) and (d) schematic of the fabrication of 2D Ti₂C MXenes via an effective thermal reduction approach³⁴ (reproduced from ref. 34 with permission from Elsevier, copyright 2020).

Alternative etching techniques to HF, such as electrochemical etching in HCl, have been developed. In dilute HCl, Al from a MAX electrode could form a MXene layer electrochemically. This fluoride-free process has a predictable byproduct, and a core–shell model was suggested to control the etching parameters and avoid over-etching.¹⁹ As for the selective removal of Al layers, the design and choice of electrolytes was significant. Furthermore, these MXenes exclusively featured –Cl terminal groups, alongside commonly found groups such as –O and –OH. Nonetheless, electrochemical etching can lead to the subsequent over-etching of parent MAX phases, resulting in the formation of carbide-derived carbon (CDC).¹⁹ Similarly, a binary aqueous system was used as the electrolyte to achieve the anodic corrosion of Ti₃AlC₂ in high yields (>90%) and large average dimension, which was comparable or even better than those techniques using HF or LiF/HCl. Chemical vapor deposition (CVD) is a versatile method for producing ultrathin, consistent Mo₂C MXene films, capable of being deposited on various substrates such as Cu/Mo bilayers, graphene, and MoO₂. The process involved exposing substrates to a precursor gas mixture in a controlled atmosphere, facilitating Mo₂C MXene layer formation via chemical reactions.^17,35,36 The one-step direct synthesis of a 2D Mo₂C-on-graphene membrane by molten copper-catalyzed CVD has been documented. High-quality and uniform Mo₂C membranes, spanning centimeters, can be grown on graphene using a Mo–Cu alloy catalyst. The graphene-templated growth of Mo₂C yielded well-faceted, large-sized single crystals with low defect density. Salt-templated growth was a method commonly used for the synthesis of various metal nitrides including Mn₃N₂, W₂N, and V₂N, which involved dissolving metal salts in suitable solvents and adding a reducing agent to prompt precipitation. The ammoniation of the 2D h-MoO₃@NaCl powder and the subsequent dissolution of the salt template resulted in the production of hydrophilic 2D MoN nanosheets with subnanometer thickness, dispersed in water.³⁷ The resulting particles were typically trapped in the template pores, creating hierarchical structures with distinctive properties.³⁷ NaCl and KCl templates could cause lattice distortion, leading to various MXene structures.³⁸ Adjusting precursors and synthesis parameters can trigger the formation of diverse 2D metal nitrides and carbides with a subnanometer thickness. Inspired by the Bayer process used in bauxite refining, an alkali-assisted hydrothermal method was developed to fabricate multilayer MXenes without F terminations. The temperature and concentration of NaOH played crucial roles in breaking the barrier of the dynamic process and eliminating the generation of oxide hydroxides.²⁰

2.1.2 Physical methods for MXene nanosheet preparation. In addition to chemical etching methods, various physical techniques were employed to expedite the rapid synthesis of MXenes. These methods include megahertz frequency acoustic excitation, ball-milling, sputtering, and thermal annealing. They leveraged substantial mechanical forces and protonation processes to selectively etch aluminum and subsequently delaminate MXene layers. Notably, these physical techniques offered significantly reduced synthesis times compared to conventional methods. Ultrafast conversion techniques demonstrated a remarkable reduction in synthesis time from 24 hours to milliseconds, as depicted in Fig. 4(c).³³ This accelerated synthesis approach has several advantages including heightened production efficiency, reduced costs, and faster material development.³³

A chemical-combined ball-milling method combines the appropriate etchants with ball milling, which is a simple and green technique for the large-scale production of MXenes. In addition, this approach enabled the production of Ti₃C₂T_x with extra-high specific surface area and hierarchical porosity, featuring solely oxygen-containing groups without over-oxidation.³⁹ Thus, this simple strategy could be extended as a universal approach for preparing various fluorine-free and porous MXenes with a significantly increased specific surface area, which was 8 times higher than that of Ti₃C₂ obtained from traditional HF treatment.³⁹

No scandium-based MXenes have ever been successfully fabricated, and magnetron sputtering was employed to fabricate semiconductive MXenes with a direct band gap, complemented by a thermal annealing process to introduce further oxidation.⁴⁰ Magnetron sputtering of Ti, Al and C can form a few-nanometer TiC incubation layer on a sapphire substrate, followed by the deposition of epitaxial Ti₃AlC₂, and then selectively etched Al from MAX. Through improved control of the deposition process to minimize or eliminate non-basal growth, there exists the potential to enhance the conductivity.⁴⁰

An innovative thermal reduction strategy was introduced for producing MXenes from sulfur-containing MAX phases. In this method, weakly bonded S atoms reacted with hydrogen, forming a volatile gas and leaving behind 2D graphene-like Ti₂C nanosheets, which is shown in Fig. 4(d).³⁴ Diverging from conventional acid etching, our approach involved conducting the etching step under reducing conditions in the presence of hydrogen gas. At an optimal treatment temperature, the weakly sandwiched sulfur layers between metal ions and carbon layers in the MAX phase crystals were nearly eliminated, resulting in weakly stacked Ti₂C MXene structures. Notably, if the reduction temperature was too low or too high, a mixture of Ti₂SC MAX phase and Ti₂C layered structures was obtained, underscoring the criticality of the reaction temperature. This innovative synthetic method represents a groundbreaking approach to producing 2D MXenes from sulfur-containing MAX phases. The controllable thermal reduction strategy is easily scalable, avoids the use of hazardous acids, and thus, holds tremendous potential for future industrial production.³⁴

MXenes were obtained through high-temperature etching of MAX phases, such as the treatment of Ti₄AlN₃ in a molten fluoride salt mixture at 550 °C under an argon atmosphere to form Ti₄N₃ MXenes.¹⁸ Temperature and atmosphere are pivotal factors in the synthesis and annealing of MXenes. At moderate temperatures, molten salts can form Lewis acids and basic molten salts containing CuCl₂ and CdCl₂, facilitating the etching of A-layer elements from the MAX phase. This process led to the generation of Cl⁻ and Br⁻⁻terminated MXenes due to the easy oxidation of A elements to produce volatile compounds.²¹ Additionally, it was feasible to modify the weak bond energy of M–Cl and M–Br bonds, enabling the design of MXenes with terminal versatility, incorporating oxygen, sulfur, and imido groups.⁴¹

2.2 Structure and chemistry of MXene nanosheets

The 2D morphology of MXenes is of particular importance due to the high aspect ratio of regular MXene nanosheets. These nanosheets endowed the resulting membranes with nanochannels, which lengthened diffusion pathways for solute transport. These nanochannels were formed by pinholes, internal spacings between parallel and wrinkled nanosheets (known as nanogalleries), and voids between the nanosheet edges, which could be considered as nanoslits. The elongation of permeation pathways allows for the differentiation of solute transport rates, forming the basis of selectivity for MXene membranes.⁴² Moreover, by engineering the interlayer spacing of 2D MXene assemblies, well-defined transport channels can be created, enabling size-exclusion-based molecular separation. The interlayer spacing between Ti₃C₂T_x nanosheets can be reduced to a range of 0.52–0.38 nm when dried and/or heated. Additionally, the lateral dimension (flake size) of MXene nanosheets is adjustable from 6 to <1 μm by varying the etching and exfoliation conditions. Furthermore, MXenes are not perfectly flat materials and can become wrinkled, adding to their microstructural properties.⁴³

The processibility of MXene nanosheets plays a crucial role in the membrane preparation process, particularly their excellent dispersity and stability in aqueous solution. The surface of MXene nanosheets typically contains abundant hydrophilic functional groups such as –OH, which help to form and stabilize dispersions.⁴⁴ Moreover, this excellent dispersity is conducive to functionalizing the MXene nanosheets. MXenes are well-suited for light-to-heat harvesting due to their high electromagnetic interference shielding effect, which is related to their prominent absorption ability. The exceptional electromagnetic wave absorption of MXenes prompted researchers to investigate their sunlight absorption ability and corresponding heat conversion. Furthermore, MXenes exhibit excellent electrical conductivity, which has the potential to modify the surface charge and control ion permeation. The high chemical and mechanical stabilities of MXenes are advantageous for water purification. Recent research has also demonstrated that MXenes possess high antibacterial efficiency towards both Gram-negative E. coli and Gram-positive B. subtilis. The versatile chemistry of MXenes allows for the control of properties for a wide range of applications including energy storage, biosensing, lubrication, and catalysis.¹⁵

3. Fabrication, characterization and existing problems of MXene-based membranes

Membranes made from 2D materials can be divided into two main types: those with in-plane pores and lamellar membranes without pores in their sheets. MXene membranes fall into two categories, namely lamellar MXene membranes and MXene nanosheets dispersed within a polymer matrix (MMMs).⁴⁵ Lamellar membranes are characterized by layered structures of stacked MXene nanosheets, while MMMs incorporated MXene nanosheets as fillers in a polymer matrix to enhance the membrane performance.⁴⁶

3.1 Fabrication of MXene-based membranes

The stacking order and arrangement of MXene nanosheets in lamellar membranes significantly affect their separation properties, determining the interlayer spacing and molecule/ion transport. Preparation methods are crucial for achieving the desired membrane structure, with techniques such as filtration, coatings, and other methods evolving to improve the fabrication of MBMs. These methods are primarily categorized into three types, as shown in Fig. 5.

Fig. 5 Fabrication methods of MXene-based membranes^47–54 (reproduced from ref. 47 with permission from Elsevier, copyright 2021; reproduced from ref. 48 with permission from Wiley Online Library, copyright 2020; reproduced from ref. 49 with permission from Springer Nature, copyright 2019; reproduced from ref. 50 with permission from Wiley Online Library, copyright 2020; reproduced from ref. 51 with permission from Elsevier, copyright 2020; reproduced from ref. 52 with permission from the American Chemical Society, copyright 2021; reproduced from ref. 53 with permission from Wiley Online Library, copyright 2023; reproduced from ref. 54 with permission from Elsevier, copyright 2015).

3.1.1 Assembly of MBMs through filtrations. The hydrophilic terminal groups (–O and –OH) on MXenes are key to producing stable and uniform suspensions, allowing effective dispersion in water and colloidal solutions suitable for the laminar membrane assembly. Uniform MXene nanosheet dispersion is crucial for achieving the desired layer orientation and stacking density in membranes.⁵⁵ In the fabrication of MXene membranes, two common filtration methods are pressure-assisted filtration and vacuum-assisted filtration. The choice of method significantly influences the resulting membrane structure. Primarily, the majority of MBMs are fabricated by vacuum-assisted filtration on porous substrates.^56,57 The quality of these MXene membranes is closely related to the pressure gradient, the concentration of dispersion, and the state of the nanosheets. As the membrane thickness increases, the vacuum-driving force gradually gets reduced, leading to the production of a loose and random structure in the top layer. Conversely, pressure filtration, due to its stable driving force, can produce a dense, uniform, and ordered structure, commonly used for thicker membranes.⁵⁴ Given the relatively high pressure exerted on MXene membranes, the use of a porous substrate is required, such as commercial polyvinylidene fluoride (PVDF) supports, poly(ether sulfones), and poly(ether sulfones).^58–60 Optimizing the operational parameters and controlling the state of the MXene nanosheets are essential for fabricating high-quality MXene membranes with the desired properties. However, these methods required a long time and large volumes of liquid to achieve the alignment of MXene nanosheets.⁶¹

3.1.2 Preparation of MBMs by coating assembly. Coating techniques such as dip coating, spin coating, spray coating, and blade coating are commonly used for membrane fabrication, with the structure of the prepared membranes dependent on surface conditions, evaporation rates, and support properties.⁵⁵ Spin coating with shear force produces a smooth, well-oriented membrane, while dip coating is suitable for constructing thin MBMs on hollow fiber substrates.^62,63 The spray coating is considered as the most useful method for the fabrication of MXene-based devices, while this method is just utilized on a small scale and mostly by hand-spraying techniques. This spray coating can quickly obtain thin membranes, but the uniformity is hard to achieve, due to the flow rate, the distance between the substrate and the nozzle, the substrate temperature, and solvent parameters.⁴⁸ In order to overcome the disadvantages and extend the application range, many techniques are equipped with spray coating, such as spin spray technology, ultrasonic spray technology, ultrasonic spray pyrolysis, mask-assisted spray coating, and automatized ultrasonic spray coater.^64–68 Blade coating allows for improved control over the alignment of flakes during processing and eliminates the need for a filtration setup, simplifying the production process and enabling large-scale manufacturing of coatings and films. The blade speed and the height between the substrate and the blade edge control the applied shear, which can induce the alignment of MXene flakes and highly densified stacking of MXene flakes.⁵¹ The slot-die coating and rod-coating are versatile methods for preparing continuous and scalable coatings.^52,53 Casting is a versatile technique that allows for the preparation of composite membranes with tailored properties by incorporating desired additives or modifications.⁶⁹ By utilizing the spin-casting method, researchers are able to create highly ordered MXene membranes with desirable properties.^70,71 Freezing in combination with casting can produce ultralight free-standing MXene aerogels with micro-sized pores determined by ice crystal morphology.⁷²

3.1.3 Assembly of MBMs through other methods. Facial electrophoretic deposition (ECD) outperforms traditional vacuum filtration, offering a higher membrane area by two orders of magnitude, a tenth of the preparation time, and the capability to adjust MXene membrane thickness from 100 nm to 1 μm. ECD's negative MXene nanosheets preferentially deposit on the cathode, creating well-ordered 2D membranes, especially with larger flakes.⁴⁷ ECD met the need for a smooth, conductive surface with average pore size, despite potentially raising costs and complexity. This approach facilitated the creation of structured membranes for targeted separation applications.

The high dispersibility of MXenes in solutions is crucial for the development of MXene-based printing inks, enabling efficient integration into ink formulations and ensuring a uniform distribution. This allows for precise and controlled printing, potentially transforming manufacturing and applications by enabling the direct printing of functional materials and structures.⁵⁰ Printing techniques are generally categorized as digital (e.g., inkjet and 3D printing) and nondigital (e.g., screen and transfer printing), with the choice for MXene membranes depending on the required structure, resolution, scalability, and method-specific capabilities.^48,49

MXene flakes, abundant in hydrophilic terminations, readily disperse in polar solvents, offering the potential for polymerization with various monomers. Furthermore, the unique channels formed by the nanosheet stacking enable precise molecular separation through prepared MXene membranes. The VAF or PAF technique, relying on interlayer van der Waals (VDW) and hydrogen bonding interactions of MXene nanosheets, is widely used as a direct approach to produce freestanding or supported membranes based on MXenes. The physicochemical characteristics of MXenes are generally unaffected by these approaches, as the interactions between MXene sheets primarily involve VDW interactions, electrostatic repulsion, and hydrogen bonding. However, these methods were notably contentious, primarily due to low stability and potential defects arising from external forces, thereby limiting the use of more robust physical cleaning methods and consequently restricting the membrane's lifespan. The principal drawbacks of these approaches included the lack of uniform membrane development and precise control of thickness. Thicker MXenes with fully exfoliated high-quality MXenes enable a highly aligned space in the horizontal direction, thereby enhancing molecular separation efficiency.¹⁵ The coating assembly can overcome the repulsive interactions between the edges of MXene sheets to form densely stacked MXene layers by using attractive capillary forces. These methods strive to scale up and commercialize MXene membranes with tailored separation performance and controllable thickness. Advanced coating assembly, such as roll-to-roll coating, can achieve the oriented assembly of MXene nanosheets with uniform nanochannels and ordered interlayer distance.⁵³ Membranes obtained through blending casting exhibit a more stable structure and a lower loss of MXenes during the separation process. Good compatibility is crucial for constructing mixed matrix membranes (MMMs) with outstanding separation performance. Nanofillers prepared by casting usually possess strong mechanical stability, effectively minimizing material loss since the nanofillers are encapsulated by the polymeric substrate. However, the uneven distribution of MXenes doped therein might occur, necessitating further regulation of the dispersion of MXene nanosheets in the polymer. The ECD (electrophoretic deposition) is an emerging method for preparing MXene-based membranes, enabling the production of membranes with a large specific surface area, facile surface modification, and tunable thermomechanical properties. The remarkable mechanical properties and flexibility of MXene nanosheets align with the characteristics of this process.⁷³ Each printing technique including inkjet, screen, and 3D printing provides distinct benefits and can be customized for specific MXene membrane needs.

3.2 Characterization of MXene-based membranes

The characterization of MXene structures and compositions often requires the use of multiple advanced techniques within a single study. Techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy/spectroscopy, and atomic force microscopy (AFM) are commonly used to assess the suitability of the precursor (MAX phase) for MXene synthesis, confirm the successful synthesis of MXene, and determine its composition, structure, and properties.^74,75

Density functional theory (DFT) calculations enable the direct visualization of the electronic properties of MXenes and the prediction of intermediate evolution kinetics by simulating the interaction of reactants with the active site.⁷⁶ For instance, DFT calculations are used to study the behavior of bilayer Ti₃C₂T₂ (T = non, F, O, and OH) MXenes in the presence of different ions between the layers. Additionally, the dynamic response of functionalized MXenes to intercalating ions was obtained through both DFT and ReaxFF MD simulations. This dynamic behavior of the MXene layers arises from the charged nature of the material, which was confirmed in simulations by injecting external charges to the O-terminated MXene layers.⁷⁷In situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been established as a new experimental technique to obtain dynamic information regarding the structure of solvated ions, enabling novel investigatory science in nanofluidics and related fields. It allowed for the first in situ observation of changes in the hydration state of solvated ions following transport across extremely confined environments.⁷⁸ The presence of –OH and –F terminations has been identified in the ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectra of MXene. Additionally, H₂O has found to be still present between the nanosheets after vacuum drying at 200 °C. Two-dimensional correlation experiments confirmed the connectivity of the –OH and –F terminations and revealed that they are all present in close vicinity to each other between the metal carbide layers. Quantitative NMR experiments showed that there were significantly fewer –OH terminations than –F and –O terminations and that the surface termination was highly sensitive to the synthesis method used.⁷⁹ ToF-SIMS was recognized for its exceptional sensitivity and ability to analyze the surfaces and interfaces with a remarkable level of spatial resolution, making it a valuable mass spectrometric technique.⁸⁰ Molecular simulation has been used to explore the performance of molecular transportation through nanochannels from a microscopic perspective, predicting membrane performance and illustrating the separation mechanism at the all-atom level. It has been found that the arrangements of solvents in the membrane are mainly regulated by the interaction between solvents and MXene surfaces.⁸¹ The extended Derjaguin–Landau–Verwey–Overbeek theory can be applied to the high interaction energy between the membrane and pollutants.⁸²

3.3 Existing problems of MXene-based membranes

Membranes made from previously mentioned preparation methods showed promise in ion and molecular selectivity. However, MBMs faced challenges in structure and applications, as shown in Fig. 6, with susceptibility to challenges such as oxidation issues, over-restacking, swelling, and fouling, which could hamper separation performance and stability.¹²

Fig. 6 Current challenges of MXene-based membranes: oxidation issue, over-restacking, swelling and fouling.

Oxidation issue. The structural and chemical stability of MXenes are highly susceptible to oxidizing atmospheres (such as air, O₂, and CO₂) and high temperatures. This stability was significantly influenced by their inherent chemical composition and microstructure. At elevated temperatures, structural variations in MXenes can occur even without the presence of any external oxidant, often due to the combination of surface terminations and trapped molecular hydrogen.⁸³ Oxidation reactions are expected to initiate from vulnerable functional group sites (e.g., OH), which leads to the potential disintegration of the lamellar structure, posing a significant challenge for long-term applications in water treatment. Furthermore, the oxidation and degradation processes of MXene nanosheets are size-dependent, with smaller flakes being less resistant to degradation. Therefore, larger nanosheets are preferred for the assembly of MXenes into membranes.⁸⁴ The instability of MXenes in air and water presents a significant hurdle for their applications. Consequently, there is a pressing need to produce environmentally stable MXenes. Research efforts had been focused on mitigating the oxidation and degradation of MXenes, including well-controlled storage conditions or by preparing MXenes completely isolated from water and O₂.⁸⁵ Thermal annealing in vacuum or under an inert atmosphere had proven to be an effective method for removing surface groups attached to MXenes and enhancing their intrinsic thermal stability.⁸³ Increasing the stability of MXene membranes in an aqueous environment can also be achieved by reducing the surface hydroxyl termination.
Over-restacking. Concurrently, the Ti₃C₂T_x nanosheets typically tended to form a dense structure through a face-to-face stacking pattern. The restacking of MXene nanoflakes and their small interlayer spacing imposed limitations on their application.⁸⁶ The challenge of over-restacking of MXene nanosheets, arising from strong interfacial interactions, was primarily propelled by interlayer van der Waals forces.⁸⁷ The inherent repulsive force between Ti₃C₂T_x nanosheets led to non-selective defects, which restricted the exposed surface area and reduced water penetration.⁸⁸ Consequently, the over-restacked MXene membrane failed to provide sufficient access for ions to contact the active material, resulting in poor rate performance. Moreover, the interlayer stacking introduced additional resistance, thereby deteriorating ionic dynamic diffusion. It was imperative to pay more attention to the inevitable stacking constraints, given the close influence of the stacked structure on the transport pathway.⁸⁹ To mitigate this issue, numerous strategies had been developed to curb over-restacking, including freeze-drying, intercalation, and crosslinking.
Swelling. The abundance of oxygen-containing functional groups such as –OH and –O on MXene nanosheets contributed significantly to their outstanding hydrophilicity and their capacity to absorb water.⁹⁰ This hydrophilic characteristic could lead to an expansion of the interlayer spacing, known as the d-spacing, when MXene nanosheets encounter an aqueous environment.⁹¹ In lamellar membranes, the size of the d-spacing is a pivotal factor influencing both permeability and rejection. It is crucial to consider the potential for overexpansion or excessive swelling of MXene nanosheets in aqueous solutions, as this can result in undulations or distortions in membranes.⁹² MXene membranes has a propensity to absorb numerous water molecules when immersed in water, salt solutions, or when subjected to hydrodynamic flow conditions. This led to an increase in interlayer spacing, membrane delamination, and a decrease in structural stability and separation selectivity. While the numerous surface end groups of MXenes serve as sites for water molecule adsorption, they also have the potential to act as sites for reducing swelling.⁹³ As a result, various research endeavors had been pursued to enhance the anti-swelling resistance of MBMs, including methods such as insertion, crosslinking, and self-crosslinking. These approaches aimed to mitigate the swelling tendencies of MBMs, thereby enhancing their stability and performance in aqueous environments.
Fouling. Membrane fouling, particularly in organic pollutant separation, poses a significant challenge. The issue of surface fouling is a critical concern due to the presence of pollutants in various feed streams, which can precipitate, adsorb, and block pores, thereby reducing the lifespan of membranes. Water treatment separation membranes are highly susceptible to fouling by organic and biological contaminants, leading to the formation of a thick, gel-like biofilm that can significantly decrease flux and degrade separation performance. Membrane fouling presents a persistent challenge in water treatment processes, often requiring the frequent use of chemical agents to mitigate fouling effects. However, this practice results in increased energy consumption and reduced membrane lifespan. Addressing biofouling was particularly crucial in wastewater treatment systems.⁹⁴ Specifically, hydrophilic MXene coatings demonstrated remarkable antibacterial activity against common waterborne bacteria including E. coli and B. subtilis. The surface oxidation of aged membranes showed a notable improvement in antibacterial activity compared to freshly prepared membranes, attributed to the synergistic effect between Ti₃C₂T_x nanosheets and TiO₂/C formed on the surface.⁹⁵ MXene-coated anti-biofouling membranes exhibited excellent antibacterial activity by causing irreparable damage to bacterial cell membranes, probably due to the direct physical contact between bacterial surfaces and highly defective MXene sharp edges, leading to physical stress and disruption of cellular membranes.⁹⁶ Additionally, the hydrophilic nature of MXene nanosheets provided self-cleaning ability, potentially extending the lifespan of MXene membranes. Furthermore, conductive MXene-based membranes could induce fouling on the membrane surface rather than allowing more contaminants to enter the membrane pores, making it easier to restore their flux after cleaning.⁹⁷

4. Modification strategies of MXene-based membranes

Accordingly, it is essential to employ tailored modification techniques on MBMs to achieve attributes such as high selectivity, superior water flux, resistance to fouling, and exceptional stability. As shown in Fig. 7, these modification strategies for MBMs briefly described the regulations of MXene nanosheets and adjustments of the membrane assembly process, which had great effects on the structure and performance of MBMs.

Fig. 7 Modification strategies of MXene-based membranes^56,58,98,99 (reproduced from ref. 56 with permission from the Royal Society of Chemistry, copyright 2018; reproduced from ref. 58 with permission from the American Chemical Society, copyright 2015; reproduced from ref. 98 with permission from Wiley Online Library, copyright 2018; reproduced from ref. 99 with permission from Wiley Online Library, copyright 2019).

4.1 Regulation of MXene nanosheets

The properties of MXene nanosheets produced by various preparation methods vary based on their morphology and structure, which in turn can influence nanosheet stacking.¹⁰⁰ To improve the filtration efficiency of MXene membranes, it is crucial to modify the nanosheets, focusing on three key areas: in-plane pores, terminal functional groups, and lateral size, as shown in Fig. 8.

Fig. 8 Main aspects for the regulation of MXene nanosheets^77,101,102 (reproduced from ref. 77 with permission from Elsevier, copyright 2022; reproduced from ref. 101 with permission from the American Chemical Society, copyright 2022; reproduced from ref. 102 with permission from Elsevier, copyright 2022).

4.1.1 Lateral size modification of MXene nanosheets. The lateral size of MXene nanosheets plays a pivotal role in determining the surface morphology of MXene lamellar membranes, consequently influencing the distribution patterns and height profiles of wrinkles and surface roughness, thereby leading to distinctive surface characteristics, which is shown in Fig. 9(a).¹⁰³ When MXene nanosheets possessed smaller lateral dimensions, they tended to demonstrate increased edge overlapping or self-folding behavior, stemming from the intrinsic flexibility and structural attributes of MXene nanosheets.¹⁰⁴ Consequently, the lateral size of MXene nanosheets within membranes could significantly impact the mass transfer pathways and subsequent transport properties. Varied lateral sizes could engender unique membrane characteristics, including the presence of vertical channels, gaps between edges, and inherent porous defects.⁹⁹ Conversely, increasing the lateral size of MXene flakes offered several advantages in terms of their assembly, interflake contacts, alignment, and the ability to explore intrinsic physical properties.¹⁰¹ Processes such as cascading centrifugation, controlled suspension freezing, or probe/tip sonication were employed to isolate nanosheets, thereby enabling control over the lateral size of flakes.¹⁰⁵ Shekhirev et al. had conducted the fabrication of ultra-large flakes of Ti₃C₂T_x MXene via soft delamination, highlighting the influence of the delamination process on the lateral size.¹⁰¹ Additionally, the micrometer lateral size of MXene nanosheets within membranes could yield advantages in terms of reducing defects and transport resistance, thereby enhancing their filtration performance and pure water flux.¹⁰⁶ The oxidation and degradation process of MXene nanosheets showed a size-dependent pattern, wherein smaller flakes exhibited lower resistance to degradation. Therefore, larger nanosheets are favorable for the assembly of MXenes into membranes, which can exhibit high chemical stability.

Fig. 9 (a) Mass transfer mechanism of methylene blue solution through MXenes with different lateral sizes¹⁰³ (reproduced from ref. 103 with permission from Elsevier, copyright 2022), (b) heterogenous simulation model for desalination: MXenes with different termination groups and electrostatic interaction sites¹⁰⁷ (reproduced from ref. 107 with permission from Elsevier, copyright 2023), and (c) schematic of the water transport and dye rejection in pristine and porous MXene membranes.¹⁰⁸ (reproduced from ref. 108 with permission from Elsevier, copyright 2021).

4.1.2 Terminal functional group modification of MXene nanosheets. The terminal functional groups of MXenes were influenced by various etchants, and the composition of these groups could be adjusted through etching and delamination conditions.¹⁰⁹ Typically, MXenes produced via etching by acidic fluoride-containing solutions would exhibit a mixture of –OH, –O, and –F terminations, thereby affecting the surface charge nature. These functional groups could be introduced and removed through substitution and delamination reactions in molten inorganic salts, leading to MXenes terminated with diverse groups such as imido, sulfur, chlorine, selenium, bromine, tellurium surface terminations, and even bare MXenes (no surface termination).⁴¹ Notably, Ti₃C₂T_x nanosheets demonstrated a higher O/F ratio when etched with HCl–LiF. The O/F ratio, the presence of –OH functional groups, and intercalating cations were significantly related to the ability to interact with water molecules.¹¹⁰ The surface charge characteristics and hydrogen bond interactions influenced the interplay between termination groups and water. Previous studies have indicated that the water angle of MXenes strongly depends on the content of terminal groups, which have significant impacts on the hydrophilicity of MXenes and further influence the hydrophilicity of MBMs.^111,112 Additionally, the terminal functional groups produce a sub-nanometer scale interlayer in the MXene membrane, making it applicable in water treatment and energy storage.¹¹³ Different terminations on the membrane's surface indicate various electrostatic interaction properties such as –F– and –O-terminated Ti₃C₂ with semiconducting properties and pristine Ti₃C₂ with metallic properties.¹¹⁴ Some metallic MXenes with –O and –OH terminations also exhibit exceptional charge transfer.¹¹⁵ Edge functional groups can obstruct the entrance space, enhancing selectivity while sacrificing water permeability.¹¹⁶ For instance, Ma et al. used molecular dynamics simulation to observe the effects of different termination groups (including –OH, –O, and –F terminations) on ion rejection efficiency and water permeation, revealing that water permeation follows the order of F > O > OH.⁷⁷ As shown in Fig. 9(b), the charge distribution at atom positions could result in two sides of MXenes with different electrostatic interactions. These two sides with strong electrostatic interactions could create coordination shells, thereby producing novel heterogeneous membranes.¹⁰⁷ The abundance of functional groups on both sides of these sheets could cause MXene nanosheets to fold and distort, resulting in a non-uniform surface within neat MXene membranes.^117,118 Additionally, the introduction of zwitterionic MXenes had been found to improve the distribution of surface functional groups, avoiding nanochannel contortions, and resulting in high permeability and low fouling propensity.¹¹⁹ The surface charge nature of MXenes is facile to adjust the structure, composition and property via chemical action with other materials. In previous work, many polymers such as polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone (PVP) and nanoparticles such as Ag particles, Fe(OH)₃ nanoparticles were incorporated into the MXenes via the formation of chemical bonds and electrostatic interaction, further to enhance the separation performance of MBMs.^56,59 The stability of MXenes significantly depends on the surface termination. Concurrently, surface terminations such as =O and OH, which unavoidably exist on the MXene surface after deintercalation from MAX to MXene, can also serve as reactive forms of O₂. These surface species have the potential to induce MXene oxidation during heat treatment, even in the absence of an external oxidant supply in the system. Research indicated that the as-synthesized MXenes with Cl terminations exhibited superior stability to those terminated with F and =O. Consequently, the influence of additional and innovative surface terminations on the stability of MXenes can impact the chemical stability of MBMs.¹²⁰

4.1.3 In-plane pore regulation of MXene nanosheets. MBMs frequently feature narrow channels and high tortuosity, potentially leading to low water permeability. To improve ion accessibility and transport pathways, the introduction of in-plane pores on MXene nanosheets had been investigated. The density of pores within the structure of 2D nanomaterials, especially in the context of laminate membranes, can be adjusted through the addition of pore formers, perforation techniques, and the stacking of multiple nanosheets.^121–123 These artificial nanopores on single-layer MXene nanosheets can be created, leading to the fabrication of porous lamellar membranes. Currently, some common strategies such as freeze-drying, hard templating and urea method are not suitable for the membrane preparation, because these pores are much larger than separated objects.¹²⁴ As reported, simple methods such as permanganate reoxidation, nitric acid oxidation, and hydrogen peroxide oxidation can be used to introduce artificial nanopores on the single-layer nanosheets, thus producing porous lamellar membranes.^125–127 These oxidation methods provide a relatively straightforward and effective approach to introduce in-plane nanopores on MXene nanosheets. Following the design concept of nanoscale ion channels, Cheng et al. developed anti-self-discharge films to mitigate the lengthy ion transport distance associated with the small-sized MXene by ultrasonic fragmentation. The in-plane nanoporous MXene not only effectively reduced the ion transport distance but also maintained exceptional mechanical strength.¹⁰² As shown in Fig. 9(c), in situ chemical etching with hydrogen peroxide had been employed to prepare porous MXene nanosheets, leading to the formation of pores that significantly impacted the transport pathways within the membrane. The presence of these pores allowed for a transformation from typical horizontal transport pathways to longitudinal-lateral three-dimensional transport pathways.¹⁰⁸ A novel “oxidation-etching” approach had been proposed for the synthesis of porous MXene sheets with a rich pore structure. Simultaneously, within the hydrothermal reaction, oxygen had the capability to substitute fluorine on the surface of MXene sheets, leading to its conversion into TiO₂; the extent of this conversion was entirely dictated by the reaction duration. The HF etching process did not compromise the inherent properties of MXene sheets. This porous strategy could significantly reduce the transmission distance and increase water permeability.¹²⁴ Large-sized MXenes with in-plane nanopores could maintain high mechanical strength, while small-sized MXene with in-plane pores had a low chemical stability.

4.2 Interlayer spacing control of MBMs

Precisely controlling the interlayer spacing and tuning the surface structure is crucial for membranes to achieve desirable selectivity and stability. Various chemical and physical approaches had been employed to address these challenges and enhance the performance of membranes.¹²⁸ Generally, it is perceived that molecules and ions are transported through interconnected interlayer spacings of adjacent stacked nanosheets in these lamellar membranes.¹²⁹ Many researchers had verified that super-fast and accurate ion selectivity in these laminar membranes is facilitated by the sizable equivalent diameter between interlayer spacing and hydrated ions.^130,131 However, the practical separation performance is far from the theoretical prediction, due to the expansion of interlayer spacing in solution.^16,28 Even though stacked MXene nanosheets could exhibit a more uniform pore structure and well-distributed nanochannels compared to other 2D nanomaterials, the transport through the interlayer nanochannels could be long and winding, leading to a reduction in water permeation and mass transport.^132,133 To address the challenge of long and winding transport pathways in stacked MXene nanosheets, it is necessary to develop ultrathin and well-established MBMs rather than ultrathin hybrid membranes with a random distribution of MXene nanosheets and free penetration of solutes.¹³⁴ The abundant terminal functional groups present on MXene nanosheets provide opportunities for introducing polymeric materials with new functionalities and inorganic particles into MXene membranes. This incorporation can lead to improvements in membrane stability and selectivity. Additionally, the adjustable layer spacing of MXene nanosheets allows for the embedding of various intercalators, enabling the formation of sandwich structures with enhanced laminar membranes.¹³⁵ As shown in Fig. 10(a), apart from these routine strategies of some crosslinkers to modify MXene membranes, the self-crosslinking and the construction of hybrid functional layers are employed to improve the performance of MBMs.

Fig. 10 Modifications of the membrane assembly process: (a) interlayer space control and (b) membrane surface adjustments.

4.2.1 Self-crosslinking of MXene nanosheets. Inspired by aluminosilicates with hydrophilic functional groups, the dehydroxylation process was adapted to MXene materials that possessed rich surface functional groups.¹³⁶ The stoichiometric ratio of hydroxyl groups on the surface of MXene sheets was relatively low, typically around 26%. Due to the low stoichiometry, the hydroxyl groups were randomly distributed and isolated on the MXene surface. This random distribution of hydroxyl groups contributed to the heterogeneous nature of the functionalization and affected the overall properties of the MXene material.¹³⁷ Temperature regulation could flexibly tune the interlayer spacing between stacked neighboring nanosheets, and self-linking is a low-cost and simple method, which can be used to lead to moisture loss and de-functionalization (–OH). Consequently, MBMs could maintain a thermodynamically stable structure during the heating or sintering process, which benefited from the strong mixed covalent/metallic/ionic chemical bonds.¹³⁸ Moreover, calcination technology has high requirements on the property of substrates. As for flexible membranes with free-standing existence or weak substrate, such as polycarbonate (PC), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF), weak mechanical strength and thermal stability limit their applications,¹³⁹ whereas the robust structure of ceramic MXenes remain stable at high temperatures. In addition, the large interlayer spacing of membranes can lead to relatively low adhesion between layers, so the laminar layers with poor structural stability are facile to fall off from the substrate in solutions, which would decrease selectivity for small molecules and ions and limit their practical applications.¹⁴⁰ Thus, it was crucial for MXene layers and underlying substrates to construct strong interfacial adhesion.

The thermal treatment was applied to fabricate MXene membranes via a self-crosslinking reaction (–OH + –OH = –O– + H₂O).¹⁴¹ These membranes showed excellent monovalent ion exclusion properties and suppressed swelling performance with favorable long-term stability, which is shown in Fig. 11(a).¹⁴¹ In addition, the self-crosslinking MXene membranes (SCMMs) possessed a steady nanochannel relying on the dehydration and self-crosslinking reaction. The size of the hydrated ions and the energy barrier of dehydration determined these hydration ions through the narrow and empty spacing of membranes. According to previous papers, partial dehydration occurs when all of the hydrated monovalent metal ions are transferred into the nanochannels of membranes.¹⁴² More importantly, the facile thermal self-crosslinking strategy provides a method to synthesize membranes with enhanced monovalent metal ion rejection and suppressed swelling performance. These ultrathin MBMs with α-Al₂O₃ tubular substrates were prepared by calcination for improving ion rejection, which reduced the interlayer spacing from 3.71 Å (60 °C) to 2.65 Å (450 °C). Below 400 °C, the reduced interlayer spacing endowed a compact layer nanostructure with controllable ion rejection properties. In addition, the wrinkled microstructure on the surface became less progressive, and the surface got relatively smooth. With the increase in sintering temperature (above 400 °C), the wrinkled micro-structure gradually disappeared.^143,144 It was observed that the model of filtration transformed from interlayer transport pathways to longitudinal nanochannels, causing a decrease of ion retention.¹⁴⁵

Fig. 11 (a) Self-crosslinking process of the MXene membranes¹⁴¹ (reproduced from ref. 141 with permission from the American Chemical Society, copyright 2019), (b) fabrication process of the MXene/BN@PDA/PEI membrane for water purification¹⁴⁶ (reproduced from ref. 146 with permission from Wiley Online Library, copyright 2022), (c) transfer and rejection mechanisms of PAN/PEI-Ti₃C₂T_x-X,¹⁴⁷ (reproduced from ref. 147 with permission from Elsevier, copyright 2016), (d) design of MLM-EDTA membranes with numerous negatively charged oxygen atoms and a 6.0 Å two-dimensional channel¹⁴⁸ (reproduced from ref. 148 with permission from Springer Nature, copyright 2023), (e) preparation of the dual-layered COF/MXene composite membranes¹⁴⁹ (reproduced from ref. 149 with permission from Elsevier, copyright 2022), and (f) LbL self-assembly of (MXene/TAEA)_n multilayers onto planar substrates¹⁵⁰ (reproduced from ref. 150 with permission from Springer Nature, copyright 2019).

4.2.2 Insertion of nanomaterials, molecules and ions. In recent research studies, various approaches have been proposed to integrate inorganic nanoparticles into MXene nanosheets to adjust the interlayer spacing of MXene membranes.¹⁵¹ For instance, a novel method involved assembling 2D MXene nanosheets and TiO₂ nanoparticles to obtain mesoporous membranes, effectively preventing aqueous sol penetration into macroporous ceramic supports.¹⁰⁶ Additionally, MXene nanosheets could serve as a matrix and reducing agent, facilitating the direct reduction of AgNO₃ on the surface, resulting in outstanding bactericidal properties and rejection of multi-valent ions.⁵⁶ Furthermore, researchers attempted to intercalate positively charged Fe(OH)₃ colloids into negatively charged MXene nanosheets, followed by a hydrochloric acid treatment to remove these Fe(OH)₃ nanoparticles.⁵⁹ Moreover, MOFs such as ZIF-8, ZIF-67, and MOF-801 had been embedded into MXene nanosheets using various methods, providing valuable insights for environmental applications.^152–155 The insertion of reduced graphene oxide between MXene layers had been shown to optimize the microstructure, leading to increased adsorption and enhanced ion reduction, thereby controlling solid–liquid interactions.¹⁵⁶ As shown in Fig. 11(b), drawing inspiration from the architectural design of column-to-beam structures in houses, the laminar Ti₃C₂T_x nanosheets, featuring an abundance of nanochannels, were conceptualized as the “beam” component.¹⁴⁶ Meanwhile, the boron nitride (BN) nanosheets were envisioned as the “brick” component within the MBMs. Additionally, polydopamine and polyethylenimine played a pivotal role as the “column” part of this innovative membrane, their functionalities being integrated through covalent cross-linking interactions.¹⁴⁶ A high-performance MXene membrane with minimal defects and an expanded d-spacing was developed, which demonstrated an increased interlayer spacing of 11 Å and exhibited high stability. The d-spacing of the MXene sheets was regulated by employing Si-based species as intercalating agents.¹⁵⁷ Here, sodium tripolyphosphate (STPP) was incorporated into Ti₃C₂T_x nanosheets to fabricate a range of modified STPP-MXene membranes via an edge-capping approach. The edge-capping technique was shown to moderately reduce the intersheet spacing between adjacent Ti₃C₂T_x nanosheets and diminish their positive electric field intensity. Additionally, by safeguarding the titanium at the edges of Ti₃C₂T_x nanosheets, the STPP-MXene membrane preserved its original two-dimensional structure, ensuring stable separation performance even after exposure to a water environment for up to 60 days.¹⁵⁸

Covalent crosslinking, in particular, had been demonstrated as a valid approach to impose restrictions on the interlayer spacing of hydrophilic laminate membranes. When selecting a crosslinker, it is beneficial to choose one with a large dimension and rich reactive terminal functional groups. Various polymers including polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polyvinylpyrrolidone (PVP) were considered promising crosslinkers for forming composite membranes due to their excellent properties such as flexibility, favorable microporous structure, and high mechanical strength. By incorporating crosslinkable functional groups or monomers into the membrane structure, the formation of covalent bonds between adjacent layers could occur, leading to a controlled interlayer spacing and enhanced stability of the membrane.^35,159,160 For example, borate and PEI molecules could be interlocked into MXene layers to regulate the interlayer spacing of MXene nanosheets, revealing the transformation from “diffusion-controlled” to “solution-controlled” channels after chemical tuning.⁹⁸ It is an interesting finding by Wu et al., where they incorporated two polymer matrices, hydrophilic polyethyleneimine (PEI) and hydrophobic polydimethylsiloxane (PDMS), into MXene nanosheets to create polymer-MXene composite membranes, which is shown in Fig. 11(c).¹⁴⁷ Covalent modification was mostly applied to adjust the channel size and functionality, but it could disrupt the structure of this membrane. Herein, a less intrusive and effective non-covalent modification by solvent treatment is reported, and then these channels are decorated by protic solvents via a hydrogen bond network to achieve membrane integrity and channel alignment. The angstrom-scale channel width facilitated the nanoconfinement effect to tune the distance and orientation of the solvent to channel walls, endowing the improvement of ion sieving and separation performance.¹⁶¹

Ren et al. confirmed that these MXene membranes could exhibit selectivity of cations with various charges and did not show the permeation cutoff, attributing to the intercalation of cations into MXene laminates.⁵⁸ These ion intercalation behaviors had nothing to do with anions, but were mainly affected by the type of cations.¹¹ Thus, cation intercalation was proposed to confine the interlayer spacing, which is regardless of the size and valences. Besides, cationic control of the interlayer spacing of MXene membranes could achieve Ångström precision using Li⁺, Na⁺, Mg²⁺, and Al³⁺ ions. The study revealed that the vacuum hot pressing molten salt intercalation technique effectively promoted the intercalation of Al³⁺ ions into MXene membranes, thereby enhancing their ability to retain consistent spacing under varying dynamic voltages.¹⁶² Many researchers attempted to modify the surface of nanosheets before the cationic intercalation to tune the ion selectivity.^19,163 For example, Wang et al. used alginate hydrogel pillars to stabilize the interlayer spacing of the MXene laminar membranes. At first, the sodium alginate (SA) molecules were homogeneously anchored on the surface of MXene nanosheets and assembled by van der Waals forces. Then, the cross-linking reaction occurred after the SA-MXene membranes were immersed in the multivalent cation solution. It was observed that the nanochannel diameters were fixed at 7.4 ± 0.2 Å by the pillar of Ca-alginate, and these membranes showed a permeation cutoff and an excellent selectivity of cations.¹³² As shown in Fig. 11, Xu et al. built cation sieving membranes by the combination of MXene nanosheets and ethylenediaminetetraacetic acid (EDTA) molecules, which were inspired by the K⁺ channel of Streptomyces A (KcsA K⁺). The biomimetic channel structure, chemical functional groups, and tunable charge density of the membrane were derived from the 6.0 Å sub-nanochannels of MXene nanosheets and numerous negatively charged oxygen atoms of EDTA molecules. These membranes exhibited an outstanding ability in sieving monovalent and divalent cations, particularly in K⁺/Mg²⁺ selectivity. These characterizations and simulations suggested that partial dehydration effects, the increasing charge density in sub-nanochannels, and the cation recognition of EDTA significantly enhanced the cation-selective sieving.¹⁴⁸

Even though masses of intercalators were inserted into these MBMs, which indeed showed expanding interlayer nanochannels and high chemical stability, the water permeance was difficult to achieve as the ideal target, owing to the steric effect of the intercalators. In addition, it could lead to a decrease in the selectivity of ions or molecules, especially for small ions.¹⁶⁴ This is because the intercalation and size effect of cross-linking agents could occupy the space of the interlayer within membranes, which would result in an increase in d-spacing and further reduce the ion rejection. Even though membranes were linked with other cross-linking agents to suppress the swelling phenomenon, these membranes showed low ion rejection, especially for these monovalent salts.^149,165

4.2.3 Introduction of hybrid functional layers. Compared with other 2D nanomaterials, although stacked MXene nanosheets exhibited a more uniform pore structure and well-distributed nanochannels, the transport through interlayer nanochannels was long and winding, leading to decreased water permeation and mass transport.^132,133 It is essential to develop ultrathin, well-established MBMs, rather than ultrathin hybrid membranes with randomly distributed MXene nanosheets and free penetration of solutes.¹³⁴ Previous studies have indicated that the surface roughness and pore sizes of substrates affected the laminar membranes during the fabrication process.¹⁶⁶ Additionally, due to the easy deformation and poor self-support of these flexible nanosheets, membranes deposited on macroporous substrates lacked stability and integrity, making it challenging to realize well-assembled MXene membranes with nanometer thickness and enhanced properties.¹⁶⁷ Thus, constructing an intermediate scaffold layer to tune the pore sizes and surface roughness of the substrate is an effective method for fabricating ultrathin composite membranes.¹⁶⁸ Furthermore, MXene membranes can be well-assembled on scaffold-deposited layers with ultrathin thickness for high water flux and ion rejection. Generally, intrinsically porous 2D nanosheets are used as nanofillers to create short and straightforward nanochannels such as organic frameworks and oxidized graphenes. The intermediate scaffold layer could provide smooth and hydrophilic substrates with a rich pore structure, facilitating the formation of well-assembled MXene layers with a nanometer thickness. The intermediate layer and the MXene were combined via electrostatic interactions, while the intermediate layer and the substrates were brought into contact with each other through electrostatic interactions. It indicated that dual-layer composite membranes could maintain an integrated structure even after numerous folds.^169–171

As demonstrated in previous papers, dual-layered COF/MXene composite membranes were fabricated by creating an intermediate COF layer on macroporous substrates and then vacuum filtering MXene nanosheets, as shown in Fig. 11(e). The smooth and hydrophilic COF layer could modify the surface properties of macroporous substrates by covering multidistributed macropores and providing intrinsic and crispation-induced pores, contributing to the fine assembly and uniform deposition to generate an integrated and ultrathin MXene layer. The construction of the COF layer not only facilitated the well-assembled MXene layer with an ultrathin thickness of 8 nm and narrow interlayer nanochannels, but also introduced intrinsic nanopores and crispation-induced pores for mass transport. Thus, the dual-layered COF/MXene composite membranes possessed a high flux due to short pathways from the porous COF layer and the ultrathin MXene layer, and high rejection due to the confined interlayer nanochannels between these well-assembled MXene nanosheets. Moreover, the content of MXene nanosheets and COFs had a crucial effect on the separation performance of the composite membranes.¹⁴⁹ Nevertheless, in most cases, these nanofillers were limited to intrinsically porous 2D nanosheets, which could limit their application. In addition, layer-by-layer (LbL) self-assembly technology could facilitate deposition on various substrates. The small molecule of tris(2-aminoethyl)amine (TAEA) could enlarge the interlayer spacing of MXene sheets by approximately 1 Å and enhance the interconnection.¹⁵⁰

In a previous work, amino-modified boron nitride nanosheets and GO nanosheets were alternated to fabricate a hybrid membrane with low defects containing edge-edge covalent bonding and plane-plane conformation. The decrease in electrostatic repulsion between the nanosheets, overlap, and dislocation contributed to strong interactions, resulting in a highly ordered and compact structure.^54,172 In order to obtain membranes with free defects or few defects, it is necessary to optimize the nanosheet stacking. For example, Tang et al. synthesized layer-by-layer repaired lamellar membranes via multiple repetitions of delayed vacuum filtration, crosslinking, glass rod rolling treatment and self-crosslinking.^89,150 The total thickness of these ultrathin layer-by-layer repaired MXene membranes was approximately 50 nm, containing very few-layer MXene nanosheets of 2 nm. Moreover, the ordered structure and smooth surface of MBMs with few defects and wrinkles were achieved through a series of craft processes. The MXene membranes, meticulously repaired layer by layer, demonstrated exceptional separation performance and remarkable stability in their interlayer spacing throughout the filtration process. Consequently, the judicious employment of various techniques proved advantageous in enhancing the anti-swelling capacity and filtering efficiency. However, the complexity of the fabrication process presented a significant challenge, making practical implementation somewhat problematic.^89,150

4.3 Membrane surface adjustments

Enhancing the permeability and selectivity of MXene membranes could be achieved through deprotonation of the surface groups on MXene nanosheets. Deprotonation involved the removal of protons (H⁺) from functional groups, leading to changes in the surface charge and properties of the MXene material.^128,173 Molecular interactions play a crucial role in achieving highly ordered stacking of nanosheets, which is essential for good molecular sieving performance. These interactions include electrostatic attractions with polymer chains and hydrogen bonding. It is evident that the surface hydrophilicity and charges of MXene membranes could be adjusted via electrostatic functionalization with cationic surfactants or cationic/neutral polymers. This tuning process could significantly impact the membrane's performance in terms of permeability and selectivity and chemical stability. As shown in Fig. 10(b), membrane surface adjustments covered three aspects, namely surface charge regulations, surface wettability regulations and surface defect controls.

4.3.1 Surface charge regulations. When assembling MXene nanosheets to obtain regularly stacked structures, polymer molecules with positive charges were introduced via electrostatic interaction. For instance, Meng et al. produced surface-charged and laminar MBMs via electrostatic interactions between negatively charged MXenes and positively charged polyethyleneimine, which is shown in Fig. 12(a).¹⁷⁴ Through nanofiltration and forward osmosis processes, it was observed that surface-charged MXene membranes effectively repel salts with high-valent co-ions. Moreover, the electrostatic attraction between the surface charges of surface-charged MXene membranes and a low-valent counter-ion was weaker than the electrostatic repulsion of these membranes against high-valent co-ions. Additionally, the presence of polyethyleneimine (PEI) coatings could restrict the swelling of MBMs in water, leading to the desired interlayer size sieving effect while enhancing the surface hydrophilicity to facilitate the passage of water molecules through the membranes. As a result of the combined effects of surface electrostatic repulsion and interlayer size sieving, the surface-charged MXene membranes demonstrated excellent salt rejection and water/salt selectivity.¹¹⁶

Fig. 12 (a) Schematic of SC-MXene membrane and their application on water desalination¹⁷⁴ (reproduced from ref. 174 with permission from Elsevier, copyright 2021), (b) schematic of the bioinspired heterogeneous MXene membrane (BHMXM)¹⁷⁵ (reproduced from ref. 175 with permission from Wiley Online Library, copyright 2022), (c) schematic of electrostatic interaction between Ti₃C₂T_x MXene nanosheet and MB dye molecules¹⁷⁶ (reproduced from ref. 176 with permission from Elsevier, copyright 2023), (d) mechanism diagram of the switchable wettability membrane for oil/water emulsion separation¹⁷⁷ (reproduced from ref. 177 with permission from Elsevier, copyright 2023) and (e) synthesis process of Ti₃C₂T_x-M and the microstructure of composite membranes.¹⁷⁸ (reproduced from ref. 178 with permission from the Royal Society of Chemistry, copyright 2019).

Furthermore, functionalization by poly(diallyldimethylammonium chloride) (PDDA) was able to reverse the surface charge of the MXene nanosheets to a positive value of +50 mV, interacting with negatively charged MXenes to produce bioinspired heterogeneous MXene membranes, exhibiting excellent osmotic energy conversion, which is shown in Fig. 12(b).¹⁷⁵ Additionally, the application of KOH to substitute –F with –OH enhanced the surface charges, resulting in outstanding ion rejection due to the hydroxylation of MXene membranes.¹⁷⁹ Moreover, to leverage silanol chemistry on –OH groups, organosiloxanes were used to graft onto the MXene nanosheets to introduce functional groups such as –NH₂, –COOR, and –C₆H₆.¹⁸⁰ Although thermal self-crosslinking could address the swelling issue, the decrease in negative charges could contribute to a reduction in ion sieving due to weakened adsorption or charge controlling effects. A membrane of poly(4-vinylphenylboric acid)-grafted MXene was created, binding interchangeable diols using dynamic boronic ester chemistry, called dynamic covalent interface engineering (DCIE). The diols anchored in the interlayer transitioned from anionic to cationic and from hydrophilic to hydrophobic states via reversible covalent bond formation and cleavage, without any residual effect from previous diols. Consequently, the membrane, with rapidly adjustable nanochannel properties, displayed exceptional switchable permeability and selectivity (98% for Congo red and 97% for Gentian violet) and exhibited high stability in various water treatment scenarios.¹⁸¹

4.3.2 Surface wettability regulations. A shift from hydrophilic to hydrophobic behavior in ceramic membranes exposed to high-temperature oil had been documented. At temperatures of 90 °C and 100 °C, deionized water droplets exhibited a Leidenfrost-like effect at the interface between the liquid and solid, resulting in water contact angles on the membrane surface of 128 ± 1.2° and 148 ± 1.2°, respectively.¹⁸² It is important to note that the hydrophilic nature of MXene nanosheets endows MXenes with self-cleaning ability, thereby offering potential for developing anti-fouling MXene membranes to extend their lifespan and improve chemical stability. Therefore, an effective solution to achieve outstanding anti-fouling and self-cleaning performance involved regulating the composite membrane surface wettability. Consequently, the search for suitable hydrophilic or hydrophobic materials became crucial for altering surface wettability. As shown in Fig. 12(c), the development of MBMs addressed interface compatibility and surface wettability mismatch, resulting in the fabrication of dual-functional superhydrophilic/underwater superoleophobic MXene composite membranes with exceptional separation performance owing to their inherent hydrophilicity and the ordered stacking of MXene nanosheets.¹⁷⁶

In the realm of superwetting materials, the hydrophilic groups on the membrane surface could rapidly absorb water molecules to generate a hydrate layer barrier, which facilitated the rapid passage of water molecules.¹⁷⁷ As shown in Fig. 12(d), Zhang et al. developed a series of Na-Bentonite@MXene (NBM) composite membranes through a one-step hydrothermal pretreatment and vacuum self-assembly on the surface of polyvinylidene fluoride membranes. These membranes, with a sandwich structure and micro-nano structures on the surface, exhibited switchable wettability. The pretreatment is dependent on the surface wettability, and the hydrophilic/hydrophobic or hydrophobic/oleophilic property arise from the treatment of water or oil.¹⁷⁷ Similarly, Feng et al. regulated the surface wettability by bioinspired polydopamine-triggered chemical crosslinking with polyethyleneimine (PEI), resulting in membranes with super-hydrophilic and underwater super-oleophobic properties.¹⁸³ Liao et al. produced multicomponent lamellar nanofilms with dual and ordered nanochannels in confined spacings through tunable ligand conformation and favorable hydrophilicity of dual-functionalized zwitterionic MXenes. The introduction of zwitterionic MXenes improved the distribution of surface functional groups and prevented nanochannel contortions, leading to high permeability and low fouling propensity.¹¹⁹

Moreover, the presence of numerous hydrophilic groups on the surface of Ti₃C₂T_x nanoparticles is noteworthy, endowing them with the potential to act as superior additives for membrane anti-fouling enhancement. While it is a prevalent approach to blend MXenes with polymer membranes to bolster their anti-fouling prowess, the distribution of MXene nanoparticles predominantly within the membrane matrix often failed to capitalize on their benefits in the critical uppermost layer - a region pivotal for membrane permeability. Conversely, the strategic employment of an external magnetic field during a phase transition effectively concentrated magnetic Ni@MXene nanoparticles atop the PES membrane. Consequently, the resulting composite membrane exhibited excellent separation properties, especially for the separation of organic pollutants.¹⁸⁴

4.3.3 Surface defect controls. It was noted that molecular passage in laminar membranes occurred through in-plane slit-like pores and then through plane-to-plane galleries.¹¹⁷ However, precisely regulating structures was challenging for laminar membranes, especially for ordered and defect-free nanochannels. The fabrication method for these membranes could lead to out-of-order structures and non-selective defects such as coating and filtration assembly, which might have contributed to the failure of selective permeation. To improve the in-plane slit-like pores, polymeric coatings were applied to cover the surface of membranes, seal defects and enhance selectivity. However, this could increase the mass transport resistance, resulting in a decrease in membrane permeability and performance stability.^185,186 Many researchers addressed possible non-selective defects by using interfacial polymerization to improve the separation performance and structural stability. Macromolecules were introduced into MXene sheets and then reacted with polymers on the surface of laminar membranes through interfacial polymerization to seal defects and adjust surface structure, such as trimesoyl chloride and poly ethylenimine.

As shown in Fig. 12(e), negatively charged MXene nanosheets electrostatically interacted with positively charged hyperbranched polyethylenimine (HPEI) to form ordered stacking structures. Subsequently, the acyl chloride groups of trimesoyl chloride (TMC) were used to interfacially polymerize with the amine groups of HPEI, further sealing non-selective in-plane slit-like pores.¹⁷⁸ Researchers investigated the role of HPEI molecules and interfacial polymerization in the process of membrane assembly, demonstrating that the introduction of HPEI created a smooth membrane surface with abundant wrinkles.¹⁷⁸ Furthermore, the surface of these membranes exhibited few visible defects and low roughness, indicating an ordered assembly of the nanosheets and numerous ordered channels.¹⁷⁸ Additionally, interfacial polymerization primarily occurred on the membrane surface to seal these interior non-selective pores, resulting in defect-free membranes with highly ordered stacking structures and sub-nanometer size channels that possessed excellent molecular separation and water/isopropanol dehydration performance. According to experimental data, Ti₃C₂T_x-HPEI/TMC membranes had a high average surface area and water-adsorbing capability, leading to a high water content in the water permeate.¹⁷⁸ Fu et al. discovered that heterostructural MXene-TiO₂ was used as the intermediate layer to prepare thin and crumpled composite nanofiltration membranes with a polyamide selective layer via interfacial polymerization reaction. These MXene-TiO₂ particles, prepared by in situ MXene oxidation, facilitated the regulation of the interlayer nanostructure and nanosheet stacking, contributing to the rapid transport of water molecules. The high specific area and numerous edges of MXene-TiO₂ accelerated the adsorption and desorption of amine monomers, leading to the formation of a crumpled structure.¹⁸⁷

5. MXene-based membranes for water and wastewater treatments

It was clear that MXenes and other polymers or inorganic materials had been utilized to produce various composite membranes, with significant efforts aimed to enhance water separation ability and long-term stability.¹⁸⁸ Several studies have focused on the separation of water and wastewater containing pollutants with varying sizes, ranging from micrometers to sub-nanometers. In addition, it was crucial to clarify the complex separation mechanisms underlying the separation behavior of MBMs.⁴⁵

5.1 Separation mechanisms of MXene-based membranes

Water and wastewater separation mechanisms generally fall into three categories: size exclusion, charge effect, and nanoconfinement effect, as depicted in Fig. 13.^189,190 The interlayer spacing between MXene nanosheets, governed by van der Waals forces and electrostatic repulsion, formed slit-shaped nanochannels that were critical for molecule transport, particularly water permeation.⁵⁸ The interlamellar distance could be chemically adjusted for selective transport, with d-spacings below 4 Å preventing water entry and the size of ions with their hydration shells determining passage through the 6 Å channel, which was effective for water purification.^45,163 Size exclusion was significantly influenced by the interlayer spacing, and pollutants larger than the nanochannel size were sieved out, while smaller ones could pass through.¹⁹⁰ The size-based separation of small molecules using MXene membranes was accomplished by adjusting the interlayer spacing between the MXene nanosheets. The flow of a solvent through MXene-based membranes can be described using the Hagen–Poiseuille (H–P) equation:¹⁹¹where h_d is the interlayer spacing of the MXene nanosheets, Δp indicates the trans-membrane pressure difference, l denotes the lateral size of the MXene nanosheets, η_s represents the solvent viscosity, and Δx denotes the membrane thickness. According to this equation, a higher water flux can be obtained by increasing the interlayer spacing and decreasing the average lateral length of the transport channels.

Fig. 13 Main separation mechanisms of MXene-based membranes.

The MXene nanosheets had numerous terminal surface groups, giving them a negative surface charge with a Zeta potential of −35 mV in water.^192,193 This negative charge allowed the separation of charged pollutants such as ions, organic dyes, and polyelectrolytes. The Donnan exclusion theory can be used to achieve the separation of charged ions and organic dyes, owing to the strongly negative charge characteristic of MXene nanosheets, as depicted by eqn (3):⁹¹

where Z_B and Z_A are the co- and counter-ion respective valences, C_B and C^m_B represent the co-ion concentrations, respectively, and C^m_X denotes the concentration of the membrane charge. In particular, co-ions, having the same charge as the membranes, were repelled from the membranes due to electrostatic interaction. When assembled into laminar membranes, counter-ions aggregated near the charged MXene membrane surface, while co-ions were repelled.^84,194 This phenomenon created an electric double layer (EDL) at the nanochannel-solution interface. The potential in the EDL region decayed over a characteristic length known as the Debye length (λ_D). Importantly, when the nanochannels were smaller than the Debye length, which typically ranges from 1 to 100 nm, external gating voltages could significantly modulate the membrane surface charge to control the transport of ions across the nanochannels. Debye length can be determined using eqn (4):¹⁹⁵where e is the basic charge of an electron, ε_r and ε₀ represent the dielectric constant of the solution and the permittivity of free space respectively, T indicates the absolute temperature in kelvins, k_B denotes the Boltzmann constant, z_i is the valence of ion i, and n_i indicates the number density of ion i in the electrolyte. The surface charge of these membranes can be regulated by the combination or charge reversal.¹⁷⁵ A phase separation method was used to produce MXene-containing amidoxime polyacrylonitrile membranes, which exhibited excellent uranium (U(VI)) extraction capacity and rapid response.¹⁹⁶ MXenes also demonstrated excellent adsorption and reusability for Ba²⁺/Sr²⁺ in model fracking flowback solutions.¹⁹⁷ In heavy metal ion removal, the adsorption capacity of MXene nanosheets is crucial, though it can lead to ultraslow permeation rates.⁴⁵

The nanoconfinement effect is crucial in explaining the differences in mass transfer within nanofluid ion channels compared to larger-scale systems. Ionic movement across MXene membranes is a two-step process involving initial dehydration followed by diffusion in the interlayer spaces. It had been found that Li⁺ ions passed through these nanoconfined channels faster than K⁺ and Na⁺ due to the tight control of interlayer spacing, which reduced Li⁺ interaction with the ionic terminal.¹⁹⁸ The rapid and selective transport of Li⁺ was enabled by electrostatic interactions, and the energy barriers for interlayer insertion varied with the charge state. The charge selectivity is a result of the charged MXene layers.¹⁶¹

The incorporation of MXene nanosheets in mixed-matrix membranes (MMMs) introduced a more complex separation mechanism, involving the synergistic effect of multiple components. While the incorporation of MXene nanosheets could increase the length of transport, the pathway could be optimized to achieve the goal of an ultra-fast and precise sieving process.¹⁹⁹ MXene nanosheets leveraged their unique advantages of electrostatic interaction and nanochannels without interfering with the diffusion mechanism or charge effect of the polymer matrix.²⁰⁰ This demonstrated the potential of MBMs in advanced separation applications.

5.2 Applications of MBMs for water and wastewater treatment

MBMs attracted widespread attention due to MXene's unique characteristics, offering exceptional selectivity and water permeation capabilities.²⁰¹ Solutions with complex chemistry, including hydration, pH, and ionic strength, significantly influenced transport pathways.⁹⁹ Hydration caused membrane swelling, and changes in pH and ionic strength altered the charge of membranes, affecting the interlayer distance of expandable laminar membranes and thus impacting its permeability.^58,159 Applied potentials could control water ion diffusion by adjusting the ionization degree of water molecules in nanochannels.²⁰² Light or heat could be used to regulate MBMs' mass transport based on their photothermal properties, as shown in the ionic flow control in MXene laminates using laser light.²⁰³ Researchers, such as Shao et al., had proposed improved post-treatment methods such as air-drying and fan-drying to achieve stable, partially dehydrated MXene membranes.²⁰⁴Table 1 presents the comparison of MBMs for separating different pollutants, showcasing their versatility across a range of separation and purification applications including desalination, and heavy metal, organic pollutant, and bacterial removal.

Table 1 Comparison of MBMs for separating various pollutants

Applications	Membrane	Pollutants	Results	Ref.
Removal of bacteria	Ti3C2Tx-pillararene	Bacitracin	Rejection: 95.8%	205
			Permeability: 350 L m^−2 h^−1 bar^−1
	Ag@MXene	Gram-negative bacteria	Rejection: 99%	56
			Permeability: 420 L m^−2 h^−1 bar^−1
	Modified Ag@MXene	Gram-negative bacteria	Rejection: 99.99%	206
Removal of organic pollutants	MXene-Si@PES	Emulsions	Rejection: 99%	157
	CuO@MXene-PAN	Emulsions	Rejection: 98%	207
			Permeability: 2365 L m^−2 h^−1 bar^−1
	MXene/ZnO	Emulsions	Rejection: >99.4%	208
			Permeability: 5133 L m^−2 h^−1 bar^−1
	MXene/COF	Congo red	Rejection: 98.2%	88
		Chrome black T	Rejection: 98.7%
			Permeability: up to 300 L m^−2 h^−1 bar^−1
		Methyl blue	Rejection: 96.4%
		Methyl orange	Rejection: 97.2%
	MXene@TiO2/PEN	Methyl orange	Rejection: 93.5%	183
		Crystal violet	Rejection: 98.06%
		Methylene blue	Rejection: 99.30%
			Permeability: up to 2507.5 L m^−2 h^−1 bar^−1
	MXene@CS/MCN	Antibiotics	Rejection: 93.4%	209
	PVDF/TiO2@MXene	Tetracycline	Rejection: 87.8%
		Meropenem	Rejection: 60.7%
			Permeability: up to 293 L m^−2 h^−1 bar^−1
Removal of heavy metals	Fe3O4@MXene	Cu^2+	Rejection: 63.2%	205
		Cd^2+	Rejection: 64.1%
		Cr^6+	Rejection: 70.2%
			Permeability: up to 125.1 L m^−2 h^−1 bar^−1
	MXene/CNFs	Pb^2+	Rejection: 89%	151
		As^3+	Rejection: 81%
			Adsorption capacity: 12.5 and 3.3 mg g^−1
	MXene	Pb^2+	Rejection: 99%	207
			Separation factor of K^+/Pb^2+: 78
	MXene/PDA/Sep@ZIF-67-1	Cu^2+	Rejection: 79.9%	210
Desalination	UiO-66-NH2@MXene	Na2SO4	Rejection: 91%	211
		MgSO4	Rejection: 87%
		MgCl2	Rejection: 79%
		NaCl	Rejection: 62%
	MXene	NaCl	Rejection: 99.5%	57
			Permeance: 85.4 L m^−2 h^−1 (vaccum: 400 Pa)
	MXene/sulfonated TFN	Na2SO4	Rejection: 96.68% (Na2SO4)	212
		NaCl	Permeability: 67.95 L m^−2 h^−1 bar^−1 (NaCl)
	SC-MXene	MgCl2	Rejection: ∼82%	174
			Permeability: ∼9 L m^−2 h^−1 bar^−1

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5.2.1 Removal of bacteria. In complex aquatic systems, physical settlement and adsorption of bacteria led to biofilm formation, which could severely reduce water permeability. This had prompted the development of bacterioselective anti-fouling membranes to ensure stable and efficient separation.⁴⁵ As shown in Fig. 14(a), direct exposure to Ti₃C₂T_x MXenes could cause disruption of cellular membranes, resulting in cellular damage and eventual fatality.²¹³ It was proposed that as the concentration of Ti₃C₂T_x increased, both E. coli and B. subtilis became ensnared or enveloped by the nanometer-thin sheets of Ti₃C₂T_x, subsequently forming clusters. Furthermore, the exposure of bacterial cells to the sharp edges of Ti₃C₂T_x resulted in membrane damage. The water contact angle on Ti₃C₂T_x films was measured at 37°, and its hydrophilicity led to effective bacterial attachment to Ti₃C₂T_x.²¹⁴ Additionally, the antibacterial effects might stem from the strong reducing activity of the MXene and its reactive surfaces.²¹⁵ The smallest Ti₃C₂T_x nanosheets could penetrate the microorganism cell through direct physical penetration or via endocytosis. Ultimately, Ti₃C₂T_x might also interact with certain molecules in the cell wall and cytoplasm of microorganisms, disrupting the cell structure and leading to their demise. The variation in bacterial sizes made membranes based on MXenes suitable for separation applications. For instance, Pandey and colleagues developed an Ag@MXene membrane with silver nanoparticles loaded through the self-reduction of silver nitride on MXene's surface, functioning as both the membrane-forming agent and the reducing agent.⁵⁶ This membrane, with an average pore size of 2.1 nanometers, showed excellent water flux (approximately 420 L m⁻² h⁻¹ bar⁻¹) and strong antibacterial activity, achieving around 60% inhibition of bacterial growth and over 99% inhibition in Gram-negative bacteria. The combination of Ag nanoparticles with TiO₂ on the composite membranes could further restrict bacterial growth, especially under UV light. However, this membrane assembly reduced the surface area, slightly diminishing average antibacterial efficacy.⁵⁶ To enhance and stabilize the antibacterial properties of the Ag@MXene hybrid, Feng et al. modified it using a bio-inspired dopamine-initiated crosslinking process with polyethyleneimine. As shown in Fig. 14(b), these modified membranes maintained high antibacterial efficiency, reaching a bacterial inhibition rate of 99.99% against Gram-negative bacteria. This suggested an effective reduction in biological fouling during separation, facilitated by these membranes.²⁰⁶

Fig. 14 (a) Schematic of the antibacterial activity of Ti₃C₂T_x MXene²¹³ (reproduced from ref. 213 with permission from the American Chemical Society, copyright 2016) and (b) separation and anti-fouling mechanism of the Ag@MXene/PEN fibrous composite membrane²⁰⁶ (reproduced from ref. 206 with permission from Elsevier, copyright 2022).

5.2.2 Removal of organic pollutants. Concerns about the toxicity of organic pollutants such as antibiotics and dyes had spurred the use of MBMs for their removal. These membranes, enhanced with functional groups and nanoparticles, helped clean the environment.¹⁴⁷ To combat the membrane fouling, research focused on creating photocatalytic composite membranes with self-cleaning abilities. The conductive and semiconductive nature of MXene materials allowed for the creation of heterostructured photocatalysts, boosting the separation of charge carriers and photocatalytic efficiency.^53,216,217 These innovative materials held the potential to transform wastewater treatment and environmental remediation.²¹⁸
Oil–water emulsions. Rapid urbanization and industrial growth led to a surge in oily wastewater and oil spills, threatening the environment and human health. MBMs had been developed to combat this issue, offering superhydrophilic/superoleophobic membranes that excelled at separating oil–water mixtures due to their low oil adhesion and hydrophilic nature.¹⁷⁶ To overcome the challenge of water barrier resistance, researchers created novel TA-ZIF-8@MXene membranes, which incorporated tannic acid-modified ZIF-8 nanoparticles to achieve high flux recovery ratios.¹⁵² The integration of hydrophilic TA-ZIF-8 nanoparticles resulted in an improved water flux, leading to a highly hydrophilic and oleophobic membrane surface.¹⁵² Additionally, 2D Na-Bentonite@MXene composite membranes with switchable wettability had been developed, demonstrating a rejection ratio of 96% and a recovery rate exceeding 86% after 8 cycling tests. These membranes could switch between hydrophilic/hydrophobic and hydrophobic/oleophobic surfaces depending on the presence of water or oil.¹⁷⁷ By modifying the pyromellitic acid group of MXene nanosheets, MBMs had been prepared, showing excellent superwettability and effective separation of harsh emulsions with a surfactant-stabilized emulsion separation efficiency of over 99.97%.²¹⁹ As shown in Fig. 15(a), MXene@TiO₂/PEN fibrous composite membranes with increased interlayer spacings and super-hydrophilic/underwater superoleophobic properties had been developed, effectively separating various surfactant-stabilized oil-in-water emulsions with high oil rejection rates.¹⁸³ Huang and colleagues developed MCE-MNR membranes, integrating MXene nanoribbons with a cellulose mix. This innovation yielded a water permeance of 15 860 L m⁻² h⁻¹ bar⁻¹ and 99% + oil rejection, credited to a surface water layer and the self-cleaning photoactivity of the membrane.²²⁰

Fig. 15 (a) Mechanism diagram of photocatalytic degradation of dyes with microscopic images of isooctane emulsion before and after separation¹⁸³ (reproduced from ref. 183 with permission from Elsevier, copyright 2016), (b) schematic of the mechanism of dye rejection for the MXene nanosheet-reinforced ANF membrane with the effect of NaCl concentration on the dye rejection²²¹ (reproduced from ref. 221 with permission from the American Chemical Society, copyright 2021), and (c) schematic of the separation mechanism of Ti₃C₂T_x/CNF membranes from antibiotic-/salt-contaminated wastewater with UV-vis spectra before and after static adsorption experiments²²² (reproduced from ref. 222 with permission from Elsevier, copyright 2016).

Dye. The textile industry is a major consumer of water, using around 150 m³ per ton of processing, resulting in over 3 billion tons of wastewater annually.²²³ MXene-based materials had proven to be effective adsorbents for cationic dye molecules, and UV light could enhance dye degradation in their presence.²¹⁵ Flexible MXene/COF membranes, self-assembled through electrostatic forces, had a hierarchical structure with micropores and mesopores, and they also possessed strong photothermal conversion abilities, showing high water permeance and excellent recyclability under UV light.²²⁴ ZnO@Ti₃C₂T_x MXene nanofiltration membranes, responsive to visible light, removed organic dyes and exhibited a high rejection rate for CR and good water permeability.⁵³ Moreover, the addition of Ti₃C₂T_x nanosheets into polyacrylonitrile (PAN) endowed the dual-functional superhydrophilic/underwater superoleophobic MXene-PAN membranes with outstanding adsorption capacity for dyes.¹⁷⁶ MXene-PANI/PES composite ultrafiltration membranes, prepared via non-solvent induced phase separation, showed high retention rates for CR and MB dyes. The application of negative voltage repelled negatively charged contaminants, reducing fouling on the membrane surface.²²⁵ Two-dimensional porous MXene membranes with perforative pore formation on nanosheets enhanced the water permeability and achieved nearly 100% rejection for small-molecule dyes like CR.¹⁰⁸ When NaCl was introduced during fabrication and application, a membrane with high salt and dye separation efficiency was obtained.²²⁶ As shown in Fig. 15(b), MXene-reinforced aramid nanofiber membranes improved rigidity and selectivity for dyes, achieving high recovery rates and low salt rejection.²²¹ As for the mechanism of dye separation, it was mainly divided into electrostatic interactions such as hydrogen bonding and π–π interaction and size sieving for passing or blocking dye molecules.²²⁷
Antibiotics. Emerging micropollutants such as antibiotics widely existed in the surface water, ground water and drinking water.²²⁸ “We will miss antibiotics when they're gone” was newspaper headlines indicating the threat of the antibiotic crisis.²²⁹ To treat wastewater containing antibiotics, membranes with well-designed pores and good anti-fouling properties are necessary.¹⁹³ MXene membranes with nanosheets having large aspect ratios had been used to fabricate laminated membranes with ordered stacking and equidistant nanosheets, demonstrating excellent separation performance and anti-fouling ability for antibiotic-containing wastewater. These membranes featured regular slit-shaped nanochannels that overcame the trade-off between selectivity and permeability for a range of antibiotics.⁹⁴ The optimized membranes, composed of Ti₃C₂T_x-pillararene hybrid nanosheets, showed high rejection rates for various antibiotics and high water permeance.²⁰⁵ The exceptional permeance and rejection are ascribed to the considerable lateral size of the nanosheets, uniform interlayer spacing, and electrostatic interaction between the membrane and the antibiotics.²⁰⁵ The optimized membranes, composed of Ti₃C₂T_x-pillararene hybrid nanosheets, showed high rejection rates for various antibiotics and high water permeance. In Fig. 15(c), the addition of carboxylated cellulose nanofibers (CNFs) stabilized the MXene laminar structure, enhancing molecular transport and anti-swelling performance.²²² The size-selective sieving, combined with the impacts of adsorption and electrostatic interaction, is predominantly accountable for the swift and efficient removal of diverse antibiotics and salts from contaminated water. The ultrafine interconnected nanofiber structure of CNFs functions similarly to “natural aquatic plants,” enabling the natural purification of water through adsorption effects.²²² To combat fouling, Yang et al. developed a ternary heterojunction photocatalytic composite membrane incorporating MXene, Bi₂MoO₆, and BiOBr nanomaterials, which demonstrated high water permeability and effective antibiotic removal, showcasing the potential of MXene-based materials for advanced wastewater treatment.²³⁰ A range of 2D g-C₃N₄@MXene composite membranes had been developed, exhibiting responsiveness to visible light. The enhanced metal conductivity of MXenes facilitated the separation of CN e⁻/h⁺ pairs, resulting in outstanding removal efficiency for various small molecules in water. Furthermore, the composite membrane displayed self-cleaning capabilities attributed to the incorporation of CN sheets, effectively mitigating membrane pollution concerns.²³¹

5.2.3 Removal of heavy metals. The quick growth of industries such as mining and metal finishing resulted in toxic metal ions such as Zn²⁺, Cd²⁺, Pb²⁺, Cr⁶⁺,and Cu²⁺ entering the natural systems, posing threats to human health and ecosystems.^120,232 MXene membranes, due to their adsorption and ion separation properties, were extensively used for heavy metal removal.²³³ For instance, Fe₃O₄@MXene membranes enhanced water flow and achieved high metal removal rates, up to 63.2% for Cu²⁺, 64.1% for Cd²⁺, and 70.2% for Cr⁶⁺.²³⁴ In Fig. 16(a), a novel adsorbent based on titanate, comprising Ti₃C₂T_x MXene-derived hydroxy-rich titanate and cellulose, had been developed to efficiently eliminate toxic heavy metal ions.²³⁵ The inherent hydroxy-rich property significantly enhanced the adsorption efficiency of the titanate. Moreover, the introduction of cellulose as a structural regulator in the fabrication of the titanate/cellulose membrane effectively addressed the challenge of recycling powdered adsorbents. Consequently, the resulting Ti₃C₂T_x-derived hydroxy-rich titanate/cellulose membrane adsorbent (M-NTO/CM) demonstrated a remarkable Cu²⁺ adsorption capacity of 2.63 mmol g⁻¹ and achieved adsorption equilibrium within a mere 5 minutes.²³⁵ The positively charged NH₂-MIL-101(Al) nanoparticles were in situ grown on the MXene nanosheets layers to form Ti₃C₂T_x MXene@MOF heterostructures that were incorporated into the PVFD membranes. Then, this membrane would possess a thin active layer and hydrophilicity, and hence, it showed a high permeation rate reaching 17.1 ± 0.2 L m⁻² h⁻¹ bar⁻¹ and a high rejection rate of heavy metals (Ni²⁺, Cd²⁺, Mn²⁺, Cu²⁺, and Zn²⁺) peaked at 95.2 ± 0.5%.²³⁶ The reduction of RGO in Ti₃C₂T_x nanosheets mitigated restacking and surface modification, enhancing wettability. The adsorption and charge transfer enhancement further improved the removal of heavy metal ions with capacities of 84, 890, 1241, and 1172 mg g⁻¹ for Cr(VI), Pd(II), Au(III), and Ag(I), respectively.¹⁵⁶ In Fig. 16(b), it introduced a novel approach for synthesizing PEI-modified GO/MXene composite membranes, demonstrating a significant enhancement in the retention of divalent metal cations.²³⁷ It was credited to the positive surface potential of the GO/MXene_PEI membranes, which created an electrostatic repulsion for metal cations. This repulsive force intensified with the increasing amount of ion charge. The removal mechanism of metal ions by the GO/MXene_PEI membranes primarily encompassed steric and electrostatic effects.²³⁷

Fig. 16 (a) Schematic of Cu²⁺ ion adsorption using M-NTO/CM with leaching rate²³⁵ (reproduced from ref. 235 with permission from Elsevier, copyright 2024), (b) schematic of GO/MXene_PEI membranes with ion rejection²³⁷ (reproduced from ref. 237 with permission from Elsevier, copyright 2021), and (c) schematic of the ion sieving mechanism with the separation of mixed ions (K⁺/Pb²⁺) for MXene membranes under different voltages²³⁸ (reproduced from ref. 238 with permission from Elsevier, copyright 2020).

The voltage-gated approach had been used to improve the rejection of ions and molecules. MXene membranes with increased hydroxylation showed excellent wettability and zeta potential, with electrical voltage further enhancing the rejection of heavy metal cations up to 99.5%.¹⁷⁹ Electrostatic membranes, leveraging the electrostatic interaction between ions and membranes, had attracted attention for their gating capabilities.^239,240 Porous membranes with small nanochannels allowed for the overlap of electrical double layers, aiding in the electrostatic control of ions.²⁴¹ In an external electric field, membranes were oxidized by positive charges or reduced by negative charges to gate rejection, such as polypyrrole-based membranes and graphene-based membranes.^191,242 Inserting cations with solvation shells between MXene layers could potentially increase the interlayer spacing of MXenes. However, it was important to note that the electrostatic attraction between the negatively charged MXene layers and the cations might counteract this effect, potentially leading to a decrease in the interlayer spacing.²⁴³ MXenes possessed excellent electronic conduction and negatively charged surface (−39.5 mV) with a tunable interlayer spacing by ion intercalation with electrical voltages.^28,192 Thus, MXene membranes had an advantage on the removal of molecules and ions from water, which was attributed to the interlayer spacing (2–3 layers of water molecules) under equilibrium in aqueous environments.²⁴⁴ Moreover, the solution concentration and ion species are the main factors for the strength of the voltage-gating effect.²⁴⁵ For example, MBMs with an external potential could effectively control the ion rejection, such as organic MB molecules and inorganic salts (MgSO₄ or NaCl).²⁰² Negative voltages led to an increase in ion rejection, while positive voltages resulted in a decrease in ion rejection across MXene membranes. In addition, the external voltage could control the swelling problem and maintain the high rejection of the heavy metal ion Pb²⁺.²³⁸ In Fig. 16(c), the hydrated K⁺ ion, with a smaller diameter of 6.62 Å, could effectively traverse the channels of the MXene membrane. Conversely, the hydrated Pb²⁺ ion, possessing a larger diameter of 8.01 Å, was hindered from diffusing, resulting in a rejection rate exceeding 99%.²³⁸

5.2.4 Desalination. In order to deal with crisis of water storage, the utilization of seawater was regarded as an effective method, which covered 96.1% of the earth.¹⁰⁷ MXene membranes could expand their interlayer spacing to approximately 6 Å in solutions, similar to the diameters of hydrated ions such as Na⁺ (7.16 Å) and K⁺ (6.62 Å).⁵⁷ The development of scalable fabrication and regenerability of MBMs had advanced desalination technologies. These ultrathin MXene membranes, with thicknesses in the tens of nanometers, exhibited a water flux of 85.4 m² h⁻¹ and a high rejection rate of 99.5% for NaCl solutions, which correlated with their lateral size and feeding temperature.

On the one hand, compared with the other membranes, exploiting the surface and edge abundant functional groups is an effective strategy to improve the 2D nanochannel structure and enhance the swelling resistance.²⁴⁶ Additionally, charged MXene membranes had been developed for desalination, with the surface-coating of polyelectrolyte layers tuning the charge and hydrophilicity without compromising the lamellar structure.¹⁷⁴ Polyelectrolytes such as poly(diallyldimethylammonium chloride) and polycyclic aromatic hydrocarbons had been used to regulate the physicochemical structure for size-based and electrostatic interactions in desalination.¹⁷³ As shown in Fig. 17(a), through the intercalation of AgNPs into the laminar structure of Ti₃C₂T_x and subsequent cross-linking with HPEI, a highly permeable and positively charged HPEI-AgNP@Ti₃C₂T_x MXene nanofiltration (NF) membrane has been developed for the first time.²⁴⁷ While the intercalation of AgNPs efficiently enhances the water permeability of the Ti₃C₂T_x MXene membrane, the prepared AgNP@Ti₃C₂T_x MXene membranes are constrained by a trade-off effect. Notably, the HPEI-Ag-0.6@MX-0.40 membrane exhibits outstanding NF performance, achieving rejections of 84.15% for MgSO₄, 77.01% for MgCl₂, 74.67% for Na₂SO₄, and 56.58% for NaCl solutions at a concentration of 1000 ppm.²⁴⁷ As shown in Fig. 17(b), a cutting-edge thin-film nanocomposite reverse osmosis (RO) membrane, integrating two-dimensional MXene Ti₃C₂T_x within the polyamide (PA) layer through in situ interfacial polymerization, had been successfully engineered.²⁴⁸ By leveraging the remarkable diffusion-regulating effect of Ti₃C₂T_x, following Fick's first law, water permeability had been enhanced to a peak value of 2.53 L m⁻² h⁻¹ bar⁻¹ while maintaining a high NaCl salt rejection of 98.5%. Through microstructural observation and mechanism analysis, this enhancement in chlorine resistance was primarily attributed to the interaction between the surface functional groups of Ti₃C₂T_x nanosheets and active chlorine, effectively shielding the PA matrix from chlorine attack.²⁴⁸

Fig. 17 (a) Schematic of the mechanism of HPEI-AgNP@MXene composite membrane with NF performance of the HPEI-AgNP@Ti₃C₂T_x MXene membrane²⁴⁷ (reproduced from ref. 247 with permission from Elsevier, copyright 2023), (b) desalination and anti-fouling process with salt rejection of different membranes²⁴⁸ (reproduced from ref. 248 with permission from Elsevier, copyright 2020) and (c) schematic of the highly efficient solar-vapor generation from the solar absorber of the Co-CNS/M foam in an actual seawater sample before and after desalination²⁴⁹ (reproduced from ref. 249 with permission from Wiley Online Library, copyright 2020).

On the other hand, combining solar-driven water evaporation with photodegradation is a significant strategy for achieving sustainable freshwater with improved energy efficiency.^250,251 Materials that possess both photocatalytic activity and cooperative photothermal conversion had been widely used for this purpose.²⁵² MXene nanosheets, in particular, had shown extended solar absorption and significant photothermal effects due to their unique localized surface plasmon resonance (LSPR) and tunable band gap.^{178,253–255} For instance, Zhang et al. developed a MXene hydrogel membrane containing porphyrin and polyvinyl alcohol, which formed a dynamic hydrophilic network.²⁵⁶ By harnessing the coupling interactions and charge redistribution between MXene and porphyrin, the hydrogel membrane had been enriched with captivating physicochemical properties, amalgamating stable hydrophilic dynamic networks, synergistically enhanced photothermal effects, and photothermal-enabled photocatalytic activity. The interaction between MXenes and porphyrin resulted in an excellent solar-driven water evaporation rate of 1.82 kg m⁻² h⁻¹ and a photocatalytic degradation efficiency of 90.5%.²⁵⁶ In addition, the flexible porphyrin-Ti₃C₂T_x MXene Janus membrane could create dual-function-enabled photothermal desalination with a stable hydrophobic/hydrophilic Janus structure.²⁵⁷ MXenes were also coated onto commercial polytetrafluoroethylene (PTFE) membranes to establish a self-heated photothermal membrane distillation process with excellent temporal responses.²⁵⁸ Zhao et al. introduced a method to prepare hydrophobic salt-blocking MXene membranes that involved modifying delaminated MXene nanosheets with trimethoxy silane to enhance sunlight harvesting, demonstrating remarkable solar evaporation performance.⁹⁶ As shown in Fig. 17(c), a sophisticated solar-absorbing architecture had been conceptualized and manufactured, featuring a 3D MXene microporous skeleton adorned with vertically aligned MXene nanosheets.²⁴⁹ This structure was further enhanced with vertical arrays of metal organic framework-derived 2D carbon nanoplates embedded with cobalt nanoparticles. Consequently, under one sun irradiation, the solar-vapor conversion efficiency of the MXene-based hierarchical design could reach approximately 93.4% and sustain over 91% efficiency for over 100 hours, facilitating the production of clean vapor for stable and continuous water desalination.²⁴⁹

6. Challenges and opportunities

The exponential growth of 2D materials had expanded the options for membrane development, with MXenes emerging as a promising solution for advanced membranes. In comparison to other 2D materials, MXenes offered several distinct advantages. For example, the uniformly distributed surface groups on MXene nanosheets ensured predictable stacking behavior and consistent channels, while their strong hydrophilicity aided in water transport. While MXene-based membranes had demonstrated exceptional separation performance, there remained significant potential for further enhancement. In Fig. 18, challenges and opportunities related to their production, preparation, mechanisms and applications needed to be addressed to advance MXene-based membranes.

Fig. 18 Current challenges and opportunities for MXene-based membranes.

1) MXenes encompassed more than 30 varieties, yet the majority of research primarily focused on Ti₃C₂T_x. Particularly in the context of membrane preparation, only Ti₃C₂T_x and Ti₂CT_x had been employed for synthesizing membranes, with over 99% of MXene membranes originating from Ti₃C₂T_x. Given the diverse properties of different MXene types, these membranes could possess unique surface microstructures and transport channels, resulting in exceptional separation processes. Therefore, the development of innovative and efficient MXene membranes could significantly rely on other types of MXene nanosheets. While Ti₃C₂T_x particles remained stable up to 800 °C in an inert atmosphere, their stability decreased significantly to only 200 °C under oxidative conditions. Even when exposed to ambient conditions, Ti₃C₂T_x sheets tended to oxidize, forming TiO₂ crystals.²⁵⁹ This susceptibility to oxidation was particularly unfavorable for membrane applications and was accelerated by exposure to aqueous media, high temperatures, oxygen-rich environments, and ultraviolet (UV) light. Thus, the storage and preparation methods are vital to the chemical stability of MXene nanosheets.

2) The prevalent approach to synthesizing MXenes entailed the application of hydrofluoric acid (HF) or other liquid etchants such as lithium fluoride (LiF) and hydrochloric acid (HCl). The production of MXenes had faced challenges, particularly in developing eco-friendly synthesis methods. Despite research efforts, key issues persisted, including safety, scalability, and cost. For instance, while various etchants were available, HF remained the highest yielding and highest quality etchant, despite its high toxicity, necessitating the use of HF-specific personal protective equipment (PPE) and gloves. Furthermore, the intercalants used were toxic, requiring proper handling.^163,260 Cost-efficient methods used inexpensive resources to fabricate MAX phases, subsequently reducing the cost of MXene phases, including the use of TiO₂, carbon recovered from waste tires, and recycled aluminum scrap.²⁶¹ For instance, reports indicated that MAX phases could be produced using a molten salt-shielded synthesis/sintering method, which could be carried out in air and at low temperatures to fabricate large-scale MAX powders.^262–264 Thus, it is necessary to explore even more green and cost-efficient methods to synthesize MXene nanosheets. As of now, while successful preparation had been achieved on a few-gram laboratory scale, efforts were underway to scale production to the kilogram level.³

3) In order to further understand the structural design principles and separation applications of MXene-based membranes, there is an urgent need for advanced characterization techniques to deeply explore transport mechanisms within confined MXene nanochannels. For example, the in situ characterization of ion dehydration during ion transport in restricted MXene nanochannels could be achieved using in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS) in combination with molecular dynamics (MD) simulations. Additionally, low-field nuclear magnetic resonance (LF-NMR) allowed for the precise detection of interlayer confined structures within the membranes. Presently, the application of simulation and modeling techniques such as density functional theory (DFT) and MD significantly advances our understanding of membrane separation mechanisms, enabling the molecular-level rational structural design of MXene-based membranes.²⁶⁵ A comprehensive exploration of the mechanisms governing pollutant behavior in wastewater is essential. This entailed performing thorough analyses using a range of available characterization tools and theoretical models to gain a deep understanding of the processes involved.

4) Numerous characteristics of MXene-based membranes had been investigated, including permeability, anti-fouling potential, and hydrophilicity. However, the biocompatibility and cytotoxicity of MXene-based membranes had received little attention. Prior to applying these membranes for water purification, it is essential to assess these characteristics to prevent potential additional harms and toxicity resulting from membrane use. The stability of the membrane hinged on the interfacial adhesion between the substrate and the MXene layer.²⁶⁶ MXene membranes were often made on thin commercial polymeric ultrafiltration filters. While the membrane size was not crucial in lab-scale settings, practical conditions required larger membranes that could endure high pressures and crossflows. To fully utilize MXene-based membranes, specific equipment tailored to these requirements must be designed and optimized for module operations. The application of these membranes is currently confined to laboratory conditions, and limited research focuses on real-world scenarios. Therefore, investigations into MXene-based membranes should be rooted in actual water environments. Moreover, greater efforts should be made to enhance the regeneration and reusability of these membranes, thereby achieving long-term membrane functionality.

In conclusion, the vast potential and challenges of MXene membranes suggested a promising future for their development. By addressing the key issues, we believe MXene-based membranes will continue to grow rapidly in the next five years, contributing to membrane design and implementation.

Abbreviation

2D	2 Dimension
AAO	Anodic aluminum oxide
AB	Alcian blue
BSA	Bovine serum albumin
B. subtilis	Bacillus subtilis
CV	Crystal violet
CdCl2	Cadmium chloride
CNT	Carbon nanotube
COF	Covalent-organic framework
CR	Congo red
CuCl2	Copper chloride
CVD	Chemical vapor deposition
DMSO	Dimethyl sulfoxide
E. coli	Escherichia coli
GO	Graphene oxide
HCl	Hydrochloric acid
HF	Hydrogen fluoride
KCl	Potassium chloride
LiF	Lithium fluoride
MB	Methylene blue
MBM	MXene-based membrane
MeB	Methylene blue
MMM	Mixed-matrix membrane
MO	Methyl orange
Mo2C	Molybdenum carbide
MOF	Metal–organic framework
MoS2	Molybdenum disulfide
NaCl	Sodium chloride
NH4HF2	Ammonium dihydrogen fluoride
PA	Polyamide
PAN	Polyacrylonitrile
PEI	Polyethyleneimine
PEN	Poly(arylene ether nitrile)
PES	Poly(ether sulfones)
PSF	Polysulfone
PVA	Poly(vinyl alcohol)
PVDF	Poly(vinylidene fluoride)
RhB	Rhodamine B
RB	Rose Bengal
TBAOH	Tetrabutylammonium hydroxide
TB	Trypan Blue
TC	Tetracycline hydrochloride
TiO2	Titanium dioxide
TMAOH	Tetramethylammonium hydroxide
rGO	Reduced graphene oxide

Data availability

No data were used for the research described in this article.

Conflicts of interest

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

Acknowledgements

This work was supported by National Key Research and Development Program of China [No. 2022YFC3703102], National Natural Science Foundation of China [No. 22276163] and Zhejiang Provincial Natural Science Foundation of China [No. LR22B070002].

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