Recent Developments in Two-Dimensional Materials-Based Membranes for Oil–Water Separation

The industrialization witnessed in the last century has resulted in an unprecedented increase in water pollution. In particular, the water pollution induced by oil contaminants from oil spill accidents, as well as discharges from pharmaceutical, oil/gas, and metal processing industries, have raised concerns due to their potential to pose irreversible threats to the ecosystems. Therefore, the effective treating of these large volumes of oily wastewater is an inevitable challenge to address. Separating oil–water mixtures by membranes has been an attractive technology due to the high oil removal efficiency and low energy consumption. However, conventional oil–water separation membranes may not meet the complex requirements for the sustainable treatment of wastewater due to their relatively shorter life cycle, lower chemical and thermal stability, and permeability/selectivity trade-off. Recent advancements in two-dimensional (2D) materials have provided opportunities to address these challenges. In this article, we provide a brief review of the most recent advancements in oil–water separation membranes modified with 2D materials, with a focus on MXenes, graphenes, metal–organic frameworks, and covalent organic frameworks. The review briefly covers the backgrounds, concepts, fabrication methods, and the most recent representative studies. Finally, the review concludes by describing the challenges and future research directions.


Introduction
The water pollution as a result of the rapid pace of industrialization in the last century has been steadily increasing and posing notable challenges worldwide [1,2]. The pollution of water by oils and organic contaminants, in particular, has been recognized as a major environmental concern since the pollution originates from multiple sources that can pose hazards not only to humans but also to terrestrial and marine ecosystems [3][4][5]. The oily wastewater can contain polyaromatic compounds and phenols that can impede the growth of species and cause cancer in humans [3]. These extensive effluents of oily wastewater are mainly originated from crude oil production and refineries, food production, textile processing, and oil spill accidents, which need to be treated in order to decelerate environmental degradation [6][7][8]. A variety of approaches have been implemented to treat oily wastewater including biological treatment, flotation, coagulation and flocculation, gravity separation, solvent extraction, and adsorption [9,10]. However, the majority of these methods may not provide an effective separation of emulsified oil-water mixtures, particularly when the size of oil droplets is lower than 20 µm [10,11]. Membrane-based technologies have been desirable for separating oil-water mixtures due to their size-exclusion mechanism that can provide selectivity for the effective separation of surfactant-stabilized emulsions [8,12]. Membrane-based separation also enables relatively low carbon footprints, scalability, continuous operation, and minimal requirements for chemical additives [13,14]. Another

Recent Developments in MXene-Based Oil-Water Separation Membranes
The abundance of hydrophilic functional groups contributes to promoting the performance of the MXene-based membranes [27]. This property can be further improved by incorporating design features that maximize the exposure of surface functional groups. A study by Huang et al., reported the modification of a mixed cellulose (MCE) membrane by utilizing fibrous MXene nanoribbons (MNRs) [56]. Figure 1a demonstrates the structure of the MXene, which resembles accordion-like morphology, whereas Figure 1b illustrates the fibrous morphology of the MNRs obtained after etching MXene by KOH [56]. The anti-adhesion capability and wettability of the membrane (MCE-MNR) were characterized by underwater oil contact angle (UOCA) measurements using diiodomethane. While MCE-MNRs demonstrated a maximum UOCA of 125 • , the MCE-MXene showed a UOCA of 111 • . The oil-water separation tests involved various types of oils: n-hexane, edible oil, gasoline, and diesel. The relatively larger pore size of MCE and MCE-MXene was found to be the main factor contributing to their lower separation performance [56]. In contrast, the nano-scale pores on the MCE-MNR membrane enabled a more effective separation with oil rejection rates of over 99% [56]. The improved performance of MCE-MNR was also attributed to the loose fibrous morphology of MNRs that enabled improved exposure of the surface functional groups and the formation of a water layer on the membrane surface that could effectively repel droplets of oil [56].
Overcoming the relatively low flux of the MXene-based membranes for oil-water separation is another arising area of interest. A study describes a fabrication approach that enables MXene lamellar membranes, which can separate oil-in-water emulsions at a high flux [57]. To enable a higher flux, expanded nanochannels were formed between the MXene nanosheets by intercalating the positively charged colloidal nanoparticles of Fe(OH) 3 into the MXene nanosheets that bear negative charges [57]. Subsequently, hydrochloric acid treatment was carried out to remove colloidal nanoparticles and obtain a MXene membrane with higher interlayer spacing. This treatment process formed additional channels that facilitated the transport of fluids and enabled a higher flux. The results showed an increase in the pure water flux by a factor of two to nine compared to a pristine MXene membrane [57].
The relatively low flux of the traditional MXene-based membranes can be partly attributed to their dense slits and longer interlayer transport [58,59]. To address this challenge, a study investigated the integration of hydrothermally synthesized ferroferric oxide-doped molybdenum disulfide (FM) with a flower-like structure into MXene laminates via the self-assembly method [58]. As demonstrated in Figure 2, the FM dispersed in MXene solution was vacuum filtered onto the mixed cellulose (MCE) membrane, which was followed by drying at room temperature to obtain the final MXene-FM membrane (MXa-FMb) [58]. Consequently, the formation of transversal nanochannels, along with the surface roughness, enabled the membrane to demonstrate the permeance of ≈4.25 × 10 2 L m −2 h −1 while separating the emulsions [58]. Overcoming the relatively low flux of the MXene-based membranes for oil-water separation is another arising area of interest. A study describes a fabrication approach that enables MXene lamellar membranes, which can separate oil-in-water emulsions at a high flux [57]. To enable a higher flux, expanded nanochannels were formed between the MXene nanosheets by intercalating the positively charged colloidal nanoparticles of Fe(OH)3 into the MXene nanosheets that bear negative charges [57]. Subsequently, hydrochloric acid treatment was carried out to remove colloidal nanoparticles and obtain a MXene membrane with higher interlayer spacing. This treatment process formed additional channels that facilitated the transport of fluids and enabled a higher flux. The results showed an increase in the pure water flux by a factor of two to nine compared to a pristine MXene membrane [57].
The relatively low flux of the traditional MXene-based membranes can be partly attributed to their dense slits and longer interlayer transport [58,59]. To address this challenge, a study investigated the integration of hydrothermally synthesized ferroferric oxide-doped molybdenum disulfide (FM) with a flower-like structure into MXene laminates via the self-assembly method [58]. As demonstrated in Figure 2, the FM dispersed in MXene solution was vacuum filtered onto the mixed cellulose (MCE) membrane, which was followed by drying at room temperature to obtain the final MXene-FM membrane (MXa-FMb) [58]. Consequently, the formation of transversal nanochannels, along with the surface roughness, enabled the membrane to demonstrate the permeance of ≈4.25 × 10 2 L m −2 h −1 while separating the emulsions [58].  Similar to the conventional separation membranes, the contamination and fouling by organic contaminants and oil phases remains a major challenge that can compromise the separation performance of MXene-based oil-water separation membranes. Self-cleaning of the MXene-based oil-water separation membranes by incorporating photocatalysis technology has recently attracted escalating interest because it is an environmentally friendly method that can degrade organic contaminants into small molecules with minimal toxicity [54,57,[60][61][62]. In a recent study, a composite MXene-based membrane was prepared for oil-water separation by incorporating MXene nanosheets and N-doped Bi2O2CO3 nanoparticles via a vacuum filtration process [54]. The results revealed that nanoparticles can enable a higher permeation flux by providing the MXene layers with additional channels for the permeation. The membrane demonstrated a water flux of 815.3 L m −2 h −1 and maintained rejection ratios of greater than 99% with oil-water emulsions Reprinted from Ref. [58], Copyright (2023), with permission from Elsevier. Similar to the conventional separation membranes, the contamination and fouling by organic contaminants and oil phases remains a major challenge that can compromise the separation performance of MXene-based oil-water separation membranes. Self-cleaning of the MXene-based oil-water separation membranes by incorporating photocatalysis technology has recently attracted escalating interest because it is an environmentally friendly method that can degrade organic contaminants into small molecules with minimal toxicity [54,57,[60][61][62]. In a recent study, a composite MXene-based membrane was prepared for oil-water separation by incorporating MXene nanosheets and N-doped Bi 2 O 2 CO 3 nanoparticles via a vacuum filtration process [54]. The results revealed that nanoparticles can enable a higher permeation flux by providing the MXene layers with additional channels for the permeation. The membrane demonstrated a water flux of 815.3 L m −2 h −1 and maintained rejection ratios of greater than 99% with oil-water emulsions prepared with different types of oils [54]. The photocatalytic capability of the membrane was evaluated by degrading different types of organic dyes in a solution phase under visible light irradiation. The dye removal reached 98.4%, 98%, and 99.9% for Rhodamine B, Trypan blue, and Congo red, respectively [54]. In addition, the self-cleaning ability of the membrane was investigated by performing the continuous filtration and photodegradation of Congo red solution. In the absence of light irradiation, the flux of dye solution through a photocatalytic membrane was reduced from 778.9 to 510.7 L m −2 h −1 after five cycles [54]. However, almost no decrease was observed in the flux of dye solution when the membrane was exposed to light irradiation. These results indicated in situ fouling removal via a photocatalysis mechanism [54].
Introducing the superwetting property is another alternative method to enhance the fouling resistance of the membranes. To capitalize on this property, a study employed Ca 2+ -crosslinking to fabricate Ti 3 C 2 T x MXene membranes intercalated with hydrophilic sodium alginate [63]. The presence of sodium alginate enabled larger MXene nanochannels and introduced superwetting properties with underwater oil contact angles measured at 151 • as an indication of very low oil-membrane adhesion [63]. While the pristine MXene membrane showed a significant flux decrease from 5.77 to 0.54 L m −2 h −1 bar −1 after five cycles of stabilized oil-water emulsion separation, the flux of the membrane modified with sodium alginate remained nearly constant, indicating a superior anti-fouling characteristic [63]. Table 1 summarizes the main findings for the most recent MXene-based oil-water separation membranes.

Graphene Materials and Their Use in Oil-Water Separation Membranes
Graphene is a single free-standing layer of sp2-hybridized carbon atoms that are bound in the hexagonal honeycomb array [64][65][66][67]. Other materials that are related to graphene include graphene oxide (GO), reduced graphene oxide (rGO), and ultrathin graphite [68,69]. Although graphene and GO have similar structures, the use of strong acids and oxidants in the preparation of GO introduces several oxygen-containing functional groups such as carboxyl, hydroxyl, and carbonyl to the GO structure [70]. The GO can be transformed into rGO via a reduction process that decreases the oxygen content and results in a structure with enhanced thermal and chemical stability [70,71]. As a result of its distinctive geometry and structure, graphene-based materials can demonstrate exceptional physico-chemical properties including a high Young's modulus, thermal and electrical conductivity, high fracture strength, chemical inertness, and tunable functionality [71][72][73][74][75][76]. In particular, graphene-based materials can offer extensive utility in environmental remediation applications, including the removal of waterborne pathogens, heavy metals, and organic contaminants due to their extremely high surface area that can even exceed the surface area of well-developed activated carbons [77,78]. Various methods have been developed to synthesize graphenes, which include micromechanical cleavage or the scotch tape method [79], liquid-phase mechanical exfoliation [80], chemical cleavage and exfoliation [81], epitaxial growth [82], and chemical vapor deposition (CVD) [66,81,83,84]. Among these synthesis approaches, the micromechanical cleavage remains a popular approach due to its simplicity that can be implemented without specialized equipment [81]. Despite simplicity, this method is inefficient in mass producing graphenes for various applications [81]. Hence, liquid-phase sonication methods have been implemented to facilitate the generation of ultrathin carbon films [81]. Furthermore, the advantage of the CVD approach is the ability to produce polycrystalline single-layer graphene over a large area with an exceptional quality [81,83,84]. Graphene-based materials have been extensively utilized to modify porous substrates in order to obtain the desired liquid wettability. Among the substrate materials, the use of polyvinylidene fluoride (PVDF) and cellulose acetate (CA) has been dominant due to their mechanical strength, oxidation resistance, and thermal stability [85,86]. In particular, the modification of PVDF substrates has been the center of investigation, as their hydrophobic wettability makes them prone to fouling and shortens their life cycle [86][87][88]. The membranes modified by graphene-based materials can be prepared mainly by two methods, vacuum filtration and electrospinning [85]. While in the vacuum filtration approach, the suspension of graphene-based nanomaterials is vacuum-filtered on a given substrate membrane, the electrospinning utilizes the precursor liquid to weave electrospun fibers into a membrane substrate [85].
The exceptional mechanical properties of graphene-based materials along with their high surface area make them suitable for fabricating membranes or modifying porous structures for applications in oil-water separation. These materials can form micron and nano-scale roughness on a surface to enable enhanced wetting properties. A main strategy in utilizing graphene-based materials is to modify meshes, aerogels, or sponges to introduce superhydrophobic-superoleophilic wettability, or to fabricate materials that enable superhydrophilic-underwater superoleophobic wettability [85].

Recent Developments in Graphene-Based Oil-Water Separation Membranes
In pursuit of performing oil-water separation at high flux and efficiency, a study by Sun et al., reported the fabrication of a composite paper by coating laser-aided reduced graphene oxide and hydrophobic polydimethylsiloxane (PDMS) onto an inexpensive paper tissue, as shown in Figure 3a,b [89]. Figure 3c-e demonstrate SEM images of uncoated paper tissue and the one after coating, respectively [89]. The laser was utilized to obtain hydrophobic graphene by reducing the graphene oxide without needing to introduce hazardous reducing chemicals. The laser irradiation also enabled the formation of defects on graphene that contributed to a 25% increase in the specific surface area [89]. The strong capillary force provided by the paper tissue along with the superhydrophobic wettability of the functional coating enabled the separation of surfactant-stabilized water-in-oil emulsions with an efficiency of up to 99.85% and flux of 4421 L m −2 h −1 [89]. paper tissue and the one after coating, respectively [89]. The laser was utilized to obtain hydrophobic graphene by reducing the graphene oxide without needing to introduce hazardous reducing chemicals. The laser irradiation also enabled the formation of defects on graphene that contributed to a 25% increase in the specific surface area [89]. The strong capillary force provided by the paper tissue along with the superhydrophobic wettability of the functional coating enabled the separation of surfactant-stabilized water-in-oil emulsions with an efficiency of up to 99.85% and flux of 4421 L m −2 h −1 [89]. In another study inspired by cell membranes, nanomesh membranes were developed with improved permeance and anti-fouling property by employing the vacuum-aided self-assembly method [90]. The nanopores were etched on graphene oxide (GO) by utilizing a partial combustion method, which reduced the length of the water transfer channels. The presence of graphene with hydrophobic wettability along with the modification using hydrophilic chitosan enabled the membrane to demonstrate low friction with the water In another study inspired by cell membranes, nanomesh membranes were developed with improved permeance and anti-fouling property by employing the vacuum-aided selfassembly method [90]. The nanopores were etched on graphene oxide (GO) by utilizing a partial combustion method, which reduced the length of the water transfer channels. The presence of graphene with hydrophobic wettability along with the modification using hydrophilic chitosan enabled the membrane to demonstrate low friction with the water permeate and a low oil adhesion underwater [90]. The enhanced water transport along with anti-fouling property enabled the membrane to show a water permeance of up to ≈4000 L m −2 h −1 bar −1 [90].
In regard to enhanced fouling resistance, Yang et al., developed a GO-based membrane that can demonstrate outstanding anti-fouling properties even at high permeance [91]. The membrane was fabricated by utilizing a GO membrane on which the layers of hydrophilic phytic acid (PA) and hydrophobic perfluorocarboxylic acids were consecutively assembled. To reduce the surface energy while tuning the surface hydration, perfluorocarboxylic acids were utilized with varied lengths of hydrophobic chains [91]. This approach resulted in a surface with an ability to demonstrate both the resistance against fouling as well as fouling release properties, which is conventionally challenging [92]. The membrane demonstrated outstanding fouling resistance as characterized by a flux decline ratio of lower than 10% at a permeance of ≈620 L m −2 h −1 bar −1 [91].
Maintaining the structure stability of the lamellar GO layer can be challenging due to its tendency to disintegrate in an aqueous environment [93]. In view of this challenge, a multifunctional fibrous composite membrane was fabricated by spray coating threedimensional (3D) TiO 2 @crumpled graphene oxide core-shell spheres on a porous support made of electrospun poly(acrylene ether nitrile) (PEN) [93]. Note that polydopamine was also utilized to mediate crosslinking and enhance the durability of the functional layer [93]. The physical stability and well-regulated nanochannels of the membranes enabled a high permeance of 3142-3514 L m −2 h −1 for oil-in-water emulsions and a rejection rate in excess of 99% [93].
Integrating switchable wettability into separation membranes has been another intriguing line of research that can enable an on-demand separation of oil and water [94][95][96]. However, fabricating graphene-based membranes that can demonstrate on-demand oilwater separation has been challenging. To address this challenge, a study investigated the in situ deposition of surfactant-modified graphene oxide (GO-CTAB) on the surface of different metal meshes by employing an electrodeposition technique [94]. The membrane demonstrated a reversible wettability switch from hydrophilic and underwater superoleophobic to superhydrophobic when exposed to organic solvents due to conformational changes in surface molecules. The membrane could retrieve the hydrophilic wettability upon applying a negative electrical potential [94]. The switchable wettability of the membrane enabled it to separate both water-in-oil and oil-in-water emulsions under gravity [94]. Table 2 summarizes the main findings for the most recent graphene-based oil-water separation membranes.

Metal-Organic Framework (MOF) Materials and Their Use in Oil-Water Separation Membranes
Metal-organic frameworks (MOFs) are an emerging category of highly crystalline and porous materials that are formed by assembling organic linkers and metal nodes [97][98][99][100]. The organic linkers can be ditopic or polytopic polyamines, phosphonates, cyano groups and carboxylates, which can form robust 2D or 3D crystalline structures when linked by the metal nodes [101,102]. The MOFs can provide highly ordered structures that enable them to demonstrate unique properties such as the high surface area, which can even exceed 6000 m 2 /g, thermal and chemical stability, low density, and versatility of the structure, all of which makes them suitable for a broad range of applications [97,99]. The structure of MOFs can be readily modified during the synthesis process by controlling the synthesis technique, synthesis conditions such as the pH and temperature, and the starting materials including the organic linkers and metal precursors [103,104]. MOFs are recognized for providing high porosity with small uniform pore size, which can be readily functionalized to benefit mass transport and separation applications, particularly membrane-based separation [103,105]. Fabricating or modifying porous membranes by MOFs has been realized using different synthesis approaches including the hydrothermal method [106], electro-spinning [107,108], layer-by-layer (LBL) coating [109,110], phase inversion [111], vacuum-aided filtration [112,113], and solvothermal method [97,114,115]. Among these approaches, the hydrothermal method can provide a more straightforward and environmentally benign synthesis route with a higher cost efficiency [106]. The LBL approach capitalizes on the sequential submerging of substrate material in solutions that contain metal ions or organic ligands [97]. The preparation of the membrane using the phase inversion method involves a well-stirred homogeneous solution that contains both organic ligands and metal ions. The solution is cast followed by pulling into a water bath to enable immersion precipitation [111]. Finally, in the case of the solvothermal method, the substrate material is submerged in solutions of organic ligands and metal ions followed by treating the substrate at an elevated temperature in an autoclave [116].
Conventionally, polymeric separation membranes have dominated the market as they can be prepared at a relatively low cost. Also, these membranes are capable of demonstrating promising permeate fluxes and mechanical flexibilities [105,117]. However, the relatively short life cycle of these membranes, low thermal and chemical stability, and low selectivity have limited the practicality of these membranes for oil-water separation [105]. In contrast, the membranes fabricated or modified by MOFs have been very appealing for oil-water separation applications due to their uniform pore structure that can be readily controlled by the interplay of organic ligands and metal ions, chemical and thermal stability, as well as their ease of functionalization [105,118].

Recent Developments in MOF-Based Oil-Water Separation Membranes
Simultaneous fouling by insoluble oil droplets and organic dyes in wastewater has been an area of continuous investigation to develop more effective membranes that can maintain consistent flux. In a study by Yin et al., a stainless-steel metal mesh was utilized as a porous substrate to be coated with MOF-303 [119]. A facile and economical hydrothermal method was utilized to coat the well-ordered structure of MOF-303 crystals. Figure 4a shows the surface of the metal mesh before the coating process [119]. Figure 4b demonstrates the surface of the metal mesh after the in situ growing Ni/Al layered double hydroxides (LDH), which served as the precursor to facilitate the nucleation of MOF-303 [119]. Subsequently, a film of MOF-303 was formed that uniformly covered the metal mesh surface, as shown in Figure 4c [119]. The unique structure of MOF-303 combined with the abundant presence of hydrophilic carboxylate functional groups enabled an excellent water affinity [119]. Consequently, the membrane could form a hydration layer to demonstrate an effective anti-fouling property. The membrane demonstrated the gravity-driven separation of the oil-water mixture with a flux of >12308 L m −2 h −1 and a separation efficiency greater than 99.35%. Furthermore, the membrane could effectively remove water-soluble dyes with >99% efficiency [119].
Integrating anti-fouling properties into oil-water separation membranes via photo-Fenton catalytic mechanism is another approach to maintain a consistent separation permeate flux and efficiency [120,121]. A study employed a quartz fibrous membrane with a high structural and thermal stability as the substrate to grow NH 2 -MIL-88B MOF via a one-step solvothermal method [114]. The prepared MOF provided the hierarchical structure necessary to achieve superhydrophilic and underwater superoleophobic wettability. Furthermore, the iron-based MOFs can provide excessive numbers of hydrophilic amino functional groups and can generate reactive •OH radicals to demonstrate self-cleaning capability [114,122,123]. The membrane could demonstrate a separation efficiency and flux of up to 99.4% and >350 L m −2 h −1 , respectively, for a range of surfactant-stabilized oil-water mixtures [114]. The reduced flux of the membrane due to the formation of oil "filter cake" could be recovered to some extent via washing by water. Ultimately, the photo-Fenton-driven self-cleaning of the membrane under visible light enabled the near-complete recovery of the water flux [114].
Membranes 2023, 13, x FOR PEER REVIEW 12 of 27 driven separation of the oil-water mixture with a flux of >12308 L m −2 h −1 and a separation efficiency greater than 99.35%. Furthermore, the membrane could effectively remove water-soluble dyes with >99% efficiency [119]. Integrating anti-fouling properties into oil-water separation membranes via photo-Fenton catalytic mechanism is another approach to maintain a consistent separation permeate flux and efficiency [120,121]. A study employed a quartz fibrous membrane with a high structural and thermal stability as the substrate to grow NH2-MIL-88B MOF via a one-step solvothermal method [114]. The prepared MOF provided the hierarchical structure necessary to achieve superhydrophilic and underwater superoleophobic wettability. Furthermore, the iron-based MOFs can provide excessive numbers of hydrophilic amino functional groups and can generate reactive •OH radicals to demonstrate self-cleaning capability [114,122,123]. The membrane could demonstrate a separation efficiency and flux of up to 99.4% and >350 L m −2 h −1 , respectively, for a range of surfactant-stabilized oilwater mixtures [114]. The reduced flux of the membrane due to the formation of oil "filter cake" could be recovered to some extent via washing by water. Ultimately, the photo-Fenton-driven self-cleaning of the membrane under visible light enabled the near-complete recovery of the water flux [114].
In addition to anti-fouling properties, the quest for membranes that can provide ultrafast oil-water separation has been on the rise to meet the demand in applications that produce large volumes of oil-water mixtures such as mining operations [124]. Recently, an ultrafast oil-water separation membrane was developed by growing 2D Cu triphenylene catecholate MOF on a copper mesh [125]. The special hierarchical structure, along with the polar nano-cavity in the MOF crystal, enabled effective superhydrophilic and underwater superoleophobic wettability [125]. Consequently, the membrane demon- In addition to anti-fouling properties, the quest for membranes that can provide ultrafast oil-water separation has been on the rise to meet the demand in applications that produce large volumes of oil-water mixtures such as mining operations [124]. Recently, an ultrafast oil-water separation membrane was developed by growing 2D Cu triphenylene catecholate MOF on a copper mesh [125]. The special hierarchical structure, along with the polar nano-cavity in the MOF crystal, enabled effective superhydrophilic and underwater superoleophobic wettability [125]. Consequently, the membrane demonstrated the gravitydriven separation of different oils including actual crude oil. The membrane enabled an ultrahigh flux of up to 329 kL m −2 h −1 during the separation process [125]. In addition, the unique hierarchical structure of the membrane contributed to an improved separation efficiency with permeates showing less than 24.6 mg L −1 of oil contents [125].
Despite the advancements in developing high-flux MOF-based membranes for oilwater separation, a majority of them can demonstrate only a single type of surface wettability, which may not meet the complex requirements in the sustainable treatment of wastewater [17]. This challenge can be partly addressed by developing membranes that can separate oil-water mixtures on demand. Incorporating switchable wettability has been a promising approach to realize on-demand separation capabilities. In such membranes, an alternation between hydrophobic and oleophobic wettability can be acquired under the influence of external triggers such as pH change [126,127], light irradiation [128,129], temperature change [130], and electricity [17,131]. In a study, a separation membrane with switchable superwettability was developed by growing CAU-10-H MOF on stainless steel mesh surface via employing the solvothermal method [17]. The interpenetrating hierarchical structure of the CAU-10-H crystals enabled dual superlyophobicity in submerged conditions. Prewetting the membrane with water resulted in the formation of a trapped layer of water that could prevent oil permeation, whereas the initial wetting by oil enabled a trapped oil layer that resisted water permeation [17]. This property enabled the membrane to demonstrate the on-demand separation of different stabilized oil-water emulsions with a flux of more than 1.85 × 10 5 L m −2 h −1 and >99.9% of separation efficiency [17].
Another line of research with rising popularity is the development of sustainable MOF-based membranes by utilizing biodegradable and renewable materials. These metalorganic frameworks are referred to as Bio-MOFs, which are synthesized by utilizing saccharides, peptides, and amino acids [132]. A study reported the preparation of a superhydrophobic Bio-MOF coating on the surface of fabric for oil-water separation [133]. The Bio-MOF was prepared by employing an electrochemical process and utilizing aspartic acid as the ligand and copper cores to obtain a rough surface, which was followed by treatment with stearic acid to lower the solid surface energy [133]. Subsequently, the Bio-MOF was sprayed onto the fabric surface to obtain the separation membrane. The membrane could demonstrate separation efficiency in the range of 95-99.4% and flux rates between 15,400 and 15,700 L m −2 h −1 for different oil-water mixtures [133]. Table 3 summarizes the main findings for the most recent MOF-based oil-water separation membranes.  Bio-MOF is a renewable material.
Fabric retains its super hydrophobicity after 55 cycles of abrasion, and it also does so after immersion in water solutions with pH range of 5 to 9.

Covalent Organic Framework (COF) Materials and Their Use in Oil-Water Separation Membranes
Covalent organic frameworks are a new class of crystalline porous materials that are formed by combining completely organic building blocks which are linked via covalent bonds to establish a highly ordered extended structure of 2D or 3D crystalline solids [134][135][136]. These organic building blocks are typically composed of light elements such as Si, C, N, O, and B [134]. While the 3D COFs are linked entirely by covalent bonds throughout the three-dimensional structure, the covalent bonds are present exclusively within the conjugated two-dimensional sheets in 2D COFs, and the layers are held by weak interactions such as hydrogen bonds and π-π stacking [137]. A covalent organic framework can offer multiple benefits including large surface area, chemical stability, low density, and ease of functionalization [137]. The reticular chemistry of COFs and their tunable functionality have diversified their applications across various fields ranging from separation to energy sectors [138]. Perhaps separation is among the applications that have most benefited from the advancements in COFs, as they have the potential to offer promising solutions to the permeability/selectivity trade-off due to their high porosity, well-defined and tunable pore size, and adjustable surface functionality [139]. Among separation methods, membrane-based separation can be significantly advanced by COFs, as the current polymer-based membranes typically suffer from the trade-off between permeability and selectivity [139]. In fact, the crystalline structure of the COFs with the orderly arranged pores act as transport channels for the liquids to permeate efficiently. Nevertheless, fabricating membranes that can take full advantage of such an orderly structure has been a challenge [139]. In particular, the in situ growth of defect-free COF layers on porous substrates has been challenging due to their limited film-forming ability [139]. Fabricating free-standing COF membranes has been a strategy to address the inadequate film-forming ability of COFs on substrates [140,141]. Consequently, a mechanically robust free-standing COF membrane requires thickness in the order of hundreds of micrometers, which can significantly compromise the permeate flux [139]. In view of these challenges, several other fabrication approaches have been implemented. For example, COFs can be integrated into common membrane fabrication methods such as non-solvent-induced phase inversion (NIPS) [142]. The presence of COFs provides additional liquid passage channels to improve the permeability, which is traditionally provided by the pores that are formed during the solvent exchange process [143]. Stacking the nanosheets of COF is another approach to fabricate COF-based membranes [143]. In this method, the bulk material is initially exfoliated in order to generate nanosheet dispersions in a solvent. Subsequently, dip coating or vacuum filtration methods are employed to form the COF layer on top of a porous substrate [143]. Interfacial polymerization (IP) is another method of fabricating COF-based membranes that utilizes a relatively mild reaction condition [144]. This method has evolved as a facile approach wherein the interface of two immiscible phases containing the monomers is utilized to perform the polymerization reaction [145,146]. It is worth noting that the final membrane structure is a function of the reactivity of the monomers [146]. The advantages of the interfacial polymerization method include the ability to fabricate thin membranes with relatively high permeability as well as the high scalability that makes it highly compatible with automatic membrane manufacturing [143,144].
Despite the use of COF-based membranes in liquid phase separation such as seawater desalination [147,148], removal of toxic ions [149,150], and organic contaminants removal from water [151,152], only a limited number of studies have reported the application of COF-based membranes in oil-water separation.

Recent Developments in COF-Based Oil-Water Separation Membranes
Incorporating selective wettability has been a promising approach to obtain a membrane for oil-water separation applications [153,154]. In a study, a dip-coating method was implemented in order to form Schiff base COFs (Tp-BD and TAPB-TPA) on electrospun nanofibers of polyacrylonitrile (PAN) [155]. The rough nanoscale structure along with hydrophobic modification using alkyl (Lauryl) groups resulted in a surface with superhydrophobic and oleophilic wettability. The water contact angle was measured as ≈167 • on the surface of the membrane [155]. The membrane could separate the oil-water mixtures that were prepared using different oil types and concentrations with efficiencies greater than 95% [155].
Preserving the stability of the COFs following the hydrophobic modification can be challenging, as it may affect the porosity, crystallinity, and morphology of the COFs. In a study by Wang et al., 2D imine-linked COFs with hydrophilic wettability were converted into aromatic quinoline-linked COFs with hydrophobic wettability by reacting enamides and imines through an aza-Diels-Alder process [156]. Figure 5a,b and Figure 5e,f show the SEM and TEM images of the imine-linked COF (COF-LZU1), respectively [156]. Figure 5c,d and Figure 5g,h, respectively, exhibit the SEM and TEM images of the quinoline-linked COF with -CF 3 groups (COF-Q-CF 3 ) [156]. The porosity and crystallinity of the COFs were maintained despite a significant increase in the hydrophobic wettability after the conversion. In particular, the quinoline-linked COF with -CF 3 groups demonstrated superhydrophobic wettability, which is attributed to the low solid surface energy of the perfluorinated groups [157]. Figure 6 shows the contact angles of water on the surface of COF-LZU1 and the quinoline-linked COF pellets [156]. The oil-water separation capability of the COF layer was tested using water-in-oil emulsions. The results showed that the oil phase in the emulsion could readily permeate the dense layer of COF, whereas the water phase was retained above the COF layer [156]. The separation efficiency reached about 99.6% for water-in-chloroform emulsion, and the oil permeation flux for different emulsions ranged from 1.1 × 10 4 to 2.2 × 10 4 L m −2 h −1 when using the quinoline-linked COF with -CF 3 groups [156].
In another study, oil-water separation membranes were prepared by growing 2D COFs on stainless steel net substrates via condensation reaction of fluorine and/or isopropyl functional groups and perfluorodialdehyde with triamines [158]. The membrane separated oil-water mixtures by allowing the permeation of oil while repelling water. Results showed that the separation efficiency and permeation flux of the membrane can reach over 99.5%, and 2.84 × 10 5 L m −2 h −1 , respectively [158].
the SEM and TEM images of the imine-linked COF (COF-LZU1), respectively [156]. Figure  5c,d and Figure 5g,h, respectively, exhibit the SEM and TEM images of the quinolinelinked COF with -CF3 groups (COF-Q-CF3) [156]. The porosity and crystallinity of the COFs were maintained despite a significant increase in the hydrophobic wettability after the conversion. In particular, the quinoline-linked COF with -CF3 groups demonstrated superhydrophobic wettability, which is attributed to the low solid surface energy of the perfluorinated groups [157].  Figure 6 shows the contact angles of water on the surface of COF-LZU1 and the quinoline-linked COF pellets [156]. The oil-water separation capability of the COF layer was tested using water-in-oil emulsions. The results showed that the oil phase in the emulsion could readily permeate the dense layer of COF, whereas the water phase was retained above the COF layer [156]. The separation efficiency reached about 99.6% for water-inchloroform emulsion, and the oil permeation flux for different emulsions ranged from 1.1 × 10 4 to 2.2 × 10 4 L m −2 h −1 when using the quinoline-linked COF with -CF3 groups [156]. In another study, oil-water separation membranes were prepared by growing 2D COFs on stainless steel net substrates via condensation reaction of fluorine and/or isopropyl functional groups and perfluorodialdehyde with triamines [158]. The membrane separated oil-water mixtures by allowing the permeation of oil while repelling water. Results The functionalization of imine-linked COFs with fluorine atoms to enhance hydrophobic wettability may be challenging due to their limited reactivity, which can make the large-scale manufacturing of the final product economically less feasible [159]. To address this challenge, a study reported a rapid microwave-aided synthesis method to form imine-linked COFs via the condensation of 1,3,5-tris(4-aminophenyl)benzene (TAB) with pyridine-based aldehyde linkers. The process imparted thioanisole functional groups into the building blocks of the COF, which resulted in hydrophobic wettability [159]. This method is synthetically more straightforward compared with introducing fluorine atoms via postsynthetic material modification approaches [159]. The resulting COF demonstrated superhydrophobic wettability to enable oil adsorption and water repellency. The adsorbed oil could be readily removed via rinsing by oleophilic solvents such as diethyl ether to regenerate the COF [159].
In another work, the carboxylated COF (COF-COOH) integrated with polydopamine (PDA) was assembled on the surface of the PVDF microfiltration membrane by employing a dip-coating method [160]. The high interfacial compatibility between COF-COOH and PDA resulted in a uniform and stable coating with high bond strength. The hierarchical structure along with the hydrophilic carboxyl groups enabled the membrane to demonstrate superhydrophilic and underwater superoleophobic wettability [160]. The membrane showed an oil rejection ratio of >98% and the permeate flux of up to 1843.48 L m −2 h −1 bar −1 [160]. Table 4 summarizes the main findings for the most recent COF-based oil-water separation membranes.

Future Directions
The separation of liquids by membranes has attracted considerable interest due to the relatively low energy requirement, continuous operation, minimum requirements for chemical additives, and size exclusion-driven selectivity. In particular, membrane-based technologies have found unparalleled application in treating oily wastewater, as their surfaces can be functionalized and undergo roughness modifications to enable the separation of various types of oil-water mixtures at different permeate fluxes. Conventionally, polymers have been utilized as the main material to develop these membranes as they can be processed using relatively straightforward fabrication techniques at reasonably low prices. Moreover, the use of polymers in these membranes enables a mechanically flexible structure that can meet the ordinary requirements in separating oil-water mixtures. However, challenges such as the relatively low chemical and thermal stability as well as the permeability-selectivity trade-off have prompted extensive research efforts to find replacements. Furthermore, with the ever-increasing need to develop more efficient solutions to treating complex industrial oily wastewater, the development of next-generation oil-water separation membranes has become inevitable. With the recent advancements in the synthesis and applications of two-dimensional (2D) materials, unprecedented opportunities have emerged to meet these new complex requirements. Two-dimensional (2D) materials can offer unique characteristics including high surface area, relatively uniform pore size, high porosity, and tunable wettability. Owing to these characteristics, 2D materials have been utilized to develop membranes, which can demonstrate promising oil-water separation efficiency and flux. Despite significant advancements, there are several key aspects of 2D materials-based membranes that require further research, as summarized below.

Membrane Thickness
Membrane thickness is a critical factor that affects the separation performance of a membrane [18]. A lower thickness of 2D materials-based membranes would not only contribute to a higher permeate flux and mass transport but will also reduce energy consumption. However, reducing the thickness of a membrane can compromise mechanical durability, as the membrane may require to be prepared free-standing and without using the porous supporting substrate. In particular, the mechanical durability becomes nontrivial when the membrane is subjected to high shear forces by the feed stream in cross-flow configurations [120]. Hence, further research is required to realize ultrathin free-standing 2D materials-based oil-water separation membranes with high mechanical durability.

Membrane Cytotoxicity and Long-Term Stability
Future research should also further study the cytotoxicity of the 2D materials-based membranes through numerous cycles of separation to investigate their practicality for actual oily wastewater treatment [27,161]. The long-term stability is also of interest for future investigations. Studies are encouraged to further investigate the long-term performance of these membranes particularly in aggressive environments with extreme pH levels, high temperatures, and high salinity.

Multifunctional Membranes
Incorporating additional functionalities into the 2D materials-based oil-water separation membranes is another line of research that can be further investigated to meet the more stringent future demands in industrial wastewater treatments. These additional functionalities include visible light-driven photocatalytic self-cleaning, removal of other contaminants (e.g., heavy metals, pesticides, pharmaceuticals), selectivity, and stimuliresponsive wettability.

Large-Scale Fabrication
Finally, research efforts are encouraged to investigate the possibility of facilitating the larger-scale fabrication of these membranes at a higher rate while lowering the nanopores defects. With the recent advancements in synthesis and manufacturing technologies including advanced robotics, precision 3D printing, and other non-conventional methods, significant breakthroughs are expected in the large-scale production of these membranes at lower prices [14,162].

Conclusions
This brief review aimed at summarizing the most recent advancements in developing membranes modified with two-dimensional (2D) materials for applications in oil-water separation. In particular, the contribution was focused on MXenes, graphene family materials, metal-organic frameworks (MOF), and covalent organic frameworks (COF) as the most common synthetic 2D materials that have been utilized to fabricate or modify the separation membranes. The characteristics of each 2D material along with the common synthesis/fabrication methods and the most recent representative studies were discussed. The 2D materials can provide unique characteristics including large specific surface area, high porosity, uniform pore size, tunable wettability, facile functionalization, tunable nanochannels, resistance to chemically and thermally aggressive environments, and mechanical durability. These unique characteristics have enabled the development of promising membranes that can demonstrate high permeate flux, improved separation efficiency, and promising regeneration capabilities when separating both free and emulsified oil-water mixtures. Despite these advancements, several limitations are identified for these membranes such as mass production, large-scale fabrication, and nanopores defects. With the anticipated tremendous progress in manufacturing technologies, materials synthesis, and computational methods, it is expected that most of these restrictions will be addressed in the near future.