1. Introduction

In response to oil-spill issues several technologies including gravity separation, air flotation separation, chemical precipitation, and adsorption separation have been proposed. However these technologies give rise to the production of secondary pollutants, and this phenomenon convinces the efficacy of environmentally benign membrane technology for treatment of oily waste water [1-5]. However membrane separation faces severe problem of fouling which can be overcome by replacing with composite membrane. Although there are numerous carbon-based nanocomposite membranes with excellent antifouling property in treating oily wastewater, the complexity and high-cost of the fabrication process limits these membranes to be applied in the industrial scale. 2D materials are excellent material as high surface area can be easily generated by simple exfoliation method. MXenes are proposed as an appropriate class of material for an effective separation membrane [6-12]. Among the MXene family, Ti₃C₂T_x has been especially highlighted for water purification applications due to its advantages including abundance, hydrophilicity, high surface area, surface functionality, facile fabrication, and environmentally benign property, and these properties of Ti₃C₂T_x are appropriate for oil/water emulsion treatment. T_x signifies the terminal functional groups that is characteristic of each MXene, and these terminal functional groups include -O, -F, -OH, and -Cl.

Fabrication of composite membrane is a multistep process. In case of formation of thin film composite membrane (TFC), skin layer has to be generated by in situ polymerization on the top of the support layer. In order to make the process simple, liquid exfoliated MXene can be very easily deposited on the surface of porous support membrane by simple vacuum filtration process and this can be easily scale up due to simplicity of the method. In this review, composite membrane of MXene and modified MXene membrane are discussed in detail. Polymers that are used in this review for composite membrane are Polyether sulfone (PES), Poly(vinyledene fluoride) (PVDF), Polyamide (PA), poly(acrylonitrile) (PAN), chitosan, etc. Fig. 1 represent the schematic representation of the classification of the review.

2. Oil/Water separation

2.1. MXene membrane

2.1.1. PES

There are numerous strategies in fabricating a membrane that resists oil fouling during oil/water separation. One strategy is incorporate hydrophilic polymer on the surface of the support membrane [13]. But due the chain flexibility of the attached polymer allows nano-sized oil droplets to pass through the barrier, eventually leading to fouling during long term operations. Another strategy is to coat inorganic materials on the membrane surface. However, the issue of compatibility of coating layer and the support layer remains as an issue. 2D titanium carbide-based MXene sheets featuring hydrophilic surface termination groups have been proposed as an appropriate material for membrane functionalization due to the tunability of the functional interlayer spacing. In this study, researchers have fabricated an effective oil-water separation membrane by incorporating thin MXene sheets onto a porous polyethersulfone (PES) support membrane through vacuum filtration. The fabricated MXene membranes displayed the highest oil separation efficiency of over 99.98% when treating oil containing salt solution, where salt ion rejection did not take place, considering the identical electric conductivity of the filtrate and the feed emulsion. XRD comparison between MXene membrane immersed in water and salt water revealed that the d-spacing value decreased 0.08 nm to 1.61 nm in salt water, with small intercalation of ions into the MXene membrane interlayer contributing to the decrease in the d-spacing. Smaller d-spacing enhanced the membrane’s capability of blocking small oil droplets from penetrating into the interlayer. The superior hydrophilicity of the MXene membrane depends on its fabrication method. When the Ti₃AlC₂ MAX precursor is etched with LiF and HCl solution, the Ti-Al bonds break easily leaving the Ti surface exposed with OH, O and/or F termination groups derived from the etchant solution. This leads to the MXene surface being heavily decorated with hydrophilic termination groups. This homogeneously hydrophilic surface enables the MXene membrane to withstand oil fouling better than its alternatives. Separation efficiency reached higher than 99.98%, and permeance value of 505 L m^-2 h^-1 bar^-1 was reached.

2.1.2. PVDF

With the presence of corrosive solutions in complex environments, the durability of the superhydrophilic material of the separation membrane is greatly shortened. One of most readily studied family of MXene membrane is Ti₃C₂T_X. Ti₃C₂T_X is fabricated by etching Al atomic layer in Ti₃AlC₂ with HF solutions, and this family of MXene membranes have demonstrated hydrophilicity being suitable for oily wastewater treatment [14]. In this study, researchers fabricated a 2D layered Ti₃C₂T_X MXene membrane that was both superhydrophilic and underwater superoleophobic. In this MXene “T” stand for fluoride group. Solutions of sodium alginates and Ti₃C₂T_X MXene were magnetically stirred to obtain an uniform suspension. This uniform suspension was then vacuum-filtered to PVDF. Sodium alginates acted as a firm adhesive between Ti₃C₂T_X MXene and PVDF. PVDF provided the overall membrane with excellent mechanical and high chemical resistance. The fabricated membrane showed excellent performance under harsh (acidic, alkalic, and saline) environments. AFM analysis displayed that the Ti₃C₂T_X MXene nanosheets were about 2.0 nm thick, and transport of water took place in the interlayer spaces between the nanosheets. The FT-IR spectrum of sodium alginates revealed that sodium alginates featured amphiphilic functional groups endowing the overall membrane with excellent hydrophilicity. The chemical stability of the fabricated membrane was tested through suspension in 3 M HCl solution, 3.5 wt% NaCl solution, and 3 M NaOH solution. Even after 10 hours of immersion into the featured solutions, the fabricated membrane was still superoleophobic, with the contact angle being greater than 150°. The fabricated membrane demonstrated separation efficiency reaching up to 99.4% and permeation flux reaching up to 887 L m^-2 h^-1 bar^-1 in treating various oil emulsions, highlighting the possibility of applying the featured membrane in treating oily wastewater under a complex environment.

2.1.3. PA

Among the numerous polymers for preparing nano fibrous mats, polyamides are the most favorable group due to their thermal stability [15]. It has been found that when nanomaterials are introduced into the surface or into the fibrous structure of the electrospun mats, separation efficiency improved. There has been numerous research proving that the usage of MXenes as nanofillers of the electrospun membranes, resistance towards oil fouling increased due to the hydrophilicity and the stacking pattern of the two dimensional lamellar structure.

In this study, researchers have deposited a dispersion of Ti₃C₂T_x particles on to the electrospun copolyamide (coPA) membrane. When the MXene coverage was low, the resulting membrane showed limited separation efficiency in treating vegetable oil. The composite membrane’s oil separation efficiency only reached a sufficient magnitude when the coverage of Ti₃C₂T_x reached 2.65 mg/cm², evidencing the requirement of minimum density of Ti₃C₂T_x on the surface of the electrospun mat. In conclusion, the deposition of Ti₃C₂T_x onto the coPA membrane acted as a crucial factor in enhancing the hydrophilicity of the overall membrane, ultimately enhancing the membrane’s anti-oil-fouling property. Separation efficiency of the membrane reached up to 99.5 %, and the flux value reached up to 11,000 L m^-2 h^-1 due to the endowed hydrophilicity.

2.1.4. Print Paper

In this study, Ti₃C₂T_x was coated on commercial print paper acting as a substrate [16]. The resulting membrane featured densely-packed and hydrophilic 2D MXene nanoflakes, and, compared to the unmodified membrane, mechanical flexibility and strength of the overall membrane was enhanced due to the paper substrate.

AFM analyses, revealed upon deposition of MXene nanoflakes on the print paper substrate through vacuum filtration, the root mean square roughness (Rq) of print paper substrate decreased from 914 nm to 200 nm. This loss in surface roughness led to a lower occurrence of membrane fouling in the separation membrane. The underwater oil contact angle (OCA) of the modified membrane, turned out to be 137 degrees. This result indicated that the modified membrane was underwater oleophobic, effectively resisting the adherence of oil droplets on the membrane surface properties of the MXene composite membrane. The fabricated membrane was able to separate the oil/water emulsion at a efficiency of 99% at a flux of 472.1 L m^-2 h^-1 held constant for 8 consecutive cycles.

2.2. Modified MXene composite membrane

2.2.1. PES

It is known that using additive nano-particles have improved the hydrophilicity, antifouling and antibacterial properties of membranes which significantly reduced the magnitude of membrane fouling. Recently, there have been efforts of using Ti₃C₂T_x nanoparticles as hydrophilicity-enhancing additives [17]. Several studies have proposed blending MXene nanoparticles with polymer membranes to enhance antifouling. Despite the simple blending process, most of the MXene nanoparticles were not sufficiently distributed on the upper layer, which essentially determines the permeability of the membrane. Addressing this issue, researchers in this study have proposed using a simple external magnetic field to sufficiently decorate the upper layer of the polyether sulfone (PES) membrane with magnetic Ni@MXene nanoparticles through a phase inversion process. MXene nanoparticles were firstly coated with Ni plating solution, providing a magnetic casting solution. The Ni@MXene nanoparticles were subjected to a phase inversion process between the water in air and solvent, leading to the Ni@MXene nanoparticles anchoring to the forming porous layer. The antifouling mechanism was evaluated through a thermodynamic method, especially evaluating through the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The XDLVO theory quantitatively evaluates the thermodynamic interaction between the membranes and the pollutants, by considering interactions including, van der Waals interaction, acid-base interaction, and static interaction and the intensity of these interactions depending on different separation distances as parameters. To understand the antifouling mechanism, the interaction energies between the membrane surface and pollutants were computed by a thermodynamic method. The interaction energy value between the pristine membrane and bovine serum albumin (BSA) was negative indicating the mutual attractiveness of PES membrane and BSA. This results in the BSA adhering onto the membrane, eventually resulting in membrane fouling. In contrast, total energy between the pollutant particles and the modified membrane was positive, evidencing the thermodynamic unfavourability of BSA particles adhering on the modified membrane surface. This led to a higher antifouling performance in the modified membranes highlighted by their higher flux recovery rate (FRR) values. The optimal fabricated membrane operated at a flux of 1181 L m^-2 h^-1 bar^-1, being 2.5 times higher than the alternative pure PES membrane.

Despite MXene-based-membrane’s superior hydrophilicity, these membranes are still limited in their antifouling performance in treating complex oily wastewater [18]. Recently there have been efforts in utilizing membrane separation technology combined with photocatalytic degradation, endowing the separation technology with photodegradation property, and semiconductor materials provide the membranes with this property. Alternately arranged layers of [Bi₂O₂]²⁺ and [CO₃]^2- comprise bismuth-based semiconductor materials (Bi₂O₂CO₃) through van der Waals forces. Although Bi₂O₂CO₃ is easier to produce and control morphology relative to other semiconductor materials, due to its limited electron transfer, application for photodegradation is limited. In this study, researchers have N doped Bi₂O₂CO₃ at room temperature in order to enhance the carrier separation efficiency and light absorption performance. Then this N- Bi₂O₂CO₃ was incorporated with MXene nanosheets through a vacuum filtration process. N-Bi₂O₂CO₃ provided more permeation channels for MXene separation layers, contributing to a higher permeation flux of membrane, and also photocatalytic ability, suggesting the reduction in fouling rate of organics. Electroparamagnetic resonance (EPR) was used to test the reactive species before and after light exposure to further investigate the mechanism of oxidative degradation of N-BMXM. Compared to other functional groups –OH^- , displayed a stronger response towards light exposure, evidencing that the – OH^- active substance contributed most to the N-BMXM degradation of dye molecule. Under visible light irradiation, the composite membranes were able to remove various dyes at a efficiency level higher than 98%, and the pure water flux under 0.1 MPa was 815.3 L m^-2 h^-1 addressing the issue of tradeoff between permeability and selectivity in treating oil and dye.

2.2.2. Chitosan

Graphene oxide (GO) is readily used as a 2D laminar membrane for filtering oily wastewater. However, hydrogen bonds that form between water molecules and functional groups of GO reduce water permeability. In addition, electrostatic repulsion between GO nanosheets in water contribute to disintegration of the GObased membrane. In order to address this instability, researchers have proposed intercalating nanoparticles to overcome this instability, and MXene is one of the major applicable nanoparticles [19]. MXenes, 2D titanium carbide nanomaterials, display structural stability in water, and there are abundant hydrophilic termination groups on the surface of MXene nanosheets possess which leads to a superior anti-fouling property. However, actual applications of 2D MXene membranes have not displayed sufficient flux and anti-fouling performance for effective oily wastewater treatment. Herein, the researchers in this study proposed a novel strategy to fabricate the two dimensional MXene lamellar membrane with high flux and high antifouling performance. Notably, in order to enhance the antifouling property, photocatalytic reaction was incorporated in the membrane to effectively degrade recalcitrant organics. Chitosan/tannic acid hydrogel was coated on the membrane surface, which endowed the membrane with oxidation resistance and catalyzed in situ mineralization of photocatalyst β-FeOOH nanoparticles. These hydrophilic β-FeOOH nanoparticles gave rise to super- hydrophilicity/underwater super-oleophobicity and enhanced self-cleaning through photocatalytic reaction. Permeation flux of the fabricated membrane reached up to as high as 1103.6 L m^-2 h^-1 with the separation efficiency being 99.3% in treating various oil mixtures.

2.2.3. Cellulose Acetate

A complex membrane functionalized with MXene, Halloysite nanotubes (Hal), and polydopamine (PDA) was fabricated that resisted such arising propensities in existing membrane technology [20]. MXene is readily used for fabricating high-performance multifunctional composite membranes. Hal, a type of raw clay minerals (Al[SiO₁₀](OH)₈⋅nH₂O) display hollow tubular structures with two open ends, and this environmentallyfriendly materials gave rise to enhanced hydrophilicity. Hal expanded the interlayer which provided more permeation materials. Polydopamine (PDA) acted as a crosslinking agent between MXene and Hal. All of these composite materials were stabilized on an acetate support through a vacuum-assisted self-assembly method, yielding the final product Hal@MXene-PDA composite membrane. The water contact angle was reduced significantly to a value of 14.5° upon modification of the pristine membrane. This occurred due to the abundance of hydroxyl groups Si-OH and Al-OH on the outer and inner surfaces of Hal. The enhancement in interlayer space and surface roughness both contributed positively to the water flux. However, excessive dopamine resulted to the nanomaterials blocking the permeation channels. Pure water flux of the fabricated membrane was 5036.2 L m^{− 2} h^{− 1} bar^{− 1} , and petroleum ether and lubricating oil were separated at an efficiency of 99.8%.This finding suggested the optimal utilization of each component of the composite membrane.

2.2.4. PAN

Recently, research has been centered on utilizing two dimensional nanoparticles for the modification of membranes and graphene oxide (GO) has been proposed as an effective example of one of these nanoparticles [21]. However, due to the presence of carboxyl groups on the surface of GO, membranes respond sensitively to aqueous media, resulting in swelling of the interlayer nanochannels, ultimately impairing the separation efficiency of the membrane. Addressing these issues, in this study researchers have proposed a method introducing 2D MXene nanosheets to the polyacrylonitrile (PAN) ultrafiltration (UF) membrane matrix, and anchoring functionalized multiwall carbon nanotube (MWCNT) within the nanosheets. The overall membrane surface was hierarchical ensuring the formation of stable channels that would not swell under an aqueous media and would not be subject to significant oil fouling. The overall composite membrane was fabricated through a simple phase inversion method with differing weighted ratios of MXene and O-MWCNT, MWCNT that underwent chemical oxidation with HNO₃ and H₂SO₄. Zeta potentials of the surface of the O-MWCNT and the MXene nanoparticles were analyzed and the results showed that they displayed values of − 30.95 and − 37.95, respectively. With these negative charges and the electrostatic repulsion between MXene and O-MWCNT, the three dimensional complex structure displayed excellent stability and uniformity in water. The optimal fabricated membrane displayed a pure water flux value of 301 L m^-2 h^-1.

2.3. Alternative

Polymeric aerogel are alternative materials for oil/water separation but hydrophobicity of need to be improved [22]. Polyimide aerogel mixed with MXene having layered structure improve the separation properties. As seen in Fig. 6, liquid exfoliated MXene are mixed with polyamide acid and then freeze dried and polyimide was formed by annealing.

Porous structure of the composite aerogel is very well visible in the SEM (Reproduced with permission from Wang et al., 22, Copyright 2019, American Chemical Society).

PI/MXene in the ratio of 5.2:1 (w/w) is very light weight with a density of 23 mg/cm3 and comes back to its original state even after using for 50 times. It has very adsorption capacity and can separate liquid paraffin, chloroform and soybean oil present in water-oil system.

Melamine sponge containing 0.1 wt % exfoliated MXene has excellent oil separation properties [22]. Amine group of melamine interact with hydroxyl functional group of 2D MXene can adsorb oil 176 times the weight of the composite. Layered structure are very well visible in the SEM image as presented in Fig. 8.

As seen in Fig. 9, the sponge has highest adsorption properties against chloroform.

MXene in the form of aerogel or sponge has very oil adsorption properties which can be applied for oil/water separation.

3. Conclusions

MXene are important class of 2D material applied in several field for purification of water by membrane separation process, capacitive deionization, heavy metal removal and surface coating. Ti₃C₂T_x is a class of MXene in which T_x is terminal functional group which are usually hydrophilic hydroxyl or fluorine. Exfoliated 2D MXene has very high surface area, hydrophilicity and very simple synthesis process which made it highly useful for various application. Heavy ion removal and oil/water separation by membrane separation method using composite membrane of MXene are highly efficient method. In this review different polymers like PI, PAN, Cellulose acetate, Chitosan, PES are mixed with 2D materials are discussed in details.