Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, has exceptional mechanical strength, high gas permeability, and molecular sieving capabilities. The nano-porous structure of graphene exhibits excellent water permeability while rejecting ions and other contaminants, making them promising candidates for desalination and water purification applications
[56,57,58][46][47][48]. Graphene-based membranes have attracted significant interest in oil–water separation applications due to their unique –properties, such as high mechanical strength, excellent chemical stability, and selective permeability. While graphene itself is a single layer of carbon atoms arranged in a hexagonal lattice, it can be incorporated into various membrane configurations to achieve effective oil–water separation
[58,59,60][48][49][50]. For instance, Prince et al. synthesized polyethersulfone (PES) modified graphene (PES-graphene)-based hollow membrane for oil–water separation. The data suggested that the PES-graphene shows excellent hydrophilicity, permeability, and selectivity, thereby effectively separating oil–water
[61][51].
3.2. Molybdenum Disulfide (MoS
2
)
MoS
2 is a layered TMDs material with unique electronic and mechanical properties that gained attention for various applications, including oil–water separation. MoS
2 membranes can be fabricated by exfoliating bulk MoS
2 into few-layered or single-layered sheets. Due to their atomically thin nanopores, these membranes show selective permeability for molecules and ions. MoS
2 membranes have the potential ability for gas separation and water purification applications, offering high permeability and selectivity. However, commercial applications require support for their stability. Therefore, immobilization or incorporation of MoS
2 into a polymeric membrane to fabricate the MoS
2-polymeric membrane enhances the stability of the composite material. Additionally, it provides some unique characteristics to the material that are useful in oil–water separation
[77][52]. MoS
2 within the polymeric membrane significantly improved the wettability, stability, and scalability. Numerous MoS
2-based polymeric membranes are effectively used for oil–water separation. For instance, Krasian et al. doped MoS
2 into polylactic acid, a fibrous biodegradable polymer. The composite material shows enhancement in oil absorption capacity and good reusability. The membrane can separate the floating oil in water by gravity-driven force
[78][53].
3.3. Boron Nitride (BN)
BN is another 2D-NMs that has gained attention for membrane synthesis. It possesses excellent chemical stability, high thermal conductivity, and good mechanical strength. BN membranes can be created by exfoliating bulk BN into thin layers or by synthesizing them through chemical vapor deposition (CVD) techniques. These membranes exhibit high water permeability while maintaining excellent selectivity, making them suitable for water desalination and filtration
[90,91,92,93,94,95][54][55][56][57][58][59]. Numerous studies have been conducted to date that suggests the BN and its hybrid membrane are effectively used for oil–water separation. For instance, Lei et al. synthesized a BN-guanidine hydrochloride (BN-GH)-based membrane to separate oil–water. The super-hydrophobicity of the porous BN-GH-based membrane makes them suitable for separating oil–water. The prepared porous BN-GH-based membrane has sturdy oxidation resistance, thereby easily recycling
[96][60].
3.4. Transitional Metal Carbide/Nitride/Carbonitrides (MXene)
MXene is an emerging 2D-NMs and belongs to the family of metals carbide, carbonitride, and nitride. MXene was first used in membrane fabrication by Professor Gogotsi et al. in 2015, which opened the door to MXene in the membrane world
[105][61]. The general chemical formula of Mxene is Mn+1XnTx, where M stands for early transition metal element, X is carbon or nitrogen, and T is the active group attached to the surface like O
−, OH
−, etc.
[106][62]. MXene has inherent characteristics, mainly negatively charged sorption sites, high surface area, chemically stable, tunable inter-layer spacing, and exceptional hydrophilicity that attract researchers for different end applications, including oil–water separation. The material has various chemical and physical properties that are desirable for oil–water separation applications. However, small interlayer space and poor mechanical strength limit its application
[107][63]. To overcome such problems in past studies, MXene was doped into the polymeric matrix. The polymeric composite enhances the interlayer space of Mxene and provides mechanical strength to the material. For instance, Zeng et al. synthesized Mxene modified by halloysite nanotubes (Hal) and polydopamine (PDA) material over cellulose acetate for membrane fabrication. Due to modifications, the composite material exhibits excellent hydrophobicity with enhanced antifouling and anti-swelling properties. The composite membrane showed excellent potential in oil–water separation
[40][37].
3.5. Tungsten Disulfide (WS
2
)
WS
2 possesses a hexagonal structure and belongs to the spatial group P63/mmc. WS
2 consists of layers in which tungsten atoms form covalent bonds with sulfur atoms. The connections between adjacent layers, specifically the S-W-S joints, are relatively weaker due to the presence of van-der Waals forces. The adjacent layers are stacked in a way that each layer aligns with the accumulation axis of tungsten atoms and rotates after the sulfur atoms, leading to a complete deviation from a flat state
[118,119][64][65]. WS
2 gives exceptional properties to the composite material that are useful in oil–water separation applications. However, the polymeric matrix provides stability and rigidity to the WS
2 leading to exploring their applicability in commercial applications. Numerous studies have been conducted on the fabrication and application of WS
2-based polymeric membranes for oil–water separation applications. For instance, Krasian et al. used a combination of two-dimensional materials (MoS
2 and WS
2) to enhance the performance of PLA fibrous mats in terms of oil adsorption and oil–water separation. The results showed that this hybrid material proved effective in separating surfactant-stabilized oil–water emulsion, with approximately 70% flux recovery achieved across multiple separation cycles
[78][53].
4. Recyclability of the 2D-NM-Incorporated Polymeric Membranes
The reusability of the 2D-NM-based polymeric composite is a very important aspect in end applications including oil–water separation. The reusability depends on numerous factors like composition of the materials (types of 2D-NMs and polymers), surface properties, and recycling process. 2D-NMs, mainly graphene and GO, exhibit unique characteristics like exceptional mechanical strength, stability, and permeability that makes them ideal candidates for the development of polymeric membranes for oil–water separation. Numerous polymeric composite materials for oil–water separation have shown excellent reusability. For instances, Rong et al. synthesized 3D-composite decorated with carbonized pollen grain for oil–water separation. The prepared hydrophobic 3D-composite effectively separate oil–water with exceptional stability. Moreover, the prepared hydrophobic composite material is efficiently reusable up to five consecutive cycles due to excellent chemical stability
[124][66].
5. Strategies to Improve Oil–Water Separation Efficiency
Researchers are actively exploring the optimization of polymeric membranes by incorporating 2D-NMs, surface modification, and their synthesis process for high oil–water separation efficiency
[55,127,128][67][68][69]. Usually, various factors affect oil–water separation efficiency. (1) Selection of appropriate membrane, some basic properties such as hydrophobicity/hydrophilicity, porous texture, and exceptional mechanical strength, are required for oil–water separation.
WeScholars must choose polymers with hydrophobicity/hydrophilicity and chemical resistance. (2) Modification of polymeric membranes, surface modification is a decisive factor affecting the oil–water separation efficiency. Numerous processes, such as surface coating, grafting, and plasma treatment, have produced hydrophobic layers that prevent oil droplets from wetting. (3) Surface texture can be optimized by the efficacy of the polymeric membrane by tuning the pore size and surface morphology of the membrane using various processes like the electrospinning process, self-assembly, and phase inversion. With the help of controlling pore size,
weit can easily improve the oil rejection ability of the membranes. (4) Stimuli-responsive polymeric composites provide a newer avenue by tuning/modification of surface properties like by changing the pH and temperature. The wetting behavior of the polymeric membranes is directly related to the liquid/solid interfaces. The hydrophobic/hydrophilic and oleophilic/oleophobic characteristics of polymeric composite might be beneficial for oil separation from water surfaces. The polymeric composite having hydrophobic or oleophilic allows oil droplets to pass through, whereas hydrophilic or oleophobic allows water molecules and blocks oil droplets. Therefore, designing the polymeric membranes with super-wettability significantly improved the separation efficiency. Interestingly, such types of materials have reversible behavior that easily switches the super-hydrophilicity and super-hydrophobicity, thereby making them potential candidates for oil–water separation. (5) Development of hybrid membranes: hybrid membranes are an important aspect of synthesizing desired polymeric membranes, as according to desired properties
wescholars can choose materials like polymers and 2D-NMs that significantly enhance the oil–water separation efficiency. It is important to mention that the specific strategies required to enhance the polymeric membranes for oil–water separation efficiency depend on the desired separation efficacy, mixture of oil–water, and operating conditions
[129,130,131][70][71][72]. Moreover, selecting 2D-NMs is also one of the decisive factors for separating oil–water.