Encyclopedia
Please note this is a comparison between Version 1 by Mohammad Ashfaq and Version 2 by Lindsay Dong.
Oil leakage and organic solvent industrial accidents harm the ecosystem, especially aquatic and marine life. Oil–water separation is required to combat this issue, which substantially enhances the ecosystem and recovery of oils from water bodies. The advent of two-dimensional nanomaterials (2D-NMs) gives newer insight in developing membranes due to their exceptional characteristics like hydrophobicity/hydrophilicity, selectivity, antifouling ability, flexibility, and stability. Incorporating 2D-NMs within the polymeric membranes makes them exceptional candidates for removing oil from water. Moreover, 2D-NMs offer rapid sorption/desorption rates and boost water transportation. Additionally, 2D-NMs provide roughness that significantly enhances the fouling resistance in the polymeric membrane.
- polymers
- 2D-NMs
- environmental remediation
- oil–water separation
- membrane
1. Introduction
Oil pollution is a common pollutant that has severe concern nowadays due to technological advancement and industrialization. Industries like textiles, petrochemicals, mining, and metals produce significant oil–water waste globally. Usually, oil contaminants are mixed with water in the form of grease, fat, and petroleum products, including emulsified oil (<20 µm), dispersed oil (15–145 µm), and free oil (>145 µm) [1][2][3][4][1,2,3,4]. Moreover, oil leakage during production and transportation probably devastates the water bodies and ecosystems. Oil pollutants also increase natural resource loss [5][6][7][8][5,6,7,8]. In this aspect, energy-efficient oil–water separation technology is required to purify water from industrial discharge.
Numerous oil–water separation techniques have been developed, including physical, chemical, and biological techniques. The physical treatment methods are flocculation-coagulation, flotation, and skimming. These processes are simple, economical, and energy-efficient [9][10][11][9,10,11]. However, these are inefficient for treating wastewater contaminated with dissolved elements and small suspended particles. Chemical processes such as ozonation [12], electrochemical [13], photocatalysis [14], and demulsification [15] have been developed for oil–water separation. These techniques are efficient for dissolved elements and small particles. However, the processes are costly and tend to produce hazardous sludge [16]. On the other hand, biochemical methods have been developed in which some enzymes, such as lipase, break down the large molecule into small molecules [17]. The biochemical technique was cost-efficient and environmentally friendly. However, the maintenance and management of the microorganism requires sophisticated instruments and skilled personnel. Therefore, it is necessary to develop newer techniques that resolve such associated issue and efficiently separate oil–water from wastewater.
Membrane filtration is an efficient alternative for oil–water separation because it can separate huge amounts of oil waste without requiring additional chemicals. Also, the membrane separation system is more straightforward than conventional methods [18][19][18,19]. The membrane separation technologies are highly efficient, easy to scale up, and do not require additional chemical or phase change. This makes membrane separation processes a highly potential technology for portable oil and gas and offshore platforms [20]. However, it has several drawbacks, such as less water permeability and a high degree of surface fouling. For that, periodic chemical cleaning must be performed, which consumes considerable energy. In this aspect, low pressure requires high permeable membranes for oil–water separation.
Polymeric membranes are extensively used for wastewater treatment due to some unique properties of the membranes, such as easy processing, high density of packing, and cost efficiency. These characteristics make polymeric membranes efficient. However, they also have some drawbacks, such as most polymeric membranes are unstable in strong acid or alkaline environments, high temperatures, and low intrinsic permeability [21][22][21,22]. Typically, altering the polymer’s composition and morphology afterwards makes it possible to change its properties, such as hydrophilicity and hydrophobicity. The hydrophilicity or hydrophobicity of a designed polymeric composite can also be adjusted by changing the polymeric composite’s surface roughness. The shape and wrapping of another polymer used to create the membrane determine how planned polymeric composites’ hydrophilic/hydrophobic surface roughness is amplified, creating a membrane with an improved surface [23][24][25][26][23,24,25,26]. The hydrophilic/hydrophobic surface roughness of planned polymeric composites is enhanced depending on the shape and wrapping of another polymer employed to create the membrane, resulting in an improved surface texture of the membranes. Hydrophilicity and hydrophobicity are important factors that affects the oil–water separation efficiency.
Recently, two-dimensional (2D) materials such as graphene, graphene oxide (GO), carbon nitride (C3N4), boron nitride (BN), and transition metal dichalcogenides (TMDs) (WS2, MoS2, MXene,), layered double hydroxides (LDHs), and black phosphorus (BP) etc. have drawn a key interest among the researchers in this field due to the unique properties of 2D-NMs such as high surface area, amenability to surface modifications, high mechanical strength, facile synthesis, and cost-effectiveness that provide a chance for scaling up the process, thereby effectively using various applications including oil–water separation [27][28][29][30][31][32][33][34][35][36][30,31,32,33,34,35,36,37,38,39]. 2D-NMs provide fast diffusion (sorption–desorption) rate and enhance water transportation. 2D-NMs improved roughness, enhancing the fouling resistance in the polymeric membrane [37][38][39][40,41,42].
2. Improvement of Polymeric Membranes Properties by Incorporating 2D-NMs
Numerous approaches have been used to fabricate polymeric membranes to achieve desired properties of the polymeric membranes like permeability, selectivity, hydrophobicity, hydrophilicity, pH, and temperature responsiveness that depend on the physicochemical characteristics of the materials. The advent of 2D-NMs offers newer paradigms in membrane technology due to their exceptional characteristics. Usually, incorporating 2D-NMs within the polymers significantly improves the membrane’s permeability. The innovative polymeric membrane is made of stimuli-responsive polymers that have exceptional characteristics, including self-cleaning, antifouling, and switchable wettability on the surface by changing their physical (like light, temperature, electric, and magnetic field) and chemical (like pH, ions, and solvent) [40][41][42][45,46,47]. ScholarsWe can develop stimuli-responsive polymeric membranes by incorporating surface functional groups and 2D-NMs at the surface of polymeric membranes. Usually, innovative smart polymeric membranes were synthesized by using various processes like blending of polymers, cross-linking of polymers, surface coating, surface grafting, and plasma treatment. Moreover, the fabrication/synthesis of polymeric membranes might be adopted according to their desired property, thereby significantly improving their applicability. It is important to mention that incorporation of 2D-NMs significantly improved hydrophobicity/hydrophilicity, permeability, stability, reusability, and scalability [37][43][40,48]. Incorporating 2D-NMs derivatives makes them more stable and rigid in oil–water separation applications. For instance, 2D-NMs like graphene are rich in oxygen-containing functional groups on their surface. These are used as the starting filter for composite formation that can drastically change the van der Walls interaction between the graphene sheets and allow easy dispersion in water [44][49]. Therefore, their property could be enhanced by making a composite with several hydrophilic polymers, such as polyvinyl alcohol (PVA), which is a biodegradable and hydrophilic polymer used by Liang et al. The composite of GO/PVA gained the enhanced thermal stability and mechanical strength. Additionally, the tensile strength and modulus of the composite also improved from 22 MPa to 42 MPa and 0.45 GPa to 121 GPa, respectively [45][50]. Figure 1 shows the schematic illustration of the fabrication of 2D-NM-dispersed polymeric membrane and its application.
Figure 1.
A schematic illustration of the fabrication of 2D-NM-dispersed polymeric membrane and its application.
3. 2D-NM-Incorporated Polymeric Membranes for Oil–Water Separation
3.1. Graphene and Its Derivative
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 [46][47][48][56,57,58]. 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 [48][49][50][58,59,60]. 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 [51][61].
