1. Introduction

In the advent of industrial advancement, the issue of wastewater disposal has caught the attention of many people. Appropriate disposal of oil contaminated wastewater is especially vital due to stringent waste water disposal standards. Coagulation, flotation, gravity and skimming, and oil absorbing are oil wastewater filtration methods, but these methods are limited by low efficiency and high operation costs. As an alternative, membrane technology is considered to be a promising candidate in oil wastewater treatment for its high efficiency and low operation costs[1-21]. However, membrane technology also has its own limitations which include reduction in separation efficiency due to fouling after multiple cycles of operation. Also, although some additives may enhance the membrane’s separation efficiency, it could cause secondary pollution. Thus, it is critical to enhance the oil-water mixture separation efficiency of the membrane while not compromising its antifouling property and biodegradability. The first half of the articles feature membranes categorized according to their components which include polyvinylidene difluoride (PVDF), zwitterion, polyketone, and other. Schematic diagram of oil water separation process is presented in Fig. 1. The summary of the oil/water sep-aration process is presented in Table 1. The second half of the articles feature composite membranes, whose components consist of metal organic frameworks, PVDF, nanofiber, CNT (carbon nanotube), and others. In all of the articles, the researchers aim to develop an oil/water separation membrane which addresses the issues mentioned above, ultimately developing a highly effective oil/water separation membrane with ideal properties.

2. Polymer Membrane

2.1. PVDF based membrane

Enhanced antifouling performance is crucial for effective ultrafiltration membranes which are subject to multiple filtrations of emulsified oil/water. There are few reports on treating acidic oil-contaminated water. This report features a hydrophilic pH-induced non-fouling membrane which could filter acidic oil-contaminated water[22]. This membrane was fabricated by grafting hyperbranched poly ethylene imine (PEI) onto PVDF blended with polyacrylic acid (PAA) grafted PVDF (PAA-g-PVDF) membrane that is prepared by a non-solvent induced phase inversion process. PEI has numerous advantages. It is biocompatible and has excellent water solubility. Its amine groups enable the PEI chain to be hydrophilic, trapping surrounding water molecules and forming a stable water layer. Also, the amine groups can protonize and deprotonize with the change of pH value of the medium, allowing the hydrophilicity of the membrane to be adjusted. When this membrane is exposed to acidic oil-contaminated wastewater the amine groups of the PEI molecule protonize and the PEI chain will extend into a full extension state, achieving a low oil-adhesion state which improves the antifouling performance of the membrane. In order to evaluate the antifouling performance of the membranes, flux recovery of permeating soybean oilin- water emulsion with different pH values was tested for several cycles. The successful anchorage of PEI onto the membrane was confirmed by using ATR-FTIR. SEM was used to observe the change of the membrane morphology before and after PEI anchorage. The resulting higher porosity of the PEI grafted membrane compared to pure membrane indicated that the PEI layer has little effect on membrane structure, ensuring the high permeating performance. Surface wettability of the membranes was measured according to the dynamic water contact angle (CA) measurement. Unlike pure PVDF membrane, the PEI grafted membrane demonstrated extremely high CA regardless of the acidity of the environment due to the hydrophilic groups on the membrane surface. Also, the improved underwater superoleophobic property of PEI grafted PVDF/PAA-g- PVDF blend membrane under acidic condition demonstrates the high pH stability and suitability for filtering acidic oil/water emulsion. When the PEI grafted membrane was tested for soybean oil permeation under neutral condition, high irreversible flux decline resulted with oil droplets adhering to the membrane. However, under acidic condition, the amount of oil adhesion to the membrane was low, and the membrane was recovered by simple water washing.

Traditional technologies of oil-water separation include air flotation, gravity separation combined skimming, oil-absorbing materials, coagulation, and flocculation[ 23]. These technologies have low separation efficiency, have high costs, require complex separation instruments, and are not effective at separating tiny amounts of oil from water. Pressure-driven filtration membranes such as ultrafiltration (UF) membranes address the problems mentioned above. After oil filtration, filtration membranes exhibit quick decline of permeation due to oil droplets clogging up the membrane pores. Also, filtration membranes are not effective at separating surfactant-free oil-water mixtures or oil-in-water emulsions due to wetting of the membrane surface during permeation. Due to the surface tension of water being higher than oil, oleophobic surfaces are also hydrophobic at most cases. So, the aim of improving the efficiency of oil/water filtration membrane is fabricating oil-removing materials with a superoleophilic property. Few reports have been given regarding the fabrication of membranes which can thoroughly separate oil from water, meeting the strict wastewater disposal standards. This report concerns a zwitterionic polyelectrolyte brush poly(3-(N-2-methacryloxyethyl-N,N-dimethyl) ammonatopropanesultone) (PMAPS) grafted poly-(vinylidene fluoride) (PMAPS-g-PVDF) through a surface-initiated atom transfer radical polymerization (SI-ATRP) process. Dispersed oil-water mixtures including isooctane-water, hexane-water, diesel-water, petroleum ether-water, and soybean oil-were sonicated and used to test oil-water separation efficiency. The ATR-FTIR spectra identified the components of the grafted polymer at different polymerization times. The 1724 cm^-1 signal peak becoming stronger and stronger along with polymerization time, indicating the graft of PMAPS polymer on the PVDF membrane. The graft of PMAPS on PVDF membrane was also identified through XPS analysis. There was no significant change in membrane surface morphology during the polymerization time within 12 hours. The surface wettability of PMAPS-g- PVDF membrane was observed by measuring the water contact angle (CA) and water flux as a function of polymerization time. With increasing polymerization time, the CA decreased continuously, achieving a minimum of 11 degrees. Water flux also increased sharply along with increasing polymerization time but started to decrease after 12 hours due to pore blocking. The oil wettability and adhesion properties of PMAPS-g- PVDF membranes were observed by measuring oil CA and adhesion force. The PMAPS-g-PVDF membrane exhibited an underwater superoleophobic property, shown by all oil CA being greater than 150 degrees. This property is attributed to the superhydrophilic zwitterionic polyelectrolyte in water. Overall, two factors that contributed to the separation efficiency of the PMAPS-g-PVDF membrane were identified. The first factor was the superhydrophilic and super oil-repelling property of the PMAPS which effectively prevented oil contact on the membrane surface. The second factor was the small pore size which effectively blocked all dispersed oil droplets. Polyvinylidene fluoride (PVDF) is often selected as the ingredient for oil/water separation membranes because of its mechanical properties and chemical stability[24]. However, PVDF is highly hydrophobic so improving the hydrophilicity of PVDF is critical in developing a more effective oil/water separation membrane. Polydopamine was utilized as an adhesive for immobilizing functional molecules onto the PVDF membrane. Although coating the membrane with polydopamine is a facile method in improving the hydrophilicity of the membrane, the exposed catechol groups on the PDA coating may cause higher protein adsorption, higher bacterial adhesion, and decreased water flux. In this study a zwitterionic random copolymer (PMEN) coating was covalently anchored on a PDA precoated PVDF microfiltration membrane through the amidation reaction. The PDA layer (PVDF/PDA/ PMEN) with hydrophilic zwitterion copolymers immobilized on the surface, formed a stable cell outer membrane mimetic interface. The resultant membrane had better superhydrophilicity and underwater superoleophobicity. In addition, the resulting membrane also exhibited outstanding antifouling performance compared to PVDF/PDA in separating various oil/water mixtures.

2.2. Other

Incorporating superwettability to membranes that can separate particles, molecules, or ions with low energy consumption can improve permeability, selectivity, fouling resistance, and stimuli responsiveness[25]. Recently several solid surfaces have been reported to have dual superlyophobicity in oil-water systems. This reversible oil-water repellency in oil-in-water (OW) and water- in-oil (WO) systems is vital in fabricating oil-water separators. The objectives of this study are as follows: 1) simply synthesizing dual superlyophobic polymeric membrane fabricated through a phase separation process (NIPS) with aliphatic polyketone (PK) 2) investigating the surface chemistry and surface morphology of the dual superlyophobic membranes and clarifying the underlying mechanism and controlling parameters 3) creating a simple design chart to assist in engineering similar superwetting materials form other materials. For the first time, interfacial systems demonstrating superwettability were prepared using PK. Recently fabricated polymeric membranes cover a wide range of pore sizes and tunabilities suitable for each substance being filtered. Nonsolvent-induced phase separation (NIPS) is one of the methods for fabricating polymeric membranes[26]. First, a homogeneous polymer dope solution is prepared by dissolving a polymer in an appropriate solution. Then phase separation is further induced by immersing the cast film in desirable shape into a coagulation bath solution containing a nonsolvent. This polymer/solvent/nonsolvent combination leads to morphological modifications to the membrane. The exchange rate of the solvent/nonsolvent determines phase separation, ultimately determining the porous structure. Aliphatic polyketone (PK) is a new polymer membrane material, whose carbonyl structure confers mechanical stability and chemical resistance against organic solvents to the membrane. Also, it is less susceptible to fouling. PK membranes that contain fibril-like texture and submicron-sized pores can be fabricated for the use of microfiltration membranes or porous support substrates for active membrane layers using the NIPS method. In this study a hydrophilic sodium alginate (SA) was utilized as an additive doped in PK polymer solution to improve the oil-in-water emulsion separation efficiency. SA is biocompatible and hydrophilic, allowing the control of membrane structures due to chains which offer ionic cross-linking. By using the SA additive in polymer dope solution would affect the solvent/ nonsolvent exchange rate, changing the membrane porosity and would inhibit SA leakage from polymer dope solution during the NIPS process, resulting in improved underwater oleophobicity and better oil-in-water emulsion separation properties. SEM images revealed that the SA additive caused the membrane structure to become more porous.

Excessive use of detergents in cleaning oil has become an environmental concern[27]. In order to address this issue, great interest has been given to the development of a surface on which oil contamination could be cleaned solely by water. Zwitterionic phosphorylcholine (PC) has been confirmed to confer the “self-cleaning” characteristic to certain cell membranes, those that resist biofouling and keep the surface bioinert. The presence of positively and negatively charged units of zwitterions enables a tightly bound water layer at the surface of the membranes. A polymer form of PC, the biomimetic polyelectrolyte, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) was used for the fabrication of the PMPC brush surface. This surface exhibited complete oil-repellency both in dry and wet conditions, evidenced by its oil contact angle in water (OCA-W) being greater than 165° when brought in tight contact with a droplet of n-hexadecane. Self-cleaning oil-water separation membranes were fabricated by grafting PMPC on top of steel meshes. To compare the effectiveness of the PMPC mesh oil-separation, petroleum- water mixtures were filtered through zinc oxide mesh and PDDA/PSS mesh. If the ZnO mesh was prewetted by oil, neither water nor oil was able to pass through, and this was also the case of the PDDA/PSS mesh. Water washing of the ZnO mesh and the PDDA/ PSS mesh did not recover the oil-water separation function. However, the PMPC mesh did not have any issues as the other filters did under the same conditions.

Materials with special wettability are thought to be the most effective solutions to separating oil/water mixtures because of their low energy requirement. Superwetting porous materials with a water contact angle greater than 150°, an oil contact angle less than 5°, and a water contact angle in oil greater than 150° can pass through oil but would retain water, allowing oil/ water mixture separation[28] (Figs. 2~5). In this study, a coating method is demonstrated to modify various membranes for oil-in-water emulsion separation. Poly [(3,3,3-trifluoropropyl) methylsiloxane] (PTFPMS) nanoparticles were dispersed in acetone/water and were used to modify the membranes through a coating process. The membranes coated with PTFPMS exhibited superhydrophobicity. Also due to PTFPMS’s resistance to most of the chemicals, the coated superwetting membranes had higher efficiency, good reusability, and good antifouling properties when separating surfactant-free or surfactant-stabilized water-in-oil emulsions. Traditionally, demulsifies have been selected for separating oil-water emulsions[29]. Recently, different materials are being tested to selectively separate water from oil or oil from water. Advanced materials such as aerogels, foam membranes, polysaccharide agents, surface modified fabrics, and inorganic meshes have been tested for effective separation of oil/water mixtures. Among these new materials, aerogel is regarded as a better choice for its sustainability, biodegradability, superhydrophilicity, and high surface area. The present study reports the use of highly porous polysaccharide chitosan-based aerogel membrane for filtering water from oil-spill and stable emulsions. Agarose was used as pore forming agent and as surface coating on the highly cross-linked chitosan network. Aerogel acted as support network and provided hydrophilicity to chitosan in this study. The selective water filtration of the membrane was tested with biodiesel/water emulsion, crude vegetable oil/water emulsion, and highly contaminated oil spill wastewater. Different samples of membrane were prepared by differing the polymer concentration ranging from 0.5% to 2% w/v while keeping the agarose/chitosan ratio constant (9 : 1 w/w). 450~ 900 mg of agarose was taken in separate beakers having 75 mL of distilled water and were solubilized by autoclaving it at 120°C for 15 minutes. In another set of beakers, 50~100 mg of chitosan was mixed with 0.05M of acetic acid. Then this solution was added to the viscous linking. Finally, each gel was cut into 0.4 mm slices and lyophilized to obtain aerogel samples. SEM images revealed that the lyophilization process induced ordered large column-like structure of the CS membrane, which stabilized the agarose gel mass in itself. To test compression breaking, recyclability, and large-scale continuous operations, membranes were tested under crossflow conditions. Agarose served as a gelling agent which allowed the aerogel membrane to be highly porous. Also, agarose enhanced the hydrophilic property by interacting with chitosan through hydrogen bonding during lyophilization. Flux rate through the membranes were consistent for several hours of continuous and repeated cycles. Under feed flow pressure, large pores completely collapsed according to the SEM images. However, the membrane regained its physical characteristics by washing in deionized water. The CS-based aerogel membranes’ biodegradability was tested by keeping the used membranes in soil for natural degradation. After 35 days of observation, the membrane mass reduced to 40~30% of its original mass. It is important to improve the durability and reusability of materials used in separating various stabilized oil-in- water emulsions[30]. Artificial self-healing material is one type of oil/water mixture separation materials. Most self-healing materials self-heal after an external stimulus is passed on to the material, releasing healants upon the stimulus. Supramolecular polymers can autonomously self-heal at ambient temperature and can self-heal for multiple cycles. The self-healing of supramolecular polymers happens at the molecular level. Inspired by self-healing and micro/ nanostructures of plants, PAN@PPH membranes which could self-heal was fabricated using electrospinning and LbL (layer by layer)-assembly technologies. SEM images revealed that upon each LbL-assembly, the fibers of the composite membrane thickened and the pore diameters decreased. With the uneven bulge micro-structure compounded with the superior hydrophilicity of HA, PEI, and PAA, the resulting PAN@PPH composite membranes demonstrated specific superhydrophilicity and underwater superoleo- phobicity with a very small oil adhesion force. Also, due to the membrane having a sub-micrometer pore size and being superhydrophilic and underwater superoleophobic, tiny oil droplets were effectively filtered. Furthermore, the membranes demonstrated antifouling, reusability and durability properties.

3. Composite Membrane

3.1. MOF

Recently superwetting materials have garnered global attention. Superwetting materials have suerhydrophobic/ superoleophobic or suerhydrophilic/superoleophobic properties, allowing effective separation of oil and water [31]. In this study, an underwater superoleophobic or under-oil superhydrophobic membrane consisting of polyacrylonitrile (PAN) nanofibers with nanocrystalline zeolite imidazole framework (ZIF-8) (PAN@ZIF-8) was fabricated through electrostatic spinning. The fabricated membrane can separate both surfactant-stabilized oil-inwater and water-in-oil emulsions solely by gravity. PAN is highly resistant to chemical reagents, and zeolite imidazole frameworks like ZIF-8 are very robust and chemically stable. ZIF-8 nanocrystals were evenly distributed in PAN nanofibers by electrostatic spinning, producing a composite membrane, which was shown through the EDS and XPS analyses. The hydrophobic nature and roughness of ZIF-8 enables the composite membrane to be superoleophobic under water and superhydrophobic under oil. PAN@ZIF-8 composite membrane’s separation capacity was tested using emulsifier stabilized oil-in-water and water-in-oil emulsions. The membrane demonstrated high flux, high separation efficiency, and high recyclability. Electrospun nanofibrous membranes have been used for separation of oil/water emulsions because of their properties which include ultrahigh porosity, large surface area, superior connectivity, tunable wettability, and controlled structure. Current membranes face the issue of fouling during the separation process[32]. Past studies demonstrated that strengthening the wettability of the membrane, by changing the surface chemical composition and microstructure, enhanced the membrane’s antifouling performance. Metal-organic frameworks (MOF’s) have large surface area, high porosity, and tunable functionality. Ammoniated zirconium (IV) dicarboxylate porous material (UiO-NH2) has hydrophilic carboxyl and amino groups and is chemically stable in aqueous media. In this study, a composite nanofibrous membrane (CNFM) is fabricated by decorating UiO-66-NH2 on polyacrylonitrile membrane (PANM). The successful incorporation of UiO-66-NH2 on polyacrylonitrile membrane (PANM) was confirmed through XPS, XRD, and FTIR analyses. The resultant membrane demonstrated excellent antifouling performance, high permeate flux, high separation efficiency, and excellent recycling capability.

3.2. PVDF

Pressure-driven membrane with switchable surface functionalities have advantageous features including high efficiency, low operation cost, and facile scalability characteristics. These advantageous features make pressure- driven membrane effective in treating oil/water mixtures[33]. The multifunctional “Janus interface membrane” with two-dimensional asymmetric surface wettability on each face addresses the issue of other membranes only being able to separate one type of oil or water from oil/water mixtures. In this study, an ultrathin surface silicification based Janus membrane was engineered onto the polyvinylidene fluoride (PVDF) substrate. Then a hydrophobic 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFTS) modification was incorporated with unilateral O₂/N₂ plasma-etching treatment (P-SiO₂/PVDF). Due to the extreme opposing wetting selectivity, the F-SiO₂ PVDF surface showed superhydrophobicity and the P-SiO₂/PVDF surface showed underwater superoleophobicity. The developed Janus membrane exhibited high separation performance both in surfactant-stabilized O/W and W/O emulsions. Due to the “oil-trapping” behavior of F-SiO₂/PVDF and hydrated behavior of P-SiO₂/PVDF surfaces, a repulsive boundary of demulsifying layer was formed, causing the Janus membrane to be have ultralow adhesion property in dispersed water/oil phases, enhancing the antifouling property. In result, a silicification based Janus membrane treated by unilateral hydrophilic/hydrophobic surface modification was fabricated and the ultrathin silicification layer with extremely low-mass-transfer resistance was introduced. The more concave topographic structures of F-SiO₂/PVDF and P-SiO₂/PVDF surfaces compared to the smooth surface of the nascent PVDF substrate shown by FESEM images confirmed the successful in-situ silicification. The resulting membrane demonstrated excellent oil/water separation and had ultrahigh permeating flux and oil/water selectivity for various O/W and W/O emulsions. Ultrafiltration is cheap and produces less waste in treating oil-contaminated wastewater. Polymeric membranes used for ultrafiltration are fabricated through a phase inversion process, and most membranes are inherently hydrophobic[34]. In order to lessen hydrophobicity and improve water flux, additives are utilized to create additional pores with desirable surface membrane morphology. It is critical to develop cost-effective degradable pore-forming additives which could tune membrane morphologies without creating any secondary pollutants. In this study, iron alkoxide, a cluster molecule, was used as an additive to improve the efficiency of the polyvinylidene fluoride (PVDF) ultrafiltration membrane. The iron clusters are harmless and are biocompatible. An inorganic cluster molecule, used as a pore-forming additive (PFA), created pores on the membrane through acid treatment. FTIR results proposing the formulae [Fe₃(OCH₃)₂ (CH₃COO)₅]⋅2CH₃OH and the FESEM images of prepared iron alkoxide both indicated that the iron compound formed is an inorganic metal cluster. The resulting membrane’s efficiency was improved.

3.3. TiO₂

Membrane technology is considered to be the most efficient method in filtering oil/water emulsions[35]. Emulsion separation membrane materials are broadly classified into two categories: polymeric membranes and inorganic membranes. Polymer membranes are easily processable and have high flexibility and tailorability. However, polymer membranes cannot withstand organic solvent dissolution and tend to degrade in strong alkaline liquids. In contrast, inorganic separation membranes can better withstand organic solvents, but most of the reported inorganic membranes cannot withstand corrosion by a strong acid or base which destroys the membranes and causes leakage of toxic substances like Cu²⁺ and Co⁺⁴. So, it is vital to develop an emulsion separation membrane that is chemically stable to organic solvents and strong corrosive liquids. Titanium dioxide (TiO₂) is environmentally friendly, semiconductive, and chemically stable. It also has a unique photoinduced superhydrophilic property which allows self-cleaning coatings and functional interfacial materials with special wettability. Surface modified TiO₂ membranes were used for separating free oil and water and two-phase organic liquids. However, a single nanofibrous membrane could not separate emulsions, because the pore sizes are too large to intercept the emulsion droplets. In this study, pine-branch-like TiO₂ membrane with hairy nanorods on nanofiber structures was fabricated. High oil contact angle (OCA) was witnessed due to the pine-branch-like structure trapping the molecules, preventing the contact of oils and membrane surfaces underwater. This fabricated membrane was able to separate emulsions with pH values ranging from less than zero to very high value effectively.

3.4. Nanofiber

Currently polymer or ceramic membranes are used for wastewater treatment[36] (Fig. 6). However, such technologies have disadvantages such as low flux, fouling, and quick decline of efficiency due to surfactant adsorption and pore plugging by oil droplets. This results in high costs for treating large volume wastewater. According to the Hagen-Poiseuille equation, a classical fluid dynamic theory, filtration rate is directly proportional to the square of the effective pore size and inversely proportional to the thickness of the membrane. So, an ideal filtration membrane should be thin as possible without sacrificing the effective pore size. This is hard to be realized in filtration systems based on polymers or ceramics. Recently an ultrathin film of single- walled carbon nanotubes (SWCNTs) that could effectively separate emulsified oil/water mixtures was developed. This material works for water-in-oil emulsions but does not work for oil-in-water emulsions due to hydrophobic and superoleophobic surface wetting properties. So, in this study, an ultrathin film made of an SWCNT network and TiO₂ nanocomposite was fabricated, possessing superhydrophilic and underwater superoleophobic properties. UV-light radiation was used to materialize. The SWCNT/TiO₂ nanocomposite film separated both surfactant-free emulsion and surfactant- stabilized oil-in-water emulsion with very high flux (up to 30000L m^-2 h^-1 bar^-1). Also, the SWCNT/TiO₂ nanocomposite film demonstrated excellent antifouling properties and self-cleaning after multiple cycles with the help of photocatalytic TiO₂ nanoparticles. Oil-containing wastewater contamination has become an urgent issue[37]. Techniques such as oil-absorbing materials, oil skimmers, and flotation have been developed to separate oil and water. Among the many membrane fabrication processes, electrospinning method is featured in this study. Electrospun nanofiber membranes (ENMs) are advantageous in oil/water emulsion separation for their high porosity, interconnected porous structure, controllable pore size, and large surface area to volume ratio. However, due to the high porosity and the weak bonding due to separation and overlap of individual fibers, ENMs have poor mechanical strength. ENM surface coating is proposed as an appropriate approach in strengthening the ENMs durability. Janus membrane, an ENM, has an anisotropic surface property, and its feasibility for oil/water separation has been tested. The asymmetric wettability of the Janus membrane on each side allows the membrane to have “switchable” behavior in oil/water emulsion separation under different conditions. This study focuses on exploiting the advantage of Janus membrane and incorporating the property into ENMS. Hydrophilic polyacrylonitrile (PAN) nanofiber membranes were prepared by electrospinning technique and was subsequently coated with an ultrathin layer of hydrophobic carbon nanotubes (CNTs) on a single side. The effects of CNTs coating on the membrane mechanical properties and wettability were observed. Compared to pure ENM membrane, the Janus CNTs@PAN_EN membrane demonstrated ultrahigh permeation flux and high separation efficiency in oil/water separation.

3.5. Others

In this work, ZnO nanowires (NWs)-coated stainless steel (SS) mesh exhibiting reversible wettability was fabricated[38] (Figs. 7, 8). This material was useful in both “oil-removing” and “water-removing” mode with an efficiency level higher than 99.9%. The water wett ability was modified through annealing in hydrogen and oxygen gas environments alternately for 1.5 and 1 hours respectively. The growth of ZnO NWs was accomplished through the chemical vapor deposition technique. The deposition was carried out without a substrate allowing for large scale production. The coating method is simple and cost-effective. Oil absorbed on the mesh surface could be easily removed by annealing at high temperature, not requiring any fouling- species-removing treatment after each cycle.

Oil contaminated water filtration membranes that originate from petroleum are expensive, non-sustainable, and non-degradable[39]. Therefore, it is important to develop environmentally friendly and sustainable microporous membranes that are biodegradable, such as poly(lactic acid) (PLA) membranes. Although antifouling and hydrophilicity of PLA membranes were improved in recent reports, superhydrophilicity and underwater super-oleophilicity with a textured surface was not achieved. This study reported an effective way of preparing a superwetting PVDF membrane with a micro/ nanoscale hierarchical structure by phase inversion and non-woven fabric (NWF) peeling off. Due to the creep deformation of flexible polymers, it has been a challenge to retain durability of polymer based micro-/ nano-scale structure. Inspired by favorable properties of fish scales and clamshells in aqueous media, mineral was coated onto the filtration membranes to enhance structural stability and hydrophilicity. However, the mineral coating was formed through weak electrostatic interactions or hydrogen bonding. In this study a PLA membrane with a hierarchical surface was prepared through phase transition and template peeling with TiO₂ nanoparticles assembled within the nano structure. SEM images showed the nanofibrils on the PLA membrane surface capturing the TiO₂ nanoparticles and confining them in the micropores to form the robust TiO₂@P(VP-VTES) coating on the PLA membrane. The resulting PLA was robust even in long-term water flushing and showed high performance in stable water flux, permeability recovery, and high rejection to bovine serum albumin (BSA) and ink.

Traditional techniques of filtering oily wastewater include gravity separation, air flotation, coagulation, and flocculation. Unfortunately, these methods are ineffective in treating oil droplets smaller than 20 micrometers [40]. Recently, there has been a trend of utilizing forward osmosis (FO), a membrane process in which osmotic pressure is used as the driving force instead of hydraulic pressure for treating oily wastewater. In contrast to conventional pressure-driven membrane technologies, FO requires no or lower operation pressure demand, and has higher fouling propensity reversibility and higher water recovery. Among the many reported surface modification methods for enhancing FO anti- fouling, poly(ethylene glycol) (PEG) is the most common method. Although PEG-based materials are less susceptible to fouling due to its hydration property, they are susceptible to damage because of the oxidative degradation and enzymatic cleavage of PEG chains. So, developing a new polyamide modification material is critical. Inspired by previous studies, this study evaluates the improvement of antifouling abilities of thin-film composite (TFC) FO membranes when sulfonated poly(arylene ether sulfone) was grafted onto the polyamide layer. The amine-terminated sulfonated poly(arylene ether sulfone) (NH₂-BPSH100) was grafted in situ via the reaction between primary amine groups and the acyl chloride groups. By adjusting the amounts of NH2-BPSH100, a superhydrophilic and underwater superoleophobic FO membrane was fabricated with a water contact angle of 10.2° and underwater oil contact angle of 155.4°. Compared with other modified membranes, the TFC membrane showed higher performance in antifouling. Fouling caused by emulsified oil was easily eliminated by simple hydraulic flushing. The water flux was 69.8% of its initial value when 80% of water recovery was reached for separating 40,000 ppm oil/water emulsion.

4. Conclusions

Extensive experiments have taken place in attempt to enhance the separation efficiency of existing oil/water separation membranes. The studies primarily lie under the category of surface modification. The surface chemistry and morphology have been modified through incorporating polymers, nanofibers, or metal organic frameworks on to the membrane surface. The resulting membranes exhibited higher separation efficiency compared to unmodified membranes. In addition, factors such as antifouling and biodegradability have also been enhanced, minimizing the loss of resources and pollution. In this review, oil water separation by different type of membranes are discussed.