Superhydrophilic and underwater superoleophobic membranes - A review of synthesis methods

Wastewaters containing oil-in-water (O/W) emulsions are generated daily in various industrial processes such as petroleum refining, petrochemical and metal finishing. Environmental and economic concerns related to the disposal of oily wastewaters emphasize demands for materials and methods that can effectively separate oil from water. Pressure-driven membrane processes have been employed for oil/water separations. The major challenge that hinders the suitability of membrane technology for industrial oily wastewater treatment is the adsorption of oil and surfactants onto the membrane surface and/or within the pores of the membrane, which severely deteriorates the membrane performance. Materials with both hydrophobic and oleophilic properties have attracted significant attention in recent years for oil removal from water. These oil-removing type of materials can be used for membrane filtration or selective and efficient absorption of oil from water. The application of oil-removing materials can successfully introduce special wettability to the oily wastewater treatment. However, these materials are fouled quickly by oil droplets because of their intrinsic oleophilic properties. The attached oil droplets on the membrane surface severely affect separation efficiency. Besides, the adhered oil droplets are hardly removable from the surface resulting in reduced life-cycle of oleophilic materials, and this thus necessitates the need for costly post-treatment processes. Therefore, it is essential to develop high-performance materials for oil/water separation with elevated separation efficiency and significant antifouling properties. This study aims to review the state-of-the-art superhydrophilic and underwater superoleophobic (SUS) membranes that are being developed to overcome the aforementioned challenges. This allows to probe the maximal potential of developing such materials, and to explore the feasibility of using these novel membranes for de-oiling processes conventionally performed using expensive and low-performance skim tanks and adsorbents. In this review, according to the various materials that have been used for membrane fabrication, development methods for membrane synthesis with SUS properties based on polymeric SUS membranes and organic-inorganic hybrid SUS membranes including grafting, non-solvent phase separation (NIPS) polymeric membrane with hierarchical glue/tape-free surface, surface coating, in situ mineralization combined with polymeric materials and surface coating of polymeric membranes with inorganic nanoparticles, respectively, are assessed, in view of suggesting the success potential of these methods.