Super-hydrophilic and underwater super-oleophobic membranes with photocatalytic self-cleaning properties for highly efficient oil-water separation: A review

The treatment of oily wastewater, emerging from different sources has been a global concern, and the subject of international statutory obligation, as it causes adverse environmental repercussions. The conventional chemical, physical and biological methods of oil water separation have inadequacies like low separation efficiency, high cost, and the need for hazardous chemicals and heavy equipment. Membrane based technology, particularly membrane-based methods that work on the principle of super-wettability of oil and water on membrane surface has emerged as a viable alternative. The combination of surface wettability decides whether the membrane is oil passing or water passing, and the choice of the surface wettability of the membrane depends on the nature of oily water sample. The major constraint of using oil passing membrane is that oil clogs the membrane, leading to the deterioration of oil water separation performance; although water passing kind can considerably do away with oil clogging, the accumulation of organic contaminants present in the oily water hampers the performance of separation. There are many research works focusing on incorporating photocatalytic materials in the matrix of the membrane coatings, and reported successful restoration of the full functionality of membrane simply by irradiating the membranes with UV or visible light. This article reviews the water passing super-hydrophilic, and underwater super-oleophobic membranes, possessing photocatalytic self-cleaning properties, which yielded high permeation flux, better oil separation efficiency, and more robust resistance to fouling. An overview of different types of membranes, namely metallic mesh support-based membranes, polymeric support-based membranes, and ceramic support-based membranes has been highlighted. The superwetting characteristics, photocatalytic self-cleaning properties, and separation performance of these membranes have been thoroughly discussed. More importantly, the conclusion, current limitations, and future outlooks on this topic have been put forward for the sake of developing more effective super-hydrophilic and underwater super-oleophobic membranes with photocatalytic self-cleaning properties for oil-water separation.