Recent advance of two-dimensional carbon-based films and their polymer functionalized membranes for oil/water separation

Oily water treatment has become an urgent task in our daily life because of the environmental and economic demands. Traditionally, gravity separation, flotation and filtration are used to separate oil/water mixtures successfully, but these methods have some drawbacks, including energy waste, low efficiency and secondary environmental pollution. In recent years, advanced materials with superwetting property involving superhydrophobic/superoleophilic separation materials, superhydrophilic and under-water superoleophobic separation materials, Janus separation materials, smart separation materials with switchable wettability emerge as a new research direction to address these problems. Because of the convenient separation and low-energy consumption, membrane separation material shows great advantages over conventional separation materials, which has a promising choice in the area of oily water treatment. Despite all these progress, most of the membranes separation materials are suffered from several limits for separating surfactant-stabilized oil/water emulsions and surfactant-free oil/water emulsions with micron or nanometer size, which may bring long-term damage to the natural environment. Furthermore, due to the presence of various harmful substances involving organic solvents, bacterial, heavy metal ions in real sewage system, the water pollution has become more serious and thus the resultant water phase is not suitable to put into the water system even after several separation cycles. From this perspective, to construct novel separation membrane materials with multifunction, good separation efficiency, high recyclability and convenient separation process for the separation of oil/water emulsions is passionately desired. Carbon-based (graphene and carbon nanotubes) membranes materials, especially polymer-functionalized carbon-based membrane materials, have gained increasingly attentions attributing to their outstanding physical and chemical properties, therefore are considered as an important choice for the separation of oily wastewater. Until now, there are tremendous advances on the carbon-based materials for oil/water separation, but most of them are focused on the development of three dimensional carbon-based separation materials, whereas the carbon-based membranes which play an important role on the treatment of oily wastewater have been almost neglected. Based on these, the research progress of polymer modified carbon-based films in wastewater treatment in recent years is reviewed in this paper. According to wettability and functions, the carbon-based membranes are divided into five parts: (1) Superhydrophobic carbon-based separation membranes, (2) superhydrophilic carbon-based separation membranes, (3) Janus carbon-based separation membranes, (4) smart carbon-based separation membranes, and (5) multi-functional carbon-based separation membranes. In each part, the influences of design routes, preparation methods and wetting behaviors on the separation efficiency of oily wastewater are discussed in detail. Finally, the development prospects and challenges of carbon-based membranes used for oil-containing wastewater purification are comprehensively discussed.