A wettability and structure gradient electrospun membrane for highly efficient emulsion separation

The treatment of oily wastewater, especially emulsions by membranes attracts growing attention nowadays. However, the handling of industrial oily wastewater, especially emulsions, remains a challenging issue and high flux and efficiency in emulsion separation usually depend on energy-consuming external driving pressure. In this work, we report a functional gradient membrane (GM) with special wettability and structure gradient in the thickness direction for highly efficient emulsion separation. The GM is prepared by a one-step electrospinning strategy using PS-PAN and GO@PAN to tune the wettability of each layer in the membrane following a specially designed fabrication protocol. The prepared GM has a wettability gradient of superhydrophilicity to hydrophobicity and a structure gradient of spindles on fibers caused by GO content variation. The wettability gradient of GM generates remarkable unidirectional liquid transport properties and significantly higher fluxes for emulsion separation than the homogeneous GO@PAN membrane while the GO and the spindles on fibers facilitate the demulsification and coalesce of demulsified oil droplets. As a result, ultrahigh separation flux and efficiency have been obtained in surfactant-free oil-in-water emulsion separation only under gravity and an energy-efficient separation of surfactant-stabilized emulsions under low gravity condition has also been achieved by the GM. In addition, the GO spindle structure in GM makes it present favorable antifouling properties. After several cycles of separation, the emulsion flux can be restored to about 100% of the initial flux after simple cleaning. Thus, the special wettability and structure gradient endows the GM with excellent energy-efficient and stable separation performance, showing tremendous potential in the field of oil–water separation.