Development of Janus membrane with controllable asymmetric wettability for highly-efficient oil/water emulsions separation

Due to the given complexity and diversity of practical oily wastewater, developing highly-efficient separation materials with multifunctional and switchable separation property is urgently needed. To advance this emerging field, a functionalized surface silicification layer based Janus membrane with asymmetric wetting selectivity was developed onto the polyvinylidene fluoride (PVDF) substrate, via the hydrophobic 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFTS) modification (F–SiO2/PVDF) followed by a unilateral plasma-etching treatment (P–SiO2/PVDF). By precisely tuning the surface hierarchical nanosphere-like architecture of the silicification layer, a robust superhydrophobicity/superoleophilicity, and underwater superoleophobicity/superhydrophilicity of F–SiO2/PVDF and P–SiO2/PVDF surfaces were obtained, reaching an extremely opposing surface wettability with a high water/underwater oil contact angle (CA) difference up to 150°. Based on the exceptionally asymmetric wetting selectivity incorporated with the ultrathin silicification layer (with the thickness of 15 nm), the as-developed Janus membrane achieved a remarkable separation property with the surfactant-stabilized oil-in-water (O/W), and water-in-oil (W/O) emulsion permeances up to 7200 and 9300 Lm−2h−1bar−1, by accompanying with their high separation efficiencies up to 99.8% and 99.0%, respectively, which shows an overall oil/water separation property superior to the state-of-the-art Janus membranes reported so far. Moreover, it should be noted that the reusability of the Janus membrane also performed excellently, where a nearly 100% recovery ratio of permeating flux was obtained after several cycles of O/W and W/O emulsion filtration tests. Overall, this work provides a perspective to explore an asymmetric Janus membrane structure with promoted separation property for highly efficient oily wastewater treatment.