A simple superhydrophilic/underwater superoleophobic PVDF-OH/AA@ZnO membrane with needle structures for efficient oil-in-water emulsions separation

Superwetting oil-water separation membranes are effective materials for treatment of wastewater containing emulsified oil, but the problem of membrane pollution remains a huge challenge during its long-term operation. In this study, we successfully developed a superhydrophilic/underwater superoleophobic membrane using a simple phase separation method. The fabrication process involved three key steps: (1) surface hydroxylation of pristine PVDF using NaOH etching to generate active hydrogen bonds and hydroxyl groups, (2) acrylic acid (AA) grafting onto the hydroxylated PVDF surface to introduce carboxyl functionalities, and (3) in-situ chelation of needle-like ZnO onto the AA-grafted matrix. The synergistic effect of the surface needle-like ZnO and the hydration layer endowed the PVDF-OH/AA@ZnO membrane with an outstanding oil-water separation efficiency of n-hexane up to 99.68 %, and higher than 98 % for various emulsions (e.g. kerosene, petroleum ether, chloroform , and dichloromethane) under gravity. Moreover, compared with the basement membrane, the tensile strength of the modified membrane increased from 106.21 kPa to 153.84 kPa, indicating its excellent mechanical properties under strong external forces. The PVDF-OH/AA@ZnO membrane demonstrated exceptional recycling performance, with a separation efficiency above 97.25 % after five cycles, and exhibited anti-fouling capability with simple water rinsing used to remove surface oil residues. Overall, this study provides a scalable strategy for the design of superhydrophilic/underwater superoleophobic separation membranes for the practical treatment of oily wastewater. • A superhydrophilic membrane was prepared via a simple phase separation. • The needle-like layered structure is conducive to demulsification and separation. • The PVDF-OH/AA@ZnO membrane shows good anti-fouling and recyclability. • The membrane demonstrates excellent mechanical property and chemical stability.