Preparation of refreshable membrane by partially sacrificial hydrophilic coating

Polyvinylidene fluoride (PVDF) is widely used as a kind of water treatment membrane material, but its high hydrophobicity leads to easily being polluted by oily wastewater, which limits further development in the field of water treatment. In this paper, a simple and green strategy based on chitosan (CS) and tannic acid (TA) was taken to transform the hydrophobic PVDF membrane into superhydrophilic one. CS possessed excellent capability of antifouling but its surface binding affinity was relatively poor, while TA enjoyed wonderful surface adhesion and certain ability of absorbing water. Herein, combining the advantages of CS and TA, a special partially sacrificial coating was designed. (3-cholropropyl)trimethoxysilane (CTS) was used as a chain connecting the PVDF substrate with CS to form the first coating layer, and then TA was introduced as the second coating layer. Because of the unstable characteristic of CS coating, part of the polluted coating can be washed away by pure water with the contaminants. The modified membrane showed superhydrophilicity (the water contact angle was 17°) and underwater superoleophobicity (the underwater oil contact angle was 158°). Moreover, the initial pure water flux of modified membrane was 8579 L m −2 h −1 , which was more than twice than that of pristine membrane (3057 L m −2 h −1 ). After the membrane was contaminated by toluene emulsion and rinsed thoroughly with pure water, the water flux recovery rate of the modified membrane reached about 98%, meaning the modified membrane possessed excellent anti-fouling and self-recovery ability. Besides, the rejection rate of toluene emulsion was 99.1%, showing the modified membrane could be used to efficiently handle the oil/water separation. During six cycles of being polluted and rinsed, the water flux recovery rate would decrease a little after each cycle, from 98% to 94.3% in the end, which demonstrated that the sacrificial layer did exist. This facile approach has great potential in practical water remediation. Graphical abstract