One-step modification of PVDF membrane with tannin-inspired highly hydrophilic and underwater superoleophobic coating for effective oil-in-water emulsion separation

Herein, hydrophobic PVDF membranes were facilely transformed into highly hydrophilic and underwater superoleophobic ones via one-step dip-coating in tannic acid (TA)/sodium periodate (NaIO 4 )-mixed solution. The employment of NaIO 4 could not only facilitate TA oxidation and deposition, but also introduce abundant hydrophilic carboxyl groups on membrane surface. Consequently, compared with the pristine PVDF membrane, the modified membrane showed much higher hydrophilicity and underwater oleophobicity with water contact angle decreasing from 121° to 32°, oil contact angle increasing from 116° to 162° and a tilt angle of just 2°. In contrast to the two-step modification (deposition of TA followed by post-oxidation by NaIO 4 ), our approach endowed the membrane with dramatically improved permeability (>2400 LMH/bar) and oil rejection (>98%) when dealing with different types of oil-in-water emulsions. Additionally, the membrane displayed excellent antifouling performance with flux recovery ratios of above 95% and favorable reusability after 5-cycle filtration tests. Moreover, the membrane possessed a stable TA coating even being subjected to harsh pH solutions or sonication. This work provides a simple, robust and cost-effective strategy for membrane modification.