Surface hydrophilic modification of PVDF membranes based on tannin and zwitterionic substance towards effective oil-in-water emulsion separation

Hydrophobic polyvinylidene fluoride (PVDF) membrane was successfully modified through a facile co-deposition strategy to improve its hydrophilicity and resistance to oil-fouling. The new zwitterion, sulfonated N, N-Diethylethylenediamine (DEDAPS), was synthesized via the ring-opening reaction and co-deposited with tannin acid (TA) on the surface of PVDF membranes. The obtained membranes were characterized by various techniques and utilized to separating oil-in-water emulsions. By controlling the concentration of DEDAPS in dipping solution, the pure water flux of the optimal membrane reached a maximum, which was 17 times that of the pristine membrane. In addition, the optimized membrane was endowed with superhydrophilicity and underwater superoleophobicity. It showed superb reusability and sustainability in separating various oil-in-water emulsions with the efficiency beyond 96%, which was attributed to the firm adhesion of co-deposition layer on the surface. This work might provide a new avenue for surface modification of membranes to be potentially used in oily wastewater purification.