Nature-inspired construction of poly (vinylidene fluoride) membranes through the coordination coating of tannic acid with copper ions for oil-in-water emulsions separation

Construction of the separation membranes with underwater super-oleophobic property proposes a promising pathway in the field of oily wastewater treatment, but it is still plagued with the bottlenecks of the time-consuming preparation and the unsatisfied stability of resultant membranes. Herein, the alkali induced hydroxylation and the nature-inspired construction through the coordination coating of copper ions with tannic acid (Cu2+/TA) were performed on the poly (vinylidene fluoride) (PVDF) membrane substrate prepared by the thermally induced phase separation (TIPS). The introduced hydroxyl groups resulted from the membrane hydroxylation is beneficial for the subsequent construction of the Cu2+/TA coordination coating layer. The excellent hydrophilicity and the specifically hierarchical roughness of the immobilized coating layer endow the modified membranes with the hydrophilic/underwater super-oleophobic property. The optimized membrane correspondingly showed the improved flux of around 2000 L m−2•h−1 accompanied with the oil rejection of above 97.0% in treating two simulated oil-in-water emulsions (toluene and n-hexane). Besides, the flux recovery ratio increased from 72.4% of the pristine membrane to 93.3% of the modified membrane after the BSA fouling and regeneration test, which is also indicative of the improved antifouling behavior as a result of the hydrophilic coordination coating. The membrane stability was further demonstrated by the results of the sandpaper abrasion-resisting test and the long-term (up to 6 months) air-exposure treatment. This work might provide a straightforward strategy for designing advanced membrane for the large-scale applications of oil/water separation.