Modification of Composite Separation Membranes with Citric Acid and Metal Ion Chelation Coatings for Oil–Water Separation

The development of advanced and efficient oil–water separation technologies is crucial, and membrane fouling remains one of the primary obstacles hindering the sustainable development of membrane technology. Separation membranes, which differ in pore size and material composition, can be selected based on specific environmental conditions and application requirements. In the study, a composite enhanced PVDF membrane with a complex coordination aggregation structure and abundant hydroxyl groups was prepared by introducing a citric acid–Fe(III) complex coating onto the PVDF surface using tannic acid as an interfacial adhesive layer. Citric acid (CA) molecules participate in competitive cross-linking. The carboxylic acid groups (-COOH) of CA form ionic bonds with Fe3+. This promotes the formation of a complex cross-linked network inside the coating. It also successfully introduces additional hydrophilic groups. Consequently, a TA-Fe/CA composite coating system is constructed. The optimized modified membrane exhibited superior performance, with a water contact angle (WCA) of 14° and complete wetting within 0.5 s. The pure water flux reached 20,473.16 L/m2·h. Compared to the pristine membrane, the modified membrane demonstrated significantly enhanced hydrophilicity, underwater oleophobicity, and antifouling properties. During the separation of surfactant-stabilized toluene emulsions, the PVDF-TA-Fe/CA membrane showed higher separation efficiency and permeate flux than both the PVDF and PVDF-TA membranes. Furthermore, the modified membrane demonstrated excellent chemical stability, long-term durability, and thermal stability.