Positively charged nanofiltration membrane prepared by polydopamine deposition followed by crosslinking for high efficiency cation separation

Novel positively charged nanofiltration membranes were facilely prepared by polydopamine (PDA) deposition followed by crosslinking on the polyethersulfone (PES) ultrafiltration (UF) membrane substrate. Specifically, small molecules DMAPA and 1,3-dibromopropane were separately selected as the bridge agent and crosslinking agent, which can not only realize the moderate crosslinking and densification of the loose structure of the PDA layer, but also further improve the hydrophilicity and positive charges of the PDA layer. Under the optimized conditions of DMAPA concentration (2 g/L), 1,3-dibromopropane concentration (10 wt%) and reaction time (9 h), the composite membrane had the surface pore size as low as 1.1 nm, water contact angle as low as 35.1° and zeta potential as high as +63.8 mV at pH = 6. As a result, the optimized positively charged NF membrane (M3) showed an outstanding permeance up to 15.7 L m−2 h−1 bar−1 while maintaining a high rejection above 95% for MgCl2. Meanwhile, M3 also showed good environmental adaptability when facing complex feeding or operating conditions. This work provided new situation in the design and construction of positive charged NF membranes for high efficiency cation separation.