A novel positively charged composite nanofiltration membrane prepared by bio-inspired adhesion of polydopamine and surface grafting of poly(ethylene imine)

Novel positively charged composite nanofiltration membranes were facilely prepared by polydopamine (PDA) deposition followed by poly(ethylene imine) (PEI) grafting on polyethersulfone (PES) substrates. Scanning election microscope (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), zeta potential measurement and water contact angle measurement were employed to characterize the surface chemical composition and morphology of the resultant membranes. The rejection of salts was increased but the pure water flux was decreased with the increase of PDA deposition time, PEI concentration, PEI reaction temperature and time. The salts rejection followed the sequence: MgCl2>CaCl2>MgSO4>Na2SO4, confirming that the membranes were positively charged. The rejection of MgCl2 could reach 73.7%, whereas the rejection of CaCl2 was 57.1%. Moreover, the membranes exhibited a superior rejection of up to 96.5% for cationic dyes.