High flux positively charged nanofiltration membranes prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone membranes

To achieve high water flux, the majority of positively charged NF membranes consist of a thin active skin deposited on a thick, permeable support, while the connection force between the active layer and the support is a physical force which may cause the membrane unstable in the long running. In general, chemical bond connection may show superior stability. Based on this consideration, a series of positively charged nanofiltration (NF) membranes were prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone ultrafiltration membranes in this work. The salts rejection order of these membranes is MgCl2 > NaCl > MgSO4 ≥ Na2SO4. Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle were employed to characterize the resulting membranes. The results indicated that the grafting degree (DG) increased with increasing the monomer concentration, prolonging the irradiation time and reducing the irradiation distance. However, the filtration performance was not well correlated with the increasing DG. The NF membrane prepared by photografting in a 1.5 M DMC solution for 5 min demonstrated high MgCl2 rejection (94.8%) accompanied with high flux (20.3 L/m2 h) at 0.2 MPa. When operation pressure increased to 0.8 MPa, the solution flux increased to 60 L/m2 h, while MgCl2 rejection nearly maintained stable about 92.4%. An interesting phenomenon was also observed in this experiment that the flux of pure water was less than that of salt solution for some NF membranes.