A novel positively charged nanofiltration membrane prepared from N,N-dimethylaminoethyl methacrylate by quaternization cross-linking

A novel method for the preparation of hydrophilic and positively charged nanofiltration (NF) membranes was developed. An active top-layer with NF characteristic was composited onto polysulfone (PSf) ultrafiltration substrate by UV-induced graft polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) followed by quaternization cross-linking with p-xylylene dichloride (XDC). Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy were employed to monitor membrane surface chemical and morphological changes after poly(DMAEMA) grafting and quaternization cross-linking. The influences of UV irradiation time, monomer DMAEMA concentration, the cross-linker XDC concentration on the separation and permeation properties of membrane were investigated. The surface hydrophilicity of the composite membranes was evaluated by contact angle measurement. It was found that membrane hydrophilicity was obviously enhanced after UV-induced grafting and quaternization cross-linking. As the UV irradiation time and DMAEMA monomer concentration were increased, salts rejection of the composite membranes increased while water permeability decreased. Under the optimized preparing conditions, water flux and MgCl2 rejection (1.0 g/L) of the composite membranes reached 60.3 L/(m2 h MPa) and 93.2%, respectively. The membrane surface charge property was measured via a streaming potential method. It was found that, at pH from 4.5 to 9, the composite membranes showed positive zeta potentials. The rejections of the composite membranes to different salts followed the order Na2SO4 < NaCl < MgSO4 < MgCl2 < CaCl2. Moreover, the composite NF membranes showed a good stability in water-phase filtration process.