Mono-valent cation selective membranes for electrodialysis by introducing polyquaternium-7 in a commercial cation exchange membrane

Ion exchange membranes are essential for electrodialysis. However, the presence of multivalent ions, such as Ca2+, Mg2+, or CO32−, SO42−, may result in a detrimental risk of membrane scaling. Mono-valent cation selective membranes may solve this problem. In this study, the surfactant N,N-dimethyl-N-2-propenyl-2-propene-1-ammonium chloride-2-propenamide (poly-quaternium-7, PQ7) is used to modificate the cation exchange membranes. Different concentrations of degraded polyquaternium-7 and sodium hydroxide are investigated to determine the optimal point. The composition and characteristics of the membranes were determined with Fourier transform infrared spectroscopy (FTIR), X-ray photo-electron spectroscopy (XPS), Scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM). Hydrophilic and functional groups of the membrane were determined by the water uptake, contact angle and ion exchange capacity. Current–voltage curves were measured to characterize the transport properties of membranes. The obtained results showed that the limiting current was reduced while the Ohmic and electro-convection resistances were increased. Diffusion dialysis experiments have demonstrated that leakage of modified membrane is lower. Furthermore, a series of electrodialysis experiments was conducted to evaluate the monovalent selectivity of the unmodified and modified CEMs. The leakage of Zn2+ was decreased from 22.0% to 14.2% and the leakage of Ca2+ and Mg2+ were both decreased while the membranes are used in seawater concentration. The obtained results indicate that the membrane has an excellent performance, particularly with respect to selectivity.