Mussel-inspired modification of ion exchange membrane for monovalent separation

Electrodialysis (ED) is a promising and widely used technique for producing drinking water. Nevertheless, the presence of multivalent ions like Mg2+ and Ca2+ can be challenging in practical applications, because this leads to precipitation in the concentrated compartment, thus decreasing the performance of the ED process. Monovalent ion exchange membranes are capable of separating monovalent ions from a solution containing both mono- and multivalent ions, which can be applied for water softening. The monovalent ion selectivity of an ion exchange membrane in electrodialysis is greatly dominated by its surface structure and properties. In this study, a simple method is proposed by co-depositing mussel-inspired polydopamine (PDA) with polyethyleneimine (PEI) to modify a self-prepared sulfonated poly (ether sulfone) (SPES) ion exchange membrane. The effects of the PEI content on the monovalent selectivity were studied. When the optimized parameters were used in ED, the permselectivity of SPES-PDA/PEI-2 was 2.5 times higher than that of the SPES membrane. Especially the flux of H+ was enhanced. Furthermore, the PDA/PEI modified ion exchange membrane shows excellent operation stability for monovalent separation performance after immersion in acid and alkaline solution for 7 days. Comparing membranes prepared with different molecular weights of PEI, results revealed that modification with a lower molecular weight PEI yields a higher selectivity. This facile strategy may provide new opportunities not only to develop monovalent ion exchange membranes but also to engineer the surface of numerous materials in energy and environmental applications.