Zwitterion structure membrane provides high monovalent/divalent cation electrodialysis selectivity: Investigating the effect of functional groups and operating parameters

Monovalent cation perm-selective membranes (MCPMs) with enhanced monovalent flux and the capability to reject multivalent cations are critical; however, their realization remains challenging in various industrial applications, such as seawater desalination and wastewater purification. To achieve this criterion, we designed and synthesized a series of MCPMs with a zwitterion structure comprising three quaternary ammonium groups, two carboxylic acids, and one sulfonic acid group. These functional groups act to balance the cation flux and perm-selectivity of the MCPM. The cation fluxes and perm-selectivity of the fabricated MCPMs were investigated for Na+/Mg2+ and Li+/Mg2+ systems. High perm-selectivities of 58.4 and 16.5 were obtained for (PMg2+Na+) and (PMg2+Li+) systems, respectively, using a QPO/DAN-SA-5 membrane. The performance of the MCPMs was compared with the state-of-art commercial monovalent cation selective membrane NeoseptaTM CIMS. The influence of feed concentration and current density on the cation fluxes and perm-selectivity was examined, and the pH was measured to investigate the effect of concentration polarization and water splitting on electrodialysis. Moreover, the presence of various functional groups in the membrane matrix provided additional insights into electrostatic repulsive and interaction effects. The zwitterion structure of the MCPM was capable of tuning the cation flux and perm-selectivity. Our study provides an effective strategy for developing MCPMs with high cation flux and ion perm-selectivity.