How bulk and surface properties of sulfonated cation-exchange membranes response to their exposure to electric current during electrodialysis of a Ca2+ containing solution

Abstract

Electrodialysis (ED) of calcium-containing solutions is a widely applied process in water treatment, dairy industry etc. The lower performance of ion-exchange membranes in these processes (in comparison, e.g. with ED of sodium-containing solutions) is known, but the reasons for this effect are still unclear. We study the behavior of two sulfonated cation-exchange membranes: a CMX (Astom, Japan) and a CJMC-5 (Hefei Chemjoy Polymer Materials, China). For both membranes, the bulk properties (conductivity, diffusion permeability, counterion permselectivity) change when the membrane is soaked in a 0.02 N CaCl 2 solution, but these changes are reversible: when the membranes are again equilibrated with a NaCl solution, the bulk properties restore and become the same as before. The exposure of the membranes to an overlimiting current does not affect the bulk properties. However, it is not the case for the properties controlled by the membrane surface. In particular, the length of the limiting current plateau of the current-voltage curve increases and electroconvection (visualized using Rhodamine 6G) decreases with increasing the time of the membrane exposure to overlimiting current in a CaCl 2 solution. The zeta potential is about −30 mV for a “fresh” membrane, and +40 mV after exposure of this membrane to an overlimiting current in a 0.02 N CaCl 2 solution; the both measurements were made in the same NaCl solution. The changes in surface-controlled properties appear to be irreversible. This effect is explained by specific adsorption of Ca 2+ ions on the surface of sulfonated membranes promoted by a strong electric field at the interface. This explanation is supported by simulations of the distribution of the electric potential and ion concentrations in the interfacial electrical double layer. • Two sulfonated cation-exchange membranes are studied. • Performances change after exposing the membranes to electric current in a CaCl 2 solution. • The change in bulk properties (conductivity, diffusion permeability) is reversible. • Changes in surface-controlled properties (e.g. zeta-potential) appear to be irreversible. • The effect is explained by specific adsorption of Ca 2+ enhanced by a strong electric field.