TRANSPORT COMPETITION BETWEEN MONOVALENT AND DIVALENT-CATIONS THROUGH CATION-EXCHANGE MEMBRANES - EXCHANGE ISOTHERMS AND KINETIC CONCEPTS

Separation of Ca2+ or Cu2+ ions from binary solutions with Na+ or protons by electrotransport through strong cation-exchange membranes (CEM) has been studied in the case where no sorbed species is present within the membrane material. The study was achieved on two conventional CEM and two CEM manufactured to be preferentially permselective to monovalent cations. Without any applied driving force, the exchange isotherms exhibit no significant increase in permselectivity for the modified CEM. Divalent cations are preferred by the membranes due to their higher electrostatic attraction with the oppositely fixed charged exchange sites. When the CEM are submitted to an electrical field, radiotracer measurements exhibit the difference of permselectivity of these two types of CEM. The apparent rate constants of the ionic interfacial transfer were calculated on the basis of unidirectional fluxes and concentration profiles of cations within the unstirred layer. The values of these kinetic constants were always greater for monovalent than for divalent cations but the difference was more important for modified membranes. Lastly, comparison between the potential profiles showed that the potential decrease within the depleted unstirred layer was more important for the solutions less concentrated in protons.