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

Separation of Ca2+ or Cu2+ ions from binary solutions with Na+ or prot ons by electrotransport through strong cation-exchange membranes (CEM) has been studied in the case where no sorbed species is present withi n the membrane material. The study was achieved on two conventional CE M and two CEM manufactured to be preferentially permselective to monov alent cations. Without any applied driving force, the exchange isother ms exhibit no significant increase in permselectivity for the modified CEM. Divalent cations are preferred by the membranes due to their hig her electrostatic attraction with the oppositely fixed charged exchang e sites. When the CEM are submitted to an electrical field, radiotrace r measurements exhibit the difference of permselectivity of these two types of CEM. The apparent rate constants of the ionic interfacial tra nsfer were calculated on the basis of unidirectional fluxes and concen tration profiles of cations within the unstirred layer. The values of these kinetic constants were always greater for monovalent than for di valent cations but the difference was more important for modified memb ranes. Lastly, comparison between the potential profiles showed that t he potential decrease within the depleted unstirred layer was more imp ortant for the solutions less concentrated in protons.