Feasibility of separation of cations in electrodialysis in relation to concentration polarization

Abstract

Separation of Cs +, Sr 2+ or La 3+ from binary solutions with Na + by electrodialysis through a strong cation-exchange membrane has been studied. The results are correlated with the concentration polarization behaviour of the membrane. Polarization curves were traced at a number of solution compositions and constant total concentrations of 0.01 and 0.05 M. Radiotracers were used to determine the separation coefficients of cations by electrodialysis, the selective uptake of cations by the membrane and the co-ion leak. The change in pH during electrodialysis was measured. Substitution of Na + by the cations of higher membrane affinity in mixtures deformed the polarization curves and raised the limiting current density in the order Cs +<La 3+<Sr 2+. The higher the valency and the lower the total concentration, the higher is the selective uptake of the preferred cations over Na +. Efficient cation separation is realized at low current densities independent of the solution composition. However, as the current density increases the efficiency decreases until Na + is preferentially transported. Concentration polarization is detrimental to the cation separation even at currents well below the limiting current density. In the presence of concentration polarization, the extent of co-ion leak is a function of the nature of cations, while the transport of hydrogen ions through the cationic membrane is negligibly small and cation independent.