On the permselectivity of di- and mono-valent cations: Influence of applied current density and ionic species concentration

Membrane permselectivity between counter-ions is crucial to deliver satisfactory performance in the ion-exchange membrane based processes like in electrodialysis (ED). Yet the influence of the applied current density and varying total concentration as well as equivalent concentration ratio between the di- and monovalent ion on their permselectivity in ED is not clearly illustrated. This is the objective of this study based on standard cation-exchange membrane. Increasing the current density (below the limiting condition) resulted in decreased transport number of Mg2+ in any case and the permselectivity over Na+. This agrees with the presence of the diffusion boundary layer where decreasing transport of Mg2+ can be ascribed to its lower diffusivity in solution. A higher initial concentration of the mixed cations and Mg2+ fraction slightly decreased the permselectivity for Mg2+. Accordingly, the membrane charge neutralization effect by the divalent cation, or its decreased partition is discussed. The influence of the above factors on permselectivity is useful in guiding a smarter operation of ED.