Strategies to increase ion selectivity in electrodialysis

We analyze selective removal of anions in electrodialysis (ED) from a multicomponent ion mixture where all ions are monovalent, both experimentally, and by a modified theory. The theory makes use of the Nernst-Planck equation to describe transport of ions across ion-exchange membranes in combination with the ion affinity, which is a parameter that accounts for the preference of membrane materials to adsorb one ion more than another. For a mixture of NO3- and Cl- anions, ion affinity was measured in an adsorption experiment, and it was found that NO3- adsorbs more in anion-exchange membranes (AEMs) than Cl-. Faster transport of NO3- relative to Cl- in batch-mode ED experiments is well described by the transport theory, for three types of AEMs that we tested. From analysis of the model, we can derive that membrane selectivity in ED between different anions can be increased further by: i) an increase in the difference in ion affinity of the anions, ii) an increase in the AEM thickness, and iii) a decrease in the charge density of the AEM. The last two strategies go against the general understanding that thin highly charged membranes are in general better for ED. Our proposed strategies follow rigorously from our combined theoretical–experimental study.