Transport mechanisms in electrodialysis: The effect on selective ion transport in multi-ionic solutions

Water reuse is one of the possible solutions to prevent depletion of freshwater resources; however, it is often limited by the accumulation of specific ions in the recirculating water. Ion selective desalination technologies can increase the potential for water reuse. Electrodialysis is a water treatment technology that is able to selectively remove ions from water. In order to enhance and further develop the selectivity of the process, a fundamental understanding of the various mechanisms governing multi-ion transport in electrodialysis is essential. In the present study, a theoretical model for multi-ionic (Na+ and K+) mass transport in electrodialysis was developed including ion-water and ion-membrane frictions, and water transport. General properties and the selectivity of ion exchange membranes towards K+ over Na+ ions were experimentally determined and included in the model. The theory was successfully validated for ion flux through the membranes, ion concentrations in the solutions, and water transport by comparing theory with batch-mode electrodialysis experiments. Contributions of different ion transport mechanisms to the selective transport of Na+ and K+ ions were evaluated by model calculations. It was found that electromigration has the largest contribution to ion transport followed by convection, and that diffusion controls the selectivity of ion transport in electrodialysis under constant voltage operation.