Comparison of Ion Selectivity in Electrodialysis and Capacitive Deionization

Ion-selective removal is an important frontier in water purification technologies. For many emerging applications, removing all ions indiscriminately can lead to excessive energy consumption, high levelized cost of water, poor effluent water quality, and increased waste brine volume. Electrodialysis and capacitive deionization are two electrochemical water purification technologies which are promising toward tunable, ion-selective purification. These technologies have fundamentally different ion removal mechanisms, as electrodialysis leverages electrodiffusion through ion-exchange membranes while capacitive deionization utilizes electrosorption into charged electrodes. We here provide a direct comparison of ion selectivity achieved by these two technologies, focusing on several important ion pairs. We highlight distinct differences in achieved selectivity between these technologies and provide theory results to connect such observations to ion removal mechanisms. Based on the experimental literature, we find that capacitive deionization demonstrates a wider range of achieved ion selectivities than electrodialysis for competing cations such as Na⁺vs Ca²⁺and Li⁺vs Na⁺, while a wider range is observed for electrodialysis when separating anion pairs such as Cl^-vs SO4_2-and Cl^-vs NO3_-. We conclude with reviewing "knobs" that can be adjusted to tune the achieved selectivities by both technologies, and emphasize important questions that should be answered in future studies to improve the selectivity of both technologies.