A Comparative Study of Membrane-Based Technologies (MCDI and ED) for Ion Selectivity: Evaluating Performance and Efficiency

Developing ion selective separation processes is crucial in resource recovery and wastewater treatment. Membrane capacitive deionization (MCDI) and electrodialysis (ED) offer various possibilities for ion selective separation processes, but the selectivity is often limited. To further understand the competition between ions with different valencies and to optimize selective removal, in the present work, we make use of a theoretical framework based on the amphoteric Donnan theory to describe multi-component (Ca2+, Na+, Cl-) mass transport in MCDI, and of a model based on the Donnan equilibrium theory to describe ion transport in ED. It is generally believed that ion selectivity is highly determined by the membranes, and therefore, there is no significant difference in selectivity between MCDI and ED. In previous work [1,2], we have shown that, in ED, ion selectivity is negatively affected by the back-diffusion of ions that are preferentially adsorbed and transported across the membrane. We hypothesize that this effect of back-diffusion can be inhibited by ion adsorption in the electrical double layers of porous carbon electrodes. As a result, these electrodes will contribute to maintaining the positive concentration gradient across the membrane, thereby enhancing ion selectivity. We will show that the significant selectivity for calcium over sodium, which is observed in the beginning of a batch-mode ED experiment, but which decreases with desalination time, can be maintained in membrane capacitive deionization.

Keywords: Mono/divalent cation selectivity; Membrane Capacitive Deionization; Ion transport modelling; Sodium removal; Desalination