Ion selectivity in brackish groundwater desalination by electrodialysis: Experiments and theory

Desalination of brackish water is crucial for addressing the growing global water shortage. Although reverse osmosis is widely used, its indiscriminate solute rejection necessitates additional post-treatment processes such as remineralization. Therefore, ion-selective desalination using electrodialysis has emerged as a promising technology. However, the mechanisms behind ion selectivity in electrodialysis are not fully understood, particularly in the desalination of natural water sources. In this study, we desalinated natural brackish groundwater using an electrodialysis system, focusing on ion selectivity under varying applied currents. We find that the system becomes more monovalent selective with increasing current densities, aligning with previous studies. To further investigate this phenomenon, we developed a theoretical model that incorporates explicit mass transport calculations in the flow channels and the ion exchange membranes. Our model reveals that the commonly assumed formation of thin stagnant diffusion layers near the membrane surface does not hold in our system. Furthermore, our findings indicate that boundary layers alone cannot fully explain the observed ion selectivity, suggesting the need for a more comprehensive understanding of additional local mechanisms. We analyzed two potential mechanisms, non-equilibrium effects on ion partitioning and ion transport within the membranes, suggesting that non-equilibrium effects on ion partitioning better explain the experimental measurements. This study underscores the importance of explicitly accounting for mass transport within the electrodialysis flow channels and highlights the need for future research on the local processes within the membranes and at the channel/membrane interfaces.