Scaling-Enhanced Scaling during Electrodialysis Desalination

Abstract

Scaling is one of the critical issues limiting the performance of electrodialysis (ED) desalination. In this study, we systematically investigated scaling during ED desalination of seawater. We observed that severe scaling occurred on the surfaces of the cathode and cation exchange membrane (CEM) facing the cathode chamber, which further induced the occurrence of scaling on the surfaces of the CEM and anion exchange membrane (AEM) facing the adjacent dilute chamber. We revealed that the formation and evolution of scaling in an entire ED stack undergo 3 sequential phases. In phase 1 during the early stage of ED desalination, divalent cations (e.g., Mg 2+ and Ca 2+ ) transport through the CEM into the cathode chamber and accumulate in the electrolyte solution, and meanwhile, OH − ions are generated via water electrolysis at the cathode, both of which synergistically increase the tendency of electrode scaling. In phase 2 after a period of desalination, fast reactions between divalent cations and OH − ions result in the occurrence of scaling in the cathode chamber. Under the typical constant-current operating mode, scaling on the CEM surface facing the cathode reduces effective membrane area, which leads to an increase in the local current density through the CEM. In phase 3, when the local current density exceeds the limiting current density, water splitting occurs on the surface of the CEM facing the dilute chamber, which induces the generation of OH − and thereby enhances the crystallization on this surface. Eventually, scaling in the cathode chamber further enhances scaling on ion exchange membranes in the adjacent dilute chamber. The mechanisms of scaling formation and evolution unveiled in this study provide important implications for scaling mitigation during ED desalination.