The Potential of Electrodialysis with Mediating Solution (EDM) for Eliminating Alkaline Scaling: Experimental Validation and Mechanistic Elucidation

Alkaline scaling in the cathode chambers of conventional electrodialysis (ED) stacks presents significant challenges when desalinating solutions containing divalent cations. This scaling, resulting from the combined effects of water electrolysis and the migration of divalent cations from the feedwater into the catholyte, further extends from the cathode chamber to the surfaces of both the cation exchange membrane (CEM) and the anion exchange membrane (AEM) in the adjacent dilute chamber. This study aims to mitigate alkaline scaling, without pretreatment or antiscalant dosing, by optimizing the ED stack design to restrict divalent cation transport toward the cathode. We evaluated three ED stack configurations, each forming the cathode chamber with a distinct ion transport control mechanism: (1) a monovalent selective cation exchange membrane (SCEM), (2) a bipolar membrane (BPM), and (3) a mediating solution chamber adjacent to the cathode chamber (EDM). Our results indicated that stacks employing the SCEM or BPM partially restricted divalent cation migration but remained vulnerable to scaling under higher feed salinities, due to weakened Donnan exclusion within the SCEM, and strong internal ion polarization at the BPM interface. In contrast, the EDM stack exhibited superior antiscaling performance by combining strong Donnan exclusion through an AEM with ionic buffering in the mediating solution chamber, effectively blocking cation transport and eliminating conditions conducive to scaling. Additionally, the EDM stack maintained low electrical resistance and high operational stability, making it a simple, efficient, and cost-effective solution for scaling mitigation in ED systems.