Elucidating the Mechanisms of Ion Transport-Dependent Scaling and Its Influence on Acid–Base Production in Bipolar Membrane Electrodialysis

Bipolar membrane electrodialysis (BMED) is a promising technology for waste brine management, enabling acid and base production alongside brine dilution. However, divalent cations (Ca²⁺, Mg²⁺) cause severe alkaline scaling. This study investigates the mechanisms of scaling formation in BMED and its impact on acid-base production, highlighting the dependence on ion transport. Bidirectional migration of divalent cations and OH^- through the cation exchange membrane (CEM) results in (1) heterogeneous surface crystallization on both sides of the CEM and the anion exchange layer of the bipolar membrane, and (2) bulk crystallization in salt and base solutions. Moreover, crystals depositing on the CEM reduce the effective membrane area and thus increase local current density, inducing local water splitting (LWS). The additional OH^- generated from LWS elevates pH near the CEM and in the salt solution, intensifying scaling on the CEM and in the salt solution. Meanwhile, H⁺ from LWS migrates to the base solution, neutralizing it, while excess OH^- accumulating in the salt solution migrates across the anion exchange membrane and neutralizes the acid. Apart from the revealed scaling-enhanced scaling and scaling-induced neutralization effects, scaling in the BMED stack also increases OH^- leakage and stack resistance, decreases current efficiency, and causes irreversible performance loss even after cleaning. Overall, these findings underscore the critical role of ion migration and local electrochemical environment change in scaling behavior and acid-base production performance.