Inorganic Scaling in Electrodialysis: Mechanistic Insights and Impact on Energy Efficiency and Ion Selectivity

Electrodialysis (ED) is an electrically driven separation technology that has been widely applied in various industrial sectors (e.g., desalination and food processing). While recent research has focused on developing novel ion-exchange membrane (IEM) materials, the detrimental effects of inorganic scaling in ED systems, particularly on energy consumption and ion selectivity, remain largely overlooked. In this study, we systematically investigated the impact of inorganic scaling (i.e., gypsum) on ED performance under varying operating conditions, including applied current densities ranging from 20% to 100% of the limiting current density and flow rates ranging from 4 to 12 mL min^-1 (linear velocities from 0.33 to 1 cm min^-1). Through precise measurements of input energy, solution composition evolution, and solid-phase characterization, we found that scaling not only increased energy consumption (up to 3-fold compared to nonscaling conditions) but also significantly changed selectivity. Specifically, we found that sodium-calcium and chloride-sulfate selectivities exhibited up to a 7-fold change relative to the initial value when the salt removal from the diluate solution reached approximately 30%. By integrating process modeling with experimental data, we confirmed that these changes in selectivity were directly attributable to gypsum formation on the surface of the IEM. These findings advance our understanding of inorganic scaling effects in ED and offer valuable guidance for optimizing real-world ED operation.