Limiting current density as a selectivity factor in electrodialysis of multi-ionic mixtures

Electrodialysis is a promising technology to remove low concentrations of target ions from multi-ionic mixtures. While the synthesis of selective membranes is a prominent topic in research, few studies have been presented on selectivity-enhancing process design. This work investigates the limiting current density as a selectivity promoter in removing dilute target ions from a concentrated solution. Ambiguities and challenges in the prevailing definitions of the limiting current density are identified, and a new approach based on the Nernst equation is proposed, the boundary-layer method. Chloride and fluoride with starting concentrations of 10 mM were removed from 1 M sodium sulfate base electrolyte with varying current density levels around the limiting value. Removal rates, separation efficiencies, and energy consumption were compared. The separation efficiencies between chloride and sulfate and fluoride and sulfate had their highest values at 0.93 and 0.81, respectively, when operating at 130 A/m2. We demonstrate that increasing the ion selectivity through the ion-specific limiting current density is possible and only requires standard current-voltage data. The experimental results suggest that process optimization is an essential supplement to membrane development to enhance the selective removal of target ions by electrodialysis.