Investigation of Acid/Base Production and Energy Consumption in Redox-based Bipolar Electrodialysis for pH Control in Lithium Recovery

This study investigates the impact of key operating conditions on acid/base production and energy consumption in a redox-based bipolar membrane electrodialysis (redox-BMED) system, which enables chemical-free pH control for lithium recovery. A redox-BMED system utilizing a ferri/ferrocyanide redox couple was constructed, and the effects of applied voltage (0.6-1.2 V), cross-flow velocity (1.25-6.25cm/min), lithium feed concentration (250-2,500 mg-Li/L), and anion type (SO<sub>4</sub><sup>2-</sup> vs Cl<sup>-</sup>) were systematically examined. Acid and base production increased with higher voltages and higher cross-flow velocities, while energy consumption decreased under lower voltages and higher cross-flow velocities. Under optimal condition (1.2V, 6.25cm/min), acid/base production exceeded 260 μmole with minimal energy consumption of 44 Wh/mole. Cl<sup>-</sup> based electrolytes showed 29% higher productivity than SO<sub>4</sub><sup>2-</sup> based ones. Moreover, increasing the lithium concentration from 250 to 2,500mg-Li/L reduced energy consumption from 57 to 46 Wh/mole, confirming higher energy efficiency at higher concentrations. The system also maintained stable operation for over 120 hours without performance degradation, highlighting its scalability and durability. Overall, this study demonstrates the potential of redox-BMED as a sustainable and efficient electrochemical platform for pH control and high-purity lithium compound production in resource circulating recovery systems.