Electrochemical Removal of Hardness and Alkalinity: Performance of an Electrolysis (ELC) Dominated Capacitive Deionization (CDI) System

This study investigates ion removal in a combined Capacitive Deionization (CDI) and Electrolysis (ELC) system for reducing hardness, alkalinity, and total dissolved solids (TDS) in groundwater. Under the applied operating conditions, the reactor behavior was interpreted to be dominated mainly by electrolysis-induced local chemical conditions, while the porous carbon layer may have provided a secondary CDI-type contribution through temporary ion accumulation and charge buffering. At the cathode, water reduction generates hydroxide ions (OH⁻), increasing pH and facilitating Ca 2+ and Mg 2+ removal through precipitation as carbonates (CO 3 2⁻ ) and OH - . Simultaneously, anode oxidation generates H + , lowering pH and promoting conversion of bicarbonate (HCO 3 ⁻) into CO 2 , thereby contributing to alkalinity reduction. Results showed that the highest center-compartment removal efficiencies under synthetic-water conditions were 77.78% for hardness at 150 C/L, 41.33% for alkalinity at 120 C/L, and 28.40% for TDS at 150 C/L. Calcium removal was generally promoted by higher alkalinity, whereas magnesium removal was lower than calcium removal under comparable conditions and improved more clearly at higher charge loading, consistent with the more favorable local conditions required for magnesium precipitation. Sodium removal was substantially lower than Ca 2+ and Mg 2+ removal, likely because Na + does not have a precipitation pathway under the present conditions and may be affected by ion redistribution and competition effects. Real-water testing showed site-dependent treatment performance, indicating that initial water composition strongly influenced removal efficiency. These findings demonstrate the reactor’s potential to reduce hardness and alkalinity with minimal chemical input, while also highlighting the need for further work on CDI versus ELC contributions, long-term electrode stability, energy consumption, and site-specific optimization.