Spatially decoupled dual-cathode system enables selective removal of Mg2+ and Ca2+ via Localized OH- accumulation and CO2 conversion.

Integrating electrochemical water softening with CO2 utilization is an emerging and promising strategy toward chemical-free water treatment and carbon neutrality. In this process, cathodic OH- generation precipitates Mg2+ as Mg(OH)2 while simultaneously converting CO2 into CO32- for Ca2+ removal as CaCO3. However, CO2 conversion competes with Mg2+ precipitation for OH-, making it difficult to coordinate Ca2+ and Mg2+ removal within a single system. Herein, we resolve this limitation by developing a dual-cathode electrochemical reactor that spatially separates the incompatible reactions along the flow direction. An upstream stainless-steel mesh cathode establishes a high-pH, carbon-limited zone for selective Mg2+ removal, while a downstream carbon felt cathode enables efficient CO2 capture and conversion for CaCO3 formation. Numerical simulations confirmed that the sequential configuration eliminates competitive interactions and enables independent regulation of alkalinity generation and carbon utilization to selective mineralization of Mg(OH)2 and CaCO3 in their respective zones. Consequently, the system achieved 90.7% total hardness removal and 54.3% CO2 capture efficiency, with an energy consumption of 22.3 kW·h·kg-1 CaCO3, approximately one-twelfth that of the conventional single-cathode system. Furthermore, the reactor demonstrated robust adaptability across a wide range of Ca2+/Mg2+ ratios through simple adjustment of current density and CO2 flux. This work demonstrates a new strategy for integrating CO2 utilization into electrochemical water softening by reconciling competing reaction pathways.