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

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 CO₃^2- 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.