Efficient Electrochemical Tap Water Softening via Lorentz-Driven Bidirectional Mass Transfer

Tap-water hardness causes persistent scaling and associated economic losses, necessitating efficient point-of-use softening. Electrochemical softening offers a reagent-free, modular route. However, tap water typically has lower hardness than industrial waters. Thus, conventional interface-dependent nucleation becomes limited by the diffusion of Ca²⁺ and Mg²⁺ to the interface, constituting a key bottleneck. Here, we report a vector-guided ion migration strategy strengthening bidirectional mass transfer. Under a magnetic field, Lorentz forces enhance Ca²⁺/Mg²⁺ transport to the interface and drive reverse transport of excess interfacial alkalinity into the bulk, thereby establishing bulk supersaturation. Over 240 h of tap water softening, the mean precipitation rate reached 50.6 g·h^-1·m^-2. This value surpasses previously reported values for electrochemical tap water softening. Furthermore, the strategy mitigated cathode scaling from 65.0% to 29.3% while cutting electricity demand by 35.0% for the same removal duty. Overall, Lorentz-driven bidirectional mass transfer offers an efficient, economically viable, and sustainable route to long-term electrochemical tap water softening.