Enhanced electrochemical softening of cooling water with three-dimensional cathodes

Electrochemically mediated precipitation is promising for cooling water softening. However, the performance of this strategy is limited by the requirement of sufficient cathode surface area. To overcome this limitation, we propose using three-dimensional (3D) cathodes, which provide more deposition sites than two-dimensional (2D) cathodes under the same dimension. We systematically evaluated the softening performance of the 3D cathode as a function of applied current density, electrode spacing, electrode size, and Ca2+/HCO3- molar ratio in water samples. The results suggest both cathode size and current intensity affect softening performance, while a lower electrode gap reduces cell voltage and energy consumption without lowering softening performance. In actual phosphonates containing cooling water, electrochemical softening with 3D Ni foam removes 65.5% Ca and 35.5% Mg in 8 h electrolysis at 80 A/m(2). We further confirmed the long-term stability of 3D system over 100 days of continuous flow operation with >50% hardness removal from synthetic solutions. The cell voltage remains relatively stable despite pronounced scaling deposition (up to 2 cm thickness). Collectively, the 3D foam cathode greatly enhances the softening performance and ensures long-term operation without maintenance. These findings are expected to provide valuable insights for the future scaling-up of 3D electrodes in water softening.