Subtle introduction of membrane polarization-catalyzed H2O dissociation actuates highly efficient electrocoagulation for hardness ion removal

Abstract

Electrocoagulation represents a promising process for hardness removal from cooling water. Nevertheless, the slow hydrolysis reaction severely restricted the floc formation, inhibiting the hardness co-precipitation and simultaneously causing secondary pollution from dissolved Al3+. Inspired by the detrimental membrane fouling phenomenon in conventional electrodialysis, we reported a rational strategy to substantially enhance the hardness removal efficiency in electrocoagulation by introducing a special membrane polarization-catalyzed H2O dissociation herein. Leveraging the electron transfer between functional groups (–SO3− and –N(CH3)3+) of ion exchange membrane (IEM) and surface-adsorbed H2O under the electric field-induced ion depletion scenario, H2O dissociation could be effectively catalyzed, with this catalytic activity more intensive in –SO3− than in –N(CH3)3+. Such a special H2O dissociation beneficially created a widely distributed and well-simulated alkalinity zone around the anodic region of IEM, which promoted the conversion of dissolved Al3+ to floc Al, thereby enhancing floc formation and circumventing secondary pollution. All these features enabled the resulting membrane-enhanced electrocoagulation (MEEC) to achieve a super-prominent hardness removal rate of 318.9 g h−1 m−2 with an ultra-low specific energy consumption of 3.8 kWh kg−1 CaCO3, considerably outperforming those of other conventional hardness removal processes reported to date. Additionally, in conjunction with a facile air-scoured washing method, MEEC exhibited excellent stability and universal applicability in various reaction conditions.