An integrated pump-suction electrocoagulation system for accelerated hardness removal: Process intensification and crystallization mechanisms

Conventional electrochemical softening processes are often constrained by the spatial decoupling of precipitation and solid-liquid separation, resulting in slow crystallization kinetics, high chemical demand, and limited process integration. To overcome these limitations, three progressively intensified systems, namely pump-suction chemical flocculation (PCF), split-type electrocoagulation (SPE), and integrated pump-suction electrocoagulation (IPE), were developed based on a membrane-free pump-suction architecture. Among them, the IPE system spatially integrates cathodic alkalinity generation and the in situ release of active Al species from a composite anode into a single reactor. Multiphysics simulations revealed that the coupling of a converging flow field and a centripetal electric potential gradient promoted the directional transport of scaling ions and OH⁻ toward the cathode region, thereby establishing a confined alkaline microenvironment with intensified reaction coupling. Compared with the single pump-suction system, the apparent removal rate constants of Mg hardness and turbidity increased by approximately 5-fold and 12-fold, respectively. Under optimal conditions, turbidity rapidly decreased from 250 NTU to 22 NTU within 2.5 min, while Ca and Mg hardness removal efficiencies finally reached 92.68% and 96.36%, respectively. Meanwhile, the specific energy consumption of the IPE system was only 12.60 kWh/kg CaCO₃, representing a 32.44% reduction compared with the SPE system. Mechanistic analyses further demonstrated that the synergistic interaction between cathode-induced precipitation and anodically released Al species facilitated nucleation, accelerated ordered crystal growth, and promoted the formation of Ca-Mg carbonate aggregates with enhanced settling behavior. Overall, this work demonstrates that rational reconstruction of the electrochemical reaction space can effectively shift the rate-limiting step of hard-water softening from nucleation to mass transfer and particle aggregation, providing a compact and low-chemical strategy for energy-efficient water softening.