Robust electrolysis system divided by bipolar electrode and non-conductive membrane for energy-efficient calcium hardness removal

In this study, an energy-efficient divided bipolar electrolysis system was developed for water softening, where two PTFE membranes were used as the separating materials and a bipolar electrode was employed to enhance the H₂O-splitting reactions. As compared with other two operation modes, the optimum calcium harness removal efficiencies of 85% and 57% could be reached in the induction cathode effluent and terminal effluent, respectively, at 8 mA cm⁻² in the mode A. Increasing the current density from 5 to 20 mA cm⁻² evidently promoted the removal of calcium hardness from 33% to 65% in the terminal effluent and the CaCO₃ precipitation rate from 743 to 1462 gCaCO₃ h⁻¹ m⁻² with the increased energy consumption from 0.53 to 2.2 kWh kg⁻¹CaCO₃. The optimized Ca²⁺/HCO₃⁻ molar ratio was 1:1.2 for the calcium hardness removal. In addition, increasing the flow rate into each cathode chamber from 10 to 40 mL min⁻¹ gradually decreased from 67% to 35%. The calcium hardness was mainly removed in the forms of vaterite and calcite in the alkaline effluents and was marginally precipitated as aragonite and calcite on the cathodes surface. Generally, present energy-efficient electrochemical water softening system showed great potential for application in industrial processes.