Na+ migration facilitated separation of hydrogen and hydroxide for efficient hardness removal via an integrated electrochemical precipitation approach

Abstract

In this study, efficient H+–OH– separation was achieved employing Na+ migration in anode chamber in a cation exchange membrane assembled electrochemical system for enhanced hardness removal. It was found that the OH– current efficiency could reach 91.3%, when hydraulic residence time (HRT) of anode chamber and the electrode distance were 1 min and 1 cm, respectively. Then, most of the retained OH– would be transported to a separated crystallizer to react with the feed water to perform hardness removal. Besides, Na+ balance could be maintained using accumulated Na+ in the crystallizer to backfill the anode circulator, so as to strengthen the current efficiency of OH– of the cathode chamber throughout the process. Specifically, at a current density and an inflow rate of 10 mA/cm2 and of 20 mL/min, hardness removal efficiency and the energy consumption reached 92.1% and 2.26 kWh/kg CaCO3, respectively. Moreover, it was found that large specific surface area of the carbon felt adopted for precipitation and crystallization could be recycled up to 15 repeated cycles (about 180 h) in terms of the pure water permeance. Finally, aragonite was characterized to be dominant at the reaction, whereas magnesium calcite could also be observed at the end of the reaction process (12 h). In conclusion, Na+ migration as well as carbon felt dependent precipitation impelled integrated electrochemical system would increase hardness removal efficiency sustainably.