Sustainable strategies of in-series AEM electrolysis and CEM electrolysis for the treatment of NF concentrates via ettringite-targeted mineral transition

A two-step ion exchange membrane electrolysis process was designed to recover divalent cations (Ca²⁺, Mg²⁺) and sulfate from nanofiltration concentrates (NFC) and investigate their removal performance and membrane fouling mechanism. As a result, the AEM electrolysis recovered divalent ions by generating calcium carbonate and magnesium hydroxide in the cathode chamber, while sulfate was enriched in the anode chamber. In the subsequent CEM electrolysis, desalination was achieved by adding chemicals to form ettringite precipitation in the cathode chamber. Meanwhile, the organics in nanofiltration concentrate were removed under the oxidation of platinum-coated titanium electrode and the precipitation adsorption of the cathode chamber. By comparing the desalination performance and organic matter removal, the dosage of ettringite was determined. When the Ca/S = 2.5 and Al/S = 1.0, it was beneficial to the recovery of ettringite. Furthermore, the membrane fouling mechanism was investigated to find that AEM fouling was more severe than CEM. Therefore, this study provides a technical idea and feasibility study for the treatment of municipal nanofiltration concentrate.