Sulfate and divalent cations recovery from municipal nanofiltration concentrate using two-step ion exchange membrane electrolysis

Municipal nanofiltration (NF) concentrate generally contains sulfate and divalent cations. In this study, the SO42−, Ca2+, and Mg2+ were recovered by the two-step ion exchange membrane (IEM) electrolysis, which included anion exchange membrane (AEM) electrolysis and subsequent cation exchange membrane (CEM) electrolysis. The results showed that the AEM electrolysis recovered calcium ions and magnesium ions by forming calcium carbonate crystals and amorphous magnesium hydroxide and simultaneously concentrated the sulfate into the anode chamber; while the CEM electrolysis promoted the formation of ettringite, amorphous magnesium hydroxide, and other precipitates with the assistance of calcium hydroxide and sodium metaaluminate, thereby recovering the sulfate and divalent cations in the NF concentrate. The calcium cation, magnesium cation, and sulfate anion concentrations were reduced by 80.0 %, 93.3 %, and 43.1 %, respectively. In addition, the dissolved organic carbon was removed by 31.0 % through the two-step membrane electrolysis. The AEM fouling was much greater than that of the CEM. The total energy consumption of the two-step IEM electrolysis process was calculated as 4.3–6.7 kWh/m3. These findings provide practical relevance for the advanced treatment of municipal NF concentrate. [Display omitted] •The two-step IEM electrolysis can effectively recover sulfate and divalent cations.•CaCO3 crystals and amorphous Mg(OH)2 dominated the precipitate in AEM electrolysis.•CEM electrolysis promoted the formation of ettringite with slight co-precipitation.•The solid contributed to organic removal of 3.66–10.96 mg TOC/g precipitate.•The CEM fouling was significantly lower than that of the AEM.