Efficient recovery of divalent metals from nanofiltration concentrate based on a hybrid process coupling single-cation electrolysis (SCE) with ultrafiltration (UF)

The concentrate of high-pressure-driven membranes is large of discharge and is rich in divalent metals, imparting it with a significant value that favors its use rather than direct discharge. In this work, a combined process coupling single-cation electrolysis (SCE) with ultrafiltration (UF) was developed to recycle calcium and magnesium from the nanofiltration (NF) concentrate. The recovery ratio of metals over the running time for the SCE-UF process was evaluated, and membrane fouling behaviors were investigated. The results showed that the combined process obtained a 92% purity of calcium after 30 min of electrolysis but that it was accompanied with a slight aggravation of UF membrane fouling. When prolonging the electrolysis time to 90 min, the purity of calcium was maintained above 80%, and membrane fouling was significantly alleviated. The mechanisms for membrane fouling mitigation were explored by real NF concentrate and synthetic solution. Membrane fouling was controlled due to molecular weight decrease and organics structural shifts in the electrooxidation process. In addition, the crystals of CaCO₃ and Mg(OH)₂ were beneficial for tuning the structure of the cake layer on the membrane surface. The total energy consumption of the SCE-UF process was approximately 0.306–0.659 kWh/m³. Therefore, the efficient recovery of calcium and magnesium were feasible in the SCE-UF process for the NF concentrate treatment.