Selective removal of fluorine and chlorine from zinc sulfate solution by pre-dilution and multi-stage nanofiltration process: Feasibility and mechanisms

To achieve efficient and simultaneous removal of fluorine (F) and chlorine (Cl) from zinc (Zn) sulfate solution, a pre-dilution and multi-stage nanofiltration process was proposed. The Zn sulfate electrolyte, sourced from a Zn regeneration enterprise in Hunan Province, China, was treated via nanofiltration. The results indicated that the average recovery rates of Zn and sulfate were 98.76% and 98.12%, respectively, while the average removal rates for F and Cl were 45.24%–46.21% and 53.18%–71.30%. During the three-stage nanofiltration process, the fluoride concentration decreased from 187.05 to 27.95 mg/L, and the chloride concentration was reduced from 1400.10 mg/L to 75.82 mg/L. The total removal rate of F and Cl was 85.06% and 94.59%, respectively. The presence of ZnF⁺ and ZnCl⁺ limited further enhancements in the average removal rates for F and Cl. The polypiperazine-based composite membrane carried a negative charge due to the rapid ionization of the carboxyl group. A higher valence state of the anion and a greater negative charge favored retention by the nanofiltration membrane. The preferential molecular retention order is ZnSO₄, ZnF₂, and ZnCl₂.