Removal of Cl(I) from zinc sulfate electrolyte by the porous Bi2O3 rich in OVs: Efficiency and mechanism

During the zinc electrolysis, the concentration of Cl(I) impurity in the zinc sulfate electrolyte needs to be controlled below 100 mg/L. In the promising approach for Cl(I) removal from the zinc sulfate electrolyte by bismuth oxide (Bi2O3), the low efficiency and high reagent dosage, arising from the limited solubility of Bi2O3, are the major technical bottleneck to be solved. In this study, an improved approach for the removal of Cl(I) from zinc sulfate electrolyte by the porous Bi2O3 rich in oxygen vacancies (OVs) was explored. For the common zinc sulfate electrolyte, a high efficiency above 90 % was always obtained under the stoichiometric mole ratio of reagent to Cl(I) under room temperature, when the Bi2O3 being irradiated for 300 min under ultraviolet (UV) was utilized. 98.9 % of the employed Bi2O3 was regenerated after the treatment using sodium hydroxide (NaOH) solution, and thus, Cl(I) was finally transferred from the zinc sulfate electrolyte to the concentrate. Thanks to the improved Bi2O3 dissolution caused by the porous surface and the adsorption of Cl(I) on OVs, the transformation process of inner Bi2O3 to BiOCl can be effectively improved, leading to the significant enhancement in Cl(I) removal efficiency and decrease in reagent dosage.