UV-Improved Removal of Chloride Ions from Strongly Acidic Wastewater Using Bi2O3: Efficiency Enhancement and Mechanisms

Strongly acidic wastewater generated from nonferrous metal smelting industries can be recycled as sulfuric acid after the contaminants have been removed, and among which, Cl^- is rather difficult to remove. Although previous studies showed that Cl^- can be removed from acidic Zn electrolyte by Bi₂O₃, this method still suffers from low efficiency when being employed for strongly acidic wastewater recycling. Otherwise, very high Bi₂O₃ dosage and H₂SO₄ concentration are required, leading to the need for improvement. In this study, UV irradiation was employed to improve the removal, and it was found that Cl^- removal efficiency was substantially enhanced from 63.9 to 98.3%, the optimum Bi₂O₃/Cl^- mole ratio was lowered from 1.5:1 to 0.5:1, and to achieve the maximum removal efficiency, the required H₂SO₄ concentration was lowered from 70 to 40 g/L. The mechanisms were also elaborated. First, Bi₂O₃ dissolves under the function of UV and H⁺, and the produced Bi³⁺ combines with H₂O and Cl^- to form BiOCl. Then, Bi₂O₃/BiOCl transforms into BiOCl(h⁺)/Bi₂O₃(e^-) under UV irradiation, and the generated h⁺ oxidizes Cl^- to Cl^•. Finally, Cl^• reacts with Bi₂O₃/e^- to produce BiOCl. This study offered a theoretical foundation for the improvement of Cl^- removal from strongly acidic wastewater.