Sulfate radical-based removal of chloride ion from strongly acidic wastewater: Kinetics and mechanism

Highlights: • Chloride ion is selectively and effectively removed by the heat-activation PS. • H⁺ with high concentration prompted the production of ·SO₄⁻ and ·OH. • The key step for Cl(-I) removal was the formation of ·Cl or ·Cl₂^–. • Above 96% of initial Cl(-I) was removed, and the residual Cl(-I) was ≤ 158 mg/L. • The product Cl₂ can be recycled as bleaching powder. Specific to strongly acidic wastewater, the traditional lime neutralization produces massive hazardous waste and present serious environmental risks. Thus, the recycling of purified wastewater after the contained contaminants being removed has been proposed. However, among these contaminants, chloride ion (Cl(-I)) is rather difficult to remove. This study proposes a new method to remove Cl(-I) using thermal activated persulfate (PS). Under optimized conditions, above 96% of initial Cl(-I) was removed from the actual wastewater, and the residual Cl(-I) was below 158 mg/L, which satisfies the requirement of Cl(-I) concentration for wastewater recycling. Furthermore, the mechanism was investigated. In the strongly acidic wastewater, the high concentration of H⁺ prompted the thermal activation process of PS through two pathways. (1) H⁺ prompted the transformation of S₂O₈^2- into HSO₄^- and SO₄, and then into HSO₅^- that was finally transformed into ·OH and ·SO₄⁻ at above 70 ℃. (2) H⁺ prompted the production of ·OH through the transformation of ·SO₄⁻ into ·HSO₄ and the cleavage of ·HSO₄. The key step for Cl(-I) removal was identified as the formation of ·Cl or ·Cl₂^– from the oxidation of Cl(-I) by ·SO₄⁻ and ·OH, and their contribution ratios were estimated to be 67.4% and 32.6%, respectively.