Simultaneous removal of chlorite and coexisting emerging organic contaminants by sulfite: Kinetics and mechanisms

The inevitable formation of chlorite (ClO₂^–) and the ineffective degradation of some emerging organic contaminants (EOCs) during chlorine dioxide disinfection have aroused increasing concern about their adverse effects on human health. In this study, the kinetics and mechanisms of simultaneous removal of ClO₂^– and coexisting EOCs by sulfite (S(IV)) were investigated. Under aerobic conditions, ClO₂^– and coexisting EOCs could be effectively removed by S(IV) at pH 6.5–7.5, and their removal rates decreased as pH increased. ClO₃^– was generated in this process but its amount decreased with increasing pH. Many lines of evidence verified that the reaction of ClO₂^– with HSO₃^– and H⁺ generated SO₃^•− and ClO^•, and the generated ClO^• was rapidly quenched by S(IV) to form OCl^– and SO₃^•−. Subsequently, SO₃^•− quickly reacted with O₂ to form SO₅^•−, which was further transformed into SO₄^•−. SO₄^•− was the dominant reactive oxidant contributing to the degradation of coexisting EOCs and the formation of ClO₃^–. Using S(IV) to simultaneously control ClO₂^– and coexisting EOCs in water treatment is a promising approach because its performance was slightly affected by the water matrix.