Probing the radical chemistry in UV/persulfate-based saline wastewater treatment: Kinetics modeling and byproducts identification

The effect of Cl^- on the oxidative degradation of Acid Orange 7 (AO7) was investigated in UV/S₂O₈^2- system to elucidate the chlorination pathways in saline wastewaters. Lower amount of Cl^- as well as Br^- enhanced the decoloration of AO7, but such promotion effect reduced gradually with the increasing halide ion dosage. The dye mineralization was found to be inhibited by Cl^-, especially under acidic conditions. Results of kinetics modeling demonstrated that the fraction of different oxidizing radicals largely depended on the content of Cl^-. At the initial pH of 6.5, Cl₂^- was much more abundant than SO₄^-. The significance of Cl₂^- for AO7 degradation increased with the increasing Cl^- concentration and overwhelmed that of SO₄^- at [Cl^-]>1mM. Without Cl^-, SO₄^- was the predominant radical for AO7 degradation under acidic conditions, while OH prevailed gradually at higher pH. Under high salinity conditions, more OH can be formed and contributed to the dye degradation especially in alkaline medium, leading to higher destruction efficiency of AO7. Several chlorinated byproducts were detected in the presence of chloride ions, and SO₄^-/Cl₂^--based degradation pathways of AO7 were proposed. This work provides further understanding of the complex reaction mechanisms for SO₄^--based advanced oxidation processes in chloride-rich environments.