Effects of exogenic chloride on oxidative degradation of chlorinated azo dye by UV-activated peroxodisulfate

Recently, the degradation of organic compounds in saline dye wastewater by sulfate radicals (SO₄^[rad]−)-based advanced oxidation processes (AOPs) have attracted much attention. However, previous studies on these systems have selected non-chlorinated dyes as model compounds, and little is known about the transformation of chlorinated dyes in such systems. In this study, acid yellow 17 (AY-17) was selected as a model of chlorinated contaminants, and the degradation kinetics and evolution of oxidation byproducts were investigated in the UV/PDS system. AY-17 can be efficiently degraded (over 98% decolorization) under 90 min irradiation at pH 2.0–3.0, and the reaction follows pseudo-first order kinetics. Cl^‒ accelerated the degradation of AY-17, but simultaneously led to an undesirable increase of absorbable organic halogen (AOX). Several chlorinated byproducts were identified by liquid chromatography-mass spectrometry (LC–MS/MS) in the UV/PDS system. It indicates that endogenic chlorine and exogenic Cl^– reacted with SO₄^[rad]− to form chloride radicals, which are involved in the dechlorination and rechlorination of AY-17 and intermediates. The possible degradation mechanisms of AY-17 photooxidative degradation are proposed. This work provides valuable information for further studies on the role of exogenic chloride in the degradation of chlorinated azo dyes and the kinetic parameters in the PDS-based oxidation process.