Effects of chloride ion on degradation of Acid Orange 7 by sulfate radical-based advanced oxidation process: Implications for formation of chlorinated aromatic compounds

► A dual effect of Cl − on Acid Orange 7 degradation in Co/PMS systems was observed. ► High concentrations of Cl − (>5 mM) did greatly inhibit dye mineralization. ► Some refractory chlorinated byproducts were identified by GC–MS measurement. ► Two possible ( SO 4 − radical-based and non-radical) reaction pathways were proposed. Sodium chloride is a common salt used during textile wet processes. Here a dual effect of chloride (i.e. inhibitory and accelerating effect) on azo dye (Acid Orange 7, AO7) degradation in an emerging cobalt/peroxymonosulfate (Co/PMS) advanced oxidation process (AOP) was reported. Compared to OH-based AOPs, high concentrations of chloride (>5 mM) can significantly enhance dye decoloration independent of the presence of the Co 2+ catalyst, but did greatly inhibit dye mineralization to an extent which was closely dependent upon the chloride content. Both UV–vis absorbance spectra and AOX determination indicated the formation of some refractory byproducts. Some chlorinated aromatic compounds, including 3-chloroisocoumain, 2-chloro-7-hydroxynaphthalene, 1,3,5-trichloro-2-nitrobenzene and tetrachlorohydroquione, were identified by GC–MS measurement in both Co/PMS/Cl − and PMS/Cl − reaction systems. Based on those experimental results, two possible branched ( SO 4 − radical-based and non-radical) reaction pathways are proposed. This is one of the very few studies dealing with chlorinated organic intermediates formed via chlorine radical/active chlorine species (HOCl/Cl 2) attack on dye compounds. Therefore, this finding may have significant technical implications for utilizing Co/PMS regent to detoxify chloride-rich azo dyes wastewater.