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

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 (>5mM) 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.