Concentration profiles of chlorine radicals and their significances in •OH-induced dye degradation: Kinetic modeling and reaction pathways

In order to further understand the mechanism details during saline wastewater treatment by
• OH-based advanced oxidation processes (AOPs), the degradation efficiency of an azo dye Acid Orange 7 (AO7) in UV/H2O2 process was investigated as a function of a wide range of salinity and pH. Kinetic modeling results demonstrated that the inhibitory effect of Cl− on AO7 degradation observed in the laboratory experiments could be attributed to both scavenging effect of Cl− on
• OH and the much lower reactivity of chlorine radicals formed, although the chlorine radicals may be more abundant than
• OH. Such retardation behavior was favored under acidic conditions due to a lower yield of
• OH generated by the dissociation of ClOH
• − to
• OH and Cl−. Traces of Br− had a greater inhibitory effect on AO7 bleaching rate than Cl−. AOX (halogenated organic compounds adsorbable on activated carbon) was found to increase with the increasing content of Cl−. Based on the intermediate products and especially several toxic halogenated byproducts identified by GC–MS, the possible degradation pathways of saline dyeing wastewater were proposed.

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► Concentration profiles of chlorine radicals were obtained by kinetic modeling.
► Several refractory halogenated byproducts were identified by GC–MS measurement.
► Possible reaction pathways involving hydroxyl/chlorine radicals were proposed.
► pH effect on chloride retardation in dye photobleaching was found.