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.