Transformation of endogenic and exogenic Cl/Br in peroxymonosulfate-based processes: The importance of position of Cl/Br attached to the phenolic ring

Transformation of halogenated organics in advanced oxidation processes (AOPs) have been extensively investigated, however, there is little information about the fate of polyhalogenated pollutants (-Cl, -Br coexist) particularly while exogenic halides (X⁻, Cl⁻ or Br⁻) are present. Herein, the oxidation of isomeric compounds of 2-bromo-4-chlorophenol (2-Br-4-CP) and 2-chloro-4-bromophenol (2-Cl-4-BP) were comparatively investigated in the presence of peroxymonosulfate (PMS) and various concentrations of exogenic Cl⁻ or Br⁻. Both 2-Br-4-CP and 2-Cl-4-BP could be efficiently degraded and gradually mineralized in the PMS/X⁻ systems. Based on Gas Chromatography-Mass Spectrometer (GC–MS) data, different chlorinated products and brominated products were identified in the PMS/X⁻ systems during the degradation of two compounds. The release of endogenic X was determined by the initial level of exogenic X⁻, where the low initial exogenic X⁻ resulted in a high AOX accumulation. Endogenic halogen seems more liable to release in PMS/Br⁻ system than PMS/X⁻ system. The faster degradation rate and higher mineralization degree of 2-Br-4-CP than 2-Cl-4-BP were successfully predicted by frontier orbital theory and mulliken atomic charge distribution. The importance of the position of halogen atoms on dehalogenation, halogenation and degradation processes is emphasized. These findings also demonstrate the necessity of 1) monitoring the X⁻ levels in wastewater prior to application of PMS-based oxidation processes and 2) precise identification of halogen position in byproducts analysis for predicting their fates.