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

•2-Br-4-CP and 2-Cl-4-BP could be effectively degraded in the PMS/X− system.•The release of endogenic X was determined by the initial level of exogenic X−.•2-Br-4-CP is more easily degraded/mineralized, in line with theoretical prediction.•The importance of endogenic X position on pollutant transformation is emphasized.