Chlorine incorporation into dye degradation by-product (coumarin)in UV/peroxymonosulfate process: A negative case of end-of-pipe treatment

Recently, UV/peroxymonosulfate (PMS)seems as a panacea for the treatment of recalcitrant organic pollutants; however, the presence of high concentration of chloride in saline wastewater indeed complicates this end-of-pipe technology. Here a negative case of UV/PMS for the treatment of one of secondary degradation byproducts of dyes (coumarin, COU)is demonstrated. The removal rate of COU is reduced by addition of Cl⁻ (0–10 mM). Further increase in Cl⁻ content favors a rapid COU degradation, whereas Cl⁻ involvement seems to open a “Pandora's box”: 1)a variety of chlorinated organic intermediates such as 4-chloroisocoumarin and 5-chloro-2-hydroxy-benzaldehyde are identified; 2)Accumulation and relative increase of absorbable organic halogen (AOX)with reaction time in the presence of high levels of chloride are observed; 3)the acute toxicity of the treated COU solution increases; 4)mineralization rate of COU decreases with the increasing [Cl⁻]. The fluorescence intensity in the UV/PMS/COU system declines with the addition of Cl⁻, implying the scavenging effects of chloride on hydroxyl radicals. The possible reaction pathways of COU are discussed. These findings highlight the imperativeness of minimizing auxiliary salt dosages in dyeing processes (i.e., source reduction)and developing new end-of-pipe technologies that can work in a saline environment.