The roles of halides in the acetaminophen degradation by UV/H₂O₂treatment: Kinetics, mechanisms, and products analysis

This study evaluated the effects of halide ions (chloride and bromide) on the degradation of acetaminophen, a widely used drug, with the UV/H2O2advanced oxidation treatment. Chloride showed minimal effects on the acetaminophen degradation, whereas increasing bromide concentration reduced the degradation rate because bromide promoted the formation of Br2-, which had a lower reaction rate than OH. Interestingly, acetaminophen degradation was enhanced when both chloride and bromide were present in the solution. This was probably attributed to the high yield of ClBr-radicals predicted by using the Kintecus model. Additional tests using five model compounds (4-chlorophenol, 4-methoxyphenol, 4-hydroxylbenzoic acid, 17β-estradiol, and bisphenol A) suggested that ClBr-radicals selectively reacted with electron-rich moieties of the organic pollutants. In the presence of natural organic matter (NOM), increasing concentration of chloride or bromide reduced the degradation rate of acetaminophen. In contrast, with a constant concentration of co-existing chloride, the acetaminophen degradation was promoted by increasing bromide concentration. The product analysis showed that the HO.and halide radicals resulted in different pathways of acetaminophen degradation. The HO.radicals favored electrophilic aromatic substitution at the ortho and para positions, forming the -OH substituted acetaminophen, whereas halide radicals preferentially led to one-electron oxidation and formation of phenoxyl radicals as intermediates. These results highlight the multiple roles of salinity in water and wastewater treatment. In addition to acting as HO.scavengers, halogen radicals may significantly contribute to the organics degradation via their high selectivity towards electron-rich moieties. The resultant intermediate products require further toxicity assessment.