Roles of Bromine Radicals and Hydroxyl Radicals in the Degradation of Micropollutants by the UV/Bromine Process

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Abstract

The inevitable occurrence of Br- in natural water affects the degradation kinetics of micropollutants in the UV/chlorine process, the radical chemistry of which, however, is largely unclear. As Br- in the UV/chlorine process first forms free bromine (HOBr/OBr-), this study investigated the radical chemistry of the UV/bromine process for the degradation of selected micropollutants resistant to bromine, i.e., ibuprofen and benzoate, to focus on the roles of radicals. The actual quantum yields of HOBr and OBr- by UV photolysis at 254 nm are 0.43 (±0.025) and 0.26 (±0.025) mol Einstein-1, respectively. Br• and HO• are generated first, and then, Br₂•- is formed, with the signal detectable at 360 nm by laser flash photolysis. Compared with Cl• in the UV/chlorine system, Br• exists at higher concentrations (∼10-12 M) in the UV/bromine system while HO• exists at similar concentrations. In the UV/bromine process, reactive bromine species (RBS) dominates the degradation of ibuprofen, while HO• dominates the degradation of benzoate. Br• and Br₂•- are reactive toward ibuprofen which second-order rate constants (k) were determined to be 2.2 × 109 and 5.3 × 107 M-1 s-1, respectively, by laser flash photolysis. Br• was the major RBS for ibuprofen degradation by the UV/bromine treatment, whereas Br₂•- increasingly contributed to ibuprofen degradation with increasing free bromine or Br- concentrations. Br• could be scavenged by HCO₃- and natural organic matter (NOM), and the k with NOM was determined to be 2.6 × 104 (mg/L)-1 s-1. Both Br• and Br₂•- prefer to react with ibuprofen via electron transfer with activation energy barriers (Δ‡G0_SET) of 1.35 and 7.78 kcal mol-1, respectively. RBS promoted the formation of hydroxylated products. Then free bromine, rather than RBS, was responsible for the formation of brominated products, increasing the total organic bromine (TOBr) and tribromomethane yields in the UV/bromine system. This study demonstrates for the first time the roles of RBS and HO• in micropollutant degradation in the UV/bromine process.

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