The UV/chlorine process is an emerging advanced oxidation process (AOP) used for the degradation of micropollutants. However, the radical chemistry of this AOP is largely unknown for the degradation of numerous structurally diverse micropollutants in water matrices of varying quality. These issues were addressed by grouping 34 pharmaceuticals and personal care products (PPCPs) according to the radical chemistry of their degradation in the UV/chlorine process at practical PPCP concentrations (1 μg L) and in different water matrices. The contributions of HO and reactive chlorine species (RCS), including Cl, Cl, and ClO, to the degradation of different PPCPs were compound specific. RCS showed considerable reactivity with olefins and benzene derivatives, such as phenols, anilines, and alkyl-/alkoxybenzenes. A good linear relationship was found between the RCS reactivity and negative values of the Hammett ∑σ constant for aromatic PPCPs, indicating that electron-donating groups promote the attack of benzene derivatives by RCS. The contribution of HO, but not necessarily RCS, to PPCP removal decreased with increasing pH. ClO showed high reactivity with some PPCPs, such as carbamazepine, caffeine, and gemfibrozil, with second-order rate constants of 9.2 × 10, 1.03 × 10, and 4.16 × 10 M s, respectively, which contributed to their degradation. Natural organic matter (NOM) induced significant scavenging of ClO and greatly decreased the degradation of PPCPs that was attributable to ClO, with a second-order rate constant of 4.5 × 10 (mg L) s. Alkalinity inhibited the degradation of PPCPs that was primarily attacked by HO and Cl but had negligible effects on the degradation of PPCPs by ClO. This is the first study on the reactivity of RCS, particularly ClO, with structurally diverse PPCPs under simulated drinking water condition.

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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‡ G 0 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.