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^-1) and in different water matrices. The contributions of HO^• and reactive chlorine species (RCS), including Cl^•, Cl₂^•-, 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_p⁺ 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 × 10⁷, 1.03 × 10⁸, and 4.16 × 10⁸ M^-1 s^-1, 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 × 10⁴ (mg L^-1)^-1 s^-1. 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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