Cl^• and Cl₂^•- radicals contribute to the degradation of trace organic contaminants (TrOCs) such as pharmaceutical and personal care products and endocrine-disrupting chemicals. However, little is known about their reaction rate constants and mechanisms. In this study, the reaction rate constants of Cl^• and Cl₂^•- with 88 target compounds were determined using laser flash photolysis. Decay kinetics, product buildup kinetics, and competition kinetics were applied to track the changes in their transient spectra. Cl^• exhibited quite high reactivity toward TrOCs with reaction rate constants ranging from 3.10 × 10⁹ to 4.08 × 10¹⁰ M^-1 s^-1. Cl₂^•- was less reactive but more selective, with reaction rate constants varying from <1 × 10⁶ to 2.78 × 10⁹ M^-1 s^-1. Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl₂^•- with TrOCs bearing phenol, alkoxy benzene, and aniline groups. The detection of Cl^•-adducts of many TrOCs suggested that Cl^• addition was an important reaction mechanism. Single electron transfer (SET) predominated in reactions of Cl^• with TrOCs bearing electron-rich moieties (e.g., sulfonamides), and their cation radicals were observed. Cl^• might also abstract hydrogen atoms from phenolic compounds to generate phenoxyl radicals. Moreover, Cl^• could react with TrOCs through multiple pathways since more than one transient intermediate was detected simultaneously. SET was the major reaction mechanism of Cl₂^•- reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl₂^•- was found to play an important role in TrOC degradation, though it has been often neglected in previous studies. The results improve the understanding of halogen radical-involved chemistry in TrOC degradation.

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