Rate Constants and Mechanisms of the Reactions of Cl• and Cl2•- with Trace Organic Contaminants

Cl-center dot and Cl-2(center dot-) 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-center dot and Cl(2)(center dot-)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-center dot exhibited quite high reactivity toward TrOCs with reaction rate constants ranging from 3.10 X 10(9) to 4.08 X 10(10) M-1 s(-1 )Cl(2)(center dot-) was less reactive but more selective, with reaction rate constants varying from <1 X 10(6) to 2.78 X 10(9) M-1 s(-1). Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl(2)(center dot- )with TrOCs bearing phenol, alkoxy benzene, and aniline groups. The detection of Cl-center dot adducts of many TrOCs suggested that Cl-center dot addition was an important reaction mechanism. Single electron transfer (SET) predominated in reactions of Cl-center dot with TrOCs bearing electron-rich moieties (e.g., sulfonamides), and their cation radicals were observed. Cl-center dot might also abstract hydrogen atoms from phenolic compounds to generate phenoxyl radicals. Moreover, Cl-center dot could react with TrOCs through multiple pathways since more than one transient intermediate was detected simultaneously. SET was the major reaction mechanism of Cl-2(center dot-) reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl-2(center dot-) 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.