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

Abstract

Cl • and Cl 2 •– 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 Cl 2 •– 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 × 10 9 to 4.08 × 10 10 M –1 s –1 . Cl 2 •– was less reactive but more selective, with reaction rate constants varying from <1 × 10 6 to 2.78 × 10 9 M –1 s –1 . Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl 2 •– 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 Cl 2 •– reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl 2 •– 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.