Mechanism, kinetics and toxicity assessment of OH-initiated transformation of triclosan in aquatic environments

• Computational approach is effective to reveal the transformation process of TCS.
• Dioxins can be formed via the cyclization of H-abstraction product at low [OH].
• When [OH] < 7.39 × 10−18 M in water, TCS will be considered as potential POPs-like.
• More toxic intermediates are produced, particularly for the chronic effects.

The mechanisms and kinetics of OH-initiated transformation of triclosan (TCS) in aquatic environments were modeled using high-accuracy molecular orbital theory. TCS can be initially attacked by OH in two ways, OH-addition and H-abstraction. Twelve OH-addition routes were reported, and the C atom adjacent to the ether bond in the benzene ring (RaddB1) was found as the most easily attacked position by OH, producing TCS–OHB1. Seven H-abstraction routes were reported, and the OH exclusively abstracted the phenolic hydroxyl (RabsOH) H atom, to form TCS(−H). The kinetics results showed that the RaddB1 and RabsOH routes would occur preferentially in aquatic environments, and the half-life depended on the OH concentration ([OH]). At low [OH], the main intermediates, TCS–OHB1 and TCS(−H), can be converted into 2,4-dichlorophenol and polychlorinated dibenzo-p-dioxins, respectively. However, when enough OH is present, such as in advanced oxidation process (AOP) systems, they would be fully decomposed. The acute and chronic toxicities of TCS and its products were assessed at three trophic levels using the “ecological structure–activity relationships” program. The toxicity of the products decreased through the RaddB1 route, while the toxicity of the products first increased and then decreased through the other degradation routes. These results should help reveal the mechanism of TCS transformation as well as risk assessment in aquatic environments, and will help design further experimental studies and industrial application of AOPs.

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