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Triclosan is a high-production volume chemical, which has become widely detected in environmental systems because of its widespread usage. Photodegradation has been identified as a major degradation pathway, but the identified photoproducts are also chemicals of concern. In this study, lower chlorinated derivatives of triclosan were synthesized to investigate the impact the chlorine substituents have on the photodegradation rate and the photoproducts produced. In addition, the photodegradation of two classes of photoproducts dibenzo-p-dioxins (DDs) and 2,2′-dihydroxylated biphenylswas also investigated. Degradation of triclosan in near-surface sunlit waters was relatively fast (t 1/2 < 5 h). Calculated degradation rates were slower for DDs and faster for dihydroxylated biphenyls in comparison to that for triclosan. In addition, the 2′-Cl substituent was critical for the high quantum yield measured for triclosan and necessary for the photodegradation mechanism that forms DDs and dihydroxylated biphenyls. The 4-Cl substituent was responsible for higher rates of light absorption and the environmentally relevant pK a . Without either of these substituents, the environmental fate of triclosan would be markedly different.

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