Dissolved oxygen inhibits the promotion of chlorothalonil photodegradation mediated by humic acid

Chlorothalonil (CT) is widely used to control fungal diseases, and it is commonly detected in the environment. Phototransformation is an important process which removes CT from aquatic environments. The kinetics and mechanisms of CT photodegradation in solutions with and without humic acid (HA) were investigated, and the influence of dissolved oxygen (DO) on the photodegradation process was also considered. The rate constant observed in pure water was only 0.017 h −1 , but it increased to 0.083 h −1 with 5.0 mgC L −1 of HA present. Reactive species generated by HA were shown to play an important role. Moreover, the degradation of CT was promoted at higher HA concentrations. But higher DO concentrations inhibits CT degradation because of the reaction between DO and excited triplet state CT ( 3 CT*) or HA ( 3 HA*). The contributions of the reactive species tested were in the sequence 3 HA* >  1 O 2  > H 2 O 2 /O 2 −  ≈  OH in a nitrogen-saturated system, but in an oxygen-saturated system the order was OH ≈ 1  O 2  > H 2 O 2 /O 2 −  >  3 HA*. 3 CT* was confirmed as the active form of CT during degradation, with 4-hydroxy-chlorothalonil and 2,4,5-trichloroisophthalonitrile as the main products. Compared to direct photodegradation, CT photodegradation mediated by HA can decrease the toxicity of an aquatic environment as demonstrated through tests with Scenedesmus obliquus.