Photodegradation of Fludioxonil and Other Pyrroles: The Importance of Indirect Photodegradation for Understanding Environmental Fate and Photoproduct Formation

Fludioxonil is a pyrrole-containing pesticide whose registration as a plant protection product is currently under review in the United States and Europe. There are concerns over its potential persistence and toxicity in the aquatic environment; however, the pyrrole moiety represents a potential reaction site for indirect photodegradation. In this study, the direct and indirect photodegradation of fludioxonil, along with pyrrole, 3-cyanopyrrole, and 3-phenylpyrrole, were investigated. Results showed that pyrrole moieties are capable of undergoing direct photoionization and sensitized photo-oxidation to form radical cation species, which then likely deprotonate and react with dissolved oxygen. Additionally, pyrrole moieties can undergo reactions with singlet oxygen (O-1(2)). Furthermore, the presence of electron-withdrawing or -donating substituents substantially impacted the reaction rate with O-1(2) as well as the one-electron oxidation potential of the pyrrole that dictates reactions with triplet states of dissolved organic matter ((CDOM)-C-3*). For fludioxonil, which can undergo both direct and indirect photodegradation, the reaction rate constant with O-1(2) alone resulted in a predicted t(1/2) < 2 days in waters under sunlit near-surface conditions, suggesting it will not be persistent in aquatic systems. These results are useful for evaluating the environmental fate of fludioxonil as well as other pyrrole compounds.