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

Abstract

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 photooxidation to form radical cation species, which then likely deprotonate and react with dissolved oxygen. Additionally, pyrrole moieties can undergo reactions with singlet oxygen ( 1 O 2 ). Furthermore, the presence of electron-withdrawing or -donating substituents substantially impacted the reaction rate with 1 O 2 as well as the one-electron oxidation potential of the pyrrole that dictates reactions with triplet states of dissolved organic matter ( 3 CDOM*). For fludioxonil, which can undergo both direct and indirect photodegradation, the reaction rate constant with 1 O 2 alone resulted in a predicted t 1/2 < 2 days in waters under sunlit nearsurface 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.

Photochemistry of the Organoselenium Compound Ebselen: Direct Photolysis and Reaction with Active Intermediates of Conventional Reactive Species Sensitizers and Quenchers

“…Even though NaN 3 quenches both 1 O 2 and • OH, the quenching reaction is associated with 1 O 2 as the expected • OH yield is low due to (1) the multistep reactions involved in • OH generation (electron transfer from 3 RB 2– * to 3 O 2 to form O 2 •– → self-dismutation of O 2 •– → H 2 O 2 irradiation (254 nm)→ • OH), (2) the low initial concentration of RB (5 μM), and (3) the wavelength employed in these experiments (365 nm). Based on the EBS and NaN 3 concentrations, the rate constant for 1 O 2 quenching by NaN 3 (1.5 × 10 9 M –1 s –1 , ), and the rate constant for 1 O 2 reaction with EBS (4.16 × 10 6 M –1 s –1 ), NaN 3 quenched more than 99.99% of 1 O 2 . Direct EBS reaction with NaN 3 was kinetically negligible.…”

Linking Triclosan’s Structural Features to Its Environmental Fate and Photoproducts

“…This light source was chosen to more accurately quantify quantum yields because of the better overlap with the absorbance spectra, and previous research that used natural sunlight to determine the quantum yield and investigate the photoproducts of triclosan found similar results . Additionally, we have previously found similar quantum yields when using a xenon arc lamp and RPR-3000A bulbs; it is therefore not expected that the light source affected the experimental results . Approximately 8 mL of the solution was added to Pyrex glass test tubes, which were placed on a merry-go-round inside the photoreactor.…”

Estimation of photochemical degradation rates of pesticides in outdoor cosm water : Guidance for inclusion in higher tier exposure assessments of the registration procedure in The Netherlands or at EU level, using the TOXSWA model

“…(i) direct photolysis, in which the reacting pesticide molecule absorbs light and the excited molecule reacts, and (ii) indirect (or 'sensitized') photolysis, where light is absorbed by a molecule, which then transfers its energy to the acceptor molecule, causing the latter to react. For many substances the rate of indirect photolysis is much larger than the rate of direct photolysis (Deneer et al, 2010, p 19, but also Burrows et al, 2002and Apell et al, 2019. In section 5.2 below indirect and direct photolysis of pesticides is considered in more detail; also the likely difference between indirect photolysis rates of pesticides found in cosm studies versus rates in real-life edge-of-field watercourses has been evaluated.…”