Direct photodegradation of aromatic carbamate pesticides: Kinetics and mechanisms in aqueous vs. non-aqueous media

Abstract

The direct photodegradation quantum yields (Φ) of five representative aromatic carbamate pesticides - carbaryl, carbofuran, propoxur, isoprocarb, and metolcarb - were examined in both aqueous and non-aqueous solutions, the latter mimicking hydrophobic environments such as leaf surfaces. For carbaryl, carbofuran, isoprocarb, and metolcarb, the Φ values generally followed the order Φ_water < Φ_MeOH < Φ_n-hexane, while propoxur showed a different trend, Φ_MeOH < Φ_n-hexane < Φ_water. Scavenging and laser flash photolysis experiments, combined with quantum chemical calculations, were used to clarify the photodegradation mechanisms. Photodegradation is primarily initiated by the singlet excited state (S*), with the triplet state (T*) also contributing in compounds with conjugated structures, such as carbaryl. Upon excitation, MCAEs generated both radical cations (S^•+) and phenoxyl radicals (S-O^•), and S^•+ would convert to S-O^• subsequently. S-O^• is predominantly generated through the cleavage of C-O bonds in ester groups, subsequently abstracting hydrogen from solvent molecules. The reactivity of hydrogen donors in these solvents follows the order: -CH₂- > -CH₃ > -OH. For propoxur, the ether group also contributes to the formation of S-O^•, which further reacts with H₂O and enhances degradation in aqueous environments. Solvent polarity had a minimal effect on photodegradation. This comparative study of degradation in aqueous and nonaqueous phases provides insights for designing and selecting pesticides that are effective during use in nonaqueous environments, such as on leaf surfaces, yet degrade rapidly in aqueous environments in the post-application phase.