Investigations of the UV-A photochemistry of the pesticide azinphos-methyl

This project focused on the investigation of the photochemistry of the pesticide azinphos-methyl (AZM) using various techniques to explore the kinetics and reaction mechanism of its photolysis with UV-A (290 to <400 nm) light sources in the laboratory and natural environment (in natural water and under direct sunlight). A key aspect of this project was the identification of the intermediate photoproduct involved in the reaction mechanism as well as the final photoproduct(s) of AZM photolysis. Fluorescence spectroscopic techniques were used to study the kinetics of the photodegradation of AZM in a number of solvents. AZM photolysis results with various excitation wavelengths in the fluorimeter showed that complete photodegradation of the pesticide occurred in the aqueous system. In other solvents, complete conversion of the AZM was not observed, at least over the time frame used. On the other hand, kinetic studies using a photoreactor with an excitation wavelength of 350 nm showed AZM to be completely degraded. It was observed that the reaction followed consecutive first-order kinetics (A → B → C) giving a highly fluorescent intermediate B and a relatively non-fluorescent final product(s) C in all the solvents of interest. The solvent polarity effect on the photolysis of the pesticide in protic and aprotic media was examined, which showed no clear dependence on solvent polarity for the growth of the highly fluorescent intermediate (A → B), but a significant correlation was observed for the further photolysis of this intermediate (B → C). In aqueous acid buffer solution, AZM photolysis was very slow compare to that in neutral solution. The observed photolysis of AZM was also observed under direct sunlight in natural water. UV-vis, fluorescence, NMR, HPLC and LFP techniques were used to investigate and identify the intermediate and the final photoproducts. A mechanism of AZM photolysis in the laboratory environment (in water and methanol) is proposed. In this study, N-methyl anthranilic acid was identified as the highly fluorescent intermediate photoproduct. This intermediate photoproduct undergoes further photodegradation processes producing aniline and/or N-methyl aniline, relatively non-fluorescent final product(s). Benzazimide was identified as one of the final photoproducts; it is also relatively non-fluorescent, but is produced by a separate pathway from that producing the N-methylanthranilic acid as the highly fluorescent intermediate. It was also proposed that AZM photolysis promoted by sunlight follows the same mechanism found in the laboratory system. The observed photodegradation of AZM by sunlight has important implications for its fate in the environment.