Photochemical nitro-nitrite rearrangement in methyl parathion decay under tropical conditions

Abstract

This work presents a study of the abiotic degradation of commercially available methyl parathion in aqueous solution at two different concentrations (88 mg/L and 200 μg/L). The effects of solar irradiation and the presence of humic acids were evaluated and revealed a synergistic response between them. The half-life of methyl parathion ranged from 4.9 to 37 days, and the experimental data also show that photochemical processes were the most relevant in this case. The only byproduct found in samples submitted to shadowed conditions was 4-nitrophenol. On the other hand, 4-nitrophenol, methyl paraoxon and a new degradation product (O,O-dimethyl O-p-hydroxyphenyl phosphorothioate) were detected when the samples were exposed directly to sunlight. This newly identified compound was prepared in the laboratory by thiophosphorylation of hydroquinone, and coelution experiments with authentic samples provided unambiguous confirmation of the presence of O,O-dimethyl O-p-hydroxy phenylphosphorothioate in samples.

Keywords:

• Photochemical rearrangements
• nitro-nitrite rearrangement
• methyl parathion byproduct
• mass spectrometry

Acknowledgments

The authors are indebted to Jason G. Taylor and Carol H. Collins for insightful discussion and for reviewing the manuscript for its English usage. P.C.M.L.M. gratefully acknowledges financial support from FAPESP and the Institute of Chemistry of UNICAMP for the analytical facilities. M.C.C. gratefully acknowledges financial support from FAPERJ and CNPq.