Thesis of Hisham K. Al Rawas

Defense of thesis Hicham K.Al Rawas - laboratory PC2A

Abstract :

Pesticides are semi volatile organic compounds used to control pests that risks crops, animals, humans, or the environment. When applied to the soil or crops, they have the potential to accumulate not only within the crops themselves but also to be transported through air, soil, and water across long distances, posing a major source of pollution within ecosystems. Pesticides can undergo biotic and abiotic degradation pathways. In the atmosphere, abiotic degradation of pesticides occurs mainly by photolysis, and chemical reactions initiated by radicals and atmospheric oxidants. In this thesis, we theoretically studied the environmental degradation by hydroxyl radical (HO^·) of three used pesticides: phosmet, chlorpyrifos (CPF), and fenpyrazamine (FPA), using DFT calculations at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level of theory. Three different reaction pathways were considered: formal hydrogen transfer (FHT), radical adduct formation (RAF), and single electron transfer (SET). The thermodynamics, mechanisms, and kinetics of the reactions, as well as the lifetime of these pesticides were examined in both gas and aqueous phases at different temperatures (i.e., 253-323 K). The further oxidation of main radical products obtained from the decomposition of the pesticides were studied towards different oxidizing agents in aquatic environment such as O₂, HO^·, ^·NO, and ^·NO₂ at 298 K. Finally, the ecotoxicity of these pesticides and their degradation by-products were estimated in the aquatic environment to understand their impact on aquatic species.

Keywords: pesticides, atmosphere, molecular simulations, HO^· radical, kinetics, ecotoxicity