Etude de la dégradation par photolyse directe de pesticides - Caractérisation structurale et toxicité potentielle des photoproduits

Abstract : Pesticides belong to the large family of organic pollutants. In general, they are intended to fight against crop pests. Distribution of pesticides in nature creates pollution in DIFFERENT compartments of the biosphere (water, soil and air) and can induce acute toxic effects on human beings of the terrestrial and aquatic living biomass. It is now shown that some pesticides are endocrine disruptors and are particularly carcinogenic and mutagenic effects in humans. Pesticides can undergo various processes of transformation in the natural life cycle (biodegradation, volatilization, solar radiation ...) or following applied in the sectors of natural water purification and treatment stations sewage treatment. The presence of degradation products of pesticides in our environment is even more alarming that their structures and potential toxicities generally unknown. Molecules belonging to two families of pesticides were selected for this study: herbicides, represented by metolachlor, and fungicides represented by procymidone, pyrimethanil and boscalid. The first part of the thesis focused on the development of an analytical strategy to characterize the structures of compounds from degradation by photolysis of pesticides. The second part focused on estimating the toxicity of degradation products using a test database in silico. Identification of degradation products was achieved through two complementary analysis techniques: the gas chromatography coupled to a mass spectrometer "multi-stage" (GC-MSn) and liquid chromatography coupled to a tandem mass spectrometer (LC-MS/MS). The estimation of the toxicity of the degradation products was performed using the TEST program QSAR recently developed to try to predict the toxicity of molecules. The strategy of the structural elucidation of degradation products of pesticides studied is based on studying of the mechanisms of fragmentation of parent molecules of the degradation products. The molar mass of parent molecules was clarified by the identification of mass-to-charge (m /z) of the pseudo molecular ions obtained using different modes of ionization in mass spectrometry: electronic ionization (EI) and chemical ionization (CI) in GC-MS and electrospray ionization (ESI) in LC-MS. The strategy used for the structural elucidation of degradation products has been very effective; most of the chemical structures of the products formed have been elucidated. A kinetic study was performed to visualize the appearance and disappearance of degradation products during the photolysis and gave us an idea of the rate of disappearance of the parent compound for each pesticide. The results showed that the photolysis méthalochlore, procymidone and boscalid were degraded under UV light after a while releasing degradation products with structures close or different from the parent molecules. While, pyrimethanil presented a high stability during 8 hours of irradiation (remaining amount after irradiation is 60%) and gave degradation products with moderate amounts. Regarding the estimation of the toxicity, most identified degradation products are equivalent or greater than those of the original molecules toxicities. The chemical analysis methods used and the estimated toxicities of the identified degradation products are proven complementary and indispensable for highlighting the presence of other toxic pollutants that emerge in the real environment without control.
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Ahmad Rifai. Etude de la dégradation par photolyse directe de pesticides - Caractérisation structurale et toxicité potentielle des photoproduits. Chimie analytique. Ecole Polytechnique X, 2013. Français. ⟨pastel-00869289⟩

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