Sandrin, Deborah (2011) Studio dei meccanismi di fotodegradazione del benzil 4-idrossibenzoato (benzilparabene). [Laurea specialistica biennale]
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: Photodegradation is an important mechanism for removal of many organic pollutants of natural waters. This thesis reports and discusses the results of a study of the photochemical reaction in an aqueous solution of an important component of many cosmetic formulations and products for personal hygiene, benzyl paraben (4-hydroxybenzoic acid benzyl ester). This compound, and more generally, parabens, concern for the possible effects on the endocrine system of humans. I made a preliminary characterization of the photophysical behavior of benzyl paraben: has led to the determination of fluorescence quantum yields (0.33%), phosphorescence (69%) and triplet excited state formation by sensitized delayed fluorescence (65%). I then conducted experiments to photodegradation under different experimental conditions with the aim to characterize the speed of the process and products and to propose a hypothesis for the mechanism of reaction. The experiments concerning the direct photolysis with UV-Vis in three different environments (air, oxygen and argon), photolysis with NUV-Vis in the presence of a photosensitizer (two have been tested, humic acid and the Rose Bengal) and photocatalysis with TiO2. In each case, were analyzed both the evolution of the kinetic reaction intermediates and products formed. Have also been conducted a few experiments to test the possible involvement of hydroxyl radical in the reaction, using two probes specific chemical (hydrogen peroxide, known source of OH radicals, and alcohol terbutilico known quencher of OH radicals). For kinetic determinations of solution samples taken at various times of treatment were analyzed by a simple measurement of absorbance at 256 nm and by HPLC-UV . The first method has often provided unreliable results due to contributions to the absorbance measurement due to photo products of the reaction, it was possible to eliminate interference in the second method of investigation. With HPLC-UV have been possible to follow not only the disappearance of the reagent but also the formation and eventual disappearance of intermediates and reaction products. A purely qualitative and preliminary characterization of these products was performed by HPLC-MS and HPLC-MS/MS using an ESI ionization source in negative mode. The main results indicate that, in experiments of direct photolysis, molecular oxygen slows the photodegradation of benzyl paraben, suggesting a mechanism of reaction through the singlet excited state produces the reactive species of benzyl paraben in the triplet state. This species undergoes a break and / or 'other of the two CO bonds of the ester group,-C (= O)-OC-forming free radicals which, through different reaction paths may explain the origin of products postulates. The results of experiments conducted in the presence of hydrogen peroxide and alcohol terbutilico agree to exclude a major involvement of the OH radical in the degradation of benzylparaben induced by direct photolysis. I also checked for radiation in the NUV-Vis benzyl paraben is stable but undergoes degradation in the presence of a photo sensitizer, humic acid is that the Rose Bengal, although with different kinetic efficiency and course. Finally, I made an experiment of photocatalysis with TiO2 for determining the kinetic of benzyl paraben and the structure of products. The kinetics suggest that the degradation of benzyl paraben occurs through two distinct processes, one of the first order, probably on the surface of the photo catalyst, the other zero-order, with the reactive species which diffuse in solution. This hypothesis is in agreement with the results of the analysis of total organic carbon, TOC. Among the reaction products were identified as phenol and other intermediate advanced stages of oxidation such as acids unsaturated maleic / fumaric.
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