Atmospheric Degradation Mechanism And Kinetics Of2,3,7,8-tetrachlorinated Dibenzo-p-dioxin Initiated By O₃and NO₃

As the most toxic congener in dioxins, the research into the degradation mechanism of2,3,7,8-TeCDD is a hot research area and of great concern. OH radical, O3, and NO3radical are important active species in the atmosphere. The concentration of O3is high in the day time, while the concentration of NO3is high in the night. Gaseous O3and NO3initiated degradation of2.3,7,8-TeCDD is researched.2,3,7,8-TeCDD is decomposed and dechlorinated to form intermediates or products with lowered toxicity. The work contributes to discovering the reaction mechanism for degradation of2,3,7,8-TeCDD in real atmosphere and reducing the emission of POPs into the environment. Besides,2-chlorophenol is an important chemical precursor of PCDD/Fs. The chemical mechanism of NO3initiated degradation of2-chlorophenol is researched to provide a reference for the degradation of PCDD/Fs.In this thesis, density functional theory in quantum chemistry, is chosen to calculate the detailed gaseous reaction properties of2,3,7,8-TeCDD and its precursor2-chlorophenol. The Rice-Ramsperger-Kassel-Marcus theory (RRKM) and canonical variational transition-state theory with small curvature tunneling contribution (CVT/SCT) is used to calculate rate constants of elementary reactions.Based on the research in this thesis, the following results are obtained.(1) Four reaction channel are found for the main ozonolysis reaction of2,3,7,8-TeCDD. Ozonolysis mechanism and kinetic data are obtained. The reactions of Criegee intermediates in the presence of NO, H2O and O3are discussed, and it is concluded that Criegee intermediates may react with H2O in atmosphere to form OH radical, which can initiate further oxidation.(2) Gas-phase degradation of2,3,7,8-TeCDD initiated by NO3radical in atmosphere at night is researched, and the degradation and dechlorination mechanism are proposed. The addition of NO3to the carbon atom on central C-0ring is the most appropriate pathways with the lower barriers, and the central ring of PCDD is opened in the subsequent reactions; Cl atom is substituted after NO3addition, and the pathway for dechlorination mechanism might not be neglected for its obvious environmental significance. The profiles of the potential energy surfaces are constructed to show the reaction pathway with the lowest potential barrier, and the most possible intermediates in the reactions are pointed out.(3) NO3initiated degradation of2-chlorophenol in atmosphere is researched, and the degradation and dechlorination mechanism are obtained. NO2addition to2-chlorophenol is also researched and the formation of nitrophenol is discussed.