The Formation And Degradation Mechanisms Of The Typical Toxic Organic Pollutants Studied With Quantum Chemical Methods

Persistent organic pollutants(POPs) present in the environment at low levels, however they have significant influents in ecosystem and human health.A large number of industry processes and man-kind activities are possible to produce these toxic organic pollutants which will be released into the environment.There are high probabilities of occupational and non-occupational exposures for humans.Therefore, it is of great significance to study the produce and transformation mechanism of these pollutants for pollution prevention and environment protection.In this thesis,high level quantum chemistry calculations have been used to study the crucial process of the transformation of four kinds of toxic organic pollutants which have fundamental influence on global environment and human health.These four kinds of toxic organic pollutants are polycyclic aromatic hydrocarbons,pesticide, dioxin-like pollutants and perfluorooctanoic acid related pollutants,respectively.All of the crucial processes were studied systematically;reaction mechanisms were elucidated in detail.The reaction rate constants of critical elementary processes were calculated using canonical variational transition-state theory(CVT) with small curvature tunneling corrections(SCT).The important and meaningful results in this thesis can be summarized as follows:1.The Mechanism for Atmospheric Oxidation of Polycyclic Aromatic AydrocarbonsPolycyclic aromatic hydrocarbons(PAHs) is one of the most aboudant environmental carcinogenic pollutants.More than 1/3 of the environmental carcinogenic pollutants were PAHs.They have drawn substantial environmental attention due to their carcinogenicity,teratogenicity,mutagenicity and bioaccumulativity,although their production volume is less than the traditional pollutants.Polycyclic aromatic hydrocarbons are ubiquitous in the atmosphere.They are emitted into the atmosphere during incomplete combustion of fossil fuels and biomass,coal gas,asphalt production and so on.Vehicle emission also represents an important source in urban areas.40 millions of tons of hydrocarbon compounds(a fraction of which were PAHs) were emitted from automobile.Furthermore,PAHs also represent an important kind of indoor pollutants.PAHs are reactive and can undergo atmospheric reactions with the ubiquitous OH radicals,O₃,NO_x and so on,which Will result in more carcinogenic or more mutagenic secondary pollutants.These secondary pollutants will deposit to the earth ground through wet/dry process and then accumulated in the soil,which will have adverse impact on the ecosystem.In this thesis,the microscopic mechanisms of various complicated reaction processes of polycyclic aromatic hydrocarbons were studied using quantum chemistry calculations. The results show that the reactions of polycyclic aromatic hydrocarbons with OH or NO₃ radicals will form OH-PAH or NO₃-PAH adducts,which will undergo further reactions with NO₂ and O₂.A H₂O molecule will be eliminated after the association of OH-PAH with NO₂ to form nitro-PAHs,while a HNO₃ will be eliminated after the association of NO₃-PAH with NO₂ to form nitro-PAHs.The elimination of H₂O was proceeded via a four member ring transition state,while the elimination of HNO₃ was proceed via a five member ring transition state.As a result,the elimination of H₂O will cross a higher potential barrier than the elimination of HNO₃.Therefore,the formation of nitro-PAH is more preferred in the NO₃ radical initiated atmospheric oxidation of polycyclic aromatic hydrocarbons than in the OH radical initiated atmospheric oxidation of polycyclic aromatic hydrocarbons.Typically,nitro-PAH is more carcinogenic than PAH.The research results were published on Chem.Phys. Lett.and Can.J.Chem.2.The Homogeneous Gas-phase Formation of PCDD/Fs from ichlorophenols PCDD/Fs are typical endocrine disrupting chemicals and persistent organic pollutants(POPs) which have been enlisted in Stockholm Convention.Furthermore, PCDD/Fs were byproducts that were never intentionally synthesized,it is of great importance to study the formation of PCDD/Fs.There was a lack of the knowledge of the mechanism of the formation of PCDD/Fs due to the high toxicity of PCDD/Fs and limitations of detection methods.The rate constants of many elementary steps were assigned to approximate values in many modeling studies due to the absence of reaction rate constants involved in the formation of PCDD/Fs,which will result in great uncertainties.2,3,7,8-TeCDD is the most toxic one among all 210 PCDD/F isomers.2,4,5-TCP is an important processor of 2,3,7,8-TeCDD.2-CP is one of the simplest chlorophenols.In this thesis,quantum chemistry calculations were used to study the formation mechanism of the PCDD/Fs.Arrhenius equations over wide range temperatures were reported firstly.The related research results were published on Environ.Sci.& Thechnol.which is a famous journal in the environmental science and engineering field.3.OH Radical Initiated Atmospheric Photooxidation of DDVPPesticide has produced important benefits on agriculture by controlling parasites and insects.However,the increasing use of pesticide is causing serious environmental pollution problems.Organic pesticide is semi-volatile and persistent.They are very likely to vaporize into atmosphere in the process of using and transporting to remote regions through atmosphere movement,which could harm human health and ecosystem seriously.Organic pesticide has been detected in the fresh snow on East Rongbuk Glacier of Mount Qomolangma during a study carried out by Peking University and Institute of Tibetan Plateau Research of Chinese Academy of Sciences. The detection frequency of DDVP in rainwater is up to 65%in Japan and highest concentration is found to be 0.33μg/L.Recently,the migration,transformation and degradation of organic pesticide have drawn substantial attention.A paper published on Environ.Sci.Techol.by Feigenbrugel reported their experimental investigation into the degradation products from the reaction of DDVP with OH radicals.However, most of the degradation products were not identified because of the absence of reference spectra.In this thesis,high level quantum chemistry calculations were used to study the degradation mechanism of DDVP in the atmosphere.Potential energy surface was constructed,and the possible dominate degradation channels were discussed.Two energetically possible degradation pathways were revealed for the first time.The additional products that have not been identified experimentally by Feigenbrugel et al.were predicted.The related research results were published on Environ.Sci.& Thechnol.in 2007.4.The Source of Arctic PFOAPerfluorinated carboxylic acids(PFCAs) are highly persistent in the environment.Some PFCAs have been shown to cause development delays and cancer. Perfluorooctanoic acid(PFOA) is a typical representative.They have been found ubiquitous in tissues of wildlife and human from midlatitudes to the poles.Local usage would not be expected to contribute significantly to contamination in the poles. Fluorotelomer alcohols(FTOHs) have been prevailingly regarded as the most significant precursors for a long time,because FTOHs are volatile and have a sufficiently long atmospheric lifetime(～20 d) to enough them transport to Arctic Pole. Long-chain fluorotelomer alcohols,F(CF₂CF₂)_nCH₂CH₂OH(n = 2,3,4),have been detected in the atmosphere and have adverse health impact.The atmospheric reactions with OH and NO₃ radicals initiate the oxidation of FTOHs.In this thesis,the reactions of FTOHs with OH and NO₃ radicals have been studied theoretically using high-level molecular orbital theory.The rate constants are deduced using canonical variational transition-state theory(CVT) with small curvature tunneling corrections(SCT) over a wide temperature range 180～370 K.The atmospheric lifetime of FTOH was formerly calculated to be～20 d by the reactions of FTOH with OH radicals.However,with the consideration of the contribution of NO₃ radicals,the atmospheric lifetime of FTOH is shortened from～20 d to～11 d.Meanwhile,it takes about 1 to 2 months to mix a species throughout a hemisphere under typical atmospheric conditions.As a result,it becomes suspectable whether the atmospheric lifetime of FTOH remains long enough to allow FTOH transport to the Arctic Pole.Therefore,the source of Arctic PFOA needs to be reconsidered.It is possible that other transport mechanisms such as oceanic transport or other PFOA precursors such as polyfluorinated sulfonamides may be at work.