Theoretical Study On The Atmospheric Degradation Mechanism Of Typical Organophosphorus Pesticides And Polycyclic Aromatic Compounds

Organophosphorus pesticides (OPs) are released to the environment to protect agricultural activities. Due to their high toxicity, mutagenicity and endocrine disruption, OPs have caused great public concern. Polycyclic aromatic hydrocarbons (PAHs) are not only the first discovered and studied, but also the most abundant environmental carcinogenic pollutants. Because of their teratogenicity, carcinogenicity, mutagenicity and biological accumulation, PAHs are listed as persistent organic pollutants (POPs). Although their concentrations are lower than those of regular pollutants, OPs and PAHs pose a great threat on public health and ecosystem. In the atmosphere, most of OPs and PAHs with 2-4 rings exist mainly in the gas phase and can undergo atmospheric reactions with the ubiquitous OH, NO3, O3 and so on. During their degradation processes, more toxic secondary pollutants may be formed. Understanding the atmospheric degradation processes of typical OPs and PAHs is important for removing those substances from the atmosphere and assessing their potential risks to non-targets.In this thesis, quantum chemical calculations have been carried out to study the degradation processes of typical OPs and PAHs, and CVT/SCT and multichannel RRKM-TST methods have been used to calculate the branch and overall rate constants. The important and meaningful results in this thesis can be summarized as follows:1. Mechanism for Atmospheric Oxidation of Typical Organophosphorus PesticidesOH-initiated atmospheric degradation processes of TEPT, chlorpyrifos and diazinon have been investigated and the detailed mechanism has lead to the following conclusions:(1) The addition of OH radicals to P atom and H abstraction are the dominant pathways for the reaction of OH radicals with these three OPs. The addition of OH radicals to pyrimidyl or pyridyl ring has relatively high energy barrier and is not the dominant reaction pathway.(2) OH radicals can transform the P=S bond to P=O bond easily. The degradation products containing P=O bond are more toxic than the parent compounds with P=S bond. For OH-TEPT adducts and OH-chlorpyrifos adducts, the unimolecular process and further reaction with O2 are both important. However, the unimolecular process is more important for OH-diazinon adducts.(3) The abstraction processes of TEPT, chlorpyrifos and diazinon are similar. The pyridyl, pyrimidyl ring or -OC2H5 group does not have an important effect on the chemistry environment of H atoms. For pesticides with -OCH2CH3, H atoms in the-CH2- moiety are more activated than those in the -CH3 moiety.2. Mechanism and Kinetic Properties for the Formation of Alkyl-nitro-naphthalenes from OH-initiated Degradation of Alkyl-naphthalenesThe formation processes of nitro-naphthalene,2-methyl-nitro-naphthalene, 2-ethyl-nitro-naphthalene and 2,6-dimethyl-nitro-naphthalene have been studied at the BB1K/6-31+G(d,p) level, and the branch and overall rate constants for the reaction of OH radicals with naphthalene and alkyl naphthalenes have been calculated with CVT/SCT and multichannel RRKM-TST methods at 180～470 K and 1 atm. The conclusions are as follows:(1) The formation processes for OH addition to the C1, C4, C5 and C8 atoms of naphthalene ring are easier than the formation processes of OH addition to another four C atoms.(2) The abstraction rate constants from -CH3 and -C2H5 groups of 2-methyl naphthalene,2-ethyl naphthalene and 2,6-dimethyl naphthalenes can be ignored.(3) For 2-methyl naphthalene,2-ethyl naphthalene and 2,6-dimethyl naphthalenes, the addition process of OH radicals to C1 atom is the dominant addition pathway. However, alkyl-nitro-naphthalenes cannot be formed from these adducts because of the lack of ortho H atoms. (4) The comparison between the reaction mechanism of 2-methyl naphthalene and naphthalene shows that the barriers for OH addition to C1, C2, C3, C5, C6, C7 and C8 of 2-methyl naphthalene are lower than those of OH addition to the corresponding atoms of naphthalene. The formation processes of 2-methyl-nitro-naphthalene initiated by OH addition to C5 and C7 atoms of 2-methyl naphthalene have lower energy barriers than corresponding processes of nitro-naphthalene.(5) The comparison between 2-methyl naphthalene and 2-ethyl naphthalene shows that -CH3 and -C2H5 have the same effect on the OH addition process and the formation of alkyl-nitro-naphthalene.(6) The comparison between 2,6-dimethyl naphthalene and naphthalene shows that the energy barriers for the OH addition process to all atoms of 2,6-dimethyl naphthalene are lower than the corresponding processes of naphthalene. However, the formation processes of 2,6-dimethyl-nitro-naphthalenes have lower energy barriers than the corresponding processes of nitronaphthalenes.3. Formation Mechanism of Nitroanthracenes from OH and NO3 Radical-Initiated Reactions of AnthraceneFormation processes of nitroanthracenes from OH- and NO3-initiated atmospheric degradation of anthracene have been studied at the BB1K/6-31+G(d,p) level and were compared with formation processes of nitrobenzene and nitronaphthalene. The conclusions are the following:(1) In atmosphere, PAHs can react with OH or NO3 radicals to form OH-PAHs or NO3-PAHs adducts, which can further react with NO2. Nitro-PAHs can be formed from the reaction of OH-PAHs with NO2 by the lost of H2O or be formed from the reaction of NO3-PAHs with NO2 by the lost of HNO3. The formation of H2O (rate-determining step for the formation of nitro-PAHs from OH-PAHs) has a four-membered transition state and the formation of HNO3 (rate-determining step for the formation of nitro-PAHs from NO3-PAHs) has a six-membered transition state. Because of smaller ring tension in six-membered ring, the formation of HNO3 is easier than the formation of H2O, leading that nitro-PAHs can be formed more easily from NO3-initiated atmospheric degradation of PAHs. The rate constants of anthracene with OH radicals are larger than those with NO3 radicals, indicating that OH-initiated atmospheric degradation is the dominant degradation process of anthracene. However, NO3-initiated formation of nitroanthracene is easier than OH-initiated formation process, because the energy barrier for the formation of nitroanthracene from NO3-anthracene adduct with NO2 is lower than that from OH-anthracene adduct with NO2.(2) The comparison between the formation of nitro-PAHs from OH- and NO3-initiated atmospheric degradation of benzene (1 ring), naphthalene (2 ring) and anthracene (3 ring) shows that the addition of aromatic ring facilitates the addition of PAHs with OH and NO3 radicals, but suppresses the formation of nitro-PAHs.(3) There are a lot of active sites in PAHs, but not all the active sites can lead to the formation of nitro-PAHs. For example, OH or NO3 radicals can add to all the C atoms of anthracene, but only the addition to C1, C2, C3, C4, C5, C6, C7 and C8 can lead to the formation of nitro-PAHs.(4) The energy barriers for the formation of nitroanthracenes from the reaction of OH-anthracene adducts with NO2 in the presence of water are 12 kcal mol-1 lower than the corresponding energy barriers without water, indicating that the presence of H2O facilitates the formation of nitro-PAHs.