Mechanism Study On Oxidative Degradation Of Typically PCDD/Fs In Atmosphere

Dioxins are the general term for polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs). Due to their properties of carcinogenesis, tetratogenesis and mutagenesis, and may enter human body through food chain and endanger the human health, PCDD/Fs have been enlisted in Stockholm Convention. The major sources of PCDD/Fs are waste incineration, chemical industry production, metal smelting and refining process, and paper manufacturing industry. The PCDD/Fs in the atmosphere are in the form of gas phase or particles, which are mainly influenced by saturated vapor pressure and temperature. At the ambient temperature, low chlorinated PCDD/Fs (mainly with six or less chlorine atoms) have high concentration in the gas phase, while high chlorinated PCDD/Fs (primarily hepta-and octa-chlorinated homologues) mainly distributed in the particle phase. In the flue gas,82%～93%of PCDD/Fs are distributed in the gas phase due to high temperature. The atmospheric fate for PCDD/Fs is removed from the atmosphere via the dry or wet deposition, photolysis and the reactions with the atmospheric oxidants (including OH radical, HO2radical, O3, NO3radical, and Cl atom). Particle-phase PCDD/Fs are removed from the atmosphere via the dry or wet deposition, while gas-phase PCDD/Fs are depleted via photolysis and degradation reactions. It is critical to know the reaction mechanism and accurate rate coefficients of PCDD/Fs with various oxidants in order to assess their dominant atmospheric fate.Due to their toxicities and the rather low vapor pressure, the reaction mechanism and direct kinetic measurements of the reactions are difficult to obtain by experimental study. Theoretical study can provide accurate predictions for the reaction mechanism by calculating the energies and can generate kinetic data of key elementary reaction steps. In this paper, the writer has done the following works:1. Mechanism of2,3,7,8-TCDD/F with HOx、O3、NO3、Cl2,3,7,8-tetrachlorinated dibenzo-p-dioxins and2,3,7,8-tetrachlorinated dibenzofuran (2,3,7,8-TCDD/F) were chosen to research the initiation reactions with OH radical, HO2radical, O3, NO3radical, and Cl atom at the level of MPWB1K /MG3S//MPWB1K/6-31+G(d,p) due to the highest toxicities and80%in the gas phase. Based on the quantum chemical information, rate constants were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) in order to assess the relative importance of the reactions initiated by various oxidation agents.The results show that2,3,7,8-TCDD/F and OH radical have two possible reaction patterns, i.e., OH radical addition and H atom abstraction, and OH radical addition is the main way. The oxygen-bonded Ca is the most favorable site for OH radical addition of2,3,7,8-TCDD, and the C1and C4are the most favorable sites for OH radical addition of2,3,7,8-TCDF. The rate constants obtained from the reactions of the TCDD/F with the OH radical are7.75×10-12and9.78×10-13cm3molecule-1s-1, respectively.2,3,7,8-TCDD/F can only react with HO2radical by H atom abstraction reaction, the rate constants at298.15K are2.53×10-25and3.61×10-25cm3molecule-1s-1, respectively.2,3,7,8-TCDD/F can react with O3radical by OH radical addition reaction, the rate constants at298.15K are3.17×10-21and6.86×10-22cm3molecule-1s"1, respectively.2,3,7,8-TCDD/F can react with NO3radical and Cl atom by OH radical addition and H atom abstraction, the OH radical addition is the main reaction way, the rate constants with NO3radical and Cl atom at298.15K are3.96×10-14/6.46×10-15and1.23×10-9/1.37×10-10cm3molecule-1s-1, respectively. The atmospheric lifetime τ of the2,3,7,8-TCDD/F with respect to OH radicals, O3, NO3radicals, and Cl atoms are estimated, and the study show that2,3,7,8-TCDD/F will be mainly scavenged by OH radical, while NO3radical is the primary oxidant at night and Cl atom dominates after sunrise in coastal and marine areas.I. Mechanism for OH-initiated Degradation of2,3,7,8-TCDD in AtmosphereThe OH-initiated atmospheric degradation reaction of2,3,7,8-TCDD was investigated using DFT calculation in order to find out favorable reaction pathways and reaction sites in the presence of O2and NO/H2O. Based on the quantum chemical information, rate constants were calculated using CVT/SCT over a suitable temperature range of250-400K. The results show that more than93%of the OH radical addition reaction for2,3,7,8-TCDD occurs on Cy. In the ensuing reactions of TCDD-OH, the bond cleavage of dioxin ring and O2addition are both important. The reaction with NO in atmosphere is the main removal way of the peroxy radical isomers TCDD-OH-O2. The carbonyl free radicals can abstract H atom from H2O to reach a stable state and the OH radical will be regenerated simultaneously, which can interprete the significantly increasing destruction efficiency of PCDD/Fs in the presence of water vapor.3. Mechanism for OH-initiated Degradation of2,3,7,8-TCDF in AtmosphereThe OH-initiated atmospheric chemical reaction mechanism and kinetics of2,3,7,8-TCDF was investigated using the DFT and CVT/SCT in the presence of O2and NO/H2O. The reaction mechanism of2,3,7,8-TCDF with OH radical and ensuing reactions including bond cleavage of furan ring, O2addition or abstraction, dechlorination process, bimolecular reaction of TCDF-OH-O2peroxy radical with NO and reaction of carbonyl free radicals TCDF-OH-O with H2O are researched. The results show that the branching ratios R of C4-addition will decrease as the temperature rises, while the branching ratios R of CA-addition and C\-addition increase noticeably and the branching ratios R of other addition reactions are elevated slightly. In the subsequent reactions of TCDF-OH, TCDF-OH(A) is kinetically favored to open up the C-O bond, while oxygen addition is more important for TCDF-OH(B), TCDF-OH(1) and TCDF-OH(4) are more likely to produce the closed shell dibenzofuranol structures and HO2radical, The dechlorinate processes of TCDF-OH(2) and TCDF-OH(3) are the dominant pathway. In the isomerization of M2d(t)-A, the furan ring of2,3,7,8-TeCDF can be turned into dioxin ring, which may explain the experimental hypothesis that PCDFs can be transformed to PCDDs. The reaction with NO in atmosphere is the main removal way of the peroxy radical isomers TCDF-OH-O2.4. QSAR Study on the Rate Constants of Polychlorinated Dibenzo-p-dioxins with OH RadicalThe rate constants of75PCDDs with OH radical at298.15K are obtained, the rate constants (kα,&β，kγ, and KOHh) are further quantified by the studies on quantitative structure-activity relationships (QSAR). According to the QSAR models, the relations of these rate constants with the numbers and positions of Cl atoms were discussed. The results show that the barrier of OH radical addition on carbon that connecting with Cl atom are higher than others, which may be due to the steric hindrance of the Cl atom. The rate constants of PCDDs that Cl atoms substitution occur on the2,3,7,8-site have the following order:kα>kβ, while others are the opposite. For the kα, Nα had limited effects on kα, and the effect of the number of relative position for these Cl atoms (mainly Np) is more than the number of Cl atoms. For the kp, it is negatively associated with Nα and kβ. The effects of relative positions of these Cl atoms are different, with the following order:N0<Nm. The effect of the number of Cl atoms (mainly Nα and Nβ) is more than the number of relative position (No and Nm) for these Cl atoms. κγ, is immune to the numbers and positions of Cl atoms. For the kOH is mainly effected by the number of Cl atoms, the kOH of high chlorinated PCDDs is less than the koH of low chlorinated PCDD/Fs.