Mechanism for OH-initiated degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins in the presence of O2 and NO/H2O

Degradation and regeneration inhibition of PCDD/Fs in incineration fly ash by low-temperature thermal technology

Low-temperature thermal degradation of PCDD/Fs for incineration fly ash (IFA), as a novel and emerging technology approach, offers promising features of high degradation efficiency and low energy consumption, presenting enormous potential for application in IFA resource utilization processes. This review summarizes the concentrations, congener distributions, and heterogeneity characteristics of PCDD/Fs in IFA from municipal, medical, and hazardous waste incineration. A comparative analysis of five PCDD/Fs degradation technologies is conducted regarding their characteristics, industrial potential, and applicability. From the perspective of low-temperature degradation mechanisms, pathways to enhance PCDD/Fs degradation efficiency and inhibit their regeneration reactions are discussed in detail. Finally, the challenges to achieve low-temperature degradation of PCDD/Fs for IFA with high-efficiency are prospected. This review seeks to explore new opportunities for the detoxification and resource utilization of IFA by implementing more efficient and viable low-temperature degradation technologies.

Multi-media environmental fate of polychlorinated dibenzo-p-dioxins and dibenzofurans in China: A systematic review of emissions, presence, transport modeling and health risks

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are notorious persistent organic pollutants (POPs) with proven toxicity to human and ecosystems. This review critically evaluates existing research, emphasizing knowledge gaps regarding PCDD/F emissions, environmental behavior, human exposure, and associated risks in China. The current emission inventory of PCDD/Fs in China remains highly uncertain, both in terms of total emissions and emission trends. Moreover, existing monitoring data primarily focus on areas near pollution sources, limiting comprehensive understanding of the overall spatiotemporal characteristics of PCDD/F pollution. To address this, we propose a novel approach that integrates the Multi-media Urban Mode (MUM) model with an atmospheric chemical transport model that includes a dual adsorption model to capture gas-particle partitioning of PCDD/Fs in the atmosphere. This coupled model can simulate the transport and fate of PCDD/Fs in multi-media environments with high spatiotemporal resolution, facilitating a nuanced understanding of the impacts of emissions, climate, urbanization and other factors on PCDD/F pollution. Additionally, dietary ingestion, particularly from animal-derived foods, is identified as the predominant source (up to 98%) of human exposure to PCDD/Fs. While the changes in dietary structure, population distribution, and age structure can influence human exposure to PCDD/Fs, their impacts have not yet been quantified. The proposed model lays the foundation for a systematic assessment of health risks from PCDD/F exposure through various pathways by further incorporating a food chain model. Overall, this review offers a comprehensive strategy for assessing PCDD/F pollution, encompassing the entire continuum from emissions to environmental impacts.

Ferrate(VI) Oxidation Mechanism of Substituted Anilines: A Density Functional Theory Investigation

Ferrate(VI) (Fe(VI)) is a promising oxidant coagulant and disinfectant for the degradation of organic micropollutants. However, it is hard to elucidate the detailed oxidation mechanism through the current experimental approaches. Substituted anilines (SANs) are important chemical compounds that are widely used in many industries. This paper presents the use of density functional theory (DFT) to understand the oxidation mechanism of SANs by Fe(VI) and the effect of substituents. The calculation results revealed that the primary oxidations of SANs follow the hydrogen atom transfer (HAT) mechanism. Interestingly, the hydroxyl oxygen of HFeO₄ ^- is more reactive than the carbonyl oxygen when reacting with SANs. The formation of the SAN radical is crucial, and all of the products are formed from it. Azobenzene is more favorable to generate the above products. In addition, the obtained results indicate that this kind of substituent has a much greater influence on the reaction rather than the position. Thus, the present study provides a valuable insight into the transformation pathways of SANs in the Fe(VI) oxidation process and the effects of the substituent on oxidation. These results will advance the understanding of Fe(VI) involved in wastewater treatment.

Mechanisms and kinetics studies of the atmospheric oxidation of eugenol by hydroxyl radicals and ozone molecules

Eugenol is a representative methoxyphenol derived from the pyrolysis of lignin containing a branched alkene group. Its concentration in the atmosphere is equivalent to guaiacol and syringol. In this present paper, the gas phase reaction mechanisms and kinetic parameters of eugenol with hydroxyl radicals (OH) and ozone molecules (O₃) were calculated at the M06-2×/6-311+G(3df,2p)//M06-2×/6-311+G(d,p) level. There are two distinct reaction types between eugenol and OH. In particular, Path2 is most favorable in the OH additions, whereas IM16 is most advantageous in H atom abstraction pathways. OH additions have more advantages than H abstraction reactions. Thus, the comprehensive and detailed reaction schemes for the further reactions of IM2 were presented. The main products generated by IM2 are methyl (Z)-3-(2-formylpenta-1,4-dien-1-yl)-2-hydroxyoxirane-2-carboxylate (P2B-4), 2-methoxy-2-oxoacetic acid (P2B-10), 2-allylmalealdehyde (P2B-11) and other carbonyl or carboxyl compounds. As for the reaction of eugenol with O₃, the cycloaddition reactions and subsequent oxidative degradation processes were also explored, which yielded the most dominant product 2-(4-hydroxy-3-methoxyphenyl) acetaldehyde (P8-1). The reaction constants of the primary reactions for eugenol with OH and O₃ under the temperature range of 225- 375 K were successively calculated by POLYRATE and MESMER program. At 298 K and 1 atm, the respective rate coefficients are 5.91 × 10^-11 and 5.48 × 10^-16 cm³ molecule^-1 s^-1 and the corresponding atmospheric lifetimes are 4.70 h and 0.72 h. The short lifetimes suggest that once eugenol enters the atmosphere, it is likely to be rapidly degraded. This work aims to provide theoretical guidance for the photochemical reaction mechanisms of eugenol with OH and O₃, and present a reference for more experimental researches.

Distribution, transformation and toxicity evaluation of 2,6-Di-tert-butyl-hydroxytotulene in aquatic environment

2,6-Di-tert-butyl-hydroxytotulene (BHT), as a significant synthetic phenolic antioxidant (SPA), has received increasing attention in the environmental field. In the present study, the BHT is confirmed to be mainly distributed in the liquid phase in the environment base on the Aspen PLUS simulation results. The mechanism and kinetics of BHT transformation initiated by OH radicals were conducted in aquatic environment using density functional theory (DFT) method. Briefly, seven initiation reactions and three detailed transformation pathways of BHT were reported. The H atoms in the t-butyl and methyl group were found more favorable to be abstracted. The C1 site of the BHT was susceptible to addition by OH radicals. Rate constants of different initial reactions were calculated and they were inhibited by temperature rise. Meanwhile, the acute and chronic toxicities of BHT and its metabolites were evaluated at three different trophic levels using the ECOSAR program. During the degradation process, the toxicities of these metabolites gradually decreased, but the toxicities of the final product 2,6-di-tert-butyl-2,5-cyclohexadien-1,4-dione (BHT-Q) were significantly increased. These results could help to reveal the transformation mechanism and risk assessment of BHT in aquatic environment, and further design the experimental and industrial applications of SPAs.

Kinetic and mechanistic insight into the OH-initiated atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin via OH-addition and hydrogen abstraction pathways: A theoretical investigation

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic polychlorinated dibenzo-p-dioxin. The OH-initiated oxidation of TCDD has been studied using the density functional, canonical transition state, and canonical Rice-Ramsperger-Kassel-Marcus theories. The kinetic data were corrected for quantum tunneling by the Wigner and Eckart models. All OH addition and hydrogen atom abstraction channels were thermodynamically exergonic. The kinetic and thermodynamic data analysis at the reliable level MPWB1K/MG3S//M06-2X/MG3S indicate that the addition of OH to the carbon atom adjacent to the oxygen atom in dioxin ring leads to the formation of predominant adduct. The calculated bimolecular rate constant for the formation of predominant adduct was ~5.97-6.75 × 10^-13 cm³ molecule^-1 s^-1, its branching ratio was ~0.955, and the overall rate constant for the OH-initiated oxidation of TCDD was ~6.25-7.08 × 10^-13 cm³ molecule^-1 s^-1. The atmospheric lifetime of TCDD determined by OH was ~8.17-9.26 days indicating the TCDD can be categorized as medium lifetime organic pollutant.

Computational investigation of the atmospheric oxidation reactions of polybrominated dibenzo-p-dioxins initiated by OH radicals

Density functional theory (DFT) method was used to study OH-initiated atmospheric oxidative reactions of some polybrominated dibenzo-p-dioxins (PBDDs). B3LYP functional and 6-311++G (2df,p) basis set were applied to optimize molecular structures of all stationary points involved in the investigated reactions. The rate constants for key elementary reactions were estimated by means of transition state theory. The computational results demonstrate that all addition reactions of PBDDs with OH radicals can occur spontaneously at standard conditions, however, the OH addition reactions are very slow due to low atmospheric concentration of OH radicals. Addition reactions occurring at γ-C position dominate in OH addition of all PBDDs. With the number of bromine atoms substituting at α-sites increases, the overall rate coefficients of OH addition decrease. The succeeding addition reactions of PBDD-OH adducts with O₂ take place hardly both thermodynamically and kinetically. Abstraction reaction of H atoms by O₂ is a governing route for PBDD-α(β)-adducts without bromine atoms at the same site, while the fused-ring CO bond fission is a main reaction channel for PBDD-γ-adducts, which will produce substituted phenoxy radicals.

Theoretical Investigations on the Reactivity of Methylidyne Radical toward 2,3,7,8-Tetrachlorodibenzo-p-Dioxin: A DFT and Molecular Dynamics Study

To explore the potential reactivity of the methylidyne radical (CH) toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the reaction mechanism between them has been systematically investigated employing the density functional theory (DFT) and ab initio molecular dynamics simulations. The relevant thermodynamic and kinetic parameters in the possible reaction pathways have been discussed as well as the IR spectra and hyperfine coupling constants (hfcc's) of the major products. Different from the reaction of the CH radical with 2,3,7,8-tetrachlorodibenzofuran, CH radical can attack all the C-C bonds of TCDD to form an initial intermediate barrierlessly via the cycloaddition mechanism. After then, the introduced C-H bond can be further inserted into the C-C bond of TCDD, resulting in the formation of a seven-membered ring structure. The whole reactions are favorable thermodynamically and kinetically. Moreover, the major products have been verified by ab initio molecular dynamics simulations. The distinct IR spectra and hyperfine coupling constants of the major products can provide some help for their experimental detection and identification. In addition, the reactivity of the CH radical toward the F- and Br-substituted TCDDs has also been investigated. Hopefully, the present findings can provide new insights into the reactivity of the CH radical in the transformation of TCDD-like dioxins.

Degradation of polychlorinated dibenzo-p-dioxins and dibenzofurans in real-field soil by an integrated visible-light photocatalysis and solvent migration system with p-n heterojunction BiVO4/Bi2O3

Degradation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in real-field soil was conducted using an integrated photocatalysis-solvent migration system of BiVO₄/Bi₂O₃ and n-hexane. The photocatalyst BiVO₄/Bi₂O₃ was synthesized, and its performance was found to be affected by the BiVO₄ content, with 20wt% BiVO₄ showing the best performance owing to its p-n heterojunction being well formed. Migration was affected by the amount of n-hexane, with 15% n-hexane giving the most effective transportation of PCDD/Fs. 37.2% of 17 PCDD/Fs was removed in 60h by the integrated photocatalysis-solvent migration system, although the reaction zone covered 8.5% of the volume of the soil. The result showed that migration via n-hexane fulfilled the aim of carrying contaminants from inside of the soil to the surface. Electron-scavenging experiments with BiVO₄/Bi₂O₃ showed an 18.4% of performance in removal compared to no-scavenging condition, which showed that the main reactions driving BiVO₄/Bi₂O₃ visible-light photocatalysis for aryl-chloride were found to be reduction-based. Owing to the hindering effect of Cl atoms, degradation by hydroxyl radical could proceed after initial dechlorination. This study establishes the applicability of integrated photocatalysis-solvent migration systems in real-field settings, and is the first report of a visible-light photocatalyst, BiVO₄/Bi₂O₃, for the degradation of PCDD/Fs in soil.

The OH-initiated atmospheric chemical reactions of polyfluorinated dibenzofurans and polychlorinated dibenzofurans: A comparative theoretical study

The atmospheric chemical reactions of some polyfluorinated dibenzofurans (PFDFs) and polychlorinated dibenzofurans (PCDFs), initiated by OH radical, were investigated by performing theoretical calculations using density functional theory (DFT) and B3LYP/6-311++G(2df,p) method. The obtained results indicate that OH addition reactions of PFDFs and PCDFs occurring at C_1∼4 and C_A sites are thermodynamic spontaneous changes and the branching ratio of the PF(C)DF-OH adducts is decided primarily by kinetic factor. The OH addition reactions of PFDFs taking place at fluorinated C_1∼4 positions are kinetically comparable with those occurring at nonfluorinated C_1∼4 positions, while OH addition reactions of PCDFs occurring at chlorinated C_1∼4 sites are negligible. The total rate constants of the addition reactions of PFDFs or PCDFs become smaller with consecutive fluorination or chlorination, and substituting at C₁ position has more adverse effects than substitution at other sites. The succedent O₂ addition reactions of PF(C)DF-OH adducts are thermodynamic nonspontaneous processes under the atmospheric conditions, and have high Gibbs free energies of activation (Δ_rG^≠). The substituted dibenzofuranols are the primary oxidation products for PCDFs under the atmospheric conditions. However, other oxidative products may also be available for PFDFs besides substituted dibenzofuranols.

Simultaneous removal of PCDD/Fs, pentachlorophenol and mercury from contaminated soil

Pentachlorophenol (PCP), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and mercury were simultaneously removed from heavily contaminated soil using a continuous pilot-scale thermal system (CPTS). Operating the system at 700 °C with 22 min of retention time ensured that the residual contaminants in remediated soil are lower in concentration than the soil standards of Taiwan EPA require. Both PCP and PCDD/Fs are effectively destroyed during the treatment at high temperatures in the CPTS, but significant dechlorination of PCDD/Fs is also found, resulting in lower net destruction efficiencies of TCDD/F and PeCDD/F-congeners, compared with those of highly chlorinated Hx-, Hp- and OCDD/F congeners. Moreover, 2,3,7,8-TetraCDD is significantly formed if the retention time is not long enough for total destruction. Inadequate reaction time (or retention time) even may lead to a rise in TEQ-value due to incomplete dechlorination. Mercury is significantly desorbed from contaminated soil and discharged through the exhaust. For PCP and PCDD/Fs, the exhaust discharge percentages including both the remediated soil and the exhaust are <0.03% and 1.14% of the input, respectively, achieved with 700 °C and 33 min retention time. In contrast, some 97.8% of input mercury rate is desorbed and discharged via the exhaust, so that the latter should be carefully cleaned via efficient air pollution control devices, whereas this contribution focuses on the conditions required for reaching adequate soil cleaning.

Theoretical study on the OH-initiated oxidation mechanism of polyfluorinated dibenzo-p-dioxins under the atmospheric conditions

Density functional theory (DFT) calculations at the B3LYP/6-311++G(2df,p) level of theory have been carried out to investigate the atmospheric oxidation mechanisms of some polyfluorinated dibenzo-p-dioxins (PFDDs), initiated by OH radical. The computed results show that all OH addition reactions of PFDDs are thermodynamically spontaneous processes and the branch ratio of the PFDD-OH adducts is determined by the magnitude of the Gibbs free energies of activation (Δ(r)G(≠)) and hence rate constants (k) for addition reactions. The OH reactions with all studied PFDDs are dominated by Cγ-addition and the total rate constants for OH addition decrease with increasing the number of fluorine atom substituting at α positions. Under the atmospheric conditions, the subsequent O2 addition reactions of PFDD-OH adducts occur hardly thermodynamically and are slow kinetically. For PFDD-α(β)-OH adducts without F atom at same positions the main reaction pathway is H abstraction by O2, while PFDD-γ-OH adducts will undergo fused-ring C-O bond cleavage, affording the substituted phenoxy radicals.

Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner

Polychlorinated naphthalenes (PCNs) are dioxins-like compounds and are formed along with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in thermal and combustion procedures. Chlorophenols (CPs) are the most important forerunners of PCNs. A comprehensive comprehension of PCN formation procedure from CPs is a precondition for reducing the discharge of PCNs. Experiments on the formation of PCNs from CPs have been hindered by PCN toxicity and short of precise detection methods for active intermediate radicals. In this work, PCN formation mechanism in gas-phase condition from 2-chlorophenol (2-CP) as forerunner was studied by quantum chemistry calculations. Numbers of energetically advantaged formation routes were proposed. The rate constants of key elementary steps were calculated over 600–1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT) method. This study illustrates formation of PCNs with one chlorine atom loss from 2-CP is preferred over that without chlorine atom loss. In comparison with formation of PCDFs from 2-CP, PCN products are less chlorinated and have lower formation potential.

Quantitative Structure-Activity Relationships Study on the Rate Constants of Polychlorinated Dibenzo-p-Dioxins with OH Radical

The OH-initiated reaction rate constants (kOH) are of great importance to measure atmospheric behaviors of polychlorinated dibenzo-p-dioxins (PCDDs) in the environment. The rate constants of 75 PCDDs with the OH radical at 298.15 K have been calculated using high level molecular orbital theory, and the rate constants (kα, kβ, kγ and kOH) were further analyzed by the quantitative structure-activity relationships (QSAR) study. According to the QSAR models, the relations between rate constants and the numbers and positions of Cl atoms, the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the difference ΔEHOMO-LUMO between EHOMO and ELUMO, and the dipole of oxidizing agents (D) were discussed. It was found that EHOMO is the main factor in the kOH. The number of Cl atoms is more effective than the number of relative position of these Cl atoms in the kOH. The kOH decreases with the increase of the substitute number of Cl atoms.

Chemical conversion pathways and kinetic modeling for the OH-initiated reaction of triclosan in gas-phase

As a widely used antimicrobial additive in daily consumption, attention has been paid to the degradation and conversion of triclosan for a long time. The quantum chemistry calculation and the canonical variational transition state theory are employed to investigate the mechanism and kinetic property. Besides addition and abstraction, oxidation pathways and further conversion pathways are also considered. The OH radicals could degrade triclosan to phenols, aldehydes, and other easily degradable substances. The conversion mechanisms of triclosan to the polychlorinated dibenzopdioxin and furan (PCDD/Fs) and polychlorinated biphenyls (PCBs) are clearly illustrated and the toxicity would be strengthened in such pathways. Single radical and diradical pathways are compared to study the conversion mechanism of dichlorodibenzo dioxin (DCDD). Furthermore, thermochemistry is discussed in detail. Kinetic property is calculated and the consequent ratio of k_add/k_total and k_abs/k_total at 298.15 K are 0.955 and 0.045, respectively. Thus, the OH radical addition reactions are predominant, the substitute position of OH radical on triclosan is very important to generate PCDD and furan, and biradical is also a vital intermediate to produce dioxin.

Mechanism and kinetic study on the ring-opening degradation of 2,3,7,8-tetrachlorinated dibenzofuran initiated by OH radicals in waste incineration

This work is about bimolecular reactions and subsequent unimolecular reactions including the ring-opening and hydrogen-transfer of 2,3,7,8-TCDF-OH adducts.

The OH-initiated chemical transformation of 1,2,4,6,8,10,11-heptachloroundecane in the atmosphere

1,2,4,6,8,10,11-Heptachloroundecane (HCU) is selected to investigate the chemical transformation of SCCPs using the density functional theory method.

Theoretical study on the degradation reaction of octachlorinated dibenzo-p-dioxin with atomic oxygen O((3)P) in dielectric barrier discharge reactor

Dielectric barrier discharges (DBD) have been used in the degradation of dioxins due to the large number of excimers and free radicals produced in discharge process. In this article, the density functional theory (DFT) is used to study the degradation mechanism of octachlorinated dibenzo-p-dioxin (OCDD) with the atomic oxygen O((3)P) in DBD reactor. The reactants, intermediates, transition states and products are optimized at the MPWB1K/6-31+G(d,p) level. The vibrational frequencies have been calculated at the same level. The reaction pathways and mechanisms are analyzed in detail. The effect of removing the chlorine atom on environment also has been discussed.

Micro-mechanism and rate constants for OH-initiated degradation of methomyl in atmosphere

The atmospheric degradation reactions of the two isomers of methomyl (MTL) initiated by OH radical in the presence of O2, NO and H2O have been investigated by density functional theory (DFT). The calculations were all carried out at MPWB1K level. The geometrical parameters and vibrational frequencies of stationary points were calculated with 6-31+G (d, p) basis sets. Single-point energy calculations were performed with 6-311+G (3df, 2p) basis sets. Profiles of the potential energy surface were constructed and all possible channels involved in the reactions were discussed. The rate constants of main elementary reactions were calculated over a temperature range of 200-400 K and mostly fitted to Arrhenius formulas. The atmospheric lifetimes of reaction species were discussed for the first time, which can be applied to the study on model simulation and management of hazardous materials.

Gas-phase tropospheric chemistry of 2,3,7,8-tetrafuorinated dibenzo-p-dioxin

Growing attention has been devoted to understanding the formation and destruction of polyfluorinated dibenzo-p-dioxins (PFDDs). High-accuracy molecular orbital calculations have been performed to investigate the tropospheric oxidation reaction of 2,3,7,8-tetrafuorinated dibenzo-p-dioxin (TFDD) initiated by OH radical, NO3 radical and O3. The rate constant of TFDD reaction triggered by the OH radical, NO3 radical and O3 is about 2.30 × 10(-11)cm(3) molecule(-l) s(-l), 3.18 × 10(-13)cm(3) molecule(-l) s(-l), and 3.30 × 10(-19)cm(3) molecule(-l) s(-l), respectively. OH radical is the major gas phase tropospheric sink for TFDD. Once TFDD-OH intermediates are produced in the initial reactions, they can react with tropospheric O2 subsequently to generate peroxy radical isomers. The TFDD-OH-O2 can further react with tropospheric NO via isomerization or combination, resulting that the dioxin ring will be ruptured completely. This study can serve as a template for tropospheric degradation of the gaseous PFDDs, which is beneficial for assessing their tropospheric behaviors.

Mechanism and kinetics study on the ozonolysis reaction of 2,3,7,8-TCDD in the atmosphere

The ozonolysis of 2,3,7,8-tetra-chlorodibenzo-p-dioxin (2,3,7,8-TCDD) is an efficient degradation way in the atmosphere. The ozonolysis process and possible reactions path of Criegee Intermediates with NO and H2O are introduced in detail at the method of MPWB1K/6-31+G(d,p)//MPWB1K/6-311+G(3df,2p) level. In ozonolysis, H2O is an important source of OH radical formation and initiated the subsequent degradation reaction. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was applied to calculate rate constants with the temperature ranging from 200 to 600 K. The rate constant of reaction between 2,3,7,8-TCDD and O3 is 4.80 x 10(-20) cm3/(mole x sec) at 298 K and 760 Torr. The atmospheric lifetime of the reaction species was estimated according to rate constants, which is helpful for the atmospheric model study on the degradation and risk assessment of dioxin.

Atmospheric degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins in the presence of NO3 at night

The density functional theory (DFT) has been applied to studies on the homogeneous gas-phase degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins (2,3,7,8-TeCDD) initiated by the NO3 radical, which is an important atmospheric species at night. The geometrical parameters and vibrational frequencies of all the stationary points were calculated at the MPWB1K/6-31+G (d,p) level. Potential energies were calculated at the MPWB1K/6-311+G (3df,2p) level. Three sites on 2,3,7,8-TeCDD react with the NO3 radical with different barriers and reaction heats. The addition of NO3 to the carbon atom on the central C–O ring is the most appropriate pathway and with the lower barriers, and the central ring of polychlorinated dibenzo-p-dioxin is opened in the subsequent reactions. Some other pathways are stressed for the dechlorination mechanism. Canonical variational transition-state theory with small curvature tunneling contribution was used to calculate the rate constants of each elementary reaction over the temperature range of 200–400 K. The Arrhenius equations were fitted to show the relationship between rate constants and temperature.

The oxidation mechanism of polychlorinated dibenzo-p-dioxins under the atmospheric conditions - a theoretical study

The atmospheric polychlorinated dibenzo-p-dioxins (PCDDs) partition appreciably in the gas phase, where they undergo rapid oxidation. The atmospheric oxidation mechanisms of a few PCDDs, initiated by OH radical, are studied using density functional theory calculations. The oxidations start with OH-addition to the aromatic rings, dominantly at γ-sites, followed by the non-chlorinated β-sites; while additions to the α-sites or chlorinated sites are negligible. For PCDDs with all β-sites being chlorinated, formation of PCDD-γ-OH adducts become virtually the only reaction path. Under the atmospheric conditions, the PCDD-β/γ-OH adducts combine with O(2) slowly at rates <1s(-1). Instead, the PCDD-β-OH adducts will react with O(2) through hydrogen abstraction at rates <50s(-1), forming PCDD-β-ol, and the PCDD-γ-OH adducts will decompose to the substituted phenoxy radicals by fused-ring C-O bond cleavage at rates of 10(3) ~10(5) s(-1). The reaction mechanisms of PCDDs are drastically different from the peroxy mechanism for the atmospheric oxidations of benzene and dibenzofuran.

Mechanism and kinetic study on the OH-initiated degradation of 2,3,7,8-tetrachlorinated dibenzofuran in atmosphere

High-accuracy molecular orbital calculation has been performed to investigate the atmospheric oxidation reaction of 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TeCDF) initiated by the OH radical in the presence of O(2) and NO/H(2)O. The possible channels involved in the reaction are discussed, and the favorable reaction pathways are revealed. The degradation occurs easily once the OH radical initiates the reaction. Two aspects need to be mentioned: one is that H(2)O in atmosphere is a source of OH radical which will initiate a new round of degradation and improve the degradation efficiency; the other is that the furan ring of 2,3,7,8-TeCDF can be turned into dioxin ring, which may explain the experimental hypothesis that polychlorodibenzofurans can be transformed to polychlorodibenzo-p-dioxins. Rate constants of the elementary reactions are calculated over a temperature range of 250-400K. Arrhenius formulas are fitted and the atmospheric lifetimes of reaction species in the troposphere are discussed for the first time, which can be applied to the study on the model simulation and the management of the hazardous materials.

Atmospheric chemical reactions of 2,3,7,8-tetrachlorinated dibenzofuran initiated by an OH radical: mechanism and kinetics study

Reactions with the OH radical are expected to be the dominant removal processes for gas-phase polychlorinated dibenzo-p-dioxins and dibenzofuran (PCDD/Fs). The OH-initiated atmospheric chemical reaction mechanism and kinetics of 2,3,7,8-tetrachlorinated dibenzofuran (TCDF) are researched using the density functional theory and canonical variational transition state theory. The reaction mechanism of TCDF with the OH radical and ensuing reactions including bond cleavage of furan ring, O(2) addition or abstraction, dechlorination process, bimolecular reaction of TCDF-OH-O(2) peroxy radical with NO, and reaction of carbonyl free radicals TCDF-OH-O with H(2)O are investigated. In the subsequent reactions of TCDF-OH, O(2) abstraction and dechlorination are most likely to predominate the process. As the main products, the HO(2) radical and the Cl atom are active and may play important roles in the atmospheric oxidation processes. The rate constants of TCDF with the OH radical are calculated, which are consistent with the reported data.