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Wang M, Liu G, Yang L, Zheng M. Framework of the Integrated Approach to Formation Mechanisms of Typical Combustion Byproducts─Polyhalogenated Dibenzo- p-dioxins/Dibenzofurans (PXDD/Fs). ENVIRONMENTAL SCIENCE & TECHNOLOGY 2023; 57:2217-2234. [PMID: 36722466 DOI: 10.1021/acs.est.2c08064] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/18/2023]
Abstract
Understanding the mechanisms through which persistent organic pollutants (POPs) form during combustion processes is critical for controlling emissions of POPs, but the mechanisms through which most POPs form are poorly understood. Polyhalogenated dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) are typical toxic POPs, and the formation mechanisms of PXDD/Fs are better understood than the mechanisms through which other POPs form. In this study, a framework for identifying detailed PXDD/Fs formation mechanisms was developed and reviewed. The latest laboratory studies in which organic free radical intermediates of PXDD/Fs have been detected in situ and isotope labeling methods have been used to trace transformation pathways were reviewed. These studies provided direct evidence for PXDD/Fs formation pathways. Quantum chemical calculations were performed to determine the rationality of proposed PXDD/Fs formation pathways involving different elementary reactions. Many field studies have been performed, and the PXDD/Fs congener patterns found were compared with PXDD/Fs congener patterns obtained in laboratory simulation studies and theoretical studies to mutually verify the dominant PXDD/Fs formation mechanisms. The integrated method involving laboratory simulation studies, theoretical calculations, and field studies described and reviewed here can be used to clarify the mechanisms involved in PXDD/Fs formation. This review brings together information about PXDD/Fs formation mechanisms and provides a methodological framework for investigating PXDD/Fs and other POPs formation mechanisms during combustion processes, which will help in the development of strategies for controlling POPs emissions.
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Affiliation(s)
- Mingxuan Wang
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People's Republic of China
- College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
| | - Guorui Liu
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People's Republic of China
- College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
- School of the Environment, Hangzhou Institute for Advanced Study, University of the Chinese Academy of Sciences, Hangzhou 310024, People's Republic of China
| | - Lili Yang
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People's Republic of China
| | - Minghui Zheng
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People's Republic of China
- College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
- School of the Environment, Hangzhou Institute for Advanced Study, University of the Chinese Academy of Sciences, Hangzhou 310024, People's Republic of China
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Lai X, Huang N, Pillai SC, Sarmah AK, Li Y, Wang G, Wang H. Formation and transformation of reactive species in the Fe 2+/peroxydisulfate/Cl - system. JOURNAL OF ENVIRONMENTAL MANAGEMENT 2022; 316:115219. [PMID: 35537272 DOI: 10.1016/j.jenvman.2022.115219] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/20/2022] [Revised: 04/24/2022] [Accepted: 05/01/2022] [Indexed: 06/14/2023]
Abstract
The influence of Cl- on the formation mechanism of active components is often neglected in the Fe2+/peroxydisulfate (PDS) system containing a large amount of ferryl ion reactive specie (Fe(Ⅳ)). In the current investigation, the effects of Cl- concentration on the removal of methyl phenyl sulfoxide (PMSO), the formation of methyl phenyl sulfone (PMSO2), the transformation of reactive species and oxidation products were investigated under different reaction conditions that included Fe2+ dosage, PDS dosage, and pH0. The results showed that Cl- complexing Fe2+ increased the formation path of sulfate radical (SO4·-) in the Fe2+/PDS system. Fe2+ dosage and pH0 value affected the content and morphology of Fe2+-Cl- complex, thus affecting the composition of reactive species. According to the experiment of free radical steady-state concentration, it was found that low concentration of Cl- reacted with SO4·- and increased the steady-state concentration of chlorine radicals (8.09 × 10-13 M [·Cl]ss at 1.41 mM Cl-), while at high concentration of Cl-, the contents of SO4·-, hydroxyl radical (·OH) and dichloride anion radicals (Cl2·-) increased and the contents of Fe(Ⅳ) and ·Cl decreased. ·Cl had strong reactivity with PMSO, and PMSO and its oxidation products were chlorinated under the combined action of ·Cl and Cl2·-. This work reveals the reaction mechanism and environmental application risks of Fe2+/PDS technology and lays the groundwork for subsequent industrial application of Fe2+/PDS system.
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Affiliation(s)
- Xiaojun Lai
- School of Environmental and Chemical Engineering, Foshan University, Foshan, 528000, China
| | - Nuoyi Huang
- Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou, 510006, China
| | - Suresh C Pillai
- Nanotechnology and Bio-Engineering Research Group and the Health and Biomedical (HEAL) Research Centre, Atlantic Technological University, ATU Sligo, Ash Lane, Sligo, F91 YW50, Ireland
| | - Ajit K Sarmah
- Department of Civil and Environmental Engineering, The Faculty of Engineering, The University of Auckland, Private Bag 92019, Auckland, 1142, New Zealand
| | - Yang Li
- College of Transportation and Environment, Shenzhen Institute of Information Technology, Shenzhen, 518172, China
| | - Guangwen Wang
- School of Environmental and Chemical Engineering, Foshan University, Foshan, 528000, China
| | - Hailong Wang
- School of Environmental and Chemical Engineering, Foshan University, Foshan, 528000, China; Key Laboratory of Soil Contamination Bioremediation of Zhejiang Province, Zhejiang A&F University, Hangzhou, 311300, China.
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Die Q, Yang J, Wang J, Wang J, Yang Y, Huang Q, Zhou Q. Occurrence and formation pathways analysis of PBDD/Fs from 2,4,6-tribromophenol under thermal reaction conditions. ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 2022; 235:113449. [PMID: 35358919 DOI: 10.1016/j.ecoenv.2022.113449] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2022] [Revised: 03/17/2022] [Accepted: 03/20/2022] [Indexed: 06/14/2023]
Abstract
Polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) are highly toxic and persistent compounds that provoke a wave of publicity. Bromophenols are important precursors for forming PBDD/Fs, and their reaction path has always been a research hotspot. In this study, the formation characteristic of PBDD/Fs from 2,4,6-TBP were studied. The yields of 2,3,7,8-substituted PBDD/Fs and 2,4,6,8-TBDF for the different thermal products ranged from 0.067 to 10.3 ng/g and 0.207-9.68 ng/g, respectively. The effects of adding Cu, Fe, and Sb2O3 were investigated and found to be more inclined to accelerate the formation of ortho-substituted PBDD/Fs than 2,3,7,8-PBDD/Fs. The formation pathways of 2,3,7,8-substituted PBDD/Fs and 2,4,6,8-TBDF were also proposed. 2,4,6,8-TBDF is generated in the C-C coupling reactions of some radical intermediates from the debromination of 2,4,6-TBP. The 2,3,7,8-PBDD/Fs are produced through more complex debromination, bromine substitution, and bromine rearrangement reactions. In addition, various catalytic effects on PBDD/F formation pathways were found, and the catalytic effect of Cu by the Ullmann reaction was the highest, while bromophenol oxidation by Fe was the highest. These results proved that both 2,3,7,8-substituted and non-2,3,7,8-substituted PBDD/Fs would be generated from 2,4,6-TBP, and the effects of the catalyst on the Br substituted position of 2,3,7,8-substituted PBDD/Fs were much lower than the Br-substituted position on bromophenol.
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Affiliation(s)
- Qingqi Die
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Jinzhong Yang
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Jianyuan Wang
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Jian Wang
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Yufei Yang
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Qifei Huang
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
| | - Qi Zhou
- State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China.
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Xu M, Zhu B, Zhao L, Sun Y, Pan Y, Yang J. Atmospheric-Pressure Pyrolysis Study of Chlorobenzene Using Synchrotron Radiation Photoionization Mass Spectrometry. J Phys Chem A 2021; 125:1949-1957. [PMID: 33651613 DOI: 10.1021/acs.jpca.0c10413] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The pyrolysis of chlorobenzene (C6H5Cl) at 760 Torr was studied in the temperature range of 873-1223 K. The pyrolysis products including intermediates and chlorinated aromatics were detected and quantified via synchrotron radiation photoionization mass spectrometry. Furthermore, the photoionization cross sections of chlorobenzene were experimentally measured. On the basis of the experimental results, the decomposition pathways of chlorobenzene were discussed as well as the generation and consumption pathways of the main products. Benzene is the main product of chlorobenzene pyrolysis. Chlorobiphenyl (C12H9Cl), dichlorobiphenyl (C12H8Cl2), and chlorotriphenylene (C18H11Cl) predominated in trace chlorinated aromatic products. Chlorobenzene decomposed initially to form two radicals [chlorophenyl (·C6H4Cl) and phenyl (·C6H5)] and the important intermediate o-benzyne (o-C6H4). The propagation processes of chlorinated aromatics, including polychlorinated naphthalenes and polychlorinated biphenyls, were mainly triggered by chlorobenzene, chlorophenyl, and benzene via the even-numbered-carbon growth mechanism. Besides, the small-molecule products such as acetylene (C2H2), 1,3,5-hexatriyne (C6H2), and diacetylene (C4H2) were formed via the bond cleavage of o-benzyne (o-C6H4).
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Affiliation(s)
- Minggao Xu
- School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, People's Republic of China.,National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, People's Republic of China
| | - Baozhong Zhu
- School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, People's Republic of China.,National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, People's Republic of China
| | - Long Zhao
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, People's Republic of China
| | - Yunlan Sun
- School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, People's Republic of China.,School of Petroleum Engineering, Changzhou University, Changzhou 213164, Jiangsu, People's Republic of China
| | - Yang Pan
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, People's Republic of China
| | - Jiuzhong Yang
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, People's Republic of China
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Teng Z, Zhao X, Wang H, Li Y, Han Y, Sun Y, Xu F. Mechanism and kinetic properties for the complete series reactions of chloro(thio)phenols with O( 3P) under high temperature conditions. RSC Adv 2021; 11:17683-17693. [PMID: 35480180 PMCID: PMC9033228 DOI: 10.1039/d1ra02407h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2021] [Accepted: 05/07/2021] [Indexed: 11/21/2022] Open
Abstract
Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs) are two groups of dioxin-like compounds with oxygen and sulfur substitution, respectively. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCDD/F and PCDT/TA formation. The formation of chlorophenoxy radicals (CPRs) and chlorothiophenoxy radicals (CTPRs) from chlorophenols (CPs) and chlorothiophenols (CTPs) with O(3P) is an important initial step for the formation of PCDD/Fs and PCDT/TAs, respectively. In this paper, the formation of CPRs/CTPRs from the complete series reactions of 19 CP/CTP congeners with O(3P) was studied using the density functional theory (DFT) method. The rate constants of each reaction were calculated using canonical variational transition state (CVT) theory along with a small-curvature tunneling (SCT) contribution over a wide temperature range of 600–1200 K. The effect of the chlorine substitution pattern on the structural parameters, thermochemical properties and rate constants in both CPs and CTPs was discussed. This study shows that the reactions between CPs and O(3P) can be affected by the chlorine substitution at the para-position, and the reactions between CTPs and O(3P) are mostly influenced by both ortho-substitutions. The thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more likely to occur than the phenoxyl-hydrogen abstraction from CPs by O(3P). Comparison of the reactivity of CP/CTPs with O(3P) with our previous work on CP/CTPs with H and OH shows that the order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H, and the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH. Under pyrolysis or combustion conditions, chlorophenols (CPs) and chlorothiophenols (CTPs) can readily form chlorophenoxy radicals (CPRs) and chlorotriophenoxy radicals (CTPRs) by abandoning the phenoxyl-H and sulfydryl-H, respectively.![]()
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Affiliation(s)
- Zhuochao Teng
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
| | - Xianwei Zhao
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
| | - Hetong Wang
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
| | - Ying Li
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
| | - Yanan Han
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
| | - Yanhui Sun
- College of Environment and Safety Engineering
- Qingdao University of Science & Technology
- Qingdao 266042
- P. R. China
| | - Fei Xu
- Environment Research Institute
- Shandong University
- Qingdao 266237
- P. R. China
- Shenzhen Research Institute of Shandong University
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Sun Y, Chen X, Xu F, Wang X. Quantum chemical calculations on the mechanism and kinetics of ozone-initiated removal of p-coumaryl alcohol in the atmosphere. CHEMOSPHERE 2020; 253:126744. [PMID: 32302911 DOI: 10.1016/j.chemosphere.2020.126744] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/13/2020] [Revised: 04/03/2020] [Accepted: 04/06/2020] [Indexed: 06/11/2023]
Abstract
p-Coumaryl alcohol (p-CMA), as the simplest lignin precursor, was determined in the process of lignin polymer degradation and wood smoke. However, its transformation and migration in the atmosphere have not been well clarified. In this work, the gas-phase reaction mechanisms and kinetic parameters of ozone-initiated removal of p-CMA were performed by using quantum chemical calculations. Seven primary addition reaction pathways were summarized. A more comprehensive and detailed reaction routes of the favorable Criegee intermediate (IM9) were presented, including the reactions with small molecules, as well as its own isomerization and decomposition reactions. p-Hydroxybenzaldehyde (P1) is the most dominant product in the further reactions of IM9 and the subsequent ozonolysis mechanisms of P1 also were elucidated. All thermodynamic calculations were investigated on the density functional theory (DFT) method at the M06-2X/6-311 + G (3df, 2p)//M06-2X/6-311 + G (d,p) level. The overall and individual rate constants have estimated by using the KiSThelP under typical atmospheric temperature (198-338 K) and pressure. The total rate constant is 3.37 × 10-16 cm3 molecule-1 s-1 at 298 K and 1 atm. In addition, the atmospheric lifetime of p-CMA by ozone-determined is 1.18 h under the average ozone concentration of 7 × 1011 molecules cm-3. The short lifetime indicates that the degradation processes of p-CMA determined by O3 cannot be ignored, especially in areas where the tip concentration of O3 molecules is high. The present study provides a synthetical investigation on ozonolysis of p-CMA for the first time and enriches our understanding of atmospheric oxidation processes of other lignin compounds.
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Affiliation(s)
- Yanhui Sun
- College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China; Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Guangzhou, 510006, PR China.
| | - Xiaoxiao Chen
- College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China
| | - Fei Xu
- Environment Research Institute, Shandong University, Qingdao, 266237, PR China
| | - Xiaotong Wang
- College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China
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Chang J, Pan W, Liu X, Xue Q, Fu J, Zhang A. The formation of PBDFs from the ortho-disubstituted phenol precursors: A comprehensive theoretical study on the PBDD/Fs formation from 2,4,6-tribromophenol. THE SCIENCE OF THE TOTAL ENVIRONMENT 2020; 713:136657. [PMID: 31958733 DOI: 10.1016/j.scitotenv.2020.136657] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/11/2019] [Revised: 01/08/2020] [Accepted: 01/10/2020] [Indexed: 06/10/2023]
Abstract
Bromophenols are known as direct precursors of the notorious polybrominated dibenzo-p-dioxin/dibenzofurans (PBDD/Fs). There is a long-held viewpoint that only the more toxic dioxin-type products could be formed from the ortho-disubstituted phenols, totally contrary to the experimental observations that both PBDDs and PBDFs are generated. To tackle the issue, the gaseous formation mechanism of PBDD/Fs from 2,4,6-tribromophenol (TBP), a typical ortho-disubstituted phenol, was investigated in this study. Firstly, the reactions between TBP and the active H radical produce three key radical species including the bromophenoxyl radical, the substituted phenyl radical and phenoxyl diradical. The self- and cross-combinations of these radical species and TBP yield not only the dioxin-type products 1,3,6,8-TeBDD and 1,3,7,9-TeBDD, but also the brominated dibenzofurans 1,3,6,8-TeBDF and 2,4,6,8-TeBDF. Notably, the reactions involving the phenyl C sites in the substituted phenyl and phenoxyl diradicals are demonstrated to be both thermodynamically and kinetically more favorable than those involving the bromophenoxyl radical and the TBP molecule. Most importantly, the findings of the present work are of great importance as it provides feasible pathways to form less toxic dibenzofuran-type products from the ortho-disubstituted phenols. These results will improve the understanding of the PBDD/Fs formation mechanism from phenol precursors.
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Affiliation(s)
- Jiamin Chang
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, PR China
| | - Wenxiao Pan
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China
| | - Xian Liu
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, PR China
| | - Qiao Xue
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China
| | - Jianjie Fu
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China
| | - Aiqian Zhang
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, PR China; Institute of Environment and Health, Jianghan University, Wuhan 430056, PR China.
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Quantum Chemical and Kinetic Study on Radical/Molecule Formation Mechanism of Pre-Intermediates for PCTA/PT/DT/DFs from 2-Chlorothiophenol and 2-Chlorophenol Precursors. Int J Mol Sci 2019; 20:ijms20071542. [PMID: 30934774 PMCID: PMC6480007 DOI: 10.3390/ijms20071542] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2019] [Revised: 03/16/2019] [Accepted: 03/22/2019] [Indexed: 12/31/2022] Open
Abstract
Polychlorinated phenoxathiins (PCPTs), polychlorinated dibenzothiophenes (PCDTs), and polychlorinated thianthrenes (PCTAs) are sulfur analogues of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/DFs). Chlorothiophenols (CTPs) and chlorophenols (CPs) are key precursors for the formation of PCTA/PT/DTs, which can react with H or OH to form chloro(thio)phenoxy radical, sulfydryl/hydroxyl-substituted phenyl radicals, and (thio)phenoxyl diradicals. However, previous radical/radical PCTA/DT formation mechanisms in the literature failed to explain the higher concentration of PCDTs than that of PCTAs under the pyrolysis or combustion conditions. In this work, a detailed thermodynamics and kinetic calculations were carried out to investigate the pre-intermediate formation for PCTA/PT/DTs from radical/molecule coupling of the 2-C(T)P with their key radical species. Our study showed that the radical/molecule coupling mechanism explains the gas-phase formation of PCTA/PT/DTs in both thermodynamic and kinetic perspectives. The S/C coupling modes to form thioether-(thio)enol intermediates are preferable over the O/C coupling modes to form ether-(thio)enol intermediates. Thus, although the radical/molecule coupling of chlorophenoxy radical with 2-C(T)P has no effect on the PCDD/PT formation, the radical/molecule coupling of chlorothiophenoxy radical with 2-C(T)P plays an important role in the PCTA/PT formation. Most importantly, the pre-PCDT intermediates formation pathways from the couplings of sulfydryl/hydroxyl-substituted phenyl radical with 2-C(T)P and (thio)phenoxyl diradicals with 2-C(T)P are more favorable than pre-PCTA/PT intermediates formation pathways from the coupling of chlorothiophenoxy radical with 2-C(T)P, which provides reasonable explanation for the high PCDT-to-PCTA ratio in the environment.
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