Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Response to Comment on “Molecular Mechanism of Dioxin Formation from Chlorophenol based on Electron Paramagnetic Resonance Spectroscopy”
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experimental conditions that were used for the in situ detection of EPR, were conducted in the tube furnace reaction system to verify our suggested PCDD formation pathways (Figure 6 in ref 1).1 The GC Q-TOF MS technique was used for screening the products formed in the thermochemical reactions, and HRGC/ HRMS was used to verify the isomer-specific PCDD products proposed in the formation pathways (Figure 6).1 The total ion chromatograms of the formation products from GC Q-TOF MS (Figure S3 in ref 1) indicated that the compounds identified for peaks eluted before 17 min included 2,3,6-TCP and other chlorophenols, chlorobenzene, and benzene. The principle formation products with the highest responses were tetrachlorodibenzo-p-dioxins.1 Thus, the GC Q-TOF MS results experimentally supported the dominant products proposed in Figure 6 in ref 1. Furthermore, the suggested four isomers of tetrachlorodibenzo-p-dioxins that showed the highest abundances in the products, of PCDD formation via oxygen−carbon coupling of TCP radicals were consistent with the experimental observations, which were verified by HRGC/ HRMS (Figure S4 in ref 1).1 This suggests that the TCP radical is an important intermediate in PCDD formation during the thermochemical reactions, and this is theoretically and experimentally supported by other researchers.4−6 In conclusion, we agree that there might be other free radical species present besides the TCP radical. We appreciate Dr. Korth’s insightful comments and the tentative identification of two potential free radicals in addition to the TCP radical that we proposed in ref 1. We think further experiments and theoretical calculations will be useful to clarify the occurrences and structures of other potential free radicals in this reaction system. However, although the number of free radicals and their structures are uncertain, the TCP radical, which is formed by widely recognized hydrogen abstraction, is definitely an important intermediate involved in PCDD formation.5−8 Therefore, the suggested formation pathways from the TCP radical to PCDD congeners in Figure 6 in our paper (ref 1) are reasonable.1
e thank Dr. Hans-Gert Korth for his interest in our paper about the pathways for formation of polychlorinated dibenzo-p-dioxins (PCDDs) from 2,3,6-trichlorophenol (TCP) over a Cu(II)O/silica matrix during thermochemical reactions.1 We appreciate the comments about the g values for the spectra in Figure 4 of ref 1.1 It need to be corrected that the g value of electron paramagnetic resonance (EPR) signal at the temperature of 350 and 523 K in Figure 4 should be labeled with 2.0049, which was regrettably mismarked as 2.0073. Although we agree with Dr. Korth’s opinions that there might be occurrence of other free radical species, it was believed that the occurrence of the 2,3,6-trichlorophenoxy radical we identified during the heating stage of the reaction system is incontrovertible.1 We insist that oxygen−carbon coupling of the TCP radical is the important pathway of PCDD formation when 2,3,6-TCP is used as a precursor as described in our paper.1 In his comments, Dr. Korth stated that besides the TCP radical, at least two other free radicals would exist during the heating stage in our reaction system. On this basis, he considered that whether our proposed PCDD formation pathways from TCP are reasonable because these other free radicals were not identified. Above all, the occurrence of TCP radical during the heating stage of the reaction system is incontrovertible as Dr. Korth and we both consent. Dr. Korth stated that 2,3,6-trichloro-p-benzosemiquinone radical anion unequivocally exist during the heating stage based on a previously reported g value and line width of 2,3,5-trichlorop-benzosemiquinone radical (3) in the alkaline alcohol.2 However, Dr. Korth used a liquid phase system as the basis for assignment of radical 3, whereas our thermochemical reactions and EPR detection were conducted in the solid phase at higher temperature of 350 K in ref 1.1 This might lead to differences in g values, stabilities, and line broadening.3 LandoltBörnstein reported that the g value of the 2,3,6-trichloro-pbenzosemiquinone radical in an alkaline alcohol was 2.0058, but a g value for the 2,3,6-trichlorophenoxy radical in the liquid phase was not given.2 Therefore, in our opinion, it may be unconvincing to conclude that free radical 3 definitely occurs in our reaction system based on a g value in an alkaline alcohol (liquid phase). Moreover, the improved simulation spectra proposed by Dr. Korth benefits from the premise of the occurrence of the so-called “unidentified radical”, whose structure remains uncertain. Because it is not easy to explain and verify the molecular mechanism of PCDD formation using a single technique, we used two powerful screening and identification techniques (gas chromatography quadrupole time-of-flight mass spectrometry, GC Q-TOF MS; and high resolution gas chromatography with high resolution mass spectrometry, HRGC/HRMS) to verify the rationality of the proposed mechanisms. Thermochemical experiments using the same reactants and matrix, and the same © XXXX American Chemical Society
Lili Yang†,‡ Guorui Liu*,†,‡ Minghui Zheng†,‡ Yuyang Zhao†,‡ Rong Jin†,‡ Xiaolin Wu†,‡ Yang Xu†,‡ †
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China
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DOI: 10.1021/acs.est.7b05787 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
‡
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University of Chinese Academy of Sciences, Beijing 100049, China
AUTHOR INFORMATION
Corresponding Author
*Phone: 86 10 62849356; e-mail:
[email protected]. ORCID
Guorui Liu: 0000-0002-8462-6734 Minghui Zheng: 0000-0001-5270-6759 Rong Jin: 0000-0001-9677-6177 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Yang, L.; Liu, G.; Zheng, M.; Zheng, Y.; Jin, R.; Wu, X.; Xu, Y. Molecular mechanism of dioxin formation from chlorophenol based on electron paramagnetic resonance spectroscopy. Environ. Sci. Technol. 2017, 51, 4999−5007. (2) Börnstein, R. New Series, Magnetic Properties of Free Radicals; Fischer, H.; Hellwege, K. H., Eds.; Springer, Berlin, Vol. II/1, 1965; p 80−84. (3) Brustolon, M., Glamello, E. Electron Paramagnetic Resonance; Wiley: NJ, 2009; p 209. (4) Zhang, Q.; Yu, W.; Zhang, R.; Zhou, Q.; Gao, R.; Wang, W. Quantum chemical and kinetic study on dioxin formation from the 2,4,6-TCP and 2,4-DCP precursors. Environ. Sci. Technol. 2010, 44, 3395−3403. (5) Evans, C. S.; Dellinger, B. Mechanisms of dioxin formation from the high-temperature pyrolysis of 2-chlorophenol. Environ. Sci. Technol. 2003, 37 (7), 1325−1330. (6) Evans, C. S.; Dellinger, B. Formation of bromochlorodihenzo-pdioxins and furans from the high-temperature pyrolysis of a 2chlorophenol/2-bromophenol mixture. Environ. Sci. Technol. 2005, 39 (20), 7940−7948. (7) Evans, C. S.; Dellinger, B. Mechanisms of dioxin formation from the high-temperature pyrolysis of 2-bromophenol. Environ. Sci. Technol. 2003, 37 (24), 5574−5580. (8) Evans, C. S.; Dellinger, B. Mechanisms of dioxin formation from the high-temperature oxidation of 2-bromophenol. Environ. Sci. Technol. 2005, 39 (7), 2128−2134.
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DOI: 10.1021/acs.est.7b05787 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
