- Title
- Thermal studies of chlorinated and mixed halogenated biphenyls
- Creator
- Hou, Song
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis provides an assessment of the toxic pollutants formed during the thermal decomposition of polychlorinated (PCBs) and mixed halogenated (i.e., simultaneously chlorinated and brominated) biphenyls (PXBs; X = Cl/Br), under gas phase conditions similar to those occurring in fires, and in other combustion processes. (For convenience, in this thesis, the term polychlorinated biphenyls also comprises monochlorinated biphenyls.) In particular, we have investigated the oxidation of 4-chlorobiphenyl (4-CB), 4,4’-dichlorobiphenyl (4,4’-DCB) and 4-bromo,4’-chlorobiphenyl (4,4’-BCB), at temperatures lower than those leading to complete conversion of PCBs and PXBs to HBr, HCl, H2O and CO2. The presence of chlorine and bromine in the molecules indicates the propensity for releasing highly toxic chlorinated and mixed halogenated volatiles (VOCs) in combustion processes. In addition, previous studies reported polychlorinated biphenyls, which were widely used as transformer oil in capacitors, to form polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). The work covered by this thesis was designed to examine the type and abundance of pollutants formed in incomplete oxidation of PCBs and PXBs and to gain an understanding into the effect of structural differences among the reactants on the reaction mechanisms responsible for the formation of pollutants. In addition, we examined the effect of three materials of construction of the reactors: alumina (99.5 %), quartz (99.95 %) and quartz with boron-oxide coating on the reactor walls. Initially, we studied the same reactant (4-CB) in the three reactors, to understand the effect of impurities (trace amount of transition metals) and surface generated singlet oxygen on the reaction mechanisms responsible for the pollutant formation. We established a laboratory-scale apparatus that consisted of a laminar flow reactor equipped with a sampling system for intercepting the volatile and condensable products. A XAD-2 cartridge served to trap the volatile organic compounds (VOCs) and PCDD/Fs. The analysis of VOCs involved high resolution gas chromatography – quadrupole mass spectrometry (GC-QMS) while HRGC – ion trap MS (GC-ITMS) quantitated the PCDD/Fs produced. The development of reaction mechanisms involved the application of Gaussian 09 suite of programs. We optimised structures and calculated the zero point vibrational energies (ZPVE) at the B3LYP/6-31G(d), B3LYP/6-311G(d,p) and M062X/6-311+G(d,p) levels of theory. Stationary points located were either minima or transition states (TSs) determined by vibrational frequency analysis wherein a transition structure contained just one imaginary frequency along the specified reaction coordinates. Intrinsic reaction coordinate (IRC) analysis afforded linking the reactants and products with their TSs. The thermal decomposition of 4-CB in the alumina reactor produces a number of volatile pollutants, including chlorophenols and chlorobenzenes, which are important precursors for the formation of PCDD/Fs. Other volatile organic compounds (VOCs) detected included (chloro)benzaldehydes, (chloro)naphthalenes, one isomer of ethylbenzaldehyde, 1-chloro-4-ethynylbenzene and dibenzofuran. The results of the thermal decomposition of 4-CB also indicate the formation of mono to octa-CDD/Fs at high temperatures, with 3-monochlorodibenzofuran (3-MCDF) being the most abundant CDD/Fs formed. Gas products were identified and quantitated by Fourier transform infrared spectroscopy (FTIR), with hydrogen chloride quantitated by ion chromatography (IC). The results from IC analysis afforded accurate quantitation of the formation of HCl in pyrolysis experiments. The oxidation of 4-CB in quartz reactor and quartz tube with boron-oxide coating on the reactor surfaces revealed the formation of similar VOCs and gas products, but in a different temperature window. The first product observed in the alumina reactor, benzaldehyde, initially appeared at around 300 °C, and at 450 °C and 500 °C in quartz and boron-oxide-coated quartz reactor, respectively. The onset temperatures of other VOCs products, such as PCBzs and PCPs formed in three reactors, rank as follow: boron-oxide-coated quartz reactor > quartz reactor > alumina reactor. In addition, we observed the formation of all homologue groups of PCDD/Fs but only in the alumina reactor. In quartz and boron-oxide-coated reactors, we only observed mono and diCDD/Fs. In all three reactors, 3-MCDF constituted the most abundant congener. The current measurements report the distinct mechanisms operating on reactor surfaces and in gas phase; the former appearing as a consequence of the transition metal impurities in the reactor walls and surface-generated singlet oxygen, which were neglected in previous studies. Our findings are of great practical significance as all studies on the thermal decomposition of PCB performed in the 20th century involved glass ampoule reactors. This means that, the results of the previous investigation, especially the product speciation, must be treated with caution, due to confounding effects of the homogenous and heterogeneous reaction mechanisms. On the basis of the results of the thermal decomposition of 4-CB, we investigated the decomposition of 4,4’-DCB in the boron-oxide-coated quartz reactor, in order to study the pollutants formed and to elucidate their reaction mechanisms. Benzene, chlorobenzene, phenol and 4-chlorophenol (4-CP) were found as the PCDD/F precursors, with chlorobenzene as the dominant species. We identified 2-chloronaphthalene as the main VOC product with smaller amount of naphthalene, and even smaller abundance of 1-chloronaphthalene. This indicates that, 2-chloronaphthalene forms directly from 4,4’-DCB, with possible 1,2-Cl shift in 2-chloronaphthalene producing 1-chloronaphthalene. Similar mechanism operates for the appearance of 4-chlorobenzaldehyde (forming benzaldehyde, and then 3-chlorobenzaldehyde), 1-chloro-4-ethynylbenzene (dechlorinating to 1-ethynylbenzene) and 1-chloro-4-(1-propynyl)-benzene (producing 1-chloro-6-(1-propynyl)-benzene, via dechlorination of 1-propynylbenzene, H abstraction and then chlorination at a different carbon atom). We found a small amount of 1-propynylbenzene in the experiments conducted at 625 and 650 ºC, and with this species acting as a source for other isomers of chloropropynylbenzenes. We also detected benzofuran from 625 – 700 °C. The formation of PCDD/Fs included only MCDF, MCDD, DCDF and a minute amount of DCDD. Note that, 3-MCDF was observed first at 550 °C above a background level of 0.2 ng/(g 4,4’-DCB). This temperature is about 250 °C higher than that of the first observation of PCDD/Fs in the alumina reactor. We also studied the thermal decomposition of PXB with 4-bromo-4’-chlorobiphenyl (4,4’-BCB) as a probe molecule to gain insights into the effect of combined sources of chlorine and bromine on formation of mixed halogenated pollutants. The oxidation of 4,4’-BCB in the B2O3-coated reactor revealed the formation of species similar to those appearing in fires of brominated flame retardants (BFR) and municipal waste combustion; i.e., in systems characterising both bromine and chlorine sources. The gas products detected included HCl, CO, CO2, C2H2 and HBr. To improve elemental balances of chlorine and bromine, we employed ion chromatography (IC) for quantitation of HCl and HBr. With increasing temperature, the concentration of HCl and HBr increased dramatically, in similar concentrations. We detected benzene, monobromobenzene, monochlorobenzene, one congener of mixed halogenated benzenes, one congener of mixed halogenated phenols, bromonaphthalenes, chloronaphthalenes, 6-bromobenzofuran, chlorobenzofurans, dibenzofuran, mixed halogenated ethynylbenzenes and chlorobenzaldehydes. Similar chlorinated species also formed in the oxidation of 4-CB and 4,4’-DCB. In addition, we observed the formation of one congener of PXDD/Fs by deploying a GC-QTOFMS (high resolution gas chromatography – quadrupole/time-of-flight mass spectrometry) to elucidate the reaction products. The accurate calculations of molecular mass and isotopic distribution revealed the formation of monobromochlorodibenzofuran. We also observed significant amount of 3-MCDF and one congener of DCDF (3,7-DCDF formed from the impurity 4,4’-DCB) in our experiments. We did not detect any PBDD/Fs. The quantum chemical calculations yielded the reaction pathways and provided reaction mechanisms for the pollutants observed in the experiments. These mechanisms comprise direct formation of VOCs and dioxins from 4,4’-BCB by its partial oxidation (for example, by adding an oxygen bridge between the two aryl rings to form dioxins) and synthesis of two- and three-ring species from primary decomposition fragments, such as chlorophenoxys and chlorobenzenes.
- Subject
- PCDD/Fs; VOCs; PCBs; thermal decomposition; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1310326
- Identifier
- uon:22026
- Rights
- Copyright 2015 Song Hou
- Language
- eng
- Full Text
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