- Title
- Formation of toxic pollutants during the thermal decomposition and oxidation of cyclodiene pesticides
- Creator
- Dharmarathne, Welayudage Nirmala Kumuduni
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis provides an assessment of the toxic products formed during the thermal decomposition of the cyclodiene group of pesticides and their main intermediate, under gas phase conditions. Cyclodiene pesticides, namely, dieldrin, aldrin, endosulfan and etc have widespread applications to control broad range of insects that spoil agricultural plantations. Thermal decompositions of the cyclodiene pesticides occurs in the fire that storage and processing facilities or in burning of biomass treated or contaminated with cyclodiene. The presence of six chlorine atoms in the molecule of cyclodiene indicates the propensity for releasing highly toxic chlorinated volatiles in combustion processes. We employed a bench type tubular reactor system to perform experiments, varying oxygen contents and temperatures. The analysis of the mechanism was assisted by the quantum chemical calculations, allowing us to propose mechanisms for the thermal decomposition of cyclodiene pesticides and their intermediates. The thermal decomposition of endosulfan (C9H6Cl6O3S) produces SO2, HCl, CO, 2,5-dihydrofuran and propene as gaseous compounds and the analysis of volatile organic compounds (VOC) identified hexachlorocyclopentadiene (HCCP), tetrachlorostyrene (TCS), pentachlorostyrene (PCS), pentachlorocyclopentadiene (PCCP), and all the range of mono to hexa chlorinated benzenes. Experimental and quantum chemical calculations indicated that the initial step of the pyrolysis of endosulfan started at 573 K, involving a retro-Diels-Alder elimination reaction which produces HCCP and 4,7-dihydro-1,3,2-dioxathiepine-2-oxide (2HDTO). At reaction temperatures of 663 K and above, fission of a Cl atom from HCCP results in the formation of endosulfan radical, which promotes further decomposition of endosulfan via a low barrier free radical pathway and arising secondary decomposition products including HCl, TCS, PCS, PCCP and polychlorinated benzenes. The oxidative decomposition of endosulfan generates substantially more PCDF than PCDD. The maximum emission factor of total PCDD/F observed at 923 K with 6% O2 content and corresponds to 64 ng TEQ–WHO2005/mg of endosulfan and a total dioxin concentration of 1131 ng/mg of endosulfan. The VOC analysis revealed that chlorinated benzenes (CBz) and chlorinated phenols (CPh) as a PCDD/F precursors. As indicated by the homologue distribution of PCDD/F, all the range of mono to octa chlorinated PCDD/F were detected in our measurements. Reducing the O2 concentration from to 1.03% causes a significant decrease in the rate of PCDD/F formation over all temperatures. Hexachlorocyclopentadiene (HCCP) is the chlorinated reagent which is a key intermediate in the production of cyclodiene pesticides via Diels-Alder addition with a substrate. The investigation of the pyrolysis of HCCP revealed the reaction is initiated by chlorine atom fission, forming the pentachlorocyclopentadienyl radical, two of which combine to form an energized bis-(pentachlorocyclopentadienyl), (S0). Following S0 undergoes a series of intramolecular rearrangements and Cl atom fissions ultimately to produce principally octachloronaphthalene and Cl2. Under oxidative conditions, Cl fission from HCCP initiates reaction but O2 bimolecular reaction with energized S0 competes with Cl fission such that 8ClNP yields decrease and hexachlorobenzene and CO yields increase with increase in temperature and O2 mole fraction up to about 10 mol%. Pyrolysis of dieldrin is initiated by Diels-Alder decomposition of HCCP and norbornadiene oxide, which further decomposes to produce CO and benzene. Above ~800 K, secondary decomposition reaction started via a decomposition of dieldrin radical mechanism and observed high temperature products, HCl, pentachlorostyrene, hexachlorostyrene and polychlorinated benzenes. However, thermal decomposition of isodrin which is an isomer of aldrin, undergoes a different reaction mechanism, an intramolecular rearrangement to a “birdcage” structure with barrier of 251 kJ/mol which eventually decomposes to HCl and soot. Both dieldrin and endosulfan undergo low temperature decomposition mechanisms eliminating products of comparatively low toxicity at temperatures below which HCCP fissions Cl atoms. Low temperatures and long residence times maximize the efficacy of remediation. Thermal desorption of either cyclodiene under the conditions above would lead to an unacceptably high emission of toxicants, with dieldrin, because of its higher activation energy for retro Diels Alder decomposition, leading to a much more elevated level of unwanted products in comparison with endosulfan. The oxidative thermal decomposition of dieldrin produces all the range of PCDD/F except OCDD. The highest emission factor of dioxin reported at 923 K with 6% O2 content corresponds to 6.24 ng TEQ WHO 2005 /mg of dieldrin. TEQ values calculated for dieldrin almost ten times lower than the endosulfan which studied previously.
- Subject
- cyclodiene pesticides; toxic pollutants; thermal decomposition; endosulfan
- Identifier
- http://hdl.handle.net/1959.13/1395621
- Identifier
- uon:33915
- Rights
- Copyright 2019 Welayudage Nirmala Kumuduni Dharmarathne
- Language
- eng
- Full Text
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