Gas Phase Thermal Oxidation of Endosulfan and Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans

Abstract

This paper investigates the thermal decomposition of technical endosulfan under oxidative conditions and the subsequent formation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F, dioxins), and their precursors. Both quantum chemical calculations and laboratory experiments were employed to investigate the pathways of oxidation of endosulfan. The laboratory scale apparatus used consists of a tubular reactor and product collection system and analysis train. The results report the effect of temperature (523-923 K) and O2 concentration on PCDD/F formation in a N2 bath gas at a residence time of 5 s. The decomposition of endosulfan produces two types of PCDD/F precursors, involving all chlorinated benzenes (CBz) and chlorinated phenols (CPh). Oxidation of endosulfan favors the formation of PCDF over PCDD. Octachlorodibenzofuran is the most abundant homologue group detected in all experiments. The maximum emission factor for PCDD/F was observed at 923 K and O2 content of 6% and corresponds to 64 ng TEQ-WHO2005 per mg of endosulfan and a total dioxin concentration of 1131 ng/mg of endosulfan.

Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides

“…From our present studies of oxidation of HCCP in both an unpassified reactor (where the influences of O 2 ( 1 Δ g ) are implicated) and a passified silica reactor, we have found that maximum rates of decomposition of HCCP and formation of Cl and of HCB occur at O 2 concentrations in the N 2 bath gas of between 6 and 10%. In our earlier study of oxidation of endosulfan, the same range of O 2 concentrations was found to produce the highest yields of PCDD/F . We now attribute this maximum in PCDD/F yields in endosulfan and other cyclodienes to conditions that maximize the rate of decomposition of hexachlorocyclopentadiene.…”

“…To eliminate the surface effect, we coated the silica reactor with boron oxide (see details in the Supporting Information) as satisfactory boron oxide coating appears transparent. The flow reactor, product collection system, and analysis train have been described in detail in our earlier publications , and associated Supporting Information. A syringe pump was used to introduce reactants into the system and dilute with carrier gas.…”

“…Volatile organic compounds were analyzed by GCMS, as described previously. 5,9 Cl 2 was analyzed by ion chromatography (IC) by first dissolving the gas in dilute aqueous sodium hydroxide, in which it forms both Cl − (aq) and ClO − (aq). Both peaks in the IC were quantified to obtain the yield of Cl 2 .…”

“…However, little is known about the toxic products emitted in fires of cyclodienes; one important exception was the 1978 study by Chopra et al who identified a large number of chlorinated benzenes and other aromatics in the pyrolysis of endosulfan at 1173 K. It might be expected that the combustion of cyclodienes would produce significant emissions of polychlorinated dibenzodioxins and dibenzofurans (PCDD/F), yet to the best of our knowledge, prior to our recent study of PCDD/F from oxidation of endosulfan, there have been no previous reports on dioxins’ emissions from cyclodienes. Researchers have thoroughly investigated the mechanistic pathways of formation of PCDD/F and related molecules via several recently discovered precursors. − In order to develop safe strategies for destruction of stockpiled cyclodienes, it is important to obtain a better understanding of the mechanism of formation of toxic products, especially dioxins and their precursors, in the oxidative degradation of cyclodienes.…”

Identification of volatile and semi-volatile organic compounds emitted during thermal degradation and combustion of triadimenol

“…In gases released during thermal decomposition of endosulfan, a polychlorinated pesticide, Dharmarathne et al [11] identified chlorinated styrenes and chlorinated benzenes, major precursors of toxic polychlorinated dibenzodioxins and dibenzofurans (PCDD/F). Moreover, during oxidative studies on that pesticide, they observed large concentrations of PCDD/F [18]. Thermal decomposition of permethrin can also be a potential source of PCDD/PCDF [12].…”