Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) by precursor pathways in oxidation of pesticide alpha-cypermethrin

This contribution assesses the oxidation of the commonly used insecticide and termiticide, alpha-cypermethrin to polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F, dioxins) and their precursors, under non-catalytic vapour-phase oxidation, in a flow reactor. No soot was produced during the reaction ruling out the de novo synthesis of PCDD/F. The results span the effect of temperature (300-650°C) and fuel equivalence ratio (of 0.01-7.0), at a residence time of 1 s. XAD-2 resin trapped the PCDD/F which were identified and quantitated by high resolution gas chromatography (HRGC)-ion trap mass spectrometry (ITMS). The reactor walls appear to have no noticeable effect on the initiation of the gas-phase oxidation reactions. The decomposition of alpha-cypermethrin produces two types of PCDD/F precursors, chlorinated benzenes (CBz)/chlorinated phenols (CP) and two-ring species, mainly diphenyl ether and chlorinated diphenyl ethers (CDPE). The CBz/CP precursors comprise chlorobenzene, o-, m- and p-dichlorobenzene, 1,2,3-trichlorobenzene, phenol, 2-monochlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-TriCP, with minor amount of 2,4,5-TriCP. The highest emission factor for PCDD/F was observed at 550°C and a fuel equivalence ratio of 0.03, corresponding to 19 ng ∑TEQ-WHO₂₀₀₅/g of alpha-cypermethrin and the total dioxins concentration of 141 μg/g of alpha-cypermethrin. These emissions are one of the highest measured from oxidation of presently used pesticides.