Formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in oxidation of captan pesticide

This study assessed the emission of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD and PCDF) from captan, a commonly used fungicide, in vapour-phase oxidative pyrolysis under conditions similar to those encountered in fires and burning biomass contaminated or treated with pesticides. The laboratory-scale apparatus consisted of a pesticide vaporiser, a tubular reactor and a product sampling system. The sampling train comprised tandem XAD-2 resin cartridges to trap PCDD/F as well as an activated charcoal tube to capture the organic volatile compounds (VOC). The analyses of PCDD/F were conducted by means of high resolution gas chromatograph (HRGC) - ion trap mass spectrometer (ITMS) and analyses of VOC by HRGC - quadrupole mass spectrometer (QMS). Substantially more PCDF formed than PCDD in the oxidative pyrolysis of captan, with higher yield of total PCDD/F observed at longer residence time. As indicated by the homologue distribution of PCDD/F, only mono to tetra chlorinated congeners were detected in our measurements, with 4-monochlorinated dibenzofuran (4-MCDF) ranking as the most abundant congener. The results of VOC analysis revealed benzene and chlorinated benzenes as important PCDD/F precursors. Combining the experimental measurements and the results of quantum chemical calculations, we established the reaction pathways for formation of PCDD/F from the vapour-phase oxidative pyrolysis of this widely employed fungicide.