Formation of chlorinated dibenzo-p-dioxins and dibenzofurans (CDD/F) and their precursors in oxidation of 4-chlorobiphenyl

摘要

This study investigates the gas-phase thermal oxidation of 4-Chlorobiphenyl (4-CB) over a temperature range of 300 to 700°C, under conditions similar to those occurring in fires and open burning of waste. The experiments deployed an isothermal flow reactor made of high purity quartz and equipped with a sample collection system to adsorb volatile and Semi-Volatile Organic Compounds (VOC/SVOC), the latter included Chlorinated Dibenzo-p-dioxins and Dibenzofurans (CDD/F). The analysis was performed by Gas Chromatography-Quadrupole Mass Spectrometry (GC-QMS) and GC-ion trap MS/MS (GC-ITMS/MS), for VOC and SVOC, respectively. Under oxidative conditions, the thermal degradation of 4-CB produced VOC species; mainly, naphthalene, styrene, and benzaldehyde. Selected Ion Monitoring (SIM), performed on the GC-QMS instrument, confirmed the formation of minor species, such as Chlorophenols (CP) and Chlorobenzenes (CBz), first observed to form in experiments conducted around 500°C. The trace analysis by GC-ITMS/MS identified mono and dichlorinated isomers of CDD/F (dioxins), with Monochlorodibenzofurans (MCDF) as the dominant homologue group. We observed the formation of 3-chlorodibenzofuran at a temperature as low as 300°C, possibly by direct oxidation of 4-CB by singlet oxygen (1Δg) generated on the walls of the quartz reactor, or by catalytic oxidation of 4-CB by the transition metals remaining in the high purity quartz at sub-ppm levels. Other CDD/F form in gas-phase reactions at temperatures in excess of 600°C.