Comparative Study on the Formation of Toxic Species from 4-chlorobiphenyl in Fires: Effect of Catalytic Surfaces

Abstract

This contribution compares the formation of toxic species, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) and their precursors such as polychlorobenzenes (PCBz) and polychlorophenols (PCP), during oxidation of 4-chlorobiphenyl (4-CB) in a laboratory-scale apparatus, under conditions similar to those that occur in fires and waste combustion. The experiments were conducted using two gas flow reactors made of 99.5% alumina and high purity quartz, respectively. A sampling system intercepted the gaseous products leaving the reactors, trapping the volatile organic compounds (VOC; i.e., PCBz and PCP) and PCDD/F on a XAD-2 cartridge. The analysis of VOC involved high resolution gas chromatography (HRGC) – quadrupole mass spectrometry (QMS) while HRGC – ion trap (IT) MS/MS quantitated the PCDD/F produced. For the experiments using the alumina reactor, VOC analysis revealed the formation of PCBz and PCP, commencing at temperatures as low as 400°C. Other products included benzaldehyde, naphthalene, 3- ethylbenzaldehyde, 1-chloro-4-ethynylbenzene and benzofuran. Gaseous species such as CO, CO2 and HCl were detected and quantitated either by Fourier transform infra-red spectroscopy (FTIR) or ion chromatography (IC). Similar products were found to form in the quartz reactor, however, their formation commenced at 500°C, with their yields significantly lower than those found for the alumina reactor. The present measurements indicate that surface reactions govern the oxidation of 4-CB between 300 and 650°C for the alumina reactor, and between 450 and 600°C for the quartz reactor. At 700°C, both reactors operate similarly, with the oxidation process dominated by the gas phase reactions. With respect to dibenzo-p-dioxins and dibenzofurans, only isomers of chlorinated monochlorodibenzofuran (MCDF) and chlorinated dichlorodibenzofuran (DCDF) were found at low temperatures (300 to 450°C), with 3-MCDF as the dominant congener. In our system, they appear to form in gas phase reactions involving 4-CB and singlet oxygen (1Δg O2), the latter generated on the reactor walls. The present results indicate that the combustion of 4-CB in fires will be dominated by catalytic surfaces of fly ash below 600°C, and by gas-phase kinetics above 700°C.