Determination of Desorption Losses of Polychlorinated Dibenzo-p-Dioxins from Municipal Incinerator Flyash by Gas Chromatography/Mass Spectrometry

Abstract

Municipal incinerator flyash was heated in a pyrex tube under a flow of high-purity nitrogen. The levels of polychlorinated dibenzo-p-dioxins (PCDD) in the heated samples were determined using gas chromatography/mass spectrometry (GC/MS) and compared to levels in unheated samples. In a time and temperature study where temperatures from 100° to 200°C and heating times from 16 to 48 hours were investigated, PCDD levels decreased to 10 to 30 percent of the levels in unheated samples. The final PCDD levels were independent of heating time and temperature under the above conditions. About 10 percent of the PCDD could not be desorbed and are thought to be chemisorbed on the surface of the flyash particles. Heating the flyash at 120°C for 2 to 6 hours desorbed 15 to 38 percent of the PCDD. The desorption rates for the PCDD homologues were estimated to be 0.037, 0.086, 0.15 and 0.23 ng/g/hr/L for tetrachloro-, pentachloro-, hexachloro- and heptachlorodibenzo-p-dioxins respectively. At constant temperature these rates appear to depend mainly on the amount of the particular PCDD homologue originally present on the flyash, not on physiochemical properties such as vapour pressure or dipole moment. When expressed as percent/hr/L, desorption rates for the PCDD homologues were not significantly different from each other. The average value was 0.78 percent/hr/L. Desorbed PCDD were adsorbed onto exhaustively extracted flyash placed in the tube downstream of the heated sample. There was no evidence of decomposition of the PCDD under the experimental conditions.