Thermal Extraction of Dioxin Surrogates and Precursors from Filter Dust: Effects of Temperature, Time, and Matrix

Abstract

This study identified potential precursors and surrogates of dioxins during thermal extraction (TE) of a sooty filter dust in a wide range of operating conditions, mainly temperature (120 degrees C-540 degrees C) and time, during direct thermal extraction (DTE) in a dedicated commercial analyzer and indirect thermal extraction (ITE) in a thermobalance used as desorber. Using identical analytical techniques, DTE products were monitored in the range of 120 degrees C-320 degrees C, and ITE products were monitored in the range of 300 degrees C-540 degrees C. Emphasis was on polychlorinated dibenzo-p-dioxins/furans potential precursors and surrogates: aromatic hydrocarbons as well as O, Cl, N, and S compounds. Aliphatic compounds were not monitored. Product distributions differ markedly between both TE techniques: matrix effects eliminate heteroatoms O, Cl, and N, rendering especially DTE less suitable for characterizing a complex sample. Effects of temperature and time were specific for each compound and tentatively explained by low-temperature desorption (up to 240 degrees C) and progressive evolution of volatiles and their eventual decomposition on the matrix at higher temperature. The particular filter dust sample was selected due to its amount of volatiles and fixed carbon; it derived from a baghouse filter, cleaning off-gas from a clean aluminum scrap-melting furnace. TE results strongly diverged from those obtained by solvent-extraction-based analysis. Hence, TE cannot be recommended for analyzing complex samples. A second article compared de novo formation from samples precleaned by TE with raw samples. Such tests may differentiate between dioxin formation and carbonaceous matter (de novo route) from the precursor route involving sorbed organics. Results suggest that there is little or no resemblance between TE and de novo products. The novelty of these articles resides in monitoring numerous dioxin structurally related compounds during (1) TE under reducing conditions and (2) de novo tests in an airflow. The scope of these tests is exploratory and the results suggested strong influence of the matrix on the products evolved.