Distribution of Mono to Octa-chlorinated PCDD/Fs in Fly Ash from a Municipal Solid-Waste Incinerator LISA LUNDIN* AND STELLAN MARKLUND Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden Received July 20, 2007. Revised manuscript received November 28, 2007. Accepted December 3, 2007. We have estimated the concentration and distribution of the mono to octa-chlorinated congeners of polychlorinated dibenzo- p-dioxins (PCDD) and dibenzofurans (PCDF) in fly ashes at various sampling points in a large-scale municipal solid waste incinerator at Umeå, Sweden, as they cooled from 700 to 170 °C. Differences between the ashes were observed, the PCDD homologue profile was found to vary with temperature. The total amount of PCDD and PCDF increased as the temperature decreased in the postcombustion zone. The increase was due to both adsorption to the fly ash and formation of PCDD and PCDF. Mono- to trichlorinated PCDD predominated at high temperatures, whereas hepta- and octachlorinated PCDD predominated at temperatures below 400 °C. PCDF predominated over PCDD in the whole temperature range. However, the changes in homologue profile for PCDF were minor. The isomer distribution within the homologue groups was not changed as the temperature decreased in the postcombustion zone. Introduction Previous studies of emissions of polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDD/F) from full-scale municipal solid waste (MSW) incinerators have concluded that emissions to air are small, and that most of the tetra- to octa-chlorinated PCDD/F species are concentrated in the solid residues, such as fly ash and flue gas cleaning products (1–3). These residues must then be remediated or deposited in landfill sites. It has also been shown that the concentrations of tetra- to octa-chlorinated PCDD/F compounds in both flue gases and fly ash increase at the lower temperatures of the postcombustion zone (2, 4–8), and that fly ash can act as a catalyst for the formation of these species (9, 10). There are four hypotheses regarding the mechanisms of formation of PCDD/F: (i) pyrosynthesis, (ii) de novo synthesis, (iii) formation from precursors, and (iv) chlorination/ dechlorination. The second and third mechanisms are generally considered the most likely, and pyrosynthesis the least. Similarities between the distribution patterns and concentrations of the PCDD/F species formed have been cited in support of both the de novo (ii) and the precursor (iii) theories. Although de novo synthesis has been defined as formation of PCDD/F from residual carbon, chlorine, and oxygen (10, 11), it is more accurately described as a series of decomposition reactions in the carbon matrix. Iino et al. (11) have shown that PCDF congeners can be formed from polycyclic aromatic hydrocarbon species (PAHs), and have suggested that chlorination and oxygenation of preformed aromatic fragments may be one of the pathways in de novo PCDF synthesis. Although the formation of PCDD from macromolecular carbon has been shown to proceed partly via chlorophenol intermediates, pathways to PCDF species do not generally appear to involve chlorophenol or chlorobenzene precursors, but rather biphenyls (12). These observations imply that the de novo and precursor pathways may be closely related (12, 13), and the traditional distinction between the two is becoming blurred (14, 15). A chlorination mechanism for PCDD formation has been shown to predominate over de novo synthesis at the relatively low temperatures of the postcombustion zone of MSW incinerators (7), where both chlorination and dechlorination are known to occur (16). Wikström et al. have also demon- strated that different reactions involving dibenzo-p-dioxins and dibenzofurans can also make substantial contributions to the formation of PCDD/F species (16, 17). The distribution of positional isomers for each PCDD/F homologue has been shown to be independent of combustion conditions such as temperature and gas velocity (17–21). However, both the overall conversion of dibenzo-p-dioxins and dibenzofurans (DD/DFs) and the degree of chlorination do vary significantly with temperature and gas velocity (19). Chlorination of lightly chlorinated congeners (17) and dechlorination of highly chlorinated congeners (22) have also been proposed as mechanisms that might influence PCDD/F isomer distributions. To the best of our knowledge, there has been no previous analysis of the lightly chlorinated (mono to trichlorinated) congeners of PCDD/F in fly ash from a full-scale MSW incinerator, or of PCCD/F in fly ash from such a plant sampled at temperatures exceeding 500 °C. The objective of this study was to map the concentrations and distribution of mono to octa-chlorinated PCDD/F isomers in fly ashes collected from three various points in a full-scale MSW incinerator as they cooled from 700 to 170 °C. Experimental Section The fly ash used in this study was from the Dåva combined heat and power plant, Umeå Energi, Umeå, Sweden, collected at three points (Figure 1). The plant has an incinerator with an effect of 65 MW and a capacity of 20 tonnes h -1 . NH 3 is added to decrease NO x levels and active carbon and Ca(OH) 2 are added before the fabric filter. During the day, the ashes were collected the combustion conditions were the following; O 2 in the economizer varied between 6 and 8%, an average of CO ∼20 mg/Nm 3 (dry gas, 11% O 2 ), an average of HCl ∼600 mg/Nm 3 (dry gas, 11% O 2 ), H 2 O varied between 16 and 17% before the textile filter. The temperature in the combustion chamber was ∼1100 °C. The ashes are individual samples and were collected during the same day but not simultaneously, and ap- proximately 1 kg of ash was collected from the bottom of the superheater, a hopper in the heat exchanger and from the fabric filter. Ash A-700 was obtained from a point where the temperature of the flue gases exceeded 700 °C, ash B-350 from a cooler zone (300–400 °C), and ash C-170 from a textile filter with a working temperature of approximately 170 °C. The temperatures referred to are the temperature in the flue gas, which is measured continuously. However, the tem- * Corresponding author e-mail: Lisa.lundin@chem.umu.se. Environ. Sci. Technol. 2008, 42, 1245–1250 10.1021/es0717919 CCC: $40.75  2008 American Chemical Society VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1245 Published on Web 01/15/2008