De Novo Synthesis of Polychlorinated Dibenzo - American Chemical

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Environ. Sci. Technol. 2001, 35, 1616-1623

De Novo Synthesis of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans on Fly Ash from a Sintering Process C EÄ L I N E X H R O U E T , * CATHERINE PIRARD, AND EDWIN DE PAUW Mass Spectrometry Laboratory, University of Lie`ge, B6c Sart Tilman, B-4000 Lie`ge, Belgium

Fly ash, collected in the electrostatic precipitator of a sinter plant in Belgium, has been examined and characterized in terms of its behavior with respect to thermal polychlorodibenzo-p-dioxins (PCDD) and polychlorodibenzofurans (PCDF) formation. Thermal experiments of the fly ash were conducted in a flow of air. The temperature was varied from 250 to 450 °C, and the reaction time varied from 30 min to 6 h. For comparison, the oxidative degradation of carbon in the fly ash was studied by differential scanning calorimetry (DSC) in the temperature range from 50 to 500 °C. Besides the known maximum of formation of PCDD/Fs around 325 °C generally found on experiments with incinerator fly ash, a second maximum of formation around 400 °C is observed on the sinter fly ash used in this study. DSC measurements on the fly ash show that the oxidative degradation of carbon appears at these two different temperatures confirming that the de novo synthesis on this kind of fly ash take place at two different optimum temperatures. About the reaction time, already after 30 min, an important quantity of PCDD/Fs is formed; the fast increase in PCDD/Fs amount is followed by a slower formation rate between 2 and 4 h. At longer reaction time, the formation slows down, and decomposition reactions become important. Analysis of homologue distribution indicates that the profile of PCDD/Fs is independent of the reaction time but that an increase of the temperature leads to a rise of lower chlorinated species. In all experiments, PCDF are formed preferentially (total PCDF/PCDD ratios larger than 5). The PCDF/PCDD ratio is clearly independent of the reaction time. Concerning the temperature, the apparently better stability of PCDF at high temperature (PCDF/ PCDD ratio higher at high temperature) results in the fact of different PCDF/PCDD ratios for the different family and modifications of homologue distribution with the temperature. The isomer distribution shows little reaction time or temperature dependency, which is an argument in favor of a thermodynamic control of the isomer distribution during de novo formation of PCDD/Fs. Differences within the isomer distribution patterns of PCDD/Fs obtained from the laboratory de novo synthesis experiments and the original fly ash, reflecting the formation under the industrial process, suggest a different mechanism of formation in the sinter plant for the PCDD and PCDF. The de novo synthesis is sufficient to explain the PCDF formation in the real 1616

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process, but synthesis from precursors must play a role for the PCDD formation.

Introduction Since the discovery of polychlorodibenzo-p-dioxins (PCDD) and polychlorodibenzofurans (PCDF) in the flue gas and fly ash of municipal waste incinerators by Kees Olie in 1977 (1), the formation of these toxic compounds has been studied intensively. Recent reviews summarized the most important trends and results (2, 3). Two different pathways have been proposed to explain the presence of PCDD/Fs in the emissions of incinerators or other combustion processes: the synthesis from precursors and the de novo synthesis. The first pathway involves reactions of chemical similar precursors to PCDD/Fs, such as chlorophenols. These are formed initially as products of incomplete combustion. The de novo synthesis consists of the PCDD/Fs formation from macromolecular carbon present in the fly ash. It concerns heterogeneous reactions between the gas phase and the fly ash catalyzed by some constituents of the fly ash such as copper and iron chlorides. Many authors postulate that this synthesis could take place essentially in the post-combustion zone of the incinerators at a temperature around 300 °C (4). The de novo formation of PCDD/Fs is supposed to be strongly correlated with the metal-catalyzed oxidation of carbon in the fly ash (5). The oxidative degradation of the carbon structure gives mainly gaseous products CO2 and CO as well as, in a minor pathway, some small aromatic compounds including PCDD/Fs. Although iron and steel industries are known to be important sources of PCDD/Fs in different countries (6, 7), most studies concerning the formation of these highly toxic compounds deal only with municipal waste incineration. All laboratory experiments use incinerator fly ash (8-18) or model mixtures (8, 15, 19, 20). Relatively few data are available for industrial and metallurgical processes, in particular for sintering plants (21-27). Considering the very large gas flow volumes discharged from many industrial processes, dioxin pollution by these sources seems to be a serious problem. In Belgium, it can be estimated from the data by Wevers and De Fre´ (28) that, in 1995, the industry sector was responsible for 34% of the total dioxin emissions; among the different industries, 24% of the dioxin emissions estimated for the industrial sector came from the sintering. This study describes the thermal behavior of fly ash from a sinter plant relating to the de novo synthesis of PCDD/Fs. The homologue and isomer distributions are also examined to get a better understanding of the formation mechanism of these compounds. Preliminary results of our investigation have been published before (29).

Experimental Section Materials. The following materials were used: solution of 2,3,7,8-Cl-substituted 13C12-labeled PCDD/Fs (EPA 1613 LCS, Campro Scientific, Veenendaal, The Netherlands); toluene (p.a., Baker); hexane (p.a., Baker); dichloromethane (p.a., Vel); dodecane (Merck); sulfuric acid (95-97%, Baker); sodium chloride (p.a., Merck); potassium hydroxide (p.a., Merck); anhydrous sodium sulfate (Baker); aluminum oxide (activated, neutral, type 507c, Aldrich); glass wool (DMCS treated, Alltech Europe); and technical dry air (Air Liquide, Belgium). * Corresponding author e-mail: [email protected]; phone: +324-366-34-22; fax: +32-4-366-34-13. 10.1021/es000199f CCC: $20.00

 2001 American Chemical Society Published on Web 03/16/2001

Fly Ash. Fly ash was collected in the electrostatic precipitator of a Belgian sintering plant. This electrofilter consists of three fields and is operating at 120-130 °C. The fly ash used in this study comes from field 3 and was stored at ambient temperature prior to lab experiments. Around 72.5 wt % of the fly ash has a size under 40 µm. Experimental Apparatus. Fly ash (5 g) was packed into a horizontal glass tube reactor (16 cm long, 3 cm diameter) with glass wool as plugs. The tube was placed in a chromatographic furnace, and the samples were heated at different temperatures (between 250 and 450 °C) under a flow of technical air (100 mL/min) for different reaction times (30 min-6 h). Products evaporating from the fly ash were collected using two washing bottles in series (100 mL of toluene cooled with ice). The toluene from the cold traps was used for the Soxhlet extraction (see Cleanup section). All experiments were conducted with extracted fly ash in order to minimize potential interferences from adsorbed organic precursors and native PCDD/Fs. Prior to experiments, all fly ashes were Soxhlet extracted with toluene (2 × 24 h), rinsed with hexane, and air-dried at room temperature. This fly ash, cleared of natives PCDD/Fs, is called “extracted fly ash” in the rest of the paper as opposed to the “original fly ash”, which refers to the fly ash coming directly from the sintering plant without any pretreatment and containing the natives PCDD/Fs. Only trace amounts of PCDD/Fs were found in the extracted fly ash. Each experiment was performed in duplicate or triplicate. Cleanup. The slightly modified EPA-8280 method was followed for classical PCDD/Fs analysis. The total fly ash sample heated in the furnace was spiked with 10 µL of the standard solution of 2,3,7,8-Cl-substituted labeled PCDD/ Fs (EPA 1613 LCS). Soxhlet extraction was performed using toluene during 24 h. At the end of the extraction, the apparatus was cooled at room temperature, and then the solvent was exchanged to a total of 25 mL of hexane. The extract was successively washed with 5% NaCl solution, 20% KOH solution (maximum of 4 washings), 5% NaCl solution again, concentrated sulfuric acid (maximum of 4 washings), and a final 5% NaCl solution. The organic layer was dried over Na2SO4 and concentrated to 2 mL. The extract was cleaned up on a 4-g neutral alumina column, with the first fraction of 10 mL of hexane:dichloromethane (92:8) discarded and the second fraction of 15 mL of dichloromethane:hexane (60:40) containing PCDD/Fs kept. A total of 50 µL of dodecane was added to the second fraction, and the extract was concentrated by blowing it down to 50 µL under a gentle stream of nitrogen prior to GC/MS analysis. Analysis. All analyses were performed by HRGC/HRMS using VG-Autospec-Q high-resolution mass spectrometer and Hewlett-Packard 5890 series II gas chromatograph. The GC conditions were optimized to separate most of the PCDD/Fs as followed: column, SP2331 capillary column (Supelco, 60 m × 0.25 mm i.d., 0.2 µm film thickness) for all PCDD/Fs except HpCDD, OCDD, and HpCDF on J&W DB-5ms (30 m); splitless injection of 2 µL of the extract at 270 °C; initial oven temperature, 150 °C; temperature programming, 150 °C, held for 1 min, then increased at 15 °C/min to 200 °C, then increased at 1.2 °C/min to 273 °C, and held 18 min. Helium was used as carrier gas. The mass spectrometer was operated in the electron impact ionization mode using selected ion monitoring. The mass spectrometer was tuned to a minimum resolution of 10000 (10% valley) and was operated in a mass drift correction mode using PFK to provide lock masses. The two most abundant ions in the chlorine clusters of the molecular ion were recorded for each isomer of native and labeled PCDD/ Fs. The electron energy was set to 45 eV, and the source temperature was set to 250 °C.

FIGURE 1. Homologue distribution of the original fly ash. Identification and Quantification. Most of the T4CDDOCDD and T4CDF-HpCDF congeners were analyzed. No analyses of the species without chlorine or less than 4 chlorines were performed. Native concentration was determined by isotopic dilution using the 2,3,7,8-Cl-substituted labeled PCDD/Fs to quantify all the native isomers within homologues assuming equal response for all isomers within an isomer group and no isomer-selective losses during the cleanup. The isomers were identified according to ref 30. DSC Measurements. The measurements by differential scanning calorimetry (DSC) were performed in a Setaram, model 131, in air. The sample (50 mg) was placed in an aluminum crucible. The heating rate was 1 °C/min from 50 to 500 °C.

Results and Discussion Fly Ash Examination. Figure 1 presents the homologue distribution of PCDD/Fs found in the original fly ash. This distribution is quite different for PCDD and PCDF. The PCDD profile presents a maximum with the hexachlorinated group, while the PCDF profile shows a decrease from the tetra- to heptachlorinated families. An other important difference is the amount found for the two groups: the original fly ash contains a total of 150 ng/g of PCDD and 617 ng/g of PCDF giving a PCDF/PCDD ratio larger than 1. The toxicity of the sample was also quantified by the analysis of the 17 2,3,7,8-chlorinated congeners: PCDD, 1.7 ng TEQ/g; PCDF, 15.1 ng TEQ/g. The main constituent of the original fly ash was obviously iron (with 49.9 wt %), followed by chlorine, potassium, and calcium with 9.5, 9.1, and 7.8 wt %, respectively. The carbon content was determined to be around 3.3 wt % in the original fly ash and around 2.7 wt % in the extracted fly ash used in the thermal experiments described in the next sections. PCDD/Fs Formation as a Function of Temperature. Optimal Temperature for Formation of PCDD/Fs on Sinter Plant Fly Ash. A first set of experiments was carried out with different temperatures between 250 and 450 °C. The different samples were analyzed for PCDD and PCDF. As shown in Figure 2 with reaction time of 2 and 4 h, the de novo formation of PCDD as well as PCDF on extracted fly ash reaches a maximum at two different temperatures. Besides the known maximum of formation of PCDD/Fs around 325 °C generally found on experiments with incinerator fly ash, a second maximum of formation is observed around 400 °C. Similar results (not shown here) were observed for other reaction time (30 min, 6 h). Considering the temperature domain 250-450 °C (with two maxima at 325 and 400 °C) found in the thermal experiments for the PCDD/Fs formation, the electrostatic precipitator of the real process operates at a too low temperature (120-130 °C) for a significant PCDD/Fs generaVOL. 35, NO. 8, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. DSC experiments on extracted fly ash, temperature rate 1 °C/min from 50 to 500 °C.

FIGURE 2. Formation of PCDD (A) and PCDF (B) on extracted fly ash as a function of the temperature; air (100 mL/min), reaction time 2 and 4 h. tion. Nevertheless, other parts of the sintering process have an appropriate temperature for a de novo synthesis. It concerns in particular the separate chambers, situated below the belt, that collect the off-gas. Depending on the chamber position along the belt, the temperature (measured below the belt) varies between 50 and 400-450 °C. There are thus zones in some chambers at appropriate temperature (between 250 and 450 °C, notably 325-400 °C) where a de novo synthesis of PCDD/Fs can take place on entrained particles or on the particles stuck on the walls. The presence of two optimum temperatures found in the laboratory experiments is in contrast with what is generally found for the incinerator fly ash: depending on the fly ash used and the experimental conditions, the optimal unique temperature of PCDD/Fs formation on incinerator fly ash is between 300 and 325 °C (4, 9, 10, 14, 16, 18). To our knowledge, only a few authors found two optimal temperatures for the formation of PCDD/Fs on incinerator fly ash: Schwarz and co-workers (31) studied the formation of different compounds including PCDD/Fs on incinerator fly ash and found a large increase of the concentration at 300 °C but also at 470 °C. Two hypotheses were postulated to explain this phenomenon: First, the maximum formation found around 325 °C is a de novo synthesis of PCDD/Fs, but the second one is a formation from precursors. Gullett et al. have shown that PCDD formation from a mixing of polychlorophenols studied between 200 and 500 °C appears around 400 °C (32). In our experiments, we can expect that precursors have been formed by de novo synthesis and react to form PCDD/Fs around 400 °C. 1618

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Second, the carbon present in the fly ash oxidizes at two different optimum temperatures and the two peaks are really two de novo syntheses of PCDD/Fs. The first hypothesis seems not realistic since the same should appear with incinerator fly ash experiments. To be able to prove the second hypothesis, DSC experiments were carried out with extracted and original fly ash. A typical DSC curve from extracted fly ash is presented in Figure 3. This curve is characterized by an exothermal reaction that starts at a temperature of 225 °C with two maxima at 350 and 420 °C. The two temperatures found correspond quite well to those found for the de novo formation of PCDD/Fs. The same curve with two maxima at the same temperatures was found for the original fly ash (not shown here). The total energy released was determined to be 450 J/gsample or 22.5 J/mgcarbon for the extracted fly ash and 607 J/gsample or 20.2 J/mgcarbon for the original fly ash. These energy values are in very good agreement with the value of 22.9 J/mgcarbon found by Stieglitz et al. on fly ash from municipal waste incinerator (10) and in the same order of the enthalpy for the reaction C + O2 f CO2 with ∆H ) -393.77 kJ/mol or 32 J/mg. The reason for the deviation from the theoretical value may be experimental errors (calibration problem) but also an uncertainty in the real amount of the carbon oxidized, which enters the calculation. Under nitrogen atmosphere no reaction was noted, which is again proof that the oxidation of carbon is the observed exothermal reaction. The oxidation of carbon to CO2 normally proceeds at temperature above 700 °C, so a catalytic effect of the fly ash on this reaction is evident. The role of copper and halide (Cl, Br, F) in this oxidation was recognized already in previous papers (33), the presence of these compounds modifying the reaction temperature. The particularity of the results found in this study on fly ash coming from a sinter plant is the presence of two temperatures for the de novo synthesis of PCDD/Fs and for the oxidation of carbon. In a previous paper (10), Stieglitz et al. found such results for DSC experiments on municipal waste incinerator fly ash but only in particular conditions: in the untreated sample (original municipal waste incinerator fly ash), a single reaction peak dominates with a temperature at 347 °C but with treated samples (annealed fly ash during different times), a second peak emerges at higher temperature. The importance of this second peak increases with annealed time. Our sinter plant fly ash shows the same DSC behavior that municipal incinerator fly ash has after thermal treatment. Stieglitz et al. propose two explanations for the presence of two different optimum temperatures for the carbon oxidation in the fly ash: the first hypothesis postulates that during the thermal treatment the carbon is not only oxidized

FIGURE 4. Homologue distribution of PCDF as a function of the temperature; air (100 mL/min), reaction time 2 h. The error bars represent the standard deviation. but also partly converted to a species for which higher temperatures are required for complete oxidation. A second explanation may be that essential reaction partners, such as catalytic metal chlorides, become deficient in the environment of the carbon particle and may be supplied to the reaction site only by diffusion processes at elevated temperatures. The authors insist however that there exists a great difference in the reactivity of fly ash from different incineration plants and therefore caution must be applied to the interpretation of laboratory experiments. Homologue Profile as a Function of Temperature. To get a better understanding of the formation mechanism of the PCDD/Fs on sinter plant fly ash, the homologue distributions were also examined. Figure 4 presents the PCDF homologue distribution for the different temperatures studied for a reaction time of 2 h (∑TCDF - HpCDF ) 100%). Some changes in the homologue distribution can be observed when increasing temperature. The lower chlorinated species (TCDF) shows a great increase between 325 and 350 °C while the PeCDF remain constant and the most chlorinated (hexa and hepta-CDF) decrease. The same behavior was found for the PCDD as well as for other reaction times (not shown), but sometimes the trend is not so clear. The decrease in degree of chlorination at higher temperatures suggests that, with increasing temperature, the lower chlorinated species are either more stable, formed preferentially, or that dechlorination reactions become more important. According to Schwarz (31), the decrease in the degree of chlorination can be explained by the relative reaction speed of decomposition on one hand and vaporization on the other. The lower chlorinated compounds evaporate, according to their lower boiling points, and survive in the impingers, whereas the high chlorinated PCDD/Fs get destroyed on the hot surface of the fly ash. PCDF/PCDD Ratios as a Function of Temperature. From the data, values for the ratio of PCDF/PCDD concentrations formed may be calculated. The results are presented in Table 1. Depending on the temperature, the total values are from 4.9 at 250 °C to 19.4-22.7 at 450 °C. In all experiments, the PCDF are thus formed preferentially, but at 250 °C, the PCDF are only formed around 5 times more than the PCDD. At 450 °C, this formation raises up to around 20 times more than the PCDD. These results are in perfect agreement with those

of Addink et al., who studied the de novo formation of PCDD/ Fs on incinerator fly ash. This suggests that the PCDF are more stable than the PCDD at higher temperatures. The PCDF/PCDD ratios for the different congeners are however very different. First, there is a big decrease between TCDF/ TCDD and HpCDF/HpCDD. For the tetrachloro species, the range of the ratio is between 25.7 and 60.0, whereas the HpCDF/HpCDD ratio is only between 1.7 and 12.3. Second, for the individual ratios, there is no clear tendency concerning the temperature. The apparently better stability of PCDF at higher temperatures (PCDF/PCDD ratios larger at higher temperatures) results from different PCDF/PCDD ratios for the different chlorofamilies and modifications of homologue distribution with the temperature (as shown in the previous section). PCDD/Fs Formation as a Function of Reaction Time. A series of experiments was carried out to study the influence of the reaction time on the formation of PCDD/Fs. The results obtained at different temperatures are presented in Figure 5 for the PCDF. For the optimal temperatures (325 and 400 °C), after 30 min, a great quantity of PCDF is formed (around 600 ng/gfly ash). At 300 and 450 °C, the amounts formed after 30 min are much lower (around 100 ng/gfly ash). Whatever the temperature studied, the fast increase is followed by a slower formation between 2 and 4 h. At longer reaction time, the formation slows down, and dechlorination/decomposition reactions become important. The interpretation of these results in terms of what happen in the sintering process itself is hazardous. The residence time of the particles in the process is indeed not easily estimated (entrained particles, particles stuck on the walls, etc.). The PCDD/Fs travel along the process adsorbed on fly ash or, depending on the temperature, also partly in the gas phase. And finally, a temperature profile exists in the industrial process and not a single temperature like in the laboratory experiments. Concerning the sinter process itself, the only evident remark that can be made from the laboratory results is that relatively short time (under 30min) at appropriate temperature is largely sufficient to generate great amounts of PCDD/Fs. Figure 6 presents a selection of the data concerning the influence of reaction time on the homologue distribution. The results presented are for PCDF at 400 °C. There is no VOL. 35, NO. 8, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. PCDF/PCDD Ratios for Different Reaction Times and Temperaturesa temperature (°C) reaction time (h)

250

300

325

350

375

400

450

0.5

nea

42.4 14.6 5.3 2.4 8.2 39.4 15.7 5.7 1.9 6.6 55.5 19.6 8.2 3.5 7.8 32.8 10.8 4.2 3.5 4.6

60.0 19.6 8.5 3.1 13.6 53.5 17.0 7.9 3.2 8.7 43.3 17.9 10.2 3.5 9.1 58.5 18.3 7.43 4.0 8.1

38.4 16.1 8.9 5.3 13.6 41.1 21.6 8.1 4.6 11.8 55.7 20.6 9.6 5.2 10.7 39.5 17.5 9.1 3.8 13.8

ne ne ne ne ne 46.4 25.5 12.9 5.9 17.4 ne ne ne ne ne ne ne ne ne ne

37.6 13.3 6.6 3.9 11.5 43.5 19.8 8.1 4.3 15.6 63.3 18.4 11.7 6.2 16.2 48.3 19.5 11.3 7.3 18.4

38.7 19.5 9.0 6.7 19.6 45.0 20.5 15.3 9.2 22.7 50.1 20.7 11.3 5.4 19.4 33.4 22.3 14.1 12.3 19.8

2

4h

6h

a

TCDF/TCDD PeCDF/PeCDD HxCDF/HxCDD HpCDF/HpCDD PCDF/PCDD TCDF/TCDD PeCDF/PeCDD HxCDF/HxCDD HpCDF/HpCDD PCDF/PCDD TCDF/TCDD PeCDF/PeCDD HxCDF/HxCDD HpCDF/HpCDD PCDF/PCDD TCDF/TCDD PeCDF/PeCDD HxCDF/HxCDD HpCDF/HpCDD PCDF/PCDD

ne ne ne ne 25.7 12.0 3.4 1.7 4.9 ne ne ne ne ne ne ne ne ne ne

ne, no such experiment carried out.

FIGURE 5. Formation of PCDF as a function of the reaction time for different temperatures; air (100 mL/min). clear tendency. The PCDF homologue distribution seems to be independent of the reaction time at 400 °C. The same results were found for all temperatures and also for PCDD (not shown) as well as for the PCDD/Fs ratios (see Table 1). Whatever is the reaction time, the different homologues of PCDD and PCDF are formed in the same proportions. Longer reaction time does not promote dechlorination reactions or selective decomposition of PCDD or PCDF. Isomer Distribution as a Function of Reaction Time or Temperature. Full isomer distribution has been measured for all the experiments. Some results are presented in Figures 7-9 as a function of the reaction time or the temperature; on the same figures, we also find the isomer distribution found in the original fly ash collected in the electrostatic precipitator of the sinter plant; these results are discussed in the next section. All the isomer distributions were calculated by setting the sum of each homologue to 100% and calculating the relative contribution of each peak. Figures 8 and 9 show the influence of the temperature on the isomer distribution for the TCDD and HxCDD families. The isomer distributions are independent of the temperature. Actually, the differences found as a result of increasing the temperature are small and in the same order of magnitude as the variation between replicates (same results were found for all the PCDD and PCDF families). Whatever is the temperature of the thermal treatment, the same pattern is obtained for the de 1620

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novo synthesis on fly ash from the sinter plant. From the Figure 7, it can be concluded that the isomer distribution for the PeCDF is nearly independent of the reaction time. Similar results were found for all the homologues of PCDF and also for the PCDD (not shown). Concerning the sinter process itself, the temperature and time independence of the isomer distribution suggests that, if the PCDD/Fs are formed by de novo synthesis, whatever is the position along the process where the material is collected, the isomer distribution found in the samples will be the same. As proposed by Addink et al. (13, 16), the time independence of the PCDD/Fs formation is an argument in favor of the thermodynamic control of the isomer distribution within homologue during de novo formation. The fact that the isomer distribution remains unchanged while increasing reaction time or temperature suggests that, after formation of the PCDD/Fs skeleton, extensive chlorination/dechlorination of the PCDD/Fs takes place and leads to an isomer distribution that is more or less fixed and a kind of equilibrium/steady state is reached. However, in their study, Addink et al. (13) have also found arguments against thermodynamic control of the isomer distribution within homologue during de novo formation: the similarity between isomer distribution measured in their experiments and predicted by theory was limited, and the lack of isomerization of 1,2,3,4,7,8-HxCDD on their incineration fly ash suggested that no equilibrium between the various isomer distribution within homologues existed. So, the thermodynamic control of the isomer distribution within homologue during de novo synthesis is still an open question since there exist arguments in favor and against it. Comparison with Original Fly Ash. To get a better understanding of the PCDD/Fs formation in the sinter plant, it is quite interesting to compare the fingerprint of the nontreated (original) fly ash, reflecting the formation under the industrial process and the results obtained in the fly ash after the laboratory thermal treatment simulating the de novo synthesis. Concerning the total PCDF amount, the 617 ng/g of PCDF found in the original fly ash can be easily explained by a de novo synthesis since, in the laboratory experiments performed at the optimum temperatures, most of the results are higher than this value. During the laboratory experiments, up to 1026 ng/g are formed (400 °C, 4 h). Concerning the

FIGURE 6. Homologue distribution of PCDF as a function of the reaction time; air (100 mL/min), temperature 400 °C. The error bars represent the standard deviation.

FIGURE 7. Part of the PeCDF isomer distribution as a function of the reaction time; air (100 mL/min), temperature 450 °C. The error bars represent the standard deviation. PCDD, the conclusions are not the same since the maximum quantity obtained during the laboratory experiments (83 ng/g at 325 °C during 2 h) is less than what is found in the original fly ash (150 ng/g). No laboratory experiments produce as much PCDD as what is found in the original fly ash. The PCDF/PCDD ratio of 4.1 found in the original fly ash is less that what is found in all the laboratory de novo experiments in which the proportion of PCDF formed is more important (see Table 1). However, the individual PCDF/PCDD ratios of the different homologue in the original fly ash follow the same decrease from the tetra- to heptachloro species than what is found in the laboratory experiments: TCDF/ TCDD, 15.8; PeCDF/PeCDD, 6.9; HxCDF/HxCDD, 2.9; HpCDF/HpCDD, 1.0. The homologue profile of the original

fly ash corresponds quite well to what is found in the laboratory experiments performed at temperatures around 325 °C. From Figure 7, it can be noticed that, for the PeCDF, the differences between the isomer distribution of the nontreated (original) fly ash, reflecting the formation under the industrial process, and the one found after the thermal treatment are small. The same tendency was found for all the isomer of PCDF whatever are the temperature or the reaction time of the thermal treatment. The de novo synthesis on sinter plant fly ash simulated by thermal treatment experiments in laboratory can thus explain the isomer distribution found in the industrial process for the PCDF. However, if we look at Figures 8 and 9, the conclusions are not the same for the VOL. 35, NO. 8, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Part of the TCDD isomer distribution as a function of the temperature; air (100 mL/min), reaction time 4 h. The error bars represent the standard deviation.

FIGURE 9. HxCDD isomer distribution as a function of the temperature; air (100 mL/min), reaction time 4 h. The error bars represent the standard deviation. PCDD. Indeed, the differences between the isomer distribution of the nontreated (original) fly ash and the one found after the thermal treatment are small for most of the isomers of PCDD except for some particular isomers that are present in bigger proportion in the original fly ash. These particular isomers are as follows: 1,3,6,8-TCDD; 1,3,7,9-TCDD; the sum of 1,2,4,6,8-PeCDD and 1,2,4,7,9-PeCDD (which coelute on SP2331 column); 1,2,3,6,8-PeCDD; and the sum of 1,2,4,6,7,9HxCDD, 1,2,4,6,8,9-HxCDD, and 1,2,3,4,6,8-HxCDD (which coelute on the SP2331 column). Different authors (34-36) have shown that this group of isomers can be explained completely as a product of chlorophenol condensation reactions. 1,3,6,8- and 1,3,7,9-TCDD are formed through condensation of 2,4,6-trichlorophenol. In a similar way 1,2,4,6,8-, 1,2,4,7,9-, and 1,2,3,6,8-PeCDD can be formed by condensation of 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol. 1,2,3,4,6,8-HxCDD is formed from 2,4,6-trichlorophenol and pentachlorophenol. These condensation reactions are postulated to take place via Smiles rearrangement with a dioxaspiro-type compound as an intermediate. These results seem to indicate that the de novo synthesis can explain the quantity, profile, and pattern of the PCDF found in the fly ash collected in the sinter plant studied. But for the PCDD, although the de novo synthesis can explain the profile of PCDD found in the original fly ash, the total amount and the isomer distribution found in the thermal experiments are not totally in agreement with the original fly ash content. Formation from precursors must thus play a role in the PCDD formation in the industrial process.

Acknowledgments The authors thank Mr. Bizzarri for the DSC measurements and Mr. J.-M. Brouhon from the C.R.M (Centre de Recherches Me´tallurgiques de Lie`ge) for critical reading of the manuscript. C.X. was funded as fellow by the F.N.R.S (Fonds National de la Recherche Scientifique Belge). 1622

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Received for review August 30, 2000. Revised manuscript received January 23, 2001. Accepted January 29, 2001. ES000199F

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