F Formation in a

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Environ. Sci. Technol. 2004, 38, 5112-5119

Potential Role of Chlorination Pathways in PCDD/F Formation in a Municipal Waste Incinerator JAE-YONG RYU,† J A M E S A . M U L H O L L A N D , * ,† JAMES E. DUNN,# FUKUYA IINO,‡ AND BRIAN K. GULLETT§ Environmental Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, Department of Mathematical Sciences, University of Arkansas, Fayetteville, Arkansas 72701, Civil and Environmental Engineering, University of Connecticut, 261 Glenbrook Road, Storrs, Connecticut 06269, and U.S. Environmental Protection Agency, E305-01, Office of Research and Development, National Risk Management Research Laboratory, Research Triangle Park, North Carolina 27711

The role of chlorination reactions in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in a municipal waste incinerator was assessed by comparing predicted chlorination isomer patterns with incinerator flue gas measurements. Complete distributions of PCDD and PCDF congeners were obtained from a stokertype municipal waste incinerator operated under 13 test conditions. Samples were collected from the flue gas prior to the gas cleaning system. While total PCDD/F yields varied by a factor of 5 to 6, the distributions of congeners were similar. A conditional probability model, dependent only on the observed distribution of monochlorinated isomers, was developed to predict the distributions of polychlorinated isomers formed by chlorination of dibenzo-p-dioxin (DD) and dibenzofuran (DF). Agreement between predicted and measured PCDF isomer distributions was high for all homologues, supporting the hypothesis that DF chlorination can play an important role in the formation of PCDF byproducts. The PCDD isomer distributions, on the other hand, did not agree well with model predictions, suggesting that DD chlorination was not a dominant PCDD formation mechanism at this incinerator. This work demonstrates the use of PCDD/F isomer patterns for testing formation mechanism hypotheses, and the findings are consistent with those from other municipal waste combustion studies.

Introduction Treatment of organic wastes in municipal waste incinerators produces complete combustion products such as CO2, H2O, and HCl. However, trace amounts of toxic combustion byproducts, such as polychlorinated biphenyls (PCBs), naphthalenes (PCNs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs), can be formed. Numerous studies of PCDD/F formation in combustion systems have been un* Corresponding author phone: (404)894-1695; fax: (404)894-8266; e-mail: [email protected]. † Georgia Institute of Technology. # University of Arkansas. ‡ University of Connecticut. § U.S. Environmental Protection Agency. 5112

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dertaken, and many formation mechanisms have been proposed to account for these incomplete combustion products under different conditions in the combustion system. Attempts to correlate thermodynamic PCDD/F isomer distributions with municipal waste incinerator measurements have generally failed (1-3); there is clear evidence of kinetic control in the measured PCDD/F distributions (4-7). Various pathways have been proposed to explain PCDD/F emissions in municipal waste incinerators. Hutzinger et al. (8) proposed that PCDDs/Fs present in the incoming feed may be incompletely destroyed or transformed during combustion. (Estimates for municipal solid waste are 6-50 ng I-TEQ/kg waste, where I-TEQ denotes “international toxic equivalent”, a quantitative measure of the combined toxicity of a mixture of dioxins.) PCDDs/Fs are formed in combustion as well. PCDD/F formation from precursors, such as chlorinated benzenes, phenols, and biphenyls, can occur both in the gas phase and on particle surfaces, with the PCDD/F product distributions bearing a mark of the precursor distributions (9-11). For example, PCDD isomers with a chlorination pattern at alternating carbon sites, e.g. 1,3,6,8and 1,3,7,9-tetrachlorodibenzo-p-dioxins, are favored due to the abundance of the 2,4,6 chlorinated phenol precursors (12). In the gas phase, the less chlorinated congeners and, in particular, dibenzofuran are favored (13, 14). Subsequent chlorination reactions can lead to a broad distribution of PCDD/F products in which 2,3,7,8 isomers are favored (15). PCDDs/Fs can also be derived from particulate carbon via de novo synthesis (16). Iino and co-workers (17) have found that octachlorinated dibenzo-p-dioxin and dibenzofuran produced by de novo synthesis can undergo surface-mediated dechlorination reactions to produce a broad distribution of PCDD/F products in which 1,4,6,9 isomers are favored. Thus, alternative PCDD/F formation routes result in different distributions of PCDD/F isomers, suggesting that measured PCDD/F isomer distributions can provide clues on how formation occurred. To be used most effectively as a characteristic pattern, or fingerprint, the complete distributions of PCDD and PCDF congeners are needed, not just the distribution of most toxic 2,3,7,8 congeners. Available municipal waste incinerator emission data that include a complete characterization of PCDD/F byproducts, including measurement of congeners with less than four chlorine substituents, are limited. Regarding PCDF distributions, Zimmermann et al. (18) found dibenzofuran concentration was much greater than total PCDF by a factor of 1000 to 10 000 from the flue gas of incinerators. At temperatures in the range of 600 to 800 °C, condensation of unsubstituted phenol readily produces dibenzofuran. Wikstrom and Marklund (19) also reported the concentration of dibenzofuran in municipal solid waste combustion exhaust gas to be high. Prior to the convective section, the dibenzofuran concentration was greater than the total PCDF concentration by a factor of 100 to 1000; after the convective section, the PCDF concentration increased by a factor of 10 to 100. These results suggest that dibenzofuran chlorination may play a role in PCDF formation in some municipal waste incinerators. Typical PCDD distributions found in municipal waste incinerator emissions, on the other hand, are not suggestive of a chlorination mechanism. Total PCDD yield is typically much lower than total PCDF concentration prior to combustion gas cooling and particle collection (20). Moreover, the distribution of PCDD products is typically skewed toward the more highly chlorinated congeners (21, 22). The low 10.1021/es0497227 CCC: $27.50

 2004 American Chemical Society Published on Web 09/02/2004

concentration of PCDD congeners with few chlorine substituents suggests that these congeners are unlikely to be a major source of more highly chlorinated congeners. Chlorination can occur by metal catalysis on the surface of fly ash. For example, copper(II) chloride (CuCl2) is known to be a very potent chlorinating agent (23). Addink et al. (24) demonstrated that CuCl2 catalyzed both ring closure and chlorination of dibenzofuran. Two possible chlorination mechanisms are Deacon chlorination and direct ligand transfer chlorination. Griffin et al. (25) proposed that HCl in the flue gas is converted into Cl2 through the Deacon process, which then can lead to gas-phase dibenzofuran chlorination. For example, CuCl2 can catalyze the Deacon mechanism.

2CuCl2 + 1/2O2 f Cu2OCl2 + Cl2

(1)

Cu2OCl2 + 2HCl f 2CuCl2 + H2O

(2)

Stieglitz et al. (26) and Taylor et al. (27) suggested that the Deacon reaction plays a minor role in the formation of dioxin. They proposed the direct transfer of chlorine by a ligand transfer oxidation mechanism, like that proposed by Nonhebel (28) for aromatic compounds, as an alternative chlorination mechanism.

ArH + CuCl2 f ArHCl* + CuCl

(3)

ArHCl* + CuCl2 f ArCl + CuCl + HCl

(4)

Hoffman et al. (29) also proposed the direct transfer of chlorine from the surface of fly ash as a chlorination mechanism. Aromatic compounds are adsorbed to the fly ash surface and chlorinated by the electrophilic aromatic substitution reaction. In this paper, we present a model in which PCDD/F isomer distributions are predicted from the monochlorinated isomer distributions based on the conditional probabilities associated with sequential chlorination. We use the predicted isomer patterns to assess the potential role of chlorination on PCDD/F formation in a municipal waste incinerator. In a previous laboratory study of dibenzo-p-dioxin and dibenzofuran chlorination, we found that the degree to which the 2,3,7,8 isomers are favored depends on reaction conditions (15). That is, the distribution of monochlorinated products varied slightly with temperature and gas velocity, although the 2- and 3-chloro isomers were always favored over the 1and 4-chloro isomers. Here, we use monochlorinated isomer distributions from a municipal waste incinerator to determine the probabilities of chlorinating the 1 and 2 carbon sites of dibenzo-p-dioxin and the 1, 2, 3, and 4 carbon sites of dibenzofuran. With these as model inputs, we predict PCDD/F isomer distributions and compare these predictions to measurements to test the hypotheses that dibenzofuran chlorination plays an important role in PCDF formation and that dibenzo-p-dioxin chlorination does not play an important role in PCDD formation.

Methods Chlorination Model. Dibenzo-p-dioxin (DD) and dibenzofuran (DF) each have eight carbon sites that can be chlorinated (the numbering convention is shown in Figure 1). Due to symmetry, only two of the eight sites of DD and four of the eight sites of DF are unique. Therefore, we define two unique probabilities for chlorinating any of the eight sites of DD and four unique probabilities for chlorinating any of the eight sites of DF. For DD, the probabilities of chlorinating the 1, 4, 6, or 9 carbon atom, P(1), P(4), P(6), or P(9), are the same, as are the probabilities of chlorinating the 2, 3, 7, or 8 carbon atom, P(2), P(3), P(7), or P(8). We define these probabilities as A

FIGURE 1. Structure of dibenzo-p-dioxin (top) and dibenzofuran (bottom). Carbon atom numbers are shown for identifying chlorinated congeners. and B, respectively. For DF, the probabilities of chlorinating the 1 and 9 carbon atoms, P(1) and P(9), are the same, as are P(2) and P(8), P(3) and P(7), and P(4) and P(6). These are defined as A, B, C, and D, respectively. Since the total probability is one, the sum of A and B for DD is 0.25, and the sum of A, B, C, and D for DF is 0.5. Thus, there is one independent parameter in the DD chlorination model, P(1) ) A, since B ) 0.25 - A, and there are three independent parameters in the DF chlorination model, P(1) ) A, P(2) ) B, and P(3) ) C, since D ) 0.5 - (A+B+C). In the modeling approach employed here, we assume that probabilities of chlorinating additional carbon sites are conditioned only on which sites are currently unoccupied. Such a process is further characterized by the fact that relative probabilities of chlorinating additional sites always remain unchanged. For example in DF formation, if the 1 carbon atom is chlorinated, the conditional probability of chlorinating the 2 carbon atom relative to the conditional probability of chlorinating the 3 carbon atom is as follows:

P[2|1]/P[3|1] ) {B/(1-A)}/{C/(1-A)} ) B/C ) P(2)/P(3) This is the same ratio of probabilities for chlorinating the 8 carbon atom relative to chlorinating the 7 carbon atom. Thus, the model represents a first-order chlorination process, where the likelihood of chlorinating an available site is unaffected by the presence of chlorine at other sites. Computation of congener distributions formed by DD and DF chlorination has been simplified by means of recursive relations represented by eqs 5 and 6.

F(i) ) S(i)P(i)

(5)

n

P(i) )

∑P(j)P(i|j)

(6)

j)1

Here, F(i) is the isomer fraction of i, or equivalently the expected fraction, of congener i; S(i) is the statistical factor associated with congener i; and P(i) is the probability of forming congener i by chlorinating one additional carbon site. For PCDDs, statistical factors can be 1, 2, or 4, which correspond inversely to symmetry numbers 4, 2, or 1, respectively. For a PCDF, the statistical factor is either 1 or 2, corresponding to symmetry of 2 or 1, respectively. The statistical number refers to the number of different combinations of carbon sites that can be chlorinated to produce the same compound. The probability of forming congener i with n chlorine atoms is calculated using eq 6 by summing n products, each being the product of the probability of forming a precursor with one less chlorine atom, P(j), and the conditional probability of chlorinating the ith site of the jth precursor, P(i|j), to form congener i. The conditional probability model for predicting PCDD and PCDF isomer distributions formed by chlorination of VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Total PCDD and PCDF Homologue Emissions in MWI Flue-Gas (ng/dscm)a MCDD DCDD T3CDD T4CDD P5CDD H6CDD H7CDD OCDD total PCDD MCDF DCDF T3CDF T4CDF P5CDF H6CDF H7CDF OCDF total PCDF PCDD+PCDF a

test 1

test 2

test 3

test 4

test 5

test 6

test 7

test 8

test 9

test 10

test 11

test 12

test 13

4.0 30.7 70.7 211 307 376 289 169 1458 152 554 1034 931 865 510 223 31.6 4300 5758

3.3 14.4 33.7 110 159 187 147 91.6 745 157 389 626 522 444 241 103 9.8 2492 3237

5.8 18.5 33.5 141 217 309 290 266 1280 221 445 674 576 486 304 189 32.5 2927 4206

2.3 8.9 17.6 47.5 71.1 87.5 67.0 47.5 349 149 300 441 322 241 112 48.4 5.6 1618 1967

2.2 19.7 33.5 103 159 251 209 157 934 124 346 513 439 400 277 170 38.9 2307 3240

0.9 3.4 7.6 19.2 26.8 74.6 67.0 48.4 248 52.0 117 185 148 136 116 64.7 8.2 827 1075

1.4 6.5 10.5 32.3 54.0 75.8 66.5 50.6 298 86.7 197 359 319 290 174 82.0 10.5 1517 1815

0.7 10.9 28.7 65.5 115 177 165 106 669 29.6 247 631 541 458 305 155 27.6 2393 3062

0.0 5.5 24.5 57.5 113 172 173 124 670 22.1 128 475 416 463 317 163 34.4 2019 2689

1.6 6.4 13.9 40.7 75.6 117 112 80.3 447 73.2 211 429 363 370 251 133 20.5 1851 2298

4.6 11.7 31.6 65.2 117 195 206 170 801 124 252 410 298 563 206 134 28.2 2015 2816

0.0 0.0 16.1 12.8 25.9 97.6 104 66.7 323 8.3 64.0 242 71.0 152 81.8 68.4 13.5 701 1024

0.0 7.8 24.6 54.2 116 222 252 191 868 118 245 483 441 996 438 289 56.8 3067 3935

These data were previously reported by Gullett et al. (30).

DD and DF, respectively, is made available as Supporting Information. Municipal Solid Waste Incinerator Data. To assess the potential role of chlorination pathways in PCDD/F formation, 13 sets of measurements from the flue gas of a stoker-type municipal waste incinerator (MWI) in Norfolk, VA, obtained in July, 1997 (30), were compared with the isomer distributions predicted by the chlorination model. In the MWI tests, the effect of cofiring coal with municipal waste on PCDD/F formation was studied. High and low sulfur coals were used. As previously reported (30), total PCDD/F emissions and the PCDD-to-PCDF ratio were affected by operating parameters. The distributions of homologues and isomers were not examined, however. Here, we examine the distribution of homologues using data from all 13 tests, and we examine the distribution of isomers using data from 9 of the 13 tests. Samples from the other four tests were not available for isomer-specific analysis. The flue gas samples were collected prior to the gas cleaning system. Sampling and analysis methods were used following EPA Method 23 protocol (31). Particle-phase and gas-phase products were combined, extracted with toluene, and analyzed using a HP 6890 gas chromatograph with SP-2331 column, coupled to a HP 5973 mass selective detector. Different temperature programs were used for monothrough trichlorinated congeners and tetra- through octachlorinated congeners. The unchlorinated congeners, DD and DF, were not measured. The 13 test conditions are described in detail elsewhere (30). While total PCDD/F emissions and the PCDD-to-PCDF ratio varied widely, the distributions of congeners were similar, as shown here. Therefore, we postulate that (1) dominant PCDD/F formation mechanisms were the same under all test conditions, and (2) formation mechanisms were different for PCDDs and PCDFs.

Results and Discussion Total PCDD and PCDF Yields from the MWI. Total PCDD and PCDF yields and total homologue yields from the flue gas samples are given in Table 1. While total PCDD and PCDF yields varied by a factor of 5.6 and the PCDD-to-PCDF ratio ranged from 0.2 to 0.5, the homologue profiles (Figure 2) were similar for the 13 tests. The most abundant PCDD products were the hexachlorinated isomers in nine of the 13 samples and heptachlorinated isomers in the other four samples. The most abundant PCDF products, on the other hand, were the trichlorinated isomers in 11 of the 13 samples. 5114

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In the other two tests (11 and 13), the pentachlorinated isomers were most abundant, and trichlorinated isomers second most abundant. DF chlorination may be important in PCDF formation at this MWI. Although not measured, it is likely that DF yields were much greater than PCDF yields, based on observations at other incinerators with similar homologue profiles (18, 22). The PCDD homologue distribution, on the other hand, does not suggest that DD chlorination plays a significant role in PCDD formation. The low yield of PCDD congeners with few chlorine substituents suggests that these congeners are unlikely to be source of the more abundant, highly chlorinated congeners. To test these hypotheses, we estimate values for the chlorination model parameters (A, B, C, and D) using the monochlorinated isomer distributions and then compare measured and predicted distributions for dichlorinated through heptachlorinated isomers. Distribution of Monochlorinated Congeners. Measured MWI monochlorinated isomer fractions are listed in Table 2 for the nine tests from which samples were available. The samples were from tests 2, 3, 5, 6, 7, 8, 9, 11, and 13. As evidenced by the low standard deviation between tests, the isomer patterns are similar. From these measurements, we obtain probabilities of 0.07 and 0.18 for chlorinating each of the 1 and 2 carbon sites of DD, respectively. The probabilities of chlorinating the 1, 2, 3, and 4 carbon sites of DF are 0.077, 0.1365, 0.175, and 0.1115, respectively. The probabilities based on MWI measurements are consistent with DD and DF chlorination experimental results in which chlorination at the lateral sites (2, 3, 7, and 8 sites) is favored over chlorination at the 1, 4, 6, and 9 sites (15). That is, the 2-MCDD isomer fraction is greater than the 1-MCDD isomer fraction, and the 2- and 3-MCDF isomer fractions are greater than the 1- and 4-MCDF isomer fractions. To evaluate the role of DD and DF chlorination in PCDD and PCDF formation, respectively, we use these parameters obtained from MCDD and MCDF isomer distributions to predict the distributions of dichlorinated through heptachlorinated isomers. Comparison of Measured and Predicted Congener Distributions. Measured and predicted distributions for PCDD and PCDF isomers are shown in Figures 3 and 4, respectively. For PCDDs, the MWI tetra- through heptachlorinated isomer patterns are significantly different than those predicted by the model. Only the dichlorinated isomer

FIGURE 2. PCDD and PCDF homologue distributions from MWI flue-gas samples. Error bars represent ( one standard deviation.

TABLE 2. Mean Observed Monochlorinated Isomer Distributions and Model Parameters

a

isomer fractiona

model parameter

1-MCDD 2-MCDD

PCDD 0.279 ( 0.05 0.721 ( 0.05

A ) 0.0697 B ) 0.1803

1-MCDF 2-MCDF 3-MCDF 4-MCDF

PCDF 0.154 ( 0.067 0.273 ( 0.024 0.350 ( 0.043 0.223 ( 0.036

A ) 0.0770 B ) 0.1365 C ) 0.1750 D ) 0.1115

Mean ( one standard deviation.

distribution and, to a lesser extent, the trichlorinated isomer distribution appear to be predicted by the model. For PCDFs, on the other hand, there is much more agreement between the measured isomer distributions and those predicted by the model. Correlation coefficients were computed to quantify the degree of agreement between mean observed isomer distributions and those predicted by the model. In Table 3, R-squared values from both linear correlations and Spearman’s rank correlations are presented for the dichlorinated through hexachlorinated isomer distributions. Analysis of

the heptachlorinated isomer distribution is not included because the number of these isomers is small. Also given in the table are Spearman’s rank correlation coefficients for the 5% significance level, indicating the level at which the correlation is 95% reliable. In the case of PCDDs, only the di- and trichlorinated isomer patterns from the MWI measurements and model predictions are significantly correlated at the 5% significance level. In the case of PCDFs, all five of the isomer patterns (di- through hexa-) are significantly correlated at this level. The PCDD isomer distributions suggest that DD chlorination may contribute to the formation of mono- through trichlorinated congeners. Distinct differences between measured and predicted tetrachlorinated distributions are observed. For example, much lower yields of the 1,3,6,8- and 1,3,7,9-T4CDDs are predicted by the chlorination model than observed in the MWI flue gas samples. These two T4CDDs are produced by condensation of two 2,4,6-trichlorophenols (32). The 2,4,6-trichlorophenol is one of the most abundant chlorinated phenols in incinerator exhaust gas (33, 34). Therefore, it is likely that phenol condensation pathways contribute to the formation of these congeners. Also abundant in incinerator exhaust gas is 2,3,4,6-tetrachlorophenol. Yields of the 1,2,4,6,8- and 1,2,4,7,9-P5CDDs, which can be produced by condensation of 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol, were underpredicted by the chlorination VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison of mean observed and predicted PCDD isomer distributions (white: MWI data; gray: DD chlorination model). Error bars represent ( one standard deviation. model, as were yields of 1,2,4,6,7,9-, 1,2,4,6,8,9-, 1,2,3,6,7,9-, and 1,2,3,6,8,9-H6CDDs, all of which are formed by condensation of 2,3,4,6-tetrachlorophenols. The PCDF congener distributions predicted by the DF chlorination model are highly correlated with those measured in the MWI flue gas samples. Spearman’s rank correlation R-squared values range from 0.85 to 0.92 for the di- through hexachlorinated isomer sets. All were greater than the 5% significance level, indicative of a statistically meaningful degree of association. The correlation coefficient was lower 5116

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for the H7CDF isomers, but the major isomer predicted by the DF chlorination model, 1,2,3,4,6,7,8-H7CDF, was the major H7CDF isomer found in the flue gas samples (see Figure 4). The results support the hypothesis that DF chlorination is a major PCDF formation pathway in this stoker-type MWI. Norttrodt et al. (35) reported that PCDF isomer distributions from several stoker-type incinerators were similar. These patterns were different than the PCDF isomer distributions from fluidized bed incinerators (36). The H7CDF congener distribution of fluidized bed incinerators was found to be

FIGURE 4. Comparison of mean observed and predicted PCDF isomer distributions. (white: MWI data; gray: DF chlorination model). Error bars represent ( one standard deviation. similar to that produced by de novo synthesis. Hatanaka et al. (37) performed experiments to investigate the role of copper(II) chloride on the formation of PCDFs and found that 1,2,3,4,6,7,8-H7CDF was the major isomer (50-70%), consistent with our results. Therefore, DF chlorination appears to be a likely PCDF formation mechanism in stokertype incinerators. The chlorination model used here is a first-order model which does not account for such effects as steric hindrance associated with chlorine substitution pattern. For example,

steric hindrance associated with chlorinating both bay sites of DF may reduce the formation of congeners with 1,9 sites chlorinated. Since the probability of chlorinating the 1 carbon site was lower than the probabilities of chlorinating the other three unique carbon sites of DF, incorporating a bay steric factor into the DF chlorination model has little effect on model performance. Results presented here demonstrate that PCDD/F isomer distributions are useful as characteristic patterns, or fingerprints, for formation mechanism hypothesis testing. A simple VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Correlation Results between Mean Observed and Predicted Isomer Distributions Spearman’s rank correlation linear correlation R-squared value

R-squared value

critical value for 5% significance level

DCDD T3CDD T4CDD P5CDD H6CDD

0.93 0.68 0.20 0.01 0.0008

PCDD 0.88 0.85 0.53 0.49 0.25

0.85 0.64 0.60 0.67 0.85

DCDF T3CDF T4CDF P5CDF H6CDF

0.71 0.88 0.80 0.73 0.58

PCDF 0.85 0.90 0.87 0.89 0.86

0.62 0.48 0.43 0.47 0.64

method was developed for predicting dichlorinated through heptachlorinated isomer distributions based on the distributions of monochlorinated isomers. We found that PCDF isomer distributions in flue gas samples from a stoker-type municipal waste incinerator were consistent with model predictions, supporting the hypothesis that DF chlorination may play an important role in PCDF formation. PCDD isomer distributions were not consistent with DD chlorination model predictions for isomers with four or more chlorine atoms.

Acknowledgments We thank the U.S. Environmental Protection Agency and National Science Foundation for financial support.

Supporting Information Available The conditional probability model for predicting PCDD and PCDF isomer distributions formed by chlorination of dibenzop-dioxin and dibenzofuran. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 20, 2004. Revised manuscript received July 28, 2004. Accepted July 29, 2004. ES0497227

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