PCDF on

Environ. Sci. Technol. 2001, 35, 3892-3898

Mechanistic Aspects of the De-Novo Synthesis of PCDD/PCDF on Model Mixtures and MSWI Fly Ashes Using Amorphous 12C- and 13C-Labeled Carbon KATHARINA HELL,* LUDWIG STIEGLITZ, AND ECKHARD DINJUS Forschungszentrum Karlsruhe, ITC-CPV, Postfach 3640, D-76021 Karlsruhe, Germany

The formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) from amorphous 12C- and 13Clabeled carbon was studied on model mixtures and real fly ashes. PCDD/F can either be formed directly (de-novo) from carbon already present in fly ash or step-by-step via condensation of two aromatic rings. Using model mixtures containing 12C- and 13C-labeled carbon in various ratios we observed the formation of the following compound classes: 12C6-PCPh, -PCBz, 13C6-PCPh, -PCBz, 12C12-PCDD/ F, 13C12-PCDD/F, and 12C613C6-PCDD/F. By examining the fraction of the mixed PCDD/F (one of the two aromatic ring is composed solely of 12C-atoms while the other contains only 13C-atoms) in the total concentration of PCDD/F, conclusions on the formation of these three ring structures are possible. From the experimental results, it can be concluded that both reaction mechanisms are operative in the formation of PCDD/F from carbon. On fly ashes approximately half of the total amount of PCDD is formed via condensation of de-novo created C6-precursors e.g. chlorophenols, while the remainder is directly released (denovo) from the carbon i.e., formed from a related C12structure. However, the condensation of intermediate aromatic C6-precursors is of minor importance in the formation of PCDF. With increasing temperature the relative amount of the 12C613C6-PCDD formed by condensation decreases due to the faster evaporation of chlorophenols. At a constant reaction temperature, the ratio of both reaction pathways is hardly influenced by reaction time. In experiments with fly ashes doped with 13C-labeled carbon, this carbon isotope shows a similar reactivity as the native carbon present on the fly ash. Thus, the used amorphous carbons are suitable models for this investigation.

Introduction Thermal treatment of fly ash from municipal solid waste incineration (MSWI) between 250 and 400 °C in the presence of air results in the formation of a variety of chlorinated aromatic compounds, among others highly toxic PCDD/F. It could be shown that these substances are formed de-novo from the residual carbon stemming from incomplete com* Corresponding author phone: 41 1 433 0466; fax: 0049-7247822244; e-mail: [email protected]. 3892

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bustion (1-4). The first experiments with 13C-carbon doped fly ashes were performed by Albrecht et al. (5) and Milligan et al. (6). Albrecht et al. performed thermal experiments with annealed fly ash (A) and annealed fly ash followed by extraction with toluene and dichloromethane (B). The annealing of the fly ash was performed to remove volatile organics from the fly ash. Both fly ashes (A and B) were doped with 13C-carbon, KCl, and CuCl2‚2H2O and kept for 2 h at 300 °C under an air flow of 40-50 mL/min and extracted afterward with toluene. The authors suggested that the precursors for PCDD/F production are organic compounds that are adsorbed onto the particulate carbon. In thermal experiments with fly ash which was exposed to ambient air they observed an increase in PCDD/F production, suggesting that some organic compounds present in the ambient air were adsorbed onto the particulate carbon and/or fly ash. Thus, Albrecht et al. reasoned that it is almost impossible to obtain fly ash which is completely free from organic material. However, the removal of 13C-compounds (originally present on 13C-carbon from the manufacture of 13C-carbon) should be possible to a limited degree. Indeed, in both cases Albrecht et al. observed about the same amounts of native PCDD/F but a drop in the production of 13C-PCDD/F by 1 order of magnitude. Milligan et al. (6) performed thermal experiments with MSWI fly ash enriched with 13C-carbon in the temperature range from 300 to 350 °C under an atmosphere of nitrogen containing 10% oxygen. They observed the formation of unlabeled (stemming from native carbon of the fly ash) and 13 C-labeled (stemming from the added 13C-carbon) chlorobenzenes and PCDD/F. The authors state that “no scrambling of the added 13C or native 12C in the resultant products was evident”. Both authors used carbon from Aldrich which might have been less reactive than carbon from the Cambridge Isotope Laboratories (CIL). Milligan et al. (6) obtained much higher 13C-PCDD/F concentrations after thermal preactivation of the carbon under an oxygen atmosphere as well as with 13C-carbon from CIL. From the experimental data Milligan et al. (6) suggested that the 13C-carbon is subject to the same reactions as native carbon. Both authors did not mention the formation of 12C613C6PCDD/F: in both cases the authors might have used the wrong mass detector (low resolution), and furthermore the concentrations in the experiments of Albrecht et al. would almost be below the detection limit. Stieglitz et al. (7) were the first who demonstrated the formation of 12C613C6-PCDD/F on fly ash, doped with 12Cand 13C-labeled carbon (from CIL). It has thus been concluded that PCDD and PCDF are formed according to the following reaction pathways: (a) PCDD are partly synthesized by condensation of intermediate monoaromatic compounds e.g. chlorophenols (8-11) and (b) PCDF are probably directly released from preformed structures (related C12-structures) in the carbon matrix, e.g. precursors with biphenyl- (12) and chlorobiphenyl substructures (13). These results lead to new questions: (i) Are these mechanisms also found on other matrices and thus generalizable? (ii) How does a variation of temperature and reaction time influence these two pathways? (iii) Does the carbon also react with added precursors? (iv) Is the used 13C-labeled carbon comparable in reactivity with the native carbon on fly ashes? Therefore, experiments were performed with a model mixture (MM) which had been enriched either with 12C- or 13C-labeled carbon or with different ratios of both carbon 10.1021/es0100266 CCC: $20.00

 2001 American Chemical Society Published on Web 09/05/2001

FIGURE 1. Experimental setup. isotopes. In this paper the results of experiments in the temperature range between 250 and 400 °C, and reaction times between 15 min and 4 h are discussed. Furthermore, the reactivity of the 13C-carbon was compared with native carbon of real fly ashes, and the PCDD/F formation from 2,4,6-Cl3Ph and 13C-carbon was investigated.

Materials and Methods Materials. The model mixtures (MM) were prepared by mixing 0.8 g of florisil (Aldrich, 60-100 Mesh, 85% SiO2, 15% MgO) with 147 mg of KCl (Merck), 11 mg of CuCl2‚2 H2O (Merck), and 40 mg of amorphous carbon (CIL, 12C-carbon: 99.95%, 13C-carbon: 99.0%). The amounts of KCl, CuCl2‚ 2 H2O, and carbon were selected in such a manner that they correspond approximately to the percentages of K, Cl, Cu, and carbon in the used real fly ashes. Experiments were performed with MM containing either 12C- or 13C-carbon or a mixture of both carbon isotopes. The following ratios were tested: 12C:13C ) 1:1, 1:2, and 2:1. For MM C (12C:13C ) 1:1), D (12C:13C ) 1:2), and E (12C:13C ) 2:1) the 12C- and 13Ccarbon mixtures were mixed individually. While each MM (A-E) was prepared separately, a further series of experiments (temperature range: 250-400 °C for 120 min and time range: 15-240 min at 350 °C) was performed with a new batch of MM C, which was prepared from the same (new) batch of a 12C:13C-carbon (1:1) mixture. For characterization and comparison of both carbon sorts the BET (Brunauer, Emmett, Teller) surface was determined with nitrogen as adsorbate. The results are comparable: the BET surface of 12C-carbon is 285 m2/g and that of 13C-carbon 338 m2/g. To show that the carbons do not contain significant amounts of the most active precursors in the formation of PCDD/F (e.g. chlorophenols and chlorobiphenyls) thermodesorption measurements were performed. These measurements yielded mainly chlorinated toluene and benzenes (7). First the concentrations of the compounds are very low (10-30 ng/g), and second they are not known as reactive compounds to form PCDD/F, so that they are negligible as precursors in the formation of PCDD/F. The used fly ashes stemmed from two different municipal solid waste incinerator (MSWI): one from a facility in Germany (Go¨ppinger fly ash, termed hereafter GP fly ash) with a residual carbon content of 4.2% and the second was obtained from the US-EPA (United States Environmental Protection Agency; termed EPA fly ash). The carbon content of the latter amounts to 1.75%. The fly ashes were preextracted with toluene for 24 h and afterward spiked with 13C-labeled carbon, so that a molar ratio 12C:13C of 1:1 resulted. Methods. The MM were heated on a glass frit in an upward gas stream of 20% O2/80% He containing 150 mg of H2O/L gas. The experimental setup is shown in Figure 1. The volatile compounds were collected in a toluene impinger. The MM

TABLE 1. Concentrations of PCPh, PCBz, and PCDD/F [ng/g] after Thermal Treatment (300 °C, 2 h) of the MM, Doped with 12 C- and 13C-Carbon, Respectively

12C

6-PCPh

12C

6-PCBz

12C

12-PCDD

12C

12-PCDF

13C

6-PCPh

13C

6-PCBz

13C

12-PCDD

13C

12-PCDF

MM-A: 12C-carbon 40 mg/g

MM-B: 13C-carbon 40 mg/g

585 112580 1940 4310 0 1230 2 3

0 6220 12 70 910 67360 1600 4130

samples were extracted with toluene for 24 h. The extract of the MM and the solution of the impinger were cleaned up separately. The PCPh and PCBz were analyzed by HRGC/ LRMS (30 m DB-5 column). PCDD/F were measured by HRGC-/HRMS (HP5890 - Fisons Autospec) with a 50 m SP2331 column in the Multi Detection Mode at a resolution of 10 000. For quantification, the following internal standards were used: 12C6-1,3,5-trichloro-2,4,6-trifluorobenzene for the 12C6and 13C6-PCBz; 12C6-2,6-dichloro-4-fluorophenol for the 12C6and 13C6-PCPh; and 12C12-1-bromo-2,3,7,8-tetrachlorodibenzofuran for the 12C12-, 13C12-, and 12C613C6-PCDD/F.

Results and Discussion Formation of PCDD/F as a Function of the Carbon Isotope Ratio. The results of the experiments with model mixtures doped either with 12C- or 13C-carbon are depicted in Table 1. In both experiments similar conversion rates of the carbon isotopes to PCDD/F were obtained. The yield of 12C12-PCDD/F from 12C-carbon is 0.0057% and that of 13C12-PCDD/F from 13C-carbon is 0.0055% (the calculation is based on a molar yield). The yields for the other compound classes are as follows: 0.00042% for 12C6-PCPh, 0.0067% for the 13C6-PCPh, 0.023% for 12C6-PCBz, 0.011% for the 13C6-PCBz. These starting experiments were only performed to show that both carbon sorts yield the desired chlorinated compounds in roughly the same concentration range. The background levels of 13C6-PCPh and 13C6-PCBz for MM-A and those of 12C6-PCPh and 12C6-PCBz for MM-B result from the impurities of 13C in 12C-carbon and 12C in 13C-carbon, respectively. Thermal experiments with MM doped with different ratios of 12C- and 13C-carbon (1:1, 1:2, 2:1) resulted in the formation of 12C6-PCPh, -PCBz, 13C6-PCPh, -PCBz, 12C12-PCDD/F, 13C12PCDD/F, and 12C613C6-PCDD/F (Table 2). VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Model Mixtures Doped with Different Molar Ratiosa of Carbon Isotopesb 12C

MM

12C:13C

12C

6-PCPh

13C

6-PCPh: 13C -PCPh 6

6-PCPh

(a) Concentrations of PCPh [ng/g] 1:1 757 1026 1:2 394 800 2:1 597 364

C D E

1.00:1.35 1.00:2.03 1.64:1.00

TABLE 3. Theoretical Product Ratios for the PCDD/F Produced Only by Condensation of C6-Precursorsa 12C :13C 6 6

1:1 1:2 2:1

condensation products

12C

12C 13C 13C 6 6: 12

12:

AA AB BA BB 25:50:25 AA AB AB′ BA B′A BB B′B BB′ B′B′ 11.1:44.4:44.4 AA A′A AA′ A′A′ AB A′B BA BA′ BB 44.4:44.4:11.1

a A ) 12C , B ) 13C ; for the mixtures with the double concentration 6 6 of either 12C or 13C, the double concentration is indicated with A′ and B′, respectively.

12C

MM

12C:13C

12C

6-PCBz

13C

6-PCBz: 13C -PCBz 6

6-PCBz

(b) Concentrations of PCBz [ng/g] 1:1 21520 22930 1:2 31170 76870 2:1 33520 20560

C D E

1.00:1.06 1.00:2.47 1.63:1.00

(c) Concentrations of PCDD/F [ng/g] PCDD 12C:13C

MM C

1:1

D

1:2

E

2:1

12C

12

12C 13C 6 6

650 34.4% 1330 25.4% 1390 49.1%

370 19.6% 1070 20.5% 480 17.0%

12C

12C 13C 6 6

13C

12

870 46.0% 2830 54.1% 960 33.9%

∑PCDD 1890 5230 2830

PCDF 12C:13C

MM C

1:1

D

1:2

E

2:1

12

1670 41.9% 2200 26.2% 3210 57.4%

60 1.5% 65 0.8% 60 1.1%

13C

12

2260 56.6% 6120 73.0% 2320 41.5%

∑PCDF 3990 8385 5590

a

The exact molar ratios result after taking into account that the used Reaction temperature: 300 °C, reaction time: 2 h; MM C: 18.9 mg 12C, 21.0 mg 13C (molar ratios 1:1, total carbon: 40 mg/g); MM D: 12.5 mg 12C, 27.5 mg 13C (molar ratios 1:2, total carbon: 40 mg/g); MM E: 25.8 mg 12C, 14.2 mg 13C (molar ratios 2:1, total carbon: 40 mg/g). 12C contained 0.05% 13C and the used 13C 1% 12C. b

The deviations of the 12C6-/13C6-PCPh and -PCBz ratios, respectively, from the used 12C:13C-carbon ratios (Table 2) are due to the higher reactivity of the 13C-carbon compared with that of 12C-carbon: the 13C12-PCDD/F concentrations are roughly 20-25% higher (based on the average results of the series of experiments in dependence of reaction temperature and time) than those of the 12C12-PCDD/F. The full-scan-measurements of the samples show no scrambling of the carbon isotopes within the chlorobenzenes and chlorophenyls (14). Among the PCPh only one isomer is formed for each degree of chlorination: 2,6-Cl2Ph, 2,4,6Cl3Ph, 2,3,4,6-Cl4Ph, and 2,3,4,5,6-Cl5Ph. For the PCBz the following isomers are preferably formed: 1,3-Cl2Bz (ca. 50100% of the Cl2Bz), 1,2,4-Cl3Bz (20-50% of the Cl3Bz), 1,2,3Cl3Bz (ca. 30-60% of the Cl3Bz), and 1,2,3,5-Cl4Bz (40-50% of the Cl4Bz). Besides the purely carbon labeled PCDD/F also 12C613C6PCDD/F are formed, in which one ring stems from 12C- and the other ring from 13C-carbon. The 12C613C6-PCDD fraction of the total PCDD concentration ranges between 17.0 and 19.6% (300 °C, 2 h), while the fraction of the 12C613C6-PCDF species represents only ca. 1% of the total PCDF concentration. The most plausible reaction way for the formation of mixed labeled PCDD/F is condensation of two different carbon labeled monoaromatic (C6-) compounds, e.g. chlorophenols. Due to the low amounts of 12C613C6-PCDF the 3894

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condensation of two C6-precursors does not play a significant role in the formation of PCDF. With the assumption that the 12C6- and 13C6-precursors have similar reactivity and that only condensation of intermediately formed phenyl rings takes place, then the product ratios summarized in Table 3 would be expected, i.e., for the MM-C (12C:13C ) 1:1) 25% 12C612C6-, 50% 12C613C6-, and 25% 13C613C6-PCDD/F. Statistically, also among isotopically pure labeled 12C12- and 13C12-PCDD/F a certain amount is formed by reaction of two phenyl rings. Due to Table 3 this amounts to half of the concentration of the 12C613C6-PCDD/F for MM-C. In the experiment with MM-C (12C:13C ) 1:1) 19.6% mixed PCDD were obtained. From this result it can be concluded, that about 40% of the total sum of PCDD (10% 12C 12C -PCDD, 20% 12C 13C -PCDD, and 10% 13C 13C -PCDD) 6 6 6 6 6 6 are formed by condensation of de-novo formed monoaromatic (C6-) compounds and the remaining 60% (24% 12C12PCDD and 36% 13C12-PCDD) are directly produced from preformed dioxin like compounds without C6-compounds as intermediate steps. Thus, the relative amount of the mixed PCDD/F of the total concentration of the PCDD/F allows conclusions with respect to the relative importance of the condensation pathway compared with the direct formation from related C12-structures out of the carbon matrix. For the other two MM this projection leads to the following results: for the MM D 45% condensation reactions (5% 12C 12C -PCDD, 20% 12C 13C -PCDD, and 20% 13C 13C -PCDD) 6 6 6 6 6 6 and for MM E 38% (17% 12C612C6-PCDD, 17% 12C613C6-PCDD, and 4% 13C613C6-PCDD), i.e., under the tested reaction conditions on average 41% ( 4% of the formed PCDD stem from condensation reactions (with monoaromatic C6compounds as direct precursors and carbon as indirect precursor). The series of experiments in dependence of reaction temperature and time was performed with the same mixture of MM C (compare also Materials and Methods and Formation of PCDD/F as a Function of the Reaction Temperature and Time). In these experiments the average yield (the calculation is based on a molar yield) of 13C12PCDD is roughly 21% higher than that of 12C12-PCDD, respectively about 24% for the PCDF. Considering the higher molar yield of 13C12-PCDD compared with that of 12C12-PCDD (i.e. subtracting 21% from the amount of substance of 13C12-PCDD and 10.5% from the amount of substance of 12C613C6-PCDD) the following decrease in the fraction of condensation reactions is obtained: MM C, 38%; for MM D, 44%; and for MM E, 37%, i.e., on average about 40% ( 4%. Even if the fraction of the condensation reaction is calculated with the amounts of substances and considering the higher molar yield of the 13C -PCDD compared with that of 12C -PCDD almost the 12 12 same result is obtained. An overview about the reaction routes in the formation of PCDD/F from carbon is given in Figure 2. Thermal treatment of a carbon free model mixture or carbon free fly ash with gaseous or solid 2,4,6-Cl3Ph gave only a few PCDD isomers (11). In our experiments we observed among the PCPh only one isomer for each degree

TABLE 4. Isomer Pattern for the PCDD [ng/g] for MM C 12C

Cl4DD

FIGURE 2. Reaction scheme for the formation of 12C12-, 12C613C6-, and 13 C12-PCDD/F from 12C- and 13C-labeled carbon on model fly ash, ‚‚‚> not significant reation pathway, f symbol for the 13C-labeled carbon.

1,3,6,8 1,3,7,9 1,3,7,8 1,3,6,9/1,2,4,7/1,2,4,8 1,2,6,8 1,4,7,8 2,3,7,8 1,2,3,7 1,2,3,4/1,2,4,6/1,2,4,9/1,2,3,8 1,2,3,6/1,2,7,9 1,4,6,9/1,2,7,8 1,2,3,9 1,2,6,9 1,2,6,7 1,2,8,9 total 12C

Cl5DD

FIGURE 3. Plausible reaction scheme for the formation of 12C12-, 12C 13C -, and 13C -PCDD/F from 12C- and 13C-labeled carbon on a 6 6 12 MM via intermediate phenol structures and chlorophenols demonstrated for 1,3,7,8-Cl4DD; D represents the carbon matrix. of chlorination: 2,6-Cl2Ph, 2,4,6-Cl3Ph, 2,3,4,6-Cl4Ph, and 2,3,4,5,6-Cl5Ph for the Cl5Ph. Assuming that each of these isomers condenses with each member of this series, a distinct number of PCDD isomers would be expected. Indeed, those PCDD isomers occur in higher concentrations compared with the remaining ones (corresponding entries are marked bold in Table 4). Furthermore, it was found that a series of further 12C 13C -PCDD isomers (underlined) are formed in relatively 6 6 high concentrations. These isomers can be subdivided into two classes. In the following this classification is only discussed for the tetrachlorinated dibenzodioxins. (1) Isomers, in which one phenyl ring is chlorinated in alternating and the other ring in adjacent positions: 1,3,7,8-, 1,2,6,8-, or 1,2,7,9-Cl4DD. (2) Isomers, in which one phenyl ring is chlorinated in the positions 1,2,3 and 1,2,4, respectively, and the other ring is only monochlorinated: 1,2,3,6-, 1,2,3,7-, 1,2,3,8-, 1,2,3,9-, 1,2,4,6-, 1,2,4,7-, 1,2,4,8-, or 1,2,4,9-Cl4DD. By examining these isomers we suggest a further reaction pathway (Figure 3): the reaction of de-novo formed chlorophenols with aromatic structures, which are still connected via C-C bonds to the carbon matrix. By condensation, e.g. of 2,4,6-Cl3Ph with a chlorophenol structure, which is still connected via two adjacent carbon atoms, an ortho-hydroxydiphenyl ether structure should result. For the last step of this reaction scheme the ligand transfer mechanism of Stieglitz et al. (15) and Weber et al. (16) is assumed. Hereby, chloride is transferred to the C-C bonds that connect the preformed molecule with the carbon matrix, resulting in the formation of C-Cl-bonds. Due to this reaction scheme condensation of 2,4,6-Cl3Ph with a chlorophenol structure, which is connected via two adjacent carbon atoms to the carbon matrix leads to 1,3,7,8-, 1,2,6,8-, or 1,2,7,9-Cl4DD (class # 1). Condensation of 2,3,4,6Cl4Ph with a chlorophenol structure, which is connected via one carbon atom with the carbon matrix leads to 1,2,3,6-,

1,2,4,6,8/1,2,4,7,9 1,2,3,6,8 1,2,4,7,8 1,2,3,7,9 1,2,4,6,9/1,2,3,4,7 1,2,3,7,8 1,2,3,6,9 1,2,4,6,7 1,2,4,8,9 1,2,3,4,6 1,2,3,6,7 1,2,3,8,9 total

1.7 0.8 1.4 1.4 0.6 0.2 0.6 0.6 1.1 0.8 2.1 1.1 0.5 0.6 0.4 13.9

24.7 15.0 8.9 11.5 5.5 10.2 2.4 2.3 3.5 1.2 5.5 6.7 97.4

Cl8DD 1,2,3,4,6,7,8,9 PCDD

13C

12C

12

89.4 101.4 190.8 12C

12

128 646

1.9 1.1 1.3 1.2 0.4 0.1 0.1 0.6 0.9 0.7 0.3 0.3 0.1 0.2 0.1 9.3 12C 13C 6 6

12

27.9 17.0 10.8 13.1 6.9 14.0 3.0 3.6 4.0 1.2 5.2 6.1 112.8 12C

1,2,4,6,7,9/1,2,4,6,8,9/1,2,3,4,6,8 1,2,3,6,7,9/1,2,3,6,8,9 1,2,3,4,7,8 1,2,3,6,7,8 1,2,3,4,6,9 1,2,3,7,8,9 1,2,3,4,6,7 total

12C 13C 6 6

12

2.9 2.0 3.0 2.5 0.9 0.4 0.9 1.2 1.5 1.7 1.6 0.3 0.3 0.5 0.4 20.1

12

Cl6DD

Cl7DD 1,2,3,4,6,7,9 1,2,3,4,6,7,8 total

13C

12

9.0 6.2 1.8 4.5 2.8 2.7 0.6 0.9 0.7 0.7 1.1 1.5 32.5 13C

12

72.1 73.0 16.0 21.4 3.0 18.7 11.3 215.5

12

80.9 80.1 17.5 24.9 3.0 20.1 11.7 238.2

12C 13C 6 6

45.4 31.7 9.5 1.5 2.0 9.8 6.9 106.8

13C 12 143.6 161.7 305.3

12C 13C 6 6

13C

12C 13C 6 6

12

195 871

54.6 65.0 119.6

102 370

1,2,3,7-, 1,2,3,8-, 1,2,3,9-, 1,2,4,6-, 1,2,4,7-, 1,2,4,8-, or 1,2,4,9Cl4DD (class # 2). Analogously, the formation of the most abundant isomers among the Cl5DD (1,2,3,6,7-, 1,2,3,8,9-, 1,2,4,6,7-, 1,2,4,7,8-, and 1,2,4,8,9-Cl5DD) can be explained by the condensation of 2,3,4,6-Cl4Ph with chlorophenol structures, which are connected via two adjacent carbon atoms with the carbon matrix. With these two suggested reaction pathways (1 and 2) the formation of more than 80% of the Cl4DD isomers and more than 95% of the isomers of the higher chlorinated homologue groups can be explained. From these facts it can be concluded that different reaction pathways are operative in the formation of PCDD/F: PCDD are partly synthesized by condensation reactions between VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 5. PCDD/F Concentrations [ng/g] after Thermal Treatment of a Model Mixture Doped with 12C- and 13C-Carbona PCDD

temp [°C]

12C

250

12

160 29.6% 650 34.4% 690 38.1% 1500 40.8% 150 48.4%

300b 300 350 400

12C 13C 6 6

120 22.2% 370 19.6% 210 11.6% 370 10.0% 30 9.7%

13C

12

260 48.2% 870 46.0% 910 50.3% 1810 49.2% 130 41.9%

∑PCDD

12C

250

12

280 35.3% 1670 41.9% 1700 40.7% 4860 43.3% 1340 47.7%

300b 300 350 400

12C 13C 6 6

4 0.5% 60 1.0% 40 0.9% 140 1.2% 30 1.1%

13C

12

510 64.2% 2260 56.5% 2440 58.4% 6230 55.5% 1440 51.2%

12-PCDD: 13C -PCDD 12

1.00:1.60

15

1890

1.00:1.34

30

1810

1.00:1.28

60

3680

1.00:1.16

120

310

1.27:1.00

240

12C

12-PCDF: 13C -PCDF 12

time [min]

794

1.00:1.64

15

3990

1.00:1.35

30

4180

1.00:1.34

60

11230

1.00:1.19

120

2810

1.00:1.01

240

∑PCDF

a 18.9 mg 12C, 21.0 mg 13C (molar ratios 1:1, total carbon: 40 mg/g); reaction temperature: 250-400 °C; reaction time: 2 h. b Duplicate run from Table 2.

intermediate aromatic C6-precurors and formed from related C12-structures. With respect to the 12C613C6-PCDD isomer pattern it can be suggested that besides the condensation of two monoaromatic rings, e.g. chlorophenols, also reactions between chlorophenols and preformed aromatic structures of the carbon matrix are plausible. In contrast, PCDF are mainly formed from related C12structures, i.e., compounds with a preexisting C-C-bond between two rings are potential precursors e.g. biphenyl, dibenzofuran, etc. related structures. Formation of PCDD/F as a Function of the Reaction Temperature and Time. The results of experiments with a model mixture doped with both carbon isotopes (12C:13C ) 1:1) in dependence of the reaction temperature and time are depicted in Tables 5 and 6. The deviation of the higher molar yield of the 13C12-PCDD compared with that of 12C12-PCDD is already discussed in Formation of PCDD/F as a Function of the Carbon Isotope Ratio. In Table 5 also the total PCDD/F concentrations at 300 °C from MM C (Table 2) are depicted. The mean value for the total concentration of PCDD/F for the duplicate runs is 5935 ( 55 ng/g (error range: ( 1%). The average concentrations for the mixed PCDD/F is 290 ( 80 ng/g for the 12C613C6PCDD (error range: ( 27%) and 50 ( 10 ng/g for the 12C613C6PCDF (error range: ( 20%). The average concentrations of the PCPh and PCBz amount to 610 ( 145 ng/g for the 12CPCPh, 861 ( 165 ng/g for the 13C-PCPh, 17 440 ( 4080 ng/g for the 12C-PCBz, 19 090 ( 3840 ng/g for the 13C-PCBz (average error range ( 21%). The fraction of 12C613C6-PCDD of the total PCDD concentration decreased from 22% at 250 °C to 15.6% (mean value) at 300 °C and to 9.7% at 400 °C. This outcome is in accordance with the decrease (due the increased evaporation and/or destruction) of the concentrations of the PCPh (sum 3896

9

time [min]

540

PCDF

temp [°C]

12C

TABLE 6. PCDD/F Concentrations [ng/g] after Thermal Treatment of a Model Mixture Doped with 12C- and 13C-Carbona

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 19, 2001

PCDD 12C

12

140 45.2% 420 37.5% 750 34.9% 1500 40.8% 720 38.5%

12C 13C 6 6

40 12.9% 150 13.4% 290 13.5% 370 10.0% 170 9.1%

13C

12

130 41.9% 550 49.1% 1110 51.6% 1810 49.2% 980 52.4%

∑PCDD

12

400 47.7% 1720 42.6% 3080 40.9% 4860 43.3% 3510 42.6%

12C 13C 6 6

8 1.0% 50 1.2% 100 1.3% 140 1.2% 80 1.0%

13C

12

430 51.3% 2270 56.2% 4360 57.8% 6230 55.5% 4640 56.4%

12-PCDD: 13C -PCDD 12

310

1.07:1.00

1120

1.00:1.24

2150

1.00:1.43

3680

1.00:1.16

1870

1.00:1.34

PCDF 12C

12C

∑PCDF

12C

12-PCDF: 13C -PCDF 12

838

1.00:1.08

4040

1.00:1.21

7540

1.00:1.31

11230

1.00:1.19

8230

1.00:1.23

a18.9 mg 12C, 21.0 mg 13C (molar ratios 1:1, total carbon: 40 mg/g); reaction temperature: 350 °C; reaction time: 30 min - 4 h.

of 12C-PCPh and 13C-PCPh) found on the solid phase: from 3450 ng/g at 250 °C to 1074 ng/g at 300 °C and 410 ng/g at 400 °C. Thus, at higher temperatures lower amounts of chlorophenols are available for condensation to PCDD yielding a smaller fraction of the condensation reactions. The subdivision of the reaction pathways for the PCDD in condensation reactions and direct formation from a related C12-structure out of the carbon gave the following results: at 250 °C 44% condensation reactions (11% 12C612C6-PCDD, 22% 12C613C6-PCDD, and 11% 13C613C6-PCDD), at 300 °C 31% ( 8%, at 350 °C 20%, and at 400 °C 19% condensation reactions. Among the PCDF a slight increase of the 12C613C6-PCDF fraction from 0.5% at 250 °C to 1.1% at 400 °C is observed. The concentrations of the PCDD/F species produced at 350 °C as a function of time (30 min - 4 h) are reported in Table 6. Increasing the reaction time at 350 °C from 15 min to 1 h has no influence on the fraction of 12C613C6-PCDD and thus on the percentage of the condensation reaction on the total reaction pathways. The amount of 12C613C6-PCDD (13.3% ( 0.4%) is constant over this time period. For reaction times > 1 h the fraction of 12C613C6-PCDD decreases slightly to 9.1%. The subdivision of the reaction pathways for the PCDD in condensation reactions and direct formation from a related C12-structure out of the carbon gave the following results: reaction time of 15 min 26% condensation reactions, 30 min 27%, 60 min 27%, 120 min 20%, and 240 min 18% condensation reactions. This suggests that with longer reaction times the condensation reaction to PCDD becomes less important. In the entire temperature and time range, the fraction of the mixed dibenzofuran species amounts to not more than 1.3%. Therefore, in the formation of PCDF from carbon the condensation of monoaromatic precursors as intermediates cannot be important.

TABLE 7. PCDD/F Concentrations [ng/g] after Thermal Treatment of a Model Mixture Doped with 12C-2,4,6-Cl3Ph (1.1 mg/g) and 13C-Carbona 12C

PCDD PCDF a

12

7100 60.4% 160 2.6%

12C 13C 6 6

1450 12.4% 200 3.2%

13C

12

3200 27.2% 5850 94.2%

total

TABLE 8. PCDD/F Concentrations [ng/g] after Thermal Treatment of the GP and EPA Fly Asha GP fly background ash level

without 13C 12C

12

11750

PCDD

27

590

6210

PCDF

14

5220

12C:13C 12C

12

12C 13C 6 6

In addition to the quantification of PCPh, PCBz, and PCDD/F we measured the concentrations of CO and CO2 online. Both carbon sorts showed similar reactivities in this main reactionsoxidation rates of 12C- and 13C-carbon were comparablesas well as in the formation of PCPh, PCBz, and PCDD/F (17). At 350 °C, 60 min 14.5% of the 12C-carbon (14.4% of the 13C-carbon) and at 350 °C, 240 min 37.8% of the 12C-carbon (29.0% of the 13C-carbon) are oxidized to 12CO2 (13CO2). Formation of PCDD/F on a Model Mixture Doped with 2,4,6-Cl3Ph and 13C-Carbon. From the isomer pattern of the 12C 13C -PCDD and the fraction of the mixed labeled PCDD/F 6 6 of total PCDD/F content we suggested that different reaction pathways are operative in the formation of PCDD/F: direct release of the PCDD/F out of the carbon matrix with related C12-structures as precursors, condensation of de-novo formed C6-aromatics, e.g. chlorophenols and condensation of chlorophenols with preformed aromatic molecules on the carbon matrix. To show that the last reaction pathway is operative, we performed an experiment with a model mixture, which was doped with 2,4,6-Cl3Ph as well as with 13C-carbon. Thus, the 12C613C6-PCDD must stem from the reaction of 2,4,6Cl3Ph either with carbon or with de-novo formed precursors of the carbon. The model mixture was composed of 0.8 g of florisil, 147 mg of KCl, 11 mg of CuCl2‚2 H2O, 40 mg of 13Ccarbon, and 1.1 mg of 2,4,6-Cl3Ph, which corresponds to a molar “12C”:13C-ratio of 1:100. We chose this high carbon ratio, as the yield of PCDD/F from 2,4,6-Cl3Ph on this model mixture is magnitudes of orders higher than that from carbon (11). The PCDD/F concentrations are summarized in Table 7. 12C 13C -PCDD 6 6

are formed in a fraction of 12.4% of the total PCDD concentration: this result corroborates that 2,4,6Cl3Ph is capable of reacting either directly with the amorphous carbon or with the intermediate de-novo formed precursors from 13C-carbon. The isomer patterns of the 12C12- and 12C 13C -PCDD differ significantly from that of the 13C -PCDD. 6 6 12 Among the 12C12-PCDD the direct condensation products of 2,4,6-Cl3Ph and those via the Smiles rearrangement (18, 19) are mainly formed. Among the 12C613C6-PCDD the condensation products of the added 2,4,6-Cl3Ph with other de-novo formed chlorophenols are preferably produced. The low 12C12and 12C613C6-PCDF fraction (roughly 3%), respectively, of the total PCDF concentration demonstrates that chlorophenols do not play a significant role in the formation of the PCDF. Formation of PCDD/F on Different Fly Ashes Doped with 13C-Carbon. Finally, we performed experiments with MSWI fly ashes which contained both native carbon and 13C-carbon. We used fly ashes from different MSWI and with different carbon contents. The fly ashes were spiked with 13C-carbon in such a manner that a molar 12C:13C-ratio of 1:1 resulted. The GP fly ash (residual carbon: 4.2%) was spiked with 45.6 mg of 13C-carbon/g and EPA fly ash (residual carbon: 1.75%) with 19 mg of 13C-carbon/g. Table 8 depicts the PCDD/F background levels of the toluene extracted fly ashes, the PCDD/F concentrations of the thermal experi-

EPA fly background ash level

without 13C 12C

12

PCDD

130

700

PCDF

27

1570

13C

total

12

430 330 470 1230 35.0% 26.8% 38.2% 2940 270 2970 6180 47.6% 4.4% 48.0%

Total 13C: 40 mg/g; reaction temperature: 300 °C; reaction time:

2 h.

) 1:1

12C:13C 12C

12

12C 13C 6 6

) 1:1 13C

total

12

340 400 440 1180 28.8% 33.9% 37.3% 1140 160 1820 3120 36.6% 5.1% 58.3%

a Reaction temperature: 350 °C; reaction time: 1 h. GP fly ash: native carbon content: 42 mg/g; spiked with 45.6 mg 13C/g. EPA fly ash: native carbon content: 17.5 mg/g; spiked with 19.0 mg 13C/g.

ments without addition of 13C-carbon, and the results obtained from 13C-carbon spiked fly ashes. Although the residual carbon is embedded within the inorganic matrix (and thus is in direct contact with active catalytic sites) and the added 13C-carbon was added mechanically to the fly ash, both carbon sorts yielded comparable amounts of 12C12- and 13C12-PCDD/F, respectively (GP fly ash, 12C -PCDD and 13C -PCDD, 450 ng/g ( 20 ng/g; 12C -PCDF 12 12 12 and 13C12-PCDF, 2955 ng/g ( 15 ng/g). Thus, due to the similar 12C -PCDD/F and 13C -PCDD/F concentrations the 13C12 12 carbon isotope represents a suitable model for this investigation. Compared with the MM used, the fractions of 12C613C6PCDD and 12C613C6-PCDF of the total sum of PCDD and PCDF, respectively, are higher by a factor of 3 for the 12C613C6-PCDD and by a factor of 4 for the 12C613C6-PCDF (compare with Table 6). This outcome suggests that on real fly ash more than 50% of the PCDD are formed by condensation reactions of intermediate monoaromatic rings (according to the assumption made in Table 3; compare this outcome also with the results obtained with MM C). For the PCDF the main reaction pathway is the direct release of preformed dibenzofuran similar structures out of the carbon lattice which is responsible for more than 90% of the PCDF formed.

Acknowledgments We would like to thank the Forschungszentrum Karlsruhe for the financial support in this project. The careful measurements by R. Will and G. Zwick are greatly appreciated.

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Received for review January 23, 2001. Revised manuscript received May 30, 2001. Accepted July 2, 2001. ES0100266