Mechanistic Aspects of the de Novo Synthesis of Polychlorinated

nitrogen is flowed through fly ash under the appropriate conditions, only very low concentrations of PCDD/F products are detected. The role of oxygen ...
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Environ. Sci. Techno/. 1995, 29, 1353-1358

llhechanistic Aspects of the de Novo Synthesis of Polychlorinated Dibenzo-p-dioxins and Furans in Fly Ash from Experiments UsinsIsotopically Labeled Reagents MICHAEL S . M I L L I G A N t A N D ELMAR R. ALTWICKER' Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590

%-activated carbon mixed with MSW incinerator fly ash and reacted with gas-phase oxygen at 300350 "C resulted in the formation of 13C-labeled chlorobenzene and PCDD/F products. Unlabeled chlorobenzene and PCDD/F derived from native carbon were also detected, but no scrambling of the added 13C or native 12C in the resultant products was evident. The [T]PCDD/F and [12C]PCDD/Fyields were both observed to have an optimum formation temperature around 325 "C, suggesting that the added %activated carbon was subject t o the same fly ash-catalyzed formation and destruction mechanisms as the native carbon. 13C-activatedcarbon added to other fly ashes had PCDD/F formation rates reflective of their respective rates from the native carbon already present. Experiments with W l a b e l e d CO and COS reacted with fly ash at 300 "C in the presence of gas-phase oxygen showed that neither is an effective precursor to PCDD/F.

Introduction The formation of PCDDlF upon annealing of exhaustively extracted municipal solid waste incinerator (MSWI)fly ash was first reported by Vogg and Stieglitz (1)and has intrigued investigators ever since (2-9). Presumably, exhaustive extraction of such fly ashes with benzene or toluene should-in principle-remove functional group organics, unless they are strongly chemisorbed or of high molecular weight. This has led to the proposal of particulate carbon-present in all fly ashes after extraction-as the most likely precursor. If true, the question then arises: What is the nature of this carbon, and how is it mobilized from the solid carbon matrix to form aromatic moieties? Carbon gasification (formation of CO and COZ in the presence of oxygen) has been studied extensively in our laboratory and was shown to proceed at temperatures of interest (200-350 "C)and to be catalyzed by MSWI fly ashes (10). Thus, it seemed reasonable to explore the following possibility: Does the formation of COICOz and PCDDlF from fly ash proceed in parallel or consecutive reactions? If the latter, are CO and COZ true intermediates, which indeed would be a surprising finding and-given the abundance of these two gases in incinerator stack gases-might lead one to expect far more PCDDlF than is actually observed. Previous work by Stromberg (11) using model fly ash mixtures suggested that COz could act as a precursor to chlorobenzenes under proper conditions via a FisherTropsch type mechanism. Carbon monoxide is known to chemisorb dissociativelyon some catalysts,perhaps freeing up carbon atoms for subsequent cyclization to aromatic compounds. Thus, one might hypothesize that carbon in the fly ash matrix could be gasified to CO and/or COZfor subsequent reactions to PCDD/F and chlorobenzenes, mediated by catalytic elements in the fly ash. Therefore, labeled CO or COz might be expected to be incorporated into the PCDD/F skeleton when flowed over fly ash at appropriate temperatures. If CO and or COZ are not found to be precursors to PCDDlF under the above conditions, new questions are raised. How is the solid carbon in fly ash converted into these aromatic compounds? What is the morphological nature of this carbon? Earlier work has shown that oxygen is required in the gas phase to initiate these de novo synthesis reactions (2-4, 7): when an inert gas such as helium or nitrogen is flowed through fly ash under the appropriate conditions, only very low concentrations of PCDD/F products are detected. The role of oxygen in this mechanism is unclear. Is gas-phase oxygen incorporated into the resultant PCDD/F products, or does oxygen instead play a role in site activation? We set out to study some of these reactions in the laboratory using the isotopically labeled reagents 13C0,l3cO2,and I3C-labeledcarbon reacted with actual MSWI fly ash with the hope of finding subsequent labeled reaction products and elucidation of potential reaction mechanisms. + Present address: Department of Chemistry, State University of New York at Fredonia, Fredonia, NY 14063.

0013-936W95/0929-1353$09.00/0

0 1995 American Chemical Society

VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY

1315

n Experiments were conducted in a laboratory system that has been described elsewhere in detail (7),utilizing a furedbed tubular reactor, a temperature-controlled clam-shell furnace, and mass flow controllers for gas mixtures. The outlet stream was bubbled through a glass impinger trap containing hexane for collection of gas-phase products. The reactor containing the requisite amount of extracted and dried fly ash (usually0.5- 1 g) was heated to the desired temperature under a helium flow. A switch was then made to an atmosphere of 10% oxygen in nitrogen, which was flowed at 80 mLlmin. After the desired run time, the clamshell heaters were shut off and opened, and helium flowed to rapidly cool the fly ash and quench the reactions. Fly ash samples were extracted, and the outlet impinger trap samples were volume-reduced before liquid chromatographic cleanup of both, followed by HRGC/LRMS analysis using a HP 5890 gas chromatograph coupled with a 5971A mass selective detector for analysis of products as described previously ( 7).

Resuks and Discussion Reactions of 13C02 and 13C0 with Fly Ash. Separate experiments were run using isotopically 13C-labeledCOz and CO reacted with fly ash to determine whether carbon monoxide and carbon dioxide can be incorporated into chlorobenzene or PCDDlF products in the presence of fly ash. In one experiment, [13ClCOz(13C, 99%) equal to 4% in a mixture of 10% 0 2 and 90% N2 was flowed through a fixed-bed of MSWI fly ash for 30 min at 300 "C. With the exception of labeled CO2, the experiment was identical to typical de novo synthesis experiments run earlier (7) that were known to generate significant quantities of chlorobenzenes and PCDDlF products. GUMS analyses of the outlet impinger sample (containing gas-phase reaction products) and the fly ash extract (containing adsorbed reaction products) showed unlabeled native carbon de novo synthesis products (chlorobenzenes and PCDDlF)with no evidence of 13C incorporation. An identical experiment was run replacing labeled CO2 with 6.3% [13ClC0(13C,99%). As was the case for [13ClCO~, unlabeled de novo synthesis products were detected with no evidence of 13C incorporation. From these results, we can conclude that in the temperature range around 300 "C, gas-phase C 0 2 and CO do not act as precursors to chlorobenzenes or PCDDlF from reactions with MSWI fly ash, at least not to an extent sufficient to explain the observed levels of PCDDlFs. Reactions of 13C Mixed with Fly Ash. In an initial experiment, an aliquot of isotopically labeled amorphous carbon (13C, 99%,Aldrich) was mixed with fly ash, and 1 g was transferred to the fured-bed reactor. The resultant mixture contained 1.9%(byweight)native (nonextractable) carbon and 2.0% I3C-labeled carbon. The labeled carbon was mixed with the fly ash by agitating the fly ash/carbon mixture together in a small vial. Oxygen (10%)was then flowed through the fly ash mixture for 30 min at 300 "C. GUMS analyses of the outlet and fly ash extract samples showed de novo synthesis products from native W, but no detectable incorporation of 13C-labeled carbon. A fresh sample of 13C-labeledamorphous carbon was then activated with 21% 0 2 at 450 "C for 16has described by Rozwadowski and Wojsz (12) and Milligan and Altwicker (10).In this type of treatment, a low temperature oxidation slowly burns away the amorphous structure, leaving behind a highly 1354 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995

TA6LE 1

Polycklorobenzene (CB) and PCDD/F Yields from l3C=ActivatedCarbon Added to Fly ASP

CB

native carbon lzC,( n d d

labeled carbon

80 000

8100

lzCl~ (ndd T&DD P&DD H&DD H7CDD O&DD PCDD T&DF PtjCDF H&DF H7CDF OsCDF PCDFf

ND ND

13 23 28 64 178

lSC,

(ndd

'3C1~

ND ND NC

3.5 4.8 8 14

185 160

28 43 54

37 800

42 181

240

a Fly ash A = 1.00 g, native carbon (W)= 7.4%, added 13C-carbon = 4.25%, T= 325 "C,reaction time= 30 min, 10% 02 = 80 mumin. Since no scrambling of the native carbon and added labeled carbon was

apparent from the GUMS data, results show polychlorobenzeneyields as "CB or 13CBand PCDD/F yields as 12C12or Y12.

porous matrix consisting of localized crystallite regions. The resulting weight loss of the 13C-labeled amorphous carbon after this treatment was 62%; 20.7 mg of this activated 13Cwas then mixed with 1.00 g of fly ash and reacted with 10%0 2 as described above. GClMS analyses again showed chlorobenzenes and PCDD/F from the native carbon, but now 13C-labeledchlorobenzenes and PCDDlF were also detected. Because the yields of labeled products in the above experiments were low and close to our detection limit, another fly ash (fly ash A)-determined earlier to be the most active fly ash to de novo synthesis in our laboratory (q-was chosen for further studies with 13C. A total of 44.2 mg of the 13C-activatedcarbon prepared as described above was mixed with 1.00 g of fly ash A, resulting in a final fly ash mixture having 4.25% 13C-activatedcarbon plus 7.4% native carbon already present. As in the previous experiments, 10%0 2 at 80 mL/min was flowed for 30 min at the optimum temperature for de novo synthesis of PCDDlF in this fly ash (325"C). The GUMS results for chlorobenzenes and PCDDlF are shown in Table 1. Here, about 9% of the detected chlorobenzenes and 10% of the PCDD/F were found as complete 13C-labeledaromatic rings; the remaining products were found as complete 12C-labeledrings derived from the native carbon. From the mass spectral data, no evidence for scrambling of 13C and 12Ccarbon atoms was apparent, Le., all of the carbon atoms in the chlorobenzene and PCDD/F products were either all 13C or all 12C. This suggests that the de novo synthesis products are generated or liberated from the solid carbon matrix as complete aromatic rings. The data and calculationsforming the basis to these conclusions are detailed in the Appendix. Because 13C-labeledproducts were produced in these experiments, 13C-labeledinternal standards could not be used. As a result, sample preparation and cleanup recoveries were estimated based on identical procedures described elsewhere (7,13). The estimated recoveries used here were as follows: tetrachlorobenzene = 30%, pentachlorobenzene = 40%, hexachlorobenzene = 50%, and all PCDDlF = 80%. Previous work in our laboratory from similar experiments

'

I

O

0

O 0

6

I

carbon as a function of temperature. 13C-Labeled chlorobenzenes were also detected in yields about an order of magnitude higher than PCDD/F. As described earlier, the mass spectral data suggested that all of the aromatic rings were either all 1% or all 1%-no measurable scrambling of the two isotopes was evident. The yields of [13C]PCDD/Fshowed a temperature peak at 325 "C, although not as pronounced as observed for the unlabeled reaction, The labeled PCDD/F seemed to be at least partially subject to the same formationand destruction mechanisms as the unlabeled PCDDIF. The control experiment using no added carbon resulted in a total PCDD/F yield of 2040 ng. The total PCDDlF (12C 13C)yield from the added carbon experiments at 325 "C was about 1160 ng, which was considerably less than the yield from the control experiment. The addition of carbon to fly ash lowered the de novo synthesis yield from the native carbon and the total yield from both carbons. . It may be that the potential of a given fly ash to produce de novo synthesis products is limited to some degree, such that adding carbon to the fly ash does not proportionally increase the total yield. In fact, in this case the total yield went down with added carbon. The added 13C may not have a morphology conduciveto formingde novo synthesis products as the native carbon. The ability of fly ash to promote the formation of de novo synthesis products is likely a complicated function of both the catalytic makeup of the fly ash and the carbon morphology. 13C Added to Other Fly Ashes. Portions of 13C were added to three different fly ashes and reacted with 10% 0 2 to test their respective activities to identical samples of labeled carbon. A new batch of 13C (Cambridge Isotopes Laboratories) was used for this set of experiments. This carbon had a noticeablydifferent morphologyfrom the 13C used in the previous experiments (from Aldrich); it was flaky in appearance, and by comparison to equal weight samples of the previous carbons was of lower density, indicative of a higher surface area. Attempts to activate this carbon at 450 "C as before resulted in complete oxidation after only 1h. Subsequent experiments showed that the new 13C as received had a reactivity to de novo synthesis almost identical to the earlier batch after activation and so was added to the fly ashes with no preliminary treatment. Three fly ashes (A, D, and coal) were compared. Fly ash A was chosen due to its relatively high potential for de novo synthesis (7). Fly ash D was chosen due to its relatively low (0.3%)concentration of native carbon, and subsequent low de novo synthesis yields. Coal fly ash was previouslyfound to be inactive to de novo synthesis of the native carbon ( 3 , and was chosen as the third fly ash. For each experiment, 18mg of 13Cwas added to a0.50-g sample of fly ash (resulting in 3.6%13Cby weight), and 10% 0 2 at 80 mL/min was flowed for 30 min at either 325 "C (fly ash A) or 300 "C (fly ash D and coal fly ash) through the fly ash samples; 325 "C was previously determined to be the optimum temperature for fly ash A, and earlier results were found for fly ash D only at 300 "C. Coal fly ash had been shown to be unreactive in the range 275-325 "C, so a middle value of 300 "C was chosen. Table 2 compiles the results for these three runs. Included are native and added carbon percentages, total [12C12]-and [13C12]PCDD/Fyields, and the rates of de novo synthesis normalized per gram of carbon initially in the fly ash (in other words, the rate here is based on the quantity

+

O i

d I

has shown maximum deviations from the above recoveries of approximately f20%; our final quantitative results can be assumed to have this same degree of uncertainty. The homolog distribution of PCDD/F observed in the 13C channel as compared to the 12C channel was quite different. For example, the dominant homolog group for [13C12]PCDFwas heptachlorodibenzofuran (H7CDF),but for [12C121PCDF,the dominant group was pentachlorodibenzofuran (P5CDF). Differences were also observed in the isomer patterns of tetrachlorodibenzofuran (TdCDF) and P&DF for the two cases. The isomer and homolog patterns of the de novo synthesis products may depend on the nature of the solid carbon precursors. Conversion of [l3C1Carbonto [13C121PCDD/Fin Fly Ash as a Function of Temperature. Previous experimentswith fly ash A showed an optimum temperature for de novo synthesis of PCDDlF at 325 "C (7). When 13C-activated carbon was added to this same fly ash at 325 "C in the experiment described above, 13C-labeledPCDDIF was also made. But, would these labeled products show the same temperature dependency as the unlabeled products derived from the native carbon? The native carbon is likely intimately imbedded within the fly ash matrix; thus, it possesses or is in contact with many potentially catalytic elements (sites). The added 13C,however, is only mixed mechanically with the fly ash so that just the surface of these carbon particles are in contact with fly ash particles; most of the reactive surface of the added carbon resides in the intraparticle regions and should not be in direct contact with potential fly ash catalysts. The PCDD/F destruction mechanism resulting in the observed temperature window for formation on fly ash, hypothesized to be surface related ( 3 ,may act differently on the 13C-labeledPCDD/F products, resultingin a different temperature dependence of PCDDlF yield. l3C-Labeled amorphous carbon (Aldrich) was activated identically as described above and added to 0.50-g samples offlyashAin4-5% portions byweight. Ten percent oxygen was flowed separately through the samples at 300,325, and 350 "C; two experiments were run at 325 "C to test for reproducibility. An identical run was made at 325 "C with no added 13C for comparison. Figure 1 shows PCDDIF yields from native carbon (12C) and added 13C-activated

VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY

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TABLE 2

[12C1& and [l3C,,lPC0D/F Kdds a d Formation Rates from Native Carbon (12C) in Fly Ash and 13C-Activatd Carbon Added to Fly Ash for Fly Ashes A and D and Coal Fly A s g fly ash A

T ("C) % '2C % 13c [12C121PCDD/F(ng) rate [I2C121PCDD/F [13C121PCDD/F(ng) rate [l3C121PCDD/F

325 7.4 3.6 824 22 319 18

D

300 0.3

3.6 4 2.7 13 0.72

coal 300 3.7 3.6 ND

0 ND

0

a Fly ash = 0.50 g, 10% O2 = 80 mumin, reaction time = 30 min. PCDD/F formation rates are expressed as pg of PCDD/F (g of carbon in fly ash)-' min.-'.

of carbon present). Note that the reactivities toward de novo synthesis from the added 13C-activatedcarbon mirrored those from the native carbon: fly ashA was again the most active, and coal fly ash was totally inactive. For fly ashes A and D, the rates of PCDDlF formation from 13C were slightly less than from 12C. MechanisticImplications. The results presented above allow us to draw some inferencesregarding the mechanism(s) of de novo synthesis of chlorobenzenes and PCDDlF in MSWI fly ash. Since in the absence of gas-phase oxygen little or no chlorobenzenes or PCDDlF are formed, oxygen may be needed as an initiator,perhaps mediated by catalysts with certain metals (7, 14-16). With oxygen in the gas phase, the ratio of PCDD to PCDF changes from fly ash to fly ash. At present, there is no direct evidence that gas phase oxygen is incorporated into the products. The nature of the native carbon-such as its surface area and concentration of chemicallybound oxygen-may be responsible for these different ratios. As shown elsewhere, MSWI fly ash is also an effective carbon gasification (oxidationto CO and COZ)catalyst (10). The apparent catalytic mechanism responsiblefor gasifymg a carbon atom from the solid matrix may also be responsible for liberating aromatic structures such as substituted benzenes, phenols, dibenzo-p-dioxins, and dibenzofurans from the solid matrix. In these experiments, we did not measure PCDDlF ratios or congener distributions as a function of reaction time. Future work addressing these issues will add to the mechanistic interpretations of de novo synthesis. An earlier study on the structure of coal carbon surfaces suggests the presence of aromatic moieties in the solid carbon matrix, including localized regions containing benzene, biphenyl, dibenzo-p-dioxin, and dibenzofuran groups (17). In this earlier work, the author combined analytical and instrumental resultsfrom elemental analysis, surface infrared spectroscopy, and X-ray analysis to arrive at the inferred structure. Present are aliphatic and aromatic regions with varying degrees of hydrogenation plus different functional groups containing oxygen. Similar carbon structures have been discussed in the recent literature. In a comprehensive review on surface reactions involving solid carbon, Leon y Leon and Radovic (18)reported on analytical studies of carbon showing the presence of ether, phenol, carbonyl, and carboxyl functional groups, among others, on carbon surfaces. Fly ash, which forms mainly in a high temperature oxidative environment, could have native 1356 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. N O . 5 , 1 9 9 5

carbon morphologiessimilar to those found in these studies with chemically bound oxygen and regions of aromaticity. Intrinsic to this carbon morphology are backbone structures that could serve as precursors to chlorobenzenes and PCDDlF. The catalytic oxidation mechanism associated with fly ash may be responsible for the release of these thermodynamically stable aromatic structures that ultimately result in chlorobenzenes and PCDD/F. In effect, the interaction of gas-phase oxygen with catalytic sites in the fly ash could lead to the preferential oxidation of the reactive aliphatic regions to CO and COZ,thus cutting out the more stable aromatic regions. The yields of PCDDlF de novo synthesis products are much lower than the yields of CO and COz from carbon in fly ash (7, 101, so that one might think of carbon gasification as being the dominate reaction and de novo synthesis as a trace reaction. The two, however, may be linked through a common mechanism of C-C bond scission. The isotope experiments support such a hypothesis. Oxygen is required in the gas phase to initiate de novo synthesis. When 13Cwas added to fly ash and reacted with 10%02,no scrambling of the native I2C and added 13Cwas detected; the aromatic structures seemed to be generated separately from the respective carbons. However, since coal fly ash does not yield PCDDlF and since the formation of these products from 13C-activatedcarbon added to other fly ashes implies the preexistence of aromatic structures, the carbon in coal fly ash either does not have comparable structures or the nature of the catalyst-or presence of inhibitors such as S or N compounds-is not effective in PCDDlF formation. Since no chlorine is present in the flowing gas stream, the chorine source for CB and PCDDlF must also be present initiallyin the fly ash matrix. At present, the mechanism(s1 of chlorination in de novo synthesis is (are) not well understood.

Conclusions Earlier studies have shown that when exhaustively extracted MSWI fly ash-normally comprised of a few percent elemental carbon-was reacted with oxygen at temperatures around 300 "C, CO and COZwere produced alongwith much lower concentrations of chlorobenzenes and PCDDIF. The production of chlorobenzenes and PCDDlF under these conditions-called de novo synthesis-must involve the combination of precursor materials in the fly ash such as carbon, chlorine, hydrogen, and perhaps oxygen to form these complex aromatic species. In this work, experiments were run in a fixed-bed tubular reactor to study the incorporation of 13C-labeledreagents such as 13C0, l3CO2, and 1%-activated carbon into these chlorobenzene and PCDDlF products when reacted or mixed with actual MSWI fly ash samples. Reactions of 13C0 and 13C02in 10% 0 2 with fly ash at 300 "C resulted in no incorporation of the labeled carbon in the chlorobenzene and PCDD/F products, suggesting that neither of these compounds acts as a precursor to these trace pollutants. When I3C-activated carbon was mixed with fly ash and reacted with 10% 0 2 at temperatures ranging from 300 to 350 "C, labeled chlorobenzene and PCDD/F products were observed along with unlabeled products derived from the native carbon. Remarkably, no scrambling of the added 13C or native 12Cin these products was evident from the mass spectral data. At least within the accuracy of the integrated GClMS data, the backbone

TABLE 3

Theoretical Ion Abundances for Different m/z Values as Function of Degree of ChlorinatioiP p (d4 m/Z

CI=2

CI = 3

CI = 4

M M f 2 M f 4 M+6 M+8 M f 10 M+ 12 M+ 14 M+ 16

0.570 0.370 0.060

0.430 0.419 0.136 0.015

0.325 0.422 0.205 0.044 0.0036

a

CI=6

CI=l

ci=a

0.185 0.361 0.293 0.127 0.0308 0.0040 2.2 10-4

0.140 0.318 0.309 0.167 0.0543 0.0106 1.1 10-3 5.3 10-5

0.106 0.274 0.31 1 0.202 0.0820 0.0213 3.5 10-3 3.2 10-4 1.3 10-5

CI = 5 0.245 0.398 0.258 0.0838 0.0136 8.8 x

M refers to the case where all substituted chlorines are the 35CIisotope.

TABLE 4

TABLE 5

Experimental and Theoretical m/z Ion Ratios for '%=Activated Carbon Added to Fly Ash A and Reacted with 10% 02 at 300 O c a

Experimental and Theoretical m/z Ion Ratios for I3C-Activated Carbon Added to Fly Ash A and Reacted with 10% 02 at 325 O c a

lZC-labeled products

MIMf2 T4CB P5CB H&B TiCDD P5CDD H&DD H7CDD OsCDD T4CDF PsCDF H&DF H7CDF O&DF

M+4/Mf2

exp

theor

exp

ND 0.61 0.51 ND ND ND ND ND 0.76 ND ND 0.40 ND

0.77 0.62 0.51 0.77 0.62 0.51 0.44 0.88' 0.77 0.62 0.51 0.44 0.88'

ND 0.63 0.78 ND ND ND ND ND 0.49 ND ND 0.95 ND

13C-labeled products

WM-I-2

theor

exp

theor

exp

0.49 0.65 0.81

ND 0.77 0.51 0.62 0.41 0.51

ND 0.56 0.74

0.49 0.65 0.81

0.49 0.65 0.81 0.98 0.66' 0.49 0.65 0.81 0.98 0.66'

ND ND ND ND ND ND ND ND 0.46 ND

0.7 0.62 0.51 0.44 0.88'

ND ND ND ND ND ND ND ND 0.94 ND

0.49 0.65 0.81 0.98 0.66' 0.49 0.65 0.81 0.98 0.66'

0.77 0.62 0.51 0.44 0.88'

theor

a Shown are ratios for "Cg and l3C6-Iabeledchlorobenzenes and [12C121and [13C121PCDD/F.Fly ash A = 0.50 g, T = 300 "C, 10% O2 = 80 mumin. For 08CDD/F, M 2/M 4 ratio is shown. For OaCDD/F, M + 6/M + 4 ratio is shown.

+

lZC-labeled products

MIM+2

M+QIM+2

+

carbon structures for the chlorobenzenes and PCDD/F were either all 12C or all 13C (neglecting the contribution from naturally occurring carbon isotopes). These results suggest that these aromatic structures were split from the solid carbon matrix, perhaps by reactions involving oxygen from the gas phase. Reactions of gas-phase oxygen with reactive carbon species in the fly ash result in the formation of CO and COz, and in the process, trace quantities of aromatic carbon species are liberated.

Acknowledgments Support of this work by the New York State Solid Waste Combustion Institute, the United States Environmental Protection Agency, the National Science Foundation, and the New York State Energy Research and Development Authority is gratefully acknowledged.

Appendix: Data Tables for 13C=Labeledde Novo

Synthesis Experiments The following discussion describes our strategy for concluding that (a) 13C-activatedcarbon atoms were incorporated into the aromatic products and that (b) no scrambling of the added 13Cwith the native 12C in the fly ash occurred in the resultant chlorobenzene and PCDD/F

T4CB P5CB H&B TiCDD P5CDD H&DD H7CDD O&DD T4CDF PsCDF H&DF H7CDF O&DF

Mf4IMf2

13C-labeled products

WM+2

Mf4IMf2

exp

theor

exp

theor

exp

theor

exp

theor

0.76 0.60 0.50 ND ND 0.51 0.45 ND 0.77 0.62 0.52 0.44 0.94'

0.77 0.62 0.51 0.77 0.62 0.51 0.44 0.94' 0.77 0.62 0.51 0.44 0.88b

0.77 0.62 0.51 ND ND 0.80 0.97 ND 0.49 0.64 0.81 0.96 0.68'

0.49 0.65 0.81 0.49 0.65 0.81 0.98 0.66' 0.49 0.65 0.81 0.98 0.66'

0.78 0.65 0.52 ND ND ND ND ND 0.79 0.65 0.53 0.43 ND

0.77 0.62 0.51 0.77 0.62 0.51 0.44 0.94' 0.77 0.62 0.51 0.44 0.88'

0.47 0.64 0.79 ND ND ND ND ND 0.55 0.65 0.81 0.96 ND

0.49 0.65 0.81 0.49 0.65 0.81 0.98 0.66' 0.49 0.65 0.81 0.98 0.66'

a Shown are ratios for "Cg- and 13C6-labeledchlorobenzenes and [lzClzl and [13C121PCDD/F.Fly ash A = 0.50 g, T = 325 "C, 10% O2 = 80 mUmin. For 08CDD/F, M + 2/M+ 4 ratio is shown. For O&DD/F, M 6/M 4 ratio is shown.

+

+

products. Shown here are results from three experiments using fly ash A, determined to be the most active to de novo synthesis. These results are representative of identical experiments using other fly ashes. Determination of [13C]Chlorobenzeneand [l3ClPCDDI F Yields. The molecular weights for l3C6-labeled chlorobenzenes are +6 from those of the unlabeled analogues; for 13C12-labeledPCDD/F, the molecular weights are +12 greater. Although the retention times for labeled and unlabeled isomers are essentially identical, we can distinguish between these two based on the mass spectral data. Most importantly, if there is no scrambling of the 12Cand 13C carbon atoms in the resultant products, then the measured M / M 2 and M 4 / M 2 ratios will match the theoretical values for both the labeled and unlabeled channels. If for a class of compounds such as T&DD the mass spectral data show the theoretical ratios for M / M + 2 = 0.77 and M 4 / M 2 = 0.49 for the cases of both [1ZC121T4CDD (where M = 320) and [13C121T&DD (whereM = 332), we can conclude that no scrambling was apparent, at least within the accuracy of the instrumental analysis (HRGCILRMS). If appreciable scrambling of the native l2C and added 13C had occurred, then the mass spectral data-comprised of an ensemble of molecules having in

+

+

+

+

+

VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY

1387

TABLE 6

Exgerimental and Tbsontiml mlz Ion Ratios fer 13C-Activated Carbon Added to Fly Ash A and Reacted with 109'0 02 at 350 O c a %-labeled products

WM+P exp T4CB P5CB H6CB T4CDD PsCDD HsCDD H7CDD O&DD TnCDF P&DF HeCDF H7CDF OsCDF

theor

M+4lM+2 exp

0.76 0.77 0.48 0.58 0.62 0.63 0.50 0.51 0.80 0.77 ND ND ND 0.62 ND ND 0.51 ND ND 0.44 ND ND 0.88b ND 0.74 0.77 0.48 0.63 0.62 0.66 0.50 0.51 0.79 0.44 0.44 0.95 0,84b 0.88b 0.62c

+

13C-labeledproducts

MIM4-2 theor

M+4lM+2 exp

theor

exp

0.49 0.65 0.81 0.49 0.65 0.81 0.98 0.66c 0.49 0.65 0.81 0.98 0.66c

0.75 0.77 0.45 0.49 0.61 0.62 0.63 0.65 0.36 0.51 0.83 0.81 ND 0.77 ND 0.49 ND 0.62 ND 0.65 ND 0.51 ND 0.81 ND 0.44 ND 0.98 ND 0.88b ND 0.66c 0.80 0.77 0.46 0.49 0.66 0.62 0.71 0.65 0.48 0.51 0.74 0.81 0.49 0.44 1.09 0.98 ND 0.88b ND 0.66c

theor

Shown are ratios for 12Cg- and 13C6-labeledchlorobenzenes and and [13CdlabeledPCDDIF. Fly ash A = 0.50 g, T = 350 "C, 10% 02=80 rnUmin. For OaCDDff, M+ 2 / M + 4ratio isshown. For08CDD/ F, M 6lM + 4 ratio is shown. a

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this case a distribution of 12Cand 13Ccombinations-would not reflect the theoretical ratios unique to the unscrambled cases. Complicating the analysis is the contribution of mass spectral peaks in the 13C-labeledchannel from the unlabeled products in certain cases. For example, unlabeled hexachlorobenzene-with six chlorines-will have mass spectral lines corresponding to M 6, M 8 , and M 10 that interfere with the corresponding M, M 2, and M 4 peaks from the I3C-labeled hexachlorobenzene. Fortunately, we can calculate what these contributions would be based on the base peak, M , for the unlabeled compound from the statistical data shown in Table 3 (15, 19). For C1 = 6, M 6 / M = 0.69, M 8 / M = 0.17, and M lO/M = 0.022. Where applicable, these ratios are multiplied by the integratedvalue of the Mpeaks from the unlabeled products and subtracted from the corresponding M , M 2, and M 4 peaks from the 13C-labeledproducts. These corrected values for the 13C-labeledproducts are then evaluated for their agreement with theoretical MIM 2 and M 4 / M 2 ratios. Mass Spectral Data from 13C de Novo Synthesis Experiments. Tables 4-6 show actual and theoretical mlz ion ratios calculated as described above from experiments where 13C-labeledcarbon was added to fly ashA and reacted

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with 10% O2 for temperatures ranging from 300 to 350 'C. Listed in adjacent columns are the measured and theoretical values for M / M 2 and M 4 / M 2 for each homolog group (except OaCDDlF where M 2 / M 4 and M 6 / M + 4 are shown). Allowing for &15% uncertainty in the integrated results (15), these data strongly support the conclusion that no scrambling of native 12Cand added 13C occurred in the chlorobenzene and PCDDlF products. In almost every case, the actual and theoretical ratios for both the 12Cand 13Cchannels were well within the experimental error; if appreciable scrambling had occurred, the actual ratios would deviate significantlyfrom the theoretical ratios expected for the unscrambled case.

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1358 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 5, 1995

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Literature Cited (1) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 373. (2) Stieglitz, L.; Vogg, H. Chemosphere 1987, 16, 1917-1922. (3) Stieglitz,L.; Zwick, G.; Beck, J.; Roth, W.; Vogg, H. Chemosphere 1989, 18, 1219-1226. (4) Stieglitz,L.; Zwick, G.; Beck, J.; Bautz, H.; Roth, W. Chemosphere 1989, 19, 283-290. (5) Stieglitz,L.; Vogg, H.; Zwick, G.; Beck, 7.; Bautz, H. Chemosphere 1991,23, 1255-1264. (6) Milligan, M. S.; Altwicker,E. R. Presentedat the 1lthIntemational Symposium on Chlorinated Dioxins and Related Compounds, Sept. 23-27, 1991, Research Triangle Park, NC. (7) Milligan, M. S.; Altwicker, E. R. Enuiron. Sci. Technol. 1993, 27, 1595-1601. (8) Altwicker, E. R.; Milligan, M. S. Chemosphere 1993,27,301-307. (9) Albrecht, I. D.; Naikwadi, K. P.; Karasek, F. W. Presented at the 12th International Symposium on Chlorinated Dioxins and Related Compounds, August 1992, Tampere, Finland Organohalogen Compd. 1992, 8, 217-220. (10) Milligan, M. S.; Altwicker, E. R. Carbon 1993, 31, 977-986. (11) Stromberg, B. Presented at the 10th International Symposium on Chlorinated Dioxins and Related Compounds, Sept 1990, Bayreuth, Germany; Organohalogen Compd. 1990,3,179-182. (12) Ronvadowski, M.; Wojsz, R. Carbon 1988, 26, 111-115. (13) Altwicker, E. R.; Konduri, R. K. N. V.; Lin, C.; Milligan, M. S. Combust. Sci. Technol. 1993, 88, 349-368. (14) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754-756. (15) Dickson, L. C.; Karasek, F. W. I. Chromatogr. 1987, 389, 127137. (16) Ross, B. J.; Naikwadi, K. P.; Karasek, F. W. Chemosphere 1989, 19,291-298. (17) Wiser, W. Am. Chem. SOC.Diu. Fuel Chem. 1975,20, 122. (18) Leon y Leon, D. C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.); Marcel Dekker: New York, 1994; Vol 24, Chapter 4. (19) Milligan, M. S. Ph.D Dissertation, Rensselaer Polytechnic Institute, Troy, NY,1994.

Received for review September 19, 1994. Revised manuscript received January 25, 1995. Accepted January 26, 1995.@ ES940576M @

Abstract published in Advance ACS Abstracts, March 1, 1995.