Environ. Sci. Technol. 1992, 26, 1822-1828
Quantitative Comparison of de Novo and Precursor Formation of Polychlorinated Dibenzo-p-dioxins under Simulated Municipal Solid Waste Incinerator Postcombustion Conditions L. C. Dickson,+D. Lenolr,*-$and 0. Hutzinger
Chair of Ecological Chemistry and Geochemistry, University of Bayreuth, Postfach 10125 1, D-8580 Bayreuth, Federal Republic of Germany Model studies were performed to determine quantitatively the predominance of two proposed pathways of polychlorinated dibenzo-p-dioxin (PCDD) formation during municipal refuse incineration: surface-catalyzed reactions of precursors occurring on fly ash and de novo synthesis of PCDD and related compounds from reactions of particulate carbon. The relative yields of PCDD formed from the model precursor compound pentachlorophenol (PCP) were 72-99000 times higher than PCDD formed from the reactions of activated charcoal, air, inorganic chloride, and Cu(I1) as catalyst under identical reaction conditions. Yields and homologue distributions of PCDD were strongly influenced by heating time, air flow rate, and temperature, with marked differences observed between the two pathways. Activated charcoal also catalyzed the formation of PCDD from PCP in the absence of Cu(I1) and promoted the dechlorination of PCDD. We speculate that Cu(I1) and carbon represent two different types of active catalytic species which could be present on the surface of municipal incinerator fly ash. H
Introduction Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) can be formed by various combustion processes (1))for example, by incineration of municipal and other waste (2, 3), by burning fuels in automobiles ( 4 ) , and by household furnaces (5). The mechanisms of formation of these compounds are not well understood. Lustenhouwer et al. (6) first postulated three possible mechanisms of PCDD and PCDF formation in municipal refuse incinerators: (1)survival of trace levels of PCDD in the fuel, (2) generation of PCDD from precursors such as polychlorinated benzenes, polychlorinated phenols, and poly(viny1chloride) (PVC) present in the fuel, and (3) the de novo synthesis of PCDD as a consequence of a complex array of thermal reactions of chemically unrelated nonchlorinated organic compounds and inorganic forms of chlorine. Recent discussions of the possible mechanisms of PCDD/F formation have focused on two hypotheses: (1) Dickson and Karasek (7, 8) have proposed that PCDD/F are formed from chloroaromatic precursors such as polychlorophenols and polychlorobenzenes by reactions which have been shown to occur by heterogeneous catalysis on the surface of fly ash particles at 250-400 "C. These products could form by Ullman-type surface reactions as recently demonstrated by Lippert et al. (9). The precursors might be already present in the fuel, or they could be formed in the higher temperature postcombustion zone by multistep reactions (10, 11), including aromatization of aliphatic compounds (12)and subsequent chlorination by Current address: Toxicology Research Center, University of Saskatchewan, Saskatoon, SK S7N OW0 Canada. t Current address: GSF-Forschungszentrum fur Umwelt und Gesundheit GmbH, Institute of Ecological Chemistry, Ingolstadter Landstrasse 1, D-8042 Meuherberg, Federal Republic of Germany. 1822
Environ. Sci. Technol., Vol. 26, No. 9, 1992
molecular chlorine formed from an equilibrium of hydrogen chloride and oxygen (13). (2) Stieglitz and co-workers have suggested a de novo synthesis of PCDD/F from active carbon particulates by gas-solid and solid-solid reactions with air, moisture, and inorganic chlorides, catalyzed by Cu(I1) ions (14-16).This theory does not require chloroaromatic precursors to be present on the fly ash or in the gas stream prior to the simultanous formation of PCDD and other chloroaromatics at 250-350 "C. Stieglitz et al. (15) stated that "...we have a de novo synthesis of organohalogen compounds with particulate carbon as the primary source and precursor to all these compounds". The use of term "precursor" can cause some confusion when these two hypotheses are discussed because it is being used in different ways. As used by Dickson and Karasek, the term precursor refers to chloroaromatic compounds, especially chlorophenols and chlorobenzenes, already present on the fly ash surface or in the gas phase prior to the critical 250-350 "C zone where the heterogenous catalyzed reactions which form PCDD/F are thought to take place. However, Stieglitz and co-workers suggested that particulate carbon is the precursor to all chloroaromatic compounds, including PCDD/F, and that reactions of carbon, oxygen, moisture, and inorganic chlorides occurring during the short time when the fly ash is in the 250-350 "C zone are responsible for the levels of these compounds detected in incinerator emissions. It is inappropriate to consider particulate carbon a precursor to PCDD, as it would be to consider chloride or oxygen to be precursors to PCDD/F. These are starting materials for the de novo synthesis of PCDD/F, as defined by Lustenhouwer et al. (6). Both formation mechanisms undoubtably occur, but what is not known is which formation pathway dominates under municipal incinerator conditions, and by how much. Knowledge of the exact nature of the pathways leading to the formation of PCDD during combustion processes is important as it will influence incinerator design and operation and the choice of pollution abatement strategies necessary to reduce the formation and emission of these toxic compounds into the environment. It was the aim of this study to quantitatively determine the predominance of either of the two pathways under identical conditions through laboratory experiments and to determine the influence of heating time, air flow rate, and temperature on the yields and homologue distribution of the resulting PCDD. A preliminary report of our results has been published (17).
Materials and Methods Materials. Silica gel (70-230 Mesh, Merck) and pulverized activated charcoal (Merck) were Soxhlet extracted with toluene for 48 and 72 h, respectively, and dried before use. Anhydrous CuClz (Purim, Fluka) was used without treatment. Two synthetic mixtures, similar to those described to Stieglitz et al. (15)were prepared in 250-mL round-bottom flasks. One mixture contained 30 g of silica
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gel, 0.7 g of powdered activated charcoal, and 0.64 g of anhydrous CuC1,. Bidistilled water (100 mL) was added to dissolve and distribute the chloride evenly over the surface of the particles. The water was removed by rotary evaporation under reduced pressure, and the mixture was dried overnight at 70 "C. This mixture contained 2.1% charcoal, 1%Cu(II), and 1.1%chloride by weight. The second mixture was prepared the same as the first, but without the addition of CuC1,. Method. The experimental apparatus is shown in Figure 1. Reactions were done in 5 mm i.d. X 200 mm borosilicate glass tubes inserted into a custom-made tubular furnace. For each experiment, the exit end of the tube was plugged with preextracted borosilicate glass wool and then 1.6 g of silica gel was introduced to give an absorbent bed 12 cm long, followed by 0.5 g of one of the two test mixtures to give a 2-cm bed. A 100-pL aliquot of a 0.92 g/L [13C6]pentachlorophenol(['%,]PCP) solution in methanol ww added to the top of the bed of the test mixture, and the solvent was allowed to evaporate. The tube was then inserted into the furnace and the upstream end of the tube attached to a supply of compressed highpurity air which was further purified by passage through a bed of silica gel. During the experiment, the lower 11 cm of the tuhe containii the silica gel absorbent remained outside the furnace at room temperature to collect unreacted [WB]PCPand reaction products that may desorb from the heated zone. Each 0.5-g aliquot of test mixture was used once and then discarded after extraction. Experiments were conducted with different combinations of air flow rates (3,11, and 19 mL/min), heating times (10, 35,60 min), and temperature (250,300, and 350 "C). Reaction producta and unreacted [13C6]PCPwere extracted by eluting with 50 mL of toluene at 1 mL/min while the tube was heated at 95 "C. Preliminary experimenta showed that this procedure would extract more than 95% of native PCDD from municipal fly ash. Extracts were concentrated under a N, stream to 100 pL. These solutions were used without deanup for GC-MSD analysis. Experiments were conducted in duplicate. Analysis. The HRGC-LRMS analyses were performed on a Hewlett-Pachd GC-MSC operated in electron impact selected-ion-monitoring mode, using a 12-m Ultra 2 fused-silica column (0.2-mmi.d., c r m - l i e d , df = 0.33 pm, Hewlett-Packard) directly coupled to the ion source. Analysis parameters: 1-2 mL/min He d e r flow; splitless
Flew 2. Ratio of yields (% ylekl ['%,,]PCW/%yield ['2C,21PCW)
of tlm two pamways to PCDD famation at live annbinams of a t ?ow
rate and heating time.
The error
bars denote average deviation.
injector and interface temperatures 300 'C; ion source temperature 200 "C; temperature program, 80 "C 2 min, 15 deg/min to 250 "C, 5 deg/min to 300 O C . Three isotope ions each were monitored for 13Clr and 12Clz-labeledtetra(TCDD),penta- (P,CDD), hexa- (HCDD), hepta- (H,CDD), and octachlorinated (OCDD) dibenzo-p-dioxins and dibenzofurans. The standard mixture used for external standard quantification contained one isomer each of [13C12]-and [12C12]PCDD.The identification of compounds was confirmed using the following criteria: (1) The chromatographic peaks must fall within a specific time window. (2) The peak areas produced by the component must exhibit the correct ratios of ion intensities. (3)The signal/noise ratio must he greater than 3. Retention time windows were determined by using mixtures of PCDD extracted from municipal incinerator fly ash. The limit of detection was 1 ng of PCDD (10 pg in 1pL injected from 100 pL). Results and Discussion
It was the aim of the study to determine the predominance of two competing pathways of PCDD formation by determining the levels of PCDD produced by both pathways occurring together in the same reaction tube. To distinguish the products of the two pathways, [13C61PCP was used a model precursor compound; 13C,z-labeled PCDD are exclusively the resulta of reactions of the labeled precursor. [WI2]PCDDare formed by de novo synthesis from reactions of activated charcoal with air and chloride were with Cu(I1) as a catalyst. No mixed ['3C61ZC6]PCDD detected in these experiments. This result demonstrates that the two pathways of PCDD formation operated independently of each other in these experiments. Formation of PCDD i n the Presence of CuCl,. (a) Influence of FIow Rate and Heating Time. Aliquots of the mixture containing silica, activated charcoal, CuCl,, and [13C,]PCP were heated at 300 OC at five combinations of air flow rate and heating time. The amounts of [WI2]PCDDformed from [13C6]PCPand the amounts of [12C12]PCDDformed from reactions of activated charcoal are given in Table I. The percentage yields of the reactions of [13C6]PCP leading to [l3CI2]PCDDwere much higher than the reactions of activated charcoal leading to [12C12]PCDD under identical conditions. The relative yields of the two pathways (% yield [13C,z]PCDD/%yield ['2Cl,]PCDD), shown in Figure 2, range from 46000 to 510. These results inEnvlron. Sci. Technol.. VoI. 26, No. 9. 1992
1823
Table I. Amounts (ng) of PCDD Formed in a Reaction Mixture Containing Silica, Activated Charcoal, Copper(I1) Chloride, and ['3C8]Pentachlorophenol Heated at 300 OC at Different Combinations of Heating Time and Air Flow Rate heating time (min) flow (mL/min)
10 3
10 19
35
[13C12]TCDD [I3C121P&DD [13C12]HCDD ['3C12]H7CDD [13C12]OCDD
nd" nd nd 73 f 27c 44000 f 3800
nd nd nd 436 24000 f 930
nd nd
sum PCDD % yield
44000 f 3800 55
24000 f 910 30
9000 f 190
[12C12] TCDD [ "C121PSCDD
nd nd nd 300 f 21 1200 f 176
nd nd
[12C12]HCDD ["C12]H7CDD [12C12]OCDD
nd nd 13b 100 I 5 290 f 13
sum PCDD % yield
400 f 2 0.001
1500 f 197 0.004
and, not detected; detection limit 1 ng. ments.
60 19
nd nd nd 5 f l 720 51
*
nd nd 13b 270b 9200 f 620
730 f 51 0.92
9400 f 760 12
240 f 7 990 f 140
nd nd 3b 49 f 20 370 f 150
nd nd 9* 170 f 37 2100 f 320
1200 f 150 0.004
420 A 180 0.001
0.007
1b
91 f 2 8900 f 190 11
l b
2200 f 350
detected in one of two experiments. CAveragedeviation of results from duplicate experi-
dicate that the activation energy of the rate-determining step for the formation of [13C12]PCDDfrom [13c6]PcPis much lower compared to that of the formation of [12C12]PCDD from activated charcoal. The experiments were reproducible; coefficients of variation (CV) between replicates were below 15%, with some exceptions. For example, the CV for [12C12]PCDD formed at 60 min heating time and 3 mL/min flow was 42%. The CV for the relative yields of the two pathways ranged from 9% to 32%. There are several possible explanations for these variations. These include inadequate control of air flow rate, variations in the composition of the reaction mixture and packing density between experiments, variations in the distribution of Cu(1I) between silica and activated charcoal, and variable irreversible adsorption and recovery of reaction products. Variations of this amount are expected for heterogenous reactions on a microscale. While the variation between replicates could conceal significant differences between some treatments, some trends can be noted (Figure 3). The yields were influenced by flow rate and heating time, but the pathways were affected differently. The yields of [13C12]PCDDdecreased Substantially with an increase in heating time from 10 to 60 min; however, the yields of [12C12]PCDDslightly increased. While heating for 10 min, increasing the air flow rate from 3 to 19 mL/min halved the yield of [l3CI2]PCDD while the yield of [12C12]PCDDincreased nearly 4-fold. The yields of both pathways increased when the air flow rate was increased while heating for 60 min. The effects of air flow rate and heating time can be explained by examination of the two pathways. The de novo synthesis of PCDD from activated charcoal is a complex, multistep process which requires oxygen for the generation of hydroxyl radicals, chlorine from chlorides (the Deacon reaction), and the oxidative degradation on the charcoal surface. These reactions lead to the formation of chloroaromatic compounds thought to be intermediates in the production of PCDD and related compounds. It is reasonable to suppose that the higher air flow rate would increase the mass-transfer rate of oxygen to the charcoal surface, thus increasing the yield of [13C12]PCDD. The formation of [13C12]PCDDfrom [13c6]PcPis a much simpler process, proceeding from the initial condensation of two PCP molecules to form nonach1oro-2-phenoxypheno1, which undergoes intramolecular cyclization to form OCDD. 1824
60 3
11
Envlron. Scl. Technol., Vol. 26, No. 9, 1992
"C-PCDD
*.O{
'.I
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3 9 ' 1 1 ' 1O(b)
@)airflow
35
3
'
60
rates (miimin); Piheating times (min)
Flgure 3. Homologue distributions of [ 13C,,]PCDD and [ '2C,,]PCDD produced at five combinations of air flow rate and heating time. The error bars denote average deviation.
We speculate that both the longer heating time and the increased mass transfer of oxygen with higher air flow contribute to oxidative degradation, other side reactions, and irreversible adsorption which could decrease the yield of PCDD from PCP. Chlorinated benzenes (18) and pentachlorophenoxy-PCDD (19) have been detected in similar experiments using municipal incinerator fly ash. There is evidence (20) that PCDD may be irreversibly adsorbed or degraded on fly ash. This may be due to bond rupture and decomposition to undetected side products. The data are not strong enough to support the creation of a quantitative kinetic model to interpret the results. In the preceding discussion it is assumed that PCDD formed by the two pathways accumulate on the reaction
Table 11. Amounts (ng) of PCDD Formed in a Reaction Mixture Containing Silica, Activated Charcoal, Copper(I1) Chloride, and [lSCB]PentachlorophenolHeated at Three Temperatures at Two Combinations of Air Flow Rate and Heating Time flow and heating time 19 mL/min for 60 min
3 mL/min for 10 min 250 "C
[W12]TCDD [13C12]P6CDD [13C12]HCDD [13C12]H7CDD [l3CI2]0CDD
nd" nd
sum PCDD % yield
300 OC
350 "C
13" 270' 9200 f 620
4900 f 2200 6.2
10000 f 730 13
9400 f 760 12
230 f 41 0.3
nd nd
nd nd
2c
13c
nd nd nd
nd nd nd
2'
100 f 5
37 f 32
290 f 13
4c 18c 140 f 93
nd nd nd 160 f 27 4700 f 400
170 f 31 2100 f 320
80 f 14 1100 f 270
39 f 32
