Environ. Sci. Technol. 1987, 21, 1080-1084
Catalytic Effects of Fly Ash from Waste Incineration Facilities on the Formation and Decomposition of Polychlorinated Dibenzo-p -dioxins and Polychlorinated Dibenzofurans Hanspaul Hagenmaler,* Mlchael Kraft, Hermann Brunner, and Roland Haag Institute of Organic Chemistry, University of Tubingen, D-7400 Tubingen, Federal Republic of Germany ~
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The thermal behavior of polychlorinated dibenzo-pdioxins (PCDD) and polychlorinated dibenzofurans (PCDF) on electrostatic precipitator ash (fly ash) from waste incinerators was studied at 300 "C under "oxygen deficient" and "oxygen surplusn conditions. Under oxygen deficient conditions a dechlorination/hydrogenation reaction is observed that is catalyzed by fly ash. The addition of octaCDD and octaCDF to fly ash and subsequent heat treatment lead to the formation of mono- to heptaCDD/CDF. Under oxygen surplus conditions an increase in PCDD and PCDF is obtained as first described by Vogg and Stieglitz (1). On the basis of evidence presented, a mechanism for this fly ash catalyzed reaction is proposed, which starts with the formation of chlorine from metal chlorides and leads finally to a de novo synthesis of PCDD and PCDF. The findings reported will have consequences for the interpretation of PCDD/PCDF formation and decomposition in waste incineration facilities and will aid in minimizing PCDD and PCDF emission from such facilities. H
Introduction Recently Vogg and Stieglitz (1)reported on the thermal behavior of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) on electrostatic precipitator ash (fly ash) from municipal waste incinerators. Heating the fly ash for 2 h at temperatures between 150 and 600 "C they found below 200 "C no change, at 300 "C a 10-fold increase in PCDD/PCDF concentration, and at 600 "C a complete destruction of PCDD and PCDF. The increase at 300 "C was attributed to a conversion of precursor compounds present in the fly ash to PCDD and PCDF, and the destruction at 600 "C was explained as a purely thermal destruction (bond rupture) at prolonged reaction times. The thermal conditions that lead to the formation and decomposition of PCDD and PCDF have been studied rather extensively in laboratory experiments. According to these experiments, the formation of PCDD and PCDF from chlorophenols, chlorobenzenes, polychlorinated diphenyl ethers, and polychlorinated biphenyls takes place at temperatures of 290-450 "C (chlorophenols) ( 2 , 3 )and 550-650 "C (chlorobenzenes and PCBs) ( 4 , 5 ) . The rate of decomposition of PCDD and PCDF at temperatures below 600 "C must therefore be slower than the rate of formation from the precursors (otherwise one would not be able to prepare PCDD and PCDF at these temperatures), and the rate of formation from pre-dioxins and pre-furans at 300 OC is fast only for chlorophenates. The explanation given by Stieglitz and Vogg for their results on the thermal behavior of PCDD and PCDF on fly ash was in our opinion not in agreement with the laboratory experiments on the thermal formation and decomposition of PCDD and PCDF. We have therefore recently investigated the thermal behavior of PCDD and PCDF on fly ash samples from a number of waste incineration facilities and report here the results obtained so far. 1080 Environ. Scl. Technol., Vol. 21, No. 11, 1987
Materials and Methods Fly Ash Samples. The fly ash samples used in the experiments described were in all cases collected at the exit of the electrostatic precipitator and were in most cases mixtures of samples collected over a period of at least 1 week. Heat Treatment. The samples (5 g) were heated at 300 "C (where indicated the actual temperature was only 280 "C) under four different conditions: Condition A. The fly ash sample was heated in an open flask as shown in Figure 1. The flask containing the sample has a tube extending about 25 cm out of the oven. The tube is filled with XAD-2 and basic alumina to adsorb any PCDD and PCDF volatilized during the heating process. Condition B. The fly ash sample was heated in a tube sealed under atmospheric pressure. Condition C. The fly ash sample was placed in a crucible and heated in the oven of a gas chromatograph with forced but uncontrolled air exchange. Condition D. The sample was placed in a porcelain boat inside a heated glass tube, through which air (or other gases) was flowing at a rate of 1.5 L/h (Figure 2). Analytical Procedures. (a) Extraction and Cleanup. After the heat treatment the fly ash sample was mixed with 200 mL of 1N HC1, stirred for 30 min, filtered, and air-dried. After the addition of 5 ng each of [l3C1,]2,3,7,8-tetraCDD, [l3CI2]-1,2,3,7,8-pentaCDD, [13C12]and 1,2,3,6,7,8-hexaCDD,[13C12]-1,2,3,4,6,7,8-heptaCDD, [13C12]-octaCDD (Promochem, Wesel, FRG), the sample was extracted in a Soxhlet apparatus with toluene for 20 h. The extract was concentrated in vacuo (7 kPa; bath temperature 45 "C; vacuum controller, Buchi, Switzerland) to about 10 mL and chromatographed on a column of basic alumina (25 g, ICN Alumina B Super I, ICN Biomedicals, Eschwege, FRG) and 20 g of Na2S04. The column was first eluted with 100 mL of benzene and 200 mL of hexane/ CHzClz(98:2) and the eluant discarded. PCDD and PCDF were then eluted with 150 mL of hexane/CHzClp (1:l). This fraction was concentrated on a rotary evaporator under vacuum control to about 2 mL and transferred to a 3-mL vial. The solvent was removed under a gentle stream of nitrogen and the PCDD/PCDF taken up in 50 pL of benzene. (b) GC/MS Analyses. Analyses for PCDD and PCDF were carried out with an HP 5890 gas chromatograph and an H P 5970 mass-selective detector, coupled with a direct interface. A 50-m Silar 1OC fused silica capillary column (0.25-mm i.d.; Quadrex Corp., New Haven, CT) was used. Sample aliquots of 1-3 pL in benzene solution were injected splitless (injector temperature 300 "C), and the column temperature was programmed as follows: 130 "C, 1min isothermal; 30 deg/min to 200 "C; 4 deg/min to 255 "C, 30 min isothermal. Carrier gas was helium at a head pressure of 150 kPa. Multiple-ion monitoring was used. Quantitative determination of PCDD and PCDF is based on direct comparison of peak heights and/or peak areas of mass fragmentograms for the (M + 2)' ion of
0013-936X/87/0921-1080$01.50/0
0 1987 American Chemical Society
Table I. Fly Ash Samples from Two Municipal Waste Incinerators (Fly Ash A and C) and a Waste Incinerator Operated b y a University (Fly Ash B) Were Heated under Condition A at 280 "C for 2 h" fly ash A, nglg untreated 280 "C, 2 h
fly ash B, n g l g untreated 280 "C, 2 h 3.2 59 88 50 22 11
fly ash untreated
c, nglg 280 "C, 2 h
0.6 12 59 110 200 458
1.6 30 29 18 15 13
2,3,7,8-TCDD tetraCDDs pentaCDDs hexaCDDs heptaCDDs octaCDD sum of PCDDs tetraCDFs pentaCDFs hexaCDFs heptaCDFs octaCDF
0.5 24 116 185 159 88
0.9 21 19 6 2 1
2.0 24 116 233 1067 6204
572
49
7644
230
111 188 123 35 26
14 12 4 1
ndb
139 393 415 844 1368
41 69 34 9 4
839 62 250 377 292 400
105 38 41 30 16 7
sum of PCDFs
483
31
3159
157
1381
132
"The PCDD/PCDF concentrations obtained are compared with those of the untreated samples. *nd = nondetectable. 60 1 U
sample
Figure 1. Schematic drawing of the experimental setup used for heating the fly ash samples under condition A. Dimenslons are In centimeters.
PCDD/PCDF isomers of a specific degree of chlorination and the corresponding (M + 2)' ion of the 13C-labeled reference compound for the same degree of chlorination. It is assumed that all PCDD and PCDF isomers of a specific degree of chlorination have the same E1 response as the 13C-labeled isomer added and that during the cleanup no isomer-selective losses have occurred.
Results and Discussion In Table I PCDD/PCDF concentrations of fly ash samples from three different municipal waste incinerators before and after heat treatment for 2 hours at 280 "C under condition A (see Materials and Methods) are shown. In all cases the heat treatment at 280 "C causes a drastic decrease in total PCDD and PCDF concentrations. The observed decrease in total PCDD/PCDF concentration is, however, not due to an equal decrease of PCDD/PCDF congener concentration. While the concentrations of the tetra- to octaCDF and the hexa- to octaCDD decrease, with the largest effect seen for the higher chlorinated congeners,
the pentaCDD concentration remains almost unchanged and the tetraCDD concentration increases. Especially, the 2,3,7,8-tetraCDD concentration increases significantly in all three experiments (the variation coefficient for the determination of 2,3,7,8-TCDD in fly ash samples in our Iaboratory is less than 20%). Since no PCDD and PCDF could be detected in the XAD-2/A1203adsorption tube, thermal desorption and volatilization could not explain these results. This was also confirmed when the experiments with the same fly ash samples were carried out in sealed tubes (condition B) and the results were identical with those obtained in the open tube. The fact that we observed not only a decrease in total PCDD/PCDF concentration but at the same time an increase in tetraCDD concentration led to the assumption that, under the experimental conditions employed, a dechlorination/ hydrogenation occurs that results in the formation of lower chlorinated PCDD and PCDF from the higher chlorinated congeners. To prove this hypothesis, a rather large amount of octaCDD was added to one of the fly ash samples and the mixture treated at 280 "C under condition A. The result of this experiment is shown in Table 11. Within 15 min at 280 "C, 99% of the octaCDD added had disappeared, and high concentrations of lower chlorinated PCDD were formed. A total of 0.7% of the octaCDD was transformed into 2,3,7,8-tetraCDD. A total of 70% of the octaCDD added could be accounted for as mono-to heptaCDD. The octaCDD sample used in this experiment contained about 2% of octaCDF. Table I1 shows that the octaCDF is dechlorinated as well. In other experiments we could show that there is no transformation of PCDD to PCDF under these reaction conditions. After 2 h at 280 "C, 95% of the PCDD and almost all of the PCDF formed from the octaCDD and octaCDF by dechlorination/hy-
synthetic a i r o r
nitrogen
ethoxyeth
Figure 2. Schematic drawing of the experimental setup used for heating the fly ash samples under condition D. Dimensions are in centlmeters.
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about 25% of the fly ash (and most of the heavy metals) are dissolved. A mixture of the remaining solid material and octaCDD (same concentration as used in the experiment described above) did not show any change in PCDD/PCDF concentrations when heated for 2 hours at 280 "C under condition B, and the octaCDD (and octaCDF) added was recovered quantitatively. In further experiments it could be demonstrated that the catalytic dechlorination/hydrogenation effect of fly ash on octaCDD and octaCDF can also be obtained with copper powder, even at temperatures as low as 120 "C (6). In order to examine whether the observed catalytic activity is a general property of fly ash from thermal waste treatment, we heated B number of electrostatic precipitator ash samples from different municipal waste incinerators at 300 "C for 2 h under condition B. All samples tested so far showed the described decomposition of PCDD/ PCDF. Some of the results are presented in Table 111. At 300 "C the decomposition of PCDD/PCDF is almost complete, and in some samples PCDD/PCDF could no longer be detected (limit of detection for single compounds, 0.01 ng/g). Fly ash from hospital waste incinerators contain, according to our analyses of samples from eight different plants, higher average PCDD/PCDF concentrations than fly ash from municipal waste incinerators (7). We were therefore interested in whether fly ash from hospital waste incinerators also catalyzes the dechlorination/ hydrogen-
Table 11. OctaCDD (2 mg), Containing about 40 pg of OctaCDF as a Contaminant, Was Added to 5 g of Fly Ash (Sample C, Table I) and Treated under Condition A at 280 OC for the Times Specified" 280 "C for 15 min, ng/g
280 "C for 2 h, ng/g
2,3,7,8-TCDD tetraCDDs pentaCDDs hexaCDDs hep taCDDs octaCDD sum of PCDDs tetraCDFs pentaCDFs hexaCDFs heptaCDFs octaCDF
2900 52000 82500 23700 9300 3700 174100 1400 3100 840 230 23
1500 12900 2600 470 210 170 17850 620 150
