Environ. Sci. Technol. 1997, 31, 776-785
Polychlorinated Dibenzo-p-dioxins and Dibenzofurans Formation in Incineration: Effects of Fly Ash and Carbon Source PETER W. CAINS,* LINDA J. MCCAUSLAND, ALWYN R. FERNANDES, AND PATRICK DYKE AEA Technology, Harwell Laboratory, Didcot, Oxon OX11 0RA, U.K.
Research aimed at understanding polychlorinated dibenzop-dioxin (PCDD) and dibenzofuran (PCDF) formation in combustion and incineration processes constitutes an important component in developing strategies for controlling their emission. Incinerator ashes from six different process sources have been examined and characterized in terms of their behavior with respect to PCDD/F formation in laboratory experiments. The effects of varying the carbon content of one ash has been investigated by replacing the native carbon content with activated charcoals and pyrocarbons from paper and PVC. Analysis of homologue totals and full isomer profiles indicate that PCDDs tend to form in preferential isomer groups, while PCDF isomers are distributed more broadly. PCDD formation is consistent with condensation of chlorophenols, together with stepwise chlorination and dechlorination. Experiments with a pentachlorophenol precursor show that fly ashes are more reactive dechlorinators than model systems, probably due to the presence of alkali elements. PCDF formation probably occurs via condensation of nonchlorinated phenol followed by chlorination of the dibenzofuran skeleton; this gives schemes of formation for both PCDDs and PCDFs that start with phenol. Demonstration of the formation of phenols in ashes and simulates is not currently satisfactory, but aromatization of residual aliphatic oils is believed to be the most probable source. A relationship was found between the amounts of such oils present on the original carbons/ashes and the quantities of PCDD and PCDF formed.
Introduction The emission of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), known popularly and collectively as “dioxins”, from industrial processes has become a subject of considerable public and scientific concern in the light of evidence of their extreme toxicity. Combustion processes, particularly those burning waste materials, are believed to represent a major source of the environmental burden of these compounds (1), and worldwide attention has focused on the best ways of controlling and regulating such emissions. Discussion of precursors has dominated recent research on the fundamentals of PCDD/F formation and proceeds from earlier investigations of catalyzed PCDD/F formation from two distinct types of starting materials: volatile aromatics and ash residues containing carbon. Of the former, chlorinated phenols and benzenes have been most intensively
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studied and are known to be present in incinerator off-gases as products of incomplete combustion (PICs) (2). A link between chlorophenols and the higher chlorinated PCDDs has been established via relatively simple condensation reactions and rearrangements (3). Recent work (4) indicates that PCDDs and PCDFs do not form directly from chlorobenzenes, and there is some evidence that their effect may be inhibitory. PCDD/F formation de novo from carbon residues (5) involves both structure formation and chlorination and requires the presence of both an oxygen and a chlorine source. In mass terms, the quantities of furans formed usually exceed the dioxins levels, a pattern reflected in most incinerator measurements. The isomer distributions of chlorinated furans do not correspond with chlorophenol precursors (3), and the most probable source is believed to be aromatic and polyaromatic structural units present in the carbon. There is, however, little reported evidence to substantiate this formation route. The main aims of the present study are twofold; to characterize a series of incinerator ashes from different provenances and compare their catalytic activities for PCDD/F formation and to investigate the effects of variations in the carbon source particularly on furan formation. To this end, a series of formation experiments have been carried out using a static bed system with both P5CP and solid carbon sources. Variations have been introduced into the carbon components of some ashes by the removal of the native carbon and its replacement either by commercially available activated carbons or by pyrocarbons prepared from various materials.
Experimental Section PCDD/F Formation Measurements. Formation experiments were carried out in a static bed set up in a laboraratory furnace. An inlet gas stream (N2/O2) was preheated by passage through a stainless steel tube of 4 mm bore and 4 m length coiled spirally along the inner surface of the furnace liner. The heated gas (nominally 100 cm3 min-1 at STP) passed upward through a bed of fly ash of 20 mm diameter and 90 mm depth. Water was introduced to the gas stream immediately upstream of the heating coil by an ultrasonic nebulizer. Cl2 addition was made downstream of the gas heater. The bed temperature was continuously monitored by a thermocouple located in a sheathed silica well at the center of the ash bed. Isothermal temperature control was (1 °C at 312 °C. The gas stream exiting the furnace tube passed through a water condenser, a toluene impinger (50 mL), and microporous filter with upstream and downstream absorption traps containing XAD-2 before discharge via a rotameter to check that no leakage occurred. Fly Ashes. Six fly ashes were obtained from waste incinerators as described in Table 1. Quantities used in experiments were typically 12-18 g. Standard treatment techniques have included drying to constant weight at 120 °C, removal of organic matter (including native PCDD/Fs) by extraction in toluene for 2 × 24 h followed by washing with hexane and drying to constant weight at room temperature, and removal of all carbon present by ignition in O2 to constant weight at 700 °C. Fused quartz was used as a blank reference material assumed to have no catalytic activity. Analysis and Examination. Combined samples from formation experiments were spiked with 13C-labeled internal standards. Quantitative analysis for the 17 2,3,7,8-chlorosubstituted isomers was performed using a 60 m DB5-MS (J&W) fused-silica GC column and a VG Autospec MS operated in SIM EI+ mode. Analysis for all T4-O8 PCDD and PCDF isomers was carried out using a 60 m SP-2331 GC column
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TABLE 1. Sources and Compositions of Fly Ashes ash ref
source/description
ash ref
source/description
A B
ESP ash from incinerator burning MSW ESP ash from incinerator burning MSW; combustion conditions better than in (A) fabric filter ash from MSW incinerator with liming as component of off-gas treatment
D E
ESP ash from incinerator burning agricultural waste cyclone ash from fluidized-bed MSW incinerator
F
baghouse ash from fluidized-bed MSW incinerator with liming of off-gas
C
loss on loss on moisturea extrctn igntn (%) (%) (%) ash ref A B C D E F a
26.1 0.09 1.8 0.19 0.34 1.5
2.5 2.3 1.4 1.1 0.9 0.9
16.7 11.3 4.8 4.4 3.7 3.0
Si (%)
Al (%)
8.8 7.6 11.5 6.1 3.3 1.8 3.1 0.51 17.9 12.8 4.0 3.0
Mg (%) 1.3 1.4 0.49 2.6 1.6 1.0
Ca (%)
K (%)
13.5 3.7 15.0 3.3 36.4 nab 11.0 30.1 13.6 na 33.0 na
Weight loss based on weight of ash as received (not dried).
b
Na (%) 2.8 2.4 na 2.5 na na
S (%)
Cl (%)
Fe (%)
1.8 6.9 4.5 3.6 0.24 15.0 6.6 4.2 0.002 0.93 0.39 13.1
Cu Mn Cr (µg/g) (µg/g) (µg/g)
1.2 758 1.9 881 0.35 307 0.48 822 2.2 3441 0.83 4617
880 996 na 2204 na na
TEQ (ng/g) PCDD
PCDF
176 14.5 16.6 335 3.4 3.2 146 0.33 0.51 51.1 0.010 0.016 519 0.037 0.048 457 0.11 0.44
na, not available.
TABLE 2. PCDD/F Homologue Totals on Native Fly Ashes A, B, and D (ng/g) ash
T4CDD
P5CDD
H6CDD
H7CDD
O8CDD
T4CDF
P5CDF
H6CDF
H7CDF
O8CDF
A B D
54.1 4.8 0.13
101.5 22.0 0.19
129.9 57.1 0.17
94.7 160.1 0.15
33.8 150.1 0.12
230.0 32.0 0.69
182.7 31.0 0.18
162.4 37.0 0.090
60.9 30.0 0.043
5.7 8.3 0.007
TABLE 3. PCDD/F Homologue Totals Formed on Ashes from P5CP (ng/g) ash
expt ref
Aa Ba C D E F SiO2b Aa Aa Aa Aa
t c d e g h i j k l
conditions
T4CDD
P5CDD
H6CDD
H7CDD
O8CDD
T4CDF
P5CDF
H6CDF
H7CDF
O8CDF
extracted extracted extracted extracted extracted extracted
27.4 (-0.3) 70.8 2085 2543 29.4 (-31.0) (-30.7) (-12.7) (-1.5) 21.8 29.2 61.3 356 339 12.0 3.6 2.9 0.73 0.57 4.1 11.6 58.8 202 436 c c c c 1.9 46.8 121 379 694 328 c c 0.65 0.49 0.65 3.7 11.3 50.8 102 176 c c c c c 3.7 5.6 10.7 24.6 20.8 c c c c c 0.12 0.24 0.72 2.8 2.0 0.05 0.02 0.02 0.02 0.06 as received, no extractiond 39.1 (-18.8) (-53.6) 44.0 231 (-66.9) (-83.1) (-10.1) (-50.5) (-4.4) ignited (no carbon) 3.1 6.7 24.9 84.5 158 0.21 0.11 0.060 0.093 0.22 ignited + Darco G60 (15%) 6.1 63.1 70.3 59.2 592 0.08 0.007 0.10 0.20 0.16 ignited + Norit A (15%) 1.2 34.5 293 1041 1496 0.10 0.052 0.20 0.38 0.32
a Background levels subtracted. b Fused quartz reference. c Values comparable with background (see text). 2) subtracted; figures relative to weight of ash on completion of experiment.
direct-interfaced to an Incos XL mass spectrometer operated in SIM EI+ mode. Analysis for organic precursors was carried out by GC/MS in scanning mode on extracts without purification. Reagents and Materials. Toluene (HPLC grade 99.8%), hexane (HPLC grade 95+%), pentachlorophenol (99%), and activated charcoals Darco G60 and Norit A were from Aldrich Chemicals. Purified XAD-2 resin was from Alltech Associates. Oxygen, nitrogen, and chlorine 500 vpm in nitrogen were from BOC gases. Pyrocarbons were prepared by heating samples of tissue paper and pure nonplasticized PVC powder (Aldrich) in a N2 atmosphere at 450 °C to constant weight.
Results Examination of Fly Ashes. Summary analytical data for fly ashes A-F are given in Table 1. All gravimetric data are related to dried ashes, except where otherwise stated. The PCDD/F loadings as received are calculated as toxicity equivalents (TEQs; I-TEF) from assays of the 2,3,7,8-chlorinated isomers. Ashes A and B stand apart as significantly more heavily contaminated than the rest. Homologue totals for the T4-O8 PCDDs and PCDFs on ashes A, B, and D are given in Table 2; ashes A and B exhibit very different homologue distributions for both dioxins and furans. Breakdowns of the contributions
d
Levels present on original ash (Table
of individual 2,3,7,8 isomers to TEQs also show differences that parallel the homologue distributions, particularly for the dioxins. Duplicated extraction and analysis of a separate sample of ash A for 2,3,7,8 isomers and homologue totals gave the same results within the limits of analytical accuracy. SEM examination identified three types of particulate material in all of the ashes. Irregularly shaped particles appeared at high magnification to have high surface area and porosity. Fibers were distinguishable from particles by their shape, deeper contrast, and higher Cl levels and probably consist of unburned or partially burned material. Spheres contained high levels of Al and Si, and exhibited smooth surfaces and a calcined appearance at high magnification. The relative quantities of these materials appeared to vary significantly, with ash A rich in fibers and poor in spheres, while ashes E and F showed very few fibers and large amounts of spheres and other calcined material. PCDD/F Formation from Pentachlorophenol. PCDD/F homologue totals formed on beds of fly ashes A-F from 1 mg of P5CP over 1.5 h at 312 °C with an inert (N2) purge gas are summarized in Table 3. The totals are expressed as the total mass of PCDD/F homologue (ng) related to the quantity (g) of dried and extracted ash in the bed. For ashes A and B, estimated background levels after extraction have been
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1.1 (-0.37) 2.3 59.2 8.4 80.6 0.82 1.5 2.8 2.3 16.8 111 44.3 296 1.33 4.3 31.3 3.4 49.2 104 109 745 0.78 3.5 30.9 2.5 130 111 148 1010 0.55 3.3 36.9 2.6 279 156 186 1880 0.36 3.7 a
Gas stream consisted of 17.5% (v) O2, 13% (v) H2O, and 100 vpm Cl2 in N2 except where indicated.
b
Background subtracted.
40.7 (-3.5) 3.0 47.5 34.1 107 3.06 12.2 9.7 22.9 11.7 143 48.2 658 1.41 26.0 45.8 25.9 32.3 273 70.7 1610 0.32 23.5 24.4 7.91 90.7 338 109 1540 0.15 10.6
H7CDD H6CDD P5CDD T4CDD
extractedb
u y p m q n w x
PCDD/F Formation from Carbon and Gaseous Precursors. Measurements were carried out on ashes and variants exposed to a flowing gas stream of 17.5% (v) O2, 13% (v) H2O, and 100 vpm Cl2 in N2 for 3 h at 312 °C. PCDD/F homologue totals are recorded in Table 4, together with experimental
ash/conditionsa
Where the carbon content of the ash was completely removed, much lower PCDD levels were obtained, of an order comparable to those obtained from the least reactive ashes E and F with relatively low carbon contents. Addition of activated carbons increased the PCDD yields across the range of homologues, but the effect varied with the carbon used.
expt ref
A further series of measurements (lower four entries in Table 3) were carried out on ash A with variations in pretreatment and carbon sources. Variations in carbon source were brought about by removing the native carbon by ignition (which may in itself modify the catalytic behavior) and replacing with equivalent amounts (15%) of the activated charcoals Darco G60 and Norit A. With unextracted ash, the quantities of H7 and O8 dioxins formed were much lower than those formed on extracted ash, and all other PCDD and PCDF species except T4CDD were present at lower levels than those found in the native ash. It appears that destruction, particularly of the furan species, occurs under these conditions. It is also possible that the presence of residual organic material on the surface of the unextracted ash may inhibit the condensation reactions leading to the higher chlorinated PCDDs.
TABLE 4. PCDD/F Homologue Totals Formed from Ashes and Gas Precursors (ng/g)
For ashes A-C and E, comparison of the yields of the two H7CDD isomers indicates that dechlorination has occurred preferentially at a ratio of around 6:4 at the position R to the phenol or ether linkage (to give the 1,2,3,4,6,7,8 isomer), while for D the corresponding ratio is close to 1:1. Full isomer distributions of the T4, P5, and H6 dioxins for ash D are given in Figure 1; coelutions are shown on the abscissas and 1,2,3,4,6,7-H6CDD was not measured.
O8CDD
Considering the first six entries in Table 3 for extracted ashes, ash A appears to be most active in producing the highest yields of H7- and O8CDD, but ash D exhibits the most marked dechlorination. The composition data in Table 1 indicate a high K level as its principal distinguishing feature. It is possible that the dechlorinating effects of the fly ashes may be related to their alkali contents, since the reported model systems (6) contained no alkali elements. Ash C contains very high Ca levels that would impart basicity, and its dechlorinating effect is much less marked than that of ash D; it thus appears that simple chlorine abstraction by a base is not a sufficient explanation.
10.1 1.50 84.2 202 96.4 1100 0.10 7.2
T4CDF
P5CDF
With the possible exception of ash B, furan formation in these experiments appears insignificant, the relatively large numerical values for ash A being comparable with background. The principal products are the H7 and O8 dioxins, but the formation of lower chlorinated dioxins also appears significant. The proportions of H7 and lower chlorinated homologues to the O8 primary condensation product are much higher than those in work reported on synthetic model ash systems (6), indicating a greater degree of dechlorination.
A B extractedb A ignited + Darco G60 (15%) A ignited + Darco G60 (15%)/no H2O in gas A ignited + Norit A (15%) A ignited + Norit A (15%)/no H2O in gas A ignited + pyrocarbon from paper (15%) A ignited + pyrocarbon from PVC (15%)
H6CDF
H7CDF
O8CDF
subtracted, giving the negative numbers (in parentheses) where such backgrounds exceed the measured quantities. Extraction efficiency data were not available for ash D, and only values larger than the native values in Table 2 by factors of 5 or more are given; the remainder (generally lower than the corresponding Table 2 values) are quoted as background. For ashes C, E, and F, where native homologue totals are unavailable, a similar comparison of the measured 2,3,7,8isomer levels (which generally follow the homologue totals) has been made. Maximum cross-contamination levels of 1.5% were measured (using a quartz sand reference) against a very low O8CDF value, with the corresponding values for the more abundant dioxins lower by more than 1 order of magnitude.
FIGURE 1. Normalized full isomer distributions of T4-, P5-, and H6CDD formed on ash D from P5CP, experiment e. conditions and variations. These homologue distributions are very different from those obtained with P5CP and inert purge gas. Table 4 also includes results from a series of experiments in which the native carbon content of ash A was removed (by ignition) and replaced by activated charcoals (Darco G60 and Norit A) and carbon residues from the pyrolysis of paper and PVC. Both charcoals yielded PCDD/F levels higher than the extracted ashes, with dioxin and furan levels of comparable magnitude in all cases. The homologue distributions for both types of species were weighted toward the lower chlorinated forms; under the standard conditions, this effect was more
marked for the Darco G60. Removal of the water from the gas stream enhanced PCDD/F yields markedly for both charcoals, and the levels recorded with Norit A under these conditions were extremely high. The residue from PVC pyrolysis yielded PCDD and PCDF levels comparable with the less active ash B (Table 1), while the paper residue produced very small quantities. Full isomer distributions have been measured for the above experiments with extracted ash A (u), Darco G60 with and without H2O in the gas stream (p, m), and Norit A without H2O (n). They are given in Figures 2 and 3 for dioxins and furans, respectively. 1,2,3,4,6,7-H6CDD and 1,2,8,9-T4CDF
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FIGURE 2. Normalized full isomer distributions for PCDD formation from carbon, ash, and gaseous precursors. were not measured. The dioxin isomer distributions (Figure 2) show considerable variations across the range of experiments, with the most marked differences between extracted ash A (u) and the cases with activated charcoals. The furan distributions in Figure 3 are more complex and difficult to analyze than the dioxins because of the larger number of isomers and coelutions. Here, significant differ-
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ences appear between all cases, but the most striking are those between extracted ash A and the activated carbons, and between the Darco G60 with and without water present. The latter also exhibited marked differences in both total yields and homologue distribution (Table 4), with high levels of lower chlorinated furans in the presence of water, and high levels of dioxins in its absence.
TABLE 5. Principal Organic Species Identified by GC/MS Scan PVC pyrocarbon as prepared ash B as received chlorinated C2-C3 alkanes and alkenesa
Norit A as received
formation expt ub
ash A as received
traces of xylene alkanes and alkenes: C9H20, C18H38, aliphatic C8-C12 hydrocarbons; methylindane; C22H44; methyl hexadecanoate diethylbenzene; benzyl formate; pinene; bibenzyl;a xylenes;a benzaldehyde;a benzyl alcohol;a cresol;a dimethylbenzaldehyde; vinyl benzyl alcohol Darco G60 as received
formation expt pc
formation expt md (rpt)
aliphatic C23-C29 aliphatic C10 hydrocarbons; xylenes;a benzaldehyde;a aliphatic alkanes/alkenes around C24; chlorinated C2-C3 alkanes and alkenesa di- and trichlorobenzenes, alkanes and alkenes benzyl alcohol;a smaller amounts of tetrabibenzyl;a chlorotoluene and hexachlorobenzenes; (mono)benzofuran; bibenzyl;a xylenes;a benzaldehyde;a benzyl alcohola a May possibly arise from solvent degradation or impurities. b With extracted ash A and gas precursors. c With ignited ash, Darco G60, and gas precursors. d With ignited ash A, Darco G60, and gas precursors without H2O.
Examination of Carbons. The infrared absorption spectra of Darco G60, Norit A, and the paper and PVC pyrocarbons, measured on pressed KBr disks, were identical, and a very similar spectrum was also obtained for extracted ash A. The most notable features of these spectra are a band of absorptions in the 1300-1700 cm-1 region that probably corresponds to adsorbed water and some absorptions in the 600-700 cm-1 region that cannot be assigned with any confidence. The principal organic species found in extracts from both starting materials and the products of selected experiments are summarized in Table 5. Species that can be associated directly with the extract solvent or background (e.g., phthalate esters) have been excluded, those that may be associated with some aspect of the treatment of the samples, e.g., from solvent impurities or degradation, are indicated by superscript a. Detection limits are around 1-10 µg g-1 in the most sensitive case, at least 3-4 orders of magnitude higher than those employed in PCDD/F analysis, and variable from species to species. Both ash A and Darco G60 showed a greater range of organic species present after formation experiments than before, and the latter produced very different results with water present and absent in the gas stream. These materials as received contained detectable quantities of aliphatic hydrocarbons and derivatives, with Norit A exhibiting significantly higher levels of such aliphatic species in the C20C30 range.
Discussion The formation experiments reported above exemplify the two principal approaches that have been taken to investigate PCDD/F formation mechanisms under incineration conditions. As in previous reports, formation from chlorophenol precursors leads to dioxin species only (3, 6, 7), while experiments in which solid ashes and carbons are exposed to gaseous oxidizing and chlorinating agents lead to both dioxins and furans (5, 8). There have been no reports of systems that enable furan formation without simultaneous dioxin formation, although some data from laboratory studies (8) and experimental incinerators (9, 10) exhibit furan to dioxin mass yield ratios as high as 4:1. Luijk et al. (3) presented a theory, based on measurements on model ashes, that PCDD formation occurs by condensation reactions and associated Smiles rearrangements of a small set of chlorophenol precursors, 2,4,6-T3CP, 2,3,4,6-T4CP, and P5CP. Our results with P5CP and fly ashes show that dechlorination also occurs, particularly with ash D, which contains a high alkali metal (K) content. Dechlorination may occur prior to condensation, reducing P5CP to T4CPs and
other lower chlorinated phenols which subsequently condense, and it may also take place on PCDDs already formed. Recent work (11) has shown that condensations of T4CP on fly ashes, particularly of 2,3,4,6-T4CP, are significantly more rapid than those of P5CP. Rapid synthesis of PCDDs other than the O8 (and arguably H7) homologues would therefore be expected where dechlorination of the P5CP precursor occurs. The data for ash D in Figure 1 show relatively high abundances for the isomers that would be formed by a direct condensation mechanism, but the effect cannot be quantified because of the coelution of 1,2,3,4,6,8-H6CDD with 1,2,4,6,7,9and 1,2,4,6,8,9-H6CDD, which cannot form directly from the above chlorophenol precursors. We also report high levels of 1,3,7,8-T4CDD and 1,2,4,7,8- and 1,2,3,7,8-P5CDDs, which cannot form directly in this way. The distributions of the two H7CDD isomers from P5CP generally show a moderate excess of the 1,2,3,4,6,7,8 species. If the H7 dioxins were formed entirely by Cl loss from O8CDD, then the rules of aromatic substitution would predict equal amounts of the two H7 isomers. Direct condensation of a P5CP moeity with 2,3,4,6-T4CP would give the 1,2,3,4,6,7,8 isomer, so the excess of this species may constitute evidence that such a mechanism is occurring. Conversion of P5CP to 2,3,4,6-T4CP involves dechlorination at the position meta to the OH group, which is unfavored for substitution. It is possible that the greater reactivity of 2,3,4,6-T4CP in relation to the other T4CP isomers may compensate for the fact that smaller quantities are formed via dechlorination. The H6CDD distribution in Figure 1 is dominated by species with 3:3 chlorination ratios on the rings, which would form from pairs of T4CP precursors, and quantities of lower chlorinated PCDDs are significantly lower (Table 3). There is thus some evidence that a single dechlorination is occurring on the P5CP precursor prior to condensation. The PCDD isomer distributions from experiments with carbon ash and gas mixtures (Figure 2) all show similar tendencies for particular isomers or coeluted groups to occur preferentially. This supports a mechanism of chlorophenol condensation of the type proposed by Luijk et al. (3), particularly since most of the more abundant isomers correspond to condensations listed. The coelution of 1,2,3,4,6,8-H6CDD, formed in this way (3), with the 1,2,4,6,7,9 and 1,2,4,6,8,9 isomers, which are not, is the most abundant identification in most cases. However, the latter are related by a single Cl substituent to the 1,2,4,6,8- and 1,2,4,7,9-P5CDDs formed by chlorophenol condensation, and their presence in relatively large quantities may indicate condensation followed by a single chlorination step.
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FIGURE 3. Normalized full isomer distributions for PCDF formation from carbon, ash, and gaseous precursors. The data on PCDD isomer and homologue distributions are thus consistent with formation via chlorophenol condensation, together with some degree of chlorination and dechlorination. If PCDDs were to form as a nonchlorinated dibenzo-p-dioxin skeleton that is subsequently chlorinated, then the chlorination would be more randomly distributed, and the homologue totals would tend to decrease with increasing number of Cl substitutions. This latter condition is usually contradicted in native fly ashes, e.g., ash B in Table 2. The importance of chlorophenols as precursors has led to detailed studies of their behavior in the presence of fly ash catalysts (12). One phenomenon reported is the “concentration” of chlorine in aromatic compounds, whereby chlorine released in the destruction of lower chlorinated species chlorinates other moieties to give more stable, higher chlorinated products. We have found the full range of chlorobenzenes with up to six substituents forming up to orders of 1-10 µg g-1 in our experiments (Table 5). The reactions of chlorophenols on fly ashes (12) also produce small quantities of PCDDs, chlorobenzenes, phenoxy-substituted PCDDs, and benzofurans, but significantly no dibenzofuran or PCDFs. The failure to produce PCDFs from chlorophenol precursors is demonstrated in Table 3 and also by other workers (3, 6), and it has been suggested that the sources of furans are associated in some way with ash or carbon residues. We have obtained widely differing PCDF (and PCDD) yields and isomer distributions from different carbon sources, all exhibiting similar basic structures (Table 4), and have examined carbon ash mixtures from our experiments for aromatic and polycyclic compounds that may give clues to the PCDF formation mechanism. For Darco G60, we find benzofuran at the 1-10 µg g-1 level (Table 5), while with ash A we find methylindane, benzyl formate, and vinyl benzyl alcohol but no indications of dibenzofuran. It is probable that the mechanisms of formation of benzofuran and dibenzofuran are entirely different, as there is no apparent way in which an aromatic ring can become ortho-fused at the benzofuran 2,3 positions. Benzofurans, and similar bicyclic compounds, are likely to form from simple aromatics via cyclic condensation and/or addition reactions
involving reactive ortho-substituted side chains (such as vinyl benzyl alcohol). The most likely route to dibenzofuran is a condensation of two nonsubstituted phenol moeities. This reaction has been reported at 28% yields from a mixture of phenol and PbO catalyst at 150 °C (13) and also in the presence of fly ash (12). The dominance of furans over dioxins in soots from the combustion of bituminous coal, with high native phenol levels (14), is also compatible with a mechanism of this type. The PCDF isomer distributions within individual homologue totals are generally more evenly distributed than the corresponding PCDD data. Such a dispersed pattern would be expected if chlorination of a preformed dibenzofuran skeleton were occurring. If this were the case, the principles of aromatic substitution would also predict two further tendencies; preferential chlorination at the 2,4,6, and 8 positions, and decreasing yields as the number of Cl substitutions increases. The latter is exhibited in all cases in Table 4, where high furan formation occurs, although many of the reductions in homologue totals from T4CDF to H6CDF are small and gradual. However, these patterns contrast markedly with the dioxins, where higher chlorinated homologues usually predominate. The pattern of decreasing PCDF homologue totals with increasing degree of chlorination is exhibited in many (but not all) incinerator measurements. The question of positional preference of substitution is more difficult to determine because of the large number of coelutions, but the data in Figure 3 indicate some evidence of preference for 2,4,6,8 substitution over the 1,3,7,9 positions: for example, the T4CDF distribution shows consistent high levels of the 2,4,6,8/ 1,2,3,8/1,4,6,7/1,2,3,6 grouping, while 1,2,3,9 and 3,4,6,7 are extremely low; and the low P5 isomers 1,3,4,7,8, 1,3,4,6,9, 1,2,3,7,9, 1,2,6,7,9, 1,2,3,6,9, 1,2,3,4,9, and 1,2,3,8,9 have at least three substitutions in unfavored positions. The directional effect would become less marked with increasing chlorination due to the ortho-/para-directing effect of the substituents. Both isomer distributions and homologue totals show considerable variation between the different conditions investigated. The cases using Darco G60 with and without
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FIGURE 4. Proposed scheme for PCDD and PCDF formation from phenol. water in the gas stream (p, m) are particularly interesting, because all other conditions in the experiments were identical. The homologue totals for T4CDF and P5CDF are higher with water present, while all other homologue totals are higher with water absent. Parallel experiments with Norit A (q, n) gave significantly higher homologue yields throughout with water absent, but the largest mass contributors where water was present were again T4CDF and P5CDF. In experiment p, three of the five individual isomers (or coelutions) making the largest mass contribution to the PCDD/F total are T4CDFs, while in (m) the five largest groups are dioxins. It is clear that the presence or absence of water exerts an important effect on PCDD/F yields and homologue distributions. Figure 4 summarizes the scheme proposed above for the formation of PCDDs and PCDFs from phenol. Both phenol and chlorophenols are known to arise in incinerator off-gases as PICs. However, formation de novo from carbon residues and ashes by such a route requires the prior formation of phenol in the ash/residue. The most obvious organic contaminants of ash A and the charcoals as received were aliphatics, generally alkanes and alkenes in the C20-C30 range (Table 5). The quantity of these materials detected decreased markedly in the order Norit A > Darco G60 > ash A, corresponding to the total PCDD and PCDF yield patterns in Table 4. The PVC pyrocarbon gave yet lower PCDD/F yields and exhibited chloroalkanes, but no hydrocarbons. Demonstration of a phenol formation route from these aliphatic hydrocarbons, together with the scheme of Figure 4, would provide a pathway from such aliphatic residues to both PCDD and PCDF formation. Evidence of aromaticization on the carbon/ash is exhibited in Table 5. A number of such reactions, including cracking, hydroforming, and other dehydrogenation, cyclization, and isomerization reactions take place over metal oxide catalysts, particularly chromia, at temperatures around 480 °C (13). It is reasonable to assume that such reactions have occurred under the conditions of the experiments, although the possibility of contamination from toluene solvents cannot be discounted entirely in all cases. Recent reports (15) have also suggested that prior extraction with toluene may contribute potential precursors as solvent residues. Chlorination of aromatics under the experimental conditions is demonstrated by the formation of chlorobenzenes on Darco G60. It is possible that the effects of water, noted above, may derive from the inhibition of aromatization and chlorination processes. Inhibition of aromatization would result in lower levels of phenol precursors, hence lower yields, while inhibition of chlorination would both increase the ratio of nonchlorinated phenol to chlorophenols, increasing the PCDF to PCDD ratio, and give a greater proportion of lower chlorinated to higher chlorinated PCDFs. What is less clear is how phenol may form under such conditions. The isomer patterns of the proposed chlorophenol precursors (3) are likely to arise via chlorination of phenol,
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rather than by hydroxy substitution of a chlorobenzene, which usually requires the substituting halogen position to be activated. We find no evidence of either phenol or chlorophenols in experimental samples, although the GC/MS technique used would not be particularly sensitive for phenol species. Luijk et al. (3) also commented on the absence of chlorophenols in their measurements on model systems and inferred that such species will be very reactive under the conditions of the ash surface. We have, however, found aromatic oxygenates such as benzyl alcohol, benzaldehyde, and benzyl esters, which are more likely to degrade to phenols than chlorobenzenes in the presence of the alkali and alkaline earth bases present in the fly ash. The correlation of high PCDD and PCDF yields with the presence of residual aliphatics is consistent with the observation that many older incinerator plants exhibit higher emission levels than newer ones. Usually, a newer and better designed plant gives much improved combustion efficiency, with a corresponding reduction in the quantity of organic matter emitted from the combustion zones, either as vapor or associated with particulates. Examination of our native fly ashes showed the high PCDD/F levels on ashes A and B to be associated with higher levels of organic matter and fiber materials and less well burned-out material.
Acknowledgments The work was funded by the Energy from Waste Programme, part of the U.K. Department of Trade & Industry’s New and Renewable Energy Programme. The views expressed in this paper are those of the authors and do not necessarily reflect those of the Department of Trade and Industry. SEM and EDX analysis was carried out by J. Fairchild, and GC/MS scans by P. Ambidge. The authors acknowledge helpful discussions with J. Unsworth & A. Felix of Shell Research (UK) Ltd.
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(10) Fa¨ngmark, I.; van Bavel, B.; Marklund, S.; Stro¨mberg, B.; Berge, N.; Rappe, C. Environ. Sci. Technol. 1993, 27, 1602. (11) Milligan, M. S.; Altwicker, E. R. Environ. Sci. Technol. 1996, 30, 225. (12) Born, J. G. P.; Mulder, P.; Louw, R. Environ. Sci. Technol. 1993, 27, 1849. (13) Finar, I. L. Organic Chemistry, 6th ed.; Longman: Harlow, U.K., 1973: Vol. 1, pp 90, 569, 833. (14) Harrad, S. J.; Fernandes, A. R.; Creaser, C. S.; Cox, E. A. Chemosphere 1991, 23, 255.
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Received for review May 30, 1996. Revised manuscript received October 29, 1996. Accepted November 5, 1996.X ES960468V X
Abstract published in Advance ACS Abstracts, January 15, 1997.
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