Mechanism of Chlorination of Aromatic Compounds Adsorbed on the Surface of Fly Ash from Municipal Incinerators Robert V. Hoffman,* Gary A. Eiceman," Yao-Ting Long, Margaret C. Collins, and Mao-Oin Lu
Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003-0001 Aromatic substrates were adsorbed on the surface of fly ash from a municipal incinerator. Treatment with gaseous hydrogen chloride gave aromatic chlorination. Relative yields and isomer distributions indicate that an electrophilic mechanism is likely for the chlorination reaction. Inverse addition experiments were used to show that a surface-bound chlorinating agent is produced by the interaction of hydrogen chloride with the fly ash surface. Recycling experiments indicate that the fly ash surface functions as a stoichiometric oxidant, and not a catalyst, in promoting aromatic chlorination by HC1. The yields of aromatic chlorination on three different fly ash samples were found to be quite different. A variety of physical and chemical characterizations revealed no simple relationship between properties and yields. I t was noted that the reaction of hydrogen chloride with the fly ash surface led to release of metal ions from the surface upon solvent extraction. It was found that the release of iron from the surface corresponded to the changing yields noted. Based on the known chlorinating ability of iron(II1) chloride, as well as other data from aromatic chlorinations on fly ash surfaces, reaction of HCl with iron(II1) sites on the fly ash surface could produce surface-bound iron(II1) chloride species that are the actual chlorinating agents.
Introduction In the last decade many studies have shown that trace amounts of chlorinated aromatic compounds, particularly polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (I),as well as other organic compounds (2), are found adsorbed to the surface of fly ash emitted from municipal incinerators. Since the amounts of these chlorinated products (PCDDs and PCDFs) adsorbed to the fly ash are greater than the amounts present in the incinerator input ( 3 , 4 ) ,there must be processes by which these products are produced during combustion. A theoretical analysis by Shaub (4) suggested that PCDDs were not formed by gas-phase reactions but were produced from substrates adsorbed on the surface of fly ash particles. It was further postulated that phenols produced from the combustion of lignins bind to the surface fly ash and are chlorinated to chlorophenols, which undergo condensation to PCDDs. Further chlorinations and/or isomerizations of the surface-bound PCDDs produce the final product distribution on the fly ash surface (4).
The first experimental support for this scenario was obtained by Eiceman and Rghei (5), who found that dibenzodioxin adsorbed on the surface of municipal incin0013-936X190/0924-1635$02.50/0
erator fly ash underwent aromatic chlorination to PCDDs when treated with hydrogen chloride (a common stack gas) at moderate temperatures (50-300 "C). When other surfaces such as firebrick or silica were used, no aromatic chlorination was detected. These studies also provided experimental verfication of early work by Townsend ( I ) suggesting that cogener ratios of PCDDs adsorbed on fly ash from municipal incinerators undergo time-dependent change in the emission plume. A variety of recent studies support the general idea that fly ash surfaces mediate the formation of condensed chlorinated aromatics as postulated by Shaub (4). Karasek (6) has found that chlorinated phenols gave PCDDs when deposited on the surface of fly ash and heated. Fly ash from different sources showed widely different reactivities, and model surfaces such as fire brick or silica were ineffective in promoting PCDD formation. Stieglitz (7) found that the amount of PCDDs on fly ash surfaces increased 10-20-fold if the fly ash was heated in air at 300 "C for 2 h before extraction and analysis. Oberg (8) recently disclosed that both chlorinated and brominated aromatic compounds were present on the surface of fly ash from an incinerator. When bromide was added to the fuel, the ratio of brominated products to chlorinated products increased proportionately. Although the general process envisioned by Shaub ( 4 ) for the production of PCDDs and PCDFs on the surface of fly ash is substantiated by these and other studies (not cited), the chemical features necessary to understand the process are largely unknown. Information pertaining to mechanisms of key reaction steps, the role of the fly ash surface in promoting these reactions, the chemical species involved in these transformations, and structural requirements of the substrates is, for the most part, fragmentary. Stieglitz (7, 9) proposed that surface-bound copper plays a key role in the formation of PCDDs when fly ash is heated in air. This proposal is based on the yields of chlorinated aromatic products (PCDDs and PCDFs) on fly ash that had been doped with metal salts or carbon sources before heating or treated with gas stream additives during heating. In another study based on the different amounts of chlorinated aromatic compounds found on fly ash from coal combustion and on fly ash from municipal incinerators, Giffn (IO)proposed that chlorides in the fuel are converted to chlorine gas, which is the reagent responsible for aromatic chlorination. The much greater levels of chloride in municipal waste result in much higher levels of PCDDs on incinerator fly ash. This view has been seconded by Oehme (11). Nestrick and Lamparski (12)
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suggested that iron(II1) chloride is the chlorinating agent on fly ash surfaces. Their model studies showed that aromatic compounds gave PCDDs when adsorbed on silica gel, treated with iron(II1) chloride, and heated to 150 "C. This view was supported by Hutzinger (131,who showed that commercial iron(II1) chloride contains PCDDs and PCDFs, presumably from contamination by organic impurities during manufacture. Chemical conclusions drawn from many of these studies are based on the changing yields of PCDDs and PCDFs ultimately extracted from the surface of the fly ash or other solid material. While the formation of these compounds on fly ash surfaces is quite important from an environmental perspective, they are the end products of a multistep process, each step of which is likely to be quite different chemically from the others in the sequence. As a consequence, each step is expected to have unique structural and mechanistic requirements that cannot be readily defined and understood by examining only the final products of the total process. A more fundamental way to gain chemical insight into the overall process is to characterize individual steps in the sequence. We have undertaken a study of an early step in the sequence, aromatic chlorination. The goals of this study have been to (a) determine if aromatic chlorination is a general reaction of aromatic substrates when adsorbed on fly ash surfaces, (b) determine the mechanism of the chlorination step, (c) identify the chlorinating agent, and (d) define the role of the fly ash surface in promoting aromatic chlorination. We have previously reported that aromatic chlorination appears to be a general reaction of aromatic compounds when they are absorbed on the surface of fly ash from municipal incinerators and treated with gaseous HCl at elevated temperatures (14). (We have also shown that the ability to promote aromatic chlorination is a general property of fly ash, since coal fly ash was found to have chlorination activity comparable to municipal incinerator fly ash (15). This result makes it possible to employ simple aromatic compounds as chlorination substrates, whose relative reactivities and isomer distributions can be used to characterize the electronic nature and perhaps mechanism of the chlorination process. Simple aromatic substrates can also be used effectively to correlate the reaction outcome with surface properties in order to define the chemical features of the fly ash surface responsible for the oxidative conversion of HC1 into the active aromatic chlorination reagent. Herein are described the results of such a study, which suggest that aromatic chlorination takes place by an electrophilic mechanism, and that the active chlorination reagent is a surface-bound species produced from a reaction between HC1 and the fly ash surface. Furthermore our results suggest that the surface-bound reagent is an iron(II1) chloride species, and we present a working hypothesis to explain the overall chlorination process. Experimental Procedures
The reaction apparatus, described in detail earlier (14), basically consists of a manifold system connected to a modified gas chromatograph. A reaction tube of 6-mm borosilicate glass with an expanded center section of 10 mm held the fly ash sample (1g). The reaction tube was positioned in the theromstated oven of a gas chromatograph. A manifold system with a mixing chamber was used to deliver gas mixtures of known composition to the reaction tube via the inlet system of the gas chromatograph. The inlet system was also used for the introduction of aromatic substrates into the front of the reaction tube. The exit from the reaction tube lead to a hood where a 1636
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flowmeter was installed to monitor the volume of gas that passed over the fly ash. Cold traps could also be inserted into this tube as it left the heated reaction zone. Samples of municipal incinerator fly ash from Chicago and Toronto, and fly ash from a coal-fired power plant in the Four Corners area, were extracted with 1:l acetonehexane to remove all organic contaminants and then dried in air and a t 25 "C overnight and a t 200 "C for 10 min before use. Control experiments showed that no volatile organic compounds could be extracted from the fly ash after this procedure. A typical chlorination reaction was carried out as follows. Fly ash from a municipal incinerator (Chicago) was mechanically seived and cleaned of soluble organic components by Soxhlet extraction with acetone-hexane (1:l). A sample of this fly ash (1.0 g) was loosely packed into the center section of the reaction tube and heated at 200 " C for 25 min with a helium flow of 100 mL/min. The tube was cooled to 100 "C and 100 pg of an aromatic substrate in dichloromethane (5 pL) was introduced into front of the reaction tube with a microliter syringe. The dichloromethane solvent was evaporated with a stream of helium (100 mL/min) for 5 min, and the aromatic substrate was flash vaporized onto the fly ash bed by rapidly heating the front portion of the reaction tube to 250 "C for 90 s with a nichrome coil with the helium flow maintained at 100 mL/min. The fly ash bed with the adsorbed aromatic was heated to the desired reaction temperature and HC1 was added to the gas stream. Temperature, gas composition, and reaction time were easily controlled for each experiment. Following a chlorination reaction, the reaction tube was cooled, removed from the apparatus, and attached to a Milton-Roy minipump. Dichloromethane (200 mL total volume, 10 mL/min) was delivered to the reaction tube to extract the products from the fly ash. An internal standard (tetradecane) was added, the extract was reduced to 1mL by rotary evaporation, and the condensed extract was transferred to a glass minivial (Wheaton) and further reduced in volume to 100 pL with a gentle stream of nitrogen gas. The products were analyzed by GC and GC/MS. Authentic monochloro aromatic products were used to standardize the GC and SIM analyses of product mixtures from toluene, anisole, mesitylene, biphenyl, naphthalene, and anthracene. The monochloro products of diphenylmethane and diphenyl ether were assigned on the basis of SIM analysis and their relative retention times. Characterization of the Fly Ash Samples. (a) pH: A 1-g sample of fly ash was suspended in water (50 mL) and sonicated in an ultrasonic bath for 2 min. The pH was measured with a glass electrode. (b) Surface area: Surface areas were measured by the BETS technique by Quantachrome Corp. (c) Ozidizing capacity: Fly ash samples (1g) were treated with 1 M potassium iodide (10 mL) and glacial acetic acid (10 mL) and sonicated under a nitrogen atmosphere for 20 min. The mixture was centrifuged and filtered through a glass frit. The filtrate was titrated for iodine with thiosulfate to a starch end point. The mequivalents of iodine produced are reported as the oxidizing power of the fly ash. (d) Elemental analysis: Elemental analyses were performed by Galbraith Laboratories. (e) Metal content: Fly ash samples (0.25 g) were dissolved in aqua regia (60 mL) overnight, filtered through a glass frit, and diluted to 100 mL with distilled water for analysis by inductively coupled plasma (ICP) techniques. Values reported are *5%. Analysis of the Residue Eroded from the Fly Ash Column after Chlorination. The chlorination of naph-
Table I. Chlorination of Aromatic Compounds (100 fig) on Chicago Fly Ash at 80 "C with HCl (800 mL)" (Yields Are Uncorrected for Incomplete Recoveries) entry
aromatic
yield: %
1 2 3
nitrobenzene methyl benzoate bromobenzene toluene diphenylmethane biphenyld naphthalened diphenyl ether anisole mesitylene anthracened
0 0 0 0.5 3.75 10.2 10.8
4
5 6 7 8 9 10 11
12
18.5 20.3 17.9
isomer distribution
ortho/para = 1:3 ortho/para/meta = 91:12 ortho/para = k3.1 1:2 = 1 6 1 para only' para only p
9:1:9,10 = 91:67
Yields are uncorrected for incomplete recoveries. bTotal yield of monochlorinated products. Errors are typically &20% for the total yields from several experiments and 1 5 % for isomer distributions. Only one monochloro product observed, which was assigned the para isomer in analogy to the results from anisole. dChlorinations carried out at 150 "C with 175 mL of HCl (23).
thalene was carried out as described above using Chicago, Coal, and Toronto fly ashes. The organic extract from each fly ash sample was evaporated to dryness, and aqua regia (6 mL) was used to dissolve the residue. The solution was diluted to 50 mL with distilled water and analyzed by ICP for metal content.
Results and Discussion A. Mechanism of Chlorination. A series of aromatic substrates was adsorbed on the surface of fly ash from a municipal incinerator, heated to 80 "C, and treated with HCl (800 mL). Extraction and analysis gave the results shown in Table I for the monochlorinated products. The fly ash used in this phase of the study (from a municipal incinerator in Chicago) was chosen because it was shown to give higher yields of chlorination products than fly ash from other municipal incinerators available to us (14). With the exception of diphenylmethane and diphenyl ether, authenic monochlorinated standards were used to calibrate both GC and GC/MS analysis, which was carried out using SIM. For diphenylmethane and diphenyl ether, products were identified by MS, and the isomers were assigned on the basis of the related compounds, toluene and anisole, respectively. In addition to monochlorinated products, which were formed in greatest abundance, small amounts of di- and trichlorinated products were observed in the product mixture as well. Measurable quantities of brominated aromatic products were also formed, as discussed previously (14). The conditions chosen for chlorination were those that gave the highest yields for the more volatile aromatic substrates. A variety of control experiments were used to validate that the yields of chlorination and isomer distributions were the result of adsorbed aromatics reacting with a chlorinating agent produced by the interaction of HC1 with the fly ash surface. (a) If the aromatic substrate was vaporized onto the fly ash surface and heated to 80 "C for the normal reaction time under a flow of helium gas to which HCl was not added, the aromatic substrate was recovered, but no aromatic chlorination was detected. Likewise if no aromatic substrate was vaporized onto the fly ash, but the fly ash was treated with HCl under the usual reaction conditions and then extracted and analyzed, no volatile chlorinated products were detected. Thus, the three components necessary to effect aromatic chlorination are the fly ash surface, the aromatic substrate, and hydrogen chloride.
(b) Cold traps (-78 "C) were positioned in the exit tube, immediately outside the heating oven. The aromatic substrate was vaporized onto the fly ash surface and heated for the normal time and temperature (80 "C, 20 min) under a flow of helium. Analysis of the contents of the cold traps showed that no substrate was eluted from the fly ash under the reaction conditions, but remained adsorbed to the fly ash surface from where it could be partially recovered by extraction. I t has been reported that fly ash surfaces strongly bind aromatic compounds (5, 14), and these control reactions illustrate this affinity. Even relatively volatile aromatics such as toluene (bp 110 "C) and bromobenzene (bp 156 "C) are retained tenaciously by fly ash. From these results it is also safe to assume that the chlorinated products are also retained on the fly ash surface after their formation. (c) The recovery of aromatic substrates and authentic monochlorinated products from the fly ash surface after heating at the normal reaction time and temperature under a stream of helium was less than quantitative. Recoveries were variable, ranging from 10 to 75%. This is further evidence that aromatic compounds undergo some irreversible binding to the fly ash surface (5, 12). While the nature of this irreversible absorption is unknown, it contributes significantly to the poor matter balances observed in these chlorination reactions. The data in Table I show that the yields of chlorination products seem directly related to the electronic character of the attached substituent. As the electron-donating ability of attached substituents increases, so does the yield of chlorinated products. The yields reported in Table I are not corrected for recovery losses. It should be noted, however, that the chlorinated products all had similar recovery efficiencies (60-75%) so that the differences in yields shown in Table I are not due to different recovery efficiencies. Naphthalene, anisole, mesitylene, and diphenyl ether (entries 7-10), which are all electron-rich aromatics, give the highest yields. Toluene and diphenylmethane (entries 4 and 5), which contain only weakly activating substituents, give low yields. Aromatic substrates with electron-withdrawingsubstituents (entries 1-3) fail to give detectable amounts of chlorinated products. While it is hazardous to equate reaction yields with chemical reactivity, the ordering of reactivities, as suggested by the yields of chlorinated products found in these data, parallels relative reactivities found in electrophilic aromatic substitutions in which an electron-deficient reagent adds to the aromatic ring in the rate-determining step. This comparison is shown in Table 11, in which the yields found in this study are compared with the relative reactivities in two well-studied electrophilic substitutions of the same substrates. The parallel trends in the three data sets are clear and they suggest that all three reactions are similar electronically; thus, chlorinations of aromatic substrates on fly ash surfaces most likely proceed by an electrophilic mechanism. The isomer distributions shown in Table I support this mechanistic assignment. A high ortho-para preference is observed for all of these activated aromatic compounds, characteristic of an electrophilic aromatic substitution (Norman and Taylor in ref 17). Production of meta isomer is only observed in the case of the weakly activated diphenylmethane, and in this case the proportion of meta isomer is extremely low. The high preference for substitution in the 1-position in naphthalene (de la Mare and Ridd in ref 17) and the 9-position in anthracene (18) is characteristic of electrophilic substitution in these polyEnviron. Sci. Technol., Vol. 24, No. 11, 1990
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Table 11. Comparison of Chlorination on Fly Ash with Electrophilic Substitutions
entry
substrate
1
nitrobenzene methyl benzoate bromobenzene toluene diphenylmethane biphenyl naphthalene diphenyl ether anisole mesitylene anthracene
2 3 4 5 6 7 8
9 10 11
yield," 70 0 0 0 0.5
3.75 10.2 10.8 12
18.5 20.3 17.9
relative reactivityb (m-NOPCBH4S020)2C BrZd
