Environ. Sci. Technol. 1993, 27, 1849-1863
Fly Ash Mediated Reactions of Phenol and Monochlorophenols: Oxychlorination, Deep Oxidation, and Condensation Jan G. P. Born,? Peter Muider, and Robert Louw'
Center for Chemistry and the Environment, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Monochlorophenols are smoothly oxidized to carbon dioxide and carbon monoxide when vapors in air are passed over fixed beds of municipal waste incinerator fly ash a t 625-725 K. Simultaneously polychlorinated benzenes, monobenzofurans, and dibenzo-p-dioxins are formed with a large fraction of the original chlorine concentrated in these products. Fly ash catalyzed oxychlorination of phenol in the presence of HC1 a t 425-725 K resulted in the formation of chlorinated phenols which, in turn, were converted above 625 K into mainly CO2 and the (poly)chloroarenes mentioned above. In contrast,under similar conditions, (chlorinated) benzenes are inert. Deep oxidation and oxychlorination of phenol have also been studied using an oxidation catalyst (CuO on alumina) or a commercial oxychlorination catalyst (basically, CuC12 on alumina). Results were comparable but, in neither case, chlorinated dibenzo-p-dioxins were detected. Kinetic data are presented and analyzed, and reaction mechanisms and the practical relevance of our results are discussed. Introduction
The formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibeGof6ans (PCDFs) in municipal waste incinerators (MWh) is a well-establishex fact ( I ) , as is the presence of these compounds on fly ash produced by these facilities (2-5). Due to the different reaction stages in MWIs, until now no clear-cut relations between normal operating conditions of various MWIs and their PCDD (and/or PCDF) content of effluent gas flows have been found (6,7).Only a limited number of testing programs with extreme operating conditions have revealed a correlation between CO emissions and PCDD/PCDF effluent gas contents as well as a relationship between operating temperature and PCDDI PCDF emissions (8, 9). It has been shown (10) that homogeneous gas-phase reactions cannot be the sole source of levels of PCDDs and PCDFs found in effluents of MWIs. In recent years, evidence has been gathered that fly ash promotes the postfurnace formation of PCDD/Fs in MWIs [especially near the electrostatic precipitator (ESP)], resulting in levels of F C D D / h in tEe stack that by far exceed those measured in the furnace (11-15). Moreover, the degree of chlorination of the PCDD/Fs increases as the effluent gas moves toward the stack (13). Accordingly, a full description of the formation of PCDDs and PCDFs should involve both gas-phase (radical) reactions-modeling the pyrolytic and flame chemistry occurring in the combustion chamber which gives rise to the organic precursors-and heterogeneous (fly ash mediated) reactions, especially the reactions a t relatively low t Present address: Waste Processing Association,P.O. Box 19300,
3501 DH Utrecht, The Netherlands. 0013-936X/93/0927-1849$04.00/0
0 1993 American Chemical Society
temperatures, in boiler sections and in the electrostatic precipitator. Societal concerns-in Western Europe, North America, and Japan-have led to very strict regulations on emissions of 'dioxins' from MWIs and chemical waste incinerators. Triggered this way-and lacking sufficient knowledge on the all-in combustion process itself-huge, costly air pollution control facilities have been developed and to some extent installed. This approach is likely to create new chemical wastes, which are difficult to dispose of. These developments can only be reversed if the combustion process itself is upgraded. A much better insight into the kinetics and mechanism of formation, and survival, of hazardous PICs under relevant conditions will be of great importance in this respect. Fly Ash Mediated Formation of PCDD/Fs. With respect to fly ash catalyzed formation of PCDD/Fs, there is still uncertainty about the nature of the organic substances acting as precursor and about the mechanism of chlorination and/or condensation. Three general pathways have been advanced for explaining the production of dibenzo-p-dioxins and dibenzofurans (16): (a) incomplete combustion of trace levels of PCDD/Fs already present in the feed (17) (b) generation from specific organic precursors such as polychlorobiphenyls (18) and chlorophenols (19-24) (c) d e nouo formation from carbon (25-27) or undefined organic fragments (28) and organic or inorganic chlorine donors On a laboratory scale it has not only been proven that, fly ash can act as a potent oxychlorination catalyst but also that MWI fly ash can engender formation of PCDDI Fs. [Note that fly ash from coal-fired power plants was also shown to catalyze chlorination reactions using HC1 (29).I
Several laboratory studies involving the surface-catalyzed formation of PCDD/Fs have revealed that these compounds are readily produced from a variety of precursors or starting materials in the temperature region 300-800 K. Three prerequisites are (30)as follows: (i) the presence of transition or heavy metal cations, (ii) the presence of oxygen in the reaction system, (iii) the presence of organic and/or inorganic carbon and chlorine sources. A decrease of the PCDD/F content of fly ash is observed upon thermal treatment (usually at ca. 575 K) in an oxygendeficient atmosphere (30-33); this reaction is believed to be copper promoted a t temperatures as low as 400 K (34). When passing vapors of pentachlorophenol (P&P) over fly ash in the absence of oxygen, a wi?e range of PCDDs is obtained (22-241, showing that dechlorination is also important under these conditions. Annealing fly ash samples for 2 h in the temperature region 400-900 K in an oxygen-containing atmosphere revealed an increase in Environ. Sci. Technoi., Vol. 27, No. 9, 1993 1849
PCDD/F content below 600 K (30, 35); above this temperature, the net destruction of PCDD/Fs was observed (35). The catalytic activity of fly ash can be reproduced best by catalysts containing CuClz and a chlorine donor (26, 27, 36). Furthermore, (activated) carbon may act as a precursor for formation of PCDD/Fs (26, 27). In fact, when adding carbon to fly ash, a linear relationship between carbon content and formation of PCDD/Fs was found upon heating (26). This de novo formation pathway is associated with carbon depletion propagated by Cu(I1) ions rendering mainly COZ (27). It has been assumed that aromatic structures existing in particulate carbon in the fly ash are released oxidatively (conducted by ”0”or “Cl”) as smaller molecules (27). However, in the presence of sufficient phenoi and chlorine sources the de nouo synthesis using carbon is overruled by fly ash catalyzed formation of PCDDs using chlorophenols as precursors (24). Very recently, Dickson (37) et al. have compared rates and efficiencies for the formation of octachlorodibenzo-p-dioxin (OCDD) via pentachlorofheno1 and carbonace&s material and conclude that, under practical conditions, the former process is by far the most important. The role of water vapor in the surface-catalyzed formation and destruction of halogenated dibenzo-p-dioxins and dibenzofurans is still unclear; it has been mentioned to engender both their destruction (32)and formation (38, 39) or was found to be of little influence (25). Pyrolysis of polychlorophenols mediated by fly ash at around 450 K produces high yields of PCDDs (but virtually no PCDFs) (40,411. For instance, heating of P5CP on fly ash produces a variety of PCDDs in a relative abundance that corresponds well with that found normally on fly ash (22,231. As a similar experiment using ground firebrick instead of fly ash only produced trace amounts of OCDD, it was concluded that metal elements on the fly ash were essential (22, 23). Apparently, phenol is activated by adsorption, possibly through the hydroxyl group, quite similar to the observation of Sandermann et al. (421,who reported much higher yields of OCDD when pyrolyzing the sodium salt of PCP rather than PCP alone. Interestingly, even in the absence of ortho-chlorine substituents in the starting phenol (e.g., 3,4,5-T3CP), the fly ash catalyzed formation of PCDDs occurs (22, 23). Fly Ash Mediated Oxychlorination Reactions. Over the last decade, it has been shown that many aromatic compounds can be halogenated on fly ash surface. The catalytic function of fly ash was demonstrated by Eiceman and Rghei (43-46). Using gaseous HC1 and fly ash impregnated with specific dibenzo-p-dioxin molecules, they were the first to prove that oxychlorination occurs by means of a heterogeneous gas-solid reaction catalyzed by fly ash above 400 K. As the formation of PCDD/Fs from activated carbon/ CuC12 depends on the concentration of copper(I1) chloride (261, CuClz seems the most plausible chlorinating agent. Although it has also been suggested that FeCl3 is the active species (47,48),a statistical study has revealed a correlation between copper and PCDD/F content in MWI fly ash, whereas the iron content was not related with PCDD/F levels (49, 50). Reports on the effect of water vapor on the rate of chlorination are ambiguous; some authors observed no effect or retardation (25),while others mention an en1850
Environ. Sci. Technol., Vol. 27,No. 9, 1993
hancement (39)of the rate of chlorination. Experimental conditions are not very well comparable however. Chlorination proceeds under both pyrolytic and oxidative conditions, using a variety of chlorine sources. The fly ash itself (22,23,25, 3 0 , 3 5 , 5 l ) may act as a chlorine source, but other sources-organic as well as inorganic [metal chlorides (25,26,36,48),HC1 (29,36,47,48),Cl2 (5211-are effective. Several mechanisms for fly ash catalyzed chlorination under various conditions have been proposed: (a) A surface-catalyzed radical mechanism has been suggested (25, 53) to explain the preferred formation of 1,2,3-trichlorobenzene in the CuClz-catalyzed oxychlorination of benzene a t 575 K:
+ CuC1, ArHC1’ + CuC1, ArH
-
ArHC1’ + CuCl
ArCl + HC1+ CuCl
(b) Based on the observation that brominated benzenes are chlorinated a t the ipso positions exclusively, a nucleophilic addition-elimination mechanism-favored by high concentrations of chloride on the fly ash-has been advanced (51). (c) Recently, Hoffman et al. (47)advanced FeCl3 rather than CuCl2 as the chlorinating agent present in MWI fly ash. In batch-like pyrolytic chlorination experiments using gaseous HC1, they noticed that the amount of iron eroded from the fly ash surface was proportional to the yield of aromatic chlorination products. Isomer distributions of products indicated an electrophilic mechanism with fly ash as a stoichiometric oxidant (47). (d) In a heterogeneous reaction system consisting of fly ash-air-DD-HC1, dibenzo-p-dioxin is chlorinated preferably at the 2-position, consonant with an electrophilic substitution mechanism (45). In order to learn more about the role of fly ash in producing PCDDs and PCDFs, we have undertaken a study on the behavior of phenol and monochlorophenols employing fixed beds of MWI fly ash, continuously passing over vapors of substrate diluted with oxygen/nitrogen mixtures together with relevant compounds such as water vapor and HCl. With respect to the relevance of this approach for the operation of MWIs, it should be kept in mind that ashes hang on, e.g., in boilers, and can be instrumental in converting products of incomplete combustion (PCIs) in a way analogous to t h d of our model experiments. Experimental Section Heterogeneous gas-solid reactions were conducted in tubular quartz reactors packed with fly ash obtained from two different Dutch municipal solid waste incinerators (denoted as MWI A and MWI B). Analytical data of the fly ash samples used are given in Table I. Reactions were performed in two different flow systems, which have been described in more detail elsewhere (54). Setup I, used for scouting experiments, operated at atmospheric pressure and typically consisted of an inlet tube, a reactor chamber, and an outlet tube. Fly ash catalyzed heterogeneous reactions were studied in quartz tubes packed with fly ash obtained from a Dutch MWI. Reactors of various lengths, typically with 8-mm internal diameter (Ld.), were employed. Plug flow character was ascribed to these packed tubular reactors.
Table I. Elemental Composition of MWI Fly Ash element Ag A1 As Au Ba Br Ca Cd Ce
c1
co Cr cs cu Eu Fe Hf Hg K Lu Mg Mn Mo Na Ni Pb Rb Re Rh Sb sc Se Sn Sr Ta Tb Th Ti U V W Yb Zn
concn MWI An (mg/kg) 23 49150 50 0.41 1569 237 119950 222 39 49890
concn MWI Bb (mg/kg) 56 38
125
20
99%) and 7wt % 1-bromonaphthalene (JanssenChimica: 99%)was prepared to serve as a co-injection standard for the offline analyses. Nitrogen (Hoekloos, 99.9 % ) and oxygen (Hoekloos, 99.9%) gases were purchased in standard cylinders. Note that HCl is produced in situ by combustion of 1,4-dichlorobutane. Results and Discussion
Fly Ash Mediated Oxidation of Chlorophenols. Experiments 1-111 of Table I1 illustrate that oxidation of o-chlorophenol proceeds readily between 600 and 700 K with residence times of 7-8 s, and almost complete destruction seems possible at temperatures well below 800 K. (Note that the residence time is based on the remaining void volume in the reactor: V,- V,& the volume occupied by the fly ash was obtained via its bed weight and specific gravity, which we established at 2.36 g/mL.) The major oxidation products are carbon dioxide and carbon monoxide in a ratio of ca. 6:l. In addition, polychlorinated -benzenes (PCBzs), monobenzofurans (PTBFs), and dibenzo-p-dioxins (PCDDsyare fzrmed. Using GC-MSD, traces of ai- and trichlorophenols together with phenoxysubstituted dibenzo-p-dioxinscould be identified. These products are also detected in the stack gas of MWIs (55) and in the effluent from heated fly ash during laboratory experiments (56). The formation of such a broad pattern of (highly) chlorinated compounds contrasts with the behavior of o-chlorophenol upon homogeneous slow combustion (21). In the latter case, mainly dichlorodibenzofurans are formed, whereas monochlorodibenzo-pdioxins are only minor products. Apparently, in fly ash mediated reactions (organic) chlorine is "concentrated" in aromatic compounds that escape from oxidative breakdown. The high C02/CO ratio and the absence of noticeable amounts of intermediate degradation products point at catalytic combustion as a prominent process; upon uncatalyzed slow combustion of chlorophenols, CO is a much more prominent product (21). Catalytic oxidation of chlorophenols produces HCl, which can be used in a parallel oxychlorination process. Environ. Sci. Technol., Vol. 27, No. 9, 1993
1853
Table XI. Fly Ash Mediated Oxidative Conversion of Monochlorophenols (Setup I) Conditions I I1 I11 IV ortho ortho ortho para 640 625 675 725 T (K) 10.Y 9 9 9 fly ash (9) residence time (s) 8.0 7.4 6.9 5.6 Inflows (mmolih) 0.95 1.85 1.83 1.83 1.83 0.11 24 24 24 48.2 138 138 138 159.6 Products (qmol/h) 70 300 1100 420 450 1860 6300 2540 0.35 1.8 43 45 618 321 278 NDb ND ND ND ND ND ND ND 10 0.095 0.31 1.1 0.058 1.7 11 11.4 0.23 0.022 0.13 0.33 0.04 0.011 0.057 0.19 0.22 0.022 ND 0.29 0.54 0.52 2.5 0.43 3.6 0.23 1.2 1.6 1.08 ND ND ND 1.8 0.099 1.1 8.8 ND 1.4 0.27 1.0 0.54 0.77 0.19 0.33 4.2 0.41 CSCl6 1.2 ND 4.1 CMCBFCsd 0.25 0.70 0.53 0.54 CDzCBFCpd ND 0.13 0.53 1.0 0.19 1.1 4.3 CT3CBFCtd 0.99 0.24 0.15 CT4CBFcrd 5.1 1.38 CPijCBFcsd 0.12 0.21 0.47 2.8 H&BFC 0.14 0.69 1.8 0.26 DF ND ND ND 1.5 0-PhOPhOH ND ND ND ND 3.3 1.6 DD 1.0 14 2-MCDD 0.29 5.9 0.20 1.5 2.1 17 54 0.57 1-MCDD 31 0.13 5.7 20 CD2CDDd 0.86 5.3 17 ND CT3CDDd 1.7 22 ND 0.15 CT4CDDd ND ND ND 8.7 CP5CDDd ND ND 9.6 ND CH&DDd experiment no. isomer
v
meta 640
VI ortho 640
10.F
10.7’
5.4
5.4
0.95 1.85
0.95 1.85
0.11
0.11
57.0 54.5 164.3 161.0 460 1770 401 8.2 19
325 1630 315 33 ND 11 5.8 0.057 0.086 0.15 0.062 0.020 ND 0.051 0.045 0.25 0.34 0.29 0.12 ND ND 0.53 0.32 ND ND 0.25 0.094 0.24 1.4 0.20 1.7 1.6 0.51 0.18 0.088 0.22 0.05 1.34 0.19 0.26 0.044 0.50 0.18 1.4 2.3 5.3 6.9 2.4 1.7 0.25 0.035 0.80 0.10 1.5 0.11 1.5 0.072 0.91 ND 0.25 2.1 1.2 ND
Fresh fly ash bed. ND, not detectable (detection limit ca. 0.1 ,umol/h). NB, Monobenzofurans. Elution order unknown and/or isomers could not be separated with the column employed (CP-Sib5 CB).
The more chlorophenol is converted-with increasing temperature-the more chlorine (HC1) becomes available for chlorination of a declining amount of pertinent organic structures. Figure 2A,B shows that production of PCBzs and PCDDs strongly increases with temperature and, hence, with the degree of conversion of the starting o-chlorophenol. In a second series of reactions (experimentsIV-VI, Table 11), t h e behavior of the three individual monochlorophenols-at 640 K-is outlined. In an attempt to suppress excessive formation of toxic PCDDs, the chlorophenol feed has been diluted (10-fold) by added phenol. Interestingly, the results are comparable to those found on combustionof pure o-chlorophenol. Apparently, added phenol does not interfere with the uconcentration” of organic chlorine as mentioned above but is independently converted, in part into unsubstituted dibenzofuran (Table II), for the major part into C02 (and water). As in the absence of fly ash, the degree of conversion of phenol is 1854
Envlron. Sci. Technol., Vol. 27, No. 9, 1993
negligible at T C650 K; this reaction appears to be catalyzed by fly ash as well. The observedproduct compositionis not very dependent on the position of C1 in the starting chlorophenol. Apparently, considerable isomerization in the “unconverted” chlorophenol occurs, rationalizing that the presence of an o-chlorine substituent is not essential for dibenzo-p-dioxin formation. Dickson and Karasek (22), too, have found that o-chlorine is not essential for dibenzop-dioxin formation upon pyrolysis of trichlorophenols on fly ash. Apparently, a chlorophenol molecule after chemisorption on the fly ash can undergo various reactions, isomerization, oxidative breakdown, repeated “conducted tour” oxychlorination, andlor condensation,before distinct products (COz, PCBzs, PCDDs) are released. Adding an excess of phenol results in the generation of dibenzofuran. Since no chlorinated dibenzofurans could be detected, chemisorbed phenol seems to be subject to either oxidative breakdown or condensation to dibenzofuran rather than to oxychlorination. Alternatively, should oxychlorinationtake place, this is continued in a conducted tour fashion prior to condensation and/or release as mentioned above. Finally, our data as such do not reveal whether chlorination advances due to prior formation of Cl2, via “deacon-like” oxidation of HC1 or by some other chlorinating agent, e.g., Cu(I1)Cl derivatives, on the fly ash surface. Fly Ash Catalyzed Oxychlorination of Phenol. Formation of Chlorophenols; Overall Reaction Rate. The results of the fly ash mediated oxidation of chlorophenol, especially the buildup of chlorine in organic molecules, reveal some sort of chlorination reaction fast enough to compete with (catalyzed) oxidation. Realizing that this oxidation of chlorophenol liberates HC1 and that HC1 becomes the most abundant chlorine derivative at high conversion, we investigated the oxychlorination of phenol using 0 2 , HC1, and fly ash. Such a reaction system resembles that in an electrostatic precipitator (ESP), although the concentr&ons Gf phenol used here by far exceed those in an ESP of a MWI. The microflow setup I1 with on-line analyses contained a quartz reactor with a fixed bed of fly ash (0.40 g, 30 X 4 mm i.d., MWI B). The reagent atmosphere was composed of nitrogen (88.4 vol %), oxygen (10.4 vol %), and phenol (0.89 vol % 1. Furthermore, an input of HC1 (0.10 vol 9%) and H2O (0.19 vol 5%) originating from preliminary combustion of 1,4-dichloromethanewas derived from the production of CO (and CO2) in that process. It was verified independently that CO and C02 are inert with respect to fly ash catalyzed reactions under our conditions. Residence times varied between 0.65 and 0.40 s, depending on temperature. Full experimental details are given elsewhere (54). Above 425 K, chlorinated phenols are progressively produced with increasing temperature. With an intake ratio of HC1:phenokOZbeing 1:9:110,the final chlorophenol production can be at best 11% ’ based on phenol input. In fact, this maximum yield is approached at 690 K (cf. Figure 3). HC1 consumption amounts to ca. 60% at the highest temperature used (ca. 690 K, Figures 3 and 4). The contribution of the oxidation channel rapidly gains in importance above ca. 600 K, ensuring over 45% phenol conversion at 690 K (to which chlorination contributes only 11%).Consequently, the consumption of 0 2 totals
7
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0675 K 0625 K
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PhC12 PhC13 PhC14 PhCI5 PhC16
0
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L M C D B DCDDs T3CDDsT4CDB PSCDDsHKDDr
Flgute 2. Formation of PCEzs (A) and PCDOs (E) in the fly ash catalyzed oxldation of Pchlorophenol. Full data are listed in Table 11.
t
a
/
I
IO 0
450
500
550
600
650
700
T (K) Flgure 3. OxychlorlnatlonOf phenol as functlon of temperature: main features.
450
500
550 T (K)
600
650
700
Flgura 4. Oxychlorination of phenol as function of temperature: formation of chlorophenols.
almost 8% at 690 K. The principal carbon-containing oxidation products are COn and CO in a ratio of 101. A closer examination of the chlorinated phenols produced reveals that chlorination involves only the ortho and parapositions. With increasingtemperature, not only are 0-and p-chlorophenol formed but also 2,4- and 2,6dichlorophenol (Figure 4).
The influence of the concentration of 0 2 , HC1, phenol, and H20on the fly ash catalyzed oxychlorination of phenol was measured in four additional sets of experiments. A fresh fly ash bed (0.49 g, 32 X 4 mm i.d., MWI A) was installed. Before each set of experiments, the fly ash was conditioned by heating overnight at 750 K in an 19/81 OdN2atmosphere. Also slightlydifferent conditionswere used. In the 02-variation experiments, performed at 525 K, the oxygen concentration ranged from 0 to 31.2 vol %, whereas other reagents were kept constant (HCI at 0.10 vol %, phenol at 0.33 vol %, and HzO at 0.54 vol %); nitrogen (67.7-98.9~01%)wasused tocompensate for the variation of the oxygen content. Using an oxygen concentration of ca. 19 vol % and other concentrations as denoted above, the HC1 intake was varied between 0 and 0.36 vol % to derive its reaction order. Analogously, the effect on reaction rate of phenol (0.21-0.03 vol %) and water (1.5-5.4 vol % ) was studied. For these latter three sets of experiments, an additional evaporator filled with water was employed to generate the water content; the fly ash temperature was kept at 475 K. Note that oxidation of phenol is unimportant at the temperatures employed in all four sets experiments. Due to the higher HCl/phenol ratio (=l)used in the present experiments with variation of [ 0 2 1 , [CsH50Hl, and [H20] compared to 0.1 for the temperature variation experiments (see above), 2,4,6-trichlorophenoInow is an additional product next to the chloropbenols mentioned above. In Figure 5A-D, the overall oxychlorination rate Le., E(OCl), the total of C-Cl bond equivalents in the polychlorophenols produced1 is plotted as a function of [Od, [HCl], [C6HsOHl, and [H201, respectively. It is immediately visible that the oxygen and HCI concentrations are the most influential to the oxychlorination rate, whereas the order in phenol-at least in the range studied, i.e., from ca. 0.025 vol % onward-is essentially zero. From Figure 5 it can also he inferred that a gradual deactivation occurs. For clarity we have numbered the experiments in each set to visualize their execution order. The sets of experiments in which LO21or [HClI was varied revealed that the deactivation is linearly related to the operating time of the fly ash bed. Since we varied the Envlron. Scl. Technol., Vd. 27. No. 9, 1993 1855
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[PhOH] (%vo~) Figure 5. Effect of concentration on overall oxychlorination. A, [02]; B, [HCI]; C, [CeH50H]; D, [H20]. Vertical axis represents [OCI],(M), the sum of C-Ci bond equivalents.
concentrations back and forth and the experiments were performed in a fixed time table (a benefit of the automatization of the analyses),the deactivation can be filtered out mathematically. As the set of experiments involving variation of [HzOI has a very narrow range both for the values of [HzOl and [OC1lt and the correlation for these data points is poor (r2 = 0.75), we refrained from using these data in the development of an overall rate expression for the oxychlorination of phenol. (The derived reaction order in [HzOI was 0.31 f 0.06.) The rate constant kocl as well as the reaction orders in [OZ],[HCII, and [CGH~OH] can be derived by evaluation of the generalized rate expression:
c log([C&OHl/M) = log(~oc~/(kocl[O,l"[HCllb))
When varying the oxygen concentration, the oxychlorination rate is divided by the other three variables (i.e., rate constant (temperature), [HClI, and [C6H50H]).The resulting (reduced) rate is a function of [021 only, and a shouldgive plot of log[Ozl vs log(~oc~~koc~~HCll~[C~H~1~)) a linear relation, with the slope representing the reaction order in oxygen. The results of the calculations are visualized in Figure 6A-D. The Arrhenius plot, Figure 6A, provides the following rate parameters (E, in kcal/mol): ~ o g ( ~ o c l / M 1 - ( Q + b + C ) a s-1, g-1 )
=
3.2 f 0.1 - (12.2 f 0.2)/2.3RT Figure 6B-D depicts the logarithm of the reduced rate of oxychlorinationvs [021, [HCll, and [C&@H]. Analysis where a, b, and c represent the respective reaction orders. of these slopes by the method of least-squares yields the Note that the weight of the fly ash bed is incorporated in following reaction orders in [Ozl, [HClI, and [CsHsOHl the generalized rate expression. for the fly ash catalyzed oxychlorination of phenol: The rate constant then is computed by evaluation of 0.37k0.03 [HC1]0.48fO.o3[C H OH]0.~f0.01 ) -1. -1 log(koc1/~ 1 - ( a + b + c -s g )= "oc1 = kOCl[O21 6 5 (M-s-l-g-') ~~~(~~~~/~~~~l~~HCllb~C~H~OHlc~} In an actual MWI, the exhaust gases leaving the The respective reaction orders are obtained from the secondary chamber of the oven contain 0 2 (8-12 vol %), results of the three sets of experiments in which the HC1 (ca. 0.03 vol %), and products of incomplete concentration of oxygen, HC1, and phenol were varied, combustion (PICs, with varyingconcentratio& dependusing the equations: ing on the quality of the combustion practice). In addition, the average level of fly ash amounts to roughly 5 g/Nm3. a log([O,l /MI = log(uocl/(k,cl[HC4 b[C6H50Hl'1) Recalling Figure 5, the temperatures, [021, and [HClI b log([HC1I/M) = log(~oci/(kocl[0,1"[C,H50H]Ic)) resemble those in the boiler and ESP of a MWI. The
uocl = k,-,cl[ OJ"[HCI]
1856
b [ C6H50H]'
Environ. Sci. Technol., Vol. 27, No. 9, 1993
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-4.2
Figure 6. Rate constants and reduced rates. Squares, uncorrected values; circles, corrected for deactivation.
reaction orders for [ 0 2 ] and [HClI, therefore, are likely to also hold for the boiler and ESP of an MWI. In fact, measurements of organochlorine emissions of incinerators have revealed a correlation between levels of HC1 in the postcombustion zone and emissions of PCDD/Fs (57,58) or PCBzs (59). Levels of PCDD/Fs also tend to increase with the operating temperature of the electrostatic precipitator. The chlorophenol concentrations in a MWI vary, but typical values of 1-10 pg/Nm3 in the stack have been reported (12,601,mainly consisting of pentachlorophenol (60). As the majority of these chlorophenols are formed in the ESP (12) at ca. 550 K, typical levels of polychlorophenols are 2 X 10-l1 M, very much lower than those used in the present lab-scale study (in on-line experiments typically S= lo4 M). The actual concentration of nonchlorinated phenol in a MWI is uncertain but probably lower than that used here. Hence, the zero order in phenol concentration does not necessarily hold in a MWI. Formation of PCDDsand Other Products. In a separate set of experiments, we studied the formation of additional products, using a new fly ash bed (0.155 g, 20 X 4 mm id., MWI A). The following conditions were used: 02 16.2 vol % , N2 83 vol %, HzO 0.39 vol % , HC10.19 vol % , and phenol 0.21 vol % Note that the initial HCl/phenol ratio in these experiments is close to unity, whereas in the experiments described in the precedingsection HCl/phenol varied from 0.1 to 1. Residence times varied between 0.45 s at 425 K to 0.29 s at 740 K. With regard to the formation of chlorinated phenols, the present experiments resemble those of the oxidation of chlorophenok already at 425 K noticeable amounts of
.
80
60 h
0
40
20
I
=1 - phenol + HCI +
1 C
-
T (K) Flgure 7. Oxychlorination of phenol; conversions. 0 - and p-chlorophenol are produced, and the amount of chlorinated phenols increases progressively with temperature. In total the yield of polychlorinated phenols (PCPs) increases from ca. 3% (61 pKenol input) 425 K to almost 40% at 600 K (Figure 7). The consumption of HC1 reaches a maximum value of 60% between 600 and 650 K. By increasing the temperature stepwise from 425 to 750 K and then lowering it to 490 K, we were able to examine possible deactivation of the fly ash catalyst in time. According to the consumption of HC1as depicted in Figure 7, it is concluded that some deactivation has occurred. The selectivity of converted phenol toward PCPs is as high as 90% at the lowest temperatures, but decreases with temperature to only 2 % at 740 K. The stabilities of
Environ. Sci. Technol., Voi. 27, No. 9, 1993
1857
the PCPs formed do not differ prominently from that of phenol itself (cf. Figure 8A,B). Consequently, above 600 K they are oxidized to COz as well, causing the net production of chlorinated phenols to decrease above that temperature. Around 600 K, the product pattern shifts from mainly PCPs to apparently more stable compounds to wit PCBzs, PCBFs, and PCDDs; see Figure 8A. By consecutive chlorination, more heavily chlorinated phenols emerge at higher temperatures (Figure 8B). For example, at 740 K, identified PCPs are (relative importance inparentheses): 2-(0.04),4- (1),2,4-di-(0.10), 2,6-di- (0.44), 2,4,6-tri- (0.50),2,3,4,6-tetra-(0.251,and pentachlorophenol (0.25). The selective chlorination of phenol at the 2- and 4-positions has also been observed in the postfurnace zone of a MWI (14). In Figure 8C, the overall ortho content of the PCPs is plotted as a function of temperature. The amount of ortho substitution (relative to para) has been calculated as: ([2-C1C&40H] + [2,4-C12C,&OH] -I2[2,6-Cl.&H,OH] + 2[2,4,6-C&C,HzOH] + 2[2,3,4,6-Cl4C6HOH1+ 2[C15C6OH])/ ([8-C1C,$40H] + [4-ClC,H,OH] + 2[2,4-C1,C6H30H] + 2[2,6-Cl,C,H3OH] + 3[2,4,6-Cl3C6HZOH] 3[2,3,4,6-Cl4C,HOH] + 3EC&C6OHI} The fraction of ortho substitution increases with temperature; at 425 K it amounts to 40% but reaches a maximum of ca. 75% at 725 K (Figure 8C). Note that the latter value is higher than the maximum statistical value of 67% in P&P or 2,4,6-T3CP. Hence, besides oxychlorination other processes influence the ortho-para ratio as well. The emergence of the full spectrum of PCBzs above 600 K points at a chlorodehydroxylation process, converting PCPs into PCBzs. As chlorobenzene itself is inert with respect to fly ash mediated oxychlorination [see also De Leer et al. (28)], polychlorobenzenes must have been formed via ipso substitution of chlorophenols rather than by chlorination of (Chlorinated) benzene(s). The formation of PCDDs is of particular interest, and we will discuss this process in some detail. Employing on-line gas analyses, only the PCDDs with a maximum of four chlorine atoms were detectable. Due to their low vapor pressures, the higher chlorinated PCDDs condensed on the fly ash bed and/or the exit tube of the reactor. As a consequence, the pressure buildup required to pass gas over the fly ash bed increased in time. This resulted in higher concentrations of reactants and longer residence times. The products condensed on the fly ash bed were analyzed qualitatively; by Soxhlet extraction using toluene as a solvent and subsequent GC-MSD analysis, all congeners of PCDDs from TdCDDs onward were detected. In Figure 9 the total ion chromatogram (TIC) of the effluent gas condensate an-d &e fly ash extract are compared. The chromatograms visualize that care should be taken when interpreting the measured effluent concentrations of the T~CDDS,since an important fraction of those T4CDDs condenses before detection. Similarly, the measured values of [T4CDD] obtained in the experiments whereby the reactor temperature was stepwise lowered are likely overrated due to "memory effects".
40
-5
30
E
E
20
f:
5
10
0
450
500
550
600
650
700
600
650
700
T (Ki
SO0
450
550
+
1858
Environ. Sci. Technol., Vol. 27,
No. 9, 1993
_1_Lj 30"
4;0
'
500 '
'
550 '
'
600 '
'
650 '
io0
T (K)
Flgure 8. Fly ash mediated oxychlorination of phenol. A, general product pattern; 6,formation of chlorophenols; C, the ortho-para ratio (chlorophenols).
Interestingly, Figure 9 reveals that the T4CDDs are dominated by the 1,3,6,8- and the 1,3,7,9-substituted isomers (1:l). This isomer distribution of the T4CDDs can adequately be explained by the condensation of two 2,4,6-T&P entities (see Summary and Conclusions for more details). Analogously, for the H&DDs, the predominant reaction appears to be condensation of P5CP with 2,4,6-T3CP favoring 1,2,3,4,6,8-H&DD, by far the most abundant peak in the cluster of H6CDDs. Figure 9 also depicts representative patterns of the T4CDDs and P5CDDs for MWIs (14,61). Isomer distributions depend on the sampling location. In the furnacewhere PCDDiF levels are relatively low-a large variety of T4CDD isomers are seen, pointing at a more or less random (thermal) mode of formation. In the boiler section, and the more so in the electrostaticprecipitator,the 1,3,6,8and 1,3,7,9-T4CDD isomers, prominently present in our product mixtures, gain in importance (14). An analogous trend is seen with the P~CDDS, of which the formation of the 1,2,4,6,8-, 1,2,4,7,9-, 1,2,3,6,8-, and 1,2,3,7,9-P5CDDs isomers is preferred going downstream in the MSW. A striking feature of the present results (and th.ose of t h e f l y ash mediated oxidation of chlorophenol) is t h e sparse formation of PCDFs relative to that o f PCDDs. Although we qualitatively identified traces of some lower chlorinated PCDFs, it appears that there is no direct pathway leading to PCDFs in the thermal reaction o€ chlorinated phenols on fly ash. This phenomenon has been observed by others as well (22-25, 40,41, 62). Above ca. 600 K, comparable quantities of tetrachloroethylene (CzCl4) and methylene chloride (CHZC12) are
1 LD#
Effluent Gas Condensate
20 10
Fly Ash Extract c i
0
500
600
700
(K)
Flgurs I O . Conversion of reactants in the CuCITcatalyzed oxychb rination of phenol ""
Flgure 9. Phenol oxychlorination. Top, PCDD congener pattern obtalned in the lab experiments of this stud). Bottom. ObseNed left-hand sMe. T,CDDs: congener pattern in the flue gas of a MWI ( 14: right-hand sue. PSCDDs.
also formed. At 700 K their molar yield is comparable to that of thechlorinatedphenols. Theseproductsvery likely result from ring opening oxidative breakdown of (poly)chlorophenols. Other studies on fly ash catalyzedreactions mention formationof C~CLaswell(25,28,63).Ittherefore deserves consideration to monitor CzClr in MWI effluent gases as a possible indicator for PCDD/F emissions. Should [C2CLl turn out to correlate well with [PCDD/Fl, its analysis must be possible at high frequency-if not continuously-and a t much lower cost than that of the tedious off-line procedure for determining PCDD/F. The mechanism of formation of monobenzofurans is as yet unknown. Presumably, chlorophenolentities condense with chlorinated CZcompounds on the fly ash surface to form PCBFs, although the combination of phenol and allylic fragments may also lead to this type of compounds. CuClz Catalyzed Oxychlorination of Phenol. We have compared the oxychlorination of phenol catalyzed by MWI fly ash with that engendered by a standard commercialoxychlorination catalyst (basically 5% CuClz on y-alumina). To bring the concentration of CuClz down to the level of Cu in fly ash, the oxychlorination catalyst was diluted 300-fold (w/w) by purified sea sand. Using 0.61 g of the resulting mixture and a reagent atmosphere consisting of 0 2 (16.2 vol %), NZ(82.9 vol %), HC1 (0.19 vol %), phenol (0.33 vol %), and H20 (0.39 vol %), the temperature was varied between 424 and 765 K. Residence times were as short as 0.63 and 0.38 s, respectively. Mono-, di-, and trichlorophenols were observed over the entire temperature region. However, above ca. 635 K,
i
I
&MCP
W CuCIZ/A1203
0 FLY ASH
I
m' 2 4 DCP
2.6~DCP 2.4.6-TCP
Flgure 11. Comparison of the isomer distribution of chlaophenols obtained in the CuCI, and fly ash catalyzed oxychlorination.
deep oxidation of phenol rapidly grew in importance. Simultaneously,the net consumption of HC1 (derived from the net formation of chlorophenols) decreases from that temperature onward (Figure 10). As a standard procedure, the temperature region was traversed upward and downward to validate the temperature dependency of the consumption of HC1. It was found that the catalyst activity was partially lost after heating above 625 K (Figure 10). This contrasts with the much higher stability of fly ash with respect to oxychlorination (preceding section). Yet, the isomer distributions of the lower chlorinated phenols, obtained upon CuClz and fly ash catalyzed oxychlorination, are alike (Figure 11). Note that, with CuClz too, chlorination preferably occurs on theorthoandparapositions,andthat theortho-pararatio is quite similar. Literature reports on the CuClz-catalyzed oxychlorination of phenol confirm this ortho-para chlorination pattern and show that under more severe conditions formation of even tetra- and pentachlorophenol is possible (64). Formation of PCDDs or other products usually found whenusingflyashwasnotobserved,suggestingthatCuClz per se does not catalyze their formation. That a CuClzbased "synthetic" fly ash does not promote PCDD formation tallies with earlier observations (25). Stability of Chlorobenzene and Phenol onFly Ash. In the experiments with chlorophenol on fly ash, a remarkable difference in stability between (chlorinated) phenols and benzenes appeared to hold. In order to verify Environ. Sci. Technol.. Val. 27, No. 9, 1993
1850
h
5E-05 -
C
3 4E-05
40
'I
/ I
i
0
2
I/
20i A/
-
> z 0 3E-05 A
2E-05
' FLY ASH OF MWI A
-
, FLY ASH ,OF MWI B I
I 1E-05
-
0
500
600
700
800
T Flgure 12. Relative stability of phenol and chlorobenzene.
this, a competition experiment has been performed in which equimolar amounts of phenol and chlorobenzene were administered to a fly ash bed under oxidative conditions. The results graphically shown in Figure 12 substantiate the large difference in stability between chlorobenzene and phenol under the experimental conditions [0.4 g of fly ash (MWI A), N2 90.6 vol %, 0 2 9.1 vol %, and chlorobenzene and phenol 0.17 vol %). The residence time ranged from 1.3 s at 500 K to 0.8 s at 870
K. Above 635 K, phenol is fully converted into mainly C02, clearly by catalytic combustion. Chlorobenzene,however, passes the fly ash bed unaffected. This deep oxidation of chlorophenol-to give COz, H20, and HC1-contrasts with the stability of phenols under comparable homogeneous reaction conditions: slow combustion or homogeneous autoxidation of phenol sets above 800 K. Due to its radical chain self-inhibition properties, the thermal stability of phenol in the homogeneous gas phase is even higher than that of chlorobenzene. In the homogeneous degradation of phenol, loss of CO by the phenoxy1 radical is a key step. Since CO is stable under the present conditions of the fly ash mediated experiments (65),the fly ash catalyzed phenol oxidation follows a different mechanism. Based on the observation that chlorobenzene is far less susceptible than phenol to fly ash catalyzed oxychlorination as well (65),the hydroxy group must be pivotal for adsorption/chemisorption at the fly ash surface to enable oxychlorinationand oxidation. Catalyzed Oxidation of Phenol;Turnover Capacity. In the preceding sections it was demonstrated that MWI fly ash not only catalyzes the oxychlorination of phenol but also its deep oxidation to H2O and C02. Although this phenomenon may seem propitious in a way that it helps to reduce the emissionsof volatile organics by MWIs, it leads t o a higher chlorine content in t h e remaining PIG. As such it is of interest to quantify the capacity of MWI fly ash to convert chlorophenol into C02 (HC1) and water. In the preceding section, it has been shown that the fly ash catalyzed deep oxidation is quite selective: around 630 K phenol is fully converted; chlorobenzene then being essentially unaffected. Consequently, f l y ash is u n a b l e t o convert a wide range of ( c h l o r i n a t e d ) hydrocarbons by catalytic oxidation, unless its temperature is considerably higher t h a n 900 K (cf. Figure 12). In order to measure the maximum capacity of fly ash to oxidize phenol, several experiments with different phenol intakes have been performed, keeping other 1860
Envlron. Scl. Technol., Vol. 27, No. 9, 1993
4E-05 6E-05 8E-05 1E-04 ISTAKE PHENOL (M) Flgure 13. Fly ash catalyzed oxidation of phenol. Oxidation capacity of fly ash at 670 K. For further conditions, see text. 0
?E-05
conditions constant. Two samples of MWI fly ash (MWI A and MWI B) were tested in separate series of experiments (bed size 0.4 g each). At constant temperature (670 K), with oxygen at ca. 12 vol % ,the phenol intake was varied between 0.26 and 0.52 vol %. Residence times were 0.67 5.
Figure 13 reveals that under these conditions full conversion (oxidation) of phenol is achieved at moderate phenol intake only. Once the initial phenol concentration exceeds 0.35 vol % , only partial conversion of phenol is achieved. Consequently, the capacity to "burn" phenol at M in 0.67 670 K can be derived from Figure 13: 6 X s with 0.4 g of fly ash or 2.2 X lo4 M-s-1.g-1. Differences between the two fly ash samples (MWI A and MWI B) are only marginal. The occurrence of a maximum turnover capacity proves that the observed deep oxidation of phenol is a catalytic process. Additional evidence for this is found in the CO/ C02 ratio which persists at a low value (ca. O.l), even at partial conversions of phenol, pointing to a "conducted tour" surface-catalyzed mechanism. Catalyzed Oxidation of Phenol; Temperature Effect. In this section the effect of temperature variation on the rate of the deep oxidation of phenol is dealt with. Using a fresh fly ash bed (0.42 g, MWI A) and a reactant atmosphere consisting of nitrogen (85.4 vol %), oxygen (14.5 vol %), and phenol (0.13 vol %), the conversion of phenol was studied over a temperature range of 150 OC (520-670 K). Residence times decreased with increasing temperature from 0.57 to 0.43 s. For comparison, a similar series of experiments was performed using CuO (10% by weight) on alumina as an oxidation catalyst (total weight of the bed, 0.44 g). The initial phenol concentration was higher (0.34 vol % ) than in the experiments with fly ash, with the oxygen and nitrogen content basically unaltered (15.5 and 84.1 vol % , respectively). Employing CuO rather than fly ash, the oxidation of phenol proceeds at lower temperatures. Therefore, the rate of oxidation was analyzed between 470 and 600 K, and residence times varied from 0.58 to 0.32 s. Both catalysts promoted the deep oxidation of phenol, albeit at widely different temperature windows. At all temperatures COa and CO were the major products with only 2,2'-dihydroxybiphenyl as a minor byproduct. Typical mass balances varied between 90 and 105%.
Scheme I
,..cu+
4
0'-
-- 3 M
..o. 0 -
e
tc
L1
9.5
1.6
,
1.7
1
,
18
,
19
1
1
2
1-
--c
21
CI
WCIJIJ 0
-
0
l000,T (K-')
Flgure 14. Comparlson of the rate constants for phenol oxidation, normallzed at grams of Cu available In the catalysts.
The respective activities of fly ash and CuO on alumina are visualized in Figure 14. By considering the oxidative breakdown of phenol as a unimolecular decomposition, pseudo-first-orderrate constants can be calculated for the various temperatures and plotted in Arrhenius form. For the calculations it was assumed that the packed reactor displayed perfect tubular behavior. For the deep oxidation of phenol on MWI fly ash the following Arrhenius expression was derived: log(k/s-'.gl) = 21 f 5 - (50 f 8)/2.3RT. The oxidation of phenol catalyzed by CuO on alumina appeared to be less temperature dependent: log(k/s-1) = 12.6 f 0.6 - (27 f 1)/ 2.3RT. Yet,asFigure 14 depicts, in the temperature region of the fly ash catalyzed phenol oxidation, absolute rates-normalized for the amount of copperpresent-are very much alike. Hence, despite differences in particle size, location, (chemical) state of copper, etc., the catalytic activity of CuO-alumina and fly ash are essentially the same. Summary and Conclusions
On the Mechanism of Formation of PCDDs. Although the fly ash catalyzed formation of PCDDs from chlorophenols is already known for a number of years, still very little is known about the mechanism involved. In part this is due to the complexity of the fly ash matrix, which effectively obscures the identity of the active catalytic sites. Anyway, the condensation of chlorinated phenols to PCDDs on fly ash surface has to be ascribed to the catalytic activity of metal sites (22, 23). The role of copper derivatives in the formation of PCDDs is hardly contradicted nowadays (26, 27,361. However, whether this is the result of mere oxychlorination reactions catalyzed by CuClz (producing precursors that form eventually PCDDs) or if it is accomplished by coppercatalyzed condensation reactions as well is unknown. In the present investigation, using HCl/Oz/phenol and supported CuClz per se, we were unable to detect PCDDs. Nevertheless, their formation is claimed in the oxychlorination of phenol using a CuCldKC1 catalyst (64). Accepting copper as the active catalyst for formation of PCDDs may well explain several other features of the fly ash catalyzed oxychlorination of phenol. For instance, apart from oxychlorination, halogen exchange, (deep) oxidation, and hydrogen transfer reactions are all promoted by copper ions (66, 67). Furthermore, the Ullmann
1,3,7,9-TCDD
1,3.6.8-TCDD c1
condensation-the copper-catalyzedformation of diphenyl ethers by reaction between halogenated benzenes and alkali metal phenolates-can be used to model the formation of PCDDs. For the Ullmann condensation, Cu(I) has been recognized as the active species, acting in a nucleophilic aromatic substitution reaction (68, 69). Although speculative, we therefore suggest that the formation of PCDDs is brought about by a reaction path as shown in Scheme I, in which for simplicity the condensation of two o-chlorophenol molecules into chlorine-free dibenzo-p-dioxin is depicted. This mode of reaction also rationalizes the predominant formation of 1,3,6,8- and 1,3,7,9-T4CDD. Recalling that oxychlorinationof phenol on fly ash surfacerenders almost exclusively ortho and para substitution, 2,4,6-T3CP is an important (intermediate) product. Adopting Scheme I, this will result in 1,3,6,8-T4CDD. The other isomer, 1,3,7,9TICDD, could emerge via a Smiles rearrangement (70,71) (Scheme 11). As pentachlorophenol is an important oxychlorination product as well, the predominance of one isomer in the congener pattern of the H6CDDs can be interpreted by condensation of P&P with 2,4,6-T3CP,yielding 1,2,3,4,6,8H6CDD. Note that the Smiles rearrangement yields indentical products in this case. In sum, the formation of highly chlorinated PCDDs in the present reaction system is brought about by condensation of polychlorinated phenols which in turn result from the fairly rapid, repeated, oxychlorination of phenolic entities. As the congener pattern of the PCDDs can be well interpreted by this sequence of reactions, the rate of oxychlorination of (chlorinated) dibenzo-p-dioxin molecules on fly ash must be relatively slow. This is under investigation in our laboratory and will be the subject of a forthcoming paper (72). Envlron. Scl. Technol., Vol. 27, No. 9, 1993 1881
Our study underscores that fly ash mediated reactions under conditions relevant for the postfurnace sections (boiler, ESP) play a prominent role in the formation of PCDD/Fs. The role of the oven itself appears to be restricted to the release of the ingredients: oxygen, HC1, organic precursors such as phenol, and ‘catalytic dust’. Proper design of the incinerator and a good combustion practice will help to reduce the level of hazardous PICs but do not guarantee sufficiently low PCDD/F emissions. Under the conditions employed, increasing levels of HC1 give rise to increasing rates of oxychlorination, with increasing chances for production of PCDD/Fs. Complete removal of chlorine-containing materials from MSW would, of course, lead to zero PCDD/F emissions. It remains to be seen if practical approaches, such as sorting out waste PVC and/or prior separation of compostables (two major carriers of the MSW chlorine load), lead to appreciably lower PCDD/F levels. This is presently studied in our laboratory, and we plan to report on our findings shortly. Finally, considering the Arrhenius-type relation between temperature and rate of oxychlorination (Figure 6A), it appears profitable to reduce the operating temperature of electrostatic precipitators. An extra advantage of lower ESP temperatures is the enhanced condensation of PCDD/ Fs on the fly ash, thus reducing the emissions even further. Acknow 1edgments Thanks are due to A. A. Sein (RIVM) and G. Tas (AVR) for providing the two representative fly ash samples of two Dutch MWIs and to Prof. J. W. Geus (University of Utrecht) for the CuO/A1203 oxidation catalyst.
Author Supplied Registry Numbers: Phenol, 10895-2; hydrogen chloride, 7647-01-0; copper, 7440-50-8; 2-chlorophenol, 95-57-8; 4-chlorophenol, 106-48-9; 2,4dichlorophenol, 120-83-2; 2,6-dichlorophenol, 87-65-0; 2,4,6-trichlorophenol, 88-06-2; 2,3,4,6-tetrachlorophenol, 935-95-5; pentachlorophenol, 87-86-5; benzofuran, 27189-6; dibenzofuran, 132-64-9;dibenzo-p-dioxin, 262-12-4; MlCDD, 35656-51-0;D&DD, 64501-00-4;T3CDD, 6976096-9; 1,3,6,8-TCDD,33423-92-6;1,3,7,9-TCDD,62470-535; TICDD, 41903-57-5;PBCDD,36088-22-9;HeCDD, 3446546-8; H,CDD, 37871-00-4; OaCDD, 3268-87-9. Literature Cited Olie, K.; Vermeulen, P. L.; Hutzinger, 0. Chemosphere 1977, 6 , 455. Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, 7, 165. Eiceman, G. A.;Clement,R. E.; Karasek,F. W. Anal. Chem. 1979,51, 2343. Lamparski, L. L.; Nestrick, T. J. Anal. Chem. 1980, 52, 2045. Viau, A. C.; Studak, S. M.; Karasek, F. W. Can. J . Chem. 1984, 62, 2140. Hahn, J. L.; VonDemfange, H. P.; Velzy, C. 0. Chemosphere 1986, 15, 1239. Benfenati, E.; Gizzi, F.; Reginato, R.; Fanelli, R.; Lodi, M.; Tagliaferri, R. Chemosphere 1983, 12, 1151. Hasselriis, F. Waste Manage. Res. 1987, 5, 311. Visalli, J. R. Hazard. Waste Manage. 1987, 37, 1451. Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983,17, 721. Goldfarb, Th. D. Chemosphere 1989, 18, 1051. Hay, D. J.; Finkelstein, A.; Klicius, R. Chemosphere 1986, 15, 1201. 1862 Environ. Sci. Technol., Vol. 27, No. 9, 1993
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Received for review November 10, 1992. Revised manuscript received March 10, 1993. Accepted April 5, 1993.
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