Envlron. Scl. Technol. 1003, 17, 717-721
imental runs for both of the volatile metals, arsenic and mercury. Table IV gives the measured values in the offgases. Typically, levels were below detection limits; but even in those samples which did give measurable levels, concentrations were sub parts per million. In order to study the effect of particle size on gaseous species, we carried out four more experiments with larger particle sizes on the shale used in runs MR-152 and MR153. The experimental parameters and results for these two additional particle sizes are depicted in Table V. Our mini-retort did not permit us to use larger than 4-cm (diameter) shale without introducing serious problems with void space, and consequently channeling problems. The values observed for COS, As, and Hg in Table V are not significantly different than those observed for smaller particles (MR-152 and -153 in Table IV). However, H2S shows a rather larger particle effect. Figure 2 shows a plot of H2S vs. particle size for retort runs with and without steam. A drop off in H2S production is observed with increasing size going from the 0.32-0.64-cm size to the 1.3-2.5-cm size; however, the effect seems to be leveling off somewhat as the size increases from 1.3-2.5 to 2.5-3.8. Concurrent with this miniretort work, Occidental participated in a joint study with the DOE Laramie Energy Technology Center. Shale equivalent to that described in Table V was retorted in their 10-ton retort. Particle sizes closer to those seen in MIS retorts were used (7.6 cm, 3 in.). H2Slevels averaged close to 7000 ppm. Even though some significant differences existed between the Laramie run conditions and those used in the Oxy retort such as higher void volume due to larger particles and nonadiabatic conditions a t the Laramie test, the sulfur results nevertheless supported the leveling off trend depicted in Figure 2.
Conclusions This study resulted in the following conclusions regarding emissions from the retorting of Cb shale. (a) The
main offgas component is hydrogen sulfide with 3-6 kg of sulfur produced per ton of oil shale when the retorted Cb oil shale has 1-2% total sulfur. (b) The presence of added steam during retorting increases the H2Sproduction only slightly. (c) The only other significant sulfur-containing offgas component pollutant is carbonyl sulfide with levels not exceeding 3% that of the hydrogen sulfide. (d) While sulfur levels in the offgas show somewhat of a trend with total sulfur in the raw shale, the levels are near to linear when correlated to the pyritic sulfur content. (e) Sulfur emissions tend to level off with increasing particle sizes. (f) Ammonia is the second most significant component in the offgas with 0.3-2.3 kglmetric ton produced during retorting of Cb shale. (g) Arsenic and mercury levels are in the sub ppm range. Registry No. Hydrogen sulfide, 7783-06-4; ammonia, 766441-7; carbonyl sulfide, 463-58-1; arsenic, 7440-38-2; mercury, 7439-97-6.
Literature Cited (1) Ridley, R. D.; Chen, R. T., I11 Oil Shale Symp. Proc. 1975, 8, 123-127. (2) McCarthy, H. E.; Cha, C. Y.; Bartel, W. J.; Borton, R. S., 68th AIChE Meeting, Los Angeles, CA, Nov 20, 1975. (3) McCarthy, H. E.; Cha, C. Y. Oil Shale Symp. Proc. 1976, 9,85-100. (4) Mahajan, R. E.; Lumpkin, R. B.; Gragg, F. M.; Fraser, R. J. Oil Shale Symp. Proc. 1977, 10, 191-199. (5) Ondov, J. M.; et. al. Lawrence Livermore National Laboratory, March 1982, Report UCRL-5326s. (6) Burnham, A. K.; Rey, N. K.; Koskinas, G. J. ACS Symp. Ser. 1981. No. 163.61-77. (7) Burnham,'A. K. Lakrence Livermore National Laboratory, 1981, Report UCID-19093. Received for review June 18,1982. Revised manuscript received February 23, 1983. Accepted June 15, 1983.
Effects of Temperature on Ultraviolet Light Disinfection Blalne F. Severin," Makram 1.Suldan,z and Rlchard S. Engelbrechtt Department of Civil Engineering, University of Illlnois at Urbana- -Champaign, Urbana, Illinois 61801
The response of pure cultures of Escherichia coli, Candida parapsilosis, and bacterial virus f2 to ultraviolet light (254-nm) radiation in a batch reactor a t pH 6.8 was studied a t 5, 20, and 35 "C. Data were analyzed by the series-event kinetic model of inactivation. Activation energies for all three organisms were below 1050 cal/(gmol). This is in the range for purely photochemical reactions and an indication that UV disinfection is relatively insensitive to temperature changes.
Introduction Recent studies with ultraviolet light (UV) a t 254 nm indicate that it has strong potential for use as an alternative to chlorine for the disinfection of municipal wastewater effluents (1-4). One of the engineering advantages often speculated for UV disinfection is its insensitivity to temperature variations (5-6). This would be 'Present address: Tennessee Eastman Company, Kingsport, TN 37662. Present address: Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
*
0013-936X/83/0917-0717$01.50/0
expected if UV disinfection were a simple photochemical reaction (7). However, the physiological act of inactivation is not well understood and is complicated by the ability of many organisms to repair UV damage inflicted to their nucleic acid (8, 9). While the collection of damage is a purely photochemical reaction, the interplay of reactions which eventually lead to inactivation may not necessarily be purely photochemical. Even when all pre- and postirradiation procedures are constant, e.g., culture time and temperature and enumeration procedures, it is possible that, during the short irradiation period in which a culture is exposed to UV, altered temperature may have effects other than on the initial photochemical reactions. One example of this might be the alteration of repair enzyme kinetics. The prediction of the effects-of temperature of UV inactivation is further complicated by the nature of DNA itself. It has been demonstrated through in vitro studies that the rate of dimer formation between adjacent thymine nucleotides in single-stranded DNA is faster a t temperatures below 25 "C than a t higher temperatures (10). At low temperatures the natural state of singlestranded DNA is a stacked configuration, and stacked DNA is more subject to dimerization than unstacked DNA.
0 1983 American Chemlcal Society
Environ. Sci. Technol., Vol. 17, No. 12, 1983 717
Because of the complexity of UV inactivation and the lack of readily available information confirming the hypothesis of the insensitivity of UV inactivation to changes in temperature, batch inactivation studies with Escherichia coli, Candida parapsilosis, and and bacterial virus f2 were performed a t 5,20, and 35 OC. Data were analyzed with the series-event kinetic model of inactivation. This model has been used with excellent success for analyzing batch UV inactivation data and as a predictive tool when extended to complete mixed, flow-through reactor systems (11). The analysis of data using this model shows the rates of UV inactivation of the test organisms are relatively insensitive to temperature changes. This is significant from an engineering standpoint in that other water disinfectants appear to be more susceptible to change in temperature than UV.
Experimental Procedures E. coli was cultured a t 37 OC in nutrient broth on a shaking water bath. An anticlumping agent, 100 ppm by volume Tween 80 (poly(oxyethy1ene)sorbitan monooxylate; ICN Pharmaceuticals, Inc., Cleveland, OH), was added to the culture broth. E. coli was enumerated by using pour plates of nutrient agar incubated a t 35 “C. Colonies were counted after 24 h. Bacterial virus f2 was enumerated on lawns of E. coli K-13 host grown on tryptone yeast extract agar and incubated a t 35 “C. Plaques were counted after 8-24 h. C.parapsilosis was cultured a t 27 OC in yeast extract malt broth on a shaking water bath and enumerated by using pour plates of yeast extract malt agar incubated at 27 OC. Colonies were counted after 72-h incubation. Detailed procedures have been presented elsewhere (11). To minimize possible effects of day to day differences in the organism cultures, all data for each organism represent results obtained by using the same growth culture. Duplicate tests for each organism were made a t each of three temperatures. To do this with as much consistency as possible, two sets of three experiments were designed for each organism. Each set included one test a t each temperature. Each set required 2-2.5 h to complete, and seta were performed back to back such that all experiments for a given organism were completed within a 5-h period. Batch disinfection data were generated by using a flat-tray reactor with a 2.54-cm water depth and a total volume of 500 mL. The UV source was a General Electric G15T8 lamp mounted in a parabolic, stainless steel reflector system utilizing a Sylvania FS-2 Glostat ballast and operated at line voltage. The lamp was placed 63 cm above the surface of the water in the tray. A shutter arrangement was used to control exposure time of the test water to UV. A magnetic stirrer provided mixing in the reactor. The UV intensity was measured to be 450 pW/cm2 a t the surface of the water with the potassium ferrioxalate chemical actinometric test (12). Test water (pH 6.8) used in the batch disinfection experiments was prepared by diluting 2.5 mL of a sterile stock phosphate buffer solution (24 g of KH2P04and 6 g of NaOH per L of stock solution) to 500 mL with sterile DI water. The temperature of the water was then equilibrated to 5, 20, or 35 OC, and the organism density was adjusted to between 2 X lo5 and 1 X loe colony- or plaque-forming unit/mL by the addition of an appropriate quantity of an organism culture. In all tests the UV absorbance of the water was below 0.01 cm-l. At this level, the average intensity through the water was approximately 97% of the input energy flux a t the water surface. Tests were performed by collecting controls (unirradiated test water) and then opening the shutter at time zero 718
Environ. Sci. Technol., Vol. 17, No. 12, 1983
to expose the water to UV. Samples were then removed by pipet a t various intervals. The total exposure time never exceeded 200 s, and the temperature of the water never changed by more than 1“C during any experiment. Series-Event Kinetics An event is assumed to be a unit of incurred damage. Events occur in a stepwise fashion, and each step is considered an integer function. The rate at which an organism passes from one event level to the next is first order with respect to UV intensity and independent of a specific event level. As long as an organism is exposed to UV, it continues to collect damage. However, a threshold level exists; organisms that have reached an event level greater than the threshold are inactivated and those that are below this level survive. The threshold may vary depending on the culturing conditions used; however, for a given set of conditions, the threshold is presumed to be constant. Finally, it is assumed that the measured response is that of single organisms, i.e., not clumps. This is expressed as a series chemical reaction:
Mo
kI +
MI
kl +
... Mi
kl +
... M,-1
kl
M,
+
kl +
...
(1)
where k is the mixed second-order reaction rate constant (cm2/(pW.s)),I is the local point light intensity (pw/cm2), Mi is an organism that has reached event level i, and n is the threshold level of the organism. The rate a t which organisms pass through level i, rNi, (number/(cm3.s)) is given by rNi = kINi-1 - kINi (2) Equation 2 is then incorporated into a material balance for a complete mixed, closed-batch reactor, and the resulting equation is solved sequentially for i = 0 to i = n - 1. The fraction of surviving organisms, Ns/NI, is then given by the fraction of organism with n - 1or less accumulated damage. (3)
A basic premise necessary for the analysis of temperature effects is that while k is temperature dependent, n is not. Since k represents the probability of causing UV damage where there are potentially unlimited reaction sites, the value of k is thus expected to be temperature dependent because it is directly related to the rate of incurring damage. The value of n, the threshold value, while expected to be dependent upon pre- and postirradiation handling of the cultures, is presumed to be independent of the temperature of irradiation. Handling conditions include growth culturing temperature, length of culturing sample holding time after UV exposure, and enumeration procedure (8,9).These conditions, when varied, allow for the organisms to “prereact” to UV stress or to be more resistant for a variety of reasons to UV stress. When preand postirradiation handling is constant, however, n is presumed not to be a variable. In this case, the only difference in culture manipulation is the range of temperature exposure given during the brief period of UV irradiation. Results and Discussion To facilitate the analysis of the inactivation data, a computer program was developed to calculate the sum of squares of deviation between observed results and model solutions. The sum of squares of deviation was defined as the sum of the squares of the difference between the
Table I. Evaluation of Temperature Effects with the Series-Event Model k X lo3, cm2/
organism E. coli
C. parapsilosis
f2 virus
data
n
all 5 "C 20 "C 35 "C all 5 "C 20 "C 35 "C all 5 "C 20 "C 35 "C
12 12 12 12 11 11 11 11 1 1 1 1
(PW.S) 1.87 1.98 2.06 0.680 0.730 0.750 0.0649 0.0718 0.0776
Ra
sample size
0.9904 0.9909 0.9910 0.9893 0,9910 0.9930 0.9913 0.9880 0.9699 0.9625 0.9850 0.9580
39 14 13 12 32 11 11 10 44 16 15 13
kI t Flgure 2. Effects of temperature on the inactivation of E . coli; as normalized to the series-event model.
L I 2 3 4 5 6 7
oooo10
k It
Figure 1. Effects of temperature on the Inactivation of f2 virus, as normalized to the series-event model.
natural logarithm of observed fraction survival a t the sample exposure time and the logarithm of model solution a t specified values of k,n,and exposure time. The best-fit value of n for each organism was obtained by searching the results at each temperature over a range of n. At each n,the best-fit value of k and the sum of squares of deviations associated with these parameters were recorded. The value of n that gave the least sum of squares of deviations when these values were summed for the results a t the three temperatures was designated as the best-fit value of n for a given organism. In all cases, the best-fit n for any temperature for a given organism was never more than one integer value away from the overall best-fit n. The values of k reported for each temperature were those which gave the best fit for each temperature a t the designated overall best-fit value of n. Table I is a summary of results as analyzed by using the series-event model. Bacterial virus f2 is best represented as a single-event organism with a coefficient of determination of R2= 0.9699. E. coli and C. parapsilosis are best represented as 12-event and 11-event organisms, respectively. In both cases coefficients of determinations exceeded R2 = 0.99. Figures 1-3 are normalized plots of fraction survival vs. the dimensionless group k l t where k
I E
0
5oc
20'C
0.001 O'OI
A
I
35oc
0,0001
0
0
.
O
IO
c
20
Q
O
30
l
~
kIt
Flgure 3. Effects of temperature on the inactivation of C. parapsi/os$, as normalized to the series-event model.
is the kinetic constant as presented in Table I. The solid lines are the expected result for the best-fit value of n. Since these figures are normalized to the model, it is appropriate to state some of the results in more conventional terms. For example the dose (MW.s/cmz) which produced 99% inactivation of each organism at 5,20, and 35 "C was respectively the following: E. coli, 11.8 X lo3, 11.2 X lo3, and 10.7 X lo3; C. purupsilosis, 30.9 X lo3, 28.8 X lo3, and 28.0 X lo3; f2 virus, 72.5 X lo3, 65.6 X lo3, and 60.7 X lo3. Figure 4 shows the approximate 95% joint confidence intervals for each organism a t each temperature. Joint confidence intervals were approximated by using a method Environ. Sci. Technoi., Vol. 17, No. 12, 1983
719
t t
I'
0.0E5
'
'
f 2 Virus
1 t
I " " " " " ' I " " " " " ' I
i
C.parapsiiosis
t
/72.5
0.08
disinfectant free chlorine monochloramine chlorine dioxide iodine iodine
0.075
Y
Table 111. Activation Energies Associated with Chemical Disinfectants
1.9
0.07
1.7
ozone 0
1
2
8
10 12
14
16
8
10
12
14
Figure 4. Joint confidence regions (95%) for reported kinetic parameters.
Table 11. Activation Energies for UV Inactivation
organism
standard error of estimate
E. coli C. parapsilosis bacterial virus f2
5 54 56 2 1023
38 126 48
presented by Berthouex and Hunter (13). It is noteworthy that, since n can only take on integer values, the intervals in Figure 4 are discontinuous. Solid-line interpretations are used for ease of presentation. From this plot it can be seen that f2 virus can only be a single-event organism. Minor overlaps of confidence regions occur for f2 virus at 5 and 20 "C and for E. coli a t 20 and 35 "C. Only one major overlap of confidence intervals occurs between the 20 and 35 "C data for C. parapsilosis. In this case there is no significant difference between the reported kinetic constants. In all other cases the reported kinetic constants are significantly different. Individual sets of data for E. coli and C. parapsilosis at each temperature never showed more than a difference of n f 1 when compared to the best-fit value of n for all data. Given this observation and accepting the stated relations of the kinetic constant to temperature, confidence intervals about n can be recalculated to include all data points for each organism, in which case, the 95% confidence interval about n is f2 for both E. coli and C. parapsilosis. Activation energies for the UV disinfection process (Table 11) were calculated from the data in Table I. All three organisms show activation energies less than 1050 cal/(g.mol). These are within the expected range of less than several thousand calories per gram mole for purely photochemical reactions (7). It is interesting to speculate that, since a wide diversity of organism types, i.e., a bacterium, a yeast, and a bacterial virus, were studied, a low activation energy associated with UV inactivation is universal. It would appear as if the effects of temperature are so slight as to be minimal compared to other factors which are imposed, inadvertently or otherwise, by the testing methods. All attempts were made to ensure that the only variable tested here was the effect of temperature during exposure to UV. As in all disinfection studies, caution should always be expressed in the extrapolation of data to other experimental conditions, especially field testing. However, the results presented are conclusive for the system defined. 720
Environ. Sci. Technol., Voi. 17, No. 12, 1983
E. coli
7
8200
14
E. coli
9
12000
15
E. coli
7
4400
16
E. coli bacterial virus f2 M. fortuiturn
6.5 7
11000 4000
17 18
6.8
13 500
19
16
Number of Events, n
Ea, cal/ (g.mol)
organism
activation energy I Ea, tal/ pH (gmol) ref
It is interesting to review the effects of temperature on the efficiencies of other disinfectants. Table I11 is a brief compilation of activated energies for free chlorine against E. coli, monochloramine against E. coli, chlorine dioxide against E. coli, iodine against bacterial virus f2,and ozone against Mycobacterium fortuitum. These values were calculated from the product of disinfectant concentration times the contact time required for 99.0% inactivation. It must be realized that this method of analysis may give results somewhat different than if the analysis were performed by using some other mathematical model. It may be generally seen that these disinfectants have activation energies from several times to more than an order of magnitude larger than that for UV. This indicates that UV disinfection is less susceptible to changes in temperature than are the chemical disinfectants, a t least under the laboratory conditions reported.
Conclusions An important engineering aspect of the design of UV inactivation reactors is an understanding of the kinetics of disinfection and the effect of temperature on the kinetic parameters. Kinetic modeling of batch reactor results with the series-event model has been presented, and the effect of temperature on microbial inactivation disinfection has been analyzed. The apparent effect of temperature on UV inactivation is very small, on the order of 1050 cal/(g.mol), or less. Due to the wide diversity of organisms studied (i.e., a bacteria, a yeast, and a bacterial virus), it is speculated that the low activation energy is a universal effect with W inactivation. A brief review of chemical disinfectants indicates that their activation energies are between several times and more than an order of magnitude greater than those observed with UV light. That the kinetics of UV inactivation are not susceptible to temperature changes can be considered to be an advantage when considering UV as an alternative disinfectant of water or wastewater to the chemical disinfectants such as chlorine or ozone. Literature Cited (1) . . Oliver, B. G.; Carey, J . H. J. Water Pollut. Control Fed. 1976,48, 2619. (2) Scheible, 0. K.; Mulligan, T.; Binkowski, G. U.S. Enuiron. Prot. Agency 1979, EPA-600/9-79-018,117-125. (3) .-, Johnson. J. D.: Aldrich. K.: Francisco, D. E.; Wolff, T.; Elliott, M. US:Enuiron. Prot. Agency 1979, EPA-6001979-018, 108-116. (4) Severin, B. F. J. Water Pollut. Control Fed. 1980, 52, 2007-2018. (5) Yip, R. W.; Konasewich, D. E. Water Pollut. Control (Don Mills, Can.) 1972, 6, 14. (6) Tonelli, F. A.; Duff,R.; Wilcox, B. "Ultraviolet Disinfection of Domestic Sewage and Stormwater"; Wastewater Treatment Section, Pollution Control Branch, Ontario
Environ. Sci. Technol. 1983, 17, 721-730
Ministry of the Environment: Ontario, Canada, 1978; paper 2046. Reinisch, R. F.; Gloria, H. R.; Androes, G. M. In “Photochemistry of Macromolecules”;Reinisch, R. F., Ed.; Plenum Press: New York, 1970; p 190. Smith, K. C.; Hannawalt, P. C. “Molecular Photobiology”; Academic Press: New York, 1969. Harms, W. ”Biological Effects of Ultraviolet Radiation”; Cambridge University Press: Cambridge, England, 1980. Rahn, R. 0. In ”Photochemistry of Macromolecules”; Reinisch, R. F., Ed.; Plenum Press: New York, 1970;p 31. Severin,B. F.; Suidan, M. T.; Engelbrecht, R. S. Water Res., in press. Calvert, J. G.; Pitts, J. N. In “Photochemistry”;Wiley: New York, 1966; p 783. Berthouex, P. M.; Hunter, W. G. J. Sanit. Eng. Diu., Am. SOC.Civ. Eng. 1971, 97 (3), 333-344. Fair, G. M.; Geyer, J. C.; Okun, D. A. “Water and Wastewater Engineering: Treatment and Water Purification and
Wastewater Treatment and Disposal”; Wiley: New York, 1968; Vol. 11, p 31-11. (15) Siders, D. L.; Scarpino,P. V.; Lucas, M.; Berg, G.; Chang, S. L. Abstracts of the Annual Meeting of the American Society for Microbiology, 1973, No. E27. (16) Cronier, S. D. Master of Science Thesis, University of Cincinnati, Cincinnati, OH, 1977. (17) Chambers, C. W.; Kabler, P. W.; Malaney, G.; Bryant, A. Soap Sunit. Chem. 1952,28 (lo), 149-165. (18) Krus6, C. W. in a final report to the Commission on Environmental Hygiene of the Armed Forces Epidemiological Board, U.S. Army Medical Research and Development, 1969, Command Contract DA-49-193-MD-2314,pp 1-89. (19) Farooq, S. Ph,D. Thesis, University of Illinois, Urbana, IL, 1976. Received for review June 21,1982. Revised manuscript received March 25,1983. Accepted June 27, 1983.
Dioxin Formation in Incinerators Walter M. Shaub” and Wlng Tsang
Chemical Kinetics Division, National Bureau of Standards, Washington, DC 20234 Processes which may contribute to the formation of polychlorinated dibenzo-p-dioxins (PCDDs) in incinerators are examined. A model mechanism has been constructed to investigate the possibility of homogeneous gas phase formation of PCDDs from chlorophenols in an incinerator environment. Numerical calculations have been made. The results lead to the conclusion that the probability of gas-phase formation of PCDDs is likely to be very low a t high temperatures, 21200 K, if mixing between fuel and air is efficient. In addition, production of PCDDs from chlorophenols is found to depend upon the square of the chlorophenol concentration. Probable sources of nonidealities in practical incinerators are examined. Effects of use of auxiliary hydrocarbon fuel and excess air are examined. The potential role of non-gas-phase effects is considered. A discussion of some of the significant problems which complicate a further understanding of PCDD formation processes in incinerators has been presented in a manner that highlights future research needs.
Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) have been reported to have been found in incinerator emissions and other combustion sources (1,2). It has been suggested that these chemicals may be ubiquitous as a consequence of trace chemical processes that occur during combustion (3). This is a controversial question (4a,b). With respect to their formation in municipal incinerators, it has been suggested that likely major routes of formation are from chemically related compounds such as chlorobenzenes, chlorophenols, and PCBs (5). There appears to be uncertainty as to the relative importance of formation from nonchlorinated precursors and inorganic forms of chlorine or from chemically unrelated chloro organic compounds such as polyvinyl chlorides (5). Laboratory experiments that deal with the formation of PCDDs and PCDFs from chlorophenols, chlorophenates, chlorobenzenes, and PCBs have been reported ( 1 , 2 , 4 b ,6, 7). Most observations concerning the formation of hazardous waste compounds such as PCDDs during thermal incineration are fundamentally qualitative
in nature in the sense that they do not provide detailed answers which can lead to process modifications that may minimize the formation of PCDDs. None of the experiments ( 1 , 2 , 6 ,7) which have been reported to date can be considered to be adequate to demonstrate that the compounds (PCDDs, PCDFs, etc.) which have been observed to form are attributable to gas-phase processes (8,9).In addition, it has previously been suggested that in some instances there seems to be evidence that formations reported in some laboratory experiments may in fact be due a t least in part to surface reactions (10, 1 1 ) . There is a substantial body of literature in which it has been explicitly demonstrated that reactions involving chlorinated organic species are frequently affected or controlled in whole or in part by the presence of reactive surfaces (8, 12-27). In the following discussion some aspects of these problems are examined by first considering the potential for gas-phase formation of PCDDs under strictly homogeneous conditions in order to establish a frame of reference. Existing information about gas-phase reaction processes is drawn upon to develop a reasonable mechanism for gas-phase formation of PCDDs from a suggested precursor, chlorophenols (2). Next, some possible gas-phase nonidealities which may develop during thermal incineration are examined. This will be followed by a consideration of potential non-gas-phase contributions. Some general conclusions will be drawn regarding the problem of understanding PCDD formation in incinerators.
Mechanism Construction The reaction steps that make up the proposed gas-phase mechanism, discussed below, are shown in Table I. The species denoted by P, P., PD, and D, respectively, represent polychlorinated phenols, polychlorinated phenoxy radicals, polychlorinated 2-phenoxyphenols (dioxin precursor ( 2 ) ) and , PCDDs. These compounds are shown in Figure 1,where C1, and Cl, denote the varying extent of chlorination of the compounds. In Table I, the species denoted by the symbol R represent any other organic fuel compounds in the gas-phase mixture. R-and P r denote a fuel molecule from which a hydrogen atom has been
Not subject to US. Copyright. Published 1983 by the American Chemical Soclety
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