Envlron. Sci. Technol. 1987, 21, 305-307
NOTES Estimation of OH Radical Reaction Rate Constants and Atmospheric Lifetimes for Polychloroblphenyls, Dlbenzo-p -dloxlns, and Dlbenzofurans Roger Atkinson
Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1
rn Room temperature rate constants for the gas-phase reactions of the hydroxyl radical with the monochlorothrough pentachlorobiphenyls, dibenzo-p-dioxin, 2,3,7,8tetrachlorodibenzo-p-dioxin, and dibenzofuran have been estimated and the atmospheric lifetimes of these compounds due to this reaction calculated. These calculated atmospheric lifetimes range from -7-8 h for dibenzo-pdioxin and dibenzofuran to -60-120 days for the pentachlorobiphenyl isomers. Introduction A large number of organic compounds are emitted into the atmosphere from anthropogenic sources. Many of these emissions, although present in the atmosphere at low concentrations, are of a hazardous or toxic nature, and a knowledge of the atmospheric residence times and products of the atmospheric transformations of these chemicals is needed to reliably assess their impact on the environment. Because of their toxicity to animals (see, for example, ref l),the chlorinated organics are receiving much attention, with respect to both their ambient atmospheric concentrations and their transport through the environment. In particular, the polychlorodibenzo-p-dioxins (PCDD’s)and the polychlorodibenzofurans (PCDF’s) are now recognized to be emitted into the troposphere as a result of incineration of chlorine-containing materials, including waste materials (2-5). In addition, polychlorobiphenyls (PCB’s) are volatilized from waste disposal sites and emitted from waste incinerators (5, 6 ) . Thus, it is estimated that the air over the U.S.contains approximately 18000 kg of PCB’s and these compounds have been identified and measured at a number of remote locations throughout the northern and southern hemispheres (8,9). Recently, Czuczwa and Hites (4) have analyzed a series of urban air particulates and Great Lakes sediment samples and concluded that, apart from a sediment from Lake Ontario contaminated by pentachlorophenol, the presence of PCDD’s and PCDF’s in the sediments is due to atmospheric transport of combustion-derived emissions. However, while it is clear that these PCB’s, PCDD’s, and PCDF’s are transported through the troposphere, the atmospheric chemistry of these classes of compounds is not quantitatively known at the present time. The vapor pressures of the PCB cogeners present in the commercial Aroclor mixtures are sufficiently high (10, 11) that the PCB’s will be partitioned mainly into the gas phase (9). While the reported room temperature vapor pressure for 2,3,7,8-tetrachlorodibenzo-p-dioxin of 7 X Torr (12) would indicate that the PCDD’s (and PCDF’s) will be partitioned more into the particle phase, Eitzer and Hites
(a,
0013-936X/87/092 1-0305$O1.50/0
(13) have recently reported that PCDD’s and PCDF’s with five or less chlorine atoms are collected almost entirely on polyurethane foam plugs, rather than on filters. This finding suggests that these compounds are also readily partitioned into the gas phase. As a result of some 2 decades of experimental laboratory studies, it is known that the chemical loss processes for organic compounds present in the atmosphere in the gas phase primarily involve photolysis and chemical reaction with OH and NO3 radicals and with O3(14),with the OH radical reaction being the most important of these loss processes for the majority of organic compounds (14,15). However, only few experimental data are presently available concerning these gas-phase reactions for the PCB cogeners (16-18), with none having been reported for the PCDD’s and PCDF’s. Thus, rate constants have been measured at room temperature for the gas-phase reactions of biphenyl and the three monochlorobiphenyl isomers with the OH radical (17-21), and an upper limit rate constant has been determined for the reaction of biphenyl with O3 (20). It has also been reported that certain di-, tri-, and tetrachlorobiphenyls are stable to photolysis by ultraviolet light under simulated atmospheric conditions (16).
These experimental data suggest that the monochlorobiphenyls will be removed from the troposphere mainly by gas-phase reaction with the OH radical, with lifetimes due to this reaction of -4-8 days (18). The more highly chlorinated PCB’s are expected to be progressively less reactive to the OH radical (18),with correspondinglylonger lifetimes. However, because of their low vapor pressures and their tendency to be adsorbed onto reaction chamber walls, experimental studies of the kinetics and mechanisms of the gas-phase reactions of the more highly chlorinated PCB’s and of the PCDD’s and PCDF’s have not been carried out. To provide a data base concerning the atmospheric lifetimes of these compounds, a technique for the calculation of rate constants for the reactions of the OH radical with monocyclic aromatics (15,19) has been extended to allow the tropospheric lifetimes of the more highly chlorinated PCB’s to be estimated. The OH radical reaction rate constants and atmosphere lifetimes of dibenzo-p-dioxin, dibenzofuran, and 2,3,7,8-tetrachlorodibenzo-p-dioxin have also been estimated. Estimation Technique The reactions of the OH radical with benzene and the chlorobenzenes proceed essentially entirely via OH radical addition to the aromatic ring (15,22),and this is expected to be the case for the PCB cogeners (18) and for the PCDD’s and PCDF’s. The estimation method proposed
0 1987 American Chemical Society
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Table I. Derivation of Room Temperature Rate Constant for Addition of OH Radicals to the CGH4ClRing (Ring B ) in the Monochlorobiphenyls
k
cm3 molecule-’ s-l ring A + ring A, ring B, obsdn calcdb ring B
chlorobiphenyl 23-
X
2.8 f 0.4 5.2 f 0.8 3.8 f 0.7
4-
1.2 2.8 1.2
1.6 f 0.4 2.4 f 0.8 2.6 f 0.7
‘From ref 15; the indicated error limits are two least-squares standard deviations. Using eq 1 and the electrophilic substituent constants for the C1 and C6H, substituent groups (23).
by Zetzsch (19) for the monocyclic aromatics utilizes the correlation between the OH radical rate constants for addition to the aromatic ring, k, and the sum of the electrophilic substituent constants, X u + , of Brown and Okamoto (23). The values of Ea+are calculated as described by Zetzsch (19). Thus, (a) steric hindrance is neglected, and the electrophilic substituent constant for the ortho position is set equal to that for the para position; (b) the total substituent constant Ea+is the sum of all substituent constants of the substituents connected to the aromatic ring; (c) the OH radical adds to the position yielding the most negative value for Ea+(preferably a free position); (d) if all positions are occupied, the ipso position is treated as a meta position. From an evaluation of the experimental data for the aromatics, dividing the recommended rate constant for biphenyl by a factor of 2 because of the two identical rings, the correlation log k (cm3 molecule-l s-l) = -11.64 - 1.31Cu+ (1) was recently derived (15). With this equation, calculated gas-phase OH radical addition rate constants are within a factor of 2 of the experimental values for 31 of the 38 aromatics for which data are available (15).
Results and Discussion In order to use eq 1to calculate rate constants for reaction of the OH radical with PCB’s, rate constants for each ring must be estimated. This necessitates the prior estimation of the u+ values for the mono- and polychlorophenyl groups. Assuming that the value of u+ for the C&&C1 substituent does not depend on the location of the C1 atom in the ring, then u+(C6H4C1)can be obtained from the experimental data for the 2-, 3-, and 4-chlorobiphenyls. Denoting the chlorine-substituted ring as ring A and the non-chlorine-substitutedring as ring B and using the C1 and C6H, electrophilic substituent constants given by Brown and Okamoto (23),then the OH radical addition rate constants given in Table I can be calculated for ring A from eq 1. Comparison of these ring A rate constants with the experimental data allows the room temperature rate constant for ring B to be derived, and the values obtained are also given in Table I. These are in reasonable agreement within the error limits, and a weighted average yields k(ring B) cm3 molecule-l s-l. From eq 1 this corre= 1.9 X sponds to u+(C6H4C1)= 0.06, and by analogy with the c6H5 group, this is the u value. By use of the observation that ACT,’ is -0.7 ACT: in going from a CH3 to a CHzCl substituent group (23),then um+(C6H4C1)N 0.28 is estimated. To calculate the OH radical addition rate constants for the polychlorobiphenyls,values of urn+and up+are needed for the polychlorophenyl groups involved, and the values used (Table 11; see paragraph at end of paper regarding +
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Table 111. Calculated Room Temperature OH Radical Addition Rate Constants for the Dichlorobiphenyl Isomers dichlorobiphenyl isomer
10l2, cm3 molecule-’ s-l
dichlorobiphenyl isomer
kX cm3 molecule-’ s-l
2,32,42,52,63,43,5-
1.8 1.8 1.8 1.8 1.8 2.9
2,2’2,3’2,4‘3,3‘3,4’4,4’-
1.4 2.1 1.4 2.7 2.1 1.4
k
X
Table IV. Calculated Room Temperature Rate Constants for OH Radical Addition to Biphenyl and the PCB’s, Together with Experimental Data and Calculated Tropospheric Lifetimes Due to This Gas-Phase Loss Process no. of C1 substituents 0
1 2 3 4 6
k x W, cm3 molecule-’ s-* calcd obsd 7.9 3.1-4.7 1.4-2.9 0.7-1.6‘ 0.4-0.gd 0.2-0.4
calcd tropospheric lifetime, days
5.8-8.2’ 2.8-5.2
3 5-11 8-17 14-30 26-60 60-120
‘From ref 15 and 18-21. bFrom ref 15 and 18. ‘Highest values for the 3,3’,5- (1.6 X ems molecule-’ 9.’) and 2’,3,5- and 3,4’5(1.4 X lo”* cm3 molecule-’ 9-l) isomers. dHighest value for the 3,3’,5,5’-isomer.
supplementary material) assume that the incremental changes in up+and urn+remain constant for each additional C1 atom substituent on the phenyl group. Hydroxyl radical rate constants were then calculated for the di-, tri-, tetra-, and pentachlorobiphenyls by use of eq 1 and these u+ values. The rate constants for the individual dichlorobiphenyl isomers are given in Table 111,while Table IV gives the ranges of OH radical reaction rate constants for the monochloro- through pentachlorobiphenyls. (Only the rate constant ranges for the isomeric PCB’s are given in Table IV, since the uncertainties in these calculated rate constants increase with the degree of chlorination, and the rate constant for any given cogener can be readily calculated.) In addition, the calculated gas-phase tropospheric lifetimes of these PCB isomers due to reaction with the OH radical are given, assuming an annual diurnally averaged OH radical concentration of 5 x lo5 molecule cm-3 (24). Table IV shows that the reactivity of the PCB’s toward OH radical reaction decreases with the degree of C1 atom substitution, by a factor of -2 per extra C1 atom substituent. The corresponding atmospheric lifetimes due to this gas-phase reaction with the OH radical are calculated to range from -3 days for biphenyl itself to -60-120 days for the pentachlorobiphenyls. Atlas and Giam (8) estimated an atmospheric lifetime of 190 days for the PCB’s typical of Aroclor 1242 from the observed variations in their ambient concentrations at Enewetak Atoll, suggesting that the calculated lifetimes given in Table IV are not unreasonable. Analogous methods can be used to calculate the OH radical reaction rate constants for dibenzo-p-dioxin and dibenzofuran and their chlorinated homologues. Assuming that the structures
-
and
m,
a”’“
Literature Cited (1) U.S. EPA Health Assessment Document for Poly-
can be approximated respectively as 3H C O@ (
and OCH3
OCH3
then the room temperature gas-phase OH radical reaction rate constants for dibenzo-p-dioxin and dibenzofuran are calculated to be 4.2 X and 3.4 X cm3molecule-l s-l, respectively, corresponding to atmospheric lifetimes due to OH radical reaction of -7 and -8 h, respectively, for an average 12-h daytime OH radical concentration of 1X lo6molecules (24). Hydroxyl radical reaction rate constants can be readily calculated for the PCDDs, since the two aromatic rings can be assumed to be independent for the purpose of the calculations (this is clearly not the case for the PCDF’s). Thus, for 2,3,7,8-tetrachlorodibenzo-p-dioxin the room temperature OH radical reaction rate constant is calculated to be -9 X cm3molecule-l s-l, corresponding to an atmospheric lifetime due to this gas-phase reaction of -3 days. This estimated OH radical rate constant for 2,3,7,8-tetrachlorodibenzo-p-dioxinis more than an order of magnitude higher than the esticm-3 molecule-l s-l cited by Podoll mated value of 5 X et al. (12) from unpublished work, for reasons that cannot presently be ascertained. Of course, for the more highly chlorinated PCB’s, PCDD’s, and PCDF’s photolysis may also contribute to their atmospheric removal (12,25,26), although no quantitative data are as yet available for the photolysis quantum yields and photolysis rates of these classes of organic compounds in the gas phase. Clearly, experimental data are needed to allow refinement of this estimation technique for these and related classes of organic compounds. However, as noted above, these data will be difficult to obtain, especially for the more highly chlorinated compounds, and for the near future OH radical reaction rate constants and atmospheric lifetime data will probably be available only from estimations. Supplementary Material Available Table I1 listing the electrophilic substituent constants urn+and up+used (1page) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St., N.W., Washington, DC 20036. Orders must state whether for photocopy or microfiche and give complete title of article, names of authors, journal issue date, and page numbers. Prepayment, check or money order for $10.00 for photocopy ($12.00 foreign) or $10.00 for microfiche ($11.00 foreign), is required and prices are subject to change. Registry No. DD, 262-12-4; DF, 132-64-9;B, 92-52-4; OH, 1746-01-6; 23352-57-6; 2,3,7,8-tetrachlorodibenzo-p-dioxin, chlorobiphenyl, 2051-60-7; 3-chlorobiphenyl, 2051-61-8; 4chlorobiphenyl, 2051-62-9; 2,3-dichlorobiphenyl, 16605-91-7; 2,4dichlorobiphenyl,33284-50-3;2,5-dichlorobiphenyl,34883-39-1; 2,6-dichlorobiphenyl,33146-45-1; 3,4-dichlorobiphenyl,2974-92-7; 3,5-dichlorobiphenyl, 34883-41-5;2,2’-dichlorobiphenyl, 13029-08-8; 2,3’-dichlorobiphenyl, 25569-80-6;2,4’-dichlorobiphenyl,3488343-7; 3,3’-dichlorobiphenyl, 2050-67-1; 3,4’-dichlorobiphenyl, 2974-90-5; 4,4’-dichlorobiphenyl,2050-68-2; pentachlorobiphenyl, 25429-29-2; chlorobiphenyl, 27323-18-8; dichlorobiphenyl, 25512-42-9;trichlorobiphenyl, 25323-68-6; tetrachlorobiphenyl, 26914-33-0.
chlorinated Dibenzo-p-dioxins;U.S.Government Printing Office: Washington, DC, Sept 1985;EPA-600/8/84-014f. (2) Brocco, D.; Cecinato, A.; Liberti, A. Proceedings of the 2nd European Symposium of the Physico-Chemical Behavior of Atmospheric Pollutants; Riedel: Dordrecht, Holland, 1982; pp 82-88. (3) Janssens, J.; Van Vaeck, L.; Schepens, P.; Adams, F. Proceedings of the 2nd European Symposium on the Physico-Chemical Behavior of Atmospheric Pollutants; Riedel: Dordrecht, Holland, 1982; pp 28-38. (4) Czuczwa, J. M.; Hites, R. A. Enuiron. Sci. Technol. 1986, 20, 195-200. (5) Nunn, A. B., I11 Gaseous HCl and Chlorinated Organic Compound Emissions from Refuse Fired Waste-to-Energy Systems; U.S.Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, March 1986; EPA-60013-841094. (6) Murphy, T. J.; Formanki, L. J.; Brownawell, B.; Meyer, J. A. Environ. Sci. Technol. 1985, 19, 942-946. (7) Polychlorinated Biphenyls; National Academy of Sciences: Washington, DC, 1979. (8) Atlas, E.; Giam, C. S. Science (Washington,D.C.) 1981,211, 163-165. (9) Eisenreich, S. J.; Looney, B. B.; Thornton, J. D. Enuiron. Sci. Technol. 1981, 15, 30-38. (10) Burkhard, L. P.; Andren, A. W.; Armstrong, D. E. Enuiron. Sci. Technol. 1985, 19, 500-507. (11) Foreman, W. T.; Bidleman, T. F. J. Chromatogr. 1985,330, 203-216. (12) Podoll, R. T.; Jaber, H. M.; Mill, T. Environ. Sci. Technol. 1986,20, 490-492. (13) Eitzer, B.; Hites, R. A. Presented at 192nd National Meeting of the American Chemical Society Anaheim, CA, Sept 7-12, 1986. (14) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiey: New York, 1986. (15) Atkinson, R. Chem. Rev. 1986,86, 69-201. (16) Crosby, D. G.; Moilanen, K. W. Chemosphere 1977, 6, 167-172. (17) Dilling, W. L.; Miracle, G. E.; Boggs, G. U. Abstracts of Papers; 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983; American Chemical Society: Washington, DC, 1983; ENVR 126. (18) Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1985, 19,462-464. (19) Zetzsch, C. 15th Informal Conference on Photochemistry, Stanford, CA, June 27-July 1, 1982. (20) Atkinson, R.; Aschmann, S. M.; Pitts, J. N., Jr. Environ. Sci. Technol. 1984, 18, 110-113. (21) Klopffer, W.; Frank, R.; Kohl, E.-G.; Haag, F. Chem.-Ztg. 1986,110, 57-61. (22) Rinke, M.; Zetzsch, C. Ber. Bunsen.-Ges.Phys. Chem. 1984, 88, 55-62. (23) Brown, H. C.; Okamoto, Y. J. Am. Chem. SOC.1958, 80, 4979-4987. (24) Crutzen, P. J. In Atmospheric Chemistry; Goldberg, E. D., Ed.; Springer-Verlag: New York, 1982; pp 313-328. (25) Choudhry, G. G.; Webster, G. R. B. Chemosphere 1985,14, 9-26. (26) Dulin, D.; Drossman, H.; Mill, T. Enuiron. Sci. Technol. 1986,20, 72-77.
Received for review May 27,1986. Revised manuscript received November 3,1986. Accepted November 11,1986. I thank the California Air Resources Board for financial support through Contract A5-104-32.
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