Environ. Sci. Technol. 1986, 20, 490-492
(24) Cleveland, J. M.; Rees, T.F.; Nash, K. L. Science (Washington, D.C.) 1983, 222, 221-223. (25) Skytte-Jenson,B. "Migration Phenomena of Radionuclides into the Geosphere"; Harwood Academic Publishers: London, 1982. (26) Neretnieks, F. "Diffusivitiesof Some Dissolved Constituents
in Compacted Wet Bentonite Clay-MX 80 and the Impact on Radionuclide Migration in the Buffer"; Royal Institute of Technology: Stockholm, Sweden, 1982; 1982-10-29. Received for review June 6, 1985. Revised mansucript received November 4, 1985. Accepted December 30, 1985.
Tetrachlorodibenzodioxin: Rates of Volatilization and Photolysis in the Environment R. Thomas Podoll," Helen M. Jaber, and Theodore Mlll Chemistry Laboratory, SRI International, Menlo Park, California 94025
rn The vapor pressure of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) at 25 "C is (7.4f 0.4) X torr. This value together with published values of aqueous solubility, octanollwater partition coefficient, and photolysis quantum yields provides a basis €or estimating the half-lives for movement and transformation of TCDD in water, air, and soil. ~
Polychlorinated dioxins (PCDDs) now are widely distributed in the environment and cause great concern because of the extreme toxicity of some of the congeners, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). PCDDs form in the manufacture of chlorinated intermediates and pesticides and in the incineration of chlorinated wastes (I). However, application of pesticides containing trace quantities of TCDD does not appear to be a significant source of these compounds (2). During the past 10 years environmental chemists have developed increasingly reliable methods for measuring rates and pathways for movement and transformation of organic chemicals in air, water, and soil (3-5). The objective of this paper is to use new quantitative information about the vapor pressure and photolysis of TCDD to estimate rate constants for its movement and transformation in the environment and, by analogy, the fate of TCDD congeners as well. The volatilization rate of dissolved TCDD from water can be estimated if Henry's constant is known (6). Henry's constant of a low-solubilityorganic compound is given with good accuracy as the ratio of the vapor pressure and aqueous solubility of the pure chemical (7). The solubility of TCDD has recently been measured at about 19.3 parts per trillion (ppt) (6 X 10-l1 M) (8). The vapor pressure of TCDD at 25 "C had not been reported prior to this investigation, and the other new reports of vapor pressure (9, 10) agree well with the value reported here. Several workers report that photolysis of TCDD in organic solvents is rapid with half-lives of 3-4 h and forms less chlorinated congeners that have significantly lower toxicity (11-13). Studies a t Saveso, Italy, following widespread distribution of TCDD in the surrounding soil and air showed that soil plots exposed to sunlight over a period of 10 days in Sept 1976 showed as much as a 10-fold decrease in TCDD content in the grass (14). Recently quantum yields for TCDD were measured for photolysis in water and in hexane (15) from which we can calculate rate constants for aqueous photolysis in sunlight and an upper limit for the atmospheric photolysis rate constant. However, no data are available for photolysis of TCDD in the atmosphere. 490
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Table I. Vapor Pressure Measurement for TCDD at 25 O C sample no.
sampling time, min
average flow rate, 10-~L/min
1 2
2832 2832 2832 2880 2880
7.6 5.9 3.2 4.6 3.0
3 4
5
volume, L
21.5 16.8
9.10 13.4 8.66
weight collected? ~ 1 O - gl ~
vapor pressure,* ~10-'0torr
260 227 111 178 11
7.0 7.7 7.1 7.7 7.7 av 7.4 f 0.4
a Includes corrections for oxidizer efficiency, background, and quench. *Calculated from P = (w/322 g/mol) (62.36 L.torr/(mol K)) (298 K/ V).
Experimental Results and Discussion Volatility. We have now measured the vapor pressure of 14C-labeledTCDD a t 25 "C using the gas saturation technique (16) which involves passing inert gas though a sample slowly enough to saturate the carrier gas with sample vapor. A known volume of nitrogen gas was passed through an activated carbon sorbent trap where the TCDD was collected and then analyzed by combustion to 14C02, and then the TCDD concentrations were quantitated. The vapor pressure was calculated by assuming that the total pressure of a mixture of gases is equal to the sum of the pressures of the component gases. The partial pressure of the sample can be calculated from the ideal gas law, P = nRT/ V, where P is the sample vapor pressure, n is the number of moles collected on the sorbent, and V is the volume of the carrier gas passed through the sorbent. These experimental parameters and results are shown in Table I. The vapor pressure of TCDD was measured at several flow rates ranging from 3 to 7.6 mL/min with no change in vapor pressure, indicating that the carrier gas was indeed saturated. Analysis of the backup charcoal sections show a breakthrough of 0-1670 with no correlation of TCDD loading to flow rate. The observed average vapor torr (5.2% SE). Rordorf pressure was (7.4 f 0.4) X (9) and Schroy et al. (10) report vapor pressures at 30-200 "C which are in good agreement with our value at 25 "C. The volatility of TCDD depends upon the value of its Henry's constant (H) which we estimated from the ratio of measured values of vapor pressure (P) and aqueous solubility (5') (7). torr/6 X M= 12 torr M-l H = P / S = 7.4 X (1)
The volatilization rate of TCDD from water can be estimated from the two-film model using estimated diffu-
00 13-936X/86/0920-0490$0 1.50/0
0 1986 American Chemical Society
~
Tale 11. Parameters Used To Estimate TCDD Volatilization Half-Lives in Water at 25 "C"
rivers L, cm kp, cm/h
kz, cm/h m n
200 8.0 2100 0.7 0.7
~~
Table 111. UV Extinction Coefficients for TCDD in Acetonitrile and Hexane
acetonitriled z, M-l cm-l
lakes or ponds 200 1.8 2100 0.7 1
Source: Smith et al. ( 6 ) .
sivities for TCDD in water and air and assumed levels of turbulence for the water and air phases. Smith et al. (6) assumed that the volatilization rate constant is given by
A, nm 299 304 309 313 314 319 324 340
hexaneb e, M-' cm-'
299 304 309 313 314 319 324 340
6130 7020 7020 6020 6020 3590 1160
0
[ T C D D l = 1.81 X
A, nm
M.
* [ T C D D l = 1.68 X
4240 5000 4850 4100 4043 2230 376 7.0
M.
Table IV. Calculated Sunlight Photolysis Rate Constants for TCDD in Water at 40" Latitude"
where the first term in parentheses accounts for liquidphase mass-transfer resistance and the second term accounts for gas-phase resistance. L is the water depth, k: is the reoxygenation rate constant for the water body, kf is the ratio of molecular diffusivity of the chemical to that of oxygen in water, k! is the mass-transfer coefficient for water vapor in the air phase, Di is the ratio of the molecular diffusivity of the chemical to that of water in air, and m and n are constants that can range from 0.5 to 1.0. For stagnant conditions m, n = 1.0, and for very turbulent conditions m, n = 0.5. The molecular diffusivity of TCDD in air was estimated to be 4.7 X cm2/s by using the WilkeLee method (19, and the molecular diffusivity in water was estimated to be 5.1 X lo4 cm2/s by using the Hayduk-Laudie method (18). Using the values suggested by Smith et at. (6) and listed in Table 11, we calculated volatilization half-lives of about 32 days for ponds and lakes and about 16 days for rivers. Mass transfer resistances in the air and water phases are estimated to be about 2 times as great in the air phase than the water phase for ponds and lakes and about 7 times as great in the air phase for rivers under the conditions listed in Table 11. Persistence of TCDD in soil also can be estimated. If the soil is wet (bulk soil water is present), partitioning of TCDD between the soil solid, water, and air phases can be described by Henry's constant, the bulk soil/water partition coefficient, and the volume fractions of the soil solid, water, and air phases (19). Sarna et al. (20) report octanol/water partition coefficients (KOw) of -3 x lo8, which suggests that TCDD will strongly sorb to the organic fraction of the soil, and other reports confirm that TCDD is tightly bound to soil (14). Thus, in wet soils, TCDD will partition predominantly in the soil solid phase because of its low Henry's constant and its high soil/water partition coefficient. We estimate that, for a soil with 1% organic carbon (per dry weight of soil) and 30% water by volume, about 99.99% of the TCDD in soil will be sorbed on the soil at equilibrium. Therefore, we would not expect TCDD to volatilize by soil air diffusion or wicking, and TCDD will infiltrate the soil very slowly during rainfall percolation. The mobility of TCDD in soil water will increase if cosolvents that can solubilize TCDD are present in the soil water. TCDD in a dry soil will be subject to volatilization in proportion to its saturated vapor pressure if present as pure solid on the soil or to its attenuated vapor pressure if sorbed. If TCDD resides in' the soil/atmosphere boundary layer, neat TCDD may volatilize relatively rapidly but leave a significant amount of residue that is tightly
season winter spring summer fall
ZLh% 63 267 364 147
kpE,
day-1
0.14 0.61 0.78 0.32
h/21
hb
118 27 21 51
"From eq 3 with spectral data from Table 11, 4 = 0.0022 (151, and LAfrom Mill et al. (4).*Averaged for 24-h day.
sorbed to the dry soil. Volatilization of TCDD that has been mixed to a depth of only a few millimeters in dry soil will be extremely slow because of the very slow unsteady-state diffusion of the vapor in the dry soil air phase. Photolysis. We calculated the half-life of aqueous dissolved TCDD in sunlight over four seasons at 40' latitude using eq 3 ( 4 4 ) . Appropriate light intensity (LA) kpE
= d'c'%LX
(3)
values are found in ref 4; 6 values are taken from Table I11 for acetonitrile solution, and 4 = 0.0022, the value reported at 313 nm (15).Table IV summarizes these data. Half-lives for dissolved TCDD in sunlight range from 118 h in winter to 21 h in summer in clear near-surface water under clear skies (24-h days). These half-lives are longer by a factor of 20 than would be observed in hexane (15). Sorption of TCDD to sediments is significant and will have the effect of slowing the apparent volatilization and photolysis rates by the factor 1/(1+ K,[S]) where K , is the sorption equilibrium constant and 5' is the mass of available sediment. TCDD and its congeners are emitted to the atmosphere from incineration sources (1);some may remain in the vapor phase and some may sorb to particulate to later return to soil or surface waters by dry or wet deposition (21). That fraction of TCDD that remains in the vapor phase can undergo photolysis in the same way as TCDD dissolved in the water column or organic solvents, and rate laws governing photolysis of chemicals in the atmosphere are similar to those for photolysis in dilute solution. If the spectral properties and quantum yield for photolysis of TCDD vapor are the same as to those in hexane, the rate constants and photolysis half-lives for TCDD vapor in sunlight should follow the relation (4, 22) d[C]/dt = k p ~ [ C=] 0.05C~hJh (4) where kpE is the atmospheric rate constant in sunlight, uh is the cross section for absorption, and Jh is the atmospheric sunlight intensity. We used uh values calculated from Table I11 and surface Jh values for summer sunlight at 40' latitude (22) to obtain a value of k, = 0.012 min-' and tl/? = 58 min. Thus, TCDD vapor will photolyze rapidly in the atmosphere if the quantum yield is the same Environ. Sci. Technol., Vol. 20, No. 5 , 1986
491
as in hexane; but this estimate is an upper limit. The rate of photolysis of TCDD which is sorbed to particulate may be quite different than that in the vapor phase but may still be important. Townsend (23) suggested that airborne microparticulates transport and mix low-volatility PCDDs and that a major loss process for them may be photolysis. We estimated the rate constant for oxidation of TCDD by OH radical, the dominant transformation process in the atmosphere (5). Rate constants for reaction of OH radical with aromatics are large (>1.0 X lo8 M-ls-l; 1.7 X molecule-’ cm3 s-l), and reactivity usually follows closely the type of substitution on the aromatic ring (24,25). By use of chlorine and phenoxy substituents on the benzene ring, it was estimated that the rate constant for reaction of TCDD vapor with OH radicals is approximately 3 X lo8 M-l s-l. By use of an average concentration of OH radical M (26),the half-life of TCDD in OH oxidation of 3 X in the atmosphere is 200 h, a value that applies only to TCDD vapor. Thus, despite low rates of diffusion of TCDD from soil or sediment, transformation of TCDD to less chlorinated congeners will occur relatively rapidly on photolysis in water and possibly in air. An accurate measurement of the vapor-phase quantum yield for TCDD is needed in order to evaluate the relative importance of photolysis and oxidation in the atmosphere.
Acknowledgments We thank John Davenport for calculating the OH rate constant and J. M. Schroy and Lee Marple for prepublication release of vapor pressure and solubility data. Registry No. TCDD, 1746-01-6.
Literature Cited Hutzinger, 0.;Frei, R. W.; Merian, E., Reggiani, G. Chemosphere 1982, 12, 425. Crosby, D., University of California, Davis, private communication, 1983. Zepp, R. G.; Cline, D. M. Enuiron. Sei. Technol. 1977,11, 359. Mill, T.; Mabey, W. R.; Bomberger, D. C.; Chou, T.-W.; Hendry, D. G.; Smith, J. H. “Laboratory Protocols for
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Evaluating the Fate of Organic Chemicals in Air and Water”. 1982, EPA Final Report EPA 60013-82-022. Atkinson, R.; Lloyd, A. C.; Winges, L. Atmos. Environ. 1982, 16, 1341. Smith, J. H.; Bomberger, D. C.; Haynes, D. L. Chemosphere 1981, 10, 281. Mackay, D.; Shiu, W. J. J. Phys. Chem.Data 1981,10,1175. Marple, L.; Brunck, R.; Throop, L. Environ. Sei. Technol. 1986 20, 180-182. Rordorf, B. F. Thermochim. Acta 1985,85, 435. Schroy, J. M.; Hileman, F. E.; Cheng, S. C., paper presented at the 8th ASTM Aquatic Toxicology Symposium, April 15-17, 1984, Fort Mitchell, KY. Crosby, D.; Wong, A. S.; Plimmer, J. R.; Woolson, E. A. Science (Washington, D.C.) 1971, 173, 748. Liberti, A,; Brocco, D.; Allegrini, I.; Bertoni, G. in “Dioxin”; Cattabeni, F.; Cavallaro, A.; Galli, G., Eds.; SP Medical Scientific Books: New York, 1978; p 195. Desideri, A,; DiMomenico, A.; Vanzati, R.; Taconi, P.; DiMuccio, A. Boll. Chim. Farm. 1979, 118, 274 (Engl.). Buser, H.-R. Chemosphere 1979, 8, 251. Dulin, D.; Drossman, H.; Mill, T. Environ Sci. Technol. 1986,20, 72-77. Fed. Regist. 1980, 45, 77345. Wilke, C. R.; Lee, C. Y. Ind. Eng. Chem. 1955,47, 1253. Hayduk, W.; Laudie, H. AIChE J. 1974,20, 611. Bomberger, D. d.; Gwinn, J. E.; Mabey, W. R.; Tse, D.; Chou, T. W. In “Models for Predicting Fate of Chemicals in the Environment”; American Chemical Society: Washington, DC, 1983; ACS Symp Ser. 225. Sarna, L. P.; Hodge, P. E.; Webster, G. R. B. Chemosphere 1984, 13, 975. Czuczwa, J. M.; Hites, R. A. Abstr. Diu. Environ. Chem. 1983, 23, 74. Peterson, J. T. “Calculated Actinic Fluxes (290-700 nm) for Air Pollution Photochemistry Applications”. 1976, EPA Final Report 60014-76-025. Townsend, D. I. Chemosphere 1983, 12, 637. Davenport, J. E.; Gu, C.-L.; Hendry, D. G.; Mill, T., unpublished results. Hendry, D. G.; Kenley, R. A. “Atmospheric Reaction Products of Organic Compounds”. 1979, EPA Find Report 5601 12-79-001. Singh, H. B. Geophys. Res. Lett. 1977, 4 , 453.
Received for review July 2, 1985. Accepted December 20, 1985. This work was supported under EPA Contract 68-03-2981.
