Environ. Sci. Techno/. 1983, 17,312-313
(5) Lugg, G. A. Anal. Chern. 1968,40, 1072. ( 6 ) Mackay, D.; Bobra, A.; Chan, D. W.; Shiw, U. Y. Enuiron. Sci. Technol. 1982, 16, 645.
( 5 ) Matsugu, R. S. M.S. Thesis, Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada, 1973.
Charles E. Felgley Department of Environmental Health Sciences School of Public Health University of South Carolina Columbia, South Carolina 29208
SIR: I wish to thank Feigley for discussing the important limitation of the assumption of an ideal solution in my procedure for multicomponent evaporation calculations (1). An earlier version of my paper contained a discussion of the calculation procedure for nonideal solutions, which was deleted for the sake of brevity. To derive equations for nonideal solutions, a modified Raoult’s law (2) can be easily applied: pi
= yixip:
where yi is the liquid activity coefficient of component i . This formulation leads to the following expressions for eq 4 and 5 in the paper:
and N
pi = x:yip;e-h&P,’t/ E [~?e-kyz~,’t] i=l
These expressions can be used to more accurately estimate evaporation rates for a nonideal solution, assuming that information on the activity coefficients yi is available. The ideal solution assumption (yi = 1)is appropriate primarily for liquid solutions containing compounds of similar molecular structure (e.g., straight-chain hydrocarbons). The estimation of the molecular diffusivities and vapor pressures for pure compounds should cause only minor errors in the evaporation calculations. Equations for the estimation of the molecular diffusivity of gases are reasonably accurate (3), and experimental vapor pressures exist for most compounds of environmental significance (4).
The assumption of 50% by weight evaporable oil in the long-term evaporation comparison (Figure 2 of the paper) was not arbitrary but was based completely on the experimental data recorded by Matsugu (5) for that particular evaporation test. After continuous evaporation of a crude oil sample in a wind tunnel for 20 days, 265 g of crude oil remained from the initial sample of 500 g, or 53% by weight nonvolatile compounds. Because the evaporation rate after 20 days was almost negligible, it was assumed that 50% by weight was a reasonable estimate of the nonvolatile fraction. Literature Cited (1) Drivas, P. J. Environ. Sci. Technol. 1982, 16, 726-728. (2) Denbigh, K. “The Principles of Chemical Equilibrium”; Cambridge University Press: London, 1968. (3) Lyman, W. J.; Reehl, W..F.; Rosenblatt, D. H. “Handbook of Chemical Property Estimation Methods”; McGraw-Hik New York, 1982. (4) Verschueren, K. “Handbook of Environmental Data on Organic Chemicals”; Van Nostrand-Reinhold: Princeton, NJ, 1977. 312
Environ. Sci. Technol., Vol. 17, No. 5, 1983
Peter J. Drivas Energy Resources Co./California La Jolla, California 92037
Comment on “Characterization of Organic Contaminants In Envlronmental Samples Associated with Mount St. Helens 1980 Volcanic Eruption” SIR: Pereira et al. ( l a ) report the detection of monoand dichlorobenzoic acid methyl ester isomers and pentachlorobiphenyl isomers (P,CBs) in samples of fly ash taken from the Mt. St. Helens ash cloud deposits and suggest “that aromatic compounds produced as a result of the pyrolysis of plant and soil organic matter, in the presence of vaporized inorganic chloride salts, were chlorinated in the high-temperature eruption zone.” They cite the work of Olie et al. ( I b ) in support of their hypothesis, while acknowledging that the ash samples could have become contaminated with PCBs. I do not believe that the data suggest de novo synthesis of chloroaromatic compounds from cellulose carbon and inorganic chloride salt vapors. Olie et al. did not establish that a de novo synthesis of polychlorinated aromatic molecules occurs in the trace chemistries of fire via experiment; rather they attribute such an experiment to Lunde and co-workers based on a personal communication. Subsequently Ahling, Bjerrseth, and Lunde (2) published the results of a study demonstrating the de novo synthesis of polychlorinated aromatics from the incomplete combustion of polyvinyl chloride. In fact Olie et al. noted that “The presence of relatively large amounts of chlorophenols in flue gas condensate makes it likely that these compounds are the precursors of chlorodibenzodioxins in the incinerator process.” The de novo formation of aromatic chlorine compounds was considered possible. Support for this route of chlorodioxin synthesis came from the relatively large quantities of polychlorinated benzenes in the fly ash samples examined, according to the authors. The formation of polychlorinated dibenzofurans (PCDFs) and polychlorinated benzenes from the pyrolysis of PCBs has been demonstrated by Buser et al. ( 3 ) . Lustenhouwer and co-workers (4) identified the most probable precursors of polychlorinated dibenzo-p-dioxins (PCDDs) found in municipal incinerator fly ash as polyvinylchloride, polychlorinated benzenes, polychlorinated phenols, PCBs, and 2,4,5-T derivatives but could not rule out the de novo synthesis of PCDDs and PCDFs from organic carbon and inorganic chloride. On the basis of a spot survey of various combustion sources, Bumb et al. (5)concluded that PCDDs in general (2,3,7,8-TCDD) and the 2,3,7,8-tetrachlorodibenzo-p-dioxin isomer in particular are generated de novo in the trace chemistries of fire from organic carbon and inorganic chloride vapors. Unfortunately, the fuel feedstocks and precombustion air were not analyzed for the precursor molecules identified above. More recent studies conducted by Dow Chemical Co. scientists identified 2,3,7,8-TCDD in the soot, tar, and ash of stoves fired with allegedly uncontaminated wood (6). Nevertheless, because the in-
0013-936X/83/0917-0312$01.50/0
0 1983 American Chemical Society
take air was not examined for the precursor molecules, the conclusion that 2,3,7,8-TCDD is generated de novo in significant quantities from organic carbon and inorganic chloride remains in great doubt. PCBs are ubiquitous (7-9). They have been detected in the ambient air from various remote and industrialized areas in the 0.1-5 ng/m3 concentration range (10). Of the Aroclor products whose manufacture predominated in the years 1963-1970,1242 through 1260 (11),the predominant PCB congener appears to be the pentachlorobiphenyl(12). The mono- through tetrachlorinated PCB congeners are relatively rapidly biodegraded (13-1.9, whereas the more heavily chlorinated congeners with chlorine substitution at the 2- and 4-positions on the phenyl are likely to be more rapidly photodechlorinated than the mono- through pentachlorinated congeners (16-18a). (More recent studies indicate that the rates of photodechlorination of PCBs in water are far more isomer specific than previously suspected, however (18b).)The availability of soil- and sediment-sorbed PCB congeners decreases with the increasing degree of chlorination (19). Differential weathering and immobilization of the various PCB congeners thus appear to favor the survival and propagation of the pentachlorobiphenyl isomers (P5CBs). PCBs sorb to foliage (20). The methyl esters of the mono- and dichlorobenzoic acids detected in the volcanic ash are probably biodegradation products of the lesser chlorinated PCBs (21). Although Dow scientists claim to have detected 2,3,7,8-TCDD in samples of gases from the combustion zone of a pure methane flame into which sodium chloride has been introduced (221,the results of these experiments have yet to be published in the peer-reviewed scientific literature. If these studies suffer from the same flaw as their earlier studies-failure to analyze the precombustion air for chlorodioxin precursor molecules-then the results will again prove inconclusive. Given the degree of contamination of the air at the Dow Michigan Division site with chlorodioxins and their photo-, pyro-, and thermochemical precursors (23-24),the presence of detectable concentrations of 2,3,7,8-TCDD in combustion source emissions at that site is not surprising. If the PCB congeners detected in the Mt. St. Helens fly ash deposits derive from modern sources of chlorine and carbon, then the ratios of 14Cto 12Cwould be consistent with those found in living things. On the other hand, if the source of the P5CBs were commercial manufacture from fossil petroleum feedstocks, the I4C to 12C ratios would be much lower (25). Were combustion the source of the P5CBs detected in the volcanic ash, shouldn’t one expect to find detectable concentrations of Olie’s hexachlorobenzene, Buser’s PCDFs, and Bumb’s 2,3,7,8-TCDD?. Given the ubiquitous contamination of the planet with PCBs, the discovery of P&B isomers in the volcanic ash of the Mt. St. Helens eruption is not evidence of the de novo synthesis of chlorinated aromatics from organic carbon and inorganic chloride. An analysis of the 14Cto 12Cratios may permit the identification of the source of the P5CB isomers detected.
Although I am an employee of the Michigan Department of Natural Resources, the views expressed and conclusions drawn herein are my own and should not be construed to be those of my employer. Literature Cited (a) Pereira, W. E.; et al. Enuiron. Sci. Technol. 1982, 16, 387-396. (b) Olie, K.; Vermueulen, P. L.; Hutzinger, 0. Chemosphere 1977, 455-459. Ahling, B.; Bjorseth, A.; Lunde, G. Chemosphere 1978,10, 799-806. Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, 9, 501-522. Lustenhouwer, J. W. A.; Olie, K.; Hutzinger, 0. Chemosphere 1980, 9, 501-522. Bumb, R. R.; et al. Science (Washington,D.C.)1980 210, 385-390. Nestrick, T.; Lamparski, L. Second International Symposium on Chlorinated Dioxins and Related Compounds, Arlington, VA, Oct 1981. Atlas, E.; Giam, C. S. Science (Washington,D.C.)1981,211, 163-165. Nisbet, I. C. T.; Sarofim, A. F. Enuiron. Health Perspect. 1972, 21-38. Risebrough, R. W.; et al. Nature (London) 1980 20, 1098-1102. Eisenreich, S. J.; Hollod, G. J.;Johnson, T. C. Enuiron. Sci. Technol. 1979,13, 569-573. Nisbet, I. C. T.; Sarofim, A. F. Environ. Health Perspect. 1972 21-38. National Research Council, “Polychlorinated Biphenyls”; National Academy of Sciences: Washington, D.C., 1979. Furukawa, K.; Tonomura, K.; Kamibayashi, A. Appl. Enuiron. Microbiol. 1978, 35, 223-227. Wong, P. T. S.; Kaiser, K. L. E. Bull. Enuiron. Contam. Toxicol. 197% 13, 249-256. Tucker, E. S.; Seager, V. W.; Hicks, 0. Bull. Enuiron. Contam. Toxicol. 1975, 14, 705-713. MacNeil, J. D., Safe, S.; Hutzinger, 0. Bull. Enuiron. Contam. Toxicol. 1976, 15, 66-77. Safe, S., et al. In “Identification and Analysis of Organic Compounds in Water”; Keith, L., Ed.; Ann Arbor Science: Ann Arbor, MI, Chapter 3, pp 35-47. (a) Safe, S.; Hutzinger, 0. Nature (London) 1971, 232, 641-642. (b) Simmons, M., personal communication, Oct 1982. Haque, R.; Schmedding, D. J. Enuiron. Sci. Health 1976, B11, 129-137. Buckley, E. H. Science (Washington, D.C.) 1982, 216, 520-522. Shiaris, M. P.; Sayler, G. S. Enuiron. Sci. Technol. 1982, 16, 367-369. Wilson, D., personal communication, May 1981. Washington, L. J., Jr.; Dow Michigan Division, letter to B. White, Michigan Department of Natural Resources, Jan 1980. Bumb, R.; et al. Science (Washington, D.C.)1980, 210, 385-390. Voorhees, K. J.; et al. Anal. Chem. 1981, 53, 1463-1465.
Larry E. Flnk West Michigan Avenue Lansing, Michigan 48917
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