Environ. Sci. Technol. 1992, 26,2301-2302
CORRESPONDENCE Comment on “Long-Term Sorption of Halogenated Organic Chemicals by Aquifer Material. 1. Equilibrium” S I R In this correspondence we would like to note two minor corrections to our previous paper on tetrachloroethene (PCE) and 1,2,4,5-tetrachlorobenzene (TeCB) sorption by aquifer solids (I)and to add some additional discussion regarding the log KO,value assumed in that work. The first correction is a typographical error in Table I (p 1224)-the value of 34.1 f 1.7 should be replaced by 3.4 f 0.17. The second correction relates to our inappropriate use of the word “solubility” in discussing concentrations of PCE in octanol (first full paragraph, p 1234). The given concentration is not a solubility (PCE is completely miscible in octanol) but rather a calculated value for equilibrium with PCE-saturated aqueous solution. Unstated assumptions were that (1) excem pure-phase PCE is absent and (2) activity coefficients in both phases are constant (concentration-independent). The calculated concentration in octanol is thus an extrapolation from dilute solution, intended solely for comparison with the similarly calculated “solubility” in natural organic matter. A more interesting matter is the appropriateness of the assumed ratio of activity coefficients,as based on literature citations of log KO, for PCE. Since publication of the paper, more recent experimental determinations of KO, have come to our attention. We therefore take the opportunity of this correspondence to examine the wide range of reported KO,values and some of the potential implications for our work. While this reexamination does not significantly alter the prior results or conclusions, the discussion demonstrates how erroneous log KO,values can propagate through the literature and affect data interpretation. In the cited paper (I), we make several calculations (including that discussed in the above correction) that are based on a KO,estimate for PCE taken from the literature [log KO,= 2.6 ( 2 ) ] .This value has been widely cited (e.g., refs 3-9), as has another slightly lower value [log KO, = 2.53 (10); cited in refs 11 and 121. The 2.6 value was reported by the original author (2)to have been calculated in accordance with an estimation method recommended by Hansch et al. (13),while the 2.53 value was experimentally determined (IO). A different calculated value (log KO,= 2.88) has also been widely cited (e.g., refs 14-17 as reported in ref 18). Other experimental estimates of log KO,include 3.38 (19),3.40 (refs 20 and 21 as reported in ref 22) and 3.78 (23). Furthermore, calculation using the most recently published method of the Pomona College Medicinal Database Project yields an estimated log KO, value for PCE of 3.48 (24). Thus, literature values of log KO,for PCE range from 2.53 to 3.78, and it is quite possible that the value (2.6) assumed in our referenced paper (I)is low. Accordingly, we have reexamined some of our results. With log KO, = 3.4, for example, extrapolation from dilute solution (assuming constant activity coefficients) implies that 450 mg 0013-936X/92/0926-2301$03.00/0
of PCE/g of odanol would be at equilibrium with aqueous solution at concentrations just below the PCE solubility limit. This is in contrast to the previous estimate of 72 mg of PCE/g of octanol and is no longer less than the calculated mass-based solubility in Borden organic matter [310 mg of PCE/g of Borden organic matter (OM) (I)]. The higher K,, estimate would also increase the calculated log KmPCE values of Table VI1 (I) by between 0.4 and 0.8 log units, making these values closer to the log K , value estimated from the Borden data [log KO, = 3.6 ( I ) ] . However, the Borden value is still higher than predicted by any of the cited correlations. From the correlation of Curtis et al. (8),for example, we predict that log K , = 2.9. Also considering the high Borden-estimated K , for TeCB (I),we are still led to the conclusion that either the organic matter in Borden aquifer material is an unusually strong sorbent or that surface adsorption is playing a significant role. Thus, a revised estimate for the KO,of PCE would not significantly alter the conclusions of our work. Nonetheless, we find it remarkable that published estimates range over more than a full order of magnitude and that some of the credible experimental estimates are not available in refereed literature. We have been reminded of the need to critically check original sources in this regard and hope that this correspondence will inspire other researchers to do the same. Acknowledgments
We thank Dr. Cary Chiou of the U.S. Geological Survey (Denver, CO) for providing comments which led to the correction on page 1234 and for alerting us to ref 19. Dr. Albert Leo of the Pomona College Medicinal Database Project graciously provided the information cited as refs 20 and 24. Dr. Karen Gruebel of Erler and Kalinowski, Inc. (San Mateo, CA) contributed several useful KO,citations. Registry No. PCE, 127-18-4;TeCB, 95-94-3.
Literature Cited
0 1992 American Chemical Society
Ball, W. P.; Roberts, P. V. Environ. Sei. Technol. 1991,25, 1223-1237. Kenaga, E. E. Partitioning and Uptake of Pesticides in Biological Systems. In Environmental Dynamics of Pesticides; Haque, R., Freed, V. H., Eds.; Plenum Press: New York, 1975. Chiou, C. T.;Freed, V. H.; Schmedding, D. W.; Kohnert, R. L.Environ. Sei. Technol. 1977, 11, 475-478. Hansch, L.; Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biology; Elsevier: Amsterdam, 1979. Schwarzenbach, R. P.; Westall, J. Environ. Sei. Technol. 1981.15, 1360-1367. Verschueren, K. Handbook of Environmental Data on Organic Chemicals, 2nd ed.; Van Nostrand R e i i o l d New York, 1983. Curtis, G. P.; Roberts, P. V.; Reinhard, M. Water Resour. Res. 1986,22, 2059-2067. Curtis, G. P.; Reinhard, M.; Roberts, P. V. In Geochemical Processes at Mineral Surfaces;ACS Sympaium Series 323; Davis, J. A., Hayes, K. F., Eds.; American Chemical Society: Washington, DC, 1986; pp 191-216. Environ. Scl. Technol., Vol. 26, No. 11, 1992
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Information Service: Springfield, VA, 1977. Gould, G.; Hansch, C. Pomona College Medicinal Database Project, Claremont, CA, unpublished experimental data. Hansch, L.; Leo, A. Medchem Project h u e No. 26. Pomona College, Claremont, CA, 1985. Howard, P. H., Ed. Handbook of Environmental Fate and Exposure Data for Organic Chemicals, Vol. 2. Soluents; Lewis Publishers: Boca Raton, FL, 1985. McDuffie, B. Chemosphere 1981,10, 73-83. CLOGP Database and Calculation Program, Daylight Chemical Information Systems, Irvine, CA, 1992.
Wllllam P. Ball" Department of Geography and Environmental Engineering Johns Hopkins University 3400 North Charles Street Baltimore, Maryland 21218-2686
Paul V. Roberts Department of Civil Engineering Stanford University Stanford, California 94305
