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CORRESPONDENCE Comment on “Interactlon of Clay Soils with Water and Organic Solvents: Implications for the Disposal of Hazardous Wastes” SIR: The article “Interaction of Clay Soils with Water and Organic Solvents: Implications for the Disposal of Hazardous Wastes” ( I ) brings out the importance of shrink-swell testing on clay liners and soils of the type encountered at storage sites for the disposal of hazardous wastes but raises some questions that we would like to address at this time. We also feel that some comments are necessary. (1)From a study of the data presented in their Table I it is felt that the liquid limit does not seem to correlate with the clay minerals present. It is well-known that montmorillonite has very high liquid limit (510 for Ca montmorillonite) (2). It has also been reported that mixtures of clay minerals or clay-sand have liquid limits proportional to their percentages (3). In the light of the above, for Ranger Shale with 50.5% of montmorillonite the reported value of liquid limit of 46% appears to be very low. Also CEC of this clay soil is 54.4 mequiv/100 g, while the dominant cation Ca itself is reported as 59 mequiv/100 g, which leads to the presence of free Ca ions. Fire clay with 44 f 1% of clay fraction is reported to be almost nonplastic (plasticity index of l%)! In general the reported values of plasticity index are rather very low for all the clay soil studied. The optimum moisture content for Kosse Kaolinite is reported as 31%, which is very high when compared to the value of 17.5% reported for Ranger Shale. The procedure employed to determine the percentages of various clay minerals would be beneficial to the readers. (2) The results of equilibrium values for swelling of the so& in water and in organic solvents have been interpreted on the basis of octanol-water partition coefficient. Similar data obtained by Sridharan and Rao ( 4 ) have been more successfully interpreted on the basis of diffused doublelayer theory and fabric of clays. If the authors have chosen pure kaolinite for their studies, the observed behavior of swelling and shrinkage in various fluids could not have been explained on the basis of log Kow alone. (3) From their Figure 4, it appears that percent swell of all the clay soils against log KOw is almost unique. We feel that the percent swelling can more satisfactorily be explained on the basis of dielectric constant itself, It has been reported that the dielectric constant of fluid plays a dominant role in changing the properties of clays (5-7). Registry No. HzO, 7732-18-5.
Literature Cited (1) Green, W. J.; Lee, G. F.; Jones, R. A.; Polit, T. Enuiron. Sci. Technol. 1983, 17, 278. (2) Lambe, T. W.; Whiteman, R. V. “Soil Mechanics”; Wiley: New York, 1969; p 33. (3) Seed, H. B.; Woodward, R. J.; Lundgren, R. J. Soil Mech. Found. Diu., Am. SOC. Civ. Eng. 1964, 90, 75. (4) Sridharan, A.; Rao, G. V. Geotechnique 1973, 23, 359. (5) Sridharan, A.; Rao, G. V. Istanbul Conf. SM. FE. 1975,1, 75. 00 13-936X/84/09 18-0217$01.50/0
(6) Sridharan, A.; Jayadeva, M. S. Geotechnique 1982,32,133. (7) Sridharan, A.; Rao, G. V. Geotechnique, 1979, 29, 177.
P. V. Sivapullalah, A. Sridharan” Department of Civil Engineering Indian Institute of Science Bangalore 560 012, India
SIR: Sivapullaiah and Sridharan raise two important issues with respect to our paper on clay soil-organic solvent interactions. The first concerns a perceived inconsistency between the reported clay content of our samples and the engineering properties which these samples exhibit. The second centers on the interpretation of our bulk swelling data. (1)Regarding the first point, there appears to be a serious misinterpretation of our Table I ( I ) . Sivapullaiah and Sridharan claim that Ranger Shale contains 50.5% montmorillonite and that its liquid limit of 46% is “very low”. In fact, the entire clay content of Ranger Shale is only 40%, 50.5% of which is montmorillonite clay. Thus, Ranger Shale contains only 20.2 % montmorillonite. Surely it is a mistake to suggest that because one sample of calcium montmorillonite has a published liquid limit of 510, all clay soils should have liquid limits proportional to this and to their montmorillonite fractions. While montmorillonite content may indeed be useful in indicating broad ranges of engineering properties, it cannot, to our knowledge, be predictive. Factors such as the type and amount of other minerals present, shape and distribution of particles, pore water chemistry, and organic content all influence Atterberg limits (2). White (3),for example, has studied a Ca montmorillonite with a liquid limit of only 177 and a plastic limit of 63, and Perloff and Baron ( 4 ) have compiled data on liquid limits for Ca montmorillonites which range from 123 to 166. Ranger Shale, with only 20% montmorillonite, should have a much lower liquid limit than these pure clay samples, and we do not find its value at all exceptional. The plasticity index (P.I.) for Fire Clay is quite low, as Sivapullaiah and Sridharan rightly point out, but this can be attributed in part to the fact that its clay fraction is largely kaolinite. Seed et al. ( 5 ) have found that kaolinite clays have low plasticity indexes, and we consider our value qualitatively consistent with their observations. In addition to clay content, Ode11 et al. (6)have shown that P.I. bears a direct dependence on percent organic carbon. The extremely low organic content of all of our clay soils is another factor contributing to their low P.I. values. In their comments, Sivapullaiah and Sridharan generally imply that a strict correlation exists between clay mineralogy and soil behavior. However, Mitchell points out that “a knowledge of composition is useful to indicate probable ranges of geotechnical properties and their variability and sensitivity to changes in environmental conditions” (2). We emphasize here ”probable ranges” and suggest that once proper attention is given to the low expandable clay content of our samples and to their low organic content, our data lie well within these ”probable ranges”. In the absence of a predictive model for Atterberg limits, there is no substitute for the actual measurements themselves, methods for which can be found in ref 7.
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(2) Concerning the second point, Sivapullaiah and Sridharan claim that percent bulk swelling of clay soils in organic solvents can be explained satisfactorily in terms of fluid dielectric constant. We have considered this possibility elsewhere (7) and have noted that a rough linear relationship does in fact exist between percent swell and dielectric constant for the soils and solvents used in our study. This relationship is particularly good for Ranger Shale (see Table I1 in our paper). However, the choice between dielectric constant and octanol-water partition coefficient (KoW)as the appropriate variable with which to model swelling behavior would seem to depend on the nature of the swelling mechanism itself. In this connection, Barshad (8) examined the effect of dielectric constant on interlayer spacing and found that the degree of swelling of dehydrated montmorillonite and vermiculite increased with increasing dielectric constant when clays were immersed in polar liquids of similar dipole moments. In these systems swelling could be explained in terms of interlayer penetration by solvent molecules. By contrast, under the conditions of our study (7), we found no X-ray evidence of interlayer penetration by organic solvent molecules. In the absence of such evidence, shrinking was attributed to the simple movement of pore waters out of the clay soils, that is, to dehydration, rather than to the collapse of expanded silicate lattices. Given this proposed mechanism, it seemed (and seems) appropriate to correlate shrinking and swelling with some measure of solvent compatibility or incompatibility with water. Kowadequately serves this purpose. It should be stressed, however, that the mechanism proposed in our paper is still hypothetical, since we did not analyze either the immersion fluid or the pore fluid during or after swelling. If shrinkage involves pore water migration out of the soil, this should be evident from the water content of the immersion fluid.
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We believe that bulk shrink-swell studies of other clay soils, over a wide range of organic solvent types, plus mechanistic studies of the shrinkage phenomenon itself, will be necessary before either dielectric constant or Kow can confidently be singled out as the essential liquid property affecting bulk clay soil behavior. Given the importance of storing and effectively containing hazardous liquid wastes, and of understanding how these wastes modify soil behavior, such studies would seem highly desirable. Registry No. HzO,7732-18-5.
Literature Cited (1) Green, W. J.; Lee, G. F.; Jones, R. A.; Palit, T. Environ. Sci. Technol. 1983, 17, 278. (2) Mitchell, J. K. "Fundamentals of Soil Behavior"; Wiley: New York, 1976; Chapter 9. (3) White, W. A. Am. Mineral. 1949, 34, 508. (4) Perloff, W. H.; Baron, W. "Soil Mechanics: Principles and Applications"; Ronald Press: New York, 1976; p 420. (5) Seed, H. B.; Woodward, R. J.; Lungren, R. J. Soil Mech. Found. Diu., Am. SOC. Civ. Eng. 1964, 90, 107. (6) Odell, R. T.; Thornburn, T. H.; McKenzie, L. J. Soil Sci. SOC.Am. Proc. 1952,24, 297. (7) Green, W. J.; Lee, G. F.; Jones, R. A. "Impact of Organic Solvents on the Integrity of Clay Linen for Industrial Waste Disposal Pits: Implication for Groundwater Contamination". Robert S. Kerr Environmental Laboratory, Ada, OK, 1980, report to U.S.EPA. (8) Barshad, I. Soil Sci. SOC.Am. Proc. 1952, 16, 176.
Wllllam J. Green, G. Fred Lee" R. Anne Jones, l e d Pallt Department of Civil Engineering Texas Tech University Lubbock, Texas 79409
