Diffusion of strontium and chloride in compacted clay-based materials
1987
Robin, M.J.L. | Gillham, R.W. | Oscarson, D.W.
This study examines the effect of pore solution composition and temperature on the measured and calculated diffusion coefficients (Dm and Dc, respectively) for Sr2+ in a compacted mixture of bentonite (50 wt%) and sand. Dm for Sr2+ and Cl- in the soil was determined at a dry bulk density of approximately 1.7 Mg m-3 using deionized water (DW) and two synthetic groundwater solutions (SGW) with ionic strengths of 0.27 and 1.4 mol L-1 to saturate the soil, and at 25, 60, and 90 degrees C. The Dc for Sr2+ was calculated from Dc = Do tau/(1 + pbKd/theta tau), where Do is the diffusion coefficient of Sr2+ in free solution; tau is a tortuosity factor, obtained for the compacted soil from Cl- diffusion experiments; pb is the dry bulk density; theta tau is the volumetric moisture content; and Kd is a distribution coefficient that accounts for the interaction of Sr2+ with the soil. Both Dm and Dc for Sr2+ increased with an increase in the ionic strength of the saturating solutions and with an increase in temperature. The Dm values were, however, always greater than Dc values by a factor of 6.2 to 8.0 for DW and 1.3 to 1.8 for the SGW with an ionic strength of 1.4 mol L-1. This difference is believed to be the result of obtaining Kd values in solutions that differed significantly from the pore solution of the compacted soil. Nevertheless, the results indicate that Dc values, which are often more convenient and less time-consuming to obtain than Dm values, can at least be used to establish relationships and trends between the diffusion rates of some contaminants in clay-based materials and the properties of these materials.
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