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 ∼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°C. The Dc for Sr²⁺ was calculated from Dc = Dₒτ/(1 + ρbKd/θᵥ), where Dₒ is the diffusion coefficient of Sr²⁺ in free solution; τ is a tortuosity factor, obtained for the compacted soil from Cl⁻ diffusion experiments; ρb is the dry bulk density; θᵥ 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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