Diffusion of Strontium and Chloride in Compacted Clay‐based Materials

Abstract

Abstract This study examines the effect of pore solution composition and temperature on the measured and calculated diffusion coefficients ( D m and D c , respectively) for Sr 2+ in a compacted mixture of bentonite (50 wt%) and sand. D m for Sr 2+ 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 D c for Sr 2+ was calculated from D c = D o τ/(1 + ρ b K d /θ v ), where D o is the diffusion coefficient of Sr 2+ in free solution; τ is a tortuosity factor, obtained for the compacted soil from Cl ‐ diffusion experiments; ρ b is the dry bulk density; θ v is the volumetric moisture content; and K d is a distribution coefficient that accounts for the interaction of Sr 2+ with the soil. Both D m and D c for Sr 2+ increased with an increase in the ionic strength of the saturating solutions and with an increase in temperature. The D m values were, however, always greater than D c 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 K d values in solutions that differed significantly from the pore solution of the compacted soil. Nevertheless, the results indicate that D c values, which are often more convenient and less time‐consuming to obtain than D m 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.