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Diffusion of Strontium and Chloride in Compacted Clay-based Materials¹

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 Sr²⁺ in a compacted mixture of bentonite (50 wt%) and sand. D_m 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 D_c for Sr²⁺ was calculated from D_c = D_o/(1 + _bK_d/_v), where D_o is the diffusion coefficient of Sr²⁺ in free solution; is a tortuosity factor, obtained for the compacted soil from Cl^- diffusion experiments; _b is the dry bulk density; _v is the volumetric moisture content; and K_d is a distribution coefficient that accounts for the interaction of Sr²⁺ with the soil. Both D_m and D_c for Sr²⁺ 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.

¹ Contribution from Dep. of Earth Sciences, Univ. of Waterloo, Waterloo, Ontario, and Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba. Paper issued as AECL-9294.

² Research Project Supervisor and Professor, respectively, Dep. of Earth Sciences, Univ. of Waterloo, Waterloo, Ontario, Canada N2L 3G1; and Research Scientist, Geochemistry and Waste Immobilization Div., Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada R0E 1L0.

Received for publication December 10, 1986.