The influence of diffusant concentration, c 0 , on effective diffusion coefficients, D e , was examined under steady-state conditions in dense clays for c 0 ranging from ~10" l° to 10~6 mol/1. Two clays, Avonlea bentonite and Lake Agassiz clay, were used, and D e values were measured for I" and Cs + using a through-diffusion method. The clays were compacted to a dry density of approximately 1.35 Mg/m 3 and saturated with a synthetic groundwater solution having an ionic strength of 0.22 mol/1. The D e values for both I" and Cs + were about 2x10" 11 m 2 /s and, in accordance with Fick's law, were not markedly affected by c 0 . The results also indicate that surface diffusion is not an important transport mechanism for Cs + . Apparent diffusion coefficients, D a , were also obtained from the experiments and were approximately 7 χ 10" 10 m 2 /s for I" and 4 χ 10 ~1 3 m 2 /s for Cs + . The lower D a value for Cs + is attributed to its greater sorption on the clay.
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