Résumé :
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The effect of exchangeable cation - Na+ and Ca2+ - on the diffusive transport of I-, Sr2+ and H-3 (as HTO) in compacted bentonite was examined using a through-diffusion method. Total intrinsic diffusion coefficients, D-i, were determined from the steady-state flux of the diffusants through the clays, and apparent diffusion coefficients, D-a, were obtained from the time lag technique. The clays were compacted to a dry bulk density of 1.3 Mg/m(3), and Na-bentonite was saturated with a solution of 100 mol NaCl/m(3) and Ca-bentonite with one of 50 mol CaCl2/m(3). The D-i values for all diffusants are 2 to 6 times higher in the Ca- than Na-day. We attribute this to the larger quasicrystal, or particle, size of Ca- compared to Na-bentonite. Hence, Ca-bentonite has a greater proportion of relatively large pores; this was confirmed by Hg intrusion porosimetry. This means the diffusion pathways in Ca-bentonite are less tortuous than those in Na-bentonite.
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