Diffusive transport through compacted Na- and Ca-bentonite

The effect of exchangeable cation — Na+ and Ca 2+ — on the diffusive transport of I−, Sr 2+ and 3H (as HTO) in compacted bentonite was examined using a through-diffusion method. Total intrinsic diffusion coefficients, Di, were determined from the steady-state flux of the diffusants through the clays, and apparent diffusion coefficients, Da, were obtained from the time lag technique. The clays were compacted to a dry bulk density of 1.3 Mg/m3, and Na-bentonite was saturated with a solution of 100 mol NaCl/m3 and Ca-bentonite with one of 50 mol CaCl2/m3. The Di values for all diffusants are 2 to 6 times higher in the Ca- than Na-clay. 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. Moreover, in some cases the effective porosity, or the porosity available for diffusive transport, may be greater in Ca-bentonite. The Da values are inversely proportional to the distribution coefficients of the diffusants with the clays.