Comparison of Measured and Calculated Diffusion Coefficients for Iodide in Compacted Clays

Abstract

AbstractAn understanding of the behaviour of contaminants in compacted clays is important in assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here the diffusion and sorption behaviour of I−with selected compacted clays is examined (129I is a relatively long-lived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients,D, and distribution coefficients,Kd, were measured for I−with four clays: bentonite, Lake Agassiz clay (a glacial lake clay composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For the diffusion experiments the clays were compacted to a dry bulk density, ρ, of ∼1.2 Mg m−3. The mean measuredDvalues,Dm, were as follows: bentonite, 310 µm2s−1; Lake Agassiz clay, 0.62 µm2s−1; illitesmectite, 190µm2s−1; and kaolinite, 74µm2s−1. The measured values were compared with those calculated,Dc, from the following model:D=Doτan/(n+ρKd), whereDois the diffusion coefficient in pure bulk water,τathe apparent tortuosity factor, andnthe solution-filled porosity of the clay. Except for the Lake Agassiz clay, whereDmwas about an order of magnitude lower thanDc, theDmandDcvalues agreed within a factor of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays, and this could affectτa, which may be overestimated in the model.