Comparison of measured and calculated diffusion coefficients for iodide in compacted clays

Abstract

A B S T R A C T: An 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 an d sorption behaviour of I-with selected compacted clays is examined (129I is a relatively longlived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients, D, and distribution coefficients, Ka, 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, p, of -1.2 Mg m -3. The mean measured D values, Din, were as follows: bentonite, 310 ~tm 2 s-l; Lake Agassiz clay, 0.62 ~m 2 s 1; illitesmectite, 190~tm2s a; and kaolinite, 74~tm2s l. The measured values were compared with those calculated, De, from the following model: D = Do~an/(n + pKd), where D O is the diffusion coefficient in pure bulk water, ra the apparent tortuosity factor, and n the solution-filled porosity of the clay. Except for the Lake Agassiz clay, where Dm was about an order of magnitude lower than De, the Dm and D~ values 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 affect ra, which may be overestimated in the model.