Diffusive transport in compacted mixtures of clay and crushed granite

Diffusion coefficients are important parameters for predicting migration rates of contaminants through earthen barrier materials used in many waste containment strategies. Apparent diffusion coefficients, D, were measured for Tc and I in compacted 1:3 mixtures of Lake Agassiz clay and crushed granite. Technetium-99 and 129I are two of the most important radioisotopes present in nuclear fuel waste. Lake Agassiz clay is a glacial lake clay composed principally of smectite, illite, quartz and kaolinite. Before being mixed with clay, crushed granite was separated into the following particle size fractions: <150, 150 to 300, 300 to 850, 850 to 2000, and 2000 to 4750 μm. The dry density of the mixes was about 2 Mg/m3 and they were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution having an effective ionic strength of 220 mol/m3. The mean D values (n = 4) for Tc in the mixes ranged from 20 μm2/s when the granite particle size was 2000 to 4750 μm to 0.19 μm2/s when it was <150 μm. In clay alone, the mean D value was 59 μm2/s. The decrease in D with decreasing granite particle size is largely attributed to an increase in the extent of reduction of Tc(VII) to Tc(IV) and its subsequent sorption on Fe(III) oxyhydroxides present in granite. The reductants are probably Fe(II)-bearing minerals, such as magnetite, in granite. The finer the granite particle size, the greater the reactive surface area. For I, clay controls the diffusion process and granite is essentially inert filler. The mean D values for I are about 3 μm2/s in all cases. The results show that, depending on the environmental conditions and the nature of the diffusant, crushed granite in compacted clay-based barriers can markedly slow mass transport. (orig.)