Diffusion of humic colloids in compacted clay and the influence on uranium migration

The diffusion of humic acid (HA) in Georgia kaolinite KGa-1B was studied. The compacted clay plug was conditioned with a 0.01 M NaClO4 + 1 mM NaN3 solution at pH 5.2. The effective porosity ε and the effective diffusion coefficient De of water were determined using tritiated water (HTO). A synthetic 14C-labeled HA type M42 with a concentration of 11.0 mg/L and a specific activity of (17.0 ± 0.5) MBq/g was used as tracer. We applied a steady-state through-diffusion technique with constant gradient. The particle size distribution of HA was determined by ultrafiltration.

The migration of HA in compacted clay is governed by diffusion. However, compared to an ionic tracer some characteristics are observed. The De values found for HA (2·10^13 to 2·10^12 m^2/s) are about two to three orders of magnitude lower than that of HTO. Likewise the rock capacity factor α(HA) amounts to only 10 % to 50 % of ε. The spatial extension of the humic colloids constricts their mobility in the narrow pore space thus increasing the tortuosity of the diffusion path and decreasing the pore volume accessible for HA. This phenomenon is referred to as size exclusion effect. In [1] a significant adsorption of HA on KGa-1B was found (Kd = 680 ml/g at pH 5.2) which would result in a value for α above unity. Obviously, the size exclusion dominates the HA migration.
The particle size distributions of the high and the low concentration reservoir differ significantly. The mean particle size shifts from about 50 kD in the high to less than 1 kD in the low concentration reservoir. Only small particles are able to pass the pore system. This is in agreement with the HA distribution in the clay plug where most of the tracer was found at the high concentration boundary.