Diffusion of Eu(III) in compacted bentonite - Effect of pH, solution concentration and humic acid

The effect of pH, Eu(III) solution concentration and humic acid on the diffusion of Eu(III) in compacted bentonite (ρ_b=1000±30kg/ m³) was studied with "in-diffusion" method at an ionic strength of 0.1M NaClO₄. The results (K_d values from the first slice and theoretical calculation, apparent and effective diffusion coefficients) derived from the new capillary method are in good agreement with the literature data under similar conditions, and fit the Fick's second law very well. The results suggest that the diffusion of Eu(III) is dependent on pH values and independent on solution concentration in our experimental conditions. Humic acid forms precipitation/complexation with Eu(III) at the surface of compacted bentonite and thus deduces the diffusion/transport of Eu(III) in compacted bentonite. The K_d values in compacted bentonite are in most cases lower than those in powdered bentonite obtained from batch experiments. The difference between the K_d values from powdered and compacted bentonite is a strong function of the bulk density of the bentonite. The results suggest that the content of interlaminary space plays a very important role to the diffusion, sorption and migration of Eu(III) in compacted bentonite.