Diffusion of Cs, Np, Am and Co in compacted sand-bentonite mixtures: evidence for surface diffusion of Cs cations

摘要

We studied the diffusive transport of Cs, Np, Am and Co in compacted sand-bentonite mixtures by using the through-diffusion method. The experiments for Cs were performed under various aqueous compositions. Effective diffusivity (D_e) values of 4.7 × 10~(-10) to 5.9 × 10~(-9) m~2 s~(-1) were obtained with a somewhat large variation. Apparent diffusivity (D_a) values, on the other hand, showed less variation, ranging from 2.0 × 10~(-12) to 6.2 × 10~(-12) m2 s~(-1). The results indicated that diffusive flux was proportional to the concentration gradient on the basis of the amount of Cs in the unit volume of the compacted sand-bentonite mixtures rather than the Cs concentration gradient in pore water. Because the former concentration gradient in the mixtures was nearly equal to that of adsorbed Cs, the diffusion of Cs in the mixtures was probably dominated by the concentration gradient of the Cs adsorbed on the mixtures. In addition, the effective/apparent diffusivity of ~(237)Np(Ⅳ) and apparent diffusivity of ~(241)Am(Ⅲ) and ~(60)Co(Ⅱ) in the mixtures were determined in 0.3/0.03 mol 1~(-1) (NH_4)_2CO_3/Na_2S_2O_4 solution.