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

Abstract

AB ST R ACT : We studied the diffusive transport of Cs, Np, Am and Co in compacted sandbentonite mixtures by using the through-diffusion method. The experiments for Cs were performed under various aqueous compositions. Effective diffusivity (D e ) values of 4.7610 À10 to 5.9610 À9 m 2 s À1 were obtained with a somewhat large variation. Apparent diffusivity (D a ) values, on the other hand, showed less variation, ranging from 2.0610 À12 to 6.2610 À12 m 2 s À1 . 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 l À1 (NH 4 ) 2 CO 3 /Na 2 S 2 O 4 solution.

Diffusion and retention behaviour of Cs in illite-added compacted montmorillonite

“…The values for D e of Cs obtained by throughdiffusion experiments were 7.2-8.0 × 10 -10 m 2 /s in simulated sea water (Suzuki et al, 2007) and 7.8 × 10 -10 m 2 /s at 0.5 mol/dm 3 NaCl (Sawaguchi et al, 2013). These experiments were carried out for a mixture of quartz sand and Kunigel V1 with a mass fraction of 3:7 compacted to a dry density of 1.6 mg/m 3 .…”

“…The surface diffusion effect is often characterized by the greater diffusivity of cations than that of electrically neutral species caused by the diffusion of mobile cations sorbed on the surface of clay minerals (Van Loon et al, 2004;Melkior et al, 2005;Suzuki et al, 2007;Wersin et al, 2008;Sawaguchi et al, 2013;Glaus et al, 2015a). In the present study, the flux of Cs attributed to the surface diffusion, J Cs SD , was defined by the following equations:…”

“…On the other hand, the diffusivities of Na + , Sr 2+ , Co 2+ and Zn 2+ have been reported to be enhanced in compacted illite (Glaus et al, 2010(Glaus et al, , 2015a. For Cs, enhanced diffusivities have been obtained in clay rocks (Van Loon et al, 2004;Melkior et al, 2005;Wersin et al, 2008) and in compacted bentonite (Muurinen et al, 1985;Kim et al, 1993;Suzuki et al, 2007;Sawaguchi et al, 2013). This phenomenon is often referred to as the surface diffusion effect, which has been explained by the increase in mobile cations concentration due to the sorption of cations on the sites to which sorbed cations are not fixed.…”

Overview of the clay mineralogy studies presented at the ‘Clays in natural and engineered barriers for radioactive waste confinement’ meeting, Montpellier, October 2012

“…19) Sawaguchi et al (2013) investigated the diffusion of Cs, Np, Am, and Co in compacted sand-bentonite mixtures. By comparing the concentration gradients of both adsorbed and bulk Cs they assumed surface diffusion to play a major role with respect to the Cs migration.…”