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Yotsuji, Kenji; Tachi, Yukio; Okubo, Takahiro*
CMS Workshop Lectures, Vol.21, p.251 - 257, 2016/06
We have developed integrated sorption and diffusion model (ISD model) for assessment of diffusion parameters consistent with sorption processes in compacted bentonite. The ISD model gives consistent consideration to porewater chemistry, sorption and diffusion processes in compacted bentonite. The diffusion component based on the electric double layer theory and the homogeneous pore model in the ISD model accounts consistently for cation De overestimation and anion exclusion in narrow pores. The current ISD model can quantitatively account for diffusion of monovalent cations and anions, however, the model predictions disagree with diffusion data for multivalent cation and complex species. To improve the applicability of the model, it is necessary to consider the atomic level interactions between solute, solvent or clay mineral, and try that we apply the current ISD model to heterogeneous pore structure. In this study we try the application of the current ISD model to multiple pore structure. As results of numerical analysis of these models, the salinity dependence of effective diffusivity for the multi-pore model is comparatively smaller than that for the homogeneous pore model and the current diffusion model is improved.
Tachi, Yukio; Suyama, Tadahiro; Yotsuji, Kenji; Ishii, Yasuo; Takahashi, Hiroaki*
CMS Workshop Lectures, Vol.21, p.241 - 250, 2016/00
Sorption and diffusion of radionuclides in argillaceous rocks are key processes in the safe geological disposal. The diffusion and sorption behavior of Ni(II), Am(III) and Se(IV) in mudstone from the Horonobe URL were investigated by experimental and modeling approaches. Effective diffusivities obtained by the through-diffusion experiments were in the sequence of Cs
, Ni
, HTO, I
, Se(SeO
), Am(Am(CO
)) by comparison with the previous study. The distribution coefficient values were consistent with those obtained by batch sorption tests. These results were interpreted by the clay-based modeling approach coupling the thermodynamic sorption model assuming key contributions of clays (smectite and illite) and the diffusion model assuming the electrical double layer theory and the simplified pore model with size distribution. This clay-based model could provide reasonable account of observed trends and could be basically applicable for various radionuclides.
Sakuma, Hiroshi*; Tachi, Yukio; Yotsuji, Kenji; Kawamura, Katsuyuki*
no journal, ,
Clay minerals are good adsorbents of many toxic elements of molecules in natural environment, because of their large surface areas and their high affinity to organic and inorganic materials. The adsorption sites of ions and molecules on the basal planes can be estimated by experiments and computer simulations based on the simple surface structure. While the adsorption of ions and molecules on the edge planes of clay minerals is poorly understood due to the absence of established model of the edge structures. In this study, the edge structures of montmorillonite were examined by the first-principles calculations based on the density functional theory. The effect of isomorphous substitution, layer charge, and positions of interlayer cations was evaluated for four different edge planes by calculating the surface energy. The acidity constant of the edges were calculated by an empirical method. We will discuss possible adsorption sites of cations on these edges.
Sato, Tsutomu*; Oda, Chie
no journal, ,
Bentonite buffer and cement fluid interaction has been a key research issue in long-term safety assessment of radioactive waste disposal. The dissolution rate of smectite, the main constituent mineral of bentonite, has therefore been investigated under hyperalkaline conditions. A dissolution rate equation of smectite has been derived considering the effect of the Gibbs free energy of reaction and is applicable to pH = 7-13 and t = 25-80
C. Reactive-transport model analyses to elucidate the consequences of coupled changes in the porewater chemistry, mineralogy and, ultimately, the mass transport properties of the bentonite buffer were conducted using the smectite dissolution rate. Except in the close proximity of the cement interface, it was found that regardless of the choice of secondary minerals, the effective diffusion coefficient and hydraulic conductivity remained largely unchanged after 100,000 years.
