Diffusion mechanism of sodium ions in compacted montmorillonite under different NaCl concentration

Abstract

Compacted bentonite, the major mineral being montmorillonite, is a candidate buffer material for geological disposal of high-level radioactive waste. The diffusion behavior of radionuclides in the compacted montmorillonite is an important issue to be clarified for the safety assessment of geological disposal. In this study, one-dimensional, non-steady diffusion experiments using 22 Na at different diffusion temperatures and the measurement of the basal spacings by XRD were conducted for Na-montmorillonite saturated with NaCl solutions of different concentrations. Some basal spacings of the montmorillonite were found to decrease from 1.88 to 1.56 nm as the NaCl concentration increased from 0 to 0.1 M. The apparent self-diffusion coefficients at 298 K obtained in this study slightly increased as the NaCl concentration increased from 0 to 0.5 M, while the activation energies, calculated from the temperature dependences of the diffusion coefficients, were found to be 14, 22, and 17 kJ mol -1 at NaCl concentrations of 0, 0.1, and 0.5 M, respectively. These NaCl concentration dependences of the diffusion coefficients and their activation energies can be explained by assuming three independent diffusion processes (diffusion in pore water, on external surface of montmorillonite, and in interlayer of montmorillonite).

Effect of exchangeable cations on apparent diffusion of Ca2+ ions in Na- and Ca-montmorillonite mixtures

“…It can be postulated that there are three potential pathways for radionuclide diffusion in solution-saturated, compacted montmorillonite, i.e., pore water, external surfaces of montmorillonite aggregates, and internal surfaces (interlayer spaces) of the montmorillonite aggregates (Kozaki et al, 2008). Pore water is the free water in the relatively large voids between the montmorillonite aggregates (interparticle space), and the external surface is the space on the montmorillonite aggregates where diffuse double layer water forms, whereas the internal surfaces is the space between montmorillonite sheets (TOT-layers).…”

“…These predictions should be based on studies of the diffusivities of Ca 2+ ions in both the Na-bentonite and the altered bentonite. Although extensive studies have been performed for the diffusion of radionuclides in bentonite (Bourg et al, 2008, Glaus et al, 2007, Gonzalez-Sanchez et al, 2008, Kozaki et al, 2008, Melkior et al, 2009, there are few studies on the diffusivities of Ca 2+ ions in compacted bentonite (Kozaki et al, 2001a) and the effects of exchangeable cations on diffusion of radionuclides (Choi and Oscarson, 1996, Kozaki et al, 1999a, 2005 The diffusion mechanism of Ca 2+ ions in compacted montmorillonite discussed in this paper is based on these experimental data.…”

Experimental and Modeling Study on Diffusion of Selenium under Variable Bentonite Content and Porewater Salinity

“…The term of G mp Á 0 mp was approximated using the values in the whole pore, 18,19,46) G p Á 0 p , determined from diffusion data of HTO, D e(HTO) , which did not interact with the surface of montmorillonite (Fig. 7).…”

“…Many researchers have developed mech-anistic diffusion models, such as the pore diffusion model that is most widely used, 11,12) the model based on the electric double layer theory, 6,13,14) and the model considering anion exclusion, 15,16) surface diffusion, [16][17][18] interlayer diffusion, [18][19][20] and Donnan equilibrium. 21) However, diffusion models for evaluation of the variable D e of anionic species depending on the montmorillonite content and porewater salinity are limited.…”

Ioncsere-folyamatok az agyagásványok „nanolaboratóriumában”

“…A bruttó migrációs együttható a háromféle pórusban való migrációból adódik össze 36 : 39 Az 1. táblázat adataiból látható, hogy a kicserélhetõ kation elsõsorban a kloridionok migrációjára van hatással, mivel nátrium-bentonitban a montmorillonit duzzadása lényegesen nagyobb mértékû, megnõ a rétegközi víz aránya. A rétegek nagy távolsága miatt a rétegek közötti az elektrosztatikus térerõsség lecsökken, tehát már nem tartható fent az a közelítés, hogy az anionok a rétegközi térbõl teljesen kizáródnak.…”