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

Abstract

Compacted Na-bentonite, of which the major mineral is montmorillonite, is a candidate buffer material for the geological disposal of high-level radioactive waste. A potential alteration of the bentonite in a repository is the partial replacement of the exchangeable cations of Na However, the apparent diffusion coefficient of Ca 2+ and the activation energy for diffusion at the same dry density were independent of the ionic equivalent fraction of Ca 2+ ions. These findings suggest that unlike HTO, which can be postulated to diffuse mainly in pore water, Ca 2+ ion diffusion could occur predominantly in interlayer spaces, of which the basal spacing was determined to be constant by the XRD technique.

Hydration and Mobility of Interlayer Ions of (Nax, Cay)-Montmorillonite: A Molecular Dynamics Study

“…Such results are in well agreement with XRD measurements. 73 Thermodynamically, the immersion energy curves reveal that all possible stable hydrated states of montmorillonite, such as the single-, double-, and triple-layer hydrates (Figure 4), are consistent with local or global immersion energy minima. 51,53−55 The energy barriers between these minima can specify the relative stabilities of different hydrated states.…”

Self-Diffusion of Water and Ions in Clay Barriers

“…The type of clay mineral (e.g., smectite vs illite) and the details of the clay mineral structural charge (e.g., whether it arises from isomorphic substitutions in the octahedral or tetrahedral sheets) also may influence the diffusion coefficients D a and D e , as suggested by the evidence that cations have a greater tendency to form inner-sphere surface complexes on tetrahedral charge sites (Chang et al, 1995). In addition to r b,clay , other parameters that influence diffusion in clay materials are temperature T (K) (González Sánchez et al, 2009), the direction of the diffusion measurement Mazurek et al, 2011), ionic strength I (mol dm À3 ) , the type of exchangeable cation (Choi and Oscarson, 1996;González Sánchez et al, 2008;Melkior et al, 2009;Kozaki et al, 2010), and the species of interest. The T-dependence of a has not been extensively studied.…”

“…Relative apparent diffusion coefficients D a / D 0 were calculated using D 0 ¼ 2.30, 2.27, and 2.24 10 À9 m 2 s À1 for H 2 O, HDO, and HTO (Mills, 1973) and 1.33, 2.07, 0.794, 2.03, 2.01, and 2.00 10 À9 m 2 s À1 for Na þ , Cs þ , Sr 2þ , Cl À , Br À , and I À at 298 K, respectively (Li and Gregory, 1974). Kozaki et al (1996Kozaki et al ( , 1997Kozaki et al ( , 1998Kozaki et al ( , 1999bKozaki et al ( , 2001Kozaki et al ( , 2005 Smectite (KP) HTO, Na þ , Cs þ , Ca 2þ 0 (Na/Ca) 288e323 AE 0.1 1.0, 1.8 D a , E a CC Kozaki et al (1999aKozaki et al ( , 2005Kozaki et al ( , 2010 Smectite ( …”

Discretization of the Water Uptake Process of Na‐Montmorillonite Undergoing Atmospheric Stress: XRD Modeling Approach

“…In this regard, montmorillonite will be exposed to several environmental constraints (temperature cycle, extremely saturated conditions, high water pressure, etc.). Under these conditions, the clay-sealing ability is influenced, and two different scenarios can take place: (1) the crystal lattices may undergo permanent changes resulting in a largely reduced swelling and gel-forming capacity, and (2) the microstructure may become temporarily or permanently altered, which affects the cation exchange process [1][2][3][4][5][6][7]. e crystal structure of smectite consists of two sheets of principally tetrahedral Si with minor Al sandwiched to the top and bottom of a layer of octahedrally coordinated cations, usually Al but with some Mg and Fe [8,9].…”