A thermodynamic surface model for caesium sorption on bentonite

Abstract

Caesium sorption on Wyoming bentonite MX-80 has been studied in solutions of NaCl, KCl, MgCl(2), CaCl(2), NaNO(3) and Ca (NO(3))(2) of concentrations varying between 0.025 and 1 mol/L, as well as in a weakly saline (I=0.004 ml/L) and a strongly saline (I=0.46 mol/L) natural groundwater. These experiments have been used to derive a thermodynamic model for the interaction of caesium with the bentonite surface in accordance with a surface chemical model, including acid/base reactions developed recently for montmorillonite. The sorption behaviour of caesium on bentonite can be described, within the experimental and model uncertainties, in terms of a one-site ion exchange model. The ion exchange constant obtained for the reaction NaX+Cs(+) left arrow over right arrow CsX+Na(+) (where X represents the ion exchange sites on montmorillonite) is log(10) K(0)(ex)=1.6. Impurities in the bentonite, influencing the concentrations of competing cations, such as Na(+), K(+), Mg(2+) and Ca(2+), have a crucial impact on the sorption of caesium. This impact can be adequately quantified with the present model. The model predictions compare well with sorption data published in the open literature on both Wyoming bentonite MX-80 and other types of bentonite. Distribution coefficients from the literature obtained from both batch and diffusion experiments and varying over four orders of magnitude are reproduced and explained successfully by the model.

Diffusion and sorption of Cs+, I− and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan: Clay-based modeling approach

“…D e trends for cations and anions could not be explained by the conventional pore diffusion model alone, considered in terms of porosity (e), tortuosity (s) and (geometric) constrictivity (d g ) factor, and the tracer diffusivity in bulk liquid water, D w (m 2 s À1 ), from Van Brakel and Heertjes (1974): Bradbury and Baeyens (2000). b Wanner et al (1996). c Tachi et al (2009b).…”

“…Maes et al (2008) and Van Loon et al (2009) also tested this three-site model to describe Cs + isotherms on Boom clay and Opalinus clay, respectively. For smectite, the one-site ion exchange model developed by Wanner et al (1996) was used. This one-site model can describe Cs + sorption in various types of smectite and bentonite, and solution conditions (Wanner et al, 1996;Ochs et al, 2001;Tachi et al, 2009a), although it has been confirmed that Cs + sorbs on smectite as inner-sphere and outer-sphere complexes by NMR (Kim et al, 1996b) and EXAFS (Bostick et al, 2002).…”

“…For smectite, the one-site ion exchange model developed by Wanner et al (1996) was used. This one-site model can describe Cs + sorption in various types of smectite and bentonite, and solution conditions (Wanner et al, 1996;Ochs et al, 2001;Tachi et al, 2009a), although it has been confirmed that Cs + sorbs on smectite as inner-sphere and outer-sphere complexes by NMR (Kim et al, 1996b) and EXAFS (Bostick et al, 2002). Model parameters, including site types, site capacities, and equilibrium constants (K) of ion exchange reactions with Na + , K + and NH 4 + , taken directly from the above literature, were expressed by the following equations (Table 4):…”

“…Cs sorption has been studied extensively because of its high importance in the safety assessment of HLW disposal. It is generally accepted that clay minerals, especially illite and smectite, sorb Cs + by cation exchange, thereby restricting its mobility (e.g., Wanner et al, 1996;Bradbury and Baeyens, 2000;Zachara et al, 2002;Van Loon et al, 2009). The distribution coefficients (K d ) obtained by batch type sorption experiments in dispersed systems are generally used directly in safety assessments of HLW disposal.…”

Transport of Strontium Through a Ca-bentonite (Almería, Spain) and Comparison with MX-80 Na-bentonite: Experimental and Modelling

“…uranium, plutonium, neptunium, americium, cesium, thorium, technetium and selenium (see, for example, references Baston et al 1995;Missana et al 2004;Hurel et al 2009;Nagasaki et al 1999;Shibutani et al 1994;Tsai et al 2001;Wang et al 2005;Wanner et al 1996;Xu et al 2006). In particular, the sorption of strontium onto bentonite has been the object of different studies in Na-and Ca-bentonite (Choi and Oscarson 1996;Eriksen et al 1999;Khan et al 1995;Galamboš et al 2009;Muurinen et al 1987;Wang et al 2004).…”