Uranium(VI) sorption on montmorillonite and bentonite: Prediction and experiments

The sorption characteristics of U(VI) on bentonite and montmorillonite were investigated in batch experiments for understanding the near-field behaviour in geological nuclear repositories. Sorption parameters were determined in batch tests. The sorption on bentonite (KWK) was studied at different dry bulk densities of the clay (1.3, 1.6, 1.9 g/cm³) at pH 8 for the U(VI) concentration dependence (10^-4 to 10^-9 m). The sorption on the pure mineral montmorillonite (SWy-1) in 0.1 M NaClO₄ was determined for the concentration dependence at pH 5.5, and pH dependence between pH 3 and 11). A scientifically founded description of sorption processes at the mineral-liquid surface is possible with the surface complexation models (SCM), the ion adsorption on surface sites as complexation reaction. We use the diffuse double layer model (DDL) to predict the sorption with the code MINTEQA2 (Version 4.03, US EPA May 2006), thermodynamic data of aqueous and solid species from the NEA-TDB [1], the mineral characterization and the respective protolysis data [2] and surface complex constants from [3].
The prediction of U(VI) sorption on montmorillonite at the pH dependence was very good. The modelling of the sorption on montmorillonite and bentonite versus U(VI) concentration shows a good agreement of measured and predicted values.

[1] Guillaumont, R. et al. (2003) “Update on the chemical thermodynamics of U, Np, Pu, Am, Tc. Chemical Thermo-dynamics”, Vol. 5 (OECD Nuclear Energy Agency, ed.), Elsevier, Amsterdam.
[4] Pabalan, R.T. et al. (1997) Aquat. Geochem. 2, 203-226.
[3] Pabalan, R.T. et al. (1998) in: Adsorption of metals by geomedia. Variables, mechanisms, and model applications; Jenne EA (Ed.), Academic Press; San Diego.

Funding by the European Commission (NF-PRO C2-ST-C-01) is gratefully acknowledged.