U(VI) sorption on granite: prediction and experiments

The sorption characteristics of U(VI) on granite was investigated in batch experiments for understanding the far-field behaviour in granitic geological nuclear repositories. The granite (from Eibenstock, Germany) was a mixture of quartz (40.9%) albite (26.7%), orthoclase (14.0%), biotite (2.0%), rubicline (0.7%), muscovite (14.7%), and hematite (1.0%) according to the ICP-MS after digestion. Except hematite and rubicline all components were found by XRD. The surface area (N₂-BET) of the granite was 0.35 m²/g. The sorption conditions were ambient atmosphere, 0.1M NaClO₄, sorption time 5 d for pH dependence (pH 3-11, at U(VI) 10^-6 M), and U(VI) concentration dependence (10^-9-10^-3 M, at pH 5).

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. The approach to real systems is the component additivity, the superposition of simultaneous sorption and competition reactions based on mineralogical composition. We used 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], and the respective protolysis data and surface complex constants for all mineralogical components of granite.

The prediction of U(VI) sorption on granite vs. pH was very good. Even the increased retention at pH 11 can be explained as precipitation of uranophane by calculation with the measured CO₂ (lower than the equilibrium CO₂). The modelling of the sorption on granite vs. U(VI) concentration shows a good agreement of measured and predicted values. At high concentrations the precipitation of well crystallized schoepite was predicted. Such a well crystallized mineral is unlikely after only 5 days sorption time. Excluding schoepite, the precipitation of metaschoepite was predicted for concentrations >10^-4 mol/L U(VI).

[1] Guillaumont, R. et al. (2003) "Update on the chemical thermodynamics of U, Np, Pu, Am, Tc. Chemical Thermodynamics", Vol. 5 (OECD Nuclear Energy Agency, ed.), Elsevier, Amsterdam.

Acknowledgement
Funding by the BMBF and BMWA (02C1144) is gratefully acknowledged.