Component Additivity Approach for Uranium Retardation in Sandstone Host Rocks

Sandstone is one of the most important host rocks for uranium ore deposits, e.g. in Germany (Königstein) or the Czech Republic (Pribram). Any remediation efforts of respective mining legacies thus require a detailed understanding of this system. Namely the sorption in this complex rock is not well enough investigated yet.
One widely accepted approach (Component Additivity – CA, cf. [1]) to describe the sorption is based on the assumption that the surface of a complex mineral assemblage is composed of a mixture of one or more phases whose surface properties are known from independent studies. An internally consistent SCM database can be developed that describes the adsorption reactions of solutes to each phase. The fitting of data of the complex system is not necessary.
We predicted the sorption of uranium(VI) on sandstone using the results of the surface characterization of the assemblage and published data for adsorption onto the pure mineral constituents. The results were compared with batch sorption experiments using natural and synthetic sandstone (mixture of the main components quartz, muscovite, and hematite).
The sorption of U(VI) on natural and two synthetic sandstone (I and II) was investigated in batch sorption experiments (air atmosphere, 0.1M NaClO4, pH 3-11, [U(VI)] 10^-9-10^-3M). We had to consider the natural uranium concentration dissolved from the sandstone at low concentrations.
The synthetic sandstone was a mixture of quartz with muscovite and hematite. The surface area (N₂-BET) of quartz was 0.047 m²/g, of muscovite 0.88 m²/g, of hematite 0.89 m²/g and of natural sandstone (mainly quartz) 0.69 m²/g.
The pH dependence shows a maximal sorption between pH 6 and 7. The U(VI) sorption varies between 95% (10^-9M) and 7% (10^-4) on natural sandstone, and between 80% (10^-9M) and 2% (10^-4M) on synthetic sandstone.
The modeling of U(VI) sorption on sandstone was performed using the CA approach. We assumed just one type of surface sites and applied a model with simple electrostatics, the Diffuse Double Layer Model (DDLM). For the modeling the code MINTEQA2 (Version 4.03, US EPA, May 2006) was used. The aqueous species were taken from the NEA-TDB [2].
As surface reactions we considered the protolysis of quartz, hematite, muscovite, the U(VI) surface complexes of quartz, hematite, muscovite (monomer, dimer) and the ternary U(VI) surface complexes of quartz and hematite with carbonate. The surface area was considered according to the mineral fraction. Despite of a slight shift of the predicted curve to lower pH in both systems, a good accordance with experimental values were found. Obviously, the sorption is strongly influenced by the small amount (0.5%) of hematite.
For the sorption behavior at varying U(VI) initial concentration, a precipitation of schoepite was predicted. As schoepite is an aged mineral, it is unlikely to be formed within the experimental duration of five days. A subsequent exclusion from the list of allowed phases led to a predicted precipitation of both soddyite and metaschoepite.
[1] Davis, J.A. et al. (1998) Application of the surface complexation concept to complex mineral assemblages. Environ. Sci. Technol. 32, 2820-2828.
[2] Guillaumont, R. et al. (2003) Update on the chemical thermodynamics of U, Np, Pu, Am, Tc. Elsevier, Amsterdam.