The ternary system U(VI) / humic acid / Opalinus Clay
Beside salt and crystalline rock also argillaceous rock is considered as possible host rock for a nuclear waste repository. Uranium represents an important radionuclide in high level radioactive waste. Thus, one objective of the work was to study the interaction of U(VI) with natural clay. For this, U(VI) sorption and diffusion experiments were performed applying the natural clay ‘Opalinus Clay’ from the underground rock laboratory in Mont Terri, Switzerland.
Beside different minerals also organic matter is contained in natural clay, which can be released in form of low molecular weight organic acids or humic substances under certain conditions. Humic substances are plant and animal degradation products. They are organic macromolecules with a variety of functional groups. They can influence the migration of actinides in the environment by forming complexes and colloids. They are also able to reduce metal ions. Thus, the influence of humic acid, as representative for organic matter, on the U(VI) / Opalinus Clay interaction was studied in the thesis.
Due to the radioactive decay of the radionuclides, elevated temperatures up to 100 °C are expected close to the waste containers in a nuclear waste repository with clay as host rock. Thus, the influence of elevated temperature on the U(VI) diffusion through Opalinus Clay was investigated
The pore water of natural clay has a complex salt composition. The present ions influence the U(VI) and humic acid speciation in solution and consequently, their sorption and diffusion properties. Here, synthetic Opalinus Clay pore water (I = 0.36 M, pH 7.6) was applied as background electrolyte for sorption and diffusion experiments. For comparison, pH-dependent sorption experiments were performed in the presence of the inert background electrolyte 0.1 M NaClO4.
In the presence of low molecular weight organic acids pore water as background electrolyte, U(VI) sorbs only weakly onto Opalinus Clay. This is due to the dissolution of calcite (mineral contained in Opalinus Clay). The calcium ions in solution affect the U(VI) speciation by forming the dominating Ca2UO2(CO3)3(aq) complex. This neutral complex shows a low sorption affinity toward Opalinus Clay and is not influenced by humic acid. Thus, the presence of humic acid has no effect on U(VI) sorption onto Opalinus Clay.
pH-dependent sorption studies applying 0.1 M NaClO4 as background electrolyte showed that at pH < 7.6 U(VI) is complexed by humic acid, which leads to an increase (pH 3-4.5) and decrease (pH 4.5-7.5) of the U(VI) sorption onto Opalinus Clay. When Opalinus Clay is contacted with 0.1 M NaClO4 in dependence on pH, different ions are leached out of Opalinus Clay. Thus, between pH 7.5 and 8 again the Ca2UO2(CO3)3(aq) complex is the dominating species in solution and humic acid presence has no effect on U(VI) speciation and U(VI) sorption.
Diffusion studies with intact Opalinus Clay bore core samples in Opalinus Clay pore water confirmed the sorption results concerning the U(VI) affinity toward Opalinus Clay. Humic acid has no significant influence on U(VI) migration through Opalinus Clay. This was confirmed also by diffusion experiments at 60 °C. However, a change of the U(VI) speciation was observed at 60 °C. Diffusion profiles of at least one colloidal and one aqueous U(VI) species were determined. Both species showed a stronger interaction with Opalinus Clay, but also a faster diffusion through Opalinus Clay compared to the U(VI) species at 25 °C. The combined effects showed for the aqueous U(VI) species that an increase of the temperature to 60 °C does not significantly influence the U(VI) migration through Opalinus Clay.
The Federal Ministry of Economics and Technology funded this work (02E10156).
- C. Joseph, L.R. Van Loon, A. Jakob, K. Schmeide, S. Sachs, G. Bernhard: “Effect of temperature and humic acid on the U(VI) diffusion in compacted Opalinus Clay”, In: The New Uranium Mining Boom (Proceedings of UMH VI), Merkel B.J., Schipek, M. (eds.), Berlin, Springer Verlag, 2011, p. 617-626.
- C. Joseph, K. Schmeide, S. Sachs, V. Brendler, G. Geipel, G. Bernhard: “Sorption of uranium(VI) onto Opalinus Clay in the absence and presence of humic acid in Opalinus Clay pore water”, Chemical Geology, 2011, 284, 240-250.
- C. Joseph, B. Raditzky, K. Schmeide, G. Geipel, G. Bernhard: “Complexation of Uranium(VI) by Sulfur and Nitrogen Containing Model Ligands in Aqueous Solution”, In: Uranium, Mining and Hydrogeology (Proceedings of UMH V), Merkel B.J., Hasche-Berger, A. (eds.), Berlin, Springer Verlag, 2008, p. 539-548.
- A. Kremleva, Y. Zhang, A.M. Shor, S. Krüger, C. Joseph, B. Raditzky, K. Schmeide, S. Sachs, N. Rösch: “Uranyl(VI) Complexation by Sulfonate Ligands: A Relativistic Density Functional and TRLFS Study”, European Journal of Inorganic Chemistry, 2012, in preparation.
- C. Joseph, K. Schmeide, S. Sachs, M. Stockmann, V. Brendler, G. Bernhard: „Sorption of U(VI) onto Opalinus Clay: Effects of pH and humic acid”, Applied Geochemistry, 2012, in preparation.
- C. Joseph, L.R. Van Loon, A. Jakob, R. Steudtner, K. Schmeide, S. Sachs, G. Bernhard: „Diffusion of U(VI) in Opalinus Clay: Influence of temperature and humic acid”, Geochimica et Cosmochimica Acta, 2012, in preparation.