The Influence of Temperature and Clay Organics on the Retention Behavior of Opalinus Clay concerning Radionuclides

Presently, argillaceous rock formations are under investigation as potential host rocks for nuclear waste repositories. The sorption of radionuclides on mineral phases is an important physicochemical process in a nuclear waste repository in the case of a water inleakage. Concerning the required long-term safety and risk assessment of the storage of high-level radioactive waste the understanding of this process is essential. Opalinus Clay (OPA) is a complex composed argillaceous rock. This natural occurring clay rock is discussed as host rock formation for nuclear waste repositories. In a repository based on argillaceous rock an initial temperature of around 100°C is expected. But although elevated temperatures are able to influence the sorption behaviour of radionuclides on mineral phases the temperature effect is still investigated insufficiently. Due to the fact that natural organic matter was found in the OPA the influence of organic matter on the radionuclide sorption hast to be investigated. The investigations concentrated on the sorption of Eu(III) and U(VI) on OPA under realistic OPA pore water conditions (OPA pore water medium, pH 7.6, I = 0.4 mol•L-1) up to 50°C. In addition to the temperature dependent investigations the influence of small organic molecules (e.g. lactate, acetate, propionate, tartrate, citrate) on the Eu(III) and U(VI) sorption on OPA was studied up to 50°C.
The Eu(III) sorption is characterized by a strong binding of Eu(III) and the U(VI) sorption is characterized by a weak binding of U(VI) to the clay surface. A significant temperature dependency for the Eu(III) and U(VI) sorption was observed. In the presence of the organic matter the Eu(III) and U(VI) sorption decreases with rising ligand concentration due to a complexation of the metal ions by the ligands in solution. The mobilization of Eu(III) and U(VI) correlates with the complex formation strength of the organic ligands.