Identification of interfacial processes and species of trivalent lanthanides and actinides on mineral surfaces

Radioactive materials are utilized for a variety of applications, including nuclear power plants, medical radiography, and material testing technologies. These uses result in radioactive waste, which poses a risk to human health and the environment due to its radiotoxic and chemotoxic properties. The widely recognized approach for the ultimate disposal of high-level waste involves the containment within deep geological repositories (DGR). A reliable modeling of potential release scenarios for radionuclides (RNs) is essential in Long-Term Safety Analysis (LSA). In addition to hydrogeological information, element-specific thermodynamic data sets are required for all RNs relevant to long-term safety. Hence, a comprehensive understanding of the molecular processes of RN potentially occurring in the near and far field of a DGR is mandatory for the improvement of the existing codes for LSA.

The purpose of this study is to investigate RN retention mechanisms such as sorption, incorporation, surface complexation, and precipitation, including their mutual interdependencies using a combination of batch sorption experiments and spectroscopic techniques. Starting with solid albite feldspar as a mineral surface and Eu(III) as an analogue of trivalent lanthanides, batch sorption experiments are performed to evaluate the extent and mechanisms of Eu(III) uptake on albite surfaces under varying conditions. These experiments provide quantitative insights into the sorption behavior, including sorption capacity, kinetics, and the influence of pH and ionic strength. In a subsequent step, this information is utilized in investigations of more complex rocks containing feldspars. Our research will identify preferred mineral surfaces for sorption and characterizes binding forms under near-natural geochemical circumstances.

The experimental data derived from batch sorption studies are complemented by the application of advanced spectroscopic techniques, including time-resolved laser induced fluorescence spectroscopy (TRLFS), infrared (IR) spectroscopy, and Raman microscopy. These techniques potentially provide the identification of radionuclide species formed at the mineral’s surface. The combination of sorption data and spectroscopic analysis provides a comprehensive understanding of the molecular-level mechanisms driving the retention of trivalent lanthanides on mineral surface.

The data, which will be derived from this study contribute to improving thermodynamic models and reaction equations for RN migration. Furthermore, we are aiming to extend the method to additional trivalent actinides such as Am(III) and Cm(III). This work will not only enhance the understanding of radionuclide behavior in complex geochemical systems but also provide crucial insights for the safe disposal of nuclear waste.