Influence of Muscovite (001) Surface Sites on Europium Adsorption

A key problem for the long-term safety of nuclear waste repositories is radionuclide migration in the geosphere. The adsorption of radionuclides onto mineral surfaces of the surrounding host rock can provide an important mechanism to retard or prevent migration from the repository to the biosphere. Due to the strong sorption potential of clay minerals, clay rock formations such as the Opalinus Clay are being considered as potential sites for nuclear waste repositories. Phyllosilicates, such as clay minerals or mica, have shown a high affinity for the adsorption of various radionuclides in several experimental studies. In natural environments, mineral surfaces are exposed to reactions (e.g., dissolution) over long periods. These processes can lead to an alteration of the surface nanotopography, thereby affecting the adsorption efficiency. In a recent study, the authors report that the nanotopography of calcite surfaces leads to heterogonous sorption of europium due to differences in the atomic configuration of the adsorption sites [1].
In this study, we investigate the influence of muscovite surface site coordination on the adsorption energy barrier and the resulting overall distribution of radionuclide adsorption on the mineral surface. Numerical methods are applied to study the adsorption of Eu(OH)3 on a muscovite (001) surface with different nanotopographic structures. Density Functional Theory (DFT) calculations are performed for eleven surface sites present on muscovite to obtain the adsorption energy barriers. The adsorption energy barrier is calculated based on a series of geometry optimizations with increasing Eu–site distance. All site-specific adsorption energy barriers are then implemented in a Kinetic Monte Carlo (KMC) model developed based on a previous study [2]. Here, larger muscovite surface portions can be simulated with structures such as dissolution etch pits for a more realistic nanotopography. Eu(OH)3 is then adsorbed on the generated muscovite surface considering the adsorption energy barriers obtained from DFT calculations. The distribution of adsorbed Eu(OH)3 and the temporal evolution of the process can be simulated with KMC and linked to the surface structures. This combined numerical approach allows us to show the effects of surface site coordination on radionuclide adsorption reactions and the resulting adsorption heterogeneity on mineral surfaces at larger scales.
References:
[1] T. Yuan, S. Schymura, T. Bollermann, K. Molodtsov, P. Chekhonin, M. Schmidt, T. Stumpf, C. Fischer, Environ. Sci. Technol. 2021, 55, 15797–15809. [2] J. Schabernack, I. Kurganskaya, C. Fischer, A. Luttge, Minerals 2021, 11, 468.