Ab initio modelling of magnetite surfaces for radionuclide retention

In many European countries (e.g., France, Switzerland), thick steel casks are foreseen for the containment of high-level radioactive waste in deep geological repositories. In contact with pore-water, steel corrodes forming mixed iron oxides, mainly magnetite (Fe3O4). After tens of thousands of years, the casks may breach allowing for leaching of the radionuclides (e.g., Tc and Pu) into pore-water. The radionuclides can be retarded by corrosion products either by adsorption or via structural incorporation [1,2]. The molecular scale mechanisms of these phenomena are investigated by ab initio simulations and X-ray absorption spectroscopy (XAS). The dominant low-index surfaces of magnetite particles and their termination at the relevant conditions were identified based on Kohn-Sham density functional theory (DFT), using the open-source CP2K code [3]. The DFT+U method was employed for the strongly correlated 3d and 5f electrons of Fe and Pu, respectively. After benchmarking the model setup, the surface energies of the (111) facet with different surface terminations and water coverage were analyzed as a function of redox conditions and pH. The Eh and pH predominance diagram could be predicted for the most stable surfaces under real repository conditions [4]. Further, we confirmed these findings for nanocrystals with approximately 2 nm size. Subsequently, ab initio molecular dynamics (MD) were applied to simulate sorption structures of radionuclides on the expected magnetite (111) surfaces based on experimental findings [2,5]. [1] R. Kirsch et al. Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite. Environmental Science & Technology, 2011, 45(17), 7267. [2] E. Yalçintaş et al. Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite. Dalton Transactions, 2016, 45(44), 17874. [3] T. D. Kühne et al. CP2K: An Electronic Structure and Molecular Dynamics Software Package - Quickstep: Efficient and Accurate Electronic Structure Calculations. The Journal of Chemical Physics, 2020, 152(19), 194103. [4] A.S. Katheras et al. Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes. Environmental Science & Technology, 2024, 58(1), 935. [5] T. Dumas et al. Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption. ACS Earth & Space Chemistry, 2019, 3(10), 2197.