Ab initio modelling of magnetite surfaces for plutonium retention

In many European countries (e.g., Switzerland, France, Sweden), 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 at the surface (Fe3O4). After tens of thousands of years, casks may breach allowing for leaching of the radionuclides by pore-water. Magnetite can retard dissolved radionuclides either by adsorption or structural incorporation [1,2]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [3].
In this computational study, we identified the dominant low-index surfaces of magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open-source code CP2K. The DFT+U method was employed for the strongly correlated 3d and 5f electrons of Fe and Pu, respectively. After revising our model and determining the Hubbard U parameter [4], we examined the preferential magnetite crystal orientation plane (111) allowing for different surface terminations, water coverage and redox conditions. Comparing the surface energy, the most stable surface can be deduced and we found the most stable magnetite (111) surfaces under real repository conditions. Subsequently, we used ab initio molecular dynamics (MD) to simulate sorption structures on the expected magnetite (111) surfaces.

[1] T. Dumas et al. (2019). Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption. ACS Earth & Space Chemistry, 2019, 3(10), 2197.
[2] R. Kirsch et al. (2011). Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite. Environmental Science & Technology, 2011, 45(17), 7267.
[3] E. Yalçintaş et al. (2016). Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite. Dalton Transactions, 2016, 45(44), 17874.
[4] A. Kéri et al. (2017). Combines XAFS Spectroscopy and Ab Inito Study on the Characterization of Iron Incorporation in Motmorillonite. Environmental Science & Technology, 51(18), 10585.