Incorporation Mechanism of Tc(IV) in Magnetite Revealed by EXAFS Measurements and Ab Initio Simulations

The reductive immobilization of Tc, a long-lived fission product present in nuclear waste, is an important geochemical mechanism regarding Tc transport in the repository near-field. Based on the ionic radius and similarity in charge, Tc(IV) is expected to substitute for Fe(II,III) in octahedral positions in magnetite, a common iron oxide. Several mechanisms have been proposed in the past for how the necessary charge compensation for the aliovalent cation substitution is obtained. In this study, we developed a reliable computational approach based on density functional theory to investigate the Tc-magnetite interaction. By comparing our simulation results to spectroscopic data, we can confirm a preference for the incorporation mechanisms, including the formation of a vacancy. For both the magnetite bulk structure as well as its commonly observed (111) surface, these findings are based on structural information and thermodynamic considerations. Our results are in line with the experimentally observed topotactic transition of the magnetite crystal structure toward the more oxidized iron mineral maghemite and reflect a component in long-term geochemical mineral transformations.