Katheras, Anita Sonja (2025). Atomic scale characterization of Tc and Pu uptake by magnetite based on atomistic simulations and X-ray absorption spectroscopy. (Thesis). Universität Bern, Bern
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Abstract
Magnetite is a common iron oxide mineral present in crystalline rocks and subsurface sediments. The mixed valence state of the Fe in magnetite allows for participation in redox reactions and redox buffering. Magnetite plays also an important role for the safe geological disposal of radioactive waste – an ongoing challenge for several countries worldwide. Many countries consider isolation of high level waste in steel casks disposed in deep underground repositories. Over time and in contact with the host-rock pore-water, magnetite will develop as a major corrosion product on the steel surface and, eventually, lead to canisters breaching. The incoming water can then mobilize the radioactive inventory, i.a. technetium and plutonium present in the waste. Experimental observations suggest that magnetite can contribute to their immobilization by participating in surface mediated reductive adsorption and incorporation of hazardous elements into the magnetite structure. This thesis aims at a detailed molecular scale understanding of these phenomena by atomistic simulations and spectroscopic investigations. The theoretical studies of magnetite as bulk and surface structures as well as nanoparticles were conducted by density functional theory (DFT). The first and fundamental step of this methodology was the development and validation by simulating the appropriate crystal structures. The Hubbard U method (DFT+U) was applied to improve the description of the electrons of octahedrally and tetrahedrally coordinated Fe cations in magnetite. In a similar approach, the Hubbard U correction for Tc and Pu was tuned by simulating oxide structures reflecting relevant oxidation states and coordination environments. A close agreement to experimental crystallographic data could be achieved using UTc(IV) = 0 eV and UPu(III) = 3.5 eV. In a second step, the thermodynamic stability of the most common magnetite (111) surface was investigated. Contrasting previous studies focusing on the stability of the magnetite surface under vacuum, environmentally relevant conditions influencing the mineral-water interface could be fully addressed. This includes oxygen-terminated surfaces with various degrees of protonation reflecting varying charges of surface-Fe cations, too. The system energies resulting from the simulations were used to calculate the surface energy as function of Eh and pH for different surface terminations. Based on the surface energy thus determined, the most stable surface termination of the magnetite (111) surface was identified. The consideration of the surface-water interaction energy was of great importance and resulted in octahedrally coordinated Fe cations with an oxidation state higher than 2.5 as the energetically most favorable surface termination. Based on this preferential (111) surface termination, 2 nm sized particles with octahedral shape were simulated. Compared to the infinite magnetite surface, they provide new geometrical features such as edges and vertices resulting in an increased solvent interaction for both water and simple electrolyte systems. In a third step, the interactions of the magnetite (111) surface with hazardous elements were investigated. The structural incorporation of Tc(IV) studied in the present thesis revealed a preferable substitution mechanism of two octahedrally coordinated Fe by one Tc coupled to the formation of a vacancy. Moreover, the Pu(III) sorption complex as proposed by X-ray absorption spectroscopy (XAS) was confirmed by ab initio simulations. The comparison with XAS – specifically using extended X-ray absorption fine structure (EXAFS) spectroscopy – signifies the main strength of the presented studies. The combination of laboratory experiment, spectroscopic characterization and the here presented atomistic modeling allows for a consistent interpretation of the experimental data and an elucidation of local structural changes as function of parameters such pH, metal loading and magnetite particle size. These findings contribute to a deeper understanding of the radionuclide-mineral interaction and provide a valuable basis for environmental safety analyses.
| Item Type: | Thesis |
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| Dissertation Type: | Cumulative |
| Date of Defense: | 27 February 2025 |
| Subjects: | 500 Science > 540 Chemistry 500 Science > 550 Earth sciences & geology |
| Institute / Center: | 08 Faculty of Science > Institute of Geological Sciences |
| Depositing User: | Hammer Igor |
| Date Deposited: | 16 May 2025 13:28 |
| Last Modified: | 16 May 2025 13:29 |
| URI: | https://boristheses.unibe.ch/id/eprint/6160 |
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