Quantum-Mechanical Investigation of the Structures and Energetics of Uranium and Plutonium Incorporated into the Magnetite (Fe3O4) Lattice

Understanding the behavior of radionuclides is key to designing and implementing effective waste storage and remediation schemes. Incorporation into minerals is one process that may reduce the mobility of these contaminants. This study uses quantum-mechanical modeling to evaluate the incorporation of U and Pu into magnetite (Fe3O4), a common mineral and steel corrosion product. The incorporation from solid and aqueous sources (e.g., PuO2(s) and UO22+(aq)) with various oxidation states (Pu +3; U and Pu +4, +5, +6) is explored. Charge balancing is achieved via Fe lattice vacancies. The incorporation energies (ΔEinc) depend strongly on the stability of different actinide oxide phases and the hydration energies of aqueous species (e.g., ΔEinc of 1.38 and 2.88 eV for U5+ from δ-U2O5(s) and UO2+(aq), respectively). The calculated bonding environment of incorporated U (U–Oax = 2.16–2.24 ？, CN = 2; U–Oeq = 2.30–2.34 ？, CN = 4) aligns with the range of experimental results. Analysis of incorporation energies as well as the charge and spin distributions within the Fe 3d and U 5f orbitals indicates that U prefers a valence state of approximately U5+ in magnetite. The physical and electronic structures of Pu-magnetite, while lacking experimental comparison, suggest that Pu (in a slightly more reduced state between Pu5+ and Pu4+) could also be incorporated, perhaps with more flexibility given the comparable reaction energetics for simple substitution of Pu3+ for Fe3+ (ΔEinc = 2.00 eV from Pu(OH)30(aq) vs 2.01 eV from Pu(OH)40(aq)). Overall, the results provide insight into potential immobilization pathways for actinides and serve as a reference for future characterization of actinide-incorporated magnetites