Using ab initio density-functional theory calculations, this study investigates the incorporation of uranium species into magnetite (Fe3O4).  Modeling of incorporation is traditionally accomplished by the calculation of solid-solid reactions in which both the contaminant source and replaced species sink are solid phases.  This study considers the incorporation of U6+ and U4+ into Fe3O4 via a variety of reactions using this method.  However, we also utilize a newly developed approach that combines periodic solid calculations with aqueous cluster calculations to describe the interactions of solid mineral phases with dissolved contaminant species.  With this approach, we are able to more realistically model the solid-aqueous reactions that may occur in the near-field environment of a radioactive waste repository or other radionuclide source.  During incorporation, U6+ and U4+ can replace Fe3+ and Fe2+ respectively in the inverse-spinel magnetite structure.  Charge balancing is achieved by a structural vacancy of either iron species, or by the reduction of an adjacent Fe3+.  Our results indicate that U6+ incorporation into the octahedral site of the magnetite structure, balanced by an octahedral Fe3+ vacancy, is the most energetically favorable solid-solid reaction (?E = 0.71 eV).  This same reaction was tested with aqueous source (UO22+(aq)) and sink (Fe3+(aq)) phases with the overall reaction energy shown to be 2.74 eV.  Overall, these reactions provide insight into potential immobilization pathways for uranium.  Further incorporation mechanisms are being analyzed for actinide incorporation into other iron oxides (e.g., goethite, FeO(OH)) and sulfides (e.g., pyrite, FeS2).

Computational study of uranium incorporation into magnetite (Fe<sub>3</sub>O<sub>4</sub>) via solid-solid and solid-aqueous reactions

248th National Meeting (2014)

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Computational study of uranium incorporation into magnetite (Fe3O4) via solid-solid and solid-aqueous reactions

Computational study of uranium incorporation into magnetite (Fe₃O₄) via solid-solid and solid-aqueous reactions