Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Martin E McBriarty, Sebastien Kerisit, Eric J Bylaska, Samuel Shaw, Katherine Morris, Eugene S. Ilton

    Research output: Contribution to journalArticlepeer-review

    Abstract

    Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe<sub>2</sub>O<sub>3</sub>) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional <i>ab initio</I> molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L<sub>3</sub>-edge EXAFS for U<sup>6+</sup>-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U<sup>6+</sup> that substitutes for structural Fe<sup>3+</sup>, which requires two partially protonated Fe vacancies situated at opposing corner-sharing lattice sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics over thermodynamics in determining the state of an impurity incorporated in a host phase under low temperature hydrothermal conditions.
    Original languageEnglish
    Pages (from-to)6282-6290
    Number of pages9
    JournalEnvironmental Science and Technology
    Volume52
    Issue number11
    Early online date14 May 2018
    DOIs
    Publication statusPublished - 14 May 2018

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