Article Cite This: Environ. Sci. Technol. 2018, 52, 6282−6290
pubs.acs.org/est
Iron Vacancies Accommodate Uranyl Incorporation into Hematite Martin E. McBriarty,*,† Sebastien Kerisit,† Eric J. Bylaska,‡ Samuel Shaw,§ Katherine Morris,§ and Eugene S. Ilton*,† †
Physical Sciences Division and ‡Environmental Molecular Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom S Supporting Information *
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-Fe2O3) 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 ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing 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 in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions.
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Octahedral coordination environments of U6+ range from uranyl with 4 equatorial ligands to Oh, with average bond lengths typically in the range of 2.05−2.10 Å.22 The coordination environment of U6+ as a trace impurity in otherwise uranium-free host minerals is the subject of ongoing research. In many such studies, extended X-ray absorption fine structure (EXAFS) spectra are interpreted by prescribing coordination shells which combine multiple U−O and U−Fe interactions of similar lengths, simulating EXAFS spectra from these shells, and then fitting the interaction distances, coordination numbers, and disorder parameters for these shells until a reasonable match to the experimental EXAFS is achieved.6,8−15,19,20 However, this shell-by-shell methodology has led to several conflicting models for the local structure of U6+ incorporated into hematite, one of the most abundant and stable iron-bearing minerals. In the earliest observation of U incorporation into FOH, Duff et al.6 aged uranyl nitrate with Fe(NO3)3 at 70 °C and pH
INTRODUCTION The interaction of nanophase iron (oxyhydr)oxides (FOHs) with the radiotoxic element uranium has been extensively studied to determine how these materials regulate uranium transport in soils and in engineered contamination mitigation systems.1−5 In this regard, there is growing evidence that uranium can be incorporated into the structure of FOHs during hydrothermal maturation or redox transformation of ferrihydrite (Fe(OH)3) to more stable phases such as goethite (αFeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4), where such phases have been proposed as suitable waste forms for the long-term containment of uranium.6−20 It follows that a deep understanding of the relationship between uranium-FOH incorporation mechanisms and the local coordination environment and chemical state of uranium impurities in FOH minerals is needed to predict its long-term behavior under sequestration conditions. Under oxidizing conditions, U6+ is often stable as the uranyl cation (UO2)2+, which features two short (∼1.8 Å) U−O bonds in a trans arrangement.21 This configuration is found in numerous minerals, molecular complexes, and aqueous solutions where coordination by 4−6 ligands in the equatorial plane perpendicular to the trans-dioxo axis is typical.22 © 2018 American Chemical Society
Received: Revised: Accepted: Published: 6282
January 16, 2018 April 7, 2018 May 14, 2018 May 14, 2018 DOI: 10.1021/acs.est.8b00297 Environ. Sci. Technol. 2018, 52, 6282−6290
Article
Environmental Science & Technology
Simulated EXAFS spectra, calculated from an ensemble of AIMD trajectories,24 were mixed in linear combination with spectra for uranyl adsorbed to FOH and fitted to experimental EXAFS spectra for U6+-hematite.10,13 Close agreement between experiment and theory reveals that U6+ adopts a coordination similar to the trans-dioxo uranyl configuration but that the proximal and more distal environment surrounding U is highly defective, resembling hydrohematite25−27 rather than pure hematite. The best fits to the experimental EXAFS data include significant contributions from structures with higher energy than the calculated ground state, indicating that growth mechanisms and kinetics must be considered when comparing theoretical results with experimental data. The example given here highlights the general role of defects in accommodating the incorporation of a symmetrically incommensurate dopant into the structure of hematite, which is a common iron mineral in soils and subsurface environments, a potential waste form for radionuclide sequestration, and a candidate material for photoelectrochemical energy conversion devices.
11 for 25 days, yielding hematite and trace goethite for low initial U concentrations. Their EXAFS data was fitted reasonably well by a model with the shortest U−O distance of 2.21 Å, and they concluded that (UO2)2+ had completely converted to relatively symmetrical octahedral coordination in the FOH structure. Ilton et al.10 performed a similar synthesis and EXAFS analysis, but they fitted the EXAFS to a more complex model, finding a significant fraction of short (1.79 Å) U−O bonds which they attributed to strongly adsorbed uranyl which persisted on the hematite surface or was occluded in nanopores. Most recently, Marshall et al.13 aged ferrihydrite in the presence of uranyl in a synthetic cement leachate (pH 10.5) at 105 °C for 45 days to yield a sample composed of >90% hematite and
