Iron Vacancies Accommodate Uranyl Incorporation into Hematite

determining the state of an impurity incorporated in a host phase under low ..... hematite crystal structure has 13 nearest cation neighbors, defined ...
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Iron Vacancies Accommodate Uranyl Incorporation into Hematite Martin E. McBriarty, Sebastien Kerisit, Eric J. Bylaska, Samuel Shaw, Katherine Morris, and Eugene S. Ilton Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00297 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Iron Vacancies Accommodate Uranyl Incorporation into Hematite Martin E. McBriarty,*,† Sebastien Kerisit,† Eric J. Bylaska,‡ Samuel Shaw,§ Katherine Morris,§ Eugene S. Ilton*,† †

Physical Sciences Division and ‡Environmental Molecular Sciences Division, Pacific Northwest

National Laboratory, Richland, Washington 99352, U.S.A. §

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 1

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be

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sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-

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Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped

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material are impacted by the local coordination environment of incorporated uranium. While

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measurements of uranium coordination in hematite have been attempted using extended X-ray

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absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields

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ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations

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for various defect configurations to generate synthetic EXAFS spectra which were combined

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with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We

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discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like

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configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated

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Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment

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included significant proportions of vacancy configurations other than the minimum-energy

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configuration, pointing to the importance of incorporation mechanisms and kinetics in

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determining the state of an impurity incorporated in a host phase under low temperature

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hydrothermal conditions.

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INTRODUCTION

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The interaction of nanophase iron (oxyhydr)oxides (FOHs) with the radiotoxic element

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uranium has been extensively studied to determine how these materials regulate uranium

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transport in soils and in engineered contamination mitigation systems.1-5 In this regard, there is

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growing evidence that uranium can be incorporated in the structure of FOHs during

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hydrothermal maturation or redox transformation of ferrihydrite (Fe(OH)3) to more stable phases

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such as goethite (α-FeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4), where such phases

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have been proposed as suitable waste forms for the long term containment of uranium.6-20 It

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follows that a deep understanding of the relationship between uranium-FOH incorporation

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mechanisms and the local coordination environment and chemical state of uranium impurities in

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FOH minerals is needed to predict its long term behavior under sequestration conditions.

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Under oxidizing conditions, U6+ is often stable as the uranyl cation (UO2)2+, which features

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two short (~1.8 Å) U—O bonds in a trans arrangement.21 This configuration is found in

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numerous minerals, molecular complexes, and aqueous solutions where coordination by 4 – 6

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ligands in the equatorial plane perpendicular to the trans-dioxo axis is typical.22 Octahedral

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coordination environments of U6+ range from uranyl with 4 equatorial ligands to Oh, with

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average bond lengths typically in the range of 2.05 – 2.10 Å.22 The coordination environment of

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U6+ as a trace impurity in otherwise non-uranium-bearing host minerals is the subject of ongoing

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research. In many such studies, extended X-ray absorption fine structure (EXAFS) spectra are

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interpreted by prescribing coordination shells which combine multiple U—O and U—Fe

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interactions of similar lengths, simulating EXAFS spectra from these shells, and then fitting the

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interaction distances, coordination numbers, and disorder parameters for these shells until a

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reasonable match to the experimental EXAFS is achieved.6, 8-15, 19-20 However, this shell-by-shell

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methodology has led to several conflicting models for the local structure of U6+ incorporated into

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hematite, one of the most abundant and stable iron-bearing minerals.

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In the earliest observation of U incorporation into FOH, Duff et al.6 aged uranyl nitrate with

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Fe(NO3)3 at 70 °C and pH 11 for 25 days, yielding hematite and trace goethite for low initial U

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concentrations. Their EXAFS data was fitted reasonably well by a model with the shortest U—O

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distance of 2.21 Å, and they concluded that (UO2)2+ had completely converted to relatively

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symmetrical octahedral coordination in the FOH structure. Ilton et al.10 performed a similar

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synthesis and EXAFS analysis, but they fitted the EXAFS to a more complex model, finding a

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significant fraction of short (1.79 Å) U—O bonds which they attributed to strongly adsorbed

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uranyl which persisted on the hematite surface or was occluded in nanopores. Most recently,

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Marshall et al.13 aged ferrihydrite in the presence of uranyl in a synthetic cement leachate (pH

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10.5) at 105 °C for 45 days to yield a sample composed of >90% hematite and