Iron Vacancies Accommodate Uranyl Incorporation into Hematite.

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

doi:10.1021/acs.est.8b00297

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.

Highlights

The interaction of nanophase ironoxides (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, 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
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 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] 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
Maintaining the U6+ oxidation state required the use of physical or chemical charge compensation schemes (CCS) to accommodate the extra charge on the site normally occupied by a trivalent cation

Summary

■ 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. Synthetic EXAFS spectra derived from AIMD simulations of U5+ in goethite, in combination with spectra from adsorbed uranyl and minority incorporated phases, were found to closely match experimental EXAFS spectra of reduced U-FOH systems.[17,18] In particular, McBriarty et al.[18] predicted that U was preferentially partitioned into the minority phase goethite, despite the solid fraction containing 90% lepidocrocite. This prediction was confirmed by atomic-resolution electron microscopy, providing further evidence that AIMD-. 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

■ MATERIALS AND METHODS

■ ACKNOWLEDGMENTS

■ REFERENCES