Incorporation of Uranium into Hematite during Crystallization from Ferrihydrite

Timothy A Marshall,Francis R Livens,Pieter Bots,J Frederick W Mosselmans,Samuel Shaw,Gareth T W Law,Katherine Morris

doi:10.1021/es500212a

Timothy A Marshall, Francis R Livens + Show 5 more

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https://doi.org/10.1021/es500212a

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Abstract

Ferrihydrite was exposed to U(VI)-containing cement leachate (pH 10.5) and aged to induce crystallization of hematite. A combination of chemical extractions, TEM, and XAS techniques provided the first evidence that adsorbed U(VI) (≈3000 ppm) was incorporated into hematite during ferrihydrite aggregation and the early stages of crystallization, with continued uptake occurring during hematite ripening. Analysis of EXAFS and XANES data indicated that the U(VI) was incorporated into a distorted, octahedrally coordinated site replacing Fe(III). Fitting of the EXAFS showed the uranyl bonds lengthened from 1.81 to 1.87 Å, in contrast to previous studies that have suggested that the uranyl bond is lost altogether upon incorporation into hematite. The results of this study both provide a new mechanistic understanding of uranium incorporation into hematite and define the nature of the bonding environment of uranium within the mineral structure. Immobilization of U(VI) by incorporation into hematite has clear and important implications for limiting uranium migration in natural and engineered environments.

Highlights

Uranium is an environmental contaminant that arises as a result of authorized and accidental releases at various stages in the nuclear fuel cycle, including from uranium ore mining activities and post-reactor operations
In many countries, uranium-containing radioactive wastes, including spent nuclear fuel and intermediate-level waste, are likely to be disposed in deep geological disposal facilities (GDF)
We provide a detailed insight into the mechanism(s) of uranium incorporation during hematite formation under conditions relevant to both geological disposal and contaminated land to determine whether significant amounts of uranium could be sequestered into this phase in the long term

Summary

■ INTRODUCTION

Uranium is an environmental contaminant that arises as a result of authorized and accidental releases at various stages in the nuclear fuel cycle, including from uranium ore mining activities and post-reactor operations. We suggest that uranium adsorbed to the surface of the ferrihydrite particles is trapped within the solid phase during the aggregation process at the early stages of the crystallization process, consistent with the incorporation mechanism of Pb into hematite during hydrothermal crystallization of ferrihydrite.[33,37] The gradual increase in U(s) during crystallization and ripening indicates that U(VI) continues to be incorporated as the iron (oxyhdr)oxide crystals form and grow This is in contrast to the behavior observed for Pb, where the contaminant was slowly released from the hematite structure during ripening because of its incompatibility (i.e., located within defect sites) with the mineral structure.[33,37] This does not occur with uranium, suggesting that it may become located within a stable crystallographic site within the newly formed mineral, in agreement with modeling simulations.[20]. The U Fe bond distances for face sharing Fe (FeF) and the nearer corner sharing Fe (FeC1) are at approximately the same distance as the Fe Fe distance in hematite, whereas an increased atomic distance to the other two Fe shells (FeE and FeC2) is observed

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■ ACKNOWLEDGMENTS

■ REFERENCES