Journal
ENVIRONMENTAL SCIENCE-NANO
Volume 4, Issue 6, Pages 1304-1313Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c7en00024c
Keywords
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Funding
- Australian Research Council Future Fellowship [FT110100067]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences (CSGB) Division through the Geosciences program at Pacific Northwest National Laboratory
- Australian Research Council Linkage, Infrastructure, Equipment and Facilities (LIEF) [LE120100104]
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Nanoscale zero-valent iron (nZVI) is a potential remediation agent for uranium-contaminated groundwaters, however, a complete mechanistic understanding of the processes that lead to uranium immobilization has yet to be achieved. In this study, the short-term anoxic reaction of U(VI) with fresh, (anoxic) aged and corroded nZVI particles was investigated under aqueous conditions conducive to the formation of thermodynamically stable U(VI)-Ca-CO3 ternary aqueous complexes. The first stage of the reaction between U(VI) and nZVI was assigned to sorption processes with the formation of surface U(VI)-carbonate complexes. Aged nZVI removed U.VI) faster than either fresh or corroded nZVI and it is hypothesized that U reduction initially occurs through the transfer of one electron from Fe(II) in the nZVI surface oxide layer. Evidence for reduction to U(V) was obtained through X-ray photoelectron spectroscopy and by determination of U-O bond distances of similar to 2.05 angstrom and 2.27 angstrom, using U L-III-edge X-ray absorption spectroscopy, which are similar to those observed for the U(V) site in the mixed U(V)/U(VI) carbonate mineral wyartite. Scanning transmission electron microscopy also demonstrated that U was present as a nanoparticulate phase after one day of reaction, rather than a surface complex. Further reduction to U(IV), as observed in previous studies, would appear to be rate-limiting and coincident with the transformation of this meta-stable U-carbonate phase to uraninite (UO2).
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