The short-term reduction of uranium by nanoscale zero-valent iron (nZVI): Role of oxide shell, reduction mechanism and the formation of U(v)-carbonate phases

Sergey Tsarev, Richard N. Collins, Eugene S. Ilton, Adam Fahy, T. David Waite

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39 Citations (Scopus)

Abstract

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 ∼2.05 Å and 2.27 Å, using U LIII-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).

Original languageEnglish
Pages (from-to)1304-1313
Number of pages10
JournalEnvironmental Science: Nano
Volume4
Issue number6
DOIs
Publication statusPublished - 2017
Externally publishedYes

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