Inhibitory effect of silicate on the Fe(II)-catalysed sequestration of U by Fe(III) oxides

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Elsevier; Cambridge Publications
It has recently been reported that the natural Fe (II)- catalysed transformation of Fe (III) minerals to more crystalline forms can result in the sequestration of uranium [1], thus potentially leading toward a solution to the universal and emotive problem of uranium contamination. While this process may reduce uranium migration, there is no clear knowledge of its viability in conditions which inhibit the transformation of iron oxides. Here we present XAS results of Fe (II)-catalysed transformations in systems containing 2-line ferrihydrite, silicate and uranium as U (VI). The chemical environment of both co-precipitated and adsorbed U (VI) bound by 2-line ferrihydrite was initiallyidentical, in both cases being associated with the iron oxides as a surface complex. Upon addition of aqueous Fe (II) in anoxic conditions, 2-line ferrihydrite with associated U (VI) transformed to goethite. Ab initio modelling of EXAFS data indicated that U (VI) associated with 2-line ferrihydrite was incorporated into the newly formed goethite mineral structure. In contrast, silicate-ferrihydrite only transformed to ferrihydrite with the associated U (VI) remaining in a form similar to its initial state. The adsorbed U (VI) did however become more resistant to reductive dissolution indicating at least a partial reduction in mobility. These results demonstrate that the Fe (II)-catalysed crystallisation of iron oxides may not always induce uranium reduction or immobilisation in relevant environmental conditions. The precise mechanism of the inhibitory effect of silicate, with a focus on how to control conditions to reduce this effect, must be resolved before this process may be considered a reliable means of preventing sub-surface uranium transport. © 2020 Elsevier B.V.
Uranium, Iron, Silicate minerals, Crystallization, Goethite, Transformations
Boland, D. D., Collins, R. N., Payne, T. E. & Waite, T. D. (2010). Inhibitory effect of silicate on the Fe(II)-catalysed sequestration of U by Fe(III) oxides. Presentation to the 20th Annual V.M. Goldschmidt Conference (Goldschmidt 2010) - "Earth, Energy and the Environment", 13th - 18th June 2010. Knoxville, Tennessee: Knoxville Convention Center. In Geochimica et Cosmochimica Acta, 74(12), A101. doi:10.1016/j.gca.2010.04.027