Is there a link between Fe(III) oxide reactivity, Fe(II)-catalysed crystallisation and U(VI) reduction?

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Elsevier; Cambridge Publications
The discovery that Fe (II) catalyses the crystallisation of highly disordered Fe (III) oxides from years to hours has dramatically changed our perception of the time-scales that may be involved in the iron redox cycle with far-reaching ramifications to the kinetics of other globally important biogeochemical cycles such as carbon and sulfur [1] as well as that of redox sensitive elements such as uranium [2]. While we still explore the mechanisms of this potentially exciting new pathway in the iron redox cycle, in very simplified systems, one must ask of the relevance of such studies when these conditions are typically far-removed from those experienced in the natural environment. Indeed, when done so, it is observed that the Fe (II)-catalysed crystallisation process may not have global applicability [3, 4]. However, obtaining information on why a process does not work is often as enlightening as direct observation of the process in motion. Here we report on experiments which demonstrate that no relationship exists between Fe (III) oxide reactivity and its propensity for Fe (II)-catalysed crystallisation when the intrinsic reactivity of the Fe (III) oxide is induced by the presence of the sulfur tetrahedral oxyanion - SO4. We further demonstrate that Fe (III) oxide crystallinity is key for the Fe (II) reduction of U (VI) and, moreover, that (FeII)-U (VI) electron transfer is related to the redox potential of the Fe (II)- Fe (III) couple. In other words, U (VI) will not be reduced by Fe (II) when both are sorbed to highly disordered Fe (III) oxides such as ferrihydrite and/or schwertmannite.
Reactivity, Crystallization, Redox process, Iron, Sulfur, Carbon
Collins, R. N., Payne, T. E., & Waite, T. D. (2010). Is there a link between Fe(III) oxide reactivity, Fe(II)-catalysed crystallisation and U(VI) reduction? Paper presented 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), A186. doi:10.1016/j.gca.2010.04.028