Uranium Uptake from Aqueous Solution by Interaction with Goethite, Lepidocrocite, Muscovite, and Mackinawite: An X-ray Absorption Spectroscopy Study

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• 作者：Lesley N. Moyes ; Richard H. Parkman ; John M. Charnock ; David J. Vaughan ; Francis R. Livens ; Colin R. Hughes ; and Anna Braithwaite
• 刊名：Environmental Science & Technology
• 出版年：2000
• 出版时间：March 15, 2000
• 年：2000
• 卷：34
• 期：6
• 页码：1062 - 1068
• 全文大小：117K
• 年卷期：v.34,no.6(March 15, 2000)
• ISSN：1520-5851

文摘

The retention of radionuclides by interaction with mineralphases has significant consequences for the planningof their short- and long-term disposal to geological systems.An understanding of binding mechanisms is important indetermining the ultimate fate of radionuclides following releaseinto natural systems and will give increased confidencein predictive models. X-ray absorption spectroscopy (XAS)has been used to study the local environment of uraniumtaken up from aqueous solution by the surfaces of goethite,lepidocrocite, muscovite, and mackinawite. On both ironhydroxides uranium uptake occurs by surface complexationand ceases when the surface is saturated. The muscovitesurface does not become saturated and uptake increaseslinearly suggesting formation of a uranium phase on thesurface. Uranium uptake on mackinawite also suggests areplacement or precipitation process. XAS indicatesthat bidentate inner-sphere surface complexes are formedon the iron hydroxides by coordination of two surfaceoxygens from an iron octahedron in the equatorial planeof the complex. Uranium uptake on muscovite may occurthrough surface precipitation, the first layer of uraniumatoms binding through equatorial coordination of two adjacentsurface oxygens from a silicate tetrahedron, with theaxial oxygens of the uranyl unit aligned across the hexagonal"cavities" created by the rings of tetrahedra. At lowconcentrations, uptake on mackinawite occurs at locallyoxidized regions on the surface via a similar mechanism tothat on iron hydroxides. At the highest concentrations,equatorial oxygen bond distances around 2.0-2.1 Å areobserved, inconsistent with the presence of uranyl species.The average number of axial oxygens also decreaseswith increasing concentration, and these results suggestpartial reduction of uranium. The nature of these differentsurface reactions plays an important role in assessingthe geochemical behavior of uranium in natural systems,particularly under reducing conditions.