Uranium uptake from aqueous solution by interaction with goethite, lepidocrocite, muscovite, and mackinawite: An X-ray absorption spectroscopy study

The retention of radionuclides by interaction with mineral phases has signi ficant consequences for the planning of their short- and long-term disposal to geological systems. An understanding of binding mechanisms is important in determining the ultimate fate of radionuclides following release into n atural systems and will give increased confidence in predictive models. X-r ay absorption spectroscopy (XAS) has been used to study the local environme nt of uranium taken up from aqueous solution by the surfaces of goethite, l epidocrocite, muscovite, and mackinawite. On both iron hydroxides uranium u ptake occurs by surface complexation and ceases when the surface is saturat ed. The muscovite surface does not become saturated and uptake increases li nearly suggesting formation of a uranium phase on the surface. Uranium upta ke on mackinawite also suggests a replacement or precipitation process. XAS indicates that bidentate inner-sphere surface complexes are formed on the iron hydroxides by coordination of two surface oxygens from an iron octahed ron in the equatorial plane of the complex. Uranium uptake on muscovite may occur through surface precipitation, the first layer of uranium atoms bind ing through equatorial coordination of two adjacent surface oxygens from a silicate tetrahedron, with the axial oxygens of the uranyl unit aligned acr oss the hexagonal "cavities" created by the rings of tetrahedra. At low con centrations, uptake on mackinawite occurs at locally oxidized regions on th e surface via a similar mechanism to that on iron hydroxides. At the highes t concentrations, equatorial oxygen bond distances around 2.0-2.1 Angstrom are observed, inconsistent with the presence of uranyl species. The average number of axial oxygens also decreases with increasing concentration, and these results suggest partial reduction of uranium. The nature of these dif ferent surface reactions plays an important role in assessing the geochemic al behavior of uranium in natural systems, particularly under reducing cond itions.