Physical Sciences Division
Research Highlights

June 2005

Formation of U(V) on mineral surfaces

Uranium was discovered in 1789, but aqueous U⁵⁺ was not identified until the 1940s during the Manhattan Project. U⁵⁺ in aqueous solution as UO²⁺ has a narrow stability field between pH 2 and 4; outside this pH regime U⁵⁺ rapidly disproportionates. The initial findings discounted the possibility of U⁵⁺ existing in the environment. Eugene Ilton and co-workers discovered that heterogeneous reduction of aqueous U⁶⁺ at ferrous mica surfaces produces and preserves U⁵⁺, as a sorbed species, over a broad range of solution compositions.

Macroscopic single crystals of annite, the near-end member ferrous trioctahedral mica, were reacted with argon-sparged aqueous solutions containing 5 µM U⁶⁺ and variable Na⁺, from pH 4.5 to 9.5. X-ray photoelectron spectroscopy (XPS) spectra of the U4f level for U sorbed on annite edge orientations were fit with three components representing U⁶⁺, U⁵⁺, and U⁴⁺. XPS yields information on the chemical state and composition of solid surfaces, where most of the signal was integrated over the top 80Å of our sample. The valence of U can be determined from the binding energies (BE) and satellite structures associated with the 4f_7/2 and 4f_5/2 levels, and the 5f intensity. In 2004, Eugene Ilton and his team demonstrated that structural Fe(II) in micas can reduce sorbed U⁶⁺, but modeled the 4f_7/2 primary peak with only two components (assumed to be U⁴⁺ and U⁶⁺). Good fits across a wide range of average U oxidation states were only possible by not including the satellite structure in the fitting and by allowing wide variations in both the full-width-at-half-maximum (FWHM) and BE separation of the component peaks. In their latest work, U⁵⁺ was discovered by explicitly including the satellite structure and three components in fitting the spectra. Schoepite, UO₃·(H₂O)₃, and uraninite, UO₂, were used to derive peak parameters for the U⁶⁺ and U⁴⁺ components. The U⁵⁺ component was derived by fitting a (U⁵⁺-U⁶⁺) oxyhydroxide, synthesized under hydrothermal conditions, with the U⁶⁺ component and solving for the U⁵⁺ peak parameters.

The finding of U(V) on mineral surfaces indicates that a key aspect of the cycling of U in the environment has been previously over looked. The results show that U⁵⁺ could play an important, but previously unidentified, role in the low-temperature geochemical cycling of uranium. This research is cover feature of the May 2, 2005 issue of Inorganic Chemistry, volume 44, issue 9.

Reference

Ilton ES, A Haiduc CO Moses, D Elbert, and DR Veblen. 2004. "Heterogeneous Reduction of Uranyl by Micas: Crystal Chemical and Solution Controls." Geochimica et Cosmochimica Acta 68(11):2417-2435. DOI:10.1016/j.gca.2003.08.010.