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 4f7/2 and 4f5/2 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 4f7/2 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.
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.