The Research Program
The SLAC-SFA will use advanced synchrotron-based techniques and innovative methods to uncover the fundamental factors that control the speciation and stability of reduced uranium in the subsurface.
Particular foci are "structure-composition-property" relationships of biogenic uraninite and of complex natural iron oxides. Collaborators from leading institutions in the U.S. and abroad will contribute to this multi-disciplinary research program.
Uraninite (UO2 + X, 0 ≤ X ≤ .25) is of great interest because it is a potential long-term sink for uranium contamination in the subsurface. Biogenic uraninite is produced by the reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)). There is much interest in driving this reaction in the subsurface by stimulating growth of subsurface microbial populations, which can reduce metal ions. Uraninite is known to be a structurally and compositionally complex material, and its nanoscale size further complicates its geochemical behavior.
Iron oxides are among the most important reactive solids in subsurface environments, acting as natural filters and as oxidants/reductants of inorganic contaminants. Ferrihydrite, a nanoscale hydrated Iron(III) oxide, is of particular importance because it is continuously produced in surface and ground waters forming grain coatings that mask the identity of the underlying sediment. Ferrihydrite is metastable and transforms to more stable iron oxides such as goethite (α - FeOOH) and hematite (α - Fe2O3). If reducing conditions are present, such as may occur in an aquifer during uranium bioremediation, ferrihydrite may transform into iron(II)-bearing minerals such as magnetite (Fe3O4) as well as goethite and other iron(III) bearing minerals. Uranium sorbed onto ferrihydrite can be remobilized into the aquifer or immobilized within the Iron oxides. Because of the abundance and ongoing formation/transformation of iron oxides in the subsurface, these processes have the potential to naturally remediate substantial amounts of subsurface uranium contamination.
Uraninite precipitated by microbes is typically nanoscale, with diameters in the 2 to 5 nm range.
Structural model of biogenic uraninite nanoparticles developed by this research program. The inner portion of the particles is well ordered and similar to stoichiometric or near-stoichiometric UO2.0, and the material consequently exhibits a solubility similar to that of bulk UO2.0. The outer periphery is contracted in resonse to surface energy at the particle terminations.
Iron oxides (redish and yellow-brown minerals) are abundant in soils as grain coatings and cements, as can been in this soil column, and in the sediment drill core from the contaminated DOE Oak Ridge