Newly recognized hosts for uranium in the Hanford Site vadose zone

Authors

Joanne E. Stubbs, Johns Hopkins University, Department of Earth and Planetary SciencesFollow
Linda A. Veblen, Johns Hopkins University, Department of Earth and Planetary Sciences
David C. Elbert, Johns Hopkins University, Department of Earth and Planetary Sciences
John M. Zachara, Pacific Northwest National LaboratoryFollow
James A. Davis, U.S. Geological Survey
David R. Veblen, Johns Hopkins University, Department of Earth and Planetary Sciences

Date of this Version

2009

Citation

Geochimica et Cosmochimica Acta 73 (2009) 1563–1576; doi:10.1016/j.gca.2008.12.004

Abstract

Uranium contaminated sediments from the U.S. Department of Energy’s Hanford Site have been investigated using electron microscopy. Six classes of solid hosts for uranium were identified. Preliminary sediment characterization was carried out using optical petrography, and electron microprobe analysis (EMPA) was used to locate materials that host uranium. All of the hosts are fine-grained and intergrown with other materials at spatial scales smaller than the analytical volume of the electron microprobe. A focused ion beam (FIB) was used to prepare electron-transparent specimens of each host for the transmission electron microscope (TEM). The hosts were identified as: (1) metatorbernite [Cu(UO₂)2(PO₄)₂•8H₂O]; (2) coatings on sediment clasts comprised mainly of phyllosilicates; (3) an amorphous zirconium (oxyhydr)oxide found in clast coatings; (4) amorphous and poorly crystalline materials that line voids within basalt lithic fragments; (5) amorphous palagonite surrounding fragments of basaltic glass; and (6) Fe- and Mn-oxides. These findings demonstrate the effectiveness of combining EMPA, FIB, and TEM to identify solid-phase contaminant hosts. Furthermore, they highlight the complexity of U geochemistry in the Hanford vadose zone, and illustrate the importance of microscopic transport in controlling the fate of contaminant metals in the environment.