Spectroscopic and Diffraction Study of Uranium Speciation in Contaminated Vadose Zone Sediments from the Hanford Site, Washington State

Authors

Jeffrey Catalano, Stanford UniversityFollow
Steven Heald, Argonne National Laboratory
John M. Zachara, Pacific Northwest National LaboratoryFollow
Gordon Brown Jr., Stanford University

Date of this Version

2004

Citation

Environ. Sci. Technol. 2004, 38, 2822-2828

Abstract

Contamination of vadose zone sediments under tank BX-102 at the Hanford site, Washington, resulted from the accidental release of 7-8 metric tons of uranium dissolved in caustic aqueous sludge in 1951. We have applied synchrotron-based X-ray spectroscopic and diffraction techniques to characterize the speciation of uranium in samples of these contaminated sediments. U L_III-edge X-ray absorption fine structure (XAFS) spectroscopic studies demonstrate that uranium occurs predominantly as a uranium- (VI) silicate from the uranophane group of minerals. XAFS cannot distinguish between the members of this mineral group due to the near identical local coordination environments of uranium in these phases. However, these phases differ crystallographically, and can be distinguished using X-ray diffraction (XRD) methods. As the concentration of uranium was too low for conventional XRD to detect these phases, X-ray microdiffraction (µXRD) was used to collect diffraction patterns on ~20 µm diameter areas of localized high uranium concentration found using microscanning X-ray fluorescence (µSXRF). Only sodium boltwoodite, Na(UO₂)(SiO₃OH)•1.5H₂O, was observed; no other uranophane group minerals were present. Sodium boltwoodite formation has effectively sequestered uranium in these sediments under the current geochemical and hydrologic conditions. Attempts to remediate the uranium contamination will likely face significant difficulties because of the speciation and distribution of uranium in the sediments.