Environ. Sci. Technol., 38 (10), 2822 -2828, 2004. 10.1021/es049963e S0013-936X(04)09963-8
Web Release Date: April 17, 2004

Copyright © 2004 American Chemical Society

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

Jeffrey G. Catalano,* Steven M. Heald, John M. Zachara, and Gordon E. Brown, Jr.

Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, PNC-CAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, Environmental Dynamics & Simulation, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-96, Richland, Washington 99352, and Stanford Synchrotron Radiation Laboratory, SLAC, MS 69, 2575 Sand Hill Road, Menlo Park, California 94025

Received for review January 7, 2004

Revised manuscript received March 9, 2004

Accepted March 16, 2004

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 LIII-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 (XRD) was used to collect diffraction patterns on ~20 m diameter areas of localized high uranium concentration found using microscanning X-ray fluorescence (SXRF). 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.

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