Web Release Date: June 1,
Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site
and
Department of Entomology, Soils, and Plant Sciences, 270 Poole Agricultural Center, Clemson University, Clemson, South Carolina 29634-0315, Lawrence Berkeley National Laboratory, Advanced Light Source, One Cyclotron Road, Berkeley, California 94720, United States Geological Survey, Water Resource Division, 345 Middlefield Road, MS 465, Menlo Park, California 94025, and Pacific Northwest Laboratory, Richland, Washington 99352
Received for review September 13, 2006
Revised manuscript received April 3, 2007
Accepted April 11, 2007
Abstract:
Uranium (U) solid-state speciation in vadose zone
sediments collected beneath the former North Process
Pond (NPP) in the 300 Area of the Hanford site (Washington)
was investigated using multi-scale techniques. In 30 day
batch experiments, only a small fraction of total U (~7.4%)
was released to artificial groundwater solutions equilibrated
with 1% pCO2. Synchrotron-based micro-X-ray fluorescence
spectroscopy analyses showed that U was distributed among
at least two types of species: (i) U discrete grains
associated with Cu and (ii) areas with intermediate U
concentrations on grains and grain coatings. Metatorbernite
(Cu[UO2]2[PO4]2·8H2O) and uranophane (Ca[UO2]2[SiO3(OH)]2·5H2O) at some U discrete grains, and muscovite at U
intermediate concentration areas, were identified in
synchrotron-based micro-X-ray diffraction. Scanning electron
microscopy/energy dispersive X-ray analyses revealed
8-10 
m size metatorbernite particles that were embedded
in C-, Al-, and Si-rich coatings on quartz and albite
grains. In - and bulk-X-ray absorption structure (-XAS
and XAS) spectroscopy analyses, the structure of
metatorbernite with additional U-C and U-U coordination
environments was consistently observed at U discrete
grains with high U concentrations. The consistency of the
- and bulk-XAS analyses suggests that metatorbernite
may comprise a significant fraction of the total U in the
sample. The entrapped, micrometer-sized metatorbernite
particles in C-, Al-, and Si-rich coatings, along with the more
soluble precipitated uranyl carbonates and uranophane,
likely control the long-term release of U to water associated
with the vadose zone sediments.
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