Synchrotron-Based Studies of Uranium Speciation in Contaminated Sediments and Related
Model System. [Jeffrey G. Catalano and Gordon E. Brown, Jr. (Stanford University and SSRL),
John M. Zachara and Steven Heald (Pacific Northwest National Laboratory)]. OBER-EMSP
(Brown).
Evaluation of the human health risks of widespread uranium contamination of soils,
sediments, and groundwater requires an understanding of the geochemical processes that control
the fate and transport of uranium in such systems. Determining the speciation of uranium at the
source of contamination, as well as the processes affecting transport, especially adsorption to
mineral surfaces, is essential in order to develop appropriate predictive transport models. We
have employed synchrotron-based X-ray spectroscopic and diffraction methods to study uranium
speciation in contaminated vadose zone sediments from the Hanford Site in Washington State
(Figure 1A), and have also applied these methods to study uranyl adsorption onto mineral
surfaces in model systems. In the study of contaminated sediments from the Hanford Site
(Catalano et al., 2004a), U LIII-EXAFS spectroscopy revealed that uranium occurred primarily in
the form of a uranophane group mineral. As the local atomic environment of uranium is similar
in all members of this group, EXAFS could not identify the specific phase present (Figure 1C).
Complementary
XRD
measurements
identified
only
sodium-boltwoodite,
Na(UO2)(SiO3OH)·1.5H2O, which has effectively sequestered uranium in these sediments under
the current geochemical and hydrologic conditions (Figure 1B). In a related study, we also
carried out detailed U LIII-EXAFS studies of a number of uranyl-containing crystalline solids for
use as model compounds in the analysis of contaminated Hanford samples (Catalano and Brown,
2004).
Two additional studies were conducted to evaluate how adsorption processes influence
uranium partitioning to solids from contaminated waters. In the first study, U(VI) adsorption
onto montmorillonite as a function of pH, ionic strength, and CO2 content was examined using
EXAFS spectroscopy (Catalano and Brown, 2004). At low ionic strength, outer-sphere
adsorption of U(VI) via cation exchange was found to be more important than predicted by
previous surface complexation models. U(VI) adsorbs to edge sites as inner-sphere uranylcarbonato ternary complexes in the presence of atmospheric CO2 concentrations. U(VI) binds
preferentially to [Fe(O,OH)6] edge sites over [Al(O,OH)6] sites in both the presence and absence
of CO2. Past surface complexation models should be modified to account for these findings, and
future studies of U(VI) transport in the environment should consider how uranium retardation
will be affected by changes in pH and ionic strength. In the second study, CTR diffraction and
GI-EXAFS spectroscopy were used to investigate U(VI) adsorption onto -Al2O3 and -Fe2O3
(1-102) surfaces at pH 5-7 (I = 0.1 M) (Catalano et al., 2004b). U(VI), in the form of uranylcarbonato ternary complexes, adsorbed in a monodentate fashion on -Al2O3 (1-102) and in a
bidentate fashion on -Fe2O3 (1-102). The -Fe2O3 (1-102) surface was found to have a higher
affinity for U(VI) adsorption under these solution conditions. The adsorption geometries
observed in this study differed from those typically found on powdered substrates using EXAFS
spectroscopy under the same conditions.
B
A
Figure 1. (A) Plumes of Cs-137,
Sb-125, and U-238 beneath the
Hanford BX-102 Tank [from A.W.
Perssons (2000) US DOE GJO-9840-TARA, GJO-HAN-19]; (B)
Portion of a microdiffraction pattern
of Hanford bore-hole sample 33A
[from Catalano et al., 2004a]
showing
evidence
for
Naboltwoodite; (C) U LIII-EXAFS
spectra (and Fourier transforms) of
four bore hole samples from beneath
the BX-102 Tank at Hanford [from
Catalano et al., 2004a].
C
33A
33
A
53A
53
A
61A
61
A
67A
67
A
References
J.G. Catalano, S.M. Heald, J.M. Zachara, and G.E. Brown, Jr., “X-ray spectroscopic and
diffraction study of uranium speciation in contaminated vadose zone sediments from the
Hanford site, Washington State, USA”, Environ. Sci. Technol. 38(10), 2822-2828 (2004a).
J.G. Catalano, T.P. Trainor, P.J. Eng, G.A. Waychunas, and G.E. Brown, Jr., “CTR diffraction
and grazing incidence XAFS study of U(VI) adsorption to -Al2O3 and -Fe2O3 (1-102)
surfaces”, Geochim. Cosmochim. Acta (2004b, submitted).
J.G. Catalano and G.E. Brown, Jr., “Uranyl adsorption on montmorillonite: evaluation of binding
sites and carbonate complexation”, Geochim. Cosmochim. Acta (2004, submitted).