Synchrotron-Based Studies of Uranium Speciation in Contaminated Sediments and Related Model System. [Jeffrey G. Catalano and Gordon E. Brown, Jr. (StanfordUniversity 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 uranyl-carbonato 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 uranyl-carbonato 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.
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).