| This dissertation examines specific (bio)geochemical aspects of U sequestration processes at the applied and fundamental levels using a combination batch sorption experiments, a suite of bulk and particle surface characterization techniques, and synchrotron-based X-ray spectroscopic and diffraction techniques.;Uranium speciation and distribution have been determined in contaminated vadose and groundwater zone sediments from the Hanford, Washington 300 Area Process Pond site. We used synchrotron-based micro-X-ray fluorescence (muXRF) imaging, micro-X-ray absorption near edge structure (muXANES) spectroscopy, and micro-X-ray diffraction (muXRD) techniques combined with bulk U L III-edge EXAFS spectroscopy to determine the distribution and speciation of U and Cu through the vadose and groundwater zones beneath North Process Pond ;Future subsurface transport of U released from the 300 Area sediments and similar sites will be strongly affected by uranium adsorption to subsurface materials, particularly clay minerals which can be a major sink for U in contaminated environments. We have used a series of batch sorption/desorption experiments combined with U LIII-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy and surface complexation modeling to investigate the dominant sorption process(es) governing uranyl uptake by chlorite. Sorption was independent of ionic strength, suggesting a dominantly inner-sphere sorption mechanism. The maximum sorption loading was 0.28 mumoles U g-1 chlorite at pH 4, whereas the maximum sorption loading was 6.3 mumoles U g-1 chlorite at pH 6.5 and pH 10. U(VI) uptake was also dependent on the solution composition, i.e., the presence of CO3 and Ca. A desorption experiment performed as a series of sequential extraction steps indicated that (1) there is little or no weakly bound U(VI) or U(VI) coprecipitated with ferrihydrite, (2) 60 to 80% of U(VI) inner-sphere sorption complexes are desorbed following a 0.1 M HCl step over one week, and (3) 100% desorption of adsorbed U(VI) is accomplished by a 1.0 M HCl step over one week. Fits of the U LIII-edge EXAFS spectra of short-term sorption samples indicate that U(VI) forms inner-sphere sorption complexes at [Fe(O,OH)6] octahedral sites in a bidentate manner, with evidence for U(VI)-CO3 sorption complexes were present, although there was no evidence for U(VI)-CO3-Ca sorption complexes. The EXAFS-derived parameters were used to constrain the type(s) of U(VI)-bearing surface species and combined with the observed batch sorption trends as input for a surface complexation model (SCM) to evaluate the relationships between aqueous U(VI) speciation and surface species. The model successfully predicts U(VI) sorption in the CO3-Ca-free and CO3-bearing system, but under-predicts U(VI) sorption by up to 30% in the CO3-Ca-bearing system. Long-term exposure of chlorite resulted in reduction of 25% of the total U. The U(IV)-bearing phase is likely an X-ray amorphous UO2-like phase. Surprisingly, the presence of Ca in solution prohibited U(VI) reduction. These results suggest that long-term exposure of chlorite to aqueous uranyl could result in U sequestration in the form of relatively insoluble, amorphous UO2, versus more transient sorption complexes in the short-term sorption samples. The results presented in this study provides additional molecular-scale information on uranyl sorption processes on chlorite as a function of pH and U(VI), Ca2+, and CO32- solution concentration. (Abstract shortened by UMI.)... |
