The Aqueous Thermodynamics and Complexation Reactions of Anionic Silica and Uranium Species to High Concentration. (Felmy ESMP-silicate, PNNL Scope #30944)
EMSL Project ID
3600a
Abstract
Highly basic tank wastes contain several important radionuclides, including 90Sr, 99Tc, and 60Co,as well as actinide elements (i.e., isotopes of U, Pu, and Am). These highly basic tank wastes are
known to have leaked into the vadose zone at the Hanford Site. Upon entering the sediments in
the vadose zone, the highly basic solutions dissolve large concentrations of silica from the silica
and aluminosilicate minerals present in the subsurface. These dissolution reactions alter the
chemical composition of the leaking solutions, transforming them from a highly basic (as high
2M NaOH) solution into a pore solution with a very high concentration of dissolved silica and a
significantly reduced pH. This moderately basic (pH 9 to 11), high-silica solution has the potential to complex radionuclides and move through the subsurface. Such strong radionuclide
complexation is a currently considered transport vector that has the potential to expedite radionuclide transport through the vadose zone. These strong complexation effects have the ability to significantly alter current conceptual models of contaminant migration beneath leaking tanks.
In this project, we are determining the aqueous thermodynamics and speciation of dissolved
silica and silica-radionuclide complexes to high silica concentration. We are also initiating
studies of U(VI) speciation under strongly basic conditions.
One part of this project is to build a prototype Laser-induced breakdown spectroscopy/detection (LIBS/LIBD) system and demonstrate its applicability in the detection of heavy metals including Pd, AS, Al, Si, as well as lanthanides and actinides. Initial testing of correlation between plasma formation laser energy threshold and targeted particle size will be carried out and confirm the observations by other research groups the ability to measure particle size based on the plasma formation energy. We’ll generate calibration curves between the plasma event and the density of particles of a given size. From there on we may initiate testing of Hanford ground waters to examine the density and possible sizes of any nano-particles present in these waters that may have a role in the migration in subsurface soils and sediments.
Another part of this project is to
Determining the near-surface composition and structural state of Co-silicates with XPS:
Objectives: The solubility of Co-silicates in aqueous solutions shows interesting incongruent behavior as a function of pH. We propose to use XPS to compliment the solution data, by tracking the Co/Si ratio of the near-surface. In addition, XPS should provide information on the chemical state of the Co-silicate surface that could yield insight into the dissolution mechanism.
XPS analysis: These goals depend on high energy resolution scans of the Co2p, Si2p, and O1s photoemission lines for reacted and unreacted Co-silicates, and a reference Co-hydroxide. The O1s line will give information on the hydration state of the near-surface by semi-quantifying the proportions of OH-, H2O (both chemisorbed and hydrogen bonded water), and O2-. The Si2p line contains information on the structural state of Si. It is well known that Si2p BEs increase systematically with increasing silica polymerization: isolated silica tetrahedra (e.g., uranyl silicates), silica tetrahedral sheets (e.g., phyllosilicates such as micas), and silica tetrahedral frame work structures (e.g., quartz) give Si2p BEs around 101.2, 102-103, and 103-104 eV, respectively. Comparison of Co2p BEs between Co-silicates and Co-hydroxide may yield information on the structural environment of Co. Carbon 1s needs to be recorded in order to reference the BE scale, the samples are electrical insulators, and to compensate for a potential C overlayer effect on measured Co/Si ratios due to the appreciably different kinetic energies of Co2p and O1s photoelectrons.
Materials: Both Co-silicate and Co-hydroxide powders will be presented as thin films on 13 mm diameter filter paper. It is hoped that the powders will be sufficiently consolidated such that “pressing” will not be necessary.
Project Details
Project type
Exploratory Research
Start Date
2004-11-11
End Date
2006-03-27
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members