The role of U(V) during heterogeneous reduction of aqueous U(VI) to U(IV).
EMSL Project ID
37692
Abstract
This proposal is being submitted in response to the Science theme call: Biogeochemistry and Subsurface Science, understanding the chemistry of radionuclides in the subsurface. It directly relates to the EMSL mission to provide resources for discovery and technological innovation in the environmental molecular sciences. The results of the proposed work will support the needs of the DOE and nation in managing uranium waste resulting from weapons production and the nuclear fuel cycle. We request standard access and the work is non-proprietary. We are proposing to study the role of the intermediate pentavalent oxidation state of uranium during heterogeneous reduction of U6+ to U4+ on mineral surfaces exposed to uranyl-bearing aqueous solutions. We will build on our previous work that showed that U5+ had measurable residence times on annite surfaces. The primary question is whether U4+ is formed by two sequential one electron transfers from the mineral to sorbed uranyl, or by disproportionation of sorbed U5+. We intend to probe the redox mechanism by monitoring the residence time of sorbed U5+ after the surface has been rendered redox inert and by varying the free energy of the reduction reaction, Gr. We will vary Gr by working with different minerals that span geochemically relevant redox potentials and by tuning the chemical potential of aqueous uranium (e.g., introduce carbonate). We also plan to investigate U5+ surface speciation. We will use a variety of microscopic and spectroscopic methods to interrogate our samples, including HRTEM (+EELS) and high resolution cryogenic XPS that are available at the EMSL. Additional characterization with XANES and EXAFS will be performed at the APS. We also propose to develop a quick and accessible method to quantify sorbed U5+ concentrations that only involves wet chemistry and optical spectroscopy. This method would make it possible for most geochemistry programs to identify U5+ without traveling to a synchrotron or having a high energy resolution XPS. We note that U5+ has significance for contaminated DOE sites in three primary ways: 1.) the U5+ to U4+ transformation is likely the rate determining step for reduction of uranyl to UO2; consequently U5+ could provide critical information on reduction rates and mechanisms, 2.) U5+ is easily mistaken for either U6+ or U4+ without careful analysis which could lead to false estimates of U transport behavior in the subsurface, and 3.) identifying and eventually quantifying U5+ will provide more accurate estimates for total energy balance during bioreduction of uranyl.
Project Details
Project type
Exploratory Research
Start Date
2009-10-01
End Date
2010-10-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members