Visualizing Uranium Atom Exchange in UO2 Driven by Biogeochemical Processes
EMSL Project ID
50768
Abstract
Redox-driven recrystallization of metal oxides is an exciting environmental phenomenon that can influence biogeochemical cycles of both nutrients and contaminants. This phenomena occurs in systems with redox gradients or fluctuating redox conditions that result in the co-occurrence of an insoluble semi-conducting mineral and a dissolved species of the same element but in a different oxidation state. Redox-driven recrystallization is an established process for Fe(III) oxides and oxyhydroxides, one of emerging understanding for Mn oxides, and an area of frontier research for UO2. Co-occurrence of U(VI) and UO2 is common in subsurface environments with biogeochemical processes that drive U(VI) reduction to U(IV) species that include UO2. This co-occurrence can result in redox-driven recrystallization of UO2 that influences its stability in subsurface environments. We are currently investigating U(VI)-catalyzed recrystallization of UO2 by reacting UO2 solids with U(VI)-enriched solution and characterizing the UO2 solids before and after reaction. The current proposal aims to elucidate the effects of interfacial electron transfer and atom exchange on the structure and reactivity of UO2 in environmental systems. A specific objective of the work is to visualize atom exchange and redox-driven recrystallization of UO2 at the atomic scale. We propose to use atom probe tomography (APT) and focused ion beam milling coupled with scanning electron microscopy (FIB/SEM) at the Environmental Molecular Science Laboratory (EMSL) to probe uranium atom exchange in UO2. The use of FIB-SEM will allow us to extract cross sections of UO2 solids via an encapsulation method. These cross sections will be further studied at EMSL using APT. With the high spatial resolution of APT, we will visualize the atom exchange front in UO2 that results from U(VI)-catalyzed recrystallization. This work will advance our understanding of spatial and temporal characteristics of redox-driven recrystallization of UO2 at the atomic scale and elucidate the electron transfer and atom exchange interaction at this important redox interface. This work will result in high-impact publications in environmental science areas, which is well-aligned with DOE OBER's stated priority science directions. The research will help EMSL fulfill its primary mission as a national scientific user facility that leads molecular-level discoveries for the DOE OBER that translate to predictive understanding and accelerated solutions for national energy and environmental challenges.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2019-10-01
End Date
2021-12-31
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members