Reduction of U6+ by magnetite and the rate of electron transfer
EMSL Project ID
25445
Abstract
The reductive adsorption of uranium(VI) by ferrous iron in magnetite (Fe3O4) is an important process for reducing the mobility of contaminants in groundwater and for sequestering radionuclides in the steel waste canisters proposed for the storage of our nation's high level nuclear waste. As part of the "Geochemistry/Biogeochemistry and Subsurface Science" call, we propose a combined experimental and atomic-scale computational investigation of the reductive adsorption of uranium(VI) on magnetite surfaces. This work is a continuation of an on-going collaboration between scientists in the EMSL and at the University of Michigan. Upon completion, this study will provide a comparison of electron transfer rates within magnetite and between magnetite and adsorbing uranyl species to determine if there is a rate-limiting step in the reduction of uranium (VI) by magnetite. We propose to combine anaerobic uranium adsorption experiments with computational molecular methods to explore the atomic-scale interaction of uranyl (UO22+) with magnetite surfaces. X-ray photoelectron spectroscopy (XPS) will be used to determine the amount of reductive adsorption observed on freshly-cleaved magnetite surfaces following exposure to uranium-bearing solution. Quantum mechanical methods will be used to determine the relaxed surface structure and availability of ferrous iron in hydrated versus vacuum-terminated magnetite surfaces. Molecular dynamics methods will be used to search for the low energy conformations of uranyl on magnetite (100) and (111) surfaces. Finally, the rate of solid-state electron hopping between ferrous and ferric iron in the octahedral sub-lattice in magnetite will be calculated and compared with the rate of interfacial electron transfer between ferrous iron and uranium (VI) in clusters developed to represent the docking of uranyl on the magnetite surface. Through these experiments and calculations we are seeking to understand the atomic-scale structure of magnetite surfaces under environmental conditions, and determine if there is a rate-limiting step in the reduction of U6+ by magnetite based on first principles and atomistic calculations performed within the framework of Marcus's theory of electron transfer.
During two previous visits to PNNL by the primary investigator, uranium adsorption experiments and XPS analyses were performed. Partial reduction of uranium was observed on the cubic (100) surface, consistent with previously published experimental results. Molecular dynamics calculations of the low energy conformations of uranyl on magnetite surfaces, and quantum mechanical charge distribution calculations in hydrated versus vacuum-terminated magnetite slabs were also initiated during these visits. In order to complete this study, we are requesting computational resources within EMSL, specifically 20,000-50,000 hours on the "Spokane" cluster. The quantum mechanical codes Crystal03 and NWChem will be used to determine the relaxed surface structures of magnetite (111) and (100) under environmental conditions, and electron transfer rates, respectively. Molecular dynamics codes will also be used to determine the low-energy conformations of uranyl on magnetite surfaces. Through this study, we will determine if there is a rate-limiting step in the two-electron transfer process for the reduction of uranium (VI) by ferrous iron in magnetite which has implications for the utility of magnetite as a barrier to contaminant transport.
General access proposal, standard access requested, non-proprietary, Geochemistry/Biogeochemistry and Subsurface Science
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-05-24
End Date
2010-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Abstract for the 2008 Goldschmidt Geochemistry Conference
Abstract for the 2009 American Chemical Society Spring Meeting
Skomurski FN, ES Ilton, MH Engelhard, BW Arey, and KM Rosso. 2011. "Heterogeneous Reduction of U6+ by Structural Fe2+ From Theory and Experiment." Geochimica et Cosmochimica Acta 75(22):7277-7290. doi:10.1016/j.gca.2011.08.006
Skomurski FN, SN Kerisit, and KM Rosso. 2010. "Structure, Charge Distribution, and Electron Hopping Dynamics in Magnetite (Fe3O4) (100) Surfaces from First Principles." Geochimica et Cosmochimica Acta 74(15):4234-4248.