Reduction of U6+ by magnetite and the rate of electron transfer

COLL 458

Frances N. Skomurski, fskomurs@umich.edu1, Sebastien Kerisit, sebastien.kerisit@pnl.gov2, Eugene S. Ilton, Eugene.Ilton@pnl.gov2, Mark Engelhard, mark.engelhard@pnl.gov3, Rodney C. Ewing1, Udo Becker1, and Kevin M. Rosso, kevin.rosso@pnl.gov2. (1) Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, 1100 N. University Ave., Ann Arbor, MI 48109, (2) Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, (3) Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352
The reduction of U6+ to U4+ by natural magnetite (FeIIFeIII2O4) was explored using a combination of experimental and computational techniques. Natural magnetite samples were cleaved and exposed to uranyl-nitrate solution at pH 4.4 under anoxic conditions for approximately 90 hours. U4+/U6+ and Fe2+/Fe3+ ratios on both clean and U-exposed surfaces were measured using XPS, and the results indicate a significant amount of U6+ adsorption, accompanied by signs of reductive adsorption. X-ray beam-induced reduction of U6+ was ruled out. Quantum mechanical calculations were used to determine the charge ordering of Fe2+/Fe3+ in the octahedral sub-lattice for vacuum-terminated and hydrated magnetite surfaces, where the near-surface availability of Fe2+ is important for the reduction of adsorbates. Electron transfer rates were calculated between near-surface ferrous iron and stable uranyl (UO22+) surface complexes predicted by molecular dynamics to determine if electron transfer could be the rate-limiting step in the reduction of U6+ by magnetite.
 

Environmental Interfaces
8:30 AM-12:40 PM, Wednesday, 13 September 2006 Sir Francis Drake -- Empire Room, Oral

Division of Colloid & Surface Chemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006