SREL Reprint #3009

Plutonium Oxidation and Subsequent Reduction by Mn(IV) Minerals in Yucca Mountain Tuff

Brian A. Powell1, Martine C. Duff2, Daniel I. Kaplan2, Robert A. Fjeld1, Matthew Newville3, Douglas B. Hunter2, Paul M. Bertsch4, John T. Coates1, Peter Eng3, Mark L. Rivers3, Steven M. Serkiz2, Stephen R. Sutton3, Ines R. Triay5, and David T. Vaniman6

1Department of Environmental Engineering and Science, Clemson University,
Anderson, South Carolina 29630
2Savannah River National Laboratory, Aiken, South Carolina 29808
3Department of Geophysical Sciences and Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637
4Savannah River Ecology Laboratory, The University of Georgia, Aiken, South Carolina 29808
5Waste Isolation Pilot Plant, Carlsbad, New Mexico 88221
6Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Abstract: Plutonium oxidation state distribution on Yucca Mountain tuff and synthetic pyrolusite (β-MnO2) suspensions was measured using synchrotron X-ray micro-spectroscopy and microimaging techniques as well as ultrafiltration/solvent-extraction techniques. Plutonium sorbed to the tuff was preferentially associated with manganese oxides. For both Yucca Mountain tuff and synthetic pyrolusite, Pu(IV) or Pu(V) was initially oxidized to more mobile Pu(V/VI), but over time, the less mobile Pu(IV) became the predominant oxidation state of the sorbed Pu. The observed stability of Pu(IV) on oxidizing surfaces (e.g., pyrolusite), is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. These findings have important implications in estimating the risk associated with the geological burial of radiological waste in areas containing Mn-bearing minerals, such as at the Yucca Mountain or the Hanford Sites, because plutonium will be predominantly in a much less mobile oxidation state (i.e., Pu(IV)) than previously suggested (i.e., Pu(V/VI).

SREL Reprint #3009

Powell, B. A., M. C. Duff, D. I. Kaplan, R. A. Field, M. Newville, D. B. Hunter, P. M. Bertsch, J. T. Coates, P. Eng, M. L. Rivers, S. M. Serkiz, S. R. Sutton, I. R. Triay, and D. T. Vaniman. 2006. Plutonium Oxidation and Subsequent Reduction by Mn(IV) Minerals in Yucca Mountain Tuff. Environmental Science and Technology 40:3508-3514.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).