Plutonium Oxidation and Subsequent Reduction by Mn(IV) Minerals in Yucca Mountain Tuff
Brian A. Powell,*,† Martine C. Duff,‡ Daniel I. Kaplan,‡ Robert A. Fjeld,† Matthew Newville,§ Douglas B. Hunter,‡ Paul M. Bertsch,ˆ John T. Coates,† Peter Eng,§ Mark L. Rivers,§ Steven M. Serkiz,‡ Stephen R. Sutton,§ Ines R. Triay,˜ and David T. Vaniman#
Abstract
Plutonium oxidation state distribution on Yucca Mountain tuff and synthetic
pyrolusite (Beta-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).
* Corresponding
author
Phone: (925) 422-0280
E-mail: powell37@llnl.gov.
Current Address: Lawrence Livermore National Laboratory, Livermore, CA
94551.
† Department
of Environmental Engineering and Science, Clemson University.
‡ Savannah River National Laboratory.
§ Department of Geophysical Sciences and Center for Advanced Radiation
Sources, The University of Chicago.
ˆ Savannah River Ecology Laborator, The University of Georgia
˜ Waste Isolation Pilot Plant, Carlsbad, NM
# Los Alamos National Laboratory
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.
