Elucidating the behavior of technetium in spinel ferrites and polyoxometalates, model systems for oxide waste forms and naturally occurring magnetite
EMSL Project ID
48597
Abstract
Technetium (Tc-99, 211 ka half-life) is the most problematic long-lived fission product due largely to the high environmental mobility of pertechnetate, TcO4-, the most stable form in aerobic environments. This mobility drives the need to understand how technetium behaves in different waste forms as well as in the environment. Spinel ferrites (MFe2O4, M = Mg, Mn, Fe, Co, Ni) are interesting from this perspective since they are potential waste forms and since the reductive immobilization of TcO4- by magnetite (Fe3O4) is a key factor in limiting its migration. The proposed research will examine the behavior of Tc in Fe3O4 and NiFe2O4 both as prepared and deliberately oxidized. In the spinel ferrites, Tc(IV) replaces Fe(III) in the octahedral lattice sites, and the charge is balanced by either substituting Fe(II) for another Fe(III), by creating cation vacancies, or by a combination of the two. These materials are readily prepared, but technetium is clustered rather than homogeneously doped into these materials. In addition, these materials contain a significant amount of Tc(V) in addition to Tc(IV). Tc(V) is postulated to lie at the surface of the ferrite nanoparticles. The goals of this research is to understand how Tc behaves in this system, especially how technetium behaves when it is present at the surface of the nanoparticles. An ideal model for these surface sites is provided by technetium doped into the defect sites of polyoxometalates (POMs). Interestingly, Tc(V) is stabilized in the technetium-doped POMs and is not readily oxidized to TcO4-, which may have significant implications for the loss of technetium from waste forms. Our goal is to characterize technetium doped spinel ferrite nanoparticles, especially technetium in the surface sites, using a combination of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy, and computational techniques. This information will enable a better understanding of technetium in spinel ferrites in particular and in nuclear waste in general, especially at the crucial interface where it meets the environment.
Project Details
Project type
Special Science
Start Date
2014-11-01
End Date
2015-09-30
Status
Closed
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