Identification and Control of Radioactive Compounds in Hanford Waste Tanks
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Outline for vitrification of Hanford
high-level waste. Separations are illustrated in red. |
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The Hanford nuclear waste site in southeastern Washington
State is one of the most contaminated sites in the DOE complex. It
stores millions of gallons of radioactive waste from the nation’s
nuclear weapons programs. High-level radioactive waste is leaking from
about a third of Hanford’s underground tanks. The waste includes
the element technetium in the radioactive form of 99Tc, which poses
a grave threat because of its long half-life (213,000 years) and
its potential to contaminate ground water and migrate toward the
Columbia River. Radioactive chemicals infiltrating the environment
are a cancer hazard to humans. The proposed solution for remediating
Tc is to chemically separate it from the tank wastes and store it
in solid glass, a process called vitrification. But separating 99Tc
into the proper phase before vitrification is proving difficult in
some of the tanks because it forms unknown compounds in some storage
conditions. To determine the identity of these unknown 99Tc compounds
in the waste, scientists performed experiments using x-ray absorption
spectroscopy, in which x-rays boost electrons in the sample to higher
energy states to provide electronic and geometric structural information.
Comparison of the experimental results to spectra from known compounds
indicates that the unknown species is a Tc(I)-carbonyl complex. These
results are extremely valuable because they indicate that technetium
separation technologies must be effective for Tc(I)-carbonyl species.
W.W. Lukens, D.K. Shuh, N.C. Schroeder,
and K.R. Ashley, “Identification of the non-pertechnetate
species in Hanford waste tanks, Tc(I)-carbonyl complexes,” Env.
Sci. Tech. 38, 229 (2004). |
Interrogating the Silent Zinc Ion in Metalloenzymes
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Zinc K-edge EXAFS as function of time. |
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Why do people need zinc in their diets? The
metal zinc is an essential mineral that stimulates the activity
of approximately 100 enzymes — substances
that promote important biochemical reactions crucial to supporting
the immune system, healing wounds, and synthesizing DNA. Changes in
the local environment of the enzymatic zinc are important for understanding
the different steps in the catalytic reaction. Classical enzymology
and structural biology have provided insights into the reaction mechanisms
of many metalloenzymes. However, little structural information is available
on the catalysis of zinc-dependent enzymes, primarily because the zinc
ion, with its fully filled 3d orbital, is “silent” for
several spectroscopic techniques. Protein crystallography fails
when investigating this problem since it depends on stable crystals;
it also lacks the resolution to determine whether water molecules
participate in the reaction. Using alcohol dehydrogenase from a
thermophilic bacterium (T.
brockii) as a representative zinc metalloenzyme, researchers
have applied time-resolved extended x-ray absorption fine structure
(EXAFS) spectroscopy to examine the structural and electronic changes
that occur at the catalytic site of a zinc metalloenzyme. The results
showed a series of changes in the number and distances of atoms
surrounding the zinc ion, and these structural changes were reflected
in the zinc’s
effective charge. Furthermore, the data suggested that the enzymatic
cycle has six steps, including the addition of a water molecule,
the binding of alcohol, the formation of the ketone, and the dissociation
of the product. The results emphasized the flexibility of zinc
sites during catalysis.
O. Kleinfeld, A. Frenkel,
J.M.L. Martin, and I. Sagi, “Active site electronic
structure and dynamics during metalloenzyme catalysis,” Nat.
Struct. Biol. 10, 98 (2003).
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Room-Temperature Semiconductor for Spintronics
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Crystal model shows Co(II) (green) substituting
for Ti(IV) (blue) in the lattice, with an oxygen (red) vacancy adjacent
to the Co(II). |
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Ferromagnetic semiconductors that remain magnetic
at and above room temperature are critical to the development of
revolutionary spin-based electronics (or "spintronics"),
technologies that manipulate electron spin, in addition to charge,
to store and transmit information. Cobalt-doped TiO2 anatase
is an oxide semiconductor that exhibits ferromagnetism well above
room temperature. The thermally robust ferromagnetism is thought
to be mediated by electrons from oxygen vacancies created by the
substitution of cobalt for titanium (the oxygen vacancy is required
to maintain local charge neutrality), but knowledge of the actual
mechanism has been elusive. An understanding of the local structure
of the magnetic dopant is critical to determining whether the magnetism
is caused by elemental cobalt nanocrystals or whether cobalt is
a magnetic dopant integrated into the host lattice. The latter
is a necessary but insufficient condition for the material being
a magnetic semiconductor. X-ray absorption near-edge structure
(XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopies
were used to probe the charge state and local structure of cobalt
dopants in TiO2 films. The researchers found that there
was a significant but incomplete structural correlation between
oxygen vacancies and substitutional Co(II) (cobalt in the +2 formal
oxidation state). The results established that there was no detectable
cobalt metal in the TiO2 films. The magnetism was correlated
with the presence of substitutional Co(II) in the anatase lattice
and free carriers resulting from excess oxygen vacancies. The results
support, but do not prove, the contention that cobalt-doped TiO2 anatase
is a true ferromagnetic semiconductor.
S.A. Chambers,
S.M. Heald, and T. Droubay, "Local Co structure in epitaxial CoxTi1-xO2-x
anatase," Phys. Rev. B 67, 100401
(2003).
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