3 research outputs found
A New Incorporation Mechanism for Trivalent Actinides into Bioapatite: A TRLFS and EXAFS Study
One of the most toxic byproducts of nuclear power and
weapons production
is the transuranics, which have a high radiotoxicity and long biological
half-life due to their tendency to accumulate in the skeletal system.
This accumulation is inhomogeneous and has been associated with the
chemical properties and structure of the bone material rather than
its location or function. This suggests a chemical driving force to
incorporation and requires an atomic scale mechanistic understanding
of the incorporation process. Here we propose a new incorporation
mechanism for trivalent actinides and lanthanides into synthetic and
biologically produced hydroxyapatite. Time-resolved laser fluorescence
spectroscopy and extended X-ray absorption fine structure have been
used to demonstrate that trivalent actinides and lanthanides incorporate
into the amorphous grain boundaries of apatite. This incorporation
site can be used to explain patterns in uptake and distribution of
radionuclides in the mammalian skeletal system
Structural Investigation of UraniumāNeptunium Mixed Oxides Using XRD, XANES, and <sup>17</sup>O MAS NMR
Uraniumāneptunium mixed dioxides
are considered as fuels
and targets for the transmutation of the minor actinides in fast neutron
reactors. Hereafter, a local and atomic scale structural analysis
was performed on a series of U<sub>1ā<i>x</i></sub>Np<sub><i>x</i></sub>O<sub>2</sub> (<i>x</i> =
0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the solāgel
external gelation method, for which longer range structural analysis
indicates that the process yields solid solutions. The oxidation state
of IV for uranium and neptunium cations was confirmed using U L<sub>III</sub> and Np L<sub>III</sub> edge X-ray absorption near edge
structure (XANES). The atomic scale structure was probed with <sup>17</sup>O magic angle spinning nuclear magnetic resonance (MAS NMR)
for the anion. Structural distortions due to the substitution of U
by the smaller Np cation were detected by <sup>17</sup>O MAS NMR
Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles
Uranium
redox states and speciation in magnetite nanoparticles
coprecipitated with UĀ(VI) for uranium loadings varying from 1000 to
10āÆ000 ppm are investigated by X-ray absorption spectroscopy
(XAS). It is demonstrated that the U M<sub>4</sub> high energy resolution
X-ray absorption near edge structure (HR-XANES) method is capable
to clearly characterize UĀ(IV), UĀ(V), and UĀ(VI) existing simultaneously
in the same sample. The contributions of the three different uranium
redox states are quantified with the iterative transformation factor
analysis (ITFA) method. U L<sub>3</sub> XAS and transmission electron
microscopy (TEM) reveal that initially sorbed UĀ(VI) species recrystallize
to nonstoichiometric UO<sub>2+<i>x</i></sub> nanoparticles
within 147 days when stored under anoxic conditions. These UĀ(IV) species
oxidize again when exposed to air. U M<sub>4</sub> HR-XANES data demonstrate
strong contribution of UĀ(V) at day 10 and that UĀ(V) remains stable
over 142 days under ambient conditions as shown for magnetite nanoparticles
containing 1000 ppm U. U L<sub>3</sub> XAS indicates that this UĀ(V)
species is protected from oxidation likely incorporated into octahedral
magnetite sites. XAS results are supported by density functional theory
(DFT) calculations. Further characterization of the samples include
powder X-ray diffraction (pXRD), scanning electron microscopy (SEM)
and Fe 2p X-ray photoelectron spectroscopy (XPS)