49 research outputs found
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Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitrate in situ: precursors to iron oxide nuclear waste forms.
The fission product, 99Tc, presents significant challenges to the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate (TcO4-), the stable Tc species in aerobic environments. Migration of 99Tc from disposal sites can potentially be prevented by incorporating it into durable waste forms based on environmentally stable minerals. Since Tc(iv) and Fe(iii) have the same ionic radius, Tc(iv) can replace Fe(iii) in iron oxides. Environmentally durable iron oxides include goethite (α-FeOOH), hematite (α-Fe2O3), and magnesioferrite (MgFe2O4). The incorporation of Tc into two of these, hematite and magnesioferrite, as well as magnetite (Fe3O4) by means of simple, aqueous chemistry is presented starting from TcO4- in 5 M nitric acid. A combination of X-ray diffraction and X-ray absorption fine structure spectroscopy reveals that Tc(iv) replaces Fe(iii) within the iron oxide structures. Following incorporation, Tc doped samples were suspended in deionized water under aerobic conditions, and the release rates of Tc were determined. The results of this work show that Tc leaches more quickly from Fe3O4 than from α-Fe2O3 or MgFe2O4. Modeling the leach rates and comparison with the leach rate of Tc from TiO2 indicate that release of Tc is controlled by solid state diffusion
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Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide.
Here, Cr(VI) effects on Tc-immobilization by Fe(OH)2(s) are investigated while assessing Fe(OH)2(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe(OH)2(s) present, where complete Cr and Tc removal requires more Fe(OH)2(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe(OH)2(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe(OH)2(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr(III) and Tc(IV) substitute for octahedral Fe(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr(III) forms oxide and/or hydroxide phases, whereas Tc(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc(IV)-oxide species. As such, to fully incorporate Tc(IV) into the goethite crystal structure, additional Fe(OH)2(s) (>200 g/L of simulant) may be required
Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation.
Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr(VI), that inhibit Tc(VII) reduction and incorporation into stable mineral phases. Here Fe(OH)2(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe(II)), and the immobilization agent to form Tc-incorporated magnetite (Fe3O4). Trace metal analysis suggests removal of Tc(VII) and Cr(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr(VI) is achieved earlier than the removal/reduction of comingled Tc(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr(III) and the formation of a Cr-incorporated spinel, Cr2O3, and Cr(OH)3 phases. Spinel (modeled as Fe3O4), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc(IV) has little effect on the spinel lattice structure. Reaction of Fe(OH)2(s) in the presence of Cr(III) results in the formation of a spinel phase that is a solid solution between magnetite (Fe3O4) and chromite (FeCr2O4)
The Deep Water Abundance on Jupiter: New Constraints from Thermochemical Kinetics and Diffusion Modeling
We have developed a one-dimensional thermochemical kinetics and diffusion
model for Jupiter's atmosphere that accurately describes the transition from
the thermochemical regime in the deep troposphere (where chemical equilibrium
is established) to the quenched regime in the upper troposphere (where chemical
equilibrium is disrupted). The model is used to calculate chemical abundances
of tropospheric constituents and to identify important chemical pathways for
CO-CH4 interconversion in hydrogen-dominated atmospheres. In particular, the
observed mole fraction and chemical behavior of CO is used to indirectly
constrain the Jovian water inventory. Our model can reproduce the observed
tropospheric CO abundance provided that the water mole fraction lies in the
range (0.25-6.0) x 10^-3 in Jupiter's deep troposphere, corresponding to an
enrichment of 0.3 to 7.3 times the protosolar abundance (assumed to be H2O/H2 =
9.61 x 10^-4). Our results suggest that Jupiter's oxygen enrichment is roughly
similar to that for carbon, nitrogen, and other heavy elements, and we conclude
that formation scenarios that require very large (>8 times solar) enrichments
in water can be ruled out. We also evaluate and refine the simple time-constant
arguments currently used to predict the quenched CO abundance on Jupiter, other
giant planets, and brown dwarfs.Comment: 42 pages, 7 figures, 4 tables, with note added in proof. Accepted for
publication in Icarus [in press
Aqueous Synthesis of Technetium-Doped Titanium Dioxide by Direct Oxidation of Titanium Powder, a Precursor for Ceramic Nuclear Waste Forms
Technetium-99 (Tc)
is a problematic fission product that complicates
the long-term disposal of nuclear waste due to its long half-life,
high fission yield, and the environmental mobility of pertechnetate,
its stable form in aerobic environments. One approach to preventing
Tc contamination is through incorporation into durable waste forms
based on weathering-resistant minerals such as rutile (titanium dioxide).
Here, the incorporation of technetium into titanium dioxide by means
of simple, aqueous chemistrydirect oxidation of titanium powder
in the presence of ammonium fluorideis achieved. X-ray absorption
fine structure spectroscopy and diffuse reflectance spectroscopy indicate
that Tc(IV) replaces Ti(IV) within the structure. Rather than being
incorporated as isolated Tc(IV) ions, Tc is present as pairs of edge-sharing
Tc(IV) octahedra similar to molecular Tc(IV) complexes such as [(H<sub>2</sub>EDTA)Tc<sup>IV</sup>](μ–O)<sub>2</sub>. Technetium-doped
TiO<sub>2</sub> was suspended in deionized water under aerobic conditions,
and the Tc leached under these conditions was followed for 8 months.
The normalized release rate of Tc (LR<sub>Tc</sub>) from the TiO<sub>2</sub> particles is low (3 × 10<sup>–6</sup> g m<sup>–2</sup> d<sup>–1</sup>), which illustrates the potential
utility of TiO<sub>2</sub> as waste form. However, the small size
of the as-prepared TiO<sub>2</sub> nanoparticles results in an estimated
retention of Tc of 10<sup>4</sup> years, which is only a fraction
of the half-life of Tc (2.1 × 10<sup>5</sup> years)
Precipitates of Al(III), Sc(III), and La(III) at the Muscovite–Water Interface
The
interaction of Al(III), Sc(III), and La(III) with muscovite–water
interfaces was studied at pH 4 and 10 mM NaCl using second harmonic
generation (SHG) and X-ray photoelectron spectroscopy (XPS). SHG data
for Sc(III) and La(III) suggest complete and/or partial irreversible
adsorption that is attributed by XPS to the growth of Sc(III) and
La(III) hydroxides/oxides on the muscovite surface. Al(III) adsorption
appears to coincide with the growth of gibbsite (Al(OH)<sub>3</sub>) deposits on the muscovite surface, as indicated by the magnitude
of the interfacial potential computed from the SHG data. This interpretation
of the data is consistent with previous studies reporting the epitaxial
growth of gibbsite on the muscovite surface under similar conditions.
The implication of our findings is that the surface charge density
of mica may change (and in the case of Al(III), even flip sign from
negative (mica) to positive (gibbsite)) when Al(III), Sc(III), or
La(III) is present in aqueous phases in contact with heterogeneous
geochemical media rich in mica-class minerals, even at subsaturation
conditions