Peter Tkac

Speciation of Molybdenum (VI) In Aqueous and Organic Phases of Selected Extraction Systems

Abstract The extraction study of molybdenum (VI) by 30% tri-n-butyl phosphate in n-dodecane and 0.2 M octyl (phenyl)-N,N-di-isobutylcarbamoylmethylphosphine oxide in 30% tri-n-butyl phosphate extraction systems was performed from aqueous solution containing HCl, HNO3 and acetohydroxamic acid. Depending on extraction conditions, acetohydroxamic acid can significantly affect the speciation of molybdenum and can increase or decrease its distribution ratio. Our investigation confirmed the strong ability of the acetohydroxamic acid to form complexes with Mo even in highly acidic solutions. UV absorption spectra confirmed that a fraction of the Mo(VI)-AHA species can be present in the organic phase after extraction.

The Effect of Acetohydroxamic Acid on Extraction and Speciation of Plutonium

Abstract The effect of acetohydroxamic acid (AHA) on speciation of transuranic elements has been investigated in the extraction and spectrophotometric experiments relevant to UREX extraction process. The distribution ratios decreased with increased concentration of added AHA rapidly, and a considerable ability of AHA to form complexes with plutonium even under strong acidic conditions was confirmed. Using FITEQL 4.0, a computer modeling program for equilibrium experimental data, the speciation distribution of tetravalent plutonium was determined. Acetohydroxamate species of Pu(IV) identified in Vis-NIR spectra of the extraction organic phase confirms formation of extractable neutral solvates of ternary acetohydroxamate-nitrate complexes of plutonium with tributyl phosphate (TBP).

Spectroscopic Study of the Uranyl–Acetohydroxamate Adduct with Tributyl Phosphate

The ultraviolet–visible (UV-Vis) and Fourier transform infrared (FT-IR) spectroscopic studies carried out for the system UO2(NO3)/AHA/TBP (uranyl–acetohydroxamate–tributyl phosphate) confirmed the presence of the adduct of UO2(NO3)(AHA) ·2TBP with 1:1 stoichiometry for UO2:AHA (acetohydroxamic acid). The spectrum of this complex is identical to the infrared spectrum of the organic phase formed in the uranium distribution experiments with 30% TBP/n-dodecane and AHA present in aqueous phase. Disappearance of the hydroxyl stretching band and a shift in the position of the carbonyl band in the infrared spectra revealed that both the hydroxyl and the carbonyl group of acetohydroxamic acid are involved in the chelate ring with uranium. Also, acetic acid, accrued after acidic hydrolysis of acetohydroxamic acid, was identified in the extraction organic phase.

Complexation Chemistry of Zirconium(IV), Uranium(VI), and Iron(III) with Acetohydroxamic Acid

The complexation of zirconium(IV), uranium(VI), and iron(III) with acetohydroxamic acid (AHA) has been analyzed spectrophotometrically in various ionic strengths at 25°C. Arsenazo III (AAIII) was used as an indicator for unbound zirconium. The SQUAD computational program was employed to evaluate the stability constants. Conditional stability constants of four zirconium complexes Zr(AAIII)3+, , Zr(AHA)3+, and were determined in 1 mol · L−1 HClO4 as log β′ = 5.09, 10.29, 12.78, and 23.13, respectively. Conditional stability constants of UO2(AHA)+, Fe(AHA)2+, , and Fe(AHA)3, in 0.1 mol · L−1 HNO3 were calculated as = 8.32, 11.00, 20.93, and 28.75, respectively.

Redox reactions of Pu(IV) and Pu(III) in the presence of acetohydroxamic acid in HNO(3) solutions.

The reduction of Pu(IV) in the presence of acetohydroxamic acid (HAHA) was monitored by vis-NIR spectroscopy. All experiments were performed under low HAHA/Pu(IV) ratios, where only the Pu(IV)-monoacetohydroxamate complex and Pu uncomplexed with HAHA were present in relevant concentrations. Time dependent concentrations of all absorbing species were resolved using molar extinction coefficients for Pu(IV), Pu(III), and the Pu(AHA)(3+) complex by deconvolution of spectra. From fitting of the experimental data by rate equations integrated by a numeric method three reactions were proposed to describe a mechanism responsible for the reduction and oxidation of plutonium in the presence of HAHA and HNO(3). Decomposition of Pu(AHA)(3+) follows a second order reaction mechanism with respect to its own concentration and leads to the formation of Pu(III). At low HAHA concentrations, a two-electron reduction of uncomplexed Pu(IV) with HAHA also occurs. Formed Pu(III) is unstable and slowly reoxidizes back to Pu(IV), which, at the point when all HAHA is decomposed, can be catalyzed by the presence of nitrous acid.

A Solution-Based Approach for Mo-99 Production: Considerations for Nitrate versus Sulfate Media

Molybdenum-99 is the parent of Technetium-99m, which is used in nearly 80% of all nuclear medicine procedures. The medical community has been plagued by Mo-99 shortages due to aging reactors, such as the NRU (National Research Universal) reactor in Canada. There are currently no US producers of Mo-99, and NRU is scheduled for shutdown in 2016, which means that another Mo-99 shortage is imminent unless a potential domestic Mo-99 producer fills the void. Argonne National Laboratory is assisting two potential domestic suppliers of Mo-99 by examining the effects of a uranyl nitrate versus a uranyl sulfate target solution configuration on Mo-99 production. Uranyl nitrate solutions are easier to prepare and do not generate detectable amounts of peroxide upon irradiation, but a high radiation field can lead to a large increase in pH, which can lead to the precipitation of fission products and uranyl hydroxides. Uranyl sulfate solutions are more difficult to prepare, and enough peroxide is generated during irradiation to cause precipitation of uranyl peroxide, but this can be prevented by adding a catalyst to the solution. A titania sorbent can be used to recover Mo-99 from a highly concentrated uranyl nitrate or uranyl sulfate solution; however, different approaches must be taken to prevent precipitation during Mo-99 production.

Effect of Gamma Irradiation on the Oxidation State of Neptunium in Nitric Acid in the Presence of Selected Scavengers

A series of experiments was performed with selected reagents that were added to inhibit the reduction of Np(VI) to Np(V) during irradiation of its solutions in HNO3. Acetamide and methylurea as nitrous acid scavengers, and vanadium(V) as a neptunium(V) oxidizer, were examined in this effort. Solutions of these reagents in 4 M HNO3 were irradiated in a Co-60 gamma irradiator. Additions of 1-10 mM of methylurea and vanadium(V) essentially had no effect on the final oxidation state of Np after irradiation with a dose of 60 kGy, while the addition of higher concentrations of methylurea (50 and 100 mM) led to an almost complete reduction of Np to the tetravalent oxidation state.

Fission Produced 99Mo without a Nuclear Reactor

99Mo, the parent of the widely used medical isotope 99mTc, is currently produced by irradiation of enriched uranium in nuclear reactors. The supply of this isotope is encumbered by the aging of these reactors and concerns about international transportation and nuclear proliferation. Methods: We report results for the production of 99Mo from the accelerator-driven subcritical fission of an aqueous solution containing low enriched uranium. The predominately fast neutrons generated by impinging high-energy electrons onto a tantalum convertor are moderated to thermal energies to increase fission processes. The separation, recovery, and purification of 99Mo were demonstrated using a recycled uranyl sulfate solution. Conclusion: The 99Mo yield and purity were found to be unaffected by reuse of the previously irradiated and processed uranyl sulfate solution. Results from a 51.8-GBq 99Mo production run are presented.

Spectroscopic identification of tri-n-butyl phosphate adducts with Pu(IV) hydrolyzed species

The complex chemistry of plutonium under low nitric acid concentration can significantly affect the extraction of tetravalent plutonium by tri-n-butyl phosphate (TBP) in n-dodecane. At low HNO3 concentrations, the main Pu(IV) species present in the aqueous solution are Pu(OH)3+ and Pu(OH)22+. Moreover, due to the disproportionation reaction of Pu(IV), a mixture of Pu(III), Pu(IV) and Pu(VI) must be considered. Also the colloidal Pu(IV) can be present under certain conditions. Vis-NIR spectroscopy revealed that in 0.1 M nitric acid, the formation of colloids and the disproportionation reaction of Pu(IV) are the main processes responsible for the distribution of plutonium between the organic and aqueous phases. Comparison of absorption spectra of Pu(IV) in TBP extracted from 0.4-10 M nitric acid concentrations confirms presence of two different Pu(IV) species. At lower nitric acid concentration (0.1 M), also colloidal Pu in TBP was observed.

MOEX: Solvent extraction approach for recycling enriched 98Mo/100Mo material

ABSTRACT Several promising pathways exist for the production of 99Mo/99mTc using enriched 98Mo or 100Mo. Use of Mo targets requires a major change in current generator technology, and the necessity for an efficient recycle pathway to recover valuable enriched Mo material. High recovery yields, purity, suitable chemical form and particle size are required. Results on the development of the MOEX – molybdenum solvent extraction – approach to recycle enriched Mo material are presented. The advantages of the MOEX process are very high decontamination factors from potassium and other elements, high throughput, easy scalability, automation and minimal waste generation.