Reducing Pu(IV) to Pu(III) with hydroxylamine in nitric acid solutions

Los Alamos National Laboratory (LANL) has evaluated different techniques to concentrate and remove plutonium from solutions stored at the Rocky Flats Environmental Technology Site (RFETS). Pu(III) oxalate precipitation was chosen to treat nitric acid solutions because it is a simple and efficient technique for removing plutonium. Reducing Pu(IV) to Pu(III) is a key process step which affects the rest of the processing sequence. Because of differences in the literature¹ over the kinetics of the reaction, additional data was obtained and compared with existing data to examine the kinetic relationship, and determine an appropriate relationship for future engineering evaluations. The results and conclusions of this work, along with new experimental data, are presented.     

Electrochemical and spectroscopic studies of Pu(IV) and Pu(III) in nitric acid solutions

Electrochemical and absorption spectroscopic properties of Pu(IV) and Pu(III) in nitric acid have been investigated by using cyclic voltammetry (CV) and UV–Visible spectroscopy. CV using a glassy carbon electrode suggested that the electrochemical reaction of Pu(IV) nitrate complexes were found to be a quasi-reversible reduction to Pu(III) species. The formal redox potentials (E ⁰) for Pu(IV)/Pu(III) couples were +0.721, +0.712, +0.706, +0.705, +0.704, 0.694, and +0.696 V (vs. Ag/AgCl) when nitric acid concentrations are 1–7 M nitric acid solutions, respectively. These results indicate that the reduction product of Pu(IV) is only Pu(III). Further details for reaction mechanism of Pu(IV) were discussed on the basis of digital simulation of the experimental cyclic voltammograms. The absorption spectroscopic properties of Pu(III) and Pu(IV) in nitric acid solutions were investigated with UV–Visible spectrophotometry. As a result, it was founds that the intensities of the characteristic absorption peaks of Pu(III) and Pu(IV) tend to decrease with increasing nitric acid concentration for 1–8 M, and the peaks positions shifted longer or shorter wavelengths depending on the complex-forming abilities of Pu(III) and Pu(IV) with an increase in the nitric acid concentration.     

Methylbutylmalonamide as an extractant for U(VI), Pu(IV) and Am(III)

We describe the operation of a Local Area Network at Nuclear Chemistry Laboratory involved in surveillance of environmental radioactivity. Detailed consideration is given separately to computer and network hardware, radiation instrument interfacing, software, as well as operations. The application of a Local Area Network offers considerable improvements in the laboratory preformance, quality assurance of radioactivity analyses, and data reporting.     

Ion exchange separation of Eu(III), Th(IV), U(VI) and Pu(IV) ions on Amberlyst A-15 in non-aqueous solutions

Distribution ratios of europium(III), thorium(IV), uranium(VI) and plutonium(IV) ions on Amberlyst A-15, a macroreticular polystyrene sulfonate resin, after extraction in HTTA-TBP-Shell Sol-T and HTTA-TOPO-benzene solutions have been determined as a function of the aqueous acidity. The affinity orders were EuPu>Th>U and Eu>Th>Pu>U in the former and the latter solutions, respectively. Separation factors were computed from the observed K_d values. A procedure for the separation of a mixture of Eu(III), Th(IV), and U(VI) ions in HTTA-TOPO-benzene solution in an ion-exchange column is described.     

Modeling Speciation and Solubility in Aqueous Systems Containing U(IV,VI), Np(IV,V, VI), Pu(III,IV, V,VI), Am(III), and Cm(III)

A comprehensive thermodynamic model, referred to as the Mixed-Solvent Electrolyte model, has been applied to calculate phase equilibria and chemical speciation in selected aqueous actinide systems. The solution chemistry of U(IV, VI), Np(IV, V, VI), Pu(III, IV, V, VI), Am(III), and Cm(III) has been analyzed to develop the parameters of the model. These parameters include the standard-state thermochemical properties of aqueous and solid actinide species as well as the ion interaction parameters that reflect the solution’s nonideality. The model reproduces the solubility behavior and accurately predicts the formation of competing solid phases as a function of pH (from 0 to 14 and higher), temperature (up to 573 K), partial pressure of CO₂ (up to \( p_{{{\text{CO}}_{2} }} \)  = 1 bar), and concentrations of acids (to 127 mol·kg^?1), bases (to 18 mol·kg^?1), carbonates (to 6 mol·kg^?1) and other ionic components (i.e., Na⁺, Ca²⁺, Mg²⁺, OH^?, Cl^?, \( {\text{ClO}}_{4}^{ - } \), and \( {\text{NO}}_{3}^{ - } \)). Redox effects on solubility and speciation have been incorporated into the model, as exemplified by the reductive and oxidative dissolution of Np(VI) and Pu(IV) solids, respectively. Thus, the model can be used to elucidate the phase and chemical equilibria for radionuclides in natural aquatic systems or in nuclear waste repository environments as a function of environmental conditions. Additionally, the model has been applied to systems relevant to nuclear fuel processing, in which nitric acid and nitrate salts of plutonium and uranium are present at high concentrations. The model reproduces speciation and solubility in the U(VI) + HNO₃ + H₂O and Pu(IV, VI) + HNO₃ + H₂O systems up to very high nitric acid concentrations (\( x_{{{\text{HNO}}_{3} }} \approx 0.70 \)). Furthermore, the similarities and differences in the solubility behavior of the actinides have been analyzed in terms of aqueous speciation.     

Ion exchange studies of U(VI), Th(IV), Pu(IV) and Eu(III) on a macroreticular resin in TBP-Shell Sol-T solutions

Ion exchange studies of uranium(VI), thorium(IV), plutonium(IV) and europium(III) ions on a macroreticular cation exchange resin, Amberlyst A-15, from solutions of 30% and 5% TBP—Shell Sol-T have been carried out. The metal ions were extracted into TBP Shell Sol-T phase from 8M NH₄NO₃ at different nitric acid concentrations. Ion exchange distribution ratios as a function of organic phase acidity of 30% and 5% TBP have been computed. Separation factors computed from the observed Kd values are plotted as a function of organic phase acidity.     

Polymerization of Pu(IV) in aqueous nitric acid solutions

The polymerization of Pu(IV) in aqueous nitric acid solutions has been studied spectrophotometrically both to determine the effects of large UO₂(NO₃)₂ concentrations on the polymerization rates and, more generally, to review the influence of other major parameters on the polymer reaction. Typically, experiments have been performed at 50°C and at 0.05M Pu in aqueous solutions of HNO₃ at concentrations ranging from 0.07 to 0.4M. An induction period usually precedes the polymer growth stage, during which time it is believed that primary hydrolysis products form and begin to aggregate. Uranyl nitrate retards the polymerization reaction by approximately 35% despite the counteracting influence of the nitrate ions associated with this solute. The rate of polymer formation at 50°C has been shown to be third order in Pu(IV) concentration.     

Bicyclic and acyclic diamides: comparison of their aqueous phase binding constants with Nd(III), Am(III), Pu(IV), Np(V), Pu(VI), and U(VI)

This report describes affinity measurements for two, water-soluble, methyl-alkylated diamides incorporating the malonamide functionality, N,N,N',N' tetramethylmalonamide (TMMA) and a bicyclic diamide (1a), toward actinide metal cations (An) in acidic nitrate solutions. Ligand complexation to actinides possessing oxidation states ranging from +3 to +6 was monitored through optical absorbance spectroscopy, and formation constants were obtained from the refinement of the spectrophotometric titration data sets. Species analysis gives evidence for the formation of 1, 4, 1, and 2 spectrophotometrically observable complexes by TMMA to An(III, IV, V, and VI), respectively, while for 1a, the respective numbers are 3, 4, 2, and 2. Consistent with the preorganization of 1a toward actinide binding, a significant difference is found in the magnitudes of their respective formation constants at each complexation step. It has been found that the binding affinity for TMMA follows the well-established order An(V) < An(III) < An(VI) < An(IV). However, with 1a, Np(V) forms stronger complexes than Am(III). The complexation of 1a with Np(V) and Pu(VI) at an acidity of 1.0 M is followed by reduction to Np(IV) and Pu(IV), whereas TMMA does not perturb the initial oxidation state for these dioxocations. These measurements of diamide binding affinity mark the first time single-component optical absorbance spectra have been reported for a span of actinide-diamide complexes covering all common oxidation states in aqueous solution.     

Absorption spectroscopic properties for Pu(III,IV and VI) in nitric and hydrochloric acid media

Absorption spectroscopic properties for various Pu oxidation states in nitric and hydrochloric acid solutions were investigated with UV-Visible spectrophotometry. As a result, it was confirmed that the intensities of the major absorption peaks had a tendency to decrease for Pu(III), Pu(IV) and Pu(VI) in HCl and HNO₃ media, and the major peak positions were shifted to longer or shorter wavelengths depending on the complexforming abilities of Pu(III), Pu(IV) and Pu(VI) with the chloride or nitrate ion with increasing acid concentrations. The values of the wavelength and the molar absorptivity for the principal peaks of Pu(III), Pu(IV) and Pu(VI) in NHO₃ and HCl solutions were similar to those reported in other works. The values of the molar absorptivity for the principal peaks of Pu(III), Pu(IV) and Pu(VI) in the HNO₃ solution were a little higher than those in the HCl solution.     

Interaction of Aluminium (III) with the f-Family Ions Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) in the Course of Simultaneous Hydrolysis

The interaction of Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) with Al(III) in solutions at pH 0–4 was studied by the spectrophotometric method. It was shown that, in the range of pH 3–4, the hydrolyzed forms of neptunyl and plutonyl react with the hydrolyzed forms of aluminium. In the case of Pu(VI), the mixed hydroxoaqua complexes (H₂O)₃PuO₂(-OH)₂Al(OH)(H₂O)₃ ²⁺ or (H₂O)₄PuO₂OAl(OH)(H₂O)₄ ²⁺ are formed at the first stage of hydrolysis. Np(VI) also forms similar hydroxoaqua complexes with Al(III). The formation of the mixed hydroxoaqua complexes was also observed when Np(IV) or Pu(IV) was simultaneously hydrolyzed with Al(III) at pH 1.5–2.5. The Np(IV) complex with Al(III) has, most likely, the formula (H₂O) _n (OH)Np(-OH)₂Al(OH)(H₂O)₃ ³⁺. At pH from 2 to 4.1 (when aluminium hydroxide precipitates), the Np(V) or Nd(III) ions exist in solutions with or without Al(III) in similar forms. When pH is increased to 5–5.5, these ions are almost not captured by the aluminium hydroxide precipitate.     

The effect of previous gamma-irradiation on the extraction of U(VI), Th(IV), Zr(IV), Eu(III) and Am(III) by various amides

The extraction behavior of U(VI), Th(IV), Zr(IV), Eu(III) and Am(III) from 3.5M nitric acid with a series of gamma-pre-irradiated symmetrical and unsymmetrical monoamides in benzene has been investigated up to a dose of 100 Mrad. The results indicated that the radiolytic stability is influenced by the structure of amides. Symmetrical monoamides seem to be less affected by radiation compared with unsymmetrical monoamides. Infrared studies identify the final products of radiolysis as the respective carboxylic acids and amines. The radiolytic degradation of the investigated monoamides has been estimated by quantitative IR spectroscopy. Extraction data obtained under similar experimental conditions for U(VI), Th(IV) and Zr(IV) with the TBP/benzene system have also been compared. This revised version was published online in August 2006 with corrections to the Cover Date.     

Extractive separation of Th(IV), U(VI), Ti(IV), La(III) and Fe(III) from Zarigan ore

In this study, the effects of various extraction parameters such as extractant types (Cyanex302, Cyanex272, TBP), acid type (nitric, sulfuric, hydrochloric) and their concentrations were studied on the thorium separation efficiency from uranium(VI), titanium(IV), lanthanum(III), iron(III) using Taguchi^??s method. Results showed that, all these variables had significant effects on the selective thorium separation. The optimum separations of thorium from uranium, titanium and iron were achieved by Cyanex302. The aqueous solutions of 0.01 and 1 M nitric acid were found as the best aqueous conditions for separating of thorium from titanium (or iron) and uranium, respectively. The combination of 0.01 M nitric acid and Cyanex272 were found that to be the optimum conditions for the selective separation of thorium from lanthanum. The results also showed that TBP could selectively extract all studied elements into organic phase leaving thorium behind in the aqueous phase. Detailed experiments showed that 0.5 M HNO₃ is the optimum acid concentration for separating of thorium from other elements with acidic extractants such as Cyanex272 and Cyanex302. The two-stage process containing TBP-Cyanex302 was proposed for separation thorium and uranium from Zarigan ore leachate.     

Sulphate and fluoride complexing of U(VI), Np(VI) and Pu(VI)

The complex formation of U(VI), Np(VI) and Pu(VI) with sulphate and fluoride ions was studied in HClO₄H₂SO₄ and HClO₄HF solutions respectively at an ionic strength of 2·0 and [H⁺] = 2·0 M by the distribution method employing the liquid cation exchanger dinonyl naphthalene sulphonic acid as the extractant. An attempt was made to explain the order of the stability constant values obtained.     

Thermodynamics of extraction of U(VI), Np(IV) and Pu(IV) with some long chain aliphatic sulphoxides

Distribution data for U(VI), Np(IV) and Pu(IV) from 2 M nitric acid medium with 0,2 M di-n-hexyl sulphoxide (DHSO) and di-n-octyl sulphoxide (DOSO) in Solvesso-100 have been obtained in the temperature range 20–50°C. From these data, the enthalpy, entropy and free energy changes associated with their extraction were evaluated. Extraction of Np(IV) and Pu(IV) with both sulphoxides is favoured by negative enthalpy and positive entropy changes whereas the extraction of U(VI) is favoured only by high negative enthalpy change. This behaviour has been explained as arising due to the higher hydration of Np⁴⁺ and Pu⁴⁺ ions as compared to the UO₂²⁺ ion.     

Spectrophotometric determination of plutonium III, IV, and VI concentrations in nitric acid solution

The determination of the concentration of various valency states of plutonium is desirable in various stages of the Plutonium/Uranium Recovery by EXtraction (PUREX) process for the effective separation and purification of plutonium. A method is optimized for the quantitative spectrophotometric determination of Pu(III), Pu(IV) and Pu(VI) existing separately or in mixed oxidation states in 1.5?M nitric acid medium. Molar absorption co-efficient (??) for the major absorption peaks with baseline correction are evaluated. With these ?? data a method is proposed for determining the molar concentration of each oxidation state.     

Extraction of U(VI), Pu(IV), Am(III) and some fission products by 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester immobilized polyvinyl alcohol hydrogels

Cross-linked hydrogel matrices immobilized with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HA), were prepared to investigate their application in the recovery of radionuclide from acidic waste solutions. Gamma-radiation was used to produce HA immobilized polyvinyl alcohol (PVA) hydrogels (HA-gel). The hydrogels with different characteristics such as: degree of cross-linking (by varying radiation dose) and quantity of extractant immobilized (by starting with aqueous PVA solution containing different amounts of HA), were synthesised. These HA-gels were investigated for solid-liquid phase extraction of U(VI), Pu(IV), Am(III) and some fission products, under various experimental conditions. The concentration of HNO₃ in the aqueous phase was found to play an important role in the extraction of these radionuclei. Extraction of U(VI) was more favourable at lower concentration of HNO₃ (∼0.001 to 0.5M), while at higher concentrations (∼0.5 to 3M HNO₃), more than 90% of Pu(IV) present in the aqueous phase, could be extracted by the HA-gel. The extraction of Am(III) was also found predominant only at lower acidities (at pH∼2 and above). Under optimized conditions, maximum metal loading capacities obtained were 19±0.8 mg, 8±0.4 mg and 11±0.5 mg per gram of swollen HA-gel, for U(VI), Pu(IV) and Am(III), respectively. Under the experimental conditions, extractions of Cs(I) and Sr(II) were observed to be negligible. No leaching out of HA from the HA-gel particles was noted even after its repetitive use for the studied ten cycles of extraction and stripping experiments, as evident from its unchanged extraction efficiency.     

Distribution of Pu(VI) from nitric acid to tri-n-butyl phosphate saturated with uranium(VI)

Solvent extraction of plutonium(VI) from nitric acid (1 to 5M) into 20% and 30% TBP in dodecane saturated with uranium(VI) (0% to 80%) has been studied. For a particular nitric acid concentration, the distribution coefficient (K _d ) is found to decrease with the increase in saturation of organic phase with uranium(VI). At a fixed organic phase the saturationK _d increased with increase in nitric acid concentration, however, the magnitude of this increase inK _d decreased with the increase in saturation.     

EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution

The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L(3) and Th-L(3) EXAFS spectroscopy for total sulfate concentrations 0.05 < or = [SO(4)(2-)] < or = 3 M and 1.0 < or = pH < or = 2.6. The sulfate coordination was derived from U-S and Th-S distances and coordination numbers. The spectroscopic results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar [SO(4)(2-)]/[UO(2)(2+)] solution, a U-S distance of 3.57 +/- 0.02 Angstrom suggests monodentate coordination, in line with UO(2)SO(4)(aq) as the dominant species. With increasing [SO(4)(2-)]/[UO(2)(2+)] ratio, an additional U-S distance of 3.11 +/- 0.02 Angstrom appears, suggesting bidentate coordination in line with the predominance of the UO(2)(SO(4))(2)(2-) species. The sulfate coordination of Th(IV) and U(IV) was investigated at [SO(4)(2-)]/[M(IV)] ratios > or = 8. The Th(IV) sulfato complex comprises both, monodentate and bidentate coordination, with Th-S distances of 3.81 +/- 0.02 and 3.14 +/- 0.02 Angstrom, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate U-S distances of 3.67 +/- 0.02 and 3.08 +/- 0.02 Angstrom, respectively. By increasing the pH value to 2, a U(IV) sulfate precipitates. This precipitate shows only a U-S distance of 3.67 +/- 0.02 Angstrom in line with a monodentate linkage between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate sulfate coordination in aqueous solutions can now be explained by differences of the [SO(4)(2-)]/[M] ratio. At low [SO(4)(2-)]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at higher ratios.