Reduction of neptunium(V) and uranium(VI) with iron(II) in bicarbonate solutions

Reaction of Np(VI) compounds with Fe(II) in bicarbonate solutions was studied. Reaction of Np(V) with Fe(II) in the presence of phthalate ions was briefly considered. Iron(II) compounds reduce Np(V) compounds in solutions saturated with Ar or CO₂ at any concentrations of bicarbonate ion. At [Na(K)HCO₃] > 0.86 M, Np(V) is reduced during mixing the reactants and recording the spectra. The reaction of Fe(II) with Np(V) in dilute bicarbonate solutions is substantially slower, probably owing to a sharp decrease in the solubility of the Np(V) carbonate complexes. The solubility of the Np(V) compounds increases after saturation of the dilute bicarbonate solutions with CO₂. However, in this case reduction remains slow. Uranium(VI) carbonate complexes are reduced with Fe(II) compounds in dilute bicarbonate solutions. The reaction products formed at elevated temperatures are UO₂ and FeOOH.     

Kinetics of Pu(VI) reduction with carbohydrazide in nitric acid

The kinetics of the Pu(VI) reduction with carbohydrazide in nitric acid solutions was studied by spectrophotometry. The reaction rate increases with increasing carbohydrazide concentration and temperature and decreases with increasing HNO₃ concentration. The reaction order with respect to Pu, carbohydrazide, and HNO₃ is 1, 2.3, and–3, respectively. The activation energy of the reaction is 111 kJ mol^–1. The final reduced Pu form is Pu(III), with Pu(V) being an intermediate.     

Kinetics of Np(VI) reduction with carbohydrazide in nitric acid

The kinetics of the Np(VI) reduction with carbohydrazide in nitric acid solutions was studied by spectrophotometry. The reaction rate increases with increasing carbohydrazide concentration and temperature and decreases with increasing HNO₃ concentration. The reaction order with respect to Np, carbohydrazide, and HNO₃ is 1, 1.15, and–1.35, respectively. The activation energy of the reaction is 85 kJ mol^–1.     

Catalytic reduction of Np(V) with hydrazine in nitric acid solutions in the presence of ruthenium catalysts

The reduction of Np(V) with hydrazine in HNO₃ solution in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The back oxidation of Np(IV) with nitric acid, also catalyzed by ruthenium, occurs concurrently with the Np(V) reduction with hydrazine. The contribution of each process is determined by the HNO₃ concentration. The mechanism of the ruthenium-catalyzed reduction of Np(V) with hydrazine was suggested on the basis of the kinetic data. The effect of the size of Ru nanoparticles on the activation energy of the catalytic reduction of Np(V), characterizing the structural sensitivity of the heterogeneous-catalytic reaction (positive size effect), was revealed.     

Coprecipitation of neptunium(IV) and plutonium(IV) with hydrolyzed iron(III) in carbonate solutions

In regeneration of Np(IV)-and Pu(IV)-containing recycled solvent by treatment with aqueous sodium carbonate contaminated with iron, some portion of Np(IV) and Pu(IV) coprecipitates with hydrolyzed iron. The degree of coprecipitation of Np and Pu depends on both the iron and sodium carbonate concentrations. The presence of n-dibutyl hydrogen phosphate in the recycled solvent before its regeneration does not noticeably affect the coprecipitation of Np and Pu. The possible mechanisms of coprecipitation of actinides with hydrolyzed iron in carbonate solutions are discussed.     

Kinetics of electrochemical oxidation of neptunium ions in nitric acid solutions containing potassium phosphotungstate K10P2W17O61

The kinetics of electrochemical oxidation of Np(IV) and Np(V) ions on a smooth platinum electrode in a 1 M HNO₃ solution containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) was studied by spectrophotometry. For the redox reactions under consideration, the rate laws were determined, the rate constants were calculated, and the schemes of transformations of Np ions were suggested. The electrochemical oxidation of Np(IV) and Np(V) is described by the first-order rate law with respect to the Np(IV) and Np(V) concentrations, respectively. The rate constants of these reactions are close and comparable with the rate constant of the Np(V) disproportionation.     

Kinetics of thermochemical reaction of TBP solutions in HCBD with nitric acid in two-phase systems

The kinetics of thermochemical reaction of 30% TBP in hexachlorobutadiene (HCBD) saturated with HNO₃ (1.35 M HNO₃) in two-phase systems with 12 M HNO₃ at temperatures from 90 to 115°C was studied. The kinetic parameters of thermolysis of organic solutions of TBP in HCBD, contacting with HNO₃ in open and closed vessels, were determined experimentally for the subsequent estimation of the explosion hazard of these systems under conditions of extraction recovery of high-level radioactive materials. It was shown that, in two-phase systems, solutions of TBP in HCBD exhibit increased resistance to thermochemical oxidation with nitric acid in comparison with hydrocarbon solutions of the extractant in the entire temperature range.     

Kinetics of thermochemical reaction of TBP solutions in HCBD with nitric acid in single-phase systems

The reaction of 30% solutions of TBP in hexachlorobutadiene (HCBD) with dissolved HNO₃ at its initial concentration of 1.2 M was studied in the range from 90 to 150°C. The kinetic parameters of HNO₃ consumption, gas release, and accumulation of dibutyl hydrogen phosphate were determined. The boundary temperature conditions for transition of oxidation processes to thermal explosion mode under conditions of extraction reprocessing of high-level radioactive materials were estimated from the kinetic data.     

Ruthenium-catalyzed oxidation of Np(IV) in nitric acid solutions

Oxidation of Np(IV) with nitric acid in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The catalytic oxidation of Np(IV) in nitric acid solutions occurs even in the presence of hydrazine. The mechanism of Ru-catalyzed oxidation of Np(IV) with nitric acid was suggested on the basis of the kinetic data. The effect of the Ru nanoparticle size on the activation energy of the catalytic oxidation of Np(IV) was revealed. It shows that the heterogeneous-catalytic reaction is structure-sensitive (positive size effect).     

Extraction of rare-earth elements from nitric acid solutions with bis(dioctylphosphinylmethyl)phosphinic acid

The distribution of microamounts of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous HNO₃ solutions and solutions of bis(dioctylphosphinylmethyl)phosphinic acid in organic diluents was studied. The stoichiometry of the extractable complexes was determined, and the diluent effect on the efficiency of extraction of rare-earth elements from nitric acid solutions was examined. Addition of a neutral solvating component, tetraphenylmethylenediphosphine dioxide, to the organic phase enhances the extraction of rare-earth elements, exerting a synergistic effect.     

Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

The kinetics of catalytic reduction of U(VI) with formic acid in H₂SO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO₂ leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.     

Kinetics of Pu(IV) reduction with tert-butylhydrazine

The rate of Pu(IV) reduction with tert-butylhydrazine in an HNO₃ solution is described by the equation-d[Pu(IV)]/dt = k[Pu(IV)]²[(CH₃)₃CN₂H ₄ ⁺ ]/[H⁺], where k = 69.4 ⊥ 3.0 l mol^?1 min^?1 at 50°C. The activation energy is E = 122 ⊥ 4 kJ mol^?1. Probable reaction mechanisms are discussed.     

Catalytic reduction of U(VI) with hydrazine on palladium catalysts in acid solutions

The stability of finely dispersed palladium supported on silica gel with respect to various acids was studied. It was shown that palladium catalysts can be used in moderately acidic media under reducing conditions. In nitric acid solutions within a wide range of experimental conditions, the palladium catalysts do not initiate reduction of U(VI) with hydrazine. The catalytic properties of palladium catalysts differing in the size of nanocrystallites of the active metal were examined in the reduction of U(VI) with hydrazine in sulfuric acid solutions. The specific activity of Pd/SiO₂ catalysts is determined solely by the size of metal nanocrystals and is independent of the metal content on the support. The negative size effect is observed, i.e., the surface Pd atoms located on large crystallites exhibit higher catalytic activity. The results obtained were interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the mechanism of U(VI) catalytic reduction with hydrazine in the sulfuric acid solutions.     

Interaction of Sr(II) and Np(V) with potassium ferrate(VI) reduction products in aqueous solution

The interaction of Sr(II) and Np(V) with potassium ferrate(VI) reduction products in aqueous solution at pH 4–8 was studied by liquid scintillation spectrometry. The ferrihydrite precipitate formed by reduction of ferrate ions quantitatively sorbs Sr(II) and Np(V) from 0.1 M NaClO₄. The precipitate particle size determined by scanning electron microscopy is in the nanometer range. The degree of recovery of the radionuclides from simulated liquid radioactive wastes containing such complexing agents as acetate, citrate, and nitrate ions is 70 ± 5 and 84 ± 2%, respectively.     

Kinetics of the Np(VI) + EDTA reaction in perchloric acid solutions

The stoichiometry of the Np(VI) + H₂C₂O₄ and Np(VI) + H4Y reactions (Y is EDTA anion) in 0.2 M HClO₄ solution was studied by spectrophotometry. With excess Np(VI), 1 mol of H₂C₂O₄ or EDTA reduces, respectively, 2 or 4 mol of Np(VI) to Np(V). In 0.1–1.0 M HClO₄ solution (the ionic strength of 1.0 was supported by adding LiClO₄) containing 3–20 mM EDTA at 20–45°C, Np(VI) at a concentration of 1 mM and higher is consumed in accordance with the first-order rate law until less than 0.4 mM Np(VI) remains in the solution, after which the reaction decelerates. The reaction rate has the order of 1 with respect to EDTA and ?1.5 with respect to H⁺ ions. The activated complex is formed with the loss of 1 and 2 H⁺ ions. The activation energy is 86.0 ± 3.5 kJ mol^?1.     

Reactivity of platinum nanoaggregates in catalytic reduction of U(VI) with hydrazine in acid solutions

Catalysts Pt/SiO₂ with different size of Pt nanoaggregates were studied in catalytic reduction of U(VI) with hydrazine in solutions of H₂SO₄, HClO₄, and HNO₃. The catalytic activity of the surface Pt atoms of Pt/SiO₂ in this reaction monotonically increases with an increase in the size of metal crystallites on the support, i.e., the negative size effect is observed. Catalytic reduction of U(VI) with hydrazine in H₂SO₄ solutions is significantly faster than in HClO₄ and HNO₃, and in HClO₄ it is slightly faster than in HNO₃. The results were interpreted within the framework of the concept of the energetic nonuniformity of the surface atoms and of the mechanisms of catalytic reduction of U(VI) with hydrazine in acid solutions.     

Kinetics of hexavalent chromium reduction by scrap iron

The kinetics of Cr(VI) reduction by scrap iron was investigated in batch system, for aqueous solutions having low buffering capacities, as a function of pH (2.10-7.10), temperature (10-40 degrees C) and Cr(VI) concentration (19.2-576.9 microM). The results obtained using only the experimental data at initial times indicate zero-order kinetics at pH 2.10 and first-order kinetics over the pH range of 2.98-7.10. The reaction order with respect to H(+) concentration, over the pH range of 4.17-7.10 and Cr(VI) concentration range of 19.2-38.4 microM, was found to be 0.31. The effects of pH, Cr(VI) initial concentration and temperature were investigated; the observed Cr(VI) reduction rates increased with decreasing pH, increasing temperature and decreasing initial Cr(VI) concentration. The observed and overall rate coefficients were determined, and a kinetic expression was developed to describe reduction of chromium by scrap iron over the pH range of 4.17-7.10 and Cr(VI) concentration range of 19.2-38.4 microM.     

Extraction of rare-earth elements from nitric acid solutions with polyalkylphosphonitrilic acid

The distribution of microamounts of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous HNO₃ solutions and solutions of polyalkylphosphonitrilic acid in a fluorinated alcohol was studied. The stoichiometry of the extractable REE complexes was determined. An increase in the concentration of fluorinated alcohol in the organic phase leads to a decrease in the REE distribution ratios, which is due to solvation of the extractant with the alcohol molecules.     

Reduction of Np(V) with formic acid in acidic solutions in the presence of palladium catalysts

The kinetics of catalytic reduction of Np(V) with formic acid in HClO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the Pd content and size of Pd nanocrystals was studied. The reaction is a structure-insenitive catalytic process, i.e., the size effect is absent. An increase in the percentage of Pd on SiO₂ leads to a decrease in the activity of the catalysis centers due to a considerable increase in the contribution of the side reaction catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The effect of the S:L ratio, concentrations of HCOOH and HClO₄, and temperature on the rate of catalytic reduction of Np(V) in the presence of palladium catalysts was examined. The suggested mechanism of the catalytic reduction of Np(V) with formic acid in the presence of Pd/SiO₂ involves a slow step of decomposition of the protonated species NpO₂H, formed by the reaction of the NpO ₂ ⁺ ion with chemisorbed hydrogen atoms Pd(H).     

Extraction of molybdenum from supersaturated solutions in nitric acid with tributyl phosphate solutions

Data were obtained on the composition of saturated Mo solutions in HNO₃ solutions of various concentrations at various temperatures, with a maximum at 5 M HNO₃. Extraction of Mo from such solutions of low acidity with concentrated solutions of TBP in xylene at room temperature was studied, and the IR spectra of the extract were measured. The extractability of Mo in the supersaturation region sharply decreases with a decrease in the TBP concentration and with an increase in the acidity of the aqueous phase. The kinetics of the Mo extraction is described by a first-order rate equation. The process apparently involves formation of molybdic acid in the aqueous phase by the reaction MoO₂(OH)⁺ + H₂O = H₂MoO₄ + H⁺. It is assumed that H₂MoO₄ dissolves in water present in TBP, instead of precipitating from the aqueous phase.