Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VIII. Kinetics of Reduction of Pu(VI) and Np(VI) with N,N-Dibutylhydroxylamine in Undiluted TBP

The kinetics of Pu(VI) and Np(VI) reduction in TBP containing HNO₃ was studied spectrophotometrically. The rate of the reduction of Pu(VI) with N,N-dibutylhydroxylamine in undiluted TBP is independent of the Pu(VI) concentration and is described by the equation -d[Pu(VI)]/dt = k[(C₄H₉)₂NOH][H₂O]⁵, with k = (2.17±0.13) × 10^-5 l⁵ mol^-5 min^-1 at 12°C. The activation energy of the reaction, E = 85.2± 4.6 kJ mol^-1, was determined from the temperature dependence of k in the range 12.0-33.5°C. Reduction of Np(VI) in undiluted TBP is approximately described by the kinetic equation -d[Np(VI)]/dt = k[Np(VI)] × [(C₄H₉)₂NOH]/[HNO₃], with k 40 min^-1 at 25°C, and in a 30% solutio of TBP in n-dodecane, by the equation -d[Np(VI)]/dt = k[Np(VI)][(C₄H₉)₂NOH]/[HNO₃]0.7 with the rate constant k = 18.4±1.8 l0.3 mol-0.3 min^-1 at 25°C.     

Kinetics of Redox Reactions of U,Np, and Pu in TBP Solutions: IX. Reduction of Np(VI) with Dibenzylhydrazine

The kinetics of reduction of Np(VI) with dibenzylhydrazine in TBP nitric acid solutions was studied. At the reductant excess Np(V), nitrogen, and benzyl alcohol were the reaction products. At low HNO₃ concentration (<0.03 M), the reaction went to completion, while at a higher acid content in TBP the equilibrium was attained, shifting to Np(VI) with increasing acidity. Taking into account direct and reverse reactions, the rate of Np(VI) to Np(V) transformation was described by the equation -d[Np(VI)]/dt = k[Np(VI)]× [(C₆H₅CH₂)₂N₂H₂][H₂O]^0.4 - k ₃[Np(V)]²[HNO₃]^1.2, where k = 64±6 l^1.4 mol^{- 1.4} min^-1 and k ₃ = 4480± 450 l^2.2 mol^{- 2.2} min^-1 at 40°C. The activation energy of the direct reaction was E = 42.7±2.2 kJ mol^{- 1}. The dilution of TBP with n-dodecane did not affect the reaction rate. The reaction mechanism was discussed.     

Kinetics of Np(VI) Reduction with Butanal Oxime

Reduction of Np(VI) to Np(V) with butanal oxime in the presence of excess reductant is presumably described by the equation 4NpO₂ ²⁺ + 2C₃H₇CHNOH + H₂O = 4NpO₂ ⁺ + 2C₃H₇CHO + N₂O + 4H⁺, and the reaction rate, by the equation -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH]/[H⁺], with k = 230±15 min^-1 at 25°C and the ionic strength of the solution = 2. This equation holds for solutions with different values of the ionic strength and HNO₃ concentration. The activation energy is 69.4±12.4 kJ mol^-1.     

Kinetics of Pu(IV) Reduction with Butanal Oxime

The reduction of Pu(IV) with butanal oxime in nitric acid solution in the presence of excess reductant follows the equation 4Pu^{4 +} + 2C₃H₇CHNOH + H₂O = 4Pu^{3 +} + 2C₃H₇CHO + N₂O + 4H⁺, and its rate is given by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²[C₃H₇CHNOH]/{[Pu(III)][H⁺]}. The rate constant is k = 3.65±0.14 min^{- 1} at 20.2°C and the solution ionic strength = 2. The activation energy is E = 88.8±10.3 kJ mol^{- 1}. The probable reaction mechanism is discussed.     

Redox Kinetics of U,Pu, and Np in TBP Solutions: VI. Reactions of Neptunium and Plutonium with Organic Reducing Agents: Determination of the Final Oxidation States and Reaction Rates

Reactions of Pu(IV) and Np(VI) with organic reducing agents of various types (substituted hydroxylamines, oximes, aldehydes, etc.) in tributyl phosphate solutions containing nitric acid were studied spectrophotometrically. The molar extinction coefficients of neptunium and plutonium in various oxidation states [Np(IV,V,VI), Pu(III,IV,VI)] in TBP solutions were determined as influenced by HNO₃ and H₂O concentrations and temperature. It was found that organic reducing agents at low HNO₃ concentration convert plutonium and neptunium to Pu(III) and Np(V), respectively. With increasing HNO₃ concentration Pu(III) and Np(V) are partly oxidized back to Pu(IV) and Np(VI), respectively, by reaction with nitrous acid. The rate constants of Pu(VI) and Np(VI) reduction and Np(V) oxidation as influenced by concentration of organic reducing agents and HNO₃ were evaluted from the kinetic data.     

Kinetics of Redox Reactions of Np and Pu Ions with Hydrazine Derivatives: XVI. Reaction of Np(V) with Phenylhydrazine

The rate of Np(V) reduction with phenylhydrazine in a perchloric acid solution is described by the equation d[Np(IV)/dt = k ₁[Np(V)][C₆H₅N₂H₄ ⁺] + k ₃[Np(V)][C₆H₅N₂H₄ ⁺][H⁺]² + k ₂[Np(V)][Np(IV)], where k ₁ = 1.27 × 10^{- 3}, 2.81 × 10^{- 3}, and 5.86 × 10^{- 3} l mol^{- 1}min^{- 1}; k ₃ = 2.32 × 10^{- 3}, 1.21 × 10^{- 2}, and 5.75 × 10^{- 2} l³ mol^{- 3} min^{- 1}; and k ₂ = 1.1, 8.3, and 50 l mol^{- 1} min^{- 1} at the ionic strength = 4 and 40, 60, and 80°C, respectively. The activation energies of three reaction pathways are E ₁ = 35±7, E ₃ = 74±17, and E ₂ = 88±1 kJ mol^{- 1}. The reaction is self-accelerated owing to formation of the reactive intermediate, hydroquinone. Its concentration in the reaction mixture is proportional to the concentration of the final product, Np(IV) ion. Probable slow stages of two main and autocatalytic pathways of the reaction are discussed.     

Kinetics of Reactions of Np and Pu Ions with Hydrazine Derivatives: XVII. Reaction of Pu(VI) with Hydroxyethylhydrazine

Reduction of Pu(VI) to Pu(III) with hydroxyethylhydrazine (HOC₂H₄N₂H₃) in HNO₃ solutions involves the following consecutive steps²: Pu(VI) + HOC₂H₄N₂H₄ Pu(V) + ...; Pu(V) + HOC₂H₄N₂H₄ ⁺ Pu(IV) + ...; Pu(V) + Pu(III) 2Pu(IV); and Pu(IV) + HOC_2H4N₂H₄ ⁺ Pu(III) + .... The overall kinetic equations of these steps were suggested, and their rate constants and activation energies were determined. The mechanisms of the four reaction steps, consistent with the experimental kinetic data, are discussed.     

Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH₂NH)₂CO in 1–6 M HNO₃ solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO₃] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.     

Kinetics of Np(V) Reduction with Carbohydrazide Catalyzed by Tc(VII) in the Presence of U(VI)

Radiochemistry - The kinetics of Np(V) reduction with carbohydrazide in nitric acid medium in the presence of Tc(VII) and U(VI) ions was studied by spectrophotometry. The reduced form of Np(IV) is...     

Features of Pu(IV) and Np(IV) Extraction from Nitric Acid Solutions of Uranyl Nitrate with 30% TBP

The extraction of Pu(IV) and Np(IV) from nitric acid solutions containing high concentrations of uranyl nitrate with 30% TBP in hydrocarbon diluent was studied. It was found that, as the Pu(IV) and Np(IV) concentration grows from tens milligrams to several grams at fixed uranyl nitrate (100 g l^-1 and higher) and nitric acid concentrations in the aqueous phase, the distribution coefficients of actinides(IV) increase (for Np to a greater extent than for Pu). This trend becomes more pronounced at higher temperatures. Correlation equations describing this effect are suggested.     

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic acid in aqueous solutions. Synthesis and structures of new crystalline compounds of U(VI), Np(VI), and Pu(VI)

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic (dipicolinic) acid in aqueous solutions was studied. All these actinides form with dipicolinic acid anion, PDC^2? 1: 1 and 1: 2 complexes. The PDC^2? ion coordinates to actinide(VI) ions in solutions in tridentate fashion. In 1: 2 complexes, the f-f transition bands in the electronic absorption spectra are very weak, which is associated with approximate central symmetry of the coordination polyhedron (CP) of the An atom. The apparent stability constants of Pu(VI) complexes were measured in a wide pH range, and the concentration stability constants of An(VI) (An = U, Np, Pu, Am) were determined. The crystalline complexes [Li₂AnO₂(PDC)₂]·2H₂O (An = U, Np, Pu) and [AnO₂(PDC)] _n (An = Np, Pu) were synthesized, and their structures were determined by single crystal X-ray diffraction. The X-ray data confirmed the conclusion that CP of An atoms in the complex ions AnO₂·(PDC) ₂ ^2? is centrosymmetrical. In the isostructural series of [Li₂AnO₂(PDC)₂]·2H₂O, the actinide contraction is manifested in shortening of the An-O distances in the “yl” groups in going from U to Pu.     

Np(VI) Reduction with Diformylhydrazine in Perchloric Acid Solutions

Radiochemistry - The stoichiometry of the reaction of Np(VI) with diformylhydrazine N2H2(CHO)2 (DFH), in 0.01 and 0.1 M HClO4 solutions was studied by spectrophotometry. At an excess of Np(VI), 1...     

Reduction of Pu(IV) and Np(VI) with hydroxylamine in solutions of low acidity with high uranium content

Decomposition of hydroxylamine in HNO₃ solutions containing 350 to 920 g l^?1 U(VI) was studied. In the absence of fission and corrosion products (Zr, Pd, Tc, Mo, Fe, etc.), hydroxylamine is stable for no less than 6 h at [HNO₃] < 1 M and 60°C. In the presence of these products, the stability of hydroxylamine appreciably decreases. The reduction of Pu(IV) and Np(VI) with hydroxylamine in aqueous 0.33 and 0.5 M HNO₃ solutions containing 850 g l^?1 U(VI) and fission and corrosion products at 60°C was studied. Np(VI) is rapidly reduced to Np(V), after which Np(V) is partially reduced to Np(IV). The rate of the latter reaction in such solutions is considerably higher than the rate of the Np(V) reduction with hydroxylamine in HNO₃ solutions without U(VI). At [HNO₃] = 0.33 M, the use of hydroxylamine results in the conversion of Pu to Pu(III) and of Np to a Np(IV,V) mixture, whereas at [HNO₃] = 0.5 M the final products are Pu(IV) and Np(V).     

Reduction of Np(VI) and Pu(VI) with anions of some substituted carboxylic acids

The reaction of Np(VI) with organic acid anions in solutions containing lithium salts of tartaric, malic, α-aminoglutaric, and trihydroxyglutaric acids was studied. Changes in the solution spectra show that Np(VI) forms complexes with organic acid anions, which is followed by the reduction of Np(VI) to Np(V). Similar processes occur in solutions containing Pu(VI) and sodium phenylglycolate or ammonium salicylate. In weakly acidic solutions, the loss of the Np(VI) and Pu(VI) concentrations is a linear function of time. The possible mechanism of the redox reactions was suggested.     

Kinetics of Np(VI) Reduction with Diformylhydrazine in Nitric Acid

The kinetics of the Np(VI) reduction with diformylhydrazine in a nitric acid solution was studied by spectrophotometry. The reaction rate increases with an increase in the reductant concentration and temperature and decreases with an increase in the HNO₃ concentration. The reaction order with respect to Np, diformylhydrazine, and HNO₃ is 1, 1.3, and–1.55, respectively. The activation energy of the reaction is 85 ± 10 kJ mol^–1.     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) Phenylacetates

Radiochemistry - Phenylacetates [AnO2(C6H5CH2COO)2], where An = U (I), Np (II), or Pu (III), were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are...     

Kinetics of Reactions of Np and Pu Ions with Hydrazine Derivatives: XVIII. Reaction between Np(V) and Hydroxyethylhydrazine

The Np(V) reduction with hydroxyethylhydrazine is described by the equation −d[Np(V)]/dt = k ₁[Np(V)][HOC₂H₄N₂H ₄ ⁺ ] + k ₂[Np(V)][Np(IV][H⁺]^1.8, reflecting its main and autocatalytic pathways. The rate constants are k ₁ = 0.31±0.04 l mol⁻¹ min⁻¹ and k ₂ = 4.04±0.11 l^2.8 mol^−2.8 min⁻¹ at 80°C and ionic strength μ = 4. The activation energies are E ₁ = 90±6 and E ₂ = 116±4 kJ mol⁻¹, respectively. The autocatalytic pathway is limited by the reaction between hydroxyethyldiazenium ions, HOC₂H₄N₂H ₂ ⁺ and protonated Np(V) ions. __________ Translated from Radiokhimiya, Vol. 47, No. 2, 2005, pp. 150–153. Original Russian Text Copyright ? 2005 by V. Koltunov, Baranov, G. Koltunov.     

Behavior of Cs, Np(V), Pu(IV), and U(VI) in pore water of bentonite

Sorption of Cs, Pu(IV), Np(V), and U(VI) with bentonite from solutions was studied. Physicochemical species of radionuclides in the liquid phase were determined, the sorption mechanisms were established, and the influence of bentonite colloids on the behavior of radionuclides was studied. It was shown that Cs is sorbed by the ion-exchange mechanism, whereas the sorption of actinides at pH > 5 is governed by the reaction with surface hydroxy groups of betonite, and at pH < 5 the competing processes are ion exchange and complex formation. Reduction of Np(V) and U(VI) to Np(IV) and U(IV) in the solution with Fe(II) compounds present in the system was proved by the extraction method. Various methods of separating the solid phase were used in studying the dependence of the distribution coefficients of Np and Pu on the ratio of pore water and bentonite; it was shown that Np and Pu are sorbed on bentonite colloids.     

Kinetics of Pu(VI) Reduction with Diformylhydrazine in Nitric Acid

The kinetics of the Pu(VI) reduction with diformylhydrazine in a nitric acid medium was studied by spectrophotometry. The reaction rate increases with an increase in the reductant concentration and temperature and decreases with an increase in the HNO₃ concentration. The reaction order with respect to Pu, diformylhydrazine, and HNO₃ is 1, 1.3, and–1.5, respectively. The activation energy of the reaction is 86.9 kJ mol^–1.     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) 2-Fluorobenzoates

The compounds NH₄[AnO₂(C₆H₄FCOO₃], where An = U (I), Np (II), or Pu (III), CgH₄COO^? is the 2-fluorobenzoate anion, were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the cubic system, space group P2₁3, Z = 4. The main structural units of I–III are mononuclear complexes [AnO₂(C₆H₄COO)₃]^? belonging to crystal-chemical group AB₃¹ (A = AnO₂²⁺, B⁰¹ = C₆H₄FCOO^?). The actinide contraction in the structures of I–III is manifested in a regular decrease in the lengths of the An=0 bonds in the AnO₂²⁺ cations and in the volumes of the Voronoi-Dirichlet polyhedra (VDPs) of the An atoms in the series U-Np-Pu. The intermolecular interactions in crystal structures of I–III were analyzed by the method of molecular VDPs.