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.     

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.     

Oxidation of Np(IV) with hydrogen peroxide in carbonate solutions

Oxidation of Np(IV) with hydrogen peroxide in NaHCO₃-Na₂CO₃ solutions was studied by spectrophotometry. In NaHCO₃ solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na₂CO₃ forms with H₂O₂ a mixed peroxide-carbonate complex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k’ is proportional to [H₂O₂] in the H₂O₂ concentration range 2.5–11 mM, but further increase in [H₂O₂] leads to a decrease in k′. The bimolecular rate constant k = k′/[H₂O₂] in solutions containing up to 11 mM H₂O₂ increases in going from 1 M NaHCO₃ to 1 M Na₂CO₃ and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide-carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of H₂O₂ oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and H₂O₂ concentrations.     

Kinetics and Mechanism of Pu(VI) Reduction with Acetaldoxime in Nitric Acid Solution

Reduction of Pu(VI) to Pu(III) with acetaldoxime (CH₃CHNOH) in an HNO₃ solution involves three consecutive steps Pu(VI) → Pu(V) → Pu(IV) → Pu(III), and also reproportionation of Pu(IV). Complete kinetics equations of these steps were derived and the rate constants and activation energies of these steps were determined by computer treatment of the experimental kinetic data for all Pu valence forms. The mechanisms of these reaction steps based on the experimental results were discussed.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 67–71.Original Russian Text Copyright © 2005 by V. Koltunov, Pastushchak, Mezhov, G. Koltunov.     

Mechanism of transformation of Np(V) into Np(IV) in sodium 1,2-cyclohexanediaminetetraacetate solutions

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5–74 mM sodium 1,2-cyclohexanediaminetetraacetate (Na₂CHDTA) or in a 96–97 mM Na₂CHDTA solution at 25–45°С was studied. The reaction rate at Na₂CHDTA concentrations in the range 5–60 mM and pH 3.5–5.9 is described by the equation V = k[Np(V)]^1.4[CHDTA], and at Na₂CHDTA concentrations in the range 70–100 mM and pH 4.1–5.2, by the equation V = k _A[Np(V)]^1.4. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)}₂ forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V).     

Reaction of Np(IV) with Orthosilicic Acid,Si(OH)4, in Acid Solutions

Reaction of Np(IV) with Si(OH)₄ within the range of pH 0-2.2 was studied spectrophotometrically. Under these conditions, a complex NpOSi(OH)₃ ^{3 +} is formed. This composition was derived from the dependences of the complex concentration on [H⁺] and [Si(OH)₄]. From the experimental electronic absorption spectra, the spectrum of NpOSi(OH)₃ ^{3 +} was reconstructed. In this spectrum, the narrow band of the aqua ion at 723.2 nm shifts to 729.2 nm and becomes weaker by approximately half. The equilibrium constant K ₁ of the complex formation reaction and the stability constant of the complex ₁ at the ionic strengths I = 0.1 and 1.0 were determined: logK ₁ = 0.71±0.05 and 0.41±0.02, ₁ = 10.52±0.05 and 10.22±0.02, respectively. The Np(IV) speciation in the acid solution in the presence of Si(OH)₄ was calculated.     

Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

Hydrolysis of Np(IV)

Hydrolysis of Np(IV) at p[H⁺] from 0 to 2.7 and ionic strength I = 0.1-1.0 was studied spectrophotometrically. In the p[H⁺] range from 0 to 2.2 and Np(IV) concentrations of 4.5 ×10^{- 5}-1.75 ×10^{- 4} M, polymerization and formation of colloids are negligible and do not noticeably affect the hydrolysis constant measurements. The hydrolysis is completely reversible. With increasing p[H⁺] to 2, only the NpOH^{3 +} complex is formed; the spectrum of this hydroxy complex was calculated. The typical narrow band of Np(IV) aqua ion occurs at 732.2 nm; in hydroxy complex, it is shifted to 729.5 nm and its intensity decreases by a factor of about 2.7. The average constant of Np(IV) hydrolysis equilibrium [Np^{4 +} + H₂O NpOH^{3 +} + H⁺] recalculated to the ionic strength I = 0 was determined as logK ₁ ⁰ = -1.23±0.06, which corresponds to the stability constant of the complex NpOH^{3 +} log₁ ⁰ = 12.77±0.06. The stability constant of the complex Np(OH)₂ ^{2 +} was calculated to be log₂ ⁰ = 24.3.     

Catalytic Reduction of Pu(IV) with Formic Acid in Nitric Acid Solutions

Pu(IV) is reduced to Pu(III) in nitric acid solutions with formic acid in the presence of urea and 1% Pt/SiO₂ catalyst. The kinetics of reduction were studied in 0.3-2.3 M HNO₃ containing 0.2-1 M HCOOH, 0.1-0.5 M (NH₂)₂CO, and 0.01-0.1 g ml^{- 1} of 1% Pt/SiO₂ at 30-60°C. At HNO₃ concentration higher than 2 M, the Pu(IV) reduction is reversible because of catalytic decomposition of urea. The reduction mechanism is discussed.     

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.     

Uranyl and Thorium(IV) Sorption from Dilute Nitric Acid Solutions with Dodecavanadic Acid

Sorption of uranyl on dodecavanadic acid proceeds by the ion-exchange mechanism at relatively low metal concentrations in the range (0.8-2.0) ×10^{- 3} M. Sorption of Th(IV) is practically pH-independent over the pH range 1.5-2.5, but depends significantly on the initial Th(IV) concentration. The maximal capacities of DDVA for uranyl and Th(IV) remarkably differ from each other (9 and 6.3 mg-equiv g^{- 1}, respectively). The uranyl compound formed on contacting DDVA with concentrated uranyl nitrate solution was identified by X-ray diffraction as UO₂(VO₃)₂·nH₂O.     

Reaction of ozone with Np(IV) and Pu(IV) oxalates in water

The reaction of the ozone–oxygen mixture with aqueous suspensions of Np(IV) and Pu(IV) oxalates was studied. Both metal cations and oxalate anions are oxidized in the process. The final products are Np(VI) and Pu(VI) hydroxides. The composition of Np(VI) hydroxide was confirmed by X-ray diffraction analysis. Oxidation of Np(IV) oxalate with oxygen leads to the accumulation of Np(V) oxalate and oxalic acid in the solution. At incomplete oxidation of Np(IV) oxalate with ozone in water, Np(V) is also accumulated. Heating considerably accelerates the ozonation. The possible reaction mechanism is briefly discussed. The Np(V) and Np(VI) ions participate in the catalytic cycle of the decomposition of oxalate ions with ozone.     

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.     

Redox Reactions of Platinum Elements: XV. Kinetics of Rhodium(III) Oxidation with Xenon Difluoride in Nitric Acid Solutions

The rate of the reaction 2Rh(H₂O)₆ ^{3 +} + XeF₂ = 2Rh(OH)₃ ⁺ + Xe + 2HF + 4H⁺ + 6H₂O is described by the second-order equation -d[Rh(III)]/dt = k ₁[Rh(III)][XeF₂], where k ₁ = 15.4±0.8 l mol^-1 min^-1 at 17.8°C and the solution ionic strength = 2. The activation energy of the reaction is E ₁ = 60.6±1.0 kJ mol^-1. This reaction is complicated by the parallel hydrolysis reaction 2XeF₂ + 2H₂O = 2Xe + O₂ + 4HF. The reaction mechanism includes a slow stage of H atom transfer from Rh(H₂O)₆ ³⁺ to XeF₂ molecule with formation of XeF^. radical as an intermediate. The subsequent stages of Rh(H₂O)₅(OH)³⁺ hydrolysis and reduction of XeF^· radical to Xe proceed rapidly.     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VII. Kinetics of Reduction of Pu(IV) and Np(VI) with Butanal Oxime in Undiluted TBP

The kinetics of reduction of Pu(IV) and Np(VI) with butanal oxime in undiluted TBP containing HNO₃ was studied spectrophotometrically. In the range [HNO₃] = 0.08-0.75 M the rate of Pu(IV) reduction is described by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²[C₃H₇CHNOH]/{[Pu(III)][HNO₃]²} with the rate constant k = 0.068±0.017 mol l^-1 min^-1 at 20°C. The kinetic equation of the reduction of Np(VI) to Np(V) in the range [HNO₃] = 0.01-0.27 M is -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH][H₂O]²/[HNO₃]0.5, where k = 0.058±0.007 l2.5 mol-2.5 min^-1 at 25°C, and the activation energy is 79±9 kJ mol^-1.     

Oxidation of Urea with Nitrous Acid in Nitric Acid Solutions

The kinetics of the reaction between urea and HNO₂ in nitric acid solution was studied spectrophotometrically. It was found that, at a constant ionic strength of the solution μ = 2, in the range of the initial concentrations of urea from 0.01 to 0.1 M, HNO₂, from 0.003 to 0.012 M, and hydrogen ions, from 0.1 to 1.5 M, the rate constant of the reaction is described by the equation -d[HNO₂]/dt = k[HNO₂][CO(NH₂)₂][H⁺] · K([H⁺]K +1)⁻¹, where the rate constant k = 15.6±0.3 l mol⁻¹ min⁻¹ and the protonation constant of urea K = 1.38 l mol⁻¹ at 15°C. From the temperature dependence of the reaction rate in the range of 15–35°C, the activation energy was determined to be 61±5 kJ mol⁻¹. The reaction mechanism involving the reaction of nondissociated HNO₂ molecules and protonated urea species NH₂CONH ₃ ⁺ was suggested.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 57–60.Original Russian Text Copyright © 2005 by Dvoeglazov, Marchenko.     

Kinetics of the reaction of Np(VI) with iminodiacetic acid

The stability of Np(VI) in 5–200 mM iminodiacetic acid (H₂IDA) solutions at 23.5–55°С was studied by spectrophotometry. In a solution with pH 2 and excess Np(VI), 1 mol of H₂IDA reduces 2 mol of Np(VI) to Np(V). In 1 and 0.5 M HClO₄ solutions containing 200 mM H₂IDA and 1 mM Np(VI), no more than 36 and 65% of Np(VI), respectively, is reduced at 44.5°С. Complete reduction of Np(VI) is observed in solutions containing 0.2 M HClO₄ and less. In the examined ranges of H2IDA concentrations and temperatures, Np(VI) is consumed in accordance with the first-order rate law. The reduction mechanism involves formation of a Np(VI) iminodiacetate complex, which is followed by intramolecular charge transfer. The generated radical reduces Np(VI). The activation energy is 107 ± 3 kJ mol^–1.     

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.     

Synthesis and crystal structure of Th(IV), U(IV), and Np(IV) tribromoacetates

Actinide(IV) tribromoacetates of the composition [An(CBr₃COO)₄(H₂O)₂]₂ (An = Th, U, Np) were synthesized and studied. Their structural feature is the formation of electrically neutral dimeric complexes. The surrounding of the An(IV) atoms in the dimers is formed by the oxygen atoms of six CBr₃COO^? anions and two water molecules; the coordination number of An(IV) is 9, and the coordination polyhedron can be described as distorted base-monocapped tetragonal antiprism. Four independent CBr₃COO^? anions in the structure are coordinated to the An(IV) atoms in different fashions: monodentate, bidentate chelate, and bidentate bridging. Hydrogen bonding links the dimers in infinite chains along [010] direction. The hydrogen bonding noticeably influences the geometric characteristics of the coordination surrounding of the An(IV) atoms.     

Uranium(IV) Complexes Formed in Extraction with Tributyl Phosphate from Nitric Acid Solutions

Speciation of U(IV) in tributyl phosphate (TBP) solutions prepared by extraction of U(IV) from 2 M HNO₃ was studied. The electronic spectra showed that in the solutions containing from 3 to 60% TBP a mixture of disolvate U(NO₃)₄(TBP)₂ and hydrates with hypothetical formula (TBP) _m ...[U(NO₃) _k · (H₂O) _n ]^(4-k)+ (k = 3 or 4) is formed. Within the 70-100% concentration range, the hexanitrate complex (TBP) _n ...2H₅O₂(H₂O) _p ⁺...[U(NO₃)₆]^{2 -} also appears. In undiluted TBP, as the concentration of uranyl nitrate increases, first the hexanitrate complex and then hydrated complexes of U(IV) gradually disappear. At uranium concentration more than 300 g l^-1, only U(IV) tetranitrate disolvate exists in the organic phase.