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 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.     

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.     

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.     

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.     

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 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.     

Kinetics of Plutonium(IV) Reduction with 3,3'-Bis(diaziridinyl) in HNO3 Solution

3,3'-Bis(diaziridinyl), H₂N₂C₂H₂N₂H₂, is oxidized with Pu(IV) ions in excess of reductant to bis(diazirinyl), N₂C₂H₂N₂, and in excess of oxidant, to nitrogen and acetic acid. The reaction rate in the HNO₃ solution at a constant ionic strength is described by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²× [H₂N₂C₂H₂N₂H₂]^{1 . 7}[H⁺]^{- 3}, where k = 28400±1400 mol^{0 . 3} l^{- 0 . 3} min^{- 1} at 35°C. The activation energy of the reaction amounts to 126±11 kJ mol^{- 1}.     

Kinetics of Pu(VI) Reaction with Ethylenediaminetetraacetic Acid

Radiochemistry - The spectrophometric method was used to examine the stoichiometry of the reaction of Pu(VI) reduction by EDTA ions in 0.05–0.5 M HClO4 solutions at a temperature of 23°C...     

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).     

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 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.     

Mechanism and Kinetics of Rh(IV) Radiation-Chemical Reduction in HNO3 Solutions

Reduction of Rh(IV) in -irradiated and nonirradiated solutions of HNO₃ (0.3-3.0 M) was studied. In both systems, Rh(IV) is completely reduced to Rh(III). The reduction rates in nonirradiated solutions amount to 50-90% of rates in irradiated solutions. Reduction of Rh(IV) with water is postulated. The rates of Rh(IV) reduction in both irradiated and nonirradiated solutions increase with [Rh(IV)] growth and decrease with an increase in [HNO₃] from 0.5-1 to 2-3 M. The dependence of the reduction rates on the dose rate is weak. Mathematical simulation was used to reveal the mechanism and kinetics of radiation-chemical reduction of Rh(IV) in HNO₃ solutions. The rate constant of Rh(IV) reduction with water was calculated by fitting to the experimental data.     

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.     

Behavior of polymeric Pu(IV) in simulated groundwater

The effect of pH, ionic strength, and supporting electrolyte on the speciation of Pu(IV) in simulated groundwater over polymeric plutonium hydroxide at fixed values of the oxidation potential Eh was examined by centrifugal ultrafiltration. Upon dilution of simulated groundwater (SGW) containing highly polymerized polymeric Pu species of high molecular weight, these species transform into those with a lower degree of polymerization. When fresh SGW portions are subjected to ultrafiltration through a compact precipitate of colloidal Pu, this precipitate becomes a source of weakly polymerized Pu species, as in the case of common dissolution of a compact precipitate of colloidal Pu in SGW. In the process, the amount of weakly polymerized forms increases by two orders of magnitude as compared to the initial SGW over polymeric Pu hydroxide. These transformations of the initial Pu species in solutions can affect its migration behavior in subsequent processes.     

Factors governing the Pu(IV) speciation in solutions

After storage of Pu(IV) hydroxide for more than 4 months, ～90% of this compound polymerizes, the remainder (～ 10%) being weakly polymerized Pu(IV). In 0.01 M NaCl solutions (pH ～4–10) being in equilibrium with mixed or polymeric Pu(OH)₄ (decantates), plutonium is mainly in the form of highly polymerized colloidal particles of molecular weight exceeding 100 kDa. Therefore, the Pu concentration in the solutions prepared by decantation or centrifugation of decanted solutions can range from 10^?4 to 10^?7 M. The content of weakly polymerized Pu in solutions varies from 10^?7 to 10^?9 M and depends on pH of the solution in the range 4–6. This dependence is virtually absent at pH 6–10.     

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.     

Disproportionation of polymeric Pu(IV) in weakly acidic solutions

Polymeric Pu(IV) in aqueous solutions in the pH range 0.5–3 disproportionates with time to form Pu(III) and Pu(VI). The arising Pu(III) is bound by hydroxyl groups of polymeric Pu(IV) and does not exhibit intrinsic absorption bands in the spectrum of a solution of polymeric Pu(IV). However, after ultrafiltration of the solution through a filter with a pore size of ∼3 nm Pu(III) is clearly identified in the filtrate by its absorption maxima. Pu(VI) occurs in the solution in the ionic state and is not bound by hydroxy groups of polymeric Pu (IV). Therefore, Pu(VI) is identified in the solution absorption spectrum both before ultrafiltration and after it. Thus, storage of solutions of polymeric Pu(IV) with pH 0.5–3 is accompanied by formation of Pu(III) and Pu(VI) ions.     

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.     

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.