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.     

Complexation of tetravalent actinides (Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid in aqueous solutions

The interaction of An(IV) ions (An = Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid (2,6-PDCA) in solutions was studied by spectrophotometry. The electronic absorption spectra of the individual complex species An(PDC)²⁺, An(PDC)₂, and An(PDC) ₃ ^2? (PDC^2? is 2,6-PDCA anion; An = U, Np, Pu) were obtained. At [2,6-PDCA] ? 3[An(IV)] + 0.01 M and [H⁺] ? 0.2 M, the prevalent An(IV) species are the complexes An(PDC) ₃ ^2? . Their overall stability constant exceeds 10²⁵ L³ mol^?3 and increases in the series from Th(IV) to Pu(IV) by ～8 orders of magnitude. Very high stability of An(IV) complexes with 2,6-PDCA anions leads to significant shifts of the redox potentials of couples involving An(IV). In particular, large difference in the stability of An(III) and An(IV) complexes is responsible for the fact that Pu(III) in the presence of 2,6-PDCA is readily oxidized with atmospheric oxygen to Pu(IV).     

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).     

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}.     

Disproportionation of Pu(V) in the CH<Subscript>3</Subscript>COOH-H<Subscript>2</Subscript>O system

The behavior of Pu(VI) and Pu(V) in CH₃COOH (HAc)-H₂O solutions was studied by spectrophotometry. The absorption spectrum of Pu(VI) does not change on adding HAc to a concentration of 5 M in the presence of 0.5–1.0 M HClO₄, but in solutions containing less than 0.001 M mineral acid, changes in the spectrum are observed at HAc concentration of 0.6 M.he major absorption band of PuO ₂ ²⁺ ions, caused by an f-f transition, with increasing [HAc] is shifted from 830.6 to 836 nm, with a simultaneous decrease in the absorption intensity, which is due to formation of 1: 1 complexes of Pu(VI) with Ac^? ions. In anhydrous HAc, the peak intensity increases again, owing to total change in the composition of the solvation shell. Pu(V) is unstable in 1–17 M HAc solutions and disproportionates to form Pu(VI) and Pu(IV). The Pu(V) loss follows a second-order rate law with respect to [Pu(V)] and accelerates with increasing HAc concentration. The reaction products exert opposite effects on the reaction rate: Pu(IV) accelerates the consumption of Pu(V), whereas Pu(VI) does not affect the process in dilute HAc solutions but decelerates the disproportionation in concentrated solutions owing to formation of a cation-cation complex with Pu(V).     

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.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

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.     

Disproportionation of Pu(V) in aqueous HCOOH solutions

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H₂O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO₄ changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu₂²⁺ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO₄ solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]² and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).     

Solubility of plutonium nitrate in uranyl nitrate hexahydrate melt

At a weight fraction below 20%, Pu(IV) nitrate is isomorphically dissolved and homogeneously distributed in the uranyl nitrate hexahydrate phase, the solubility being independent of the HNO₃ concentration in the mixed U-Pu melt up to the acid concentration of 7 M. In dissolving solid Pu(IV) nitrate in 1–6 M HNO₃, Pu(IV) disproportionates to form Pu(III) and Pu(VI). With increasing HNO₃ concentration to above 10 M, Pu(IV) passes into solution as Pu(NO₃) ₆ ^2? , and no disproportionation is observed.     

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.     

Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Published data on reactions of Np ions with O₂, H₂O₂, HNO₂, and HNO₃ in solutions of various compositions in a wide pH range are considered. O₂ oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. H₂O₂ exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75?C1 M NaHCO₃ it oxidizes Np(V) to Np(VI), whereas in dilute HClO₄ and HNO₃ and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO₃ solutions, H₂O₂ converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates H₂O₂ (in addition to the H₂O₂ arising as primary product of water radiolysis). In HNO₃ solutions, oxidation of Np ions occurs only in the presence of HNO₂ arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO ₂ ^? , NO₂, and NO⁺ present in equilibrium with HNO₂.     

Tc(VII)-catalyzed oxidation of U(IV) with nitric acid in solution of TBP in <Emphasis Type="Italic">n</Emphasis>-dodecane

Oxidation of U(IV) with nitric acid in 30% solution of TBP in n-dodecane is catalyzed by Tc ions; the rate-determining steps are 3U(IV) + 2Tc(VII) → 3U(VI) + 2Tc(IV) and Tc(IV) + Tc(VII) → Tc(V) + Tc(VI). Oxidation of U(IV) is inhibited by the reaction product, HNO₂, which partially binds Tc(IV) ions (TcO²⁺) in an inert complex. The overall rate equation of U(IV) oxidation is-d[U(IV)]/dt = k ₁[U(IV)][Tc][HNO₃]^?3 ? k ₄[U(IV)]²[HNO₂]²[HNO₃]^?1, where k ₁ = 4.8 ± 1.0 mol²1^?2 min^?1 and k ₄ = (2.4 ± 1.0) × 10⁵ 1² mol^?2 min^?1 at 25°C, [H₂O] = 0.4 M ([Tc] is the total Tc concentration in the reaction mixture). Water and U(VI) have no effect on the reaction rate.     

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.     

Oxidation of Pu(IV) to Pu(VI) with an XeO<Subscript>3</Subscript> + H<Subscript>2</Subscript>O<Subscript>2</Subscript> Mixture in a Perchloric Acid Solution

In a perchloric acid solution, XeO₃ does not oxidize Pu(IV), but the addition of H₂O₂ leads to the accumulation of Pu(VI). It is assumed that Pu(IV) forms a complex with XeO₃. The reaction of the complex with hydrogen peroxide generates OH radicals, which oxidize Pu(IV) to Pu(V). The latter disproportionates to Pu(IV) and Pu(VI).     

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.     

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.     

Oxidation of uranium(IV) with oxygen in NaHCO<Subscript>3</Subscript> solution

The kinetics of U(IV) oxidation with atmospheric oxygen in NaHCO₃ solutions was studied by spectrophotometry. In 1 M NaHCO₃ at [U(IV)]₀ = 20 mM, an induction period is observed, which virtually disappears with decreasing [U(IV)]₀ to 1.0 mM. The induction period is caused by the fact that initially U(IV) exists in a weakly active polymeric form. Addition of U(VI) to the initial solution accelerates the oxidation. In a 1 M NaHCO₃ solution containing 0.1–1.0 mM U(IV), the U(IV) loss follows the first-order rate law with respect to U(IV) and O₂. The pseudo-first-order rate constants, bimolecular rate constants, and activation energy of the U(IV) oxidation were calculated. In dilute NaHCO₃ solutions (0.5–0.01 M), the hydrolysis and polymerization of U(IV) become more pronounced. The autocatalysis mechanism presumably involves formation of a complex [U(IV) · U(VI)] with which O₂ reacts faster than with U(IV). Oxidation of U(IV) occurs by the two-electron charge-transfer mechanism.     

Catalytic Reduction of U(VI) with Hydrazine and Formic Acid in HNO3 Solutions

Catalytic reduction of 0.75 M U(VI) with hydrazine in HNO₃ solutions was studied under various conditions. At 58°C up to 0.55 M U(IV) is accumulated within 2 h in solutions containing 1-1.5 M N₂H₅ ⁺ and 2 M HNO₃ in the presence of 1% Pt/SiO₂ (S : L = 1 : 10). The reduction is decelerated with decreasing N₂H₅ ⁺ concentration to 0.75 M or with increasing HNO3 concentration to 3-4 M. 1.2 mol of U(IV) is formed per mole of oxidized hydrazine. Uranium(VI) reduction with formic acid in the presence of 1% Pt/SiO₂ and 1% Pt/VP-1An anion exchanger was studied. There exists a threshold N₂H₅ ⁺ concentration below which U(VI) is not reduced. The reduction in solutions containing 1-2 M HCOOH, 1-2 M HNO₃, and 0.1 M N₂H₅ ⁺ is faster than in solutions free of formic acid and containing 1-1.5 M N₂H₅ ⁺.     

Separation of transuranium and transplutonium elements on S-957 sorbent

Sorption of Pu⁴⁺, UO₂²⁺, NpO₂⁺, Am³⁺, and Eu³⁺ ions on S-957 cation exchanger from 2–7 M HNO₃ solutions was studied. The following selectivity series was obtained: Pu⁴⁺ > UO₂²⁺ > NpO₂⁺ > Am³⁺ ≈ Eu³⁺. The static and dynamic capacities of the sorbent for Pu were determined, and the eluent composition for the efficient desorption was chosen. The possibility of separating Pu(IV)-Am(III) and Pu(IV)-Np(V) pairs on the sorbent in the column chromatography mode was demonstrated.