Kinetics of Pu(V) disproportionation in CH<Subscript>3</Subscript>COOH-CH<Subscript>3</Subscript>COOLi aqueous solutions

The behavior of Pu(IV–VI) in CH₃COOH-CH₃COOLi solutions was studied by spectrophotometry. The Pu(VI) absorption spectrum changes essentially with an increase in the CH₃COOLi concentration. Owing to formation of Pu(VI) acetate complexes, the maximum of the main absorption band is shifted from 830.6 (in HClO₄ solution) to 845 nm, with the band intensity decreasing by a factor of approximately 8. The Pu(V) and Pu(IV) absorption spectra at low concentrations of acetate ions vary insignificantly relative to the spectra in noncomplexing media. With an increase in the acetate concentration in the system to 1–3 mM, the effect of Pu(V) complexation on its absorption spectrum becomes noticeable (the absorption intensity considerably decreases), whereas the Pu(IV) absorption spectra remain essentially unchanged. Solutions containing 1–2 mM Pu(V) and 0.2–0.5 M CH₃COOLi remain unchanged at 18–25°C for 2 days. In solutions with [CH₃COOLi] = 1–3 M, Pu(V) disproportionates with the formation of soluble Pu(VI) complexes and a suspension of Pu(IV) hydroxide. Introduction of CH₃COOH to a concentration of 0.1–1.0 M prevents the formation of a suspension of Pu(IV) hydroxide, but only up to a temperature of 45°C. The Pu(V) loss follows a second-order rate law, with the reaction products, Pu(IV) and Pu(VI), accelerating the Pu(V) consumption. The reaction rate at a constant concentration of acetate ions is proportional to [H⁺]. The reaction order with respect to Ac⁻ ions is close to 1.6. The activation energy of the Pu(V) disproportionation in the range 19–45°C is estimated at 74.5 kJ mol⁻¹. It is assumed that the disproportionation mechanism involves the formation of dimers from Pu(V) acetate complexes and aqua ions, their protonation, and decomposition with the transformation into Pu(IV) and Pu(VI).     

Formal oxidation potentials of actinide couples in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

The formal oxidation potentials of the M(VI)/M(V), M(V)/M(IV), and M(IV)/M(III) couples for actinides from U to No and of the M(IV)/M(III) couples for some actinides in 1 M H⁺ or 1 M Na⁺ (pH ～5–5.5) solutions containing K₁₀P₂W₁₇O₆₁ were calculated from the data on stability of complexes of f element ions with the unsaturated heteropolytungstate anion P₂W₁₇O ₆₁ ^10? . In some cases, the previously accepted values were subjected to major revision, especially the potentials of the An(V)/An(IV) couples. Problems arising in measuring the potentials of the couples involving Np(III) and Pu(III) which react with the heteropolyanion to form a heteropoly blue are discussed. The potentials of some M(III)/M(II) couples are estimated.     

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).     

Kinetics of Np(V) transformation in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The behavior of Np(V) in concentrated HNO₃ solutions containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) at various concentrations of HNO₃ (1.0–3.0 M) and KPW [(1–5) × 10^?3 M] was studied. Under the examined experimental conditions, the final products of Np(V) transformation are Np(IV) and Np(VI). The reaction follows a first-order rate equation with respect to the Np(V) concentration.     

Behavior of Np(VI) and Np(V) Ions in NaHCO<Subscript>3</Subscript> Solutions Containing H<Subscript>2</Subscript>O<Subscript>2</Subscript>

The behavior of Np(VI) and Np(V) in NaHCO₃ and NaHCO₃ + Na₂CO₃ solutions containing H₂O₂ was studied spectrophotometrically. In 0.75–1.0 M NaHCO₃, hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na₂CO₃ is added. In 1.0 M Na₂CO₃, 0.5 M NaHCO₃ + 0.5 M Na₂CO₃, or 0.01–0.5 M NaHCO₃, hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO₃ solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H₂O₂ yields the excited *Np(V) ion. Then it reacts with another H₂O₂ molecule and forms a carbonate-peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O ₂ ^2? ligand with formation of two Np(VI) ions.     

Stability of Pu(VI) and Pu(V) in aqueous formate solutions

The behavior of Pu(VI), Pu(V), and Pu(IV) in K(Li,Na)HCO₂ and HCOOH + Li(Na)HCO₂ solutions was studied by spectrophotometry. Changes in the spectra of a Pu(VI) solution, observed on adding alkali metal formates, suggest formation of Pu(VI) formate complexes. Changes in the absorption spectra of Pu(V), observed with an increase in the concentration of LiHCO₂ or NaHCO₂, suggest the appearance of Pu(V) formate complexes. The absorption spectra of Pu(IV) indicate that, in a wide range of formate concentrations, the composition of the Pu(IV) formate complexes under the examined conditions is constant. The Pu(VI) content in formate solutions decreases at a rate exceeding the rate of the Pu(VI) disappearance in 0.5–2 M HClO₄ under the action of the ²³⁹Pu α-radiation. The tendency of Pu(V) to reduction and disproportionation in formate solutions depends in a complex fashion on the formate ion concentration and kind of the alkali metal. The kinetics of the Pu(V) consumption in HCOOH + Li(Na)HCO₂ solutions was studied. The reaction starts with the formation of a Pu(V) formate complex, which interacts with Pu(V) aqua ions and Pu(V) formate complex to form dimers, with their subsequent protonation and transformation into Pu(VI) and Pu(IV).     

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).     

Behavior of Am(V) in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The Am(V) disproportionation in (2–10) × 10^?3 M K₁₀P₂W₁₇O₆₁ (KPW) solutions in 1–7 M HNO₃ was studied spectrophotometrically. Under the experimental conditions, Am(VI) and Am(III) are the final products of Am(V) transformation; the process is described by the rate equation of a reversible reaction. The direct reaction follows the first-order rate equation with respect to Am(V) concentration, and the reverse reaction, the zero-order equation.     

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.     

Effect of complexation on crystallization of plutonium(IV) oxalate in the system Pu(NO<Subscript>3</Subscript>)<Subscript>4</Subscript>-HNO<Subscript>3</Subscript>-H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The published data on complexation in the system Pu(NO₃)₄-HNO₃-H₂C₂O₄ were treated on the basis of a unified approach to determination of the oxalate ion concentration. Because of discrepancies between results published by different researchers, additional experiments on crystallization of Pu(IV) oxalate were carried out at widely varied excess and deficiency of oxalic acid. These experiments confirmed high stability of the complex cations PuC₂O ₄ ²⁺ . The upper boundary of the field of metastable supersaturated solutions of Pu oxalate at the initial Pu concentration of 15–50 g l^?1 was determined.     

Disproportionation of Pu(V) in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

In solutions of unsaturated heteropolytungstate K₁₀P₂W₁₇O₆₁, Pu(V) disproportionates in a wide pH range; it is a first-order reaction with respect to Pu(V), and its rate only slightly changes in the pH range from 0.7 to 4.0. The activation energy E _a of Pu(V) disproportionation was determined as 78.6±2.0 and 64.2±3 kJ mol^?1 at pH 2.0±0.1 and 4.0±0.2, respectively. The thermodynamic parameters of activation ΔH ^≠ and ΔS ^≠ were evaluated. Published data on disproportionation of Np(V) and Am(V) in K₁₀P₂W₁₇O₆₁ solutions were analyzed.     

Behavior of plutonium in various oxidation states in aqueous solutions: II. Behavior of Pu(V) and Pu(III) at concentrations of 10<Superscript>−5</Superscript>–10<Superscript>−8</Superscript> M in solutions at pH ∼8

Pu(V) does not exist for a long time in solutions at pH ～8 and concentrations of 10^?5–10^?8 M because of disproportionation to polymeric Pu(IV) and Pu(VI), which form a common polymeric structure that subsequently does not significantly change. Plutonium(III) at its concentrations from 10^?5 to 10^?8 M in solutions at pH ～8 undergoes oxidation, transforming into polymeric Pu(IV). The data obtained can be useful for interpretation of the plutonium behavior under natural conditions.     

Reaction of Np(VI) with cyclohexanediaminetetraacetic acid in HClO<Subscript>4</Subscript> solutions

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H4chdta) in 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO₄ solutions (the ionic strength of 1.0 was supported with LiClO₄) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of–1.2 with respect to [H⁺]. The activated complex is formed with the loss of one and two Н⁺ ions. The activation energy is 82.3 ± 3.8 kJ mol^–1.     

Sorption properties of silicate materials based on Ca<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

Sorption studies showed that a silicate material formed in the course of metallurgical slag reprocessing, containing dicalcium silicate as the major component, exhibits high affinity (logK _d [ml g⁻¹] > 4) for U(VI), Pu(IV), REE(III), and Th(IV) cations and for simple anions and appreciable affinity (logK _d [ml g⁻¹] > 2) for Cs(I) and Sr(II) cations in their sorption from river water. The sorbent can be of practical interest for separation, preconcentration, neutralization, and recovery of harmful chemical elements and radioactive substances from aqueous solutions, followed by their cementation (solidification) and safe disposal.     

Behavior of plutonium in various oxidation states in aqueous solutions: I. Behavior of polymeric Pu(IV) and of Pu(VI) at 10<Superscript>−5</Superscript>–10<Superscript>−8</Superscript> M concentrations in solutions with pH ∼8

Pu(V) does not exist for a long time in solutions at pH ∼8 and concentrations of 10⁻⁵–10⁻⁸ M because of disproportionation to polymeric Pu(IV) and Pu(VI), which form a common polymeric structure that subsequently does not significantly change. Plutonium(III) at its concentrations from 10⁻⁵ to 10⁻⁸ M in solutions at pH ∼8 undergoes oxidation, transforming into polymeric Pu(IV). The data obtained can be useful for interpretation of the plutonium behavior under natural conditions.     

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.     

Crystal structure of double acetates SrAnO<Subscript>2</Subscript>(OOCCH<Subscript>3</Subscript>)<Subscript>3</Subscript>·3H<Subscript>2</Subscript>O [An = Np(V), Pu(V)]

The crystal structure of isostructural Pu(V) and Np(V) acetates of the general composition SrAnO₂Ac₃ · 3H₂O (Ac = CH₃COO^?) was determined. The structures are based on complex anions [AnO₂Ac₃]^2? and Sr²⁺ cations combined into a three-dimensional framework with water molecules located in framework cavities. The An(V) atoms are characterized by the hexagonal-bipyramidal oxygen surrounding; the equatorial plane is formed by the O atoms of three acetate groups. The coordination surrounding of the Sr atom is a tetragonal antiprism formed by the O atoms of acetate ions and water molecules. The bond lengths within the coordination sphere decrease in passing from Np(V) to Pu(V): the average An=O and An-O bond lengths are 1.828(5) and 2.549(6) Å for Np and 1.811(4) and 2.530(4) Å for Pu, respectively.     

Reaction of Am(VI) with Na<Subscript>4</Subscript>XeO<Subscript>6</Subscript> in Alkaline Solutions in the Presence of Ozone

The reaction of Am(VI) with perxenate ions XeO ₆ ^4? in 1 M NaOH solutions was studied. A solid compound [Am(VI) : Na₄XeO₆ = 1 : 1] is formed in reaction of 1 mM Am(VI) with solid Na₄XeO₆; its ozonation in a thin layer yields an Am(VII) compound.     

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).     

Synthesis and properties of complexes of hexavalent actinides with dimethyl sulfoxide [AnO<Subscript>2</Subscript>(DMSO)<Subscript>5</Subscript>](ClO<Subscript>4</Subscript>)<Subscript>2</Subscript> (An = U,Np, Pu)

The complexes [NpO₂(DMSO)₅](ClO₄)₂ (1) and [PuO₂(DMSO)₅](ClO₄)₂ (2), isostructural to the known uranyl complex, were synthesized in the form of single crystals. Their crystallographic characteristics were determined by single crystal X-ray diffraction. The IR and electronic absorption spectra of the crystalline U(VI), Np(VI), and Pu(VI) complexes were measured and analyzed.