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).     

Disproportionation of Pu(VI) and reproportionation of Pu(V), Pu(VII) in aqueous alkali solutions

Disproportionation of Pu(VI) and reproportionation of Pu(V) and Pu(VII) in aqueous NaOH solutions was studied. With an increase in the NaOH concentration in solution over 7.5 M, the equilibrium of the reaction Pu(VII) + Pu(V)?2Pu(VI) is gradually shifted toward formation of Pu(V) and Pu(VII) as products of Pu(VI) disproportionation, and at [NaOH] + 13 M, Pu(VI) disproportionates virtually completely. At [NaOH] + 7.5 M, the equilibrium of the above reaction is shifted toward formation of Pu(VI). Based on the experimental data, the equilibrium constants of the reaction at various alkali concentrations in the solution and the formal potentials ?_{f[Pu(VII)/Pu(VI)]} were calculated. The data obtained showed that, with respect to reduction with water, Pu(VII) is stable in aqueous alkali solutions at NaOH concentrations exceeding 7.5 M.     

Interaction of Pu(VI) with Aluminosilicate in Alkaline Solution

Behavior of Pu(VI) in the course of crystallization of aluminosilicate in 2 and 3 M NaOH was studied. Plutonium(VI) inhibits aluminosilicate crystallization. At the Al : Si : Pu molar ratio of 10 : 40 : 2 in the initial mixture, only a minor amount of the X-ray amorphous phase is formed. Partial sorption of Pu(VI) on the aluminosilicate precipitate depends on the alkali concentration in the solution. As determined by spectrophotometry, only neutral and low-charged Pu(VI) hydroxo complexes are sorbed on the aluminosilicate; anionic complexes like [PuO₂(OH)₄]^{2 -} formed in more alkaline solutions are not sorbed. Plutonium(IV) formed by reduction of Pu(VI) is sorbed on aluminosilicate from 3 M NaOH.     

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).     

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).     

Mechanism of Pu(VI) and Pu(V) autoreduction in aqueous solutions

Published data on the stability of Pu(VI) and Pu(V) in solutions of mineral and organic acids and their salts are analyzed. The hypothesis that Pu(VI) in acid solutions disappears owing to the disproportionation to Pu(VII) and Pu(V) cannot be accepted because of high redox potential of the Pu(VII)/(VI) couple. Plutonium( VI) is reduced owing to radiation-chemical reactions induced by its α-radiation and to the formation of a dimer (so-called excimer) by an excited Pu(VI) ion with an unexcited Pu(VI) ion, which rapidly decomposes to Pu(V) and H₂O₂. Plutonium(V) disappears owing to disproportionation and radiation-chemical processes.     

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.     

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).     

Interaction of U(VI), Np(VI), Pu(VI), and Np(V) with benzenetricarboxylic acids: Complexation in aqueous solutions, synthesis, and crystal structure of uranyl complexes [UO2(H2O)2{C6H3(COO)2(COOH)}]2·2H2O and [(UO2)3(H2O)4{C6H3(COO)3}2]

The complexation of U(VI), Np(VI), and Pu(VI) and of Np(V) with 1,2,3- and 1,2,4-benzenetricarboxylic acids (BTC) in aqueous solutions was studied in wide ranges of pH and actinyl ion concentrations. The compositions of the forming hexavalent actinide complexes were determined. Their apparent stability constants β₁′ depend on pH of the solution: in the pH range 2–4, logβ₁′ from 2 to 4 for the complexes of U(VI), Np(VI), and Pu(VI) with 1,2,3-BTC and from 1.5 to 3.5 for the complexes with 1,2,4-BTC. For Np(V), the β₁′ values are close with both acids, and at equal pH values the Np(V) complexes are less stable than the An(VI) complexes (An = U, Np, Pu). With an increase in pH from ～3 to 6.2–6.9, logβ₁′ of the Np(V) complexes increases approximately from 0.5 to 3. Solid U(VI) complexes with 1,2,3- and 1,2,4-benzenetricarboxylic acids were synthesized by the hydrothermal method, their crystal structure was determined, and the IR spectra were examined.     

Reduction of neptunium(V) and uranium(VI) with iron(II) in bicarbonate solutions

Reaction of Np(VI) compounds with Fe(II) in bicarbonate solutions was studied. Reaction of Np(V) with Fe(II) in the presence of phthalate ions was briefly considered. Iron(II) compounds reduce Np(V) compounds in solutions saturated with Ar or CO₂ at any concentrations of bicarbonate ion. At [Na(K)HCO₃] > 0.86 M, Np(V) is reduced during mixing the reactants and recording the spectra. The reaction of Fe(II) with Np(V) in dilute bicarbonate solutions is substantially slower, probably owing to a sharp decrease in the solubility of the Np(V) carbonate complexes. The solubility of the Np(V) compounds increases after saturation of the dilute bicarbonate solutions with CO₂. However, in this case reduction remains slow. Uranium(VI) carbonate complexes are reduced with Fe(II) compounds in dilute bicarbonate solutions. The reaction products formed at elevated temperatures are UO₂ and FeOOH.     

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.     

Synthesis and study of complexes of Pu(V) and Np(V) formates and acetates with α,α′-bipyridine

Crystalline NpO₂OOCH·C₁₀H₈N₂·nH₂O and NpO₂OOCCH₃·C₁₀H₈N₂·H₂O were synthesized by the addition of a small excess of α,α′-bipyridine to neutral Np(V) formate and acetate solutions. In the wet state and after drying in air, these compounds have essentially different powder X-ray patterns. In the dry state, these compounds are monohydrates. In formate solutions with addition of α,α′-bipyridine, Pu(V) behaves similarly, whereas attempts to prepare the related Pu(V) acetate complexes failed. The Np(V) and Pu(V) compounds AnO₂OOCCH₃·C₁₀H₈N₂·nH₂O are isostructural. According to the results of spectrophotometric studies, the coordination polyhedron of the central atom in all the compounds studied is a pentagonal bipyramid. The IR spectra were recorded and the thermal behavior of the compounds was studied.     

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.     

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.     

Extraction and sorption preconcentration of U(VI), Am(III), and Pu(IV) from nitric acid solutions with alkylenediphosphine dioxides

The extraction of U(VI), Am(III), and Pu(VI) from nitric acid solutions in the form of complexes with alkylenebis(diphenylphosphine) dioxides and their sorption with POLIORGS F-6 sorbent prepared by noncovalent immobilization of methylenebis(diphenylphosphine) dioxide (MDPPD) on a KhAD-7M? polymeric matrix were studied. The preconcentration conditions and distribution coefficients of U(VI), Am(III), and Pu(IV) in their sorption from 3 M HNO₃ were determined. The possibility of concentrating actinides from multicomponent solutions was demonstrated. The composition and nature of complexes of U(VI) with MDPPD were determined from the ³¹P NMR data.     

Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

Formation of peroxy compounds of Pu(IV) is possible in concentrated alkali solutions despite instability of hydrogen peroxide under these conditions. The resulting peroxy compound of Pu(IV) is fairly stable in alkaline solutions. The [Pu]: [O₂⁻] ratio in the compound is close to 1: 2. The formation of the peroxy compound favors a decrease in the content of colloidal polymeric Pu(IV) species in solution, thus making the Pu precipitation from solution more complete.     

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).     

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of Fe(III) oxides and hydrous oxides and MnO<Subscript>2</Subscript>

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (α-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and amorphous MnO₂ was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced upon sorption on α-FeOOH, but not on α-Fe₂O₃ and γ-Fe₂O₃. On the MnO₂ surface, Pu occurs as Pu(VI). From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O ₂ ⁺ and Pu(V)O ₂ ⁺ with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with α-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order \(K_{MnO_2 } > K_{\alpha - FeOOH} > K_{\alpha - Fe_2 O_3 } \sim K_{\gamma - Fe_2 O_3 } \).     

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.