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.     

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.     

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).     

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.     

Formation of polymeric Pu(IV) hydroxide structures in aqueous solutions

Forms of occurrence of polymeric Pu(IV) in simulated groundwater (SGW) were studied spectrophotometrically and by the method of centrifugal ultrafiltration through filtering inserts permeable to polymeric Pu species with different molecular weights. The dependences of the fractions of definite Pu(IV) forms on the total Pu content in the solution were found. The possibility of formation of Pu(IV) quasipolymeric structures in aqueous solutions was considered in relation to the problem of the transfer of radioactive contaminants with underground water. Equilibrium distribution of Pu(IV) polymers depending on the total Pu(IV) concentration in the solution was analyzed theoretically. From the experimental data obtained, the parameter allowing determination of the weight distribution of the polymeric particles in relation to the total Pu(IV) concentration was theoretically calculated, and their equilibrium distributions depending on the total Pu(IV) concentration were found.     

Mechanisms of Pu(VII) reduction in aqueous alkaline solutions in the presence of catalysts

Published data on the Pu(VII) stability in alkaline media suggest that one-electron oxidation of OH^? ion or H₂O with Pu(VII) ions is thermodynamically impossible. The more probable pathway of Pu(VII) reduction is formation of a dimer from thermodynamically excited and nonexcited Pu(VII) ions, followed by decomposition of the dimer into two Pu(VI) ions and H₂O₂ molecule, which reacts with Pu(VII) and Pu(VI). The released energy is spent for Pu(VII) excitation. In the presence of difficultly soluble hydroxides of Fe(III), Co(III), and other d elements, and also of Pt and PbO₂, hydroxides of elements in higher oxidation states appear at the surface of colloidal particles and electrode materials under the action of Pu(VII). The neighboring OH groups dimerize and are eliminated in the form of H₂O₂.     

Synthesis of Pu(V) sulfates

Synthesis of Pu(V) sulfates from aqueous solutions was studied, and previously unknown Co(NH₃)₆PuO₂(SO₄)₂·2H₂O was prepared and characterized by X-ray diffraction analysis. It was found that it is isostructural to neptunyl(V) analog. Thermal behavior of this compound in air was studied, and its IR spectrum was recorded and analyzed. The factors preventing formation of Cs₃PuO₂(SO₄)₂·2H₂O and simple sulfates of the composition (PuO₂)₂SO₄·xH₂O were examined.     

Synthesis and study of Pu(V) formates

New double formates MPuO₂(OOCH)₂ (M = NH₄ and K) were isolated from neutral aqueous solutions. These compounds are isostructural to the Np(V) analogs. The unit cell parameters of the salts were determined by powder X-ray diffraction. The symmetry of the crystal lattice decreases from rhombic to monoclinic in going from the ammonium to the potassium salt. The electronic absorption spectra were recorded and analyzed. The thermal behavior of the Pu(V) compounds was studied. Their IR spectra were recorded. The hexagonal bipyramidal oxygen surrounding in the solid compounds was determined. Preparation of simple Pu(V) formates was attempted. The reasons preventing preparation of these compounds were analyzed.     

A spectrophotometric study of the reduction of Pu(VIII) and Pu(VII) in alkaline solutions

The reduction of Pu(VIII) with water in ozonized 15 and 6 MNaOH solutions after termination of the ozone bubbling was studied by spectrophotometry. Kinetic curves reflecting the whole set of processes that occur in NaOH solutions of different concentrations were obtained. Calculations performed with the experimental curves made it possible to construct the kinetic curves describing the behavior of plutonium in higher oxidation states in solution and to obtain the individual absorption spectra of Pu(VII) and Pu(VIII). The processes occurring in ozonized solutions of Pu(VI) in 6 and 15 M NaOH after termination of the ozone bubbling were found to occur by different mechanisms, yielding in 4 days Pu(VI) and Pu(V), respectively.     

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.     

Synthesis and study of new complex Pu(V) and Np(V) acetates

Double Pu(V) and Np(V) acetates of the compositions BaPuO₂Ac₃·2H₂O and SrAnO₂Ac₃·3H₂O with An = Pu and Np were synthesized in a well-crystallized form. In the dry state the compounds are stable in prolonged storage and are not hygroscopic. The plutonyl(V) and neptunyl(V) salts of the similar composition are isostructural. The IR spectra of the compounds were measured, and their behavior in heating was studied.     

Electronic absorption spectra of crystalline Pu(V) compounds

Electronic absorption spectra of different crystalline Pu(V) compounds in the range 400–1200 nm were recorded. The correlation between the spectra of these compounds and their structure was discussed. The intensities of the absorption bands of solid compounds with the pentagonal bipyramidal surrounding of the Pu atom are similar to those observed in solution, whereas the centrosymmetrical hexagonal-bipyramidal surrounding is characterized by substantially decreased intensity of the absorption bands up to their complete disappearance. Comparison of the optical spectra of Pu(V) compounds with known structure and those of aqueous Pu(V) solution shows that the hydration number of PuO ₂ ⁺ is 5.     

Behavior of Pu(VI) and Np(VI) in malonate solutions

Complexation of PuO ₂ ²⁺ in solutions containing malonate anions C₃H₂O ₄ ^2? (L^2?) is studied by spectrophotometry. Mono-and bimalonate complexes are formed. The monomalonate complex was isolated as PuO₂L · 3H₂O. It is isostructural to UO₂L · 3H₂O and forms rhombic crystals with the unit cell parameters a = 9.078(2), b = 7.526(2), and c = 6.2005(15) Å, space group Pmn2₁. The electronic absorption spectrum of the monomalonate complex is characterized by a strong band at 843 nm. In malonate solutions, Pu(VI) is slowly reduced to the pentavalent state even in the cold. The reduction of Np(VI) is considerably faster and more sensitive to increasing temperature. Some kinetic features of the reduction are discussed.     

Synthesis and study of Np(V) and Pu(V) benzoate complexes with bipyridine

Heteroligand compounds AnO₂(bipy)OOCC₆H₅ (An = Np, Pu; bipy = α,α-bipyridine, C¹⁰H⁸N²) were synthesized and studied. It follows from powder X-ray patterns that these compounds are isostructural. Their unit cell parameters, determined by indexing of the powder X-ray patterns, are as follows: a = 9.2162 (7), b = 10.2339(8), c = 17.4083(17) Å, and β = 96.48(1)° for Np and a = 9.1983(18), b = 10.2052(18), c = 17.370(3) Å and β = 96.51(1)° for Pu. The compounds crystallize in the monoclinic system space group P2₁/n, Z = 4. The electronic absorption spectra of crystalline compounds suggest pentagonal-bipyramidal surrounding of the central atom and the prescence of cation-cation bonds with AnO ₂ ⁺ ions acting as monodentate ligands with respect to each other. The IR spectra of the compounds were recorded, and their thermal behavior in air was studied.     

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).     

Removal of Cr(VI) and As(V) from aqueous solutions by HDTMA-modified zeolite Y

The synthesized zeolite NaY from rice husk ash (RHA) and the commercial zeolite NaY both modified with surfactants in amounts equal to 50%, 100% and 200% of their external cation exchange capacity (ECEC) were used to remove chromate and arsenate anions from aqueous solutions. While the unmodified zeolite Y had little or no affinity for the Cr(VI) and As(V) anionic species, the surfactant-modified zeolite Y (SMZY) showed significant ability to remove of these anions from the aqueous solutions. The highest chromates and arsenates adsorption efficiency was observed from solutions of pH values 3 and 8, respectively because of the dominance of the univalent species of both anions. The adsorption equilibrium data were best fitted with the Langmuir isotherm model with the highest removal capacities observed for the SMZY initially prepared considering the hexadecyltrimethyl ammonium (HDTMA) amount equal to the 100% of the ECEC of zeolite Y. Synthesized SMZY remove Cr(VI) and As(V) more than the corresponding commercial one due to its lower silica to alumina ratio. Thus, the HDTMA-covered modified zeolite Y synthesized using RHA can be used to remove Cr(VI) and As(V) from water.     

Oxidation of Pu(VI) with ozone and stability of the oxidation products,Pu(VII) and Pu(VIII), in concentrated alkali solutions

Oxidation of Pu(VI) with ozone and stability of the oxidation products, Pu(VII) and Pu(VIII), in 4–15 M NaOH solutions were studied. In a wide range of alkali concentrations, from 1 to 15 M, the Pu(VI) ozonation yields a mixture of Pu(VII) and Pu(VIII). It was proved that Pu(VII) exists in aqueous alkali solutions in the form different from that suggested previously. Pu(VII) is readily reduced with ?2? in aqueous alkali solutions with the NaOH concentration of up to 8 M, whereas at [NaOH] + 8 M it is fairly stable. On the contrary, Pu(VIII) is noticeably reduced with water at room temperature throughout the examined range of NaOH concentrations from 1 to 15 M.     

Synthesis and properties of complexes of Np(V) and Pu(V) benzoates with phenanthroline

Complexes of 1,10-phenanthroline (phen) with Np(V) and Pu(V) benzoates of the compositions NpO₂(phen)(OOCC₆H₅) and PuO₂(phen)(OOCC₆H₅) were synthesized. A powder X-ray diffraction study showed that these compounds, depending on the preparation conditions, exist in the form of two phases having essentially the same composition but different powder patterns. The phases isolated from hot (70–100°C) solutions are isostructural with the previously described complexes AnO₂(bipy)(OOCC₆H₅) (An = Np and Pu, bipy = 2,2′-bipyridine), i.e., in their structure there are dimeric (AnO₂) ₂ ²⁺ cations formed by mutual coordination of two AnO ₂ ⁺ ions via “yl” oxygen atoms. The compounds AnO₂(phen)(OOCC₆H₅) prepared by slow crystallization in the cold or at weak (up to 45°C) heating are isostructural with each other but appreciably differ in the structure from the high-temperature phases. The electronic absorption spectra of the compounds and their thermal behavior were examined.     

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).     

Behavior of Np(V) in HCOOH Solutions Containing H2O and HClO4

Radiochemistry - Spectrophotometry has been used to examine the kinetics of Np(V) disproportionation in HCOOH solutions containing H2O and HClO4 at temperatures of 20–45°C. The reaction...