Oxidation of Np(IV) with hydrogen peroxide in carbonate solutions

Oxidation of Np(IV) with hydrogen peroxide in NaHCO₃-Na₂CO₃ solutions was studied by spectrophotometry. In NaHCO₃ solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na₂CO₃ forms with H₂O₂ a mixed peroxide-carbonate complex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k’ is proportional to [H₂O₂] in the H₂O₂ concentration range 2.5–11 mM, but further increase in [H₂O₂] leads to a decrease in k′. The bimolecular rate constant k = k′/[H₂O₂] in solutions containing up to 11 mM H₂O₂ increases in going from 1 M NaHCO₃ to 1 M Na₂CO₃ and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide-carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of H₂O₂ oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and H₂O₂ concentrations.     

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.     

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.     

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.     

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.     

Recycling of Ni(II)-citrate complexes using precipitation in alkaline solutions

When the excess of Ni(II) ions as compared to citrate concentration is used both Ni(II) ions and citrate can be precipitated in alkaline solutions. The ratio between Ni(II) and citrate in the precipitate and completeness of citrate precipitation depends on the ratio between the Ni(II) and citrate concentrations in the initial solution and its pH. The data of chemical analysis, potentiometric titration, FT-IR as well as visible spectrophotometric investigations suggest that Ni(II) in the insoluble compound is bound with three -COO- groups and -OH group of the citrate. The insoluble compound also contains SO4(2-) and hydroxides. The treatment of this precipitate with H2SO4 enables to recover a soluble Ni(II)-citrate complex, which can be reused in practice, and to remove the excess of Ni(II) in the form of insoluble Ni(OH)2.     

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.     

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 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.     

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.     

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.     

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.     

Sorption of Pu and Np on chitin-containing materials from strongly alkaline solutions

Sorption of Pu and Np on chitin, chitosan, and chitin-containing materials from strongly alkaline (1–4 M NaOH) solutions was studied under the static and dynamic conditions in the presence of large amounts of NaNO₃. The Pu(IV) distribution coefficient (K _d) between the solutions with the NaOH concentration ranging from 1 to 4 M and different chitin-containing sorbents varies from 3000 to 6000 cm³ g^?1. The K _d of Np(V) with different sorbents decreases from 2000–4000 to 600–1200 cm³ g^?1 with increasing NaOH concentration.     

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).     

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 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.     

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.     

Extraction of U(VI), Th(IV), Pu(IV), and Am(III) from nitric acid solutions with polyfunctional organophosphorus acids

Extraction of microamounts of U(VI), Th(IV), Pu(IV), and Am(III) nitrates from aqueous HNO₃ solutions with solutions of (diphenylphosphinylmethyl)phenylphosphinic, (di-p-tolylphosphinylmethyl)phenylphosphinic, and (dioctylphosphinylmethyl)phosphinic acids and of butyl hydrogen (diphenylphosphinylmethyl)phosphonate in organic diluents was studied. The metal: extractant stoichiometric ratio in the extractable complexes was determined, and the diluent effect on the extraction efficiency was examined. The possibility of using a macroporous polymeric sorbent impregnated with (dioctylphosphinylmethyl)phenylphosphinic acid for concentrating metal ions from HNO₃ solutions was demonstrated.     

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.