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.     

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.     

Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

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.     

Oxidation of Np(V) and Np(IV) with Fe(CN) 6 3− ions in carbonate solutions

The formal potential of the Fe(CN) ₆ ^3? /Fe(CN) ₆ ^4? couple in 1 M NaHCO₃ and 1–2 M Na₂CO₃ solutions was determined. It is equal to 505 and 510 mV, respectively, exceeding the potentials of the Np(VI)/(V) and Np(V)/(IV) couples in carbonate solutions. The equilibrium of the reaction Np(V) + Fe(CN) ₆ ^3? = Np(VI) + Fe(CN) ₆ ^4? was studied. Fe(CN) ₆ ^3? ions oxidize Np(IV) to Np(V) and then to Np(VI). The arising Np(VI) oxidizes Np(IV). The Np(IV) oxidation accelerates in going from NaHCO₃ to Na₂CO₃. An increase in [Na₂CO₃] or in the ionic strength (by adding neutral salts) decelerates the oxidation. Np(IV) introduced in an HCl solution reacts with Fe(CN) ₆ ^3? or with Np(VI) faster than Np(IV) introduced in a Na₂CO₃ solution. The activation energy of the reaction of Np(IV) with Fe(CN) ₆ ^4? in the range 20–45°C is 107 kJ mol^?1. The reaction mechanism involves formation of the activated complex from ions of Np(IV) hydroxocarbonate and oxidant.     

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.     

Photochemical Reactions of Neptunium Ions in Aqueous Carbonate Solutions

Photochemical reactions of Np(VI), Np(V), and Np(IV) in aqueous solutions containing HCO₃ ^- and CO₃ ^2- anions were studied spectrophotometrically. Two parallel photoreactions, oxidation of Np(V) to Np(VI) and reduction of Np(VI) to Np(V), were revealed. The quantum efficiency of Np(VI) photoreduction in 1.95 M Na₂CO₃ is 0.003. As the pH of the solution decreases, Np(VI) photoreduction is decelerated and Np(V) photooxidation is accelerated. The photoconversion of Np(V) into Np(VI) in 1 M NaHCO₃ is 94-95%. The presence of oxygen has no effect on the oxidation. Neptunium(IV) is oxidized to Np(V) and Np(VI).     

Behavior of neptunium ions in organic media

Published data on the effect of organic solvents on the hydrolysis of Np(IV) and redox reactions of Np(IV?CVI) are analyzed. In aqueous-organic solutions, Np(IV) ions undergo hydrolysis at higher acidity than in aqueous solutions. With respect to the effect on hydrolysis, the solvents can be ranked in the order methanol > ethanol > dioxane > acetone > acetonitrile. Dimethyl sulfoxide suppresses the hydrolysis. The Np(V) disproportionation in CH₃OH and CH₃OH + C₆H₆ solutions in the presence of HCl, HDEHP, or TTA and in a TBP solution was studied. The influence of the solution composition, including the H₂O concentration, on the reaction kinetics was examined. The reactions occur faster than in aqueous solutions. The reaction mechanism is the same in all the media: Two solvated Np(V) ions form a complex decomposing upon protonation into Np(IV) and Np(VI). The role of the solvent in the Np(V) reproportionation was examined. In mixed water-ethylene glycol, water-methanol, and water-acetone solvents, with an increase in the fraction of the organic component, the Np(IV) + Np(VI) reaction rate passes through a maximum, which is due to combined effect of two factors: Np(IV) hydrolysis (acceleration) and decrease in [H₂O] (deceleration). In TBP solutions, the Np(IV) + Np(VI) reaction decelerates in proportion to [HNO₃]^?2 and [H₂O]. The course of the Np(VI) + H₂O₂ and Np(IV?CVI) + HNO₂ reactions in TBP differs from that in aqueous solutions. Deceleration of the Np(VI) reduction and acceleration of the Np(V) oxidation, compared to aqueous solutions, are associated with a decrease in the formal potential of the Np(VI)/(V) couple in going from H₂O to TBP. In solutions of KOH in aqueous methanol, Np(VI) rapidly disproportionates to Np(VII) and Np(V). A decrease in the H₂O concentration shifts the equilibrium toward Np(VII).     

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.     

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 and Mechanism of Np(IV) Oxidation with Nitric Acid

Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions of low acidity (<∼0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k ₁[Np(IV)]/[H⁺]² + k ₂[Np(IV)]²[U(VI)]/[H⁺]³, in which k ₁ = (2.0±0.3) × 10⁻⁵ mol² l⁻² min⁻¹ and k ₂ = (5.50±0.47) × 10⁻² mol l⁻¹ min⁻¹ at 50°C and solution ionic strength μ = 0.5. The activation energies of the two pathways are 148±31 and 122±12 kJ mol⁻¹. The reaction along the main pathway (with the rate constant k ₂) is limited by disproportionation of Np(IV) involving NpOH³⁺ and Np(OH)₂UO ₂ ⁴⁺ complex ions.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 228–233.Original Russian Text Copyright © 2005 by Koltunov, Taylor, Marchenko, Savilova, Dvoeglazov, Zhuravleva.     

Mechanism of transformation of Np(V) into Np(IV) in sodium 1,2-cyclohexanediaminetetraacetate solutions

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5–74 mM sodium 1,2-cyclohexanediaminetetraacetate (Na₂CHDTA) or in a 96–97 mM Na₂CHDTA solution at 25–45°С was studied. The reaction rate at Na₂CHDTA concentrations in the range 5–60 mM and pH 3.5–5.9 is described by the equation V = k[Np(V)]^1.4[CHDTA], and at Na₂CHDTA concentrations in the range 70–100 mM and pH 4.1–5.2, by the equation V = k _A[Np(V)]^1.4. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)}₂ forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V).     

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.     

Interaction of An(VI) (An = U, Np, Pu) and Np(V) with 2,3-pyridinedicarboxylic (quinolinic) acid (H2Quin): Complexation in aqueous solutions, synthesis and structure of the complexes [UO2(HQuin)2], [(NpO2)2(HQuin)2(C6H4NO3)2]·2H2O, and [PuO2Quin(H2O)]

Complexation of An(VI) (An = U, Np, Pu), and Np(V) with 2,3-pyridinedicarboxylic (quinolinic, H₂Quin) acid in aqueous solutions was studied. Np(V) can form 1: 1 and 1: 2 complexes, and An(VI), also 1: 3 complexes (at pH ? 6 and [H₂Quin] ? 0.1 M). Quinolinate ion can coordinate to actinide(VI) and (V) ions in solutions in different modes. The apparent stability constants of the complexes in a wide pH range and the concentration stability constants of the An(VI) complexes were measured. In the series from Pu(VI) to U(VI), the stability of the complexes slightly increases. Crystalline complexes [UO₂(HQuin)₂], [(NpO₂)₂(HQuin)₂(HL)₂]·2H₂O (HL is N-protonated 2-hydroxypyridine-3-carboxylic acid anion), and [PuO₂Quin(H₂O)] were synthesized, and their structures were determined by single crystal X-ray diffraction. Different types of coordination of quinolinate ions to actinide ions are also observed in the crystalline complexes.     

Kinetics of Redox Reactions of U,Np, and Pu in TBP Solutions: IX. Reduction of Np(VI) with Dibenzylhydrazine

The kinetics of reduction of Np(VI) with dibenzylhydrazine in TBP nitric acid solutions was studied. At the reductant excess Np(V), nitrogen, and benzyl alcohol were the reaction products. At low HNO₃ concentration (<0.03 M), the reaction went to completion, while at a higher acid content in TBP the equilibrium was attained, shifting to Np(VI) with increasing acidity. Taking into account direct and reverse reactions, the rate of Np(VI) to Np(V) transformation was described by the equation -d[Np(VI)]/dt = k[Np(VI)]× [(C₆H₅CH₂)₂N₂H₂][H₂O]^0.4 - k ₃[Np(V)]²[HNO₃]^1.2, where k = 64±6 l^1.4 mol^{- 1.4} min^-1 and k ₃ = 4480± 450 l^2.2 mol^{- 2.2} min^-1 at 40°C. The activation energy of the direct reaction was E = 42.7±2.2 kJ mol^{- 1}. The dilution of TBP with n-dodecane did not affect the reaction rate. The reaction mechanism was discussed.     

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 Am(IV) with Water in KHCO3 + K2CO3 Solutions

Reduction of Am(IV) with water in KHCO₃, K₂CO₃ solutions (pH 8.5-10.5) was studiedspectrophotometrically at 54-70°C. The Am(IV) concentration decreases, following the first-order rate law.The reduction rate increases with pH (logk/pH = 0.4), but decreases with increase in (HCO₃ ^- + CO₃ ^{2 -}) concentration. It was assumed that the thermally excited Am(IV) ion forms a dimer with unexcited Am(IV). The dimer dissociates into two Am(III) ions and H₂O₂. Hydrogen peroxide reduces two more Am(IV) ions. In this process, the excited Am(III) ion appears, which transfers the excitation to Am(IV) at collision. Thus, a chain process is initiated. This scheme can also explain the kinetics of Am(VI) and Np(VII) reduction in carbonate solutions.     

Behavior of Np(VII,VI, V) in silicate solutions

Properties of Np(VII, VI, V) in silicate solutions were studied spectrophotometrically. In noncomplexing media, the Np(VII) cation transforms into the anionic species at pH 5.5–7.5. In the presence of carbonate ions, this rearrangement occurs at pH 10–11.5, and in silicate solutions, at pH 10.5–12.0. These data show that Np(VII) cation forms complexes with carbonate and silicate ions, the latter being stronger. From the competitive reactions of Np(VI) complex formation with carbonate and silicate ions, the stability of NpO₂SiO₃ complex was estimated (log = 16.5) using the known stability constant of NpO₂(CO₃) ₃ ^4– . Complexation of Np(V) with SiO ₃ ^2– ions was not detected by the methods used.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 527–530.Original Russian Text Copyright © 2004 by Shilov, Fedoseev, Yusov, Delegard.     

Catalytic Reduction of Np(VI,V) with Formic Acid in Perchloric Acid Solutions

Neptunium(VI) is successively reduced with formic acid to Np(V) and Np(IV) in perchloric acid solutions in the presence of 1% Pt/SiO₂ catalyst. The kinetic features of Np(VI,V) reduction with formic acid in 0.1-4.0 M HClO₄ in the presence of 0.01-0.1 g ml^-1 of 1% Pt/SiO₂ at [HCOOH] = 0.001-1.0 M and T = 40-70°C were studied. The rate-determining steps of reduction of Np(VI) to Np(V) and Np(V) to Np(IV) are diffusion and decomposition of the activated complex adsorbed on the catalyst surface, respectively. The mechanisms of both processes are discussed.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.