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.     

A Spectrophotometric Study of the Interaction of Uranyl Ions with Orthosilicic Acid and Polymeric Silicic Acids in Aqueous Solutions

The interaction of UO ₂ ²⁺ ions with orthosilicic acid Si(OH)₄ and polymeric silicic acids (PSAs) was studied spectrophotometrically. The equilibrium constant of the reaction UO ₂ ²⁺ + Si(OH)₄ = UO₂OSi(OH) ₃ ⁺ + H⁺ in solutions with the ionic strength I = 0.1–0.2 is log K = −2.56±0.09 (log K ⁰ = −2.29±0.09 recalculated to I = 0); the stability constant of the complex UO₂OSi(OH) ₃ ⁺ (I = 0) is log β⁰ = 7.52±0.09. Formation of small oligomers (degree of polymerization n ≤ 4) has virtually no effect on the apparent constant K. When high polymers are formed (n > 100), the apparent equilibrium constant decreases by a factor of 2–3, and the “true” equilibrium constant recalculated to the actual concentration of silanol groups increases by a factor of 3–4. The absorption spectrum of the complex UO₂OSi(OH) ₃ ⁺ was obtained by treatment of the experimental spectra; it has an absorption maximum in the visible range at 422.5 nm, ɛ_422.5 = 35±2 l mol⁻¹ cm^{− 1}. At pH higher than 5–6, complexes of UO ₂ ²⁺ with PSAs of the composition UO₂(≡SiO)₂(≡ SiOH) _{m − 2} are formed. The absorption spectrum of such a complex was obtained.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 315–321.Original Russian Text Copyright © 2005 by Yusov, Fedoseev.     

Structure and Some Properties of a Double Neptunyl(V) Cesium Molybdate,Cs[NpO<Subscript>2</Subscript>MoO<Subscript>4</Subscript>(H<Subscript>2</Subscript>O)]

A double neptunyl(V) cesium molybdate, Cs[NpO₂MoO₄(H₂O)], was studied by single crystal X-ray diffraction. Crystal data: rhombic system, a = 9.478(2), b = 7.900(1), c = 10.499(2) Å; space group Pnna, Z = 4, d = 5.05 g cm^?3, R = 0.030. The compound has a framework structure; the coordination polyhedron of the Np atom is a distorted pentagonal bipyramid with the equatorial positions occupied by four O atoms of four molybdate groups and an O atom of the coordinated water molecule. The IR and visible absorption spectra of this compound and of Cs₂[(NpO₂)₂Mo₂O₈] whose structure had been determined previously were measured. The NpO ₂ ⁺ stretching vibration frequencies in the IR spectra of these compounds virtually coincide. Incorporation of the O atom of the Mo-O-Mo bridge into the first coordination sphere of the neptunyl(V) ion in Cs₂[(NpO₂)₂Mo₂O₈] exerts the same disturbing effect on the electronic absorption spectrum as does the cation-cation interaction with one of the O atoms acting as a bridge between two Np atoms.     

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).     

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.     

Formation of a sol of mixed U(IV) · U(VI) hydroxide in aqueous solutions with pH 1.5–4

containing U(IV) polymer start to form. With an increase in pH from 1.5 to 4 or in temperature, the induction period becomes shorter. Under anaerobic conditions, the colloidal solution is stable for more than a month. Centrifugation at 8000 rpm (5500g) allows separation of the colloidal particles from the liquid phase. The colloid slowly dissolves in mineral acids saturated with argon or in a K₂CO₃ solution, whereas precipitates of individual freshly prepared U(IV) and U(VI) hydroxides dissolve rapidly. Short UV irradia-of a UO₂(ClO₄)₂ solution saturated with argon and containing ethanol (pH 2.5) results in the appearance of U(V) which then disproportionates, and U(IV) forms with U(VI) a black colloid similar to that arising on mixing U(IV) and U(VI) solutions.     

Reaction of Np(VI) with nitrilotriacetic acid in perchloric acid solutions

Stoichiometry of the reaction of Np(VI) with N(CH₂COOH)₃ (NTA) in a 0.05 M HClO₄ solution was studied by spectrophotometry. With excess Np(VI), 1 mol of NTA reduces 2 mol of Np(VI) to Np(V). In 0.05–0.98 M HClO4 solutions (the ionic strength I = 1.0 was maintained by adding LiClO4) containing 5–30 mmol of NTA, at 25–45°С Np(VI) at a concentration of 0.3–2 mM is consumed in accordance with a firstorder rate law until less than 1/3 of Np(VI) remains in the solution. After that, the reaction decelerates. The reaction is first-order with respect to NTA and has an order of–2 with respect to Н⁺ ions. The activated complex is formed with the loss of two Н⁺ ions. The activation energy of the reaction is 100 ± 2 kJ mol^–1.     

Crystal structure of mixed-valent Np(IV)/Np(V) chloride, [Np(NpO2)6(H2O)8(OH)Cl9]·H2O

A new mixed-valent Np(IV)/Np(V) chloride, [Np(NpO₂)₆(H₂O)₈(OH)Cl₉]·H₂O, was synthesized, and its crystal structure was determined. The crystals consist of NpO ₂ ⁺ and Np⁴⁺ cations, of Cl^? and OH^? anions, of coordination-bonded water molecules, and of water molecules of crystallization. The Np(V) atom, Np(1), has pentagonal bipyramidal coordination surrounding with O atoms in the apical position and with the equatorial plane formed by three Cl^? anions, O atom of the adjacent NpO ₂ ⁺ cation, and O atom of water molecule. The mutual coordination of the neptunyl(V) ions, cation-cation (CC) interaction, links the Np(1) coordination polyhedra via common vertices into rings around sixfold axes, with the Np(V)?Np(V) distance in these fragments of 4.276 Å. The ring fragments are linked with each other via common equatorial edges of the bipyramids into layers perpendicular to c-axis. The Np(IV) atom, Np(2), has coordination surrounding in the form of tricapped trigonal prism (CN 9) with the tetragonal lateral faces formed by the O atoms of the NpO ₂ ⁺ cations and the capping positions occupied by the O atoms of two water molecules and one hydroxy group. The Np(2) atoms act in CC interactions as coordination center for six NpO ₂ ⁺ ions, with the Np(IV)?Np(V) distance of 4.183 Å. The Np(2) polyhedra are arranged in the crystal between layers of the Np(1) coordination polyhedra, linking them with each other.     

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.     

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).