Uranium(III) in aqueous solutions: Preparation,properties, synthesis of solid compounds

Published data on the preparation procedures, stability, and complexation of U(III) in aqueous solutions are summarized and correlated. Reactions with inorganic and organic free radicals studied by the flash radiolysis method, the spectroscopic properties, the extraction and ion-exchange behavior of U(III), and methods for isolation of solid U(III) compounds from aqueous solutions are discussed.     

Adaptive terrain triangulation using the representation of quad trees by vertex textures and wavelet estimation of vertex significance

A method of adaptive terrain triangulation is proposed that can be implemented in hardware. The method is based on an estimate of the static error of a quad tree nodes using wavelet transforms and on the representation of the resulting quad tree by a vertex texture. The proposed method has the following characteristic features: the adjacent nodes of the generated adapted mesh can differ in any number of hierarchical levels; the triangulation process is not limited by the size of the decomposition segments, which solves the problem of joining segments without inserting additional nodes; the multiscale terrain representation used in the method makes it possible to store the levels of detail in the graphics processor memory as a multilevel vertex texture; thus, the costliest part of the algorithm can be efficiently implemented using a vertex shader. When constructing the triangulation, the algorithm takes into account both local features of the terrain and the camera location; also, it has a natural support of geomorphing.     

Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe₂O₃, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe₂O₃ was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.     

Complexation and Disproportionation of Np(V) in K10P2W17O61 Solutions

Complexation of NpO₂ ⁺ and NpO₂ ^{2 +} with unsaturated K _n P₂W₁₇O₆₁ ^{n - 1 0} (L ^x-) heteropolyanion and disproportionation of Np(V) in the presence of L ^x- were studied spectrophotometrically. The logarithms (logK) of the formation constants of NpO₂ ^VL and NpO₂ ^{V I}L are 3 and 7, respectively. The K⁺ and Na⁺ cations bind the L ^x- anions, thus decreasing the yield of the complexes. Neptunium(V) disproportionation in K₁₀P₂W₁₇O₆₁ solutions containing 1 M (HClO₄ + NaClO₄) (pH from 0 to 4) and free from NaClO₄ (pH 2-6.5) was studied. The disproportionation rate is described by the equation -d[Np(V)]/dt = k[Np(V)][L ^x-]. The pH dependence of the rate constant passes through a maximum at pH 1. The rate constant decreases with increasing [Na⁺]. The reaction is inhibited by its product, Np(IV). The Np(V) complex is not involved in disproportionation; the reactive species is NpO₂ ⁺ aqua ion, which is probably converted into NpO^{3 +}L ^x-. Then NpO^{3 +}L ^x- rapidly reacts with NpO₂ ⁺, which occurs simultaneously with, or is preceded by release of the second oxygen atom.     

Complexation of Np(V) with Silicate Ions

It was shown that Np(V) forms complexes with anions of orthosilicic acid and other silicate ions at pH higher than 8–8.5. At pH < 9.5, the reaction is mainly described by the equation NpO ₂ ⁺ + OSi(OH) ₃ ⁻ ⇄ NpO₂OSi(OH)₃; the stability constant of the NpO₂OSi(OH)₃ complex is equal to log β₁ = 2.1 ± 0.3. Thus, interaction is weak and hardly significant under real conditions. Carbonate ions in equilibrium with air at pH > 8.5 are the substantially stronger ligands for NpO ₂ ⁺ , and in their presence it is impossible to reveal Np(V) complexation with silicate ions.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 39–43.Original Russian Text Copyright © 2005 by Yusov, Fedoseev, Isakova, Delegard.     

Selective Recovery of Chromium from Precipitates Containing d Elements and Actinides: I. Effect of O2

Oxidation of Cr(III) hydroxides, double Fe(III)-Cr(III) hydroxides, and some examples of spinel phases NiCr₂O₄ and Fe(Cr,Fe)₂O₄ in alkaline suspensions (0.2-0.5 M NaOH) under the action of air and pure oxygen (1-3 atm) was studied. The reaction rate increases with increasing concentration of alkali, temperature, and oxygen pressure. Under these conditions, Pu(IV) sorbed on chromium hydroxides is not oxidized with oxygen and remains in the precipitate.     

Interaction of tetravalent actinides and Np(V) with some hydroxy carboxylic acids

Interaction of actinides(IV) with hydroxyisobutyric acid (HHIB) in aqueous solutions and in the course of crystallization of solid compounds was studied. The complexes ML _n ^(4-n)+ (M = U, Np, Pu; L^? is hydroxyisobutyrate anion; n = 1, 2, 3) exist in solution. Their apparent stepwise stability constants K?? _i were measured, and the overall concentration stability constants ??₃ of the complexes ML ₃ ⁺ were calculated. For U(IV) and Np(IV), log??₃ is close to 13.3?C13.4, and for Pu(IV), log??₃ = 14.5 ± 0.9 (ionic strength I = 0.1?C0.3). In the course of crystallization in air, complexes of U(IV) with hydroxyisobutyric acid, as well as those with citric acid, undergo oxidative degradation, which can be accompanied by complete oxidation of U(IV). The crystalline compounds formed in the process are oxalates of U(IV) or U(VI). The complexation of Np(V) with HHIB was studied. NpO ₂ ⁺ forms with HHIB the complexes NpO₂L and NpO₂L ₂ ^? . Their concentration stability constants are logK ₁ = 2.04 ± 0.15 and logK ₂ = 0.71 ± 0.10 (I = 0.4), i.e., log??₂ = 2.75 ± 0.25.     

Potential of the NP(VI)/Np(V) couple in solutions of various alkalis

The formal potentials of the Np(VI)/Np(V) couple E ^f in alkaline solutions were measured potentiometrically. In 1 M LiOH, NaOH, KOH, CsOH, and (CH₃)₄NOH, the potentials are equal to 0.163⊥0.004, 0.125⊥0.005, 0.112⊥0.005, 0.107⊥0.005, and 0.109⊥0.005 V, respectively. In solutions of MOH+MCl [M=Li, Na, K, Cs, and (CH₃)₄N] at the ionic strength of 1, the dependence of E ^f on log[OH^?] is a straight line with a slope of 0.118⊥0.010, i.e., two OH^? ions participate in the electrochemical reaction between Np(VI) and Np(V). Taking into account the well-known structure of Np(VI), it can be stated that Np(V) in solutions with [OH^?]=1 M and less exists in the form of the NpO₂(OH) ₂ ^? anion. In 2–4 M LiOH and 2–11 M NaOH or KOH, the potential decreases with increasing alkali concentration. In these media, the anion NpO₂(OH) ₃ ^2? is formed.     

Complexation of actinides(VI) with furan-2-carboxylic and thiophene-2-carboxylic acids in aqueous solutions and synthesis of crystalline compounds

Crystalline uranyl compounds with furan-2-carboxylic (F2C) and thiophene-2-carboxylic (T2C) acids containing the complex anions [UO₂(OOCC₄H₃O)₃]^? and [UO₂(OOCC₄H₃S)₃]^? were synthesized and studied by single crystal X-ray diffraction. A previously unknown compound of a transuranium element, Pu(VI), with T2C, also containing the complex anions [PuO₂(OOCC₄H₃S)₃]^?, was synthesized. With Pu(VI) taken as example, the complexation of hexavalent actinides with F2C and T2C in aqueous solutions was studied, and the stability of the complexes PuO₂L⁺ (L is F2C or T2C anion) was determined. The Pu(VI) complexes with F2C are slightly more stable than those with T2C but less stable than Pu(VI) monoacetate complexes. The spectral characteristics of the complexes are discussed.     

Oxidation of U(VI) with oxygen in weakly acidic and neutral solutions

The kinetics of U(IV) oxidation with atmospheric oxygen in solutions with pH 2–7 was studied. In the kinetic curves there is an induction period, which becomes shorter with increasing pH. The induction period is caused by accumulation of U(VI), whose initial presence in the working solution accelerates oxidation. The pseudo-first-order rate constants and bimolecular rate constants of U(IV) oxidation with oxygen were evaluated. The mechanism of U(IV) oxidation is considered. At pH higher than 3, formation of a polymer of hydrolyzed U(IV) with U(VI) plays an important role in oxidation of U(IV), since this prevents formation of U(V). Heating accelerates oxidation of U(IV) at pH 2–2.5, but at a higher pH the process becomes difficultly controllable.