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.     

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.     

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.     

A Spectrophotometric Study of the Interaction of Np(VI) with Orthosilicic Acid and Polymeric Silicic Acids in Aqueous Solutions

The interaction of NpO ₂ ²⁺ ions with orthosilicic acid Si(OH)₄ and polymeric silicic acids (PSAs) in aqueous solutions was studied spectrophotometrically. The interaction at pH ≤ 4.5 is described by the equation NpO ₂ ²⁺ + Si(OH)₄ = NpO₂OSi(OH) ₃ ⁺ + H⁺ with the equilibrium constant log K = − 2.88±0.12 at the ionic strength I = 0.1–0.2 (log K ⁰ = −2.61±0.12 recalculated to I = 0); the stability constant of the complex NpO₂OSi(OH) ₃ ⁺ (I = 0) is log β⁰ = 7.20± 0.12. At pH > 5, a second complex of NpO ₂ ²⁺ with PSAs of the presumed composition NpO₂(≡ SiO)₂(≡SiOH) _{m − 2}, where (≡SiOH)_m denotes a PSA molecule with surface Si-OH groups, is formed. The absorption spectra of the complexes NpO₂OSi(OH) ₃ ⁺ and NpO₂(≡ SiO)₂(≡SiOH) _{m − 2} were obtained. In contrast to the hydroxo complexes, they have pronounced maxima at 560 – 600 nm with the molar extinction coefficients of about 25–30 l mol⁻¹ cm⁻¹, which is several times higher compared to the Np(VI) aqua ion.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 322–327.Original Russian Text Copyright © 2005 by Yusov, Shilov, Fedoseev, Astafurova, Delegard.     

Oxidation of uranium(IV) with oxygen in NaHCO<Subscript>3</Subscript> solution

The kinetics of U(IV) oxidation with atmospheric oxygen in NaHCO₃ solutions was studied by spectrophotometry. In 1 M NaHCO₃ at [U(IV)]₀ = 20 mM, an induction period is observed, which virtually disappears with decreasing [U(IV)]₀ to 1.0 mM. The induction period is caused by the fact that initially U(IV) exists in a weakly active polymeric form. Addition of U(VI) to the initial solution accelerates the oxidation. In a 1 M NaHCO₃ solution containing 0.1–1.0 mM U(IV), the U(IV) loss follows the first-order rate law with respect to U(IV) and O₂. The pseudo-first-order rate constants, bimolecular rate constants, and activation energy of the U(IV) oxidation were calculated. In dilute NaHCO₃ solutions (0.5–0.01 M), the hydrolysis and polymerization of U(IV) become more pronounced. The autocatalysis mechanism presumably involves formation of a complex [U(IV) · U(VI)] with which O₂ reacts faster than with U(IV). Oxidation of U(IV) occurs by the two-electron charge-transfer mechanism.     

Selective Recovery of Chromium from Precipitates Containing d Elements and Actinides: II. Effect of H2O2

Oxidation of various Cr(III) hydroxides and mixed Cr(III)-Ni(II) and Cr(III)-Fe(III) hydroxides with hydrogen peroxide was studied. The initial reaction rate increases as the Cr(III) content in the suspension and H₂O₂ concentration are increased and nonmonotonicaly decreases with increasing NaOH concentration within the 0.2-2.0 M range. The activation energy in 0.5 M NaOH is equal to 82 kJ mol^{- 1} (30-90°C). The oxidant consumption substantially exceeds the stoichiometry.     

Corrosion of uranium and its low content Zr, Nb, and Ru alloys in aqueous solutions

Corrosion of uranium and its alloys with low content (0.5–5.0 at %) of Zr, Nb, and Ru in water and bicarbonate aqueous solutions is studied; the effect of hydrogen peroxide, the main product of radiation processes, on the corrosion rate is elucidated. The rate of the primary corrosion process U +(2 +n)H₂O=UO₂·nH₂O+ 2H₂↑ is measured by electrochemical methods in anaerobic and aerobic conditions for uranium metal and its alloys containing 0.5 to 5.0 at % of Zr, Nb, and Ru. It is shown that the corrosion rates for the alloys are lower than that of reactor-grade uranium; however, the difference is rather close to the measurement error. The corrosion mechanism is studied; U(III) is shown to be rather unstable in neutral solutions when uranium(III) hydroxide is precipitated and no significant amount of U(III) and UH₃ is present among the products of the metallic uranium corrosion in water. The kinetics of the second corrosion stage U(IV) + O₂→U(VI) is studied by spectrophotometric method. It is shown that the reaction of U(IV) oxidation by atmospheric oxygen is similar in weakly acid solutions (pH 1.5–4.0) and in bicarbonate media: in particular, it has an induction period for uranium (IV) accumulation, after which the reaction accelerates; it is formally a first-order reaction with respect to uranium. The reaction mechanisms differ in the two media: in weakly acid solutions, after the appearance of U(VI), the reproportionation reaction proceeds; thus formed U(V) interacts with O₂ faster than U(IV). In the bicarbonate medium, the acceleration of the reaction is due to the formation of a [U(IV)ΣU(VI)] complex whose reactivity is higher than that of uranium (IV). In the absence of bicarbonate, of great importance is the formation of a copolymer of U(IV) and U(VI), which at pH≥4 prevents formation of U(V). It is shown that on the introduction of hydrogen peroxide to aqueous solutions, the metallic uranium surface becomes transpassive, which increases the rate of corrosion process by at least an order of magnitude,. The introducing of oxidants and platinum mesh lowers the hydrogen accumulation at 120–150°C and, hence, the hydrogen-explosion danger of the uranium-water-corrosion-products system. Methods of deposition of metal oxide (Tc, Ru, Mo, W) films onto uranium surfaces by immersing uranium metal into Tc(VII), Ru(VI), or Mo and W heteropoly compound solutions are studied. Original Russian Text ? V.F. Peretrukhin, A.G. Maslennikov, A.Yu. Tsivadze, C.H. Delegard, A.B. Yusov, V.P. Shilov, A.A. Bessonov, K.E. German, A.M. Fedoseev, L.P. Kazanskii, N.Yu. Budanova, A.V. Kareta, A.V. Gogolev, K.N. Gedgovd, G.S. Bulato, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 3, pp. 229–251.     

Selective Recovery of Chromium from Precipitates Containing d Elements and Actinides: III. Effect of S2O82-

Reaction of Cr(III) hydroxides and mixed Fe(III)-Cr(III) and Ni(II)-Cr(III) hydroxides with persulfate ion in alkaline solutions was studied by spectrophotometry. The Cr(VI) yield (at oxidant deficiency) corresponds to the S₂O₈ ^{2 -} : Cr(VI) molar ratio close to 1.5. The initial reaction rate V ₀ is described by the kinetic equation -d[Cr(III)]/d = k[Cr(III)][S₂O₈ ^{2 -}][NaOH]. The activation energy is 53 kJ mol^{- 1} within the 41.5-95°C range. V ₀ is higher than the rate of thermal decomposition of persulfate ion, i.e., Cr(III) reacts directly with S₂O₈ ^{2 -}. Oxidation of NiCr₂O₄·nH₂O and mixed Fe(III)-Cr(III) hydroxides proceeds faster than oxidation of pure Cr(III) hydroxide. This is due to the catalytic effect of Fe(III) and Ni(II). Additions of Co(II) and Cu(II) also accelerate the process. Pu(IV) in alkali solution under the action of persulfate is converted into a more soluble oxidized species, which can be reduced back to Pu(IV) with appropriate reductants.