Catalytic reduction of Np(V) with hydrazine in nitric acid solutions in the presence of ruthenium catalysts

The reduction of Np(V) with hydrazine in HNO₃ solution in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The back oxidation of Np(IV) with nitric acid, also catalyzed by ruthenium, occurs concurrently with the Np(V) reduction with hydrazine. The contribution of each process is determined by the HNO₃ concentration. The mechanism of the ruthenium-catalyzed reduction of Np(V) with hydrazine was suggested on the basis of the kinetic data. The effect of the size of Ru nanoparticles on the activation energy of the catalytic reduction of Np(V), characterizing the structural sensitivity of the heterogeneous-catalytic reaction (positive size effect), was revealed.     

Oxidation of Am(III) to Am(IV) with ozone in solutions of heteropolyanions

Oxidation of Am(III) in the presence of K₁₀P2W₁₇O₆₁ and K₈SiW₁₁O₃₉ (L) at 20dGC was studied by spectrophotometry. Am(III) is oxidized with ozone to Am(IV) only in the pH range 3.5–1.0. In the case of formation of a complex with Am: L = 1: 1, the reaction is approximately first-order with respect to the metal, and its rate decreases with a decrease in pH from 3.5 to 1.0. Am(IV) in solution in the presence of ozone is slowly reduced with products of water α-ray radiolysis, predominantly with ?₂?₂. The rate constant of the reaction of Am(IV) with ?₂?₂ was estimated.     

Kinetics of Np(VI) reduction with carbohydrazide in nitric acid

The kinetics of the Np(VI) reduction with carbohydrazide in nitric acid solutions was studied by spectrophotometry. The reaction rate increases with increasing carbohydrazide concentration and temperature and decreases with increasing HNO₃ concentration. The reaction order with respect to Np, carbohydrazide, and HNO₃ is 1, 1.15, and–1.35, respectively. The activation energy of the reaction is 85 kJ mol^–1.     

Kinetics of Pu(VI) reduction with carbohydrazide in nitric acid

The kinetics of the Pu(VI) reduction with carbohydrazide in nitric acid solutions was studied by spectrophotometry. The reaction rate increases with increasing carbohydrazide concentration and temperature and decreases with increasing HNO₃ concentration. The reaction order with respect to Pu, carbohydrazide, and HNO₃ is 1, 2.3, and–3, respectively. The activation energy of the reaction is 111 kJ mol^–1. The final reduced Pu form is Pu(III), with Pu(V) being an intermediate.     

Influence of the origin of H2O2 on the rate of its reaction with Pu(VI) and Am(V) in acid solutions

Analysis of published data shows that H₂O₂ arising in radiolysis of acidic aqueous solutions is in the excited state and reacts with Pu(VI) and Am(V) faster than in the case when in it is introduced into the system with actinide ions from the outside.     

Extraction and sorption preconcentration of U(VI), Am(III), and Pu(IV) from nitric acid solutions with alkylenediphosphine dioxides

The extraction of U(VI), Am(III), and Pu(VI) from nitric acid solutions in the form of complexes with alkylenebis(diphenylphosphine) dioxides and their sorption with POLIORGS F-6 sorbent prepared by noncovalent immobilization of methylenebis(diphenylphosphine) dioxide (MDPPD) on a KhAD-7M? polymeric matrix were studied. The preconcentration conditions and distribution coefficients of U(VI), Am(III), and Pu(IV) in their sorption from 3 M HNO₃ were determined. The possibility of concentrating actinides from multicomponent solutions was demonstrated. The composition and nature of complexes of U(VI) with MDPPD were determined from the ³¹P NMR data.     

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.     

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic acid in aqueous solutions. Synthesis and structures of new crystalline compounds of U(VI), Np(VI), and Pu(VI)

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic (dipicolinic) acid in aqueous solutions was studied. All these actinides form with dipicolinic acid anion, PDC^2? 1: 1 and 1: 2 complexes. The PDC^2? ion coordinates to actinide(VI) ions in solutions in tridentate fashion. In 1: 2 complexes, the f-f transition bands in the electronic absorption spectra are very weak, which is associated with approximate central symmetry of the coordination polyhedron (CP) of the An atom. The apparent stability constants of Pu(VI) complexes were measured in a wide pH range, and the concentration stability constants of An(VI) (An = U, Np, Pu, Am) were determined. The crystalline complexes [Li₂AnO₂(PDC)₂]·2H₂O (An = U, Np, Pu) and [AnO₂(PDC)] _n (An = Np, Pu) were synthesized, and their structures were determined by single crystal X-ray diffraction. The X-ray data confirmed the conclusion that CP of An atoms in the complex ions AnO₂·(PDC) ₂ ^2? is centrosymmetrical. In the isostructural series of [Li₂AnO₂(PDC)₂]·2H₂O, the actinide contraction is manifested in shortening of the An-O distances in the “yl” groups in going from U to Pu.     

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.     

Mechanism of Am(III) oxidation with ozone in carbonate solutions

The reaction of ozone with Am(III) in bicarbonate and carbonate solutions was studied by spectrophotometry. On adding ozone-saturated water to a 2 × 10⁻⁴ M Am(III) solution in 1 M NaHCO₃, about 1/3 of Am remains in the trivalent state and 2/3 is converted to Am(V), with no accumulation of Am(IV). The reaction of Am(III) with ozone involves replacement of H₂O molecules in the coordination sphere of Am(III) by O₃ molecule, followed by elimination of the O₂ molecule. The third O atom remains bonded with Am, converting it to the pentavalent state.     

Catalytic reduction of Np(VI) with formic acid in the presence of platinum nanoparticles

The kinetics of catalytic reduction of Np(VI) with formic acid in the presence of Pt nanoparticles of different types (“brown” colloid stabilized with sodium polyacrylate and nonstabilized “gray” colloid) was studied. In both cases in the examined range of conditions ([Np(VI)]₀ = 2.80 × 10^?4?9.03 × 10^?4 M; [HCOOH] = 0.03–1.0 M; [Pt] = 4 × 10^?7 ?2 × 10^?5 M; T = 18–60°C) the reaction is zero-order with respect to [Np(VI)] and first-order with respect to [HCOOH]. The catalytic activity of the nonstabilized “gray” colloid exceeds by almost an order of magnitude that of the “brown” colloid, due to the blocking effect of stabilizing polyelectrolyte molecules on the active catalytic centers. The dependence of the reaction rate on the sodium polyacrylate concentration in the range 1 × 10^?4?1 × 10^?2 M is nonmonotonic, due to deflocculation of the nanoparticles. The mechanism of the catalytic reduction of Np(VI) with formic acid in the presence of Pt colloids is discussed; it involves a slow step of dissociative chemisorption of HCOOH molecules on the nanoparticle surface.     

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.     

Photoreduction of chromium(VI) in the presence of algae, Chlorella vulgaris

In this thesis, the photochemical reduction of hexavalent chromium Cr(VI) in the presence of algae, Chlorella vulgaris, was investigated under the irradiation of metal halide lamps (lambda = 365 nm, 250 W). The affecting factors of photochemical reduction were studied in detail, such as exposure time, initial Cr(VI) concentration, initial algae concentration and pH. The rate of Cr(VI) photochemical reduction increased with algae concentration increasing, exposure time increasing, initial Cr(VI) concentration decreasing and the decrease of pH. When pH increased to 6, the rate of Cr(VI) photochemical reduction nearly vanished. When initial Cr(VI) concentration ranged from 0.4 to 1.0 mg L(-1) and initial algae concentration ranged from ABS(algae) (the absorbency of algae) = 0.025 to ABS(algae) = 0.180, According to the results of kinetic analyses, the kinetic equation of Cr(VI) photochemical reduction in aqueous solution with algae under 250 W metal halide lamps was V0 = kC(0)(0.1718)A(algae)(0.5235) (C0 was initial concentration of Cr(VI); A(algae) was initial concentration of algae) under the condition of pH 4.     

Kinetics of neptunium(V) reduction with iron(II) sulfamate in nitric acid solutions

The kinetics of chemical reduction of Np(V) with iron(II) sulfamate in HNO₃ solutions was studied by spectrophotometry. The reduction process can be described by the equation NpO ₂ ⁺ + Fe²⁺ + 4H⁺ ? Np⁴⁺ + Fe³⁺ + 2H₂O and follows the first- and zero-order rate law with respect to the Np(V) concentration. The reaction rate constants were determined in relation to the HNO₃ concentration (1–3 M), iron(II) sulfamate concentration [(0.59–2.94) × 10^?3 M], and temperature (298.2–319.2 K). The activation energy (E _a) and the thermodynamic functions of formation of the activated complex (activation free energy ΔG ^≠, enthalpy ΔH ^≠, entropy ΔS ^≠) were calculated.     

Extraction of REEs(III), U(VI), and Th(IV) from nitric acid solutions with carbamoylmethylphosphine oxides in the presence of an ionic liquid

The extraction of REEs(III), U(VI), and Th(IV) from nitric acid solutions with solutions of diphenyl(dialkylcarbamoylmethyl)phosphine oxides is considerably enhanced in the presence of an ionic liquid, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide. The extraction efficiency varies in a wide range depending on substituents at the P and N atoms and on the structure of the fragment linking the coordinating groups in the extractant molecule.     

Mechanism of Np(VI) oxidation with ozone in alkaline solutions

Bubbling of an ozone-oxygen mixture containing 0.1?C0.5 vol % O₃ at a rate of 15?C20 l h^?1 through 13 ml of a 2 × 10^?5?1 × 10^?4 M solution of Np(VI) in 0.1 and 1 M LiOH leads to the formation of Np(VII). The initial rate increases approximately in proportion to [Np(VI)] and [O ₃ ^gas ]^0.5. Up to 80% of Np(VI) is oxidized at maximum. At the O₃ concentration in the gas phase increased to 1?C4 vol %, Np(VI) is oxidized completely. Under the same conditions, Np(VI) in a concentration of (1?C5) × 10^?3 M is oxidized to almost 100%. Analysis of published data and additional experiments on the reaction of O₃ with Np(VI) ions in LiOH solutions allow a conclusion that the ozonation involves the reactions O₃ + OH^? = HO ₂ ^? + O₂, O₃ + HO ₂ ^? + OH^? = O ₃ ^? + O ₂ ^? + H₂O, and O₃ + O ₂ ^? = O ₃ ^? + O₂, followed by O ₃ ^? + NpO₂(OH) ₄ ^2? = O₂ + NpO₄(OH) ₂ ^3? + H₂O. In addition, HO ₂ ^? reduces Np(VII) and Np(VI) and reacts with O ₃ ^? . Certain contribution is made by the reaction Np(VI) + O₃ = Np(VII) + O ₃ ^? . The dependence of the Np(VII) accumulation rate on [O ₃ ^gas ]^0.5 was interpreted in terms of the concept of a heterogeneous-catalytic process.     

Sorption recovery of U(VI), Pu(IV), and Am(III) from nitric acid solutions with solid-phase extractants based on Taunit carbon nanotubes and polystyrene supports

Sorption procedures were developed for recovering U(VI), Pu(IV), and Am(III) with solid-phase extractants (SPEs) prepared by impregnation of Taunit carbon nanotubes and polystyrene supports with diphenyl( dibutylcarbamoylmethyl)phosphine oxide (CMPO) and tri-n-octylphosphine oxide (TOPO) The impregnation and actinide recovery were performed in the batch mode and using microcolumns. Procedures for support impregnation and SPE preparation are described. Conditions were found for sorption recovery of U(VI), Pu(IV), and Am(III) from 3 M HNO₃ solutions. The possibility of actinide desorption was demonstrated. The effect of macrocomponents on the degree of actinide recovery was examined.     

Electrochemical Oxidation of Am(V) Ions in HNO3 Solutions

Oxidation potentials (E ₀p) of the Am(VI)/Am(V) couple were measured and the kinetics of electrochemical oxidation of Am(V) on platinum electrode in concentrated solutions of nitric acid (1-7 M) containing potassium phosphotungstate K_{1 0}P₂W_{1 7}O_{6 1} (KPW) was studied by the potentiometric method with spectrophotometric control of the oxidation states. The potential E ₀p is independent of the concentrations of HNO₃ and KPW and is shifted toward the negative region by 70 mV (E ₀p) as compared to 1 M HClO₄. The extent of Am(V) oxidation into Am(VI) under the experimental conditions studied remains almost constant and comprises 90%. Electrochemical oxidation of Am(V) is described by the kinetic equation of the first and zero orders with respect to the Am(V) concentration: -dC _{A m ( V )}/dt = k'₁ C _{A m ( V )} - k'₀.     

Extraction of U(VI), Th(IV), Pu(IV), Am(III), and rare-earth elements from nitric acid solutions with diphenyl(dialkylcarbamoylmethyl)phosphine oxides substituted in the methylene bridge

The extractive power and selectivity of diphenyl(dialkylcarbamoylmethyl)phosphine oxides in extraction of U(VI), Th(IV), Pu(IV), Am(III), and rare-earth elements from nitric acid solutions was studied, as influenced by substitution of one or two hydrogen atoms in the methylene bridge with alkyl, cycloalkyl, CH₂P(O)Ph₂, and CH₂C(O)NBu₂ groups.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 427–432.Original Russian Text Copyright © 2004 by Turanov, Karandashev, Yarkevich, Safronova, Kharitonov, Radygina, Fedoseev.     

Reduction of Np(V) with formic acid in acidic solutions in the presence of palladium catalysts

The kinetics of catalytic reduction of Np(V) with formic acid in HClO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the Pd content and size of Pd nanocrystals was studied. The reaction is a structure-insenitive catalytic process, i.e., the size effect is absent. An increase in the percentage of Pd on SiO₂ leads to a decrease in the activity of the catalysis centers due to a considerable increase in the contribution of the side reaction catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The effect of the S:L ratio, concentrations of HCOOH and HClO₄, and temperature on the rate of catalytic reduction of Np(V) in the presence of palladium catalysts was examined. The suggested mechanism of the catalytic reduction of Np(V) with formic acid in the presence of Pd/SiO₂ involves a slow step of decomposition of the protonated species NpO₂H, formed by the reaction of the NpO ₂ ⁺ ion with chemisorbed hydrogen atoms Pd(H).