Selective extraction of uranium from hydrochloric acid solutions with macrocyclic endoreceptors

Extraction of uranyl ion from aqueous hydrochloric acid solutions with solutions of cis,syn,cis-dicyclohexyl-18-crown-6 ether (DCH18C6-A) in organic solvents was studied. The distribution ratio D of UO ₂ ²⁺ ions in these systems depends on the HCl concentration in the initial aqueous solutions and is maximal in 6–8 M HCl. The ratio D grows with an increase in the solvent polarity, reaching 1000 with a mixture of 1,2-dichloroethane and nitrobenzene (DCH18C6-A concentration 0.01 M). The extractable complex contains two DCH18C6-A molecules and two H₃O⁺ ions per UO ₂ ²⁺ ion. The uranyl ion is quantitatively backwashed from the organic phase with an equal volume of deionized water. Nitrate and sulfate ions decrease D in extraction from HCl solutions.     

Sorption of actinide ions onto mesoporous phosphorus-containing silicas

Equilibrium and kinetic characteristics of template mesoporous silicas containing phosphonic acid residues in sorption of various actinide ions were studied. The sorption equilibrium involving these sorbents is attained within 20 min after introducing the sorbent into the solution. The calculated values of the internal diffusion coefficient \((\bar D)\) and half-exchange time (τ_0.5) in sorption of uranium were ～3.5 × 10^?16 m² s^?1 and ～390 s, respectively. Mesoporous phosphorus-containing silicas efficiently sorb from acid solutions uranyl ions, Th(IV), and Pu(IV). In sorption of uranium from sulfuric acid solutions, the capacity of the sorbents is 125–132 mg g^?1, and in sorption from nitric acid solutions (0.5–3.0 M HNO₃), 276–299 mg g^?1. In sorption of Th(IV) from nitric acid solutions, the capacity of the sorbents is 60–66 mg g^?1. In sorption of microamounts of ²³⁹Pu(IV), the distribution coefficient reaches 4500 cm³ g^?1. Phosphorus-containing silicas in nitric acid solutions do not noticeably sorb ²⁴¹Am, which allows using them for efficient separation of the Pu/Am pair with the separation factor of no less than 2 × 10₃.     

Immobilization of uranyl ions from dilute solutions by contact with ultrabasic and basic rocks

Immobilization of uranyl ions from 0.01 M NaCl and NaHCO₃ solutions with Fe(II)-rich samples of ultrabasic (dunite) and basic (olivine gabbro, gabbronorite) rocks was studied. From NaCl solutions, uranium is removed to approximately equal extent with all the samples, whereas with NaHCO₃ the immobilization efficiency differs by a factor of several units, being directly dependent on the FeO content. Uptake of large amounts of U (0.13–0.16 μmol g^?1 from solutions initially containing 9 μM UO ₂ ²⁺ ) from NaHCO₃ solutions, even at high pH values (9.0), make dunite and olivine gabbro promising as materials for making artificial geochemical barriers for dissolved uranium.     

Reduction of U(VI) in POCl<Subscript>3</Subscript>-lewis acid solutions

Features of the behavior of uranyl ions in POC1₃-MCl _x -²³⁵UO ₂₊ ² solutions (M = Ti, Si, Zr, Sn, Sb) were considered. Irreversible accumulation of U(IV) in the course of synthesis of POCl₃-SnCl₄-²³⁵UO ₂₊ ² solutions prepared from water-containing U(VI) compounds, excluding UO₂(ClO₄)₂ · 5H₂O, was found. The reaction rate increases with increasing uranyl concentration, k _l[U(IV)] ～ (1.6±0.2) × 10^?6 s^?1 (T = 380 K). The U(IV) accumulation was also observed on heating (T = 360–380 K) POCl₃-SnCl₄-²³⁵UO ₂₊ ² and POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions hermetically sealed in glass cells and on irradiating them by the light of a xenon lamp. In POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions prepared from UO₂(C1O₄)₂ · 5H₂O, U(IV) disappears within several days after stopping the irradiation. The reduction of U(VI) is caused by formation of uranyl dichlorophosphate complexes and by deactivation of uranyl excitation with chlorine-containing agents.     

Corrosion and impedance studies on magnesium alloy in oxalate solution

Corrosion behavior of AZ91E alloy was investigated in oxalate solution using potentiodynamic polarization and electrochemical impedance measurements (EIS). The effect of oxalate concentration was studied, where the corrosion rate increases with increasing oxalate concentration. The effect of added ions (Br⁻, Cl⁻ or SiO₃²⁻) on the electrochemical behavior of magnesium alloy in 0.1 M Na₂C₂O₄ solution at 298 K, was investigated. It was found that the corrosion rate of 0.1 M oxalate solution containing silicate ion is lower than the blank (0.1 M Na₂C₂O₄). This was confirmed by scanning electron microscope (SEM) observations. However, for the other added ions Br⁻ or Cl⁻, the corrosion rate is higher than the blank.     

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.     

Effect of complexation on crystallization of plutonium(IV) oxalate in the system Pu(NO<Subscript>3</Subscript>)<Subscript>4</Subscript>-HNO<Subscript>3</Subscript>-H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The published data on complexation in the system Pu(NO₃)₄-HNO₃-H₂C₂O₄ were treated on the basis of a unified approach to determination of the oxalate ion concentration. Because of discrepancies between results published by different researchers, additional experiments on crystallization of Pu(IV) oxalate were carried out at widely varied excess and deficiency of oxalic acid. These experiments confirmed high stability of the complex cations PuC₂O ₄ ²⁺ . The upper boundary of the field of metastable supersaturated solutions of Pu oxalate at the initial Pu concentration of 15–50 g l^?1 was determined.     

Catalytic decomposition of organic anions in alkaline radioactive waste: III. Oxidation of oxalate and glycolate

Decomposition of oxalate and glycolate ions in alkaline solutions under the action of O₃, H₂O₂, and Na₂S₂O₈ was studied spectrophotometrically and titrimetrically. At 20°C, ozone slowly decomposes oxalate in 0.05 M NaOH. In 1 M NaOH, heating at 90°C is required to oxidize oxalate with ozone. Glycolate is readily oxidized with ozone at 20°C in 0.05–1 M NaOH, predominantly into oxalate. Hydrogen peroxide is ineffective reagent for oxalate and glycolate decomposition. Persulfate oxidizes oxalate ion in 0.5–5 M NaOH at 90°C. The reaction of persulfate with glycolate proceeds at 50°C and higher temperatures and is characterized by an induction period, which shortens with increase in concentration of S₂O ₈ ^2? , OH^?, and temperature, or in the presence of AgNO₃ and K₄Fe(CN)₆. Oxidation involves thermal dissociation of persulfate ions into radical ions followed by a chain reaction.     

Structure and hydrolytic durability of a glass containing waste from spent tributyl phosphate reprocessing

The structure of the glass obtained by incorporation of the residue from reprocessing of the spent extractant, tributyl phosphate, into sodium aluminophosphate glass and the structural state of uranium in this glass were studied by vibration (IR, Raman) and X-ray absorption (XAFS) spectroscopy. An IR and Raman spectroscopic study shows that the structural network of the glasses is formed by ortho- and pyrophosphate groups linked by tetrahedral AlO₄ units. As follows from the XAFS data, uranium is present in the glass in the form of uranyl ions and of separate UO_2+x particles. The glass has high hydrolytic durability: The uranium leach rate determined in accordance with GOST (State Standard) R 52 126–2003 is of the order of 10^–8 g cm^–2 day^–1.     

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).     

Preparation of uranium oxides in nitric acid solutions by the reaction of uranyl nitrate with hydrazine hydrate

UO₂·nH₂O formed by thermal denitration of uranyl nitrate in solutions under the action of hydrazine hydrate can be converted in air to UO₃ at 440°C and to U₃O₈ at 570–800°C, and also to UO₂ in an inert or reducing atmosphere at 280–800°C. After the precipitation of hydrated uranium dioxide, evaporation of the mother liquor at 90°C in an air stream allows not only evaporation of water, but also complete breakdown and removal of hydrazine hydrate and NH₄NO₃. The use of microwave radiation considerably reduces the time required for complete thermal denitration of uranyl nitrate in aqueous solution to uranium dioxide, compared to common convective heating.     

Sol–gel synthesis of 2D and 3D nanostructured YSZ:Yb<Superscript>3+</Superscript> ceramics

This paper presents results of a detailed study of fundamental aspects of the formation of 2D and 3D nanostructured YSZ:Yb³⁺ ceramics with a cubic structure through a key synthesis step in aqueous solutions of zirconium-containing hydroxy nanoparticles (1–2 nm) modified by Y³⁺ and Yb³⁺ ions, with the use of a sol–gel method and subsequent calcination of the resultant xerogels at temperatures above 350°C. As starting chemicals for the synthesis of ceramic powders, we used zirconyl, yttrium, and ytterbium nitrates and chlorides and aqueous ammonia. Using mixed solutions of these salts and a procedure developed by us, we synthesized sols, gels, and xerogels. To examine the effect of temperature on solid-state transformations, the xerogels were calcined according to a predetermined program in a muffle furnace at temperatures in the range from 350 to 1350°C (rarely, up to 1650°C). We focused primarily on ceramic powders close in composition to 0.86ZrO₂ · 0.10Y₂O₃ · 0.04Yb₂O₃. The ceramics were characterized by high-resolution transmission electron microscopy, electron microdiffraction, electronic diffuse reflectance spectroscopy, energy dispersive X-ray microanalysis, and X-ray fluorescence analysis.     

Extraction of uranyl,La(III), and Y(III) nitrates with a composite solid extractant based on a polymeric support impregnated with trialkylamine

Extraction of uranyl, La(III), and Y(III) nitrates from aqueous solutions containing 0–4 M sodium nitrate with a composite solid extractant based on a polymeric support impregnated with trialkylamine (C₇-C₉) was studied. The extraction isotherms were analyzed assuming that La(III), Y(III), and uranyl nitrates are extracted with the solid extractant in the form of complexes (R₃NH)₃[Ln(NO₃)₆] and (R₃NH)₂[UO₂(NO₃)₄], respectively. The extraction constants were calculated. The joint extraction of uranyl and La(III) [Y(III)] nitrates with the solid extractant from aqueous salt solutions was studied. The composite solid extractant based on a polymeric support impregnated with trialkylamine can be used for purification of aqueous solutions of rare-earth metal nitrates to remove uranium impurities.     

Synthesis and structure of the complex of uranyl citraconate with urea

Crystals of [UO₂(C₅H₄O₄)(urea)₂], where C₅H₄O₄^2– is citraconate ion, were prepared and studied by IR spectroscopy and single crystal X-ray diffraction. The structure of the crystals is formed by chains having the same composition as the compound as a whole and belonging to the crystal-chemical group AТ¹¹M₂¹ (A = UO₂²⁺, Т¹¹ = C₅H₄O₄^2–, M¹ = urea) of uranyl complexes. Comparative analysis of nonvalent interactions in the structures of uranyl complexes with citraconate ions was performed by the method of molecular Voronoi–Dirichlet polyhedra.     

Reduction of U(VI) in laser liquids POCl3-SnCl4-235UO 2 2+ -Nd3+

U(IV) is irreversibly accumulated during synthesis of laser liquids POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ prepared from various initial Nd(III) and U(VI) compounds, irrespective of the way of their introduction. The rate of U(IV) accumulation in POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions increases with increasing UO ₂ ²⁺ and Nd³⁺ concentrations; for laser liquids with the Nd³⁺ luminescence lifetime τ > 150 μs the observed rate constant of U(IV) accumulation by the second-order reaction k ₂[U⁴⁺] is equal to (3 ± 1) × 10^?5 1 mol^?1 s^?1 at T = 380 K. U(IV) is accumulated during storage of POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions in hermetically sealed glass cells at room temperature and upon irradiation of solutions by xenon lamp light in the spectral region of UO ₂ ²⁺ absorption. The U(VI) reduction proceeds by chemical and photochemical activation of uranyl with formation of stable U⁴⁺ complexes with dichlorophosphate ions and also with Nd³⁺. Deactivation of the uranyl ion excitation with proton-and chlorine-containing impurities prevents U(VI) reduction.     

Extraction of uranyl, La(III), and Y(III) nitrates with a composite solid extractant based on a polymeric support impregnated with trialkylmethylammonium nitrate

Extraction of uranyl, La(III), and Y(III) nitrates from aqueous solutions containing 0–4 M sodium nitrate with a composite solid extractant based on a polymeric support impregnated with trialkylmethylammonium nitrate (Aliquat-336) was studied. The extraction isotherms were analyzed assuming that uranyl, La(III), and Y(III) nitrates are extracted with the solid extractant in the form of complexes (R₄N)₂[Ln(NO₃)₅] and (R₄N)₂[UO₂(NO₃)₄], respectively. The calculated extraction constants decrease in the order La³⁺ > UO ₂ ²⁺ > Y³⁺. The joint extraction of uranyl, La(III), and Y(III) nitrates with the solid extractant from aqueous salt solutions was studied. The La/U separation factor increases with increasing total concentration of uranyl and La(III) nitrates in aqueous phase. Original Russian Text A.K. Pyartman, V.A. Keskinov, V.V. Lishchuk, A.V. Konstantinova, V.V. Belova. 2007, published in Radiokhimiya, 2007, Vol. 49, No. 3, pp. 237–240.     

Extraction of REE(III), U(VI), and Th(IV) from Nitric Acid Solutions with Carbamoylmethylphosphine Oxides in the Presence of Bis[(trifluoromethyl)sulfonyl]imide Ions

Extraction of microamounts of REE(III), U(VI), and Th(IV) with solutions of carbamoylmethylphosphine oxides (CMPOs) in organic diluents from aqueous HNO₃ solutions containing lithium bis[(trifluoromethyl) sulfonyl]imide (LiTf₂N) was studied. The efficiency of the REE(III), U(VI), and Th(IV) extraction from nitric acid solutions with CMPO solutions considerably increases in the presence of Tf₂N^– ions in the aqueous phase. The stoichiometry of the extractable complexes was determined, and the influence of the structure of the CMPO molecule, kind of organic diluent, and aqueous phase composition on the efficiency of the U(VI), Th(IV), and REE(III) extraction into the organic phase was considered.     

Formation of Excited Uranyl Ion in Oxidation of U(IV) with Xenon Trioxide in Aqueous Solutions of H2SO4 and HClO4: II. Influence of H2O2 on the Reaction Kinetics

The kinetics and mechanism of chemiluminescent oxidation of U(IV) with xenon trioxide [monoexponential decay of chemiluminescence (CL)] dramatically changes in the presence of H₂O₂. First the CL intensity slightly decreases (induction period), then increases by a factor of several units (autocatalytic oxidation), and finally sharply decreases by several orders of magnitude after reaching a maximum. Radical chain mechanism of the reaction was proposed. The elementary step of formation of CL emitter, electronically excited (UO₂ ^{2 +})*, is electron transfer from the uranyl(V) ion (UO₂ ⁺) to the oxidizing agent (OH radical). In the presence of H₂O₂ in the solution, OH radicals are formed non only by reaction of U(IV) with XeO₃ but also by reaction of H₂O₂ with XeO₃, which complicates the CL kinetics.     

Kinetics of dissolution of stoichiometric uraninite

The kinetics of dissolution of stoichiometric uraninite (UO₂) synthesized by electroreduction (electrolysis) of uranyl chloride in a melt of an eutectic mixture of alkali metal salts was studied. It follows from the results obtained that, in the initial step of uraninite dissolution under both dynamic and static conditions, schoepite is not an intermediate phase. It is formed, apparently, in the further steps by oxidation of uranium dioxide. The diffusion coefficients of uranium in solutions over uraninite and schoepite were determined. It was found that, at least in the initial step of the dissolution, the forming uranyl hydroxide occurs in the solution either in the dissociated state or in the form of mononuclear hydroxo complexes.     

Mechanism of cerium(III) oxidation with ozone in sulfuric acid solutions

The kinetics of Ce(III) oxidation with ozone in 0.1–3.2 M H₂SO₄ solutions was studied by spectrophotometry. The reaction follows the first-order rate law with respect to each reactant. The rate constant k slightly increases with an increase in the acid concentration, which is associated with an increase in the O₃/O ₃ ^? oxidation potential. The activation energy in the range 17–35°C is 46 kJ mol^?1. With excess Ce(III), the stoichiometric coefficient Δ[Ce(IV)]/Δ[O₃] increases from 1.6 to 2 in going from 0.1 to 1–3.2 M H₂SO₄. The extent of the Ce(III) oxidation is 78% in 0.1 M H₂SO₄ and reaches 82% in 1 M H₂SO₄. The ozonation involves the reactions Ce(III) + O₃ → Ce(IV) + O ₃ ^? , O ₃ ^? + H⁺ → HO₃, HO₃ → OH + O₂, OH + HSO ₄ ^? → H₂O + SO ₄ ^? , OH + Ce(III) → OH^? + Ce(IV), and SO ₄ ^? + Ce(III) → SO_4/2? + Ce(IV). Low stoichiometric coefficient of the Ce(III) oxidation is associated with the hydrolysis of Ce(IV). The excited Ce(IV) ion arising from oxidation of Ce(III) with OH radical forms with the hydrolyzed Ce(IV) ion a dimer whose decomposition yields Ce(III) and H₂O₂. After the ozonation termination, Ce(IV) is relatively stable in sulfuric acid solution, with only 5–7% of Ce(IV) disappearing in 24 h.