Causes of uranyl ion nonlinearity in crystal structures

The stereochemical features of U(VI) in the structure of crystals containing UO _a or UO _b N _c coordination polyhedra (a or b + c = 5, 6, 7, 8, or 9) were examined using Voronoi-Dirichlet polyhedra (VDPs). The volume of VDP of U atoms in UO _a complexes is independent of the coordination number of U atoms. An increase in the number of N atoms in UO _b N _c complexes is accompanied by a regular increase in the volume of VDP of the U atom. The nonlinearity of uranyl ions in the crystal structures correlates with the displacement of the nuclei of the U(VI) atoms from the center of gravity of their VDP.     

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.     

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.     

Physicochemical principles of preparation of U(VI) carbonate solutions for extraction reprocessing in the Carbex process

Physicochemical principles of preparation of U(VI) carbonate solutions in the step of oxidative dissolution of U₃O₈ and UO₂ in the Carbex process are considered. Carbonate solutions with the U(VI) concentration higher than 100 g L^–1, suitable for subsequent final purification of uranium by extraction, can be prepared under the conditions of formation of U(VI) carbonate–peroxide complexes in the course of dissolution with prevention of hydrolysis of U(VI) compounds. The behavior of impurities simulating some fission products in the course of oxidative dissolution was studied, and the decontamination factors of U(VI) from the chosen simulated fission products were determined.     

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.     

Synthesis and properties of complexes of hexavalent actinides with dimethyl sulfoxide [AnO<Subscript>2</Subscript>(DMSO)<Subscript>5</Subscript>](ClO<Subscript>4</Subscript>)<Subscript>2</Subscript> (An = U,Np, Pu)

The complexes [NpO₂(DMSO)₅](ClO₄)₂ (1) and [PuO₂(DMSO)₅](ClO₄)₂ (2), isostructural to the known uranyl complex, were synthesized in the form of single crystals. Their crystallographic characteristics were determined by single crystal X-ray diffraction. The IR and electronic absorption spectra of the crystalline U(VI), Np(VI), and Pu(VI) complexes were measured and analyzed.     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Solubility of UO2HPO4 in seawater

The solubility of UO₂HPO₄ in normal seawater of 35‰ salinity was studied. The solubility of U(VI) phosphates in seawater increases nonlinearly with decreasing acidity (by two orders of magnitude per pH unit) due to formation of strong hydroxo, carbonate and, probably, phosphate complexes. At pH > 7.5, UO₂HPO₄ dissolves congruently. With decreasing pH to 6.9–7.2, UO₂HPO₄ is converted to UO₂(H₂PO₄)₂. The solubility of U(VI) phosphates in seawater considerably exceeds the natural uranium concentrations in seawater and pore solutions of the bottom sediments. Therefore, these compounds cannot form intrinsic minerals in the oceanic bottom sediments. The possibility of accumulation of U(VI) in sea phosphorites due to its chemisorption in the form of surface uranyl phosphate complexes is discussed.     

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.     

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.     

Reaction of ozone with uranium(IV) oxalate in water

Decomposition of aqueous suspensions of uranium(IV) oxalate under the action of an ozone–oxygen mixture was studied. The process occurs in two steps. In the first step, the U(IV) oxidation with the formation of oxalic acid uranyl solutions prevails. The second step involves decomposition of oxalate ions and hydrolysis of uranyl ions. An increase in temperature accelerates the transformation of uranium(IV) oxalate into uranium(VI) hydroxide compounds. In solutions containing KBr or UO₂Br₂, the following reaction occurs: O₃ + Br^– → O₂ + BrO^–. The arising hypobromite ions and hypobromous acid oxidize uranium(IV) oxalate extremely efficiently. The possible mechanism of ozonation of aqueous uranium(IV) oxalate suspensions is discussed.     

Complexation of hexavalent U,Np, and Pu with cyclopropanecarboxylate anions. Crystal structure of the uranyl complexes {[UO2(bipy)(cpc)]2O2} and [UO2(cpc)2(H2O)2] (bipy = 2,2′-Bipyridine,cpc− = cyclopropanecarboxylate anion)

The complexation of hexavalent U, Np, and Pu with cyclopropanecarboxylate anions, cpc^?, in aqueous solutions was studied. The stepwise concentration stability constants of the complexes PuO₂(cpc) _i ^2?i (i = 1, 2, 3) are as follows: logK _1,2,3 = 2.63 ± 0.20, 1.61 ± 0.20, 1.43 ± 0.20, respectively; the overall concentration stability constant of the complex PuO₂(cpc) ₃ ^? is logβ₃ = 5.67 ± 0.60. The complexing properties of the cpc^? anion are very close to those of butyrate and isobutyrate anions. Two crystalline uranyl compounds were synthesized: {[UO₂(bipy)(cpc)]₂O₂} (bipy = 2,2′-bipyridine) and [UO₂(cpc)₂(H₂O)₂]. The specific feature of the first complex is that it contains peroxide ion. Its appearance may be due to the formation of the cationic moiety via hydrolytic uranyl dimer. The second compound forms a 3D structure, with the complexes linked via hydrogen bonds.     

Complexation in Binary Systems Uranyl(VI)-8-Quinolinethiol in Gelatin-immobilized (UO2)2[Fe(CN)6] Matrices

Complexation processes occurring in gelatin-immobilized uranyl(VI) hexacyanoferrate(II) matrixsystems on contact with aqueous alkaline (pH 12.0) solutions of 8-quinolinethiol, its 5-chloro, 5-bromo' and 5-methylthio derivatives were studied. Incorporation of the ligand into the inner coordination sphere ofUO₂ ^{2 +} is preceded by alkali transformation of gelatin-immobilized (UO₂)₂[Fe(CN)₆] to uranic acid (H₂UO₄). In the course of complexation in each of the uranyl(VI)-ligand systems studied, only coordination compounds UO₂L₂ (L^- is the deprotonated form of the ligand) are formed.     

Disproportionation of Pu(V) in the CH<Subscript>3</Subscript>COOH-H<Subscript>2</Subscript>O system

The behavior of Pu(VI) and Pu(V) in CH₃COOH (HAc)-H₂O solutions was studied by spectrophotometry. The absorption spectrum of Pu(VI) does not change on adding HAc to a concentration of 5 M in the presence of 0.5–1.0 M HClO₄, but in solutions containing less than 0.001 M mineral acid, changes in the spectrum are observed at HAc concentration of 0.6 M.he major absorption band of PuO ₂ ²⁺ ions, caused by an f-f transition, with increasing [HAc] is shifted from 830.6 to 836 nm, with a simultaneous decrease in the absorption intensity, which is due to formation of 1: 1 complexes of Pu(VI) with Ac^? ions. In anhydrous HAc, the peak intensity increases again, owing to total change in the composition of the solvation shell. Pu(V) is unstable in 1–17 M HAc solutions and disproportionates to form Pu(VI) and Pu(IV). The Pu(V) loss follows a second-order rate law with respect to [Pu(V)] and accelerates with increasing HAc concentration. The reaction products exert opposite effects on the reaction rate: Pu(IV) accelerates the consumption of Pu(V), whereas Pu(VI) does not affect the process in dilute HAc solutions but decelerates the disproportionation in concentrated solutions owing to formation of a cation-cation complex with Pu(V).     

Synthesis and X-ray diffraction study of uranyl malonate monohydrate

Uranyl malonate monohydrate [UO₂(mal)(H₂O)], where mal^2– is malonate ion, was synthesized and studied by single crystal X-ray diffraction. The main structural units are electrically neutral [UO₂(mal)(H₂O)] layers belonging to crystal-chemical group AQ²¹М¹ (A = UO ₂ ²⁺ , Q²¹ = mal^2–, M¹ = H₂O) of uranyl complexes. Specific features of the packing of the uranium-containing complexes are discussed on the basis of Voronoi–Dirichlet tessellation. The structural differences between the monohydrate synthesized and the known uranyl malonate trihydrate are considered.     

Complex of uranyl citraconate with dimethylacetamide: Synthesis and structure

The complex [UO₂(C₅H₄O₄)(Dmа)] (I), where C₅H₄O₄ ^2– is citraconate ion and Dmа is dimethylacetamide, was synthesized and studied by single crystal X-ray diffraction analysis. The complex has layered structure and belongs to crystal-chemical group AQ²¹M¹ (A = UO₂ ²⁺, Q²¹ = C₅H₄O₄ ^2–, M¹ = Dmа) of uranyl complexes. Structural features of all the known uranyl complexes with citraconate anions are considered. Nonvalent interactions in the crystal structure of I are characterized using the method of molecular Voronoi–Dirichlet polyhedra. The IR spectrum of I is analyzed.     

Sorption of U(VI) from aqueous solutions with carbon sorbents

The sorption characteristics of ordinary and oxidized sorts of synthetic (SKN) and kernel (KAU) carbons and also carbon fabric oxidized with HNO₃ (AUTo) with respect to U(VI) were studied. The influence of solution pH on the sorption capacity of carbon materials with respect to uranium was elucidated. The influence of chlorine and sulfate anions on the sorption rate and sorption capacity was studied. Based on kinetic curves and sorption isotherms of uranyl ions and their derivatives, possible mechanisms of uranium adsorption with carbon sorbents were considered. It was shown that carbon sorbents can be used for treatment of aqueous media, among them drinking water, to remove U(VI) compounds.     

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.     

Sorption of U(VI) onto layered double hydroxides and oxides of Mg and Al,prepared using microwave radiation

Layered double hydroxides of Mg and Al, containing CО₃^2– ions in the interlayer space (LDH-Mg-Al-CО₃), and layered double oxides of Mg and Al (LDO-Mg-Al) were prepared using microwave radiation (MWR). The use of MWR allows not only acceleration of the synthesis of both LDH and LDO, but also preparation of compounds with high kinetic characteristics of the U(VI) sorption. The degree of U(VI) sorption (α) from 10^–2 M aqueous U(VI) solutions at a sorption time of 4 h and V/m = 50 mL g^–1 exceeds 99.0%. In sorption from more concentrated (10^–1 M) aqueous U(VI) solutions under similar conditions, α on all the samples does not exceed 37.5%.     

Role of U(VI) complexes in the chemiluminescent reaction U(IV) + O3 in H2SO4 solution

Data on the effect of U(VI) on the reaction U(IV) + O₃ in H₂SO₄ solution are analyzed. The chemiluminescence enhancement is caused by the formation of a complex of an excited U(VI) ion with an unexcited U(VI) ion, so-called excimer. The decomposition of the excimer to two U(V) ions and H₂O₂ is followed by the reaction of U(V) with ozone, giving rise to the excited U(VI) ion. Thus, a chain reaction develops.