Mechanism of UO<Subscript>2</Subscript>(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O decomposition under the action of microwave radiation: Part 2

Products of UO₂(NO₃)₂·6H₂O decomposition under the action of microwave radiation (MWR) were studied by thermal gravimetric analysis, X-ray phase analysis, IR spectroscopy, and electron microscopy. The results of physicochemical studies of these decomposition products were compared to the published data for various uranium compounds, including UO₂(NO₃)₂·6H₂O. Apart from gaseous products, the final products of decomposition of 2–10 g of UO₂(NO₃)₂·6H₂O under the action of MWR for 35 min (the maximal process temperature, 170–320°C, is attained in the first 2–5 min of irradiation) are uranyl hydroxonitrate UO₂(OH)NO₃ and uranium trioxide UO₃ or their hydrates. The results obtained are consistent with the mechanism suggested in our previous paper and involving the reactions (1) UO₂(NO₃)₂·6H₂O → UO₂(OH)NO₃ + 5H₂O + HNO₃ and (2) UO₂(OH)NO₃ → UO₃ + HNO_3. The physicochemical study confirms the conclusions on the composition of products of UO₂(NO₃)₂·6H₂O decomposition under the action of MWR, made previously on the basis of chemical studies. The only precursor of UO₃ in microwave treatment of UO₂(NO₃)₂·6H₂O is UO₂(OH)NO₃ (or its hydrates). This is the main difference between the courses of uranyl nitrate decomposition under the conditions of microwave and convection heating. In the latter case, uranyl nitrate and its hydrates also participate in the formation of UO₃.     

Mechanism of UO<Subscript>2</Subscript>(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O decomposition under the action of microwave radiation

The effect of microwave radiation (MWR) on the decomposition of UO₂(NO₃)₂·6H₂O was studied. Determination of [UO ₂ ²⁺ ] and [NO ₃ ^? ], and also of the molar ratio NO ₃ ^? : UO ₂ ²⁺ in various fractions of the decomposition product showed that the mechanism of the UO₂(NO₃)₂·6H₂O decomposition under the action of MWR differs from the mechanism of its decomposition under common convection heating. The main precursor of UO₃ as product of UO₂(NO₃)₂·6H₂O decomposition under the action of MWR is uranyl hydroxonitrate UO₂(OH)NO₃ formed already in the first minutes of the irradiation. In contrast to the thermolysis under convection heating, UO₂(NO₃)₂ or its hydrates were not detected as intermediates. The mechanism of the UO₂(NO₃)₂·6H₂O decomposition under the action of MWR can be presented by the reactions UO₂(NO₃)₂·6H₂O → UO₂(OH)NO₃ + 5H₂O + HNO₃ and UO₂(OH)NO₃ → UO₃ + HNO₃. The solubility of UO₂(OH)NO₃ in H₂O at 20°C was estimated experimentally at 6.83 × 10^?2 M.     

Solubility of tributyl phosphate and its degradation products in concentrated uranyl nitrate solutions and of uranyl nitrate in tributyl phosphate at elevated temperatures

Solubility of UO₂(NO₃)₂, TBP, and its degradation products in the organic and aqueous phases of the system TPB-H₂O-HNO₃-UO₂(NO₃)₂ at 25–128°C over the uranyl nitrate concentration range 200–1200 g 1^–1 is studied. The solubility of TBP and its degradation products in uranyl nitrate hexahydrate melt decreases in the order H₂MBP > HDBP > TBP > H₃PO₄. At the boiling point of the melt, their solubility ranges from 100 to 0.5 g l^–1. The solubility of uranyl nitrate in the TBP phase under the same conditions is caused by formation of complexes with a composition close to that of the monosolvate UO₂(NO₃) ₂ TBP.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 436–439.Original Russian Text Copyright © 2004 by Usachev, Markov.     

Decomposition of uranyl nitrate in the silica gel matrix under the action of microwave radiation

Decomposition of aqueous solutions of uranyl nitrate in a matrix of granulated silica gel of KSKG grade under the action of microwave radiation (MWR) was studied. Microwave irradiation leads not only to formation of solid decomposition products UO₃, UO₂(OH)NO₃, and their hydrates in pores of KSKG granules, but also to accumulation of gaseous NO _x and H₂O. The presence of NO _x in KSKG pores leads to HNO₃ formation in the course of washing of sorbent granules with water. This prevents hydrolysis of uranyl nitrate and formation of UO₂(OH)₂·H₂O in KSKG pores. Washout of uranium with water and HClO₄ solutions from the KSKG fraction containing products of decomposition of 2 and 10 g of the initial UO₂(NO₃)₂·6H₂O under the action of MWR (hereinafter denoted as KSKG-P-I) was studied. Upon ∼7-day contact of the solid and liquid phases at the total ratio S : L = 1 : 20, from 5 to 14% of U passes into the aqueous phase from KSKG-P-I samples obtained in experiments with 10 and 2 g of UO₂(NO₃)₂·6H₂O, respectively. In the course of repeated treatments of KSKG-P-I with water, pH of the wash water increased from 3 to 6, owing to removal of NO _x from KSKG pores. Then an insoluble phase of uranyl hydroxide UO₂(OH)₂·H₂O, which can also be presented as hydroxylated uranium trioxide UO₃·2H₂O, is gradually formed from the solution obtained by treatment of KSKG-P-I with water. On treatment of KSKG-P-I with HClO₄ solutions (pH 1–2), virtually all uranium species formed by MWR treatment of aqueous uranyl nitrate solutions in KSKG matrix dissolve (at a contact time of the solid and liquid phases of ∼21 days, the amount of U that passed into HClO₄ solutions is ∼90%). The amount of the U form that is not extracted with HClO₄ solutions and remains in KSKG granules is ∼12% of its initial amount. X-ray phase analysis suggests that the uranium species remaining in KSKG are silicate compounds formed by sorbent saturation with a uranyl nitrate solution and subsequent MWR treatment.     

Addition of Urea or Biuret on Synthesis of Rhabdophane-Type Neodymium and Cerium Phosphates

Urea or biuret was added to the thermal synthetic system of Rhabdophane-type neodymium and cerium phosphates. The mixture of a rare earth compound, a phosphorus compound, and an additive [CO(NH₂)₂ or NH(CONH₂)₂] was heated at 150°C or 300°C for 20 hr, and the thermal products were analyzed by the XRD, FT-IR, and BET methods. H₃PO₄ and (NH₄)₂HPO₄ were used for phosphorus compounds, and for rare earth compounds, Nd₂O₃, Nd(NO₃)₃ · 6H₂O, NdCl₃ · 6H₂O, Nd₂(CO₃)₃ · 8H₂O, CeO₂, Ce(NO₃)₃ · 6H₂O, CeCl₃ · 7H₂O, and Ce₂(CO₃)₃ · 8H₂O were used. Urea and biuret worked not only as a dispersing agent but also as a reactant. By the addition of biuret, the thermal products changed from cerium oxide to Rhabdophane-type cerium phosphate in the system using CeCl₃ · 7H₂O and (NH₄)₂HPO₄. Addition of urea or biuret influenced the specific surface area of Rhabdophane-type neodymium and cerium phosphates. Furthermore, to increase the reactivity of the raw solid materials, mechanical treatment was performed. The mixture of diammonium hydrogenphosphate and a rare earth compound was ground with water and then heated. The influence of the addition of urea or biuret was also studied in these systems.     

Mechanism of decomposition of UO2(NO3)2·6H2O + Fe(NO3)3·9H2O mixture under the action of microwave radiation

Products of decomposition of the UO₂(NO₃)₂·6H₂O + Fe(NO₃)₃·9H₂O mixture under the action of microwave radiation (MWR) were studied by thermal gravimetric, X-ray phase, and chemical analyses. The results obtained were compared to the published data for various U and Fe compounds. The final products of decomposition of the UO₂(NO₃)₂·6H₂O + Fe(NO₃)₃·9H₂O mixture under the action of MWR for 3?C5 min (the maximal temperature of the process, equal to 140?C150°, is attained within 2?C3 min of irradiation), apart from gaseous products, are UO₂(OH)NO₃, UO₃, and Fe₂O₃. The action of MWR on the UO₂(NO₃)₂·6H₂O + Fe(NO₃)₃·9H₂O mixture under the examined conditions does not lead to the formation of uranyl ferrite.     

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.     

Partially ordered organic-inorganic nanocomposites in the system UO<Subscript>2</Subscript>SeO<Subscript>4</Subscript>-H<Subscript>2</Subscript>O-NH<Subscript>3</Subscript>(CH<Subscript>2</Subscript>)<Subscript>9</Subscript>NH<Subscript>3</Subscript>

The compound [NH₃(CH₂)₉NH₃]₂[(UO₂)₃(SeO₄)₅(H₂O)₂](H₂O)_x (1) was prepared by isothermal evaporation from aqueous uranyl selenate solutions containing 1,9-diaminononane. A structural study showed that the compound is a partially ordered organic-inorganic nanocomposite. The structural model of the inorganic complex was determined by single crystal X-ray diffraction a = 19.5572(5), c = 47.878(2) Å, V= 15859.1(9) ų, Z= 12; R₁ = 0.1318, wR₂ = 0.3186 for 2808 reflections with |F_o| ≥ 4σ_F). The structure consists of double hydrogen-bonded [(UO₂)₃(SeO₄)₅(H₂O)₂]^2- layers parallel to the (001) plane. The disordered protonated 1,9-diaminononane molecules and water molecules are arranged between the layers. The inorganic layered complex [(UO₂)₃(SeO₄)₅(H₂O)₂]^2- belongs to a new type that was not observed previously in the structures of inorganic and organometallic compounds.     

Extraction of uranyl nitrate with a binary extractant based on trialkylbenzylammonium and higher isomeric α,α′-branched carboxylic acids

A binary extractant based on trialkylbenzylammonium and higher isomeric α,α′-branched carboxylic acids (R₄NA) was synthesized. The extraction of uranyl nitrate with 0.52 M solutions of R₄NA and R₄NNO₃ in toluene was studied. The extraction isotherms were constructed. The physicochemical and mathematical models of the extraction of uranyl nitrate were developed. The following extractable species were shown to be formed in the organic phase: (R₄N)₂[UO₂(NO₃)₄], R₄N[UO₂A(NO₃)₂], and (R₄N)₂[UO₂A₂(NO₃)₂] The extraction constants were calculated. The organic phase was examined by IR spectroscopy.     

Synthesis,Structure, and Properties of Inorganic Nanotubes Based on Uranyl Selenates

Two new U(VI) compounds, K₅[(UO₂)₃(SeO₄)₅](NO₃)(H₂O)_3.5 and (C₄H₁₂N)₁₄[(UO₂)₁₀(SeO₄)₁₇·(H₂O)], containing porous nanotube formations, were synthesized; their structure and properties were studied.     

A single crystal X-ray diffraction study of Na4(UO2)4(i-C4H9COO)11(NO3)·3H2O

Synthesis and the results of IR and single crystal X-ray diffraction study of Na₄(UO₂)₄(i-C₄H₉COO)₁₁·(NO₃)·3H₂O are reported. The crystals are monoclinic; the unit cell parameters at 100 K are as follows: a = 13.697(2), b = 20.285(3), c = 15.991(3) Å, β = 103.760(3)°, space group P2₁, Z = 2, R = 0.0650. The uraniumcontaining structural units are mononuclear moieties [UO₂(i-C₄H₉COO)₃]^? and [UO₂(NO₃)(i-C₄H₉COO)₂]^?, belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = i-C₄H₉COO^? and NO ₃ ^? ) of uranyl complexes. The IR data are consistent with the results of the single crystal X-ray diffraction study. The influence of the carboxylate ligand volume on the structure of Na[UO₂L₃]·nH₂O crystals (L = acetate, n-butyrate, isovalerate ion) is analyzed.     

Phase equilibria in the liquid ternary system [Th(NO3)4(TBP)2]-[UO2(NO3)2(TBP)2]-Isooctane at different temperatures

The phase diagrams of the ternary systems [Th(NO₃)₄(TBP)₂]-[UO₂(NO₃)₂(TBP)₂]-isooctane in the temperature range 298.15–333.15 K were constructed. These diagrams contain the field of homogeneous solutions and the field of separation into two liquid phases (I, II). Phase I is enriched in [Th(NO₃)₄(TBP)₂] and [UO₂(NO₃)₂(TBP)₂], and phase II is enriched in isooctane. With increasing temperature from 298.25 to 333.15 K, the mutual solubility of Th(NO₃)₄(TBP)₂ and isooctane does not change noticeably, but the two-phase fields somewhat contract. In the two-phase systems [UO₂(NO₃)₂(TBP)₂] is preferentially distributed in phase I, although the binary system [UO₂(NO₃)₂(TBP)₂]-isooctane is single-phase over the entire temperature range examined. The preferential accumulation of [UO₂(NO₃)₂(TBP)₂] in phase I causes the redistribution of [Th(NO₃)₄(TBP)₂] and isooctane into phases II and I, respectively. The compositions of the ternary systems in the critical points at different temperatures were determined. The electronic absorption spectra of uranyl nitrate solvate with TBP in the homogeneous and two-phase systems were recorded and analyzed. Original Russian Text ? A.K. Pyartman, V.A. Keskinov, V.V. Lishchuk, Ya.A. Reshetko, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 5, pp. 420–422.     

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 crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

Synthesis,Structure, and Physicochemical Properties of AI 4[UO2(CO3)3]·nH2O (AI = Li,Na, K,NH4)

Procedures for the synthesis of Li₄[UO₂(CO₃)₃]·1.5H₂O, Na₄[UO₂(CO₃)₃], K₄[UO₂(CO₃)₃], (NH₄)₄[UO₂(CO₃)₃], and K₃Na[UO₂(CO₃)₃] were optimized. The structures of these compounds and their thermolysis were studied by X-ray diffraction, precision IR spectroscopy, and thermal analysis. The standard enthalpies of formation of these compounds at 298.15 K were determined by reaction calorimetry.     

Crystal Structure of Uranyl Malonate Trihydrate [UO<Subscript>2</Subscript>(OOC)<Subscript>2</Subscript>CH<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)]·2H<Subscript>2</Subscript>O

Single crystals of [UO₂(OOC)₂CH₂(H₂O)]·2H₂O (I) were prepared by recrystallization of finely crystalline uranyl malonate trihydrate under hydrothermal conditions. The crystal structure of I consists of electroneutral [UO₂(OOC)₂CH₂(H₂O)]_n layers and water molecules located between them. The uranium coordination number is 7. The uranium coordination polyhedron is a distorted pentagonal bipyramid with the oxygen atoms of the uranyl group in the apices. The equatorial plane is occupied by four O atoms of three malonate ligands and the water molecule. The malonate anion is coordinated in the bidentate fashion to one uranyl ion to form a six-membered ring and in the monodentate fashion to two other uranyl ions.     

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.     

Synthesis, crystal and molecular structure, and spectral properties of guanidinium salt of complex uranyl 2-methoxybenzoate

A crystalline uranyl 2-methoxybenzoate, [C(NH₂)₃][UO₂L₃]·H₂O, was synthesized. Its structure was determined by single crystal X-ray diffraction, and the electronic and IR spectra were recorded. The coordination number of the U atom is 8, with methoxyl O atom not involved in coordination bonding with uranyl. The structure contains a system of hydrogen bonds with water molecule of crystallization and guanidinium cation acting as proton donors. The electronic absorption spectrum of the crystalline complex has a pronounced vibronic structure, whereas in the solution spectrum all the lines are strongly broadened. This may be due to dissociation of the complex anion in solution, which leads to superposition of several spectra. In the IR spectrum, there is a set of band characteristic of guanidinium cations. The uranyl group in [C(NH₂)₃][UO₂L₃]·H₂O has almost symmetrical structure. Therefore, only the band of its antisymmetric vibrations is observed in the IR spectrum.     

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.     

Synthesis and single crystal X-ray diffraction study of U(VI), Np(VI), and Pu(VI) perchlorate hydrates

The compounds [AnO₂(H₂O)₅](ClO₄)₂ (An = Np, Pu) and [NpO₂(ClO₄)₂(H₂O)₃] were prepared as single crystals, which were studied by X-ray diffraction at 100 K. The structural type at room temperature was determined. The low-temperature modification of [UO₂(H₂O)₅](ClO₄)₂ was found and structurally studied. The coordination polyhedra in [AnO₂(H₂O)₅]²⁺ are weakly distorted pentagonal bipyramids with averaged interatomic distances An-O of 1.754, 1.744, and 1.732 Å in the “yl” groups and of 2.415, 2.416, and 2.409 Å in the equatorial planes for U, Np, and Pu, respectively. Hence, in the complex cations [AnO₂(H₂O)₅]²⁺ the actinide contraction is manifested only in regular shortening of the An-O interatomic distances in the “yl” groups. The compound [NpO₂(ClO₄)₂(H₂O)₃], isostructural to its known uranyl analog, appeared to be the first, proved by single crystal X-ray diffraction, example of a compound with coordination interaction between the perchlorate ion and the neptunyl(VI) cation.