Mechanism of chemiluminescent reaction of U4+ with oxygen in weakly acidic aqueous solutions

The mechanism of the chemiluminescent reaction of U(IV) oxidation with atmospheric oxygen is substantiated using published data. The suggested mechanism includes the initiating step of successive oxidation of U(IV) and U(V) with the formation of the excited ion [UO ₂ ²⁺ ·aq]*, which forms with an unexcited ion UO ₂ ²⁺ ·aq an excimer [UO ₂ ²⁺ ·aq?UO ₂ ²⁺ ·aq]*. The decay of the excimer yields H₂O₂ and two U(V) ions, causing the development of the chain process of the U(IV) oxidation.     

Disproportionation of polymeric Pu(IV) in weakly acidic solutions

Polymeric Pu(IV) in aqueous solutions in the pH range 0.5–3 disproportionates with time to form Pu(III) and Pu(VI). The arising Pu(III) is bound by hydroxyl groups of polymeric Pu(IV) and does not exhibit intrinsic absorption bands in the spectrum of a solution of polymeric Pu(IV). However, after ultrafiltration of the solution through a filter with a pore size of ∼3 nm Pu(III) is clearly identified in the filtrate by its absorption maxima. Pu(VI) occurs in the solution in the ionic state and is not bound by hydroxy groups of polymeric Pu (IV). Therefore, Pu(VI) is identified in the solution absorption spectrum both before ultrafiltration and after it. Thus, storage of solutions of polymeric Pu(IV) with pH 0.5–3 is accompanied by formation of Pu(III) and Pu(VI) ions.     

Specific features of reactions of Am(V) with reductants in weakly acidic solutions

In EDTA solutions with pH ??5 at 25°C, Am(V) in a concentration of 5 × 10^?4?3 × 10^?3 M slowly transforms into Am(III). The Am(V) reduction and Am(III) accumulation follow the zero-order rate law. In the range 60?C80°C, the reaction is faster. In some cases, an induction period is observed, disappearing in acetate buffer solutions. In the range pH 3?C7, the rate somewhat increases with pH. In an acetate buffer solution, an increase in [EDTA] accelerates the process. The activation energy is 47 kJ mol^?1. Zero reaction order with respect to [Am(V)] is observed in solutions of ascorbic and tartaric acids, of Li₂SO₃ (pH > 3), and of hydrazine. The process starts with the reaction of Am(V) with the reductant. The Am(III) ion formed in the reaction is in the excited state, *Am(III). On collision of *Am(III) with Am(V), the excitation is transferred to Am(V), and it reacts with the reductant: *Am(V) + reductant ?? Am(IV) + R₁ and then Am(IV) + reductant ?? *Am(III) + R₁, Am(V) + R₁ ?? Am(IV) + R₂. A branched chain reaction arises. The decay of radicals in side reactions keeps the system in the steady state; therefore, zero reaction order is observed.     

Extraction of REEs(III), U(VI), and Th(IV) with acidic phosphoryl-containing podands from nitric acid solutions

The extraction of microamounts of REEs(III), U(VI), and Th(IV) from HNO₃ solutions with solutions of acidic phosphoryl-containing podands in 1,2-dichloroethane was studied. The stoichiometry of the extractable complexes was determined. The influence of the extractant structure and aqueous phase composition on the efficiency and selectivity of the recovery of REEs(III), U(VI), and Th(IV) into the organic phase was considered.     

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.     

Sorption of U(VI) and Ra from aqueous solutions with analcime-containing rock

The influence of the phase contact time, pH, and initial concentrations of U and Ra on their sorption from aqueous solutions with analcime-containing rock was studied. The sorption capacity and the strength of sorption of the above radionuclides were determined. Possible sorption mechanisms are considered.     

Sorption of U(VI) from sulfate solutions with spherically granulated chitosans

Sorption of U(VI) from sulfate solutions with spherically granulated chitosans was examined. It was shown that the best sorption and kinetic characteristics are exhibited by freshly formed and cross-linked chitosan granules. The total static exchange capacity for the freshly formed granules was 0.7 mmol of U(VI) per gram of dry sorbent. The diffusion coefficients and similarity criteria describing the diffusion process parameters were calculated. It was found that, irrespective of the method by which the granules were prepared (freshly formed, cross-linked, air-dried), the dominating process is U(VI) sorption on the surface, mainly controlled by external diffusion. Physicomechanical tests of the films of the irradiated chitosan and cellophane (absorbed dose 100–250 kGy) revealed higher strength characteristics of the irradiated chitosan.     

Catalytic reduction of U(VI) with hydrazine on palladium catalysts in acid solutions

The stability of finely dispersed palladium supported on silica gel with respect to various acids was studied. It was shown that palladium catalysts can be used in moderately acidic media under reducing conditions. In nitric acid solutions within a wide range of experimental conditions, the palladium catalysts do not initiate reduction of U(VI) with hydrazine. The catalytic properties of palladium catalysts differing in the size of nanocrystallites of the active metal were examined in the reduction of U(VI) with hydrazine in sulfuric acid solutions. The specific activity of Pd/SiO₂ catalysts is determined solely by the size of metal nanocrystals and is independent of the metal content on the support. The negative size effect is observed, i.e., the surface Pd atoms located on large crystallites exhibit higher catalytic activity. The results obtained were interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the mechanism of U(VI) catalytic reduction with hydrazine in the sulfuric acid solutions.     

Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

The kinetics of catalytic reduction of U(VI) with formic acid in H₂SO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO₂ leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.     

Reactivity of platinum nanoaggregates in catalytic reduction of U(VI) with hydrazine in acid solutions

Catalysts Pt/SiO₂ with different size of Pt nanoaggregates were studied in catalytic reduction of U(VI) with hydrazine in solutions of H₂SO₄, HClO₄, and HNO₃. The catalytic activity of the surface Pt atoms of Pt/SiO₂ in this reaction monotonically increases with an increase in the size of metal crystallites on the support, i.e., the negative size effect is observed. Catalytic reduction of U(VI) with hydrazine in H₂SO₄ solutions is significantly faster than in HClO₄ and HNO₃, and in HClO₄ it is slightly faster than in HNO₃. The results were interpreted within the framework of the concept of the energetic nonuniformity of the surface atoms and of the mechanisms of catalytic reduction of U(VI) with hydrazine in acid solutions.     

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.     

Interaction of U(VI), Np(VI), and Pu(VI) ions with unsaturated heteropolytungstates of the 17th and 11th series in aqueous solutions

The stability constants of the complexes of U(VI), Np(VI), and Pu(VI) with the heteropolyanions (HPAs) P₂W₁₇O ₆₁ ^10? , SiW₁₁O ₃₉ ^8? , and PW₁₁O ₃₉ ^7? in solutions with pH from ?0.3 (2 M H⁺) to 5–5.5 in the presence of Na or K salts (up to 2 M) and without them were measured. All the complexes have exclusively the 1: 1 composition; their stability constants β^M(VI) in neutral solutions at a low ionic strength are close to 10⁸ 1 mol^?1. In 0.1–2.0 M acid solutions, log β^M(VI) for the complexes with P₂W₁₇O ₆₁ ^10? is within 1.4–3.9. The slope of the pH dependence of log β^M(VI) does not exceed 1.75; this fact suggests that no more than two protons are displaced from HPA upon complexation in acid solutions. In the presence of 1–2 M sodium salts, the β^M(VI) values reach a maximum at pH ～3 and drastically decrease with a further increase in pH. Actinides(VI) interact with HPAs appreciably more weakly than do actinides(III), which is apparently due to the fact that the denticity of HPAs in the complexes with An(VI), apparently, does not exceed 2.     

Behavior of Pu(VI) and Np(VI) in malonate solutions

Complexation of PuO ₂ ²⁺ in solutions containing malonate anions C₃H₂O ₄ ^2? (L^2?) is studied by spectrophotometry. Mono-and bimalonate complexes are formed. The monomalonate complex was isolated as PuO₂L · 3H₂O. It is isostructural to UO₂L · 3H₂O and forms rhombic crystals with the unit cell parameters a = 9.078(2), b = 7.526(2), and c = 6.2005(15) Å, space group Pmn2₁. The electronic absorption spectrum of the monomalonate complex is characterized by a strong band at 843 nm. In malonate solutions, Pu(VI) is slowly reduced to the pentavalent state even in the cold. The reduction of Np(VI) is considerably faster and more sensitive to increasing temperature. Some kinetic features of the reduction are discussed.     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) Phenylacetates

Radiochemistry - Phenylacetates [AnO2(C6H5CH2COO)2], where An = U (I), Np (II), or Pu (III), were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are...     

制备方法对氧化石墨烯氧化程度及对Th(IV)、U(VI)吸附的影响

采用Hummers方法、优化Hummers方法及改进Hummers方法合成氧化石墨烯, 并通过FT-IR、TGA、XRD、XPS、SEM以及元素分析等手段对制备产物进行了表征。结果表明, 利用优化Hummers方法制备得到的氧化石墨烯具有较高的氧化程度。三种产物对Th(IV)、U(VI)的等温吸附实验结果表明, 采用优化Hummers方法制备的氧化石墨烯对Th(IV)的最大吸附量为192.3 mg/g, 相比于Hummers方法制备产物的吸附能力提高了38.5%; 对U(VI)的最大吸附量为156.2 mg/g, 吸附能力提高了28.1%, 三种样品对Th(IV)、U(VI)的吸附都更加符合Langmuir等温吸附模型。此外, 考察了优化Hummers方法制备的氧化石墨烯吸附Th(IV)、U(VI)的动力学和热力学参数, 证实氧化石墨烯吸附Th(IV)、U(VI)符合准二级动力学方程, 是自发吸热行为。     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) 2-Fluorobenzoates

The compounds NH₄[AnO₂(C₆H₄FCOO₃], where An = U (I), Np (II), or Pu (III), CgH₄COO^? is the 2-fluorobenzoate anion, were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the cubic system, space group P2₁3, Z = 4. The main structural units of I–III are mononuclear complexes [AnO₂(C₆H₄COO)₃]^? belonging to crystal-chemical group AB₃¹ (A = AnO₂²⁺, B⁰¹ = C₆H₄FCOO^?). The actinide contraction in the structures of I–III is manifested in a regular decrease in the lengths of the An=0 bonds in the AnO₂²⁺ cations and in the volumes of the Voronoi-Dirichlet polyhedra (VDPs) of the An atoms in the series U-Np-Pu. The intermolecular interactions in crystal structures of I–III were analyzed by the method of molecular VDPs.

     

Oxidation of Pu(VI) with ozone and stability of the oxidation products,Pu(VII) and Pu(VIII), in concentrated alkali solutions

Oxidation of Pu(VI) with ozone and stability of the oxidation products, Pu(VII) and Pu(VIII), in 4–15 M NaOH solutions were studied. In a wide range of alkali concentrations, from 1 to 15 M, the Pu(VI) ozonation yields a mixture of Pu(VII) and Pu(VIII). It was proved that Pu(VII) exists in aqueous alkali solutions in the form different from that suggested previously. Pu(VII) is readily reduced with ?2? in aqueous alkali solutions with the NaOH concentration of up to 8 M, whereas at [NaOH] + 8 M it is fairly stable. On the contrary, Pu(VIII) is noticeably reduced with water at room temperature throughout the examined range of NaOH concentrations from 1 to 15 M.     

Extraction and sorption preconcentration of U(VI) and Th(IV) from nitric acid solutions using bis(diphenylphosphorylmethylcarbonylamino)alkanes

Extraction of microamounts of U(VI) and Th(IV) from HNO₃ solutions in the form of complexes with polyfunctional neutral organophosphorus compounds, bis(diphenylphosphorylmethylcarbonylamino)alkanes [Ph₂P(O)CH₂C(O)NH]₂(CH₂) _n (n = 3, 5, 8), was studied. The influence of the extractant structure on the efficiency and selectivity of the extraction of U(VI) and Th(IV) was examined, and the stoichiometry of the extractable complexes was determined. The examined compounds exhibit higher extraction ability and selectivity to metal ions in HNO₃ solutions, compared to the alkylamide Ph₂P(O)CH₂C(O)NHC₉H₁₉. The possibility of selective recovery and preconcentration of U(VI) and Th(IV) from nitric acid solutions with a complexing sorbent prepared by noncovalent immobilization of bis(diphenylphosphorylmethylcarbonylamino)pentane on a macroporous polymeric matrix was demonstrated.     

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.