Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

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.     

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.     

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.     

Reaction of Np(VI) with nitrilotriacetic acid in perchloric acid solutions

Stoichiometry of the reaction of Np(VI) with N(CH₂COOH)₃ (NTA) in a 0.05 M HClO₄ solution was studied by spectrophotometry. With excess Np(VI), 1 mol of NTA reduces 2 mol of Np(VI) to Np(V). In 0.05–0.98 M HClO4 solutions (the ionic strength I = 1.0 was maintained by adding LiClO4) containing 5–30 mmol of NTA, at 25–45°С Np(VI) at a concentration of 0.3–2 mM is consumed in accordance with a firstorder rate law until less than 1/3 of Np(VI) remains in the solution. After that, the reaction decelerates. The reaction is first-order with respect to NTA and has an order of–2 with respect to Н⁺ ions. The activated complex is formed with the loss of two Н⁺ ions. The activation energy of the reaction is 100 ± 2 kJ mol^–1.     

Ruthenium-catalyzed oxidation of Np(IV) in nitric acid solutions

Oxidation of Np(IV) with nitric acid in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The catalytic oxidation of Np(IV) in nitric acid solutions occurs even in the presence of hydrazine. The mechanism of Ru-catalyzed oxidation of Np(IV) with nitric acid was suggested on the basis of the kinetic data. The effect of the Ru nanoparticle size on the activation energy of the catalytic oxidation of Np(IV) was revealed. It shows that the heterogeneous-catalytic reaction is structure-sensitive (positive size effect).     

Mechanism of transformation of Np(V) into Np(IV) in sodium 1,2-cyclohexanediaminetetraacetate solutions

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5–74 mM sodium 1,2-cyclohexanediaminetetraacetate (Na₂CHDTA) or in a 96–97 mM Na₂CHDTA solution at 25–45°С was studied. The reaction rate at Na₂CHDTA concentrations in the range 5–60 mM and pH 3.5–5.9 is described by the equation V = k[Np(V)]^1.4[CHDTA], and at Na₂CHDTA concentrations in the range 70–100 mM and pH 4.1–5.2, by the equation V = k _A[Np(V)]^1.4. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)}₂ forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V).     

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.     

Kinetics of the Np(VI) + EDTA reaction in perchloric acid solutions

The stoichiometry of the Np(VI) + H₂C₂O₄ and Np(VI) + H4Y reactions (Y is EDTA anion) in 0.2 M HClO₄ solution was studied by spectrophotometry. With excess Np(VI), 1 mol of H₂C₂O₄ or EDTA reduces, respectively, 2 or 4 mol of Np(VI) to Np(V). In 0.1–1.0 M HClO₄ solution (the ionic strength of 1.0 was supported by adding LiClO₄) containing 3–20 mM EDTA at 20–45°C, Np(VI) at a concentration of 1 mM and higher is consumed in accordance with the first-order rate law until less than 0.4 mM Np(VI) remains in the solution, after which the reaction decelerates. The reaction rate has the order of 1 with respect to EDTA and ?1.5 with respect to H⁺ ions. The activated complex is formed with the loss of 1 and 2 H⁺ ions. The activation energy is 86.0 ± 3.5 kJ mol^?1.     

Kinetics of Np(VI) Reduction with Butanal Oxime

Reduction of Np(VI) to Np(V) with butanal oxime in the presence of excess reductant is presumably described by the equation 4NpO₂ ²⁺ + 2C₃H₇CHNOH + H₂O = 4NpO₂ ⁺ + 2C₃H₇CHO + N₂O + 4H⁺, and the reaction rate, by the equation -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH]/[H⁺], with k = 230±15 min^-1 at 25°C and the ionic strength of the solution = 2. This equation holds for solutions with different values of the ionic strength and HNO₃ concentration. The activation energy is 69.4±12.4 kJ mol^-1.     

Reduction of Pu(IV) and Np(VI) with hydroxylamine in solutions of low acidity with high uranium content

Decomposition of hydroxylamine in HNO₃ solutions containing 350 to 920 g l^?1 U(VI) was studied. In the absence of fission and corrosion products (Zr, Pd, Tc, Mo, Fe, etc.), hydroxylamine is stable for no less than 6 h at [HNO₃] < 1 M and 60°C. In the presence of these products, the stability of hydroxylamine appreciably decreases. The reduction of Pu(IV) and Np(VI) with hydroxylamine in aqueous 0.33 and 0.5 M HNO₃ solutions containing 850 g l^?1 U(VI) and fission and corrosion products at 60°C was studied. Np(VI) is rapidly reduced to Np(V), after which Np(V) is partially reduced to Np(IV). The rate of the latter reaction in such solutions is considerably higher than the rate of the Np(V) reduction with hydroxylamine in HNO₃ solutions without U(VI). At [HNO₃] = 0.33 M, the use of hydroxylamine results in the conversion of Pu to Pu(III) and of Np to a Np(IV,V) mixture, whereas at [HNO₃] = 0.5 M the final products are Pu(IV) and Np(V).     

Np(VI) Reduction with Diformylhydrazine in Perchloric Acid Solutions

Radiochemistry - The stoichiometry of the reaction of Np(VI) with diformylhydrazine N2H2(CHO)2 (DFH), in 0.01 and 0.1 M HClO4 solutions was studied by spectrophotometry. At an excess of Np(VI), 1...     

Potential of the NP(VI)/Np(V) couple in solutions of various alkalis

The formal potentials of the Np(VI)/Np(V) couple E ^f in alkaline solutions were measured potentiometrically. In 1 M LiOH, NaOH, KOH, CsOH, and (CH₃)₄NOH, the potentials are equal to 0.163⊥0.004, 0.125⊥0.005, 0.112⊥0.005, 0.107⊥0.005, and 0.109⊥0.005 V, respectively. In solutions of MOH+MCl [M=Li, Na, K, Cs, and (CH₃)₄N] at the ionic strength of 1, the dependence of E ^f on log[OH^?] is a straight line with a slope of 0.118⊥0.010, i.e., two OH^? ions participate in the electrochemical reaction between Np(VI) and Np(V). Taking into account the well-known structure of Np(VI), it can be stated that Np(V) in solutions with [OH^?]=1 M and less exists in the form of the NpO₂(OH) ₂ ^? anion. In 2–4 M LiOH and 2–11 M NaOH or KOH, the potential decreases with increasing alkali concentration. In these media, the anion NpO₂(OH) ₃ ^2? is formed.     

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.     

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.     

Synthesis and properties of Np(VI) and Pu(VI) monophthalates

Neptunium(VI) and plutonium(VI) monophthalates were prepared and characterized. The complexes AnO₂ (COO)₂C₆H₄ 2H₂O were isolated from cold solutions, and AnO₂ (COO)₂C₆H₄ 1.33H₂O, from hot solutions. NpO₂ (COO)₂C₆H₄ b. 2H₂O and PuO₂ (COO)₂C₆H₄ 2H₂O crystalize in the triclinic and monoclinic systems, respectively. The complexes AnO₂(COO)₂C₆H₄ 1.33H₂O are isostructural and crystallize in the rhombohedral system. The thermal behavior of these complexes was studied. Their IR and electronic absorption spectra were recorded. The properties of these complexes were compared to those of known U(VI) monophthalates.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 389–395.Original Russian Text Copyright © 2004 by Krot, Bessonov, Grigorev, Charushnikova, Makarenkov.     

Kinetics of Np(VI) Reduction with Diformylhydrazine in Nitric Acid

The kinetics of the Np(VI) reduction with diformylhydrazine in a nitric acid solution was studied by spectrophotometry. The reaction rate increases with an increase in the reductant concentration and temperature and decreases with an increase in the HNO₃ concentration. The reaction order with respect to Np, diformylhydrazine, and HNO₃ is 1, 1.3, and–1.55, respectively. The activation energy of the reaction is 85 ± 10 kJ mol^–1.     

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.     

Reduction of Np(VI) and Pu(VI) with anions of some substituted carboxylic acids

The reaction of Np(VI) with organic acid anions in solutions containing lithium salts of tartaric, malic, α-aminoglutaric, and trihydroxyglutaric acids was studied. Changes in the solution spectra show that Np(VI) forms complexes with organic acid anions, which is followed by the reduction of Np(VI) to Np(V). Similar processes occur in solutions containing Pu(VI) and sodium phenylglycolate or ammonium salicylate. In weakly acidic solutions, the loss of the Np(VI) and Pu(VI) concentrations is a linear function of time. The possible mechanism of the redox reactions was suggested.     

Reaction of ozone with Np(IV) and Pu(IV) oxalates in water

The reaction of the ozone–oxygen mixture with aqueous suspensions of Np(IV) and Pu(IV) oxalates was studied. Both metal cations and oxalate anions are oxidized in the process. The final products are Np(VI) and Pu(VI) hydroxides. The composition of Np(VI) hydroxide was confirmed by X-ray diffraction analysis. Oxidation of Np(IV) oxalate with oxygen leads to the accumulation of Np(V) oxalate and oxalic acid in the solution. At incomplete oxidation of Np(IV) oxalate with ozone in water, Np(V) is also accumulated. Heating considerably accelerates the ozonation. The possible reaction mechanism is briefly discussed. The Np(V) and Np(VI) ions participate in the catalytic cycle of the decomposition of oxalate ions with ozone.