Behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The behavior of Np(VI) in HNO₃ solutions containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) at various concentrations of HNO₃ (1–3 M) and KPW [(1–3) × 10⁻³ M] was studied by spectrophotometry. The rate law and the Np(VI) transformation scheme were determined, and the effective rate constants of the Np(VI) reduction and Np(V) reproportionation were calculated.     

Kinetics of Np(V) transformation in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The behavior of Np(V) in concentrated HNO₃ solutions containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) at various concentrations of HNO₃ (1.0–3.0 M) and KPW [(1–5) × 10^?3 M] was studied. Under the examined experimental conditions, the final products of Np(V) transformation are Np(IV) and Np(VI). The reaction follows a first-order rate equation with respect to the Np(V) concentration.     

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.     

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).     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VII. Kinetics of Reduction of Pu(IV) and Np(VI) with Butanal Oxime in Undiluted TBP

The kinetics of reduction of Pu(IV) and Np(VI) with butanal oxime in undiluted TBP containing HNO₃ was studied spectrophotometrically. In the range [HNO₃] = 0.08-0.75 M the rate of Pu(IV) reduction is described by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²[C₃H₇CHNOH]/{[Pu(III)][HNO₃]²} with the rate constant k = 0.068±0.017 mol l^-1 min^-1 at 20°C. The kinetic equation of the reduction of Np(VI) to Np(V) in the range [HNO₃] = 0.01-0.27 M is -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH][H₂O]²/[HNO₃]0.5, where k = 0.058±0.007 l2.5 mol-2.5 min^-1 at 25°C, and the activation energy is 79±9 kJ mol^-1.     

Kinetics of the reaction of Np(VI) with iminodiacetic acid

The stability of Np(VI) in 5–200 mM iminodiacetic acid (H₂IDA) solutions at 23.5–55°С was studied by spectrophotometry. In a solution with pH 2 and excess Np(VI), 1 mol of H₂IDA reduces 2 mol of Np(VI) to Np(V). In 1 and 0.5 M HClO₄ solutions containing 200 mM H₂IDA and 1 mM Np(VI), no more than 36 and 65% of Np(VI), respectively, is reduced at 44.5°С. Complete reduction of Np(VI) is observed in solutions containing 0.2 M HClO₄ and less. In the examined ranges of H2IDA concentrations and temperatures, Np(VI) is consumed in accordance with the first-order rate law. The reduction mechanism involves formation of a Np(VI) iminodiacetate complex, which is followed by intramolecular charge transfer. The generated radical reduces Np(VI). The activation energy is 107 ± 3 kJ mol^–1.     

Catalytic Reduction of Np(VI,V) with Formic Acid in Perchloric Acid Solutions

Neptunium(VI) is successively reduced with formic acid to Np(V) and Np(IV) in perchloric acid solutions in the presence of 1% Pt/SiO₂ catalyst. The kinetic features of Np(VI,V) reduction with formic acid in 0.1-4.0 M HClO₄ in the presence of 0.01-0.1 g ml^-1 of 1% Pt/SiO₂ at [HCOOH] = 0.001-1.0 M and T = 40-70°C were studied. The rate-determining steps of reduction of Np(VI) to Np(V) and Np(V) to Np(IV) are diffusion and decomposition of the activated complex adsorbed on the catalyst surface, respectively. The mechanisms of both processes are discussed.     

Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH₂NH)₂CO in 1–6 M HNO₃ solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO₃] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.     

Kinetics of Redox Reactions of U,Np, and Pu in TBP Solutions: IX. Reduction of Np(VI) with Dibenzylhydrazine

The kinetics of reduction of Np(VI) with dibenzylhydrazine in TBP nitric acid solutions was studied. At the reductant excess Np(V), nitrogen, and benzyl alcohol were the reaction products. At low HNO₃ concentration (<0.03 M), the reaction went to completion, while at a higher acid content in TBP the equilibrium was attained, shifting to Np(VI) with increasing acidity. Taking into account direct and reverse reactions, the rate of Np(VI) to Np(V) transformation was described by the equation -d[Np(VI)]/dt = k[Np(VI)]× [(C₆H₅CH₂)₂N₂H₂][H₂O]^0.4 - k ₃[Np(V)]²[HNO₃]^1.2, where k = 64±6 l^1.4 mol^{- 1.4} min^-1 and k ₃ = 4480± 450 l^2.2 mol^{- 2.2} min^-1 at 40°C. The activation energy of the direct reaction was E = 42.7±2.2 kJ mol^{- 1}. The dilution of TBP with n-dodecane did not affect the reaction rate. The reaction mechanism was discussed.     

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.     

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.     

Reaction of Np(VI) with cyclohexanediaminetetraacetic acid in HClO<Subscript>4</Subscript> solutions

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H4chdta) in 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO₄ solutions (the ionic strength of 1.0 was supported with LiClO₄) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of–1.2 with respect to [H⁺]. The activated complex is formed with the loss of one and two Н⁺ ions. The activation energy is 82.3 ± 3.8 kJ mol^–1.     

Reaction of Np(VI) with <Emphasis Type="Italic">N</Emphasis>-(2-Hydroxyethyl)ethylenediaminetriacetic acid in perchloric acid solutions

The stoichiometry of the reaction Np(VI) + H₃hedta [hedta is N-(2-hydroxyethyl)ethylenediaminetriacetate, HEDTA, anion] in a 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of HEDTA reduces 4 mol of Np(VI) to Np(V). In 0.125–1.0 M HClO₄ solutions (the ionic strength of 1.0 was maintained constant by adding LiClO₄), containing 3–29.2 mM HEDTA, at 20–45°С Np(VI) at a concentration of 0.4–3.5 mM is consumed in accordance with a first-order rate law until approximately 40% of Np(VI) remains. Then, the reaction decelerates. The reaction rate has first order with respect to [HEDTA] and the order of–1.6 with respect to [Н⁺]. The activated complex arises with the loss of one and two Н⁺ ions. The activation energy is 88.4 ± 5.3 kJ mol^–1.     

Solubility of mixed-valence U(IV–VI) and Np(IV–V) hydroxides in simulated groundwater and 0.1 M NaClO4 solutions

The kinetics of U(VI) accumulation in the phase of U(IV) hydroxide and of Np(V) in the phase of neptunium(IV) hydroxide, and also the solubility of the formed mixed-valence U(IV)-U(IV) and Np(IV)-Np(V) hydroxides in simulated groundwater (SGW, pH 8.5) and 0.1 M NaClO₄ (pH 6.9) solutions was studied. It was found that the structure of the mixed U(IV–VI) hydroxide obtained by both oxidation of U(IV) hydroxide with atmospheric oxygen and alkaline precipitation from aqueous solution containing simultaneously U(IV) and U(VI) did not affect its solubility at the U(VI) content in the system exceeding 16%. The solubility of mixed-valence U(IV–VI) hydroxides in SGW and 0.1 M NaClO₄ is (3.6±1.9) × 10^?4 and (4.3 ± 1.7) × 10^?4 M, respectively. The mixed Np(IV–V) hydroxide containing from 8 to 90% Np(V) has a peculiar structure controlling its properties. The solubility of the mixed-valence Np(IV–V) hydroxide in SGW [(6.5 ± 1.5) × 10^?6 M] and 0.1 M NaClO₄ [(6.1±2.4) × 10^?6 M] is virtually equal. Its solubility is about three orders of magnitude as high as that of pure Np(OH)₄ (10^?9–10^?8 M), but considerably smaller than that of NpO₂(OH) (～7 × 10^?4 M). The solubility is independent of the preparation procedure [oxidation of Np(OH)₄ with atmospheric oxygen or precipitation from Np(IV) + Np(V) solutions]. The solubility of the mixed-valence Np hydroxide does not increase and even somewhat decreases [to (1.4±0.7) × 10^?6 M] in the course of prolonged storage (for more than a year).     

Oxidation of Np(V) with Xenon Trioxide in an HClO<Subscript>4</Subscript> Solution

Oxidation of Np(V) to Np(VI) with xenon trioxide in a 0.5–1.4 M HClO₄ solution was studied by spectrophotometry. The reaction rate is described by the equation–d[Np(V)]/dt = k[Np(V)][XeO₃], where k = 4.6 × 10^–3 L mol^–1 s^–1 in 1 M HClO₄ at 92°С. The activation energy is close to 92 kJ mol^–1. The activated complex is formed in contact of NpO ₂ ⁺ and ХеО₃ without participation of Н⁺ ions. The activated complex transforms into NpO ₂ ²⁺ and the products: ОН, Хе, and О₂. The ОН radical oxidizes Np(V). Admixtures of Со²⁺ and especially Fe³⁺ accelerate the Np(V) oxidation.     

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.     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VIII. Kinetics of Reduction of Pu(VI) and Np(VI) with N,N-Dibutylhydroxylamine in Undiluted TBP

The kinetics of Pu(VI) and Np(VI) reduction in TBP containing HNO₃ was studied spectrophotometrically. The rate of the reduction of Pu(VI) with N,N-dibutylhydroxylamine in undiluted TBP is independent of the Pu(VI) concentration and is described by the equation -d[Pu(VI)]/dt = k[(C₄H₉)₂NOH][H₂O]⁵, with k = (2.17±0.13) × 10^-5 l⁵ mol^-5 min^-1 at 12°C. The activation energy of the reaction, E = 85.2± 4.6 kJ mol^-1, was determined from the temperature dependence of k in the range 12.0-33.5°C. Reduction of Np(VI) in undiluted TBP is approximately described by the kinetic equation -d[Np(VI)]/dt = k[Np(VI)] × [(C₄H₉)₂NOH]/[HNO₃], with k 40 min^-1 at 25°C, and in a 30% solutio of TBP in n-dodecane, by the equation -d[Np(VI)]/dt = k[Np(VI)][(C₄H₉)₂NOH]/[HNO₃]0.7 with the rate constant k = 18.4±1.8 l0.3 mol-0.3 min^-1 at 25°C.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

Electrochemical Oxidation of Am(V) Ions in HNO3 Solutions

Oxidation potentials (E ₀p) of the Am(VI)/Am(V) couple were measured and the kinetics of electrochemical oxidation of Am(V) on platinum electrode in concentrated solutions of nitric acid (1-7 M) containing potassium phosphotungstate K_{1 0}P₂W_{1 7}O_{6 1} (KPW) was studied by the potentiometric method with spectrophotometric control of the oxidation states. The potential E ₀p is independent of the concentrations of HNO₃ and KPW and is shifted toward the negative region by 70 mV (E ₀p) as compared to 1 M HClO₄. The extent of Am(V) oxidation into Am(VI) under the experimental conditions studied remains almost constant and comprises 90%. Electrochemical oxidation of Am(V) is described by the kinetic equation of the first and zero orders with respect to the Am(V) concentration: -dC _{A m ( V )}/dt = k'₁ C _{A m ( V )} - k'₀.     

Behavior of Np(VI) and Np(V) Ions in NaHCO<Subscript>3</Subscript> Solutions Containing H<Subscript>2</Subscript>O<Subscript>2</Subscript>

The behavior of Np(VI) and Np(V) in NaHCO₃ and NaHCO₃ + Na₂CO₃ solutions containing H₂O₂ was studied spectrophotometrically. In 0.75–1.0 M NaHCO₃, hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na₂CO₃ is added. In 1.0 M Na₂CO₃, 0.5 M NaHCO₃ + 0.5 M Na₂CO₃, or 0.01–0.5 M NaHCO₃, hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO₃ solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H₂O₂ yields the excited *Np(V) ion. Then it reacts with another H₂O₂ molecule and forms a carbonate-peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O ₂ ^2? ligand with formation of two Np(VI) ions.