Electrochemical Oxidation of Am(III) Ions in HNO3 Solutions

Formal oxidation potentials (E0 ^p) of the Am(IV)/Am(III) couple were measured and the kinetics of electrochemical oxidation of Am(III) on platinum electrode in concentrated solutions of nitric acid (1-6 M) containing potassium phosphotungstate K_{1 0}P₂W_{1 7}O_{6 1} (KPW) was studied. The formal potential E0 ^p only slightly depends on the concentration of HNO₃ and is shifted toward the negative region by 1.0 V as compared with the standard values. The extent of Am(III) oxidation increases with increasing KPW concentration and decreasing concentration of nitric acid. Electrochemical oxidation of Am(III) is accompanied by radiochemical reduction of Am(IV) and is described by the equation -dC _{A m ( I I I )}/dt = (k + k ₁)C _Am(III) - k ₁ C ₀ - k ₀, where k is the apparent rate constant of electrochemical oxidation of Am(III), k ₁ is the apparent rate constant of Am(IV) reduction, and k ₀ is the constant of radiation-chemical reduction of Am(IV).     

Electrochemical Oxidation of Oxalate Ions in Aqueous Solutions

Radiochemistry - Electrochemical oxidation (ECO) of oxalate ions in aqueous solutions has been studied with the use of an MB–11T-06 electrochemical unit element. It was shown that the ECO...     

Interaction of Aluminum with Am(III) and Certain REE(III) Ions in Alkaline Solutions

Precipitates prepared by addition of ammonia or NaOH into solutions containing Al and REE(III) (La, Nd) or Al and Am(III) were studied by thermogravimetry, UV-Vis and IR spectroscopy, X-ray diffraction analysis, and measurement of the dissolution rate. It was shown that the properties of the precipitates containing Al and the second element noticeably differ from the properties of straight aluminum hydroxide precipitate. In the IR spectra, new vibrational bands appear. In the electronic spectra, small shifts of Nd(III) and Am(III) absorption bands relative to the bands of the individual f-element hydroxides are observed. The rates of dissolution of the precipitates in HCl solutions noticeably differ. With increasing pH of coagulation to 13-14, a part of aluminum is captured with the precipitate. However, this fraction is small. In the precipitates obtained from 0.1-1 M NaOH, the interaction of f elements with aluminum is practically lacking.     

Mechanism and Kinetics of Rh(IV) Radiation-Chemical Reduction in HNO3 Solutions

Reduction of Rh(IV) in -irradiated and nonirradiated solutions of HNO₃ (0.3-3.0 M) was studied. In both systems, Rh(IV) is completely reduced to Rh(III). The reduction rates in nonirradiated solutions amount to 50-90% of rates in irradiated solutions. Reduction of Rh(IV) with water is postulated. The rates of Rh(IV) reduction in both irradiated and nonirradiated solutions increase with [Rh(IV)] growth and decrease with an increase in [HNO₃] from 0.5-1 to 2-3 M. The dependence of the reduction rates on the dose rate is weak. Mathematical simulation was used to reveal the mechanism and kinetics of radiation-chemical reduction of Rh(IV) in HNO₃ solutions. The rate constant of Rh(IV) reduction with water was calculated by fitting to the experimental data.     

Oxidation of Am(III) to Am(IV) with ozone in solutions of heteropolyanions

Oxidation of Am(III) in the presence of K₁₀P2W₁₇O₆₁ and K₈SiW₁₁O₃₉ (L) at 20dGC was studied by spectrophotometry. Am(III) is oxidized with ozone to Am(IV) only in the pH range 3.5–1.0. In the case of formation of a complex with Am: L = 1: 1, the reaction is approximately first-order with respect to the metal, and its rate decreases with a decrease in pH from 3.5 to 1.0. Am(IV) in solution in the presence of ozone is slowly reduced with products of water α-ray radiolysis, predominantly with ?₂?₂. The rate constant of the reaction of Am(IV) with ?₂?₂ was estimated.     

Electrochemical Oxidation of Am(III) and Am(V) Ions in 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 kinetics of electrochemical oxidation of Am(III) and Am(V) on Pt electrode in solutions of concentrated nitric acid containing potassium phosphotungstate was studied spectrophotometrically. The processes occurring in the system follow the rate law for reversible reaction. The direct reaction is first-order with respect to the concentration of Am(III) ions, and the reverse process is a zero-order reaction.     

Catalytic Reduction of U(VI) with Hydrazine and Formic Acid in HNO3 Solutions

Catalytic reduction of 0.75 M U(VI) with hydrazine in HNO₃ solutions was studied under various conditions. At 58°C up to 0.55 M U(IV) is accumulated within 2 h in solutions containing 1-1.5 M N₂H₅ ⁺ and 2 M HNO₃ in the presence of 1% Pt/SiO₂ (S : L = 1 : 10). The reduction is decelerated with decreasing N₂H₅ ⁺ concentration to 0.75 M or with increasing HNO3 concentration to 3-4 M. 1.2 mol of U(IV) is formed per mole of oxidized hydrazine. Uranium(VI) reduction with formic acid in the presence of 1% Pt/SiO₂ and 1% Pt/VP-1An anion exchanger was studied. There exists a threshold N₂H₅ ⁺ concentration below which U(VI) is not reduced. The reduction in solutions containing 1-2 M HCOOH, 1-2 M HNO₃, and 0.1 M N₂H₅ ⁺ is faster than in solutions free of formic acid and containing 1-1.5 M N₂H₅ ⁺.     

Reduction of Am(IV) with Water in KHCO3 + K2CO3 Solutions

Reduction of Am(IV) with water in KHCO₃, K₂CO₃ solutions (pH 8.5-10.5) was studiedspectrophotometrically at 54-70°C. The Am(IV) concentration decreases, following the first-order rate law.The reduction rate increases with pH (logk/pH = 0.4), but decreases with increase in (HCO₃ ^- + CO₃ ^{2 -}) concentration. It was assumed that the thermally excited Am(IV) ion forms a dimer with unexcited Am(IV). The dimer dissociates into two Am(III) ions and H₂O₂. Hydrogen peroxide reduces two more Am(IV) ions. In this process, the excited Am(III) ion appears, which transfers the excitation to Am(IV) at collision. Thus, a chain process is initiated. This scheme can also explain the kinetics of Am(VI) and Np(VII) reduction in carbonate solutions.     

Extraction of f Elements and Technetium from HNO3 Solutions with Solutions of Alkyl (N,N-Diethylcarbamoylmethyl)phenylphosphinates

Phosphinic acid derivatives, alkyl (N,N-diethylcarbamoylmethyl)phenylphosphinates (ADPs) were synthesized, studied, and tested as extractants for f elements and technetium at the Mayak Production Association. The distribution factors of these elements D were studied in relation to the ADP and HNO₃ concentrations. The influence exerted by the structure of the alkoxy group at the P atom on DAm and DEu in extraction of Am(III) and Eu(III) (chosen as examples) from nitric acid solutions with solutions of ADPs in dichloroethane was examined. The differences between the distribution factors of Am(III), Pu(IV), U(VI), and Tc(VII) are sufficiently high for complete separation of these elements with a high degree of purification in one step of extraction with ADP solutions. The changes observed in the IR spectrum of the 2-ethylhexyl ADP upon complexation with Pr(III) suggest high complexing power of this reagent toward lanthanide ions. The spectrometric data suggest bidentate coordination of the metal with the chelating groups of the reagent.     

Features of Electrolysis of Nitric Acid Solutions of Silver: I. Behavior of Ag(II) in HNO3 Solutions

The current efficiency of Ag(II) formation in the course of electrochemical oxidative dissolution of PuO₂ was determined. It approaches 72% at the anodic potentials from 1.9 to 2.1 V (vs. SHE) and the anodic current densities of 0.006-0.013 A cm^{- 2}. The behavior of Ag(II) in 4 M HNO₃ at 8, 25, and 40°C was studied. The reaction of Ag(II) with water can be described by the equation d[Ag(II)]/dt = kapp[Ag(II)] ⁿ , where n 2 at 8-25°C and 2-1.5 at 39-41°C. The apparent rate constants of the reaction are 1.5 and 3.8 l mol^{- 1} min^{- 1} at 8.5°C and 25°C, respectively. At 39-41°C, the reaction order with respect to Ag(II) ranges from 2 to 1.5, and the rate constant is about 1.2 l^{1 / 2} mol^{- 1 / 2} min^{- 1}.     

Extraction of Actinides from HNO3 Solutions with Solutions of Dialkyl Methylphosphonates

Extraction of actinides with dialkyl methylphosphonates was studied with the aim to find a compromise between the use of highly effective but expensive chemicals, on the one hand, and weakly effective but cheap and readily available chemicals, on the other hand, and also to find possible ways of utilization of methylphosphonic dichloride, which is the starting substance in syntheses not only of dialkyl methylphosphonates, but also of chemical weapons. In systems with 3 M HNO₃ as the aqueous phase and a solution of tributyl phosphate (TBP), diisoamyl methylphosphonate (DIAMP), or bidentate diphenyl(carbamoylmethyl)phosphine oxide (Ph₂Bu₂) in dichloroethane as organic phase, at equal concentrations of the organophosphorus compounds, the distribution coefficients of Am(III) increase in the order TBP <; DIAMP < Ph₂Bu₂ in a proportion of 1 : 50 : 5 × 10⁴. Mixtures of dialkyl methylphosphonates and acidic phosphonates show a synergistic effect, which, however, can be utilized only at [HNO₃] > 0.1 M and [HIAMP] < 1.5 M (HIAMP is isoamyl hydrogen methylphosphonate).     

Anodic Oxidation of Graphite in 10 to 98% HNO3

Systematic studies into the anodic oxidation of natural fine-particle graphite in 10 to 98% HNO₃are described. The concentration ranges of the formation of stage I–IV graphite nitrates are determined. Below C _{HNO 3} = 55%, no graphite intercalation compounds are detected by x-ray diffraction. The electrochemical oxidation of graphite in dilute HNO₃yields graphite oxide phases. Their formation seems to be preceded by intercalation and hydrolysis, which occur in parallel. The properties of the oxidation and hydrolysis products are shown to depend strongly on the process parameters.     

Effect of ionic liquids on the extraction of Am from HNO3 solutions with diphenyl(dibutylcarbamoylmethyl)phosphine oxide in dichloroethane

Addition of small (1?C5 vol %) amounts of ionic liquids (ILs) increases the Am(III) distribution ratio in the system with diphenyl(dibutylcarbamoylmethyl)phosphine oxide (Ph₂Bu₂) by more than 2 orders of magnitude. With 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄mim]⁺Tf₂N^?) used as IL, the Am(III) extraction is possible even from 8 M HNO₃, which is important in radiochemical analysis of process and environmental samples, because many procedures are based on the transfer of solid samples into 8 M HNO₃. The extraction data show that the extractable Am(III) complex contains three Ph₂Bu₂ ligands, an IL anion, and two NO ₃ ^? anions.     

Specific Features of Electrochemical Oxidation of Pu(III) in a Diaphragmless Cell in Hydrazine-Containing Nitric Acid Solutions

Oxidation of Pu(III) in a diaphragmless cell with a Ti cathode and a Pt anode in 1.0–3.9 M HNO₃ solutions containing 4.5 × 10^?2–1.8 × 10^?1 M hydrazine was studied. The final solutions contain, along with Pu(IV), also a small amount of Pu(VI), increasing with an increase in the current density. Methods allowing minimization of the Pu(VI) amount in the solutions after the electrolysis were suggested. The possible process mechanism involving reactions of Pu ions at the electrodes and in the bulk of the solution with HNO₂ was considered.

     

Complexation of Np(V) with Silicate Ions

It was shown that Np(V) forms complexes with anions of orthosilicic acid and other silicate ions at pH higher than 8–8.5. At pH < 9.5, the reaction is mainly described by the equation NpO ₂ ⁺ + OSi(OH) ₃ ⁻ ⇄ NpO₂OSi(OH)₃; the stability constant of the NpO₂OSi(OH)₃ complex is equal to log β₁ = 2.1 ± 0.3. Thus, interaction is weak and hardly significant under real conditions. Carbonate ions in equilibrium with air at pH > 8.5 are the substantially stronger ligands for NpO ₂ ⁺ , and in their presence it is impossible to reveal Np(V) complexation with silicate ions.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 39–43.Original Russian Text Copyright © 2005 by Yusov, Fedoseev, Isakova, Delegard.     

Electrochemical Oxidation in Aqueous Solutions of Organic Complexing Agents and Surfactants

Radiochemistry - The electrochemical oxidation (ECO) of aqueous solutions of organic complexing agents [ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA),...     

Structure and Properties of Anodic Aluminum Oxide Films Produced in HNO3 Solutions

Amorphous films produced by electrochemical oxidation of aluminum in an aqueous 10 M HNO₃ solution were studied. The parameters of the nearest neighbor atomic configuration were evaluated from x-ray scattering intensities, and the surface morphology of the films was examined by scanning electron microscopy. The results demonstrate that the electrochemical dissolution of the Al substrate prevails over the oxide film growth, which results in a large specific surface area of the films.     

Extraction of Am(III) from Nitric Acid Solutions with Bis(diphenylphosphinylmethyl)phosphinic Acid

Extraction of trace quantities of Am(III) from aqueous HNO₃ solutions with solutions of bis(diphenylphosphinylmethyl)phosphinic acid (I), bis[2-(diphenylphosphinyl)phenoxymethyl]phosphinic acid (II), and bis[2-(diphenylphosphinylmethyl)phenoxymethyl]phosphinic acid (III) in dichloroethane was studied. The stoichiometry of the extractable complexes was determined; the effect of organic diluent on the extraction was considered. The extractive power of the reagents toward Am(III) grows in the order III < II < I. Acid I surpasses in the extractive power a neutral bidentate extractant, tetraphenylmethylenediphosphine dioxide (TPMDPD), by three orders of magnitude. A synergistic effect in extraction of Am with mixtures of TPMDPD and acid I was revealed.     

Stability of Neptunium(V) in Concentrated Acetic Acid Solutions

Np(V) acetate in glacial acetic acid and aqueous acetic acid solutions is resistant to disproportiona-tion and occurs in equilibrium with the NpO₂(Ac)·H₂O precipitate. Within the time of dehydration with excess acetic anhydride, Np(V) disproportionates. The inverse reproportionation process occurs after adding excess water to anhydrous acetic acid containing acetic anhydride, Np(IV), and Np(VI). Addition of HClO₄ causes rapid disproportionation of Np(V) in anhydrous acetic acid and in concentrated acetic acid solutions. The spectra of suspensions containing Np(V) and potassium acetate in concentrated acetic acid contain absorption bands with maxima at 990 and 1020 nm, assigned to NpO₂(Ac)₃^2- anions and K₂NpO₂(Ac)₃ particles, respectively.

     

Electrochemical properties and dissolution of U-5 wt % Zr Alloy in HNO3 solutions

The main parameters of dissolution of U-5 wt % Zr alloy in HNO₃ solutions were determined by linear voltammetry. Without applying external potential, the electrode potential E ⁰ in 0.5–6.0 M HNO₃ solutions increases. Its value is determined by the formation of a film containing hydrated Zr(IV) and U(IV) oxides and by the cathodic reduction of the acid. The reduction of NO₃ — ions on the surface of U-5 wt % Zr alloy in 0.5–6.0 M HNO₃ occurs with high overvoltage. In experiments without applying external potential, an increase in the HNO₂ yield in accordance with this reaction leads to an increase in the alloy dissolution rate with an increase in the HNO₃ concentration in the electrolyte from 0.5 to 6.0 M. At potentials higher than 450 mV vs. Ag/AgCl electrode, the U-5 wt % Zr electrode undergoes transpassivation. Data on the electrolysis at controlled potential in the region close to the transpassivation potential showed that an increase in the electrode potential at the given HNO₃ concentration leads to an increase in the alloy dissolution rate.