A. V. Gogolev

A pulse radiolysis study of the reactivity of neptunoyl ions relative to inorganic free radicals

Conclusions1. Pulse radiolysis was used to find the rate constants for the reactions of OH, HSO4, NO3, and Cl2 radicals with neptunoyl ions.Change in the NO3 and H+ ion concentrations do not affect the term k[NO3 + NpO2+], while k[Cl2− + NpO2+] increases with increasing chloride concentration due to the formation of neptunoyl ion chlorocomplexes.

Redox reactions of neptunium and plutonium in alkaline aqueous solutions upon gamma radiolysis

The paper is a brief review of data obtained by the authors from the study on redox reactions of neptunium and plutonium ions upon γ-radiolysis of their aerated alkaline aqueous solutions. It includes the information on radiolytic reduction of Np(V), Np(VI), and Pu(VI) ions under various experimental conditions. It was found that the values of Np(VI) and Pu(VI) reduction yields do not depend on alkali concentration. The values considerably increase in the presence of some organic compounds (EDTA and formate were investigated). The formation of the Np(V) peroxo complex was observed in the γ-radiolysis of alkaline aqueous solutions of Np(VI) and Np(V) in the presence of nitrate. The mechanism of radiolytic redox reactions of the ions is discussed in some detail.

Reduction of neptunium(V) and uranium(VI) with iron(II) in bicarbonate solutions

Reaction of Np(VI) compounds with Fe(II) in bicarbonate solutions was studied. Reaction of Np(V) with Fe(II) in the presence of phthalate ions was briefly considered. Iron(II) compounds reduce Np(V) compounds in solutions saturated with Ar or CO2 at any concentrations of bicarbonate ion. At [Na(K)HCO3] > 0.86 M, Np(V) is reduced during mixing the reactants and recording the spectra. The reaction of Fe(II) with Np(V) in dilute bicarbonate solutions is substantially slower, probably owing to a sharp decrease in the solubility of the Np(V) carbonate complexes. The solubility of the Np(V) compounds increases after saturation of the dilute bicarbonate solutions with CO2. However, in this case reduction remains slow. Uranium(VI) carbonate complexes are reduced with Fe(II) compounds in dilute bicarbonate solutions. The reaction products formed at elevated temperatures are UO2 and FeOOH.

Catalytic Decomposition of Organic Anions in Alkaline Radioactive Waste: 1. EDTA Oxidation

Decomposition of ethylenediaminetetraacetate in alkaline solutions with H2O2, Na2S2O8, NaClO, and NaBrO was studied titrimetrically. EDTA is oxidized in solutions heated above 60°C in the presence of cobalt salts at stepwise addition of excess H2O2. The reaction between persulfate and EDTA has an induction period decreasing with increasing NaOH concentration and temperature and with decreasing initial EDTA content or with adding AgNO3, K4Fe(CN)6, or NaNO2. The process involves thermal dissociation of the persulfate ions into radical ions and the subsequent development of a chain reaction. Hypochlorite ions oxidize EDTA in 0.5-5.0 M NaOH at 25-60°C. The process efficiency can be improved by fractional addition of the oxidant in the presence of Co(II) or Ni(II) salts. EDTA is oxidized in alkaline solutions with hypobromite ions only on heating to 95°C. Salts of Co(II), Ni(II), and Cu(II) accelerate the process.

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/O3− 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) + O3−, O3− + H+ → HO3, HO3 → OH + O2, OH + HSO4− → H2O + SO4−, OH + Ce(III) → OH− + Ce(IV), and SO4− + Ce(III) → SO4/2− + 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.

Speciation, stability, and reactions of Np(III–VII) in aqueous solutions

Published data on the structure of Np ions in acid and alkali solutions, on hydrolysis of Np ions and their complexation with anions and cations, on their redox reactions with water and with each other (disproportionation, reproportionation), and on the effect of anions on this process are analyzed. Possible directions of research in chemistry of Np ions are outlined.

Oxidation of U(VI) with oxygen in weakly acidic and neutral solutions

The kinetics of U(IV) oxidation with atmospheric oxygen in solutions with pH 2–7 was studied. In the kinetic curves there is an induction period, which becomes shorter with increasing pH. The induction period is caused by accumulation of U(VI), whose initial presence in the working solution accelerates oxidation. The pseudo-first-order rate constants and bimolecular rate constants of U(IV) oxidation with oxygen were evaluated. The mechanism of U(IV) oxidation is considered. At pH higher than 3, formation of a polymer of hydrolyzed U(IV) with U(VI) plays an important role in oxidation of U(IV), since this prevents formation of U(V). Heating accelerates oxidation of U(IV) at pH 2–2.5, but at a higher pH the process becomes difficultly controllable.

Catalytic decomposition of organic anions in alkaline radioactive waste: III. Oxidation of oxalate and glycolate

Decomposition of oxalate and glycolate ions in alkaline solutions under the action of O3, H2O2, and Na2S2O8 was studied spectrophotometrically and titrimetrically. At 20°C, ozone slowly decomposes oxalate in 0.05 M NaOH. In 1 M NaOH, heating at 90°C is required to oxidize oxalate with ozone. Glycolate is readily oxidized with ozone at 20°C in 0.05–1 M NaOH, predominantly into oxalate. Hydrogen peroxide is ineffective reagent for oxalate and glycolate decomposition. Persulfate oxidizes oxalate ion in 0.5–5 M NaOH at 90°C. The reaction of persulfate with glycolate proceeds at 50°C and higher temperatures and is characterized by an induction period, which shortens with increase in concentration of S2O82−, OH−, and temperature, or in the presence of AgNO3 and K4Fe(CN)6. Oxidation involves thermal dissociation of persulfate ions into radical ions followed by a chain reaction.

Kinetics of the radiation chemical reactions of tervalent and quadrivalent actinides and lanthanides in carbonate solutions

The reactions of the ions of tervalent and quadrivalent actinides and lanthanides with hydrated electrons eaq− and CO3− radicals in concentrated carbonate solutions have been studied by microsecond pulse radiolysis, using spectrophotometric recording of short-lived particles. It is shown that the rate of the reactions of eaq− with carbonato complexes of Ce(IV), Pu(IV), and Np(IV) is controlled by diffusion. The carbonato complex of Eu(III) reacts with eaq−appreciably more slowly. A linear relationship is obtained between the logarithm of the rate constant for the reactions of CO3− with the carbonato complexes of Am(III), Ce(III), and Pu(III) and the redox potential of the complexes. The rate of the reaction of CO3− with Np(III) in carbonate solutions is controlled by diffusion.

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.