V. P. Shilov

Development of Alkaline Oxidative Dissolution Methods for Chromium (III) Compounds Present in Hanford Site Tank Sludges

The high-level radioactive waste sludge in the underground storage tanks at the Hanford Site contains various chromium(III)solid phases. Dissolution and removal of chromium from tank waste sludges is desirable prior to high-level waste vitrification because increased volume is required to incorporate the residual chromium. Unfortunately, dissolution of chromium from the sludge to form Cr(OH){sub 4}{sup {minus}} through treatment with heated NaOH solution (also used to dissolve aluminum phases and metathesize phosphates to sodium salts) generally has been unsuccessful in tests with both simulated and genuine Hanford waste sludges. Oxidative dissolution of the Cr(III) compounds to form soluble chromate has been proposed as an alternative chromium solid phase dissolution method and results of limited prior testing have been reported.

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.

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.

A small-angle X-ray scattering study of aqueous solutions of uranium(IV) complexes with lacunar heteropolytungstates

The behavior of aqueous and acid solutions of U(IV) complexes with lacunar heteropolyanions was studied by small-angle X-ray scattering. The polytungstate forms in solutions and in crystals are generally similar. The polyanionic complexes in solutions can exhibit ordering with a period of approximately 5.6–7.8 nm. Such species are probably precursors of the crystalline form.

Tricarbonate complex of hexavalent Am with guanidinium: synthesis and structural characterization of [C(NH2)3]4[AmO2(CO3)3]·2H2O, comparison with [C(NH2)3]4[AnO2(CO3)3] (An=U, Np, Pu)

Abstract A formation of Am(VI) carbonate complexes has been observed under ozonation of Am(III) in guanidinium carbonate solutions. New [C(NH2)3]4[AmO2(CO3)3]·2H2O tricarbonate complex has been isolated and studied by the X-ray analysis. The Am atom has oxygen environment as distorted hexagonal bipyramid with three CO32− ions in equatorial plane and oxygen atoms of the AmO2 group in the apical positions. Averaged Am–OCO3 bond length is 2.431(13) Å, the length of Am=O bond is 1.750(12) Å. In crystal hydrogen bonds between [C(NH2)3]+ cations, water molecules and [AmO2(CO3)3]4− complex anions link the fragments of structure into the three-dimensional network. To compare the peculiarities of the structure of tricarbonate complexes it was prepared [C(NH2)3]4[AnO2(CO3)3], where An = U(VI), Np(VI) and Pu(VI). In structure of [C(NH2)3]4[AnO2(CO3)3] an actinide contraction appears as a slight shortening of the bond lengths in the actinyl groups AnO2: 1.80 Å for U, 1.78 Å for Np 1.76 Å Pu. The An–OCO3 bond length in the equatorial plane of hexagonal bipyramids remains almost unchanged in the series from U to Am. An analysis of the structures showed that hydrogen bonding played an important role in crystal formation.

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.

Study of reactivity of neptunium ions toward oh radicals in perchloric acid solutions by pulse radiolysis method

ConclusionsThe rate constants for the reactions of OH radicals with Np(IV) and Np(V) ions in aqueous perchloric acid solutions are practically independent of the acidity of the medium and are quite high, in order to exclude the recombination of the OH radicals in the presence of these ions.

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.