M. V. Nikonov

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.

Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

Formation of peroxy compounds of Pu(IV) is possible in concentrated alkali solutions despite instability of hydrogen peroxide under these conditions. The resulting peroxy compound of Pu(IV) is fairly stable in alkaline solutions. The [Pu]: [O2−] ratio in the compound is close to 1: 2. The formation of the peroxy compound favors a decrease in the content of colloidal polymeric Pu(IV) species in solution, thus making the Pu precipitation from solution more complete.

On existence and properties of plutonium(VIII) derivatives

Abstract The existence of Pu(VIII) was shown in alkaline solutions and in nonpolar solvents (CCl4 and CHCl3) on the ground of such experimental facts like extraction of plutonium species, obtained by means of ozonization of Pu(VI) alkaline solutions into CCl4 and CHCl3; volatility of Pu compounds out of aqueous alkaline solutions, and the mentioned solvents. The product of interaction between PuO3 · 0.8H2O and XeF2 in CCl4 and CHCl3 was of the same chemical form for both cases and identical with the form of the product of volatility of Pu out of these organic solutions. The electronic absorbance spectra of these products in CCl4 and CHCl3 were fairly similar to the ones for OsO4 and RuO4. The band of absorption in these spectra with the maximum at 332 nm corresponds to the charge transfer of metal-ligand type [π(O)→Pu], as the value of molar extinction coefficient ε was found to be higher than 2 · 103 L/mole cm. A satisfactory agreement between the energies for the transitions of PuO4 molecule designed by Xα-SW method and experimental data was observed. A similar accordance was observed for the designed vibration frequency (DFT-method) with the one found as result of analysis of the vibration structure of the experimental spectra.

A spectrophotometric study of the reduction of Pu(VIII) and Pu(VII) in alkaline solutions

The reduction of Pu(VIII) with water in ozonized 15 and 6 MNaOH solutions after termination of the ozone bubbling was studied by spectrophotometry. Kinetic curves reflecting the whole set of processes that occur in NaOH solutions of different concentrations were obtained. Calculations performed with the experimental curves made it possible to construct the kinetic curves describing the behavior of plutonium in higher oxidation states in solution and to obtain the individual absorption spectra of Pu(VII) and Pu(VIII). The processes occurring in ozonized solutions of Pu(VI) in 6 and 15 M NaOH after termination of the ozone bubbling were found to occur by different mechanisms, yielding in 4 days Pu(VI) and Pu(V), respectively.

Americium volatility from ozonized alkaline solutions

The Am volatility from 4 M NaOH solutions in the course of their ozonation at 80°C was studied. The formation of volatile Am compounds under these conditions was proved by α-ray spectrometry. The whole set of data on the volatility and kinetics of reduction of ozonized Am solutions suggest possible formation of short-lived hydroxo complexes of Am in higher oxidation state, including Am(VIII), along with Am(VI) hydroxo complexes, with AmO4 possibly existing in equilibrium with these Am(VIII) species. No Cm volatility was detected under the same conditions. The observed differences in the behavior of Pu and Am, on the one hand, and Np and Cm, on the other hand, under identical ozonation conditions are attributed to the ability of Pu and Am to be oxidized under these conditions to unstable octavalent state, responsible for their volatility.

Disproportionation of Pu(VI) and reproportionation of Pu(V), Pu(VII) in aqueous alkali solutions

Disproportionation of Pu(VI) and reproportionation of Pu(V) and Pu(VII) in aqueous NaOH solutions was studied. With an increase in the NaOH concentration in solution over 7.5 M, the equilibrium of the reaction Pu(VII) + Pu(V)⇄2Pu(VI) is gradually shifted toward formation of Pu(V) and Pu(VII) as products of Pu(VI) disproportionation, and at [NaOH] + 13 M, Pu(VI) disproportionates virtually completely. At [NaOH] + 7.5 M, the equilibrium of the above reaction is shifted toward formation of Pu(VI). Based on the experimental data, the equilibrium constants of the reaction at various alkali concentrations in the solution and the formal potentials ?f[Pu(VII)/Pu(VI)] were calculated. The data obtained showed that, with respect to reduction with water, Pu(VII) is stable in aqueous alkali solutions at NaOH concentrations exceeding 7.5 M.