Behavior of neptunium ions in organic media

Abstract

Published data on the effect of organic solvents on the hydrolysis of Np(IV) and redox reactions of Np(IV–VI) are analyzed. In aqueous-organic solutions, Np(IV) ions undergo hydrolysis at higher acidity than in aqueous solutions. With respect to the effect on hydrolysis, the solvents can be ranked in the order methanol > ethanol > dioxane > acetone > acetonitrile. Dimethyl sulfoxide suppresses the hydrolysis. The Np(V) disproportionation in CH3OH and CH3OH + C6H6 solutions in the presence of HCl, HDEHP, or TTA and in a TBP solution was studied. The influence of the solution composition, including the H2O concentration, on the reaction kinetics was examined. The reactions occur faster than in aqueous solutions. The reaction mechanism is the same in all the media: Two solvated Np(V) ions form a complex decomposing upon protonation into Np(IV) and Np(VI). The role of the solvent in the Np(V) reproportionation was examined. In mixed water-ethylene glycol, water-methanol, and water-acetone solvents, with an increase in the fraction of the organic component, the Np(IV) + Np(VI) reaction rate passes through a maximum, which is due to combined effect of two factors: Np(IV) hydrolysis (acceleration) and decrease in [H2O] (deceleration). In TBP solutions, the Np(IV) + Np(VI) reaction decelerates in proportion to [HNO3]−2 and [H2O]. The course of the Np(VI) + H2O2 and Np(IV–VI) + HNO2 reactions in TBP differs from that in aqueous solutions. Deceleration of the Np(VI) reduction and acceleration of the Np(V) oxidation, compared to aqueous solutions, are associated with a decrease in the formal potential of the Np(VI)/(V) couple in going from H2O to TBP. In solutions of KOH in aqueous methanol, Np(VI) rapidly disproportionates to Np(VII) and Np(V). A decrease in the H2O concentration shifts the equilibrium toward Np(VII).