Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Abstract

Published data on reactions of Np ions with O2, H2O2, HNO2, and HNO3 in solutions of various compositions in a wide pH range are considered. O2 oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. H2O2 exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75–1 M NaHCO3 it oxidizes Np(V) to Np(VI), whereas in dilute HClO4 and HNO3 and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO3 solutions, H2O2 converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates H2O2 (in addition to the H2O2 arising as primary product of water radiolysis). In HNO3 solutions, oxidation of Np ions occurs only in the presence of HNO2 arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO2−, NO2, and NO+ present in equilibrium with HNO2.