Oxidation of Np(IV) with hydrogen peroxide in carbonate solutions

Abstract

Oxidation of Np(IV) with hydrogen peroxide in NaHCO3-Na2CO3 solutions was studied by spectrophotometry. In NaHCO3 solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na2CO3 forms with H2O2 a mixed peroxide-carbonate complex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k’ is proportional to [H2O2] in the H2O2 concentration range 2.5–11 mM, but further increase in [H2O2] leads to a decrease in k′. The bimolecular rate constant k = k′/[H2O2] in solutions containing up to 11 mM H2O2 increases in going from 1 M NaHCO3 to 1 M Na2CO3 and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide-carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of H2O2 oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and H2O2 concentrations.