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

Oxidation of Np(IV) with hydrogen peroxide in NaHCO3-Na 2CO3 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 Na 2CO3 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′/[H 2O2] in solutions containing up to 11 mM H 2O2 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 H 2O2 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. © 2013 Pleiades Publishing, Ltd.