Abstract
The behavior of Np(VI) and Np(V) in NaHCO3 and NaHCO3 + Na2CO3 solutions containing H2O2 was studied spectrophotometrically. In 0.75–1.0 M NaHCO3, hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na2CO3 is added. In 1.0 M Na2CO3, 0.5 M NaHCO3 + 0.5 M Na2CO3, or 0.01–0.5 M NaHCO3, hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO3 solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H2O2 yields the excited *Np(V) ion. Then it reacts with another H2O2 molecule and forms a carbonate-peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O22− ligand with formation of two Np(VI) ions.
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