Reaction of Np(VI) with H2O2 in carbonate solutions

Abstract

The kinetics and stoichiometry of the reaction of Np(VI) with H2O2 in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na2CO3, the reaction 2Np(VI) + H2O2 = 2Np(V) + O2 occurs, as Δ[Np(VI)]/Δ[H2O2] ≈ 2. In Na2CO3 + NaHCO3 solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H2O2 follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H2O2 concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na2CO3 solution containing 1 mM H2O2, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H2O2] = 1 mM, k passes through a minimum at [Na2CO3] = 0.5–0.1 M. The activation energy in a 0.5 M Na2CO3 solution is 48 kJ mol−1.