Mechanism of Np(VI) oxidation with ozone in alkaline solutions

Bubbling of an ozone-oxygen mixture containing 0.1–0.5 vol % O3 at a rate of 15–20 l h−1 through 13 ml of a 2 × 10−5−1 × 10−4 M solution of Np(VI) in 0.1 and 1 M LiOH leads to the formation of Np(VII). The initial rate increases approximately in proportion to [Np(VI)] and [O3gas]0.5. Up to 80% of Np(VI) is oxidized at maximum. At the O3 concentration in the gas phase increased to 1–4 vol %, Np(VI) is oxidized completely. Under the same conditions, Np(VI) in a concentration of (1–5) × 10−3 M is oxidized to almost 100%. Analysis of published data and additional experiments on the reaction of O3 with Np(VI) ions in LiOH solutions allow a conclusion that the ozonation involves the reactions O3 + OH− = HO2− + O2, O3 + HO2− + OH− = O3− + O2− + H2O, and O3 + O2− = O3− + O2, followed by O3− + NpO2(OH)42− = O2 + NpO4(OH)23− + H2O. In addition, HO2− reduces Np(VII) and Np(VI) and reacts with O3−. Certain contribution is made by the reaction Np(VI) + O3 = Np(VII) + O3−. The dependence of the Np(VII) accumulation rate on [O3gas]0.5 was interpreted in terms of the concept of a heterogeneous-catalytic process.