Mechanisms of Plutonium(III–VI) Redox Reactions in Solutions with pH above 1

The published data were analyzed on the stability of 239 Pu(VI, V, IV) in aerated solutions with pH > 1 up to pH 14 during long-term storage. Under the action of the products of α-radiolysis of water, mainly H 2 O 2 , plutonium (80–85%) converts into Pu(IV) hydroxide. Н 2 О 2 partially oxidizes Pu(IV), which leads to a stationary concentration of Pu(V), while in a 0.1 M NO 3 – solution, to a stationary concentration of Pu(VI). In deaerated solutions of 0.1 M NaCl containing 0.1–0.4 mM 242 Pu(VI), the decrease in Pu(VI) at pH 3.48 is 8%/day instead of the expected 0.1%/day. This fact is attainable with the presence of impurities in the solution. It is likely that impurities arise in the course of water purification from salts. In solutions with pH 7.30 and 9.56, Pu(VI) is stable, which contradicts other publications. In solutions of 1 M ethylenediaminetetraacetic acid (EDTA, H 4 Y) with pH 5.99–9.55, Pu(VI) in a complex with Y 4– transforms into Pu(V) in 6 days. The resulting EDTA fragments convert Pu(V) to Pu(IV) completely in solution with pH 5.99 in 34 days, with pH 8.31 in 110 days, with pH 8.89 by 30% in 110 days, in solution with pH 9.55 Pu(IV) is not generated. Pu(III) in an EDTA solution after an induction period of 110 days is completely oxidized to Pu(IV) by H + ions and water. The oxidation mechanism includes thermal excitation of Pu III Y – · m H 2 O, formation of a dimer from an excited and unexcited Pu III Y – ·m H 2 O ion, and decomposition of the dimer into H 2 , Y 4– , and hydrated PuO 2 .