Solubility and redox reactions of Pu(IV) hydrous oxide: Evidence for the formation of PuO_2+x(s, hyd)

The solubility and redox reactions of Pu(IV) hydrous oxide were analyzed by comparing total Pu concentrations, oxidation state distributions and simultaneously measured redox potentials under air and under Ar containing only traces of oxygen. At pH>3 the aqueous Pu concentration is dominated by Pu(V) for both solubility studies under air and argon. Combining all information strongly indicates that PuO_2+x (s,hyd), mixed valent (Pu^V)_2x (Pu^IV)_1-2x O_2+x(s,hyd) or (PuO_2.5)_2x (PuO₂)_1-2x (s,hyd) with x = 0.003 (present study) and x = 0.05–0.06 (literature studies under air), is the solubility controlling solid phase. It can be formed by the oxidation of PuO₂(s,hyd) with the oxygen in the system and by co-precipitation of Pu(V) and Pu(IV). The Pu⁴⁺ concentration is given by the known solubility product of Pu(IV) hydrous oxide and the PuO₂⁺ concentration is described by the solubility product (K_sp = [PuO₂⁺][OH^-]) for the fraction of Pu(V) in PuO_2+x(s,hyd): logK°_sp = −14.0±0.8 at zero ionic strength. Small Pu(IV) colloids/polymers present in neutral to alkaline solutions at a constant level of log[Pu(IV)]_coll = −8.3±1.0 play an important role for the redox potentials in these systems. Similar Pu(V) concentrations and redox potentials were reached from oversaturation in initially Pu(VI) solutions.