Self-reduction of dioxoplutonium VI: Characterization and modeling of radiolytic and disproportionation pathways

Disproportionation of Pu(IV): A reassessment of kinetic and equilibrium properties

“…Disproportionation of Pu(VI) yields Pu(V) as the only product over a wide pH range [10,11]. Occurrence of that reaction provides a possible explanation for forming Pu(V) directly from Pu(VI), but kinetic analysis does not support that conclusion.…”

“…4) show that Pu(V) formed at a rate greater than 3 · 10 À5 mol l À1 d À1 and that the deficit in [Pu(VI)] did not increase detectably during the four day period after the steady state was reached [6]. Application of kinetic data [10] to the steady-state 0.20 M H + solution with 3 · 10 À4 M [Pu(VI)] gives a disproportionation rate (2 · 10 À6 mol l À1 d À1 ) consistent with a time-insensitive [Pu(VI)] beyond 0.8 day, but unable to account for the relatively rapid formation of Pu(V) at shorter times. The rate of Pu(V) formation tracks the rate of Pu(IV) disproportionation, suggesting that products of the slow step are involved.…”

“…The net reaction does not simply redistribute electrons because charges do not balance. The nature of the reaction and the origin of near-static concentrations identified as the equilibrium state [7] is suggested by comparing the Pu(VI) reaction rate at 4.5 day (8.7 · 10 À6 mol l À1 d À1 ) with that (8.8 · 10 À6 mol l À1 d À1 ) calculated for reduction of Pu(VI) to Pu(V) by disproportion-driven reaction of 0.28 mM Pu(VI) in 1.3 M acid [10]. The Pu(VII) product formed by Pu(VI) disproportionation is instantaneously reduced back to Pu(VI) by reaction with H 2 O and Pu(V) remains as the only observed product.…”

“…Additional impetus for the effort arose from preliminary results showing that Àd[Pu(IV)]/dt is not proportional to [Pu(IV)] 2 as required for a bimolecular disproportionation reaction. In addition to examining kinetic and equilibrium properties of aqueous Pu(IV), this study provides new evidence that the redox chemistry of plutonium solutions is not determined by equilibrium thermodynamics, but by kinetically controlled reactions that depend on formation of hydroxide complexes [10,11,19]. The length of this report reflects the need to show both that prior interpretations are inadequate and that behavior is accurately described by an alternative approach.…”

“…Early studies show that Pu(IV) forms Pu(III), Pu(V), and Pu (VI) in acidic media [4][5][6]. Disproportionation of Pu(V) forms Pu(III) plus Pu(VI) in some cases [1,7] and Pu(IV) plus Pu(VI) in others [1,8,9] with reaction rates varying by more than 10 6 over the pH 0-15 range [10,11]. Recent studies show that disproportionation, not alpha-induced reduction by peroxide, is the primary pathway for Pu(VI) instability over a broad pH range [10,11].…”