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

Abstract

Kinetic results for disproportionation of Pu(VI) and reaction of Pu(III) with Pu(VI) show that rates of 3 Pu 4 + ( aq ) + 2 H 2 O ( l ) = 2 Pu 3 + ( aq ) + PuO 2 2 + ( aq ) + 4 H + ( aq ) in HNO 3 and HClO 4 solutions are described by trimolecular rate laws consistent with involvement of trimeric hydroxo complexes as reactive intermediates in the slow mechanistic steps. Product ratios and modeling of concentration–time curves reveal that Pu(V) is formed by reduction of Pu(VI) product in a secondary reaction. Results do not support the accepted interpretation that attributes reversible reaction and Pu(V) formation to a two-step bimolecular process. Secondary redox reactions driven by disproportionation of Pu(VI) prevent attainment of equilibrium in 1 M H + and determine long-term redox chemistry. The equilibrium constant (0.00051) defined by forward and reverse rate constants for 1 M HClO 4 agrees with that (0.00049) derived from concentration data for 1 M HNO 3, but not with prior results. Disagreement of these values with that calculated from thermodynamic data suggests that steady-state Pu concentrations are controlled by kinetics. Possible pathways of secondary reactions are identified and a mechanism for reversible oxygenation of plutonium ions is described.