Kinetics of Pu(V) disproportionation in CH3COOH-CH 3COOLi aqueous solutions

The behavior of Pu(IV-VI) in CH3COOH-CH3COOLi solutions was studied by spectrophotometry. The Pu(VI) absorption spectrum changes essentially with an increase in the CH3COOLi concentration. Owing to formation of Pu(VI) acetate complexes, the maximum of the main absorption band is shifted from 830.6 (in HClO4 solution) to 845 nm, with the band intensity decreasing by a factor of approximately 8. The Pu(V) and Pu(IV) absorption spectra at low concentrations of acetate ions vary insignificantly relative to the spectra in noncomplexing media. With an increase in the acetate concentration in the system to 1-3 mM, the effect of Pu(V) complexation on its absorption spectrum becomes noticeable (the absorption intensity considerably decreases), whereas the Pu(IV) absorption spectra remain essentially unchanged. Solutions containing 1-2 mM Pu(V) and 0.2-0.5 M CH 3COOLi remain unchanged at 18-25°C for 2 days. In solutions with [CH3COOLi] = 1-3 M, Pu(V) disproportionates with the formation of soluble Pu(VI) complexes and a suspension of Pu(IV) hydroxide. Introduction of CH3COOH to a concentration of 0.1-1.0 M prevents the formation of a suspension of Pu(IV) hydroxide, but only up to a temperature of 45°C. The Pu(V) loss follows a second-order rate law, with the reaction products, Pu(IV) and Pu(VI), accelerating the Pu(V) consumption. The reaction rate at a constant concentration of acetate ions is proportional to [H+]. The reaction order with respect to Ac- ions is close to 1.6. The activation energy of the Pu(V) disproportionation in the range 19-45°C is estimated at 74.5 kJ mol-1. It is assumed that the disproportionation mechanism involves the formation of dimers from Pu(V) acetate complexes and aqua ions, their protonation, and decomposition with the transformation into Pu(IV) and Pu(VI). © 2009 Pleiades Publishing, Ltd.