Disproportionation of Pu(V) in aqueous HCOOH solutions

Abstract

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H2O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO4 changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu 2 2+ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO4 solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]2 and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).