Spectroscopic study of Pu(IV) oxidation to Pu(VI) in 3 mol/dm³ nitric acid at 373 K

Abstract

Optical absorption spectra of Pu(IV) in 3 mol/dm³ (M) nitric acid at 373 K were measured using an optical quartz cell equipped with a thin-film heater. Pu(IV) was oxidized to Pu(VI) by heating at 373 K, and the concentrations of Pu(IV) and Pu(VI) gradually decreased and increased, respectively, with the heating time. The ratio of Pu(IV) oxidized to Pu(VI) was 37% at a heating time of 394 min, and no Pu(III) or Pu(V) characteristic peaks were observed throughout the experiment. The oxidation rate of Pu(IV) in 3 M nitric acid at 373 K could be described by the equation -d [Pu(IV)]_t/d t= k′₁ [Pu(IV)]_t-k′₂ [Pu(VI)]_t, where [Pu(IV)]_t and [Pu(VI)]_t were the concentrations of Pu(IV) and Pu(VI) at a heating time of t min. The apparent rate constants for Pu(IV) oxidation (k′₁) and Pu(VI) reduction (k′₂) at 373 K were calculated to be 2.3 × 10³ min^-1 and 3.4 × 10³ min^-1, respectively.