The oxidation of uranium(IV) ions by nitrous acid in 30% tri-butyl phosphate

The kinetics of the oxidation of U(IV) ions by nitrous acid in a 30% TBP solution have been determined. The rate equation was found to be: - d[U(IV)] / dt = k2a [U(IV)][HNO2][HNO3][H2O] / [HNO3]2 + β3[HNO3][H2O] + β4[H2O]2, where, k2a = 0.405 ± 0.055 M-1 min-1 at 55 C (β3 ∼ 0.08; β4 ∼ 0.007) and the activation energy was E = 112 ± 17 kJ mol-1. The reaction mechanism appeared to involve interaction with the 1st hydrolysis product of U(IV)-UOH3+. The data is compared with a previous study of the nitric acid oxidation of U(IV) in 30% TBP. This reaction is autocatalytic due to the formation of nitrous acid during the reaction. The kinetics of the decomposition of HNO2 in 30% TBP (in the absence of U(IV)) have also been reported. (orig.)