PNNL

Abstract

The kinetics of uranium(IV) oxidation by atmospheric oxygen in NaHCO3 solutions has been studied by sampling and analysis. In 1 M NaHCO3 solution with an initial 20 mM U(IV) concentration, an induction period is observed; the induction period is almost absent at an initial (U(IV) concentration of 1.0 mM. The induction period is related to the U(IV) that initially is present as poorly reactive polymer species. Introduction of U(VI) to the initial solution accelerates oxidation. In 1 M NaHCO3 solution containing 0.1 to 1.0 M U(IV), the rate of U(IV) decrease obeys first-order rate law relative U(IV) and O2 concentrations. The pseudo first-order and bimolecular rate constants and the activation energy of U(IV) oxidation by O2 have been determined. Hydrolysis and polymerization of U(IV) have been shown to increase as NaHCO3 concentration decreases from 0.5 to 0.01 M. Based on the experimental results, the reaction is shown to be autocatalytic and proceeds through formation of a [U(IV)•U(VI)] complex with which O2 reacts faster than with U(IV). Uranium(IV) oxidation proceeds through a two-electron charge transfer mechanism.