Oxidation of Uranium(IV) with Oxygen in NaHCO3 Solution

The kinetics of U(IV) oxidation with atmospheric oxygen in NaHCO3 solutions was studied by spectrophotometry. In 1 M NaHCO3 at [U(IV)](0) = 20 mM, an induction period is observed, which virtually disappears with decreasing [U(IV)](0) to 1.0 mM. The induction period is caused by the fact that initially U(IV) exists in a weakly active polymeric form. Addition of U(VI) to the initial solution accelerates the oxidation. In a 1 M NaHCO3 solution containing 0.1-1.0 mM U(IV), the U(IV) loss follows the first-order rate law with respect to U(IV) and O-2. The pseudo-first-order rate constants, bimolecular rate constants, and activation energy of the U(IV) oxidation were calculated. In dilute NaHCO3 solutions (0.5-0.01 M), the hydrolysis and polymerization of U(IV) become more pronounced. The autocatalysis mechanism presumably involves formation of a complex [U(IV)center dot U(VI)] with which O-2 reacts faster than with U(IV). Oxidation of U(IV) occurs by the two-electron charge-transfer mechanism.