Oxidation of U(IV) by atmospheric oxygen in pH 1.5-7.4 aqueous solutions

U(IV) oxidation by oxygen in weakly acidic and neutral solutions at pH 1.5-7.4 has been found to proceed by a pseudo-first order reaction with an induction period dependent on the accumulation of U(VI). The presence or the addition of U(VI) accelerates U(IV) oxidation. The rate constants of U(IV) oxidation by air have been determined. The oxidation mechanism involves the interaction of hydrolyzed U(OH)22+ species with O2, HO2, H2O2, and OH, the formation of U(V), and the reaction of U(V) with the listed oxidants. After the accumulation of sufficient U(VI), the reaction U(VI) + U(IV) ⇄ 2 U(V) becomes rate-controlling. The formation of colloidal polymer solution has been observed at pH 1.5-4 during the oxidation of U(IV) by air, during photochemical reduction of U(VI), or by mixing of U(VI) and U(IV) solutions. The colloidal polymer solution has been found to be stable at 20-100 °C under anaerobic conditions. The polymer was isolated from the solution by centrifugation at 5500 × g. Chemical and X-ray photoelectron spectroscopy studies have shown the polymer to be a sparingly soluble mixed U(VI)-U(IV) hydroxide.