A flash photolytic study of the photo-oxidation of some inorganic anions by the uranyl ion

Abstract

Transient species produced on photolysis of aqueous solutions of UO 2+ 2 and some inorganic anions have been studied by flash photolysis. With I −, CNS − and Br − absorptions are observed which are assigned to the X?? 2 radical anions formed by one-electron photo-oxidation by uranyl followed by reaction with the corresponding anion. Both intra- and inter-molecular mechanisms are suggested for this photo-oxidation. Decay of the I?? 2 radical anion is second-order in radical anion, but the observed rate constant also shows a dependence on uranyl ion concentration. It is suggested that I?? 2 may complex with UO 2+ 2. With (CNS)?? 2 and Br?? 2 the kinetic decay is found to be first-order in radical anion and to show an apparent dependence on uranyl ion concentration. Thermodynamic analysis suggests that the apparent dependence in UO 2+ 2 concentration is actually a dependence in UO + 2 (the reduction product of uranyl ion) and that the radical anion decays by oxidizing uranium (V) back to uranium (VI). The difference between these and iodide provides a ready explanation for why photolysis of UO 2+ 2 with Br − or CNS − shows no overall reaction, whereas the uranyl—iodide system leads to formation of stable photo-products. Flash photolysis of an aqueous solution of uranyl perchlorate and sodium bicarbonate yields a broad absorption assigned to CO − 3, formed by one-electron oxidation of bicarbonate. However, the result was somewhat irreproducible, suggesting this system may be sensitive to the extent and type of complexation.