Reduction of the photoexcited uranyl ion by water - 1. U-IV and H2O2 formation

Photooxidation of water by the uranyl ion was studied. Solutions of uranyl in 0.01-4.0 M H2SO4, HClO4, or 0.1-1.0 M Na2SO4 and NaClO4 containing "lacu nary" heteropolytungstate (HPT) K10P2W17O61 or K8SiW11O39 were irradiated w ith a nitrogen laser, a mercury or xenon lamp, or visible light. Spectropho tometric analysis showed that the irradiation results in the accumulation o f U-IV. Simultaneously the formation of H2O2 proceeds. The quantum yield Ph i of the reaction increases as the concentration of the acid or salt increa ses. For aerated solutions of 1 M H2SO4 or 1 M HClO4, irradiation by light with lambda = 337.1 Phi is close to (1.5-2) . 10(-3). The irradiation of so lutions with pH similar to 4 for many days leads to an almost quantitative transformation of UO22+ into U-IV. When the irradiation was carried out in the absence of HPA, U-IV was not detected, although hydrogen peroxide was o bserved in the solution.