Photoluminescence studies of the uranyl carbonate system

Abstract

An investigation was carried out on the effects of hydrolysis and complex formation on the optical absorption spectra, photoluminescence and excited state lifetimes of various uranyl species formed under different pH conditions and various carbonate to uranyl concentration ratios in aqueous solutions. The hydrolysis increases both the quantum efficiency of luminescence Φ L and the lifetimes τ of the excited states up to a pH of about 6.2; the exact pH depends on the total uranyl concentration in the solution. The effect is attributed to the formation of large polynuclear species with hydroxo bridges, such as (UO 2) 3(OH) 7 −, which introduce a rigidity into the molecular structure of the emitting species and inhibit the collisional quenching due to their large size and high molecular weight. At a pH of greater than 6.2, at a total uranyl concentration of 5 × 10 −4 M, both Φ L and τ decline continuously. This is explained as being due to the transformation of polynuclear species with hydroxo bridges to species with oxo bridges, with a concomitant reduction in the molecular rigidity. Complex formation between uranyl(VI) species and carbonate ion always leads to the quenching of uranyl luminescence. The Φ L value decreases with incresing carbonate to uranyl concentration ratio, so much so that the UO 2(CO 3) 3 4− complex is non-luminescent both in solution and in the solid state. Experimental evidence suggests that the quenching of uranyl luminescence by carbonate takes place by an intramolecular mechanism. At a pH close to 4.8, where the formation of very large polynuclear species such as (UO 2) 11(OH) 12(CO 3) 6 2− has been reported in the literature, kinetic such as (UO 2) 11(OH) 12(CO 3) 6 2– has been reported in the literature, kinetic evidence for the excited state dissociation of higher polynuclear species into smaller fragments is obtained. On dilution, uranyl carbonato complexes show enhanced luminescence which is interpreted as being due to the hydrolytic reactions.