Study on the Species Distributions of Dissolved U(VI) and Adsorbed U(VI) on Silica Surface

Dissolved hexavalent uranium can exist in the form of several different chemical species. Furthermore, species distributions depend on the pH value of the aqueous solution. Representatively, UO2 2+, UO2OH+, (UO2)2(OH)2 2+, and (UO2)3(OH)5 + species coexist in solutions at acidic and circumneutral pH values. When amorphous silica particles are suspended in an aqueous solution, the dissolved chemical species are easily adsorbed onto silica surfaces. In this study, it was examined whether the species distribution of the adsorbed U(VI) on a silica surface followed that of the dissolved U(VI) in an aqueous solution. Time-resolved luminescence spectra of three different dissolved species (UO2 2+, UO2OH+, and (UO2)3(OH)5 +) and two different adsorbed species (≡SiO2UO2, ≡SiO2(UO2)OH–, or ≡SiO2(UO2)3(OH)5–) were measured in the pH range 3.5−7.5. The spectral shapes of these chemical species were compared by changing the pH value; consequently, it was confirmed that the species distribution of the adsorbed U(VI) species was different from that of the dissolved U(VI) species.