Uranyl Surface Speciation on Silica Particles Studied by Time-Resolved Laser-Induced Fluorescence Spectroscopy

The sorption of uranyl ions onto amorphous silica has been studied in the presence of atmospheric CO₂ by laser-induced time-resolved fluorescence spectroscopy at trace concentrations (1.0 and 0.1 M). Two fluorescent uranyl surface complexes have been identified in the pH range 4 to 9. Both complexes could be differentiated by lifetimes (170±25 s at low pH and 360±50 s at high pH) and fluorescence emission spectra. Within the constant capacitance model framework they are described by mononuclear (1 : 1) complexes with release of two and three protons, respectively. When fluorescence data were compared to wet chemistry sorption data, a third “silent” ternary uranyl–silica–carbonate surface complex had to be postulated to account partly for adsorption between pH 8.0 and 9.0. Three independent data sets led therefore to the identification of three surface complexes, postulated as &tbond;SiO₂UO₂°,&tbond;SiO₂UO₂OH^-, and &tbond;SiO₂UO₂OHCO₃ ^3-.