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 muM). Two fluorescent uranyl surface complexes have been identified in the pH range 4 to 9. Both complexes could be differentiated by lifetimes (170 +/- 25 mus at low pH and 360 +/- 50 mus 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 drop SiO(2)UO(2)degrees, drop SiO2UO2OH-, and drop SiO2UO2OHCO33-.