U(VI)-Kaolinite Surface Complexation in Absence and Presence of Humic Acid Studied by TRLFS

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was applied to study the U(VI) surface complexes on kaolinite in presence and absence of humic acid (HA). Two uranyl surface species with fluorescence lifetimes of 5900  700 and 42500  1700 ns and 4400  600 and 30900  3600 ns were identified in the binary (U(VI)-kaolinite) and ternary system (U(VI)-HA-kaolinite), respectively. The fluorescence spectra of adsorbed uranyl surface species were described with six and five fluorescence emission bands in the binary and ternary system, respectively. Positions of peak maxima are shifted significantly to higher wavelengths compared to the free uranyl ion in perchlorate medium. HA has no influence on positions of the fluorescence emission bands. In the binary system, both surface species can be attributed to adsorbed bidentate mononuclear surface complexes, which differ in the number of water molecules in their coordination environment. In the ternary system, U(VI) prefers direct binding on kaolinite rather than via HA, but it is sorbed as a uranyl-humate complex. Thus, the hydration shell of the U(VI) surface complexes is partly displaced with complexed HA, which is simultaneously partly distributed between kaolinite particles. Aluminol binding sites are assumed to control the sorption of U(VI) onto kaolinite.