Uranyl Sorption Onto Gibbsite Studied by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS)

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was combined with batch experiments to study the sorption of uranium(VI) onto gibbsite (γ-Al(OH)₃). The experiments were performed under ambient conditions in 0.1 M NaClO₄ solution in the pH range from 5.0 to 8.5 using a total uranium concentration of 1 x 10-5 M, and a solid concentration of 0.5 g/40 mL. Two uranyl surface species with fluorescence lifetimes of 330 ± 115 ns and 5600 ± 1640 ns, respectively, were identified. The first species was dominating the more acid pH region whereas the second one became gradually more prominent towards higher pH values. The fluorescence spectra of both adsorbed uranyl(VI) surface species were described with six characteristic fluorescence emission bands situated at 479.5 ± 1.1, 497.4 ± 0.8, 518.7 ± 1.0, 541.6 ± 0.7, 563.9 ± 1.2, and 585.8 ± 2.1 nm. The surface species with the short-lived fluorescence lifetime of 330 ns is attributed to a bidentate mononuclear inner-sphere surface complex in which the uranyl(VI) is bound to two reactive OH- groups at the broken edge linked to one Al. The second surface species with the significant longer fluorescence lifetime of 5600 ns was attributed to small sorbed clusters of polynuclear uranyl(VI) surface species. The longer fluorescence lifetime of the long-lived uranyl surface species at pH 8.5 is explained with the growing average size of the adsorbed polynuclear uranyl surface species.