Interaction of uranium(VI) with silicic acid in aqueous solutions studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

We investigated for the first time the complex formation in the uranyl-silicate system by fluorescence spectroscopy. The interaction between soluble species of uranium(VI) with silicic acid was studied in 0.3 M NaClO4 solution using time-resolved laser-induced fluorescence spectroscopy (TRLFS). Tetramethylorthosilicate (TMOS) was used as source of silicic acid in order to minimize the formation of polymeric silicon species. Fluorescence lifetimes were obtained for (a) the free uranyl cation of 1.7+/-0.3 mu s, and (b) the 1:I silicato-uranyl complex of 19.0+/-4 mu s. The main fluorescence bands of the complex UO2OSi(OH)(3)(+) are centered at 500, 521, 544 and 570 nm. The stability constant of the complex UO2OSi(OH)(3)(+) was determined at pH 3.9, as log beta degrees = -(1.67 +/- 0.20) using an uranyl concentration of 2.3 x 10(-5) M, and a monosilicic acid concentration up to 5.4 x 10(-3) M. (C) 1998 Elsevier Science S.A.