Luminescence Properties of U(VI) Surface Species on Amorphous Silica

U(VI) (hexavalent uranium) in an aqueous solution is easily adsorbed onto solid surface under near neutral pH condition. Thus, the interactions of U(VI) with a variety of minerals have been widely studied to understand its migration behavior in groundwater system. The sorption characteristics of U(VI) onto SiO2 surfaces were investigated at a molecular scale using several spectroscopic techniques, such as X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy, and time-resolved laser fluorescence spectroscopy (TRLFS). Among these techniques, TRLFS has received considerable attention because of its superior speciation sensitivity. A lot of data on the luminescence characteristics (peak wavelengths in the spectrum and lifetimes of excited state) of U(VI) species adsorbed onto SiO2 surface has been accumulated in literatures. However, the values of luminescence lifetimes of U(VI) surface species obtained by different groups differ significantly. Until now, the reason for these inconsistent luminescence lifetime values had not been clarified. The objective of the present TRLFS study is to understand the origin of the inconsistent luminescence lifetime values of U(VI) species on SiO2 surface reported in previous studies. We focus on the luminescence lifetimes of the U(VI) surface species which depend on the specific surface area (SSA) of SiO2 samples and the incident laser pulse energy at UV wavelength. Other experimental parameters (aging effect of the sample and several instrumental parameters) are also examined. 2. Experimental Conditions