Adsorption of uranyl carbonate species onto alumina, silica, and kaolinite

Abstract

Aqueous uranyl species coordinating carbonate ligands such as $UO_2(CO_3)_3^{4-}$, $CaUO_2(CO_3)_3^{2-}$, and $Ca_2UO_2(CO_3)_3(aq)$ are predicted to predominant and mobile species in the aquifer of the deep geological repository for nuclear spent fuel. To investigate into the effect of these uranyl carbonate species to U(VI) adsorption, Time-resolved laser fluorescence spectroscopy (TRLFS) was applied to U(VI) surface speciation on alumina, silica, and kaolinite. In prevailing carbonate conditions, the fluorescence spectra of adsorbed U(VI) species onto alumina with blue-shifted emission bands were observed. The deviation to low wavelength is considered as carbonate-like feature found in aqueous uranyl carbonate species and uranyl carbonate minerals that have at least two carbonate ligands as well. The carbonate-like spectral feature indicates uranyl carbonate surface complexation onto alumina. Additionally, the carbonate-like spectral characteristic was found in the aqueous systems where ternary calcium uranyl carbonate species predominantly exist. On the other hand, U(VI)-hydroxo surface complexes on silica and kaolinite surfaces were characterized in spite of the strong abundance of uranyl tricarbonate moiety. In processes of adsorption on both silica and kaolinite, the carbonate ligands may be detached from uranium(VI). Using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, the two absorption peaks were observed and the spacing between two peaks indicates that the carbonate ligands complex with uranyl ion in a bidentate-fashion. According to our findings and previous studies, the uranyl carbonate surface complexation tends to be favorable on the surfaces having high point of zero charge $(pH_{pzc})$ such as aluminum and iron oxides. Regarding U(VI) retention in aquifer, the results in this work may provide new insight and comprehensive interpretation of U(VI) adsorption in groundwater system containing abundant carbonate ions.