Surface speciation of uranyl(VI) on gibbsite: A combined spectroscopic approach

Sorption phenomena are important immobilization processes to be considered in the design of nuclear waste repositories. Confidence in respective performance assessments requires a mechanistic understanding of the dominant surface reactions. Highly sensitive and non-invasive techniques such as Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) and Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy are valuable tools to reveal the true surface speciation.
Here TRLFS and EXAFS were applied to study the species of uranyl(VI) adsorbed onto gibbsite particles. This system is important as a model for the characterization of the edge sites of aluminosilicates, relevant as potential host rock and as part of engineered barrier systems.
The experiments were performed with both normal and CO2-free atmosphere, in the pH range 4 to 9, with a total uranium concentration of 10 µM, an ionic strength of 0.01 M (NaClO4), a solid concentration of 4.15 and 12.5 g/L, and using a grain size of 0.2 – 12 µm.
TRLFS at room temperature provided evidence for two adsorbed uranium(VI) surface species. The two species showed similar positions of the fluorescence emission bands and different fluorescence lifetimes indicating a different coordination environment for the two species. The first surface species with the shorter fluorescence lifetimes was assigned to a mononuclear surface complex, where EXAFS indicated (AlO)2UO2 distance of 3.6 Å. The second species with the longer fluorescence lifetimes was attributed to polynuclear surface species supported by a U – U distance of 4.2 Å obtained by EXAFS at cryogenic temperature. Cryogenic TRLFS experiments at 10 K implied the presence of a third surface species. The significant shift of the fluorescence emission bands to shorter wavelength (approximately 16 nm) points to a ternary uranyl carbonato surface species, supported by an EXAFS-derived U – C distance of 2.9 Å. These results will help to formulate more realistic surface reactions on aluminosilicates.