„TRLFS fingerprinting for uranyl(VI) surface species on clay model substances“

The U(VI) binding to complex systems such as clays, which play an important role in nuclear waste disposal design, is still to be clarified. An efficient tool to study the relevant speciation in-situ is required, thus TRLFS (time-resolved laser-induced fluorescence spectroscopy) was applied. Silica gel (providing higher binding site concentrations as quartz) and gibbsite were used as model substances for the investigation of silianol and aluminol binding sites, respectively, in clays. Kaolinite, offering both types of surface groups, served as a further model system.

All measurements were using 10 mg silica gel in 40 ml 0.1 M NaClO4 and a uranium concentration of 5•10-6 mol/l. The pH was varied between 4.5 and 9. For the TRLFS measurements the solid phase was centrifuged and resuspended with a NaClO4 solution with pH and ionic strength identical to the original but without uranium. A Nd:YAG diode laser with an excitation wavelength of 266 nm was used to study the sorbed species. Spectra were recorded by a diode array in the wavelength range between 460 and 620 nm. The delay times covered 30 ns up to 200 µs. For further details of the experimental set-up see [1]. All measurements were performed under oxygen atmosphere and at room temperature.

For silica gel, at least three U(VI) surface complexes with fluorescence decay constants  of 47 µs, 185 µs and 299 µs were observable. Peak maxima were red-shifted by 10-16 nm compared to the free uranyl cation. Concerning gibbsite, two species with fluorescence lifetimes of 2.4 and 13.2 µs were present throughout the whole pH range, with a third surface species occurring at pH > 7 ( = 51 µs). Finally, the investigation with kaolinite yielded two surface species with lifetimes of 5.9 and 42.5 µs. For pH values below 7 this is closer to the gibbsite results, indicating that aluminol sites dominate uranyl sorption onto alumosilicates.

[1] Sachs, S. et al. (2007) Radiochim. Acta 95, 103-110.