Investigations of uranyl sorption onto gibbsite

The application of surface complexation models (SCM) towards real-world problems requires not only a reliable parameter database but also information about the chemical structure of surface species and the crystallographic location of binding sites. This study focused on the sorption behavior the uranyl cation UO₂ ²⁺ on aluminol and silanol groups in model substances for clays, namely gibbsite: γ-Al(OH)₃.
Batch sorption experiments were carried out at an ionic strength of 0.1 M NaClO)₄ and at different pH values with a stepping of 0.5 from 3.5 to 9.5 under air. After a contact time of 2 days in the overhead shaker gibbsite was separated by centrifugation and the uranium contend in the separated solution was measured by ICP-MS. The concentration of uranium adsorbed on gibbsite was calculated in accounting for uranium sorbed on the container wall. The sorption curve indicates a maximum sorption between pH 5.5 and 7.5.
For the spectroscopic investigations with TRLIF (Time-resolved laser-induced fluorescence) the gibbsite from the batch experiments was re-suspended in a solution with pH and ionic strength being identical to the original solution. So it was guaranteed, that the fluorescence signals were only caused from uranium sorbed on the gibbsite, and not from uranium in solution. After a second centrifugation the solution was also measured to ensure the absence of dissolved uranyl species.
The TRLIF spectra, excluding the wavelength range between 525 and 540 nm influenced by the laser dispersion peak, were integrated and then fitted to a sum of exponential decay terms. The best approximation for the fluorescence decay gave a bi-exponential decay function yielding two fluorescence decay times: t)₁. in a range between 200 and 450 ns (this indicates the formation of UO)₂.(OH))³⁺) and t)₂. between 3200 and 7900 ns (indicates UO)₂.(OH))₂.).
The peak maxima (at approximately 498, 521, 543 and 557 nm) differ slightly at varying pH, but do not show systematic shifts. The uranium surface species are therefore assumed to be similar throughout the investigated pH range. They should have identical numbers of hydroxyl groups in their first coordination sphere and differ only in the respective water content. This is valid both for the short lived and the long lived species.