Influence of Humic Acid on U(VI) Sorption Onto Kaolinite Studied by TRLFS and EXAFS

Understanding the migration behavior of actinides is important for the reliable long-term risk assessment of potential nuclear waste repositories. Depending on geochemical conditions different materials and processes can influence the behavior of such pollutants in natural aquifer systems. Humic acids (HA) comprise an important part of natural organic materials. HA are soluble in the pH range of natural waters and have the ability for complex and colloid formation. Due to these properties HA can affect the speciation of actinide ions, and therefore, their migration in the environment. In the present study we investigated the influence of HA on the U(VI) sorption onto kaolinite.
Results from our previous batch experiments were combined with spectroscopic measurements to obtain molecular-level information on the interaction of U(VI) with HA and kaolinite (KGa-1b) in natural systems.
From our batch experiments resulted that the sorption of U(VI) on kaolinite is influenced by experimental conditions such as pH, U(VI) concentration, CO2 presence and HA concentration. HA affects U(VI) adsorption onto kaolinite over the entire pH range. At pH < 5 the presence of HA enhances the U(VI) uptake relative to the HA-free system due to the formation of additional binding sites for U(VI). In the pH range between pH 5 and pH 8.5 the U(VI) sorption decreases in the presence of HA because of desorption of HA from the kaolinite surface resulting in the formation of dissolved U(VI)-HA complexes. At pH > 8.5 uranium sorption increases again relative to the HA-free system. Possibly, ternary U(VI)-humate complexes, e.g. uranyl-carbonato-humate complexes, are formed, which can interact with the kaolinite surface and therefore, enhance the U(VI) sorption onto kaolinite in the presence of HA in this pH region.
We used time-resolved laser-induced fluorescence spectroscopy (TRLFS) in order to characterize the species of U(VI) adsorbed onto kaolinite in the absence and presence of HA. Two adsorbed U(VI) surface species on kaolinite were identified in the pH range of pH 5.0 to pH 8.5. No EXAFS was measured in the system U(VI)-HA-kaolinite, therefore we performed also EXAFS measurements under different experimental conditions with U(VI)-HA-kaolinite sorbates in order to improve the knowledge about the surface complexes of U(VI). Results of the spectroscopic measurements are interpreted and compared with the results in the binary system without HA [1] to study the influence of HA on the near-neighbor surrounding of U(VI) in the kaolinite surface complexes. Obtained results help to improve the understanding of the geochemical interactions of hexavalent actinides in environment.

[1] Amayri et al.: EXAFS - Untersuchungen zur U(VI) - Sorption an Kaolinit, presentation on BMWA Project Meeting, Saarbrücken, May 2004.