Investigation of the Influcence of Humic Acid on the Interaction of U(VI) with Kaolinite: Sorption and Spectroscopic Studies

Humic acids (HA) comprise an important part of natural organic materials. By the formation of soluble complexes with radionuclides, HA can affect the transport of these contaminants. Therefore, the understanding of the impact of HA on the actinide migration in geologic environments is essential for a reliable safety assessment of nuclear waste disposal sites. In the present study we investigated the influence of HA on the U(VI) sorption onto kaolinite. Kaolinite represents a well-defined model substance for clay-rich host formations.
Batch experiments were combined with EXAFS spectroscopy to obtain molecular-level information on the interaction of U(VI) with HA (14C-labeled) and kaolinite (KGa-1b) in natural systems.
From our batch experiments it is known that the sorption of U(VI) on kaolinite is influenced by experimental conditions such as pH, U(VI) concentration, CO2 and HA presence. U(VI) sorption increases with pH up to pH 7.5, then it decreases. In the absence of CO2, no decrease is observed. In the presence of CO2 HA effects 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 because the adsorbed HA forms 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 due to 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.
Previously, EXAFS spectroscopy was applied to study the structure of the U(VI) adsorbed on kaolinite [1], but no EXAFS was measured in the system U(VI)-HA-kaolinite. Therefore, we performed EXAFS measurements with U(VI)-HA-kaolinite sorbats in order to characterize the surface complexes of U(VI) in the presence of HA. Results of the spectroscopic measurements are interpreted and compared with the EXAFS results in the binary system without HA to obtain the information on the influence of HA on the near-neighbor surrounding of U(VI) in the kaolinite surface complexes.

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