Study of the Influence of Humic Acid on the U(VI) Sorption onto Kaolinite

Humic acids (HA) comprise an important part of natural organic materials. Due to their interactions with organic and inorganic pollutants produced by human activity, they play a significant role in biochemical cycles of ecosystems. By the formation of soluble anionic complexes with the range of more or less toxic metals, including radionuclides, HA can affect the transport of these contaminants. Therefore, the understanding of their impact on the actinide migration in geologic environments is essential for a reliable safety assessment of nuclear waste disposal sites. In the present study the influence of HA on the U(VI) sorption onto kaolinite was studied. 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 HA) and kaolinite (KGa-1b).
Three kinds of batch experiments were performed under different experimental conditions: HA sorption in the absence of U(VI), U(VI) sorption in the absence and U(VI) sorption in the presence of HA. The sorption curves have shown that the U(VI) sorption 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 8.5, 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 (Fig.1).
We performed EXAFS measurements with U(VI)-HA-kaolinite sorbates 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 [1] to obtain 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.