Uranium(VI) Adsorption onto Synthetic Humic Substance-Kaolinite Sorbates

The reliable risk assessment of future nuclear waste repositories requires knowledge on the actinide migration behavior in potential host rock formations. Clay formations as possible host rocks are closely associated with natural organic matter, e.g., humic substances, which are known as efficient complexing agents for metal ions. Therefore, the immobilization as well as the release of humic substances by clays can affect the mobilization of actinide ions in the environment.
The impact of humic acid on the U(VI) sorption onto kaolinite was investigated in the ternary model system U(VI) – kaolinite - synthetic humic acid [1]. In the present work, the U(VI) sorption onto a synthetic kaolinite sorbate with humic substances is studied in order to approach natural conditions. For that, we synthesized humic acid M42 [2] in presence of Georgia kaolinite KGa-1b. As synthesis product we obtained a synthetic humic substance - kaolinite sorbate (M42-KS). Due to the fact that this humic substance - kaolinite sorbate was formed during the humic acid synthesis, a different composition of the humic material and different interactions between the humic substances and the kaolinite can be expected compared to the model system investigated in [1]. However, these conditions are probably closer to those found in natural clay systems.
The organic carbon content of two synthetic humic substance - kaolinite sorbates was determined with 5 and 24 mg C/g representing about 9 and 44 mg humic substances/g, respectively. To quantify the influence of the humic material associated with kaolinite we studied the U(VI) sorption onto M42-KS ([U(VI)]: 1· 10-6 M, S/L: 4 g/L, I: 0.01 M NaClO4) in the pH range between pH 3.0 and pH 10.0 by batch experiments. Compared to the humic substance-free system the U(VI) adsorption onto M42-KS is significantly increased between pH 3 and pH 5 which is ascribed to the association of humic substances with kaolinite. Up to pH 8 the U(VI) sorption onto M42-KS is reduced in contrast to the system without humic materials. Under these conditions M42-KS releases humic acid resulting in the formation of soluble U(VI) humate complexes. A further reduction of the U(VI) sorption was observed between pH 8 and 10. This is attributed to the formation of soluble U(VI) humate complexes and/or the formation of U(VI) carbonato complexes that dominate the U(VI) sorption in absence of humic material under these conditions. Differences were found for the U(VI) sorption onto humic substances - kaolinite sorbates with various contents on organic carbon. In addition to that, the U(VI) sorption onto M42-KS differs from that observed in the ternary model system U(VI) – kaolinite - humic acid M42. Probably, these dissimilarities are due to a different composition of the humic materials resulting in different functionalities of the humic substances. These can effect a varying interaction behavior in the system U(VI) - humic substance – kaolinite. Our results indicate that the release of humic materials from natural clays can play an important role for the mobilization of U(VI).

[1] Křepelova, A. et al., In: Forschungszentrum Rossendorf, Wissenschaftlich-Technische Berichte, FZR-419, Rossendorf 2005, p. 50.
[2] Pompe, S. et al., Radiochim. Acta 74, 135 (1996).