Interaction of Neptunium(V) with Humic Acids of Different Functionalities

In order to improve models used for the predictive modeling of the actinide migration in natu-ral aquifer systems it is necessary to get more precise information about the interaction process between humic acids and actinide ions. This includes for instance a more profound knowledge about humic acid binding sites interacting with actinide ions at different pH values. In addition, natural humic acids contain small amounts of inorganic components, e.g., iron, that cannot be removed by usual purification methods. It is not known whether or not these components may influence the interaction between humic acids and actinide ions.
Applying humic acid model substances and natural humic acids with different functionalities we continue our studies concerning the interaction between humic acids and actinides [e.g., 1,2]. In order to specify humic acid binding sites interacting with actinide ions in the neutral pH range, we study the influence of phenolic OH and other acidic OH groups on the interaction process. We investigate the neptunium(V) complexation in neutral solution by near infra-red absorption spectroscopy and X-ray absorption spectroscopy applying modified synthetic and natural humic acids with blocked phenolic and other blocked acidic OH groups. The influence of inorganic humic acid components on the actinide complexation by humic acids is studied using an "ultra-pure" synthetic humic acid model substance. The neptunium(V) com-plexation of this humic acid is compared to the complexation behavior of natural and synthetic humic acids having small amounts of non-removable inorganic components.

[1] Pompe, S. et al.: Determination and Comparison of Uranyl Complexation Constants with Natural and Model Humic Acids. Radiochim. Acta 82, 89 (1998).
[2] Pompe, S. et al.: Investigation of Humic Acid Complexation Behavior with Uranyl Ions Using Modified Synthetic and Natural Humic Acids. Radiochim. Acta 88, 553 (2000).