Influence of humic acids on the actinide migration in the environment: Suitable humic acid model substances and their application

Humic acids (HA) are soluble in the pH range of natural water, possess the ability for complex and colloid formation and show redox properties that can impact the redox state of metal ions. Therefore, HA can influence the speciation of metal ions, e.g., actinides, and thus, their migration in the environment. Knowledge of the impact of HA on the actinide migration is required for the long-term risk assessment for potential nuclear waste repositories in deep geological environments. The complex nature of HA complicates the description of their interaction with metal ions. Thus, there are a lot of difficulties in the thermodynamic description of the geochemical interaction processes of HA. A more basic understanding of the interaction processes of HA can be obtained by model investigations with model ligands representing building blocks of HA as well as with specific HA model substances. These should be characterized by similar operational and chemical properties, a more defined functionality, and a higher homogeneity compared to natural HA. Furthermore, they should offer the possibility for specific variations of functional and structural properties and for the stable isotopic labeling (e.g., 13C, 14C, 15N) of the HA structure. Model substances which achieve these requirements can be applied to study the interaction behavior of HA with metal ions in detail and to study the fate of HA in sorption and migration experiments. This work focuses on selected types of HA model substances, their synthesis and characterization in comparison to natural HA. Their use in geochemical studies will be shortly discussed.