Humic acids (HA) can influence the speciation of metal ions, e.g., actinide ions, and thus their migration in the environment.
Therefore, knowledge of the impact of HA on the actinide migration is required to assess their transport in natural systems.
However, due to the complex and heterogeneous nature of HA, there are a lot of difficulties in the thermodynamic description
of their geochemical interaction behavior. A more basic understanding of the interaction processes of HA can be obtained by
investigations applying HA model substances with more specific and tailored properties. This work gives a review of selected
types of HA model substances (HA-alike melanoidins, synthetic HA with pronounced redox functionality, modified HA with blocked
phenolic/acidic OH groups, synthetic humic substance-clay-associates), their synthesis, isotopic labeling, and characterization
in comparison to isolated natural HA. Examples for their application in various geochemical studies, such as complexation,
redox, sorption and migration studies with uranium as representative for actinides are presented.