Specific Humic Acid Model Substances for the Study of the Redox Behavior of Humic Acids in the Environment

The mobility of actinides in the environment is determined by their speciation which is strongly influenced by their oxidation states. Humic acids (HA) influences the speciation of actinides by complexation. However, they have also significant redox properties which can affect the oxidation states of actinides and consequently their migration in the environment. Thus, it is very important to understand both the complexation and the redox properties of HA in natural systems.
In order to study the redox behavior of HA in more detail we synthesized HA model substances with distinct redox functionalities based on the oxidation of diphenolic compounds in the absence and presence of amino acids. These humic acid-like model substances are characterized by elemental compositions and structural elements that are comparable to natural HA. However, they show phenolic/acidic OH group contents that are higher than those of natural HA. For characterization of the redox properties of these synthetic products, their Fe(III) and ferricyanide redox capacities (RC) were determined at pH 3 and pH 9.2, respectively, and compared to that of purified natural HA from Aldrich. The synthetic products show RC that are up to a factor of 11 and 4 higher than that of Aldrich HA at pH 3 and pH 9.2, respectively. Additionally, we modified, i.e., etherified the phenolic OH groups of these HA by methylation with diazomethane. Applying these modified HA we verified the dominating role of phenolic OH groups in the redox behavior of these HA.
The synthesized HA model substances with distinct redox functionalities are applied to further improve the knowledge on the impact of HA on the redox stability of actinides in the environment. We study the redox stability of uranium(VI) complexes of these HA depending on pH. Uranium(IV) that is formed due to the redox process is spectroscopically determined both directly by laser-induced photoacoustic spectroscopy and indirectly in form of the uranium(IV) arsenazo(III) complex.