Humic acid model substances with pronounced redox functionality for the study of environmentally relevant interaction processes of metal ions in the presence of humic acid

Humic acid (HA) model substances with pronounced redox functionality were synthesized by oxidation of hydroquinone or catechol in the presence of glycine or glutamic acid and characterized concerning their elemental, structural and functional properties. In order to characterize the redox properties of the synthetic products, formal redox potentials and Fe(III) reducing capacities were determined and compared to purified Aldrich HA (AHA). Furthermore, the reduction of U(VI) in the presence of HA was studied. The synthetic products show elemental, functional and structural properties comparable to natural HA, however, they are characterized by high amounts of phenolic/acidic OH groups (5.3–6.6 meq/g). Furthermore, the synthetic HA show significantly higher reducing capacities for Fe3+ and [Fe(CN)6]3− at pH 3.0 (8.8–14.5 meq/g) and at pH 9.2 (27.5–36.9 meq/g), respectively, than AHA (pH 3.0: 1.2 ± 0.1 meq/g; pH 9.2: 7.2 ± 1.9 meq/g). The highest reducing capacities were obtained for HA Cat-Gly (pH 3.0: 14.5 ± 1.6 meq/g; pH 9.2: 36.9 ± 0.2 meq/g), an oxidation product from catechol and glycine, which is characterized by the lowest formal redox potential (E0⁎ = 517 ± 12 mV) of all studied HA (E0⁎ = 517–571 mV). Indications for a slight reduction of U(VI) in the presence of HA were observed, whereby, HA Cat-Gly exhibits again the highest reducing capacity (pH 6: 0.065 ± 0.002 meq/g). Using modified HA with blocked phenolic/acidic OH groups the importance of these functional groups for the redox behavior of HA was confirmed. Synthetic HA with pronounced redox functionality can be used to study the redox behavior of HA and the redox stability of metal ions in the presence of HA and furthermore, to stabilize redox-sensitive metal ions against oxidation in complexation and transport studies with HA. This contributes to a better understanding of interaction processes of metal ions with humic substances in soils, sediments and waters.