Impact of Humic Acid on the Uranium Migration in the Environment

The remediation of contaminated areas and facilities of the former uranium mining and milling in Saxony and Thuringia (Germany) requires detailed knowledge on the uranium migration in natural aquifer systems. Humic acids (HA), polyelectrolytic organic macromolecules, are of importance for the mobilization of radiotoxic and toxic metals in the environment due to their ability for complex and colloid formation. In addition, HA are characterized by redox properties that can influence the oxidation state of metal ions which effects their speciation and consequently their migration behavior.

In order to improve the knowledge on the interaction between HA and uranium we characterized the redox properties of different HA (Eh values, Fe(III) redox capacities) and studied the stability of the oxidation state of U(VI) in presence of HA on a time-scale of several weeks. We obtained a spectroscopic proof for the reduction of U(VI) to U(IV) by HA with pronounced redox functionalities applying laser-induced photoacoustic spectroscopy.

This demonstrates a possible relevance of HA for the migration of tetravalent actinides. Therefore, we investigated the transport behavior of the redox couple U(IV)/U(VI) in presence of HA in quartz sand by column experiments. Retardation factors and eluate recoveries were determined. We found that HA influences the transport of both, U(IV) and U(VI). In presence of HA both redox species migrate nearly as fast as the groundwater flow. In case of U(VI) HA exhibits a significant mobilizing effect. There are strong indications for a similar impact on the U(IV) transport.