First reporting of the scientific-technical outcome of WP 4.2: Chemical and redox behavior of the investigated radionuclides in the different systems through microbial mediated processes

Within the framework of ReCosy redox processes in biofilms, grown in-situ in radionuclide contaminated environments, will be studied by oxygen and redox potential microsensors. First in-situ measurement activities have already taken place in a former uranium mine in Saxony (Germany). Since the mining activities had stopped in 1990, the uranium mine has been partially flooded for remediation. This resulted in encroaching release of heavy metals and radionuclides as contaminants in acid, sulphate-rich waters. Huge biofilms with a thickness of more than 10 centimeters are currently growing in drainage channels (so far not flooded) under acid conditions and in the presence of uranium concentrations of approximately 1x10-4 M. Concentration profiles of oxygen were measured in these biofilms in-situ by means of electrochemical microsensors of the Clark design (Unisense, Denmark) with a tip diameter of 10 µm. The sensors were connected to a picoammeter and fixed in a holder on a motor-driven micromanipulator stage, connected with a motor controller, for a precise small-scale positioning and for automated measurements in 50 µm steps in Z-axis. Due to the hetero-geneities of the biofilm, numerous microprofiles were measured at different points of the biofilm. The results revealed a fast decrease of the oxygen concentration in the biofilm profiles. Already at a depth of approximately 2 mm no oxygen was detectable. It can be assumed that the microorganisms of the biofilm battle the toxic effects of aqueous uranium in a similar way as the biofilms described in Krawczyk-Bärsch et al. (2008). The biofilm community is reacting to the exposition of uranium in environmentally relevant concen-trations with an increase of the respiratory activity of the microorganisms. The increased metabolic activity of the microorganisms, as a result of an activation of heavy metal resistance mechanisms, will lead to larger anoxic zones (reducing zones) within the biofilms, which may induce redox processes. In these zones uranium (VI) can be reduced to highly insoluble uranium (IV) through abiotic processes. The precipitations of uranium (IV) solids will result in an increased removal of uranium and immobilisation of uranium from the solution.

References:

Krawczyk-Bärsch, E. et al. (2008) Geochimica et Cosmochimica Acta 72, 5251–5265