The application of microsensors for the determination of redox processes in biofilms from uranium contaminated acidic mine drainage waters

Microsensors were used in order to perform profiles of redox potential and pH within biofilms, which have formed in small drainage channels of uranium contaminated acidic mine drainage (AMD) waters from an underground uranium mine in Saxony (Germany). The results clearly showed significant differences between the redox potential and pH of the AMD water in comparison to the measured values in the biofilm, indicating that the biofilms have built up their own microenvironment. The redox potential of 921 ± 17 mV inside the biofilm was approximately 200 mV higher than in the surrounding water. The pH of the biofilm was characterized by a pH of 1.9 ± 0.1, whereas the pH of the AMD water showed a higher pH of 2.6 ± 0.1. The results were plotted into the pH-Eh diagram for the U-S-O-H-C system, which was constructed by using the analytical data of the drainage water for the calculation of the predominance fields of different uranium species. It clearly shows that an aqueous uranium(VI) sulfate complex exists in the biofilm as well as in the water under the ambient conditions. Laboratory experiments, simulating the first state of flooding of the uranium mine by increasing the pH of the AMD water, showed the influence of the Ferrovum myxofaciens dominated biofilms on the migration of uranium. Due to homeostatic mechanisms the microbes maintain their intracellular pH even when the pH of the water is increased. Consequently, an immobilization of uranium in the biofilms as well as in the AMD water is inhibited as long as Ferrovum myxofaciens will be the dominant bacteria of the biofilms.