Reduction of Uranium(VI) by isolated, anaerobic microorganisms from the flooding water of the former uranium mine Königstein (Saxony/Germany)

The former uranium mine Königstein (Saxony, Germany) is currently in the process of remediation. The under-ground is flooded in a controlled way, and the flooding water is cleaned up in an elaborate waste water treat-ment plant. Despite high uranium concentrations up to 13 mg/L and a low pH of 2.9, these waters contain a high microbial diversity as detected by culture-independent methods [1]. Microorganisms are known to interact with metals and radionuclides in different ways [2]. As a result, microorganisms can change the chemical behavior of metals and radionuclides. Anaerobic bacteria which are able to gain energy from the reduction of several metals are known to change the redox state of radionuclides. For instance, anaerobic sulfate-reducing bacteria (SRB) reduce U(VI) to U(IV) and thus change the migration behavior from the more soluble U(VI) into the insoluble U(IV) [3]. For that reason, these anaerobic living bacteria are able to use U(VI) as energy source. Genomic sequence analysis of the flooding water of Königstein revealed the presence of such anaerobic SRB. By culture dependent methods it was possible to isolate anaerobic microorganisms from the flooding water. They were then incubated with 10 mM glycerol using the flooding water as background solution. The uranium concentration was about 13 mg/L. During an incubation time of six weeks the redox potential decreased from 660 mV to 300 mV. After four and six weeks of incubation, the cells were separated from the incubation medium by centrifugation and then analyzed by U-LIII edge EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) measurements. By Iterative Target-Factor Analysis (ITFA) we determined that
100 % of U(VI) was reduced to U(IV). Simultaneously, investigations of the supernatant with UV-vis resulted in the same findings. The results show that naturally occurring anaerobic microorganisms within the flooding water of the former uranium mine Königstein are able to reduce U(VI) to U(IV).

REFERENCES

1. Zirnstein, I., Charakterisierung der mikrobiellen Biozönose im sauren Grubenwasser des ehemaligen Uranbergwerks Königstein, Institut für Ressourcenökologie, Dissertation, 2015, TU Dresden: Dresden.
2. Lloyd, J.R.M., L. E. , Interactions of microorganisms with radionuclides. Elsevier Science. 2002.
3. Lovley, D.R., et al., Enzymatic Iron and Uranium Reduction by Sulfate-Reducing Bacteria. Marine Geology, 1993. 113(1-2): p. 41-53.