Immobilization of Uranium from Drinking Water by Bacillus Sphaericus JG-A12

Uranium ores are very common in several regions of Germany, e.g. Saxony, Thuringia and Bavaria. Weathering and ore mining are processes which result in mobilization of uranium. So far only a general radiation dose limit for drinking water exists in Germany but no limit value for uranium itself. The latter is a radioactive and also toxic element and therefore discussions have raised to adopt a limit value for uranium in Germany. Against this background the development of novel strategies for cleaning uranium contaminated water became an increasingly interesting subject. In the past the Bacillus sphaericus strain JG-A12, recovered from the uranium mining waste pile “Haberland” nearby the town of Johanngeorgenstadt, was demonstrated to bind selectively several heavy metals including uranium [1]. Molecular and structural analyses of the uranium complexes formed by the cells, spores and the surface layer (S-layer) protein of B. sphaericus JG-A12 via extended X-ray absorption fine structure (EXAFS) spectroscopy identified carboxyl and phosphate groups as binding sites for uranium [2]. Moreover sol-gel techniques were used for immobilization of B. sphaericus JG-A12 cells in a porous silicate matrix. [4, 5]. The obtained biological ceramic (biocer) is particularly suitable for the use as selective filter material for the immobilization of uranium from polluted industrial waters.

This work is aimed to investigate the capability of biocers containing B. sphaericus JG-A12 cells to remove uranium from contaminated drinking water. For the experiments four different tap water samples supplemented with 30 µg uranium per liter and one Hungarian mineral water with a uranium content of 142 µg uranium per liter were selected. Uranium sorption by the biocer showed the removal of over 99% of dissolved uranium from the tap water samples. In the case of the heavy metal containing and highly saline mineral water 86% of uranium and 76% of zinc were immobilized.

In order to improve the uranium binding capacity of the biocers, the uranium sorption by B. sphaericus JG-A12 cells was tested depending on the amount of phosphorus present in the growth medium. Highest binding capacities were achieved with biomass grown in phosphate rich media, containing at least 1.4 mM phosphorus. In contrast, the binding capacity was remarkably lower with biomass grown in phosphate limited medium. Furthermore uranium binding of the isolated and purified S-layer of B. sphaericus JG-A12 was investigated. Interestingly, the S-layer of B. sphaericus JG-A12 possesses even at low concentration of uranium higher affinity to uranium than the S-layer protein of its closest relative B. sphaericus NCTC 9602 or the reference protein bovine serum albumine. Recent analyses of the S-layer genes of B. sphaericus JG-A12 [6] allow to perform in future genetic modifications of the cell surface of B. sphaericus JG-A12. This displays a further possibility to enhance the capacity and the selectivity of the uranium binding by the next generation of biocers.

Acknowledgement
This work was founded through Project DFG SE 671/7-2 from the German Research Community (DFG) and trough EC grant GRD1-2001-40424. We thank R. Getzlaff and I. Plumeier (GBF Braunschweig) for their assistence and U. Soltmann (GMBU, Dresden) for the preparation of the biocer.

References
[1] S. Selenska-Pobell et al. 1999. FEMS Microbiology Ecology 29, 59-67
[2] J. Raff et al. 2004. In: R.B. Wanty and R.R. Seal II. Water-Rock Interaction. 1, 697-701
[4] J.Raff et al. 2003. Chemistry of Materials 15, 240-244
[5] U. Soltmann et al. 2003. Journal of Sol-Gel Science and Technology 26, 1209-1212
[6] K. Pollmann et al. (in preparation)