Uranium binding by the natural isolate Bacillus Sphaericus JG-A12 and its application for bioremediation

Bacillus sphaericus JG-A12 is a natural isolate recovered from a uranium mining waste pile near the town of Johanngeorgenstadt in Saxony, Germany. Similarly to some other Bacillus isolates from this site, cells and spores of B. sphaericus JG-A12 bind selectively and reversibly large amounts of different toxic metals, such as uranium /1/. The cells of the B. sphaericus strain JG-A12 are covered by a surface layer (S-layer) protein. The latter forms a highly ordered lattice with distinct structural and chemical properties. Because it was shown that S-layers may work as protective coat, matrix for biomineralization, molecular sieve or as ion and molecule trap /2, 3, 4/ their special importance for the interaction of cells with radionuclides and heavy-metals in the surrounding environment is evident. As a consequence of the above mentioned properties, B. sphaericus JG-A12 was investigated with regard to its suitability to work as a binding matrix for bioremediation of uranium mining waste waters. A fundamental requirement for biotechnological application is the complete immobilization of the biomass in a porous structure. In this work, sol-gel techniques were used for embedding B. sphaericus JG-A12 cells, spores and S-layer protein in a SiO2-matrix in order to obtain a novel kind of biological ceramic (biocer) /5/. To select the appropriate filter material, uranium sorption and desorption experiments were carried out with free and immobilized cells, spores and S-layer protein of B. sphaericus JG-A12. Besides quantification of the available binding sites, the formed uranium complexes were investigated also by using infrared and extended X-ray absorption fine structure (EXAFS) spectroscopy. The latter revealed that the binding of uranium by free cells, spores and S-layer occurred via phosphate and carboxyl groups. The presence of phosphate groups was confirmed by different methods not only for cells and spores but also for the S-layer protein. The spectroscopic analysis of the uranium complexes formed by the biocer demonstrates the metal binding via phosphate groups and the formation of a uranium precipitate, what is of special interest for the binding capacity of the biocer. In case of the xerogel uranium was bound to silanol groups. Analyses of the uranium sorption by cells, spores and S-layer protein of B. sphaericus JG-A12 and of the xerogel itself (without biocomponent) showed highest binding capacity for spores followed by cells, S-layer protein and xerogel. In case of embedded biocomponents only the binding capacity of spores is significantly reduced and is lower than for cell- and S-layer-ceramics. Uranium bound to biocers can be completely removed by washing with citric acid.

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