Removal of soluble strontium into biogenic carbonate minerals from a highly saline solution using halophilic bacterium, Bacillus sp. TK2d
Applied and environmental microbiology | 13 Aug 2017
T Horiike, Y Dotsuta, Y Nakano, A Ochiai, S Utsunomiya, T Ohnuki and M Yamashita
Radioactive strontium ((90)Sr) leaked into saline environments, including the ocean, from the Fukushima Daiichi Nuclear Power Plant after a nuclear accident. Since the removal of (90)Sr using general adsorbents (e.g., zeolite) is not efficient at high salinity, a suitable alternative immobilization method is necessary. Therefore, we incorporated soluble Sr into biogenic carbonate minerals generated by urease-producing microorganisms from a saline solution. An isolate, Bacillus sp. strain TK2d, from marine sediment removed > 99% Sr after contact for four days in a saline solution (1.0 × 10(-3) mol L(-1) Sr, 10% marine broth, and 3 weight% NaCl). Transmission electron microscopy and Energy-dispersive X-ray spectroscopy showed Sr and Ca were accumulated as phosphate minerals inside the cells and adsorbed at the cell surface at two days of cultivation, and then carbonate minerals containing Sr and Ca developed outside the cells after two days. Energy dispersive spectroscopy revealed that Sr, but not Mg, was present in the carbonate minerals even after eight days. X-ray absorption fine structure analyses showed that a portion of the soluble Sr changed its chemical state to strontianite (SrCO3) in biogenic carbonate minerals. These results indicated that soluble Sr was selectively solidified into biogenic carbonate minerals by the TK2d strain in highly saline environments.ImportanceRadioactive nuclides, (134, 137)Cs and (90)Sr leaked into saline environments, including the ocean, by the Fukushima Daiichi Nuclear Power Plant accident. Since the removal of (90)Sr using general adsorbents, such as zeolite, is not efficient at high salinity, a suitable alternative immobilization method is necessary. Utilizing the known concept that radioactive (90)Sr is incorporated into bones by biomineralization, we got the idea of removing (90)Sr into biominerals. In this study, we revealed the Sr removal ability of the isolated ureolytic bacteria under highly salinity condition and the mechanism of Sr incorporation into a biogenic calcium carbonate over a longer duration. These findings indicated the mechanism of the biomineralization by urease-producing bacterium and the possibility of the biomineralization application for a new purification method for (90)Sr in highly saline environments.
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