황산염환원박테리아(SRB) 지하미생물을 이용한 용존 세슘 제거 연구

Abstract

Radioactive cesium (Cs-137) is one of the main radioactive materials causing severe ecological damage and environmental pollution because it has a relatively long half-life (30.17 years), emitting the high energy of gamma-rays. When the ocean was contaminated by Cs-137, resulting from the accidents of the nuclear power plants such as Chernobyl and Fukushima nuclear reactors, the removal of Cs-137 from seawater has been difficult causing great costs because it is not easily separated from water with highly-concentrated salt components. The popular process for the Cs-137 removal from water has been the adsorption and/or the precipitation technologies, however the development of the outstanding adsorbents or precipitant for Cs-137 from seawater has been very limited in previous researches. This study focused on the development of the effective and the environment-friendly Cs-137 removal technology by using the biogenic sulfide generation process, which uses a sulfate reducing bacterium (SRB), D. vulgaris, in here rather than commercial inorganic absorbents. D. vulgaris was cultivated in an incubator (30℃) for the experiment and cesium, iron and lactate with various concentration were injected into the SRB media. The batch experiment for a feasibility study was performed to remove Cs-137 from seawater by the sulfide mineralization process and to define its optimal operation conditions. At first, several batch experiments for removing stable cesium (Cs-133) from both the freshwater and the seawater were performed with different cesium concentrations (0.01 - 10 mg/L), iron concentrations (0.5 - 3 mM), and two lactate concentrations (3 mM and 6 mM). Another batch experiment containing radioactive Cs (Cs-137; 133 Bq/mL) was duplicated for the seawater under the condition of iron concentration of 3 mM and lactate concentration of 6 mM and their results were compared with those of the commercial absorbent (R9160-G), which is now used for the Cs-137 removal from seawater at Fukushima nuclear power plant in Japan. From the results of SEM-EDS and XRD analyses, we found that the sulfate ions dissolved in both seawater and freshwater were transformed into a sulfide mineral, mackinawite (FeS), by the biomineralization of D. vulgaris. The dissolved Cs-137 (or Cs-133) was removed during the experiment, suggesting that Cs was successfully separated from the water as a solid form. For the freshwater experiment, the stable cesium (Cs-133) removal efficiency was the highest under the condition of iron concentration of 1 mM and lactate concentration of 3 mM. At Cs-133 concentration of 10 mg/L and 1 mg/L, the removal efficiency was 17 % and 47 %, respectively. At 0.1 mg/L of Cs-133, the removal efficiency was 96 % and in the lowest concentration (0.01 mg/L), it was almost 100 %, suggesting that Cs-133 removal efficiency was inversely proportional to Cs concentrations in freshwater. For the seawater experiment, the Cs removal efficiency was the highest under the condition of iron concentration of 3 mM and lactate concentration of 6 mM. At Cs-133 concentration of 10 mg/L and 1 mg/L, the removal efficiency was 21 % and 39 %, respectively. At 0.1 mg/L of Cs-133, the removal efficiency was 86 % and in the lowest concentration (0.01 mg/L), it was almost 100 % in seawater. Interestingly, the Cs removal efficiency increased with the decrease of Cs concentration in seawater, which was similar to the results obtained from freshwater. For the seawater experiment, the Cs-137 removal efficiency of R9160-G was 71.3 % and the removal efficiency using D. vulgaris was 96.1 %. For the experiment with the Cs biomineralization using D. vulgaris, the amount of produced sludge as a precipitate after the experiment was 0.3 D.W. g/L, which was only one tenth of that with the usage of R9160-G, suggesting that the radioactive cesium was successfully removed by the biotechnology, producing a very small volume of radioactive Cs waste. In addition, this experiment verified that the Cs-137 removal process by using sulfate reducing bacteria (SRB) can effectively treat a large volume of seawater that was contaminated with cesium.