Bioleaching of Co and Cs from soil by sulfur-oxidizing bacteria

Abstract

The Korea government has announced that Korea will phase out the nuclear power plants (NPP). Decommissioning of NPP and site remediation will be followed with this political keynote. The soil and groundwater in vicinity area of NPP can be contaminated by radionuclides emitted from NPP. Consequently, human health can be affected by radionuclides such as 137Cs and 60Co (half-life of 30.1 and 5.3 years, respectively) because these radionuclides stay in the environment for a long time. One of remarkable methods for decontamination of this contaminated soil is the bioleaching. It is biological method catalyzed by a living organism, whereby an insoluble metal is extracted to a soluble form from soil. In this study, we estimated the applicability of sulfur-oxidizing bacteria, Acidithiobacillus thiooxidans for bioleaching of Co and Cs in the soil. Since there are little studies regarding the bioleaching of radionuclides, we tried to assess the tolerance of A.thiooxidans to the Co, Cs and gamma radiation. The sulfur-oxidizing bacteria were exposed to stable isotope Co and Cs conditions, where the Co and Cs concentrations were 10 mg/L, 50 mg/L, 100 mg/L, and 1000 mg/L. Tolerance to gamma radiation was assessed separately with 4 Gy and 400 Gy of irradiation to the bacteria. The growth of sulfur-oxidizing bacteria was measured indirectly by measuring the pH and sulfate concentration in the medium. Then the bioleaching experiments were conducted with different soil from the vicinity area of Hanbit NPP (HB8), the vicinity area of Gori NPP (GR5) and northeast area of Jeju island (JJ11). Only in case of the HB8, it was artificially contaminated in order to simulate the accidental exposure of radionuclides such as Fukushima accident. Abiotic experiment, where nitric acid was added instead of bacteria inoculum to lower the medium pH similarly with biotic experiment, was carried out together to distinguish the effect of pH during bioleaching. The concentration of Co and Cs in the soil was analyzed with modified BCR sequential extraction before and after leaching. The removal efficiencies of Co and Cs were confirmed depending on elemental fraction in these soil. In order to optimize the bioleaching process, the ratio among medium, soil, sulfur powder and bacteria inoculum was varied and the ratio for the highest efficiency was determined. As the results of tolerance experiments, the sulfur-oxidizing bacteria had no adverse effect below 10 Co mg/L, therefore, it can be applied below 10 Co mg/L. On the other hand, the bacteria can be applied regardless of the Cs concentration. Though the bacteria had adverse effect on the growth after 400 Gy of gamma-ray irradiation, it still had worked and not stop to metabolize. Therefore, the sulfur-oxidizing bacteria can be applied below 400 Gy of gamma-ray irradiation. As the results of bioleaching experiments, artificially contaminated HB8, which contains high percentage of fraction 1 (Soluble species, carbonates and cation exchange sites) and fraction 2 (Iron and manganese oxyhydroxides), showed high removal efficiency of Co and Cs, 96.8% and 79.1%, respectively. On the other hand, GR5 and JJ11, which contain high percentage of faction 4 (Residual), showed relatively low removal efficiency. The efficiencies of Co and Cs for GR5 were 41.2% and 1.9%, respectively and for JJ11 were 57.1% and 0.3%, respectively. The fraction 1 and fraction 2 can be extracted easily by bioleaching even just by adjusting the pH of medium, while the fraction 4 cannot be extracted by both. In case of fraction 3(Organic matter and sulfides), it can be extracted by bioleaching but cannot be extracted by pH adjustment. These results demonstrate that the bioleaching is effective to the soil that contains high percentage of fraction 1, 2 and 3. The optimum soil:liquid ratio for bioleaching was 1:10 (g:mL), where the Co and Cs removal efficiencies were 94.7% and 79.2%, respectively. In case of sulfur powder and bacteria inoculum, a little amount were enough for bioleaching. The removal efficiency of Co and Cs remained above 90% and 80%, respectively where the amount of sulfur powder and inoculum were reduced to 0.01 g and 0.01 mL each for 40 mL of medium. These results support the economical benefits of bioleaching. Through above results, we can potentially conclude that the sulfur-oxidizing bacteria can be applied for remediation of radionuclides-contaminated soil.