Toru Nagaoka

Radiocesium contaminations of 20 wood species and the corresponding gamma-ray dose rates around the canopies at 5 months after the Fukushima nuclear power plant accident

Radiocesium ((134)Cs + (137)Cs) deposition from the Fukushima nuclear power plant accident was measured in 20 woody plants (12 evergreen and 8 deciduous species) grown in Abiko (approximately 200 km SSW from the NPP). Leaves (needles) and twigs were sampled from each of three foliar positions (top, middle, and bottom) in the plant canopy in early August 2011. At the time, soils around the plants were also sampled, and gamma radiation dose rates were measured at each sampling position. The average radiocesium activity in the observed leaves of the evergreen species was 7.7 times that in the leaves of the deciduous species. Among the observed evergreen coniferous species, the activity in pre-fallout-expanded leaves was 2.4 times that in the post-fallout-expanded leaves. Notably, a distinct variation in the activity among the evergreen coniferous species could be observed for the post-fallout-expanded leaves but not for the pre-fallout-expanded leaves. Although these differences depend on whether the leaves had expanded at the time of the fallout, it is probable that a considerable amount of radiocesium was translocated to newly developed leaves at a species-specific rate. In addition, it was demonstrated that dose rates around woody plants were not consistent with the prevailing prediction that general dose rates correspondingly decrease with monitoring height from the ground. Thus, the dose rates in the top foliar layer of the deciduous species decreased more than predicted, whereas those in the top foliar layer of the coniferous species did not decrease. This may be due to differences in the balance between the attenuation resulting from a shielding effect of the plant bodies and the higher radiocesium accumulation in the leaves.

Growth of Thiobacillus ferrooxidans on hydrogen by the dissimilatory reduction of ferric iron under anaerobic conditions

Thiobscillus ferrooxidans is generally considered to be an autotrophic bacterium that can grow aerobically on soluble ferrous iron or sulfur compounds. We now report that the bacterium can grow on hydrogen by the dissimilatory reduction of soluble ferric iron under anaerobic conditions. A series of growth experiments was carried out under aerobic or anaerobic conditions with or without ferric iron using the strains. Strain IFO 14262 was shown to be capable of oxidizing hydrogen when grown under aerobic condition. This strain also grew on hydrogen with the reduction of ferric iron to ferrous iron under anaerobic conditions. The accumulation of ferrous iron was readily observed when the bacterium was grown by the dissimilatory reduction of ferric iron. Strain JCM 7811 did not grow on hydrogen under aerobic conditions, but could grow when incubated with hydrogen and soluble ferric iron under anaerobic conditions. Growth correlated with the amount of reduced ferric iron. It was found that four out of six different strains of T ferrooxidans, including strains IFO 14262 and JCM 7811, were able to grow anaerobically on hydrogen with ferric iron as the sole electron acceptor. The dissimilatory reduction of ferric iron may play an important role for growth under oxygen-limited conditions by T ferrooxidans.

A novel mineral processing by flotation using Thiobacillus ferrooxidans

Oxidative leaching of metals by Thiobacillus ferrooxidans has proven useful in mineral processing. Here, we report on a new use for T. ferrooxidans whereby bacterial adhesion is used to remove pyrite from mixtures of sulfide minerals during flotation. Under control conditions, the floatabilities of 5 sulfide minerals tested (pyrite, chalcocite, molybdenite, millerite and galena) ranged from 88 to 99%. Upon addition of T. ferrooxidans , the floatability of pyrite was significantly suppressed to less than 20%. In contrast, addition of the bacterium had little or no effect on the floatabilities of the other minerals, even when they were present in relatively large quantities: Their floatabilities remained in the range of 70 to 94%. T. ferrooxidans thus appears to selectively suppress pyrite floatability. As a consequence, 84 to 95% of pyrite was removed from mineral mixtures, while 73 to 100% of non-pyrite sulfide minerals were recovered. The suppression of pyrite floatability was caused by bacterial adhesion to pyrite surfaces. The number of cells adhering to pyrite was significantly larger than the number adhering other minerals. These results suggest that flotation with T. ferrooxidans may provide a novel approach to mineral processing in which the biological functions involved in cell adhesion play a key role in the separation of minerals.