Geochemical and grain-size distribution of radioactive and stable cesium in Fukushima soils: implications for their long-term behavior

Abstract

Availability and mobility of radioactive cesium, 137Cs, in soils are crucial for recovery from the accident in the Fukushima Dai-ichi Nuclear Power Plants. In this study we investigated the geochemical and grain-size distribution of 137Cs in 11 soil samples collected in the eastern area of Fukushima Prefecture after the accident. Sequential extractions were performed to evaluate the distribution of 137Cs having different geochemical interactions with soil components. The result was further compared with that of the stable cesium, 133Cs, which had occurred in the soils before the accident. The distribution of 137Cs in different grain-size fractions was also determined. Radioactive cesium was predominantly found in the extract obtained by strong-acid dissolution and the extraction residue and was more concentrated in silt and clay grains. X-ray diffraction analyses revealed that micaceous minerals as well as kaolin minerals were predominantly dissolved by the strong-acid treatment. Correlation between the fraction of 137Cs and the content of micaceous minerals in different grain-size fractions of soil minerals suggests that micaceous minerals are responsible for the fixation of 137Cs in the soils. The isotopic ratio of 137Cs and 133Cs in the extract by strong-acid dissolution was more than three times smaller than those in the extracts by water, ion exchange, and reductive dissolution. This indicates that the distribution of 137Cs was not in the steady state in 2 y after the accident due to relatively slow fixation by the soil clay minerals.