Category: Behaviors of radionuclides in environments

Desorption mechanisms of cesium from illite and vermiculite

Background Vast areas of the eastern part of Japan have been contaminated by a short-lived radioisotope of cesium (Cs), 137Cs, whose half-life is 30.1 years, from the accident of the Fukushima Dai-ichi Nuclear Power Plant in 2011. Most of contaminated soils in residential areas have been stripped and gathered for future disposal. On the other hand, air dose rates in forests are still high. Understanding the behaviors of Cs+ in soils is thus crucial for the safety assessments of the disposal of contaminated soil in future and the evaluation of long-term exposure of residents living near forest areas. It is known that Cs+ is strongly sorbed to micaceous minerals. However, the desorption of Cs+ at a trace sorption level with time in the presence of different salt ions is not well understood. In this study, we conducted long-term sorption and desorption experiments of Cs+ with illite and vermiculite, which are micaceous minerals, at room temperature to study the effects of sorption time and co-existing cations on the desorption. Method A small amount of Cs+ (50 nM Cs+ spiked with 900 Bq 137Cs) was sorbed to the illite and vermiculite in the presence of 1 mM K+ or Ca2+, or 1 mM K+ and 100 mM Ca2+ over 8 weeks, which was then desorbed in the presence of Prussian blue (PB) […]

Evaluation of labile fraction of radioactive Cs by diffusive gradients in thin films (DGT)

Background A large area of Fukushima Prefecture was contaminated by radioactive nuclides, radioactive cesium in particular, released by the accident of the Fukushima Daiichi nuclear power plants in Mar., 2011. Most of residential area has been decontaminated by now; nevertheless, a large part of forests Environmental behaviors of radioactive Cs is crucial to predict long-term changes of its distribution in soil or river environments. It is known that Cs is selectively bound to micaceous minerals in nature and inert for chemical exchange reactions. Nevertheless, dynamics of labile fractions of Cs in a given environment if any must play an important role for its environmental behaviors, as it directly contributes to the mobility and bioavailability of Cs. In this study, we perform in-situ evaluation of the labile Cs in soil and aqueous environments of Fukushima Prefecture by diffusive gradients in thin films (DGT), which is a technique designed for the lability measurement of trace elements in nature. Research contents Two types of DGT devices, one for aqueous environments and the other for soil environments, were developed and their applicability for environments have been tested by laboratory experiments. With the developed devices we have revealed that the labile fractions of radioactive Cs In river environments exceeds the dissolved ones, suggesting that the desorption of Cs from suspended particles modulates its behaviors in […]

Long-term desorption of radioactive Cs from soilsLong-term desorption of radioactive Cs from soils

Introduction Vast areas of north-eastern regions of Japan has been contaminated by radionuclides released from the accident of the TEPCO’s Fukushima Dai-ichi nuclear power plants. In particular, radioactive cesium, namely 137Cs, cause many problems due to its relatively long half-life (30.1 years), including decontamination and the management of contaminated soil wastes. To settle such problems, we need to know long-term behaviors of radioactive Cs in soils. Cesium is reported to be strongly fixed micaceous to so-called frayed edge sites (FES) in minerals in soils by interlayer collapse. Nevertheless,the long-term desorption behaviors is of great importance, which is not yet fully understood in terms of the mechanisms, partly because of its slowness and also of the existence of re-sorpton. In this research, we performed the long-term desorption experiments of 137Cs and 133Cs from the real contaminated soils in the presence of varying concentrations of competing cations. The resorption of desorbed Cs was diminished by adding cation-exchange resin as an adsorbent. Experimental The contaminated soils collected in Fukushima prefecture after 40 days since the accident were used. Soils were suspended in dilute KCl solution together with a dialysis bag containing cation exchange resins (Dowex 50w-X8，K+ form).The dialysis bags were exchange 10 times in 3 months with new ones. 133Cs and 137Cs released from the soils and captured by the resigns were quantified […]

Speciation of radioactive Cs in Fukushima soils

Radioactive cesium (<sup>137</sup>Cs) released from the accidence of the Fukushima Dai-ichi nuclear power plant contaminated areas of the eastern part of Japan, particularly Fukushima prefecture and has affected the life of local residents in many ways. During the release events, 137Cs (b.p. 678˚C) was emitted to the atmosphere as CsI in aerosols. After a couple of years soil is likely the most significant reservoir for it. As <sup>137</sup>Cs emits 652 keV γ-ray, <sup>137</sup>Cs in top-layers of soils may contribute to external exposure of local residents and biospheres. It can be absorbed by plants from contaminated soils, which may lead to internal exposure. Thus, It is of great importance to understand behaviors of <sup>137</sup>Cs. In this research, we’ve evaluated the geochemical and grain-size distribution of <sup>137</sup>Cs in contaminated soils sampled in Fukushima Prefecture after the accident. The obtained results have been also compared with those of stable cesium (<sup>133</sup>Cs) to infer the variation of the geochemical distribution in the near future. <sup>137</sup>Cs are most extracted to the acid-digestible factions and the extraction residues. It is also found that <sup>137</sup>Cs is associated with relatively small silt and clay  grains, which are rich in micaceous clay minerals and kaolins according to X-ray diffraction. We’ve found positive correlation between <sup>137</sup>Cs content in each fraction and the amount of micaceous minerals, suggesting crucial roles of the clay minerals on […]