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 by ICP-MS and Ge detector.

Results & discussion
From all samples, more sup>137Cs than that in short-term ion exchange experiments with high concentration of competing ions (ex. K⁺ and NH₄⁺) was desorbed after 3 months. In the initial period, the desorption of ¹³⁷Cs was suppressed by high concentration of K⁺; the effect was diminished in time, and the desorption at the high concentration of K⁺ exceeded that at the low concentration. Desorption of ¹³³Cs was different from that of ¹³⁷Cs, which suggested that these isotopes exist in different chemical forms, reflecting the differences in their residence time in soil.
A desorption model with 3 sites were fit to the observed desorption curves. The half-lives of the desorption from the slowest site fell into the range of years, indicating that a part of ¹³⁷Cs is strongly fixed soils and is desorbed slowly.

Murota, K., Saito, T.*, Tanaka, S., “Desorption kinetics of cesium from Fukushima soils”, J. Environ. Radioact. 153, 134-140 (2016).

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 by ICP-MS and Ge detector.

Results & discussion
From all samples, more sup>137Cs than that in short-term ion exchange experiments with high concentration of competing ions (ex. K⁺ and NH₄⁺) was desorbed after 3 months. In the initial period, the desorption of ¹³⁷Cs was suppressed by high concentration of K⁺; the effect was diminished in time, and the desorption at the high concentration of K⁺ exceeded that at the low concentration. Desorption of ¹³³Cs was different from that of ¹³⁷Cs, which suggested that these isotopes exist in different chemical forms, reflecting the differences in their residence time in soil.
A desorption model with 3 sites were fit to the observed desorption curves. The half-lives of the desorption from the slowest site fell into the range of years, indicating that a part of ¹³⁷Cs is strongly fixed soils and is desorbed slowly.

Murota, K., Saito, T.*, Tanaka, S., “Desorption kinetics of cesium from Fukushima soils”, J. Environ. Radioact. 153, 134-140 (2016).