The retention of radioactive cesium (Cs) in soil is significantly related to the types of clay minerals, while the weathering process affects the irreversible adsorption sites in clay minerals. In this study, the effect of weathering (exposure duration of Cs and repeated wetting and drying cycles) on fractionation of Cs in soils was investigated using fractionation analysis by the sequential extraction. The residual fraction of Cs increased slowly with exposure time but increased rapidly by repeated wetting and drying cycles. XRD analysis shows that a 1.43 nm of interlayer size for vermiculite is shortened to 1.00 nm, i.e., similar to that of illite. The change implies the potential that the structure of expandable clay minerals is transformed to the non-expandable structure by weathering process after Cs retention. Based on the result, the residual fraction of Cs, most stable form of Cs in the soil, reached relatively rapidly to a maximum. However, the process is much slower kinetically in the field because the bench-scale weathering process used in this study is more aggressive. This study implies that Cs fractionations in the soil are converted into a more stable fraction by weathering processes in the soil. Therefore, Cs removal should be conducted as soon as possible after accidental release of Cs in an environmental side.
Keywords: Cesium fractionation; Clay minerals; Soil contamination; Weathering; Wetting and drying cycles.