The fate of caesium-137 in a soil environment controlled by immobilization on clay minerals

Caesium-137 (Cs), with its high release rate and long half life, is the most important longterm contributor to environmental contamination of all the radionuclides released by the accident at the Fukushima Dai-ichi nuclear power plant in March 2011. There have been many surveys of the fate of Cs in terrestrial environments, especially after the atmospheric nuclear tests of the 1950s and 60s and the Chernobyl accident in 1986. Previous surveys revealed that most of the Cs deposited on land remains in the surface soil for decades and that Cs rarely exchanges with other cations (atoms that have lost an electron to become positively charged). The immobilization of Cs in soil is mostly ascribed to selective adsorption on clay minerals, particularly on illitic minerals. Cs is irreversibly retained at the fringe of the layered structure of illitic minerals, called the frayed-edge site, with much higher selectivity than other cations. Comparison of the radiocaesium interception potential (RIP) between soils and clay minerals indicates that the difference in the frayed-edge site capacity is largely dependent on the degree of weathering and the degree of hydroxy-Al interlayering in illitic minerals. Here we investigate the difference in the RIP values between mineral soils to predict the level of contamination of crops grown in Cs contaminated fields.