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

Abstract

Caesium-137 (137Cs), 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 137Cs 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 137Cs deposited on land remains in the surface soil for decades and that 137Cs rarely exchanges with other cations (atoms that have lost an electron to become positively charged). The immobilization of 137Cs in soil is mostly ascribed to selective adsorption on clay minerals, particularly on illitic minerals. 137Cs 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 137Cs contaminated fields.