Affinity of radioactive cesium and strontium for illite and smectite clay in the presence of groundwater ions

Sorption behaviour of hazardous nuclides such as Cs-137 and Sr-90 onto illite and smectite clay was studied in the presence of major groundwater ions (Ca2+, Mg2+, K+, Na+ and HCO3-). Illite and smectite clay can selectively sorb small hydrated Cs+ ion in the presence of groundwater cations except K+ Son. In contrast, sorption of larger and stable hydrated Sr2+ ion onto illite and smectite clay is highly competed by background groundwater cations. The sorption characteristics of Cs+ and Sr2+ onto illite and smectite clay are closely related to hydration properties of ions and complex forms of ions (inner-sphere or outer-sphere complex) in the siloxane cavity of clay minerals, The mechanism of Cs+ and Sr2+ sorption of illite is ion exchange with an interlayer cation K+, and K+, Ca2+ and Mg2+ respectively, The effect of bicarbonate ion on Sr sorption of illite is due to the change of pH and the precipitation of a solid phase (SrCO3), The zeta potentials of illite and smectite clay as a function of pH reflect surface charge characteristics of two clay minerals. The results of this study indicate that the transport of Cs-137 in geologic barrier of a radioactive waste repository would be greatly retained by illite and smectite clay, and that the fate of Sr-90 would be significantly controlled by ionic strength of groundwater cations and bicarbonate ion as a complex agent.