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

Abstract

Sorption behaviour of hazardous nuclides such as ¹³⁷Cs and ⁹⁰Sr 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⁺ ion. 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 Sr²⁺ 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 ¹³⁷Cs in geologic barrier of a radioactive waste repository would be greatly retained by illite and smectite clay, and that the fate of Sr would be significantly controlled by ionic strength of groundwater cations and bicarbonate ion as a complex agent.

Key words:

• Sorption
• Nuclides
• Groundwater Ions
• Clay Minerals