Sorption and retardation processes of Cs in granite under groundwater conditions

The dynamics of Cs⁺ sorption and migration behavior in granite under groundwater conditions were investigated using a granite sample collected from Inada, Japan. A Cs⁺ sorption isotherm for the Inada granite was obtained originally by batch sorption experiment with a powdered rock sample. The extended Langmuir model with two sorption sites fit the adsorption isotherm fairly well. The Cs⁺ sorption behavior of the dominant sorption mineral, biotite, was observed by means of phase-shift interferometry (PSI) as an in-Situ probe. Sorption of Cs⁺ from the edge to the interlayer of biotite, but not deep inside, was observed under groundwater conditions. Cs ⁺ sorption was considered to occur near the edge of the biotite layer, where K⁺ ions in the layer were replaced by Na⁺ ions in the groundwater first, followed by exchange with Cs⁺. Breakthrough behavior of intact granite (32 mm in diameter × 25 mm length) for Cs⁺ was observed over six months in a flow-through experiment using a centrifuge system. The Cs⁺ breakthrough curve showed a plateau. The migration behavior of Cs⁺ in the granite was modeled based on the series of results observed in the flow-through experiment, PSI analysis and the batch sorption experiment. A mi model with cascade-type dual modes of kinetic sorption was examined to interpret the Cs⁺ breakthrough data.