Cs selectivity and adsorption reversibility on Ca-illite and Ca-vermiculite

For understanding and predicting the Cs behavior in soils and groundwater, Cs adsorption properties of illite and vermiculite were investigated under various pH conditions and Cs concentrations. Cs adsorption and desorption experiments have been conducted with Ca-homoionic illite and Ca-vermiculite in CaCl₂ solution with an ionic strength of 0.03 and of 0.06 mol.L^-1 respectively, by focusing on cation exchanges between Cs, proton and calcium at thermodynamic equilibrium. Ca-illite displayed more affinity for Cs than Ca-vermiculite. Cs adsorption was non-linear for both clay minerals and a multi-site exchange model approach was adopted to interpret and model adsorption isotherms. Each mineral reactivity was described by their sorption site properties expressed by their exchange capacity and ionic selectivity. High-selective and low-capacity sites were shown to control Cs uptake at concentrations below 10^-8-10^-7 mol.L^-1 for both Ca-illite and Ca-vermiculite. Three high-capacity sites dominated Cs adsorption at higher concentrations. Cs adsorption reversibility was demonstrated for illite at Ca concentrations ranging from 5 10^-2 to 10^-9 mol.L^-1. The partial irreversibility of Cs adsorption onto vermiculite at Cs concentrations greater than 10^-5 mol.L^-1 was related to interlayer collapse. Reversible adsorption may occur as long as the interlayer space stays open. The irreversible adsorbed fraction was quantified and taken into account in the modeling approach to calculate the selectivity coefficient of each site.