Adsorption of strontium and caesium onto an Na-illite and Na-illite/Na-smectite mixtures: Implementation and application of a multi-site ion-exchange model

Abstract : The application of a multi-site ion-exchange (MSIE) model to the prediction of contaminant behaviour onto soils or sediments requires a coherent adsorption database. Following the same approach exposed in preceding works on smectite (Siroux et al., Appl. Geochem. 87, 167; Wissocq et al., Appl. Geochem., 93, 167), a database is here implemented to allow modelling the adsorption of radionuclides $^{90}$Sr and $^{137}$Cs onto Na-illite, one of the major clay mineral encountered in the terrestrial environment. This database allows describing the adsorption properties of Sr$^{2+}$ and Cs$^+$ competing with major cations H$^+$$-$considered as the reference cation$-$and Na$^+$ onto an Na-illite. To obtain the adsorption site capacities and selectivity coefficients of illite sites for Na$^+$ towards H$^+$ , a saturation curve of Na$^+$ on Na-conditioned illite was acquired. Adsorption isotherms issued from literature have been reinterpreted using the MSIE model to obtain the adsorption properties of Sr$^{2+}$ and Cs$^+$ onto purified and Na-conditioned illite. Afterwards, the predictability and robustness of the database were verified simulating experimental adsorption of Sr$^{2+}$ and Cs$^+$ onto Na-illite/Na-smectite mixtures.