Evaluation of component additive modelling approach for europium adsorption on 2:1 clays: Experimental, thermodynamic databases, and models.

This study evaluates the component additive approach for Eu adsorption on mixtures of smectite and illite, which are the most common clays used as barriers for contaminant retention in waste repositories. A thorough set of Eu adsorption data for Na-exchanged smectite and illite that encompasses a wide range of pH values, ionic strengths, and Eu concentrations was provided. This database is likely one of the largest sorption databases available for Eu in 2:1 clays, making it appropriate for sorption model calibration. The main adsorption mechanisms considered were surface complexation, on weak and strong clay edge sites, and cation exchange. Further, the role of principal ions, which are naturally leached from clays, as competitive factors for Eu retention, was evaluated in the modelling calculations. The main uncertainties related to the modelling procedures and the use of different thermodynamic data on sorption modelling were outlined. The reactions and parameters successful in modelling Eu adsorption on individual clays were used without any modification to model Eu adsorption on illite/smectite mixtures, wherein only the relative mineral proportions were considered. The fit of the sorption data in the mixed clay system was satisfactory, indicating that, in 2:1 clays, Eu sorption is an additive process, which stresses the predictive capacity of the component additive approach in these systems. This is an important support for assessing the performance of barrier materials for contaminant migration under different geochemical conditions.