Sorption of selenium on minerals mixtures: role of minor phases in the modelling part

The sorption processes were largely studied and modelled when single mineral surfaces (iron oxides, clay etc) were considered. In the presence of mineral mixtures, the interest is to determine which mineral will predominate in the adsorption processes. The aim of this study was to perform adsorption experiments of Se(IV) on single minerals, binary, ternary, and quaternary mineral mixtures in order to assess the impact of each surface. The composition of the solid mixture was chosen to be representative of the composition of the argilite found in the Bure site (France) (montmorillonite/iron oxides/calcite/ pyrite), and the water used was chosen to be equilibrated with the solid mixture. A surface complexation model based on the component additivity approach was chosen to predict the sorption process when mineral mixtures were considered. In this model, the stoichiometries and surface complexation constants determined for single mineral were used, without any fitting, to describe the adsorption behaviour of Se(IV). For every mixtures, it appeared that only the montmorillonite surface was responsible for the adsorption of selenium. However, the model tended to overestimate the sorption when complex mixtures were studied. Taking into account the dissolution of silicate from the montmorillonite, it was possible to weaken the estimation. The presence of calcite, pyrite, and iron oxide did not influence the total amount of selenium sorbed on the mixture, even if the sorption of Se on the neat surface of iron oxide is significant when single iron oxide surface is used in experiment. (authors)