Effects of gamma-alumina nanoparticles on strontium sorption in smectite: Additive model approach

Strontium sorption was analysed in binary mixtures of smectite and gamma-alumina nanoparticles under different pH, ionic strength and Sr concentration. The aims were to verify if gamma-alumina nanoparticles enhance Sr sorption in smectite and to analyse whether a component additive model satisfactorily described Sr sorption in the mixtures. In smectite, Sr sorption mainly occurs by cation exchange but surface complexation was also accounted for. In both solids, surface complexation was described with a non-electrostatic model. The addition of gamma-Al2O3 nanoparticles to smectite improved Sr uptake under alkaline pH and high ionic strength, and the additive model successfully reproduced experimental data. In contrast, under acid pH and low ionic strength, no sorption improvement was observed upon adding the nanoparticles and the additive model overestimated Sr sorption. The competition of Al3+ ions, coming from gamma-Al2O3 dissolution, partially explained the differences between data and model. Nevertheless, surface interactions between alumina particles and smectite layers may be shielding the charge, hindering contaminant access to exchangeable sites in smectite.