Analysis of the improvement of selenite retention in smectite by adding alumina nanoparticles

Abstract

Smectite clay is used as barrier for hazardouswaste retention and confinement. It is a powerful material to retain cations, but less effective for retaining anionic species like selenite. This study shows that the addition of a small percentage of γ-Al2O3 nanoparticles to smectite significantly improves selenite sorption. γ-Al2O3 nanoparticles provide high surface area and positively charged surface sites within a wide range of pH, since their point of zero charge is at pH 8–9. An addition of 20 wt% of γ-Al2O3 to smectite is sufficient to approach the sorption capacity of pure alumina. To analyze the sorption behavior of the smectite/oxide mixtures, a nonelectrostatic surface complexation model was considered, accounting for the surface complexation of HSeO3 − and SeO3 2−, the anion competition, and the formation of surface ternary complexes with major cations present in the solution. Selenite sorption in mixtures was satisfactorily described with the surface parameters and complexation constants defined for the pure systems, accounting only for the mixture weight fractions. Sorption in mixtures was additive despite the particle heteroaggregation observed in previous stability studies carried out on smectite/ γ-Al2O3 mixtures.