Analysis of anion adsorption effects on alumina nanoparticles stability

Abstract

Nanoparticles and colloids can be relevant in contaminant migration if the contaminant is strongly adsorbed onto the particles and they are stable and mobile. The main conditions required for a colloidal

system to be considered as ‘‘stable’’ are: (1) low ionic strength of the groundwater (<1 · 10-3 M) and (2)

pH far from the point of zero charge (pHPZC). These conditions however, are too simplified to describe the

colloidal behaviour in real cases; in fact, specific adsorption of different ions may also have an important

impact on colloid stability.

In particular, in this study we analyse the effects of the adsorption of inorganic anions on Al2O3 oxide

nanoparticles (alumina NPs) stability, combining batch adsorption studies with electrophoretic and

dynamic light scattering measurements. Selenite adsorption was studied in a wide range of pHs (3–11), ionic strengths (5 · 10-4 – 1 · 10-1 M) and selenite concentration. Different electrolytes were used

to understand the competitive effects for selenite sorption of different anions in solution (ClO4-

, NO3-, HCO3-, SO42-) and especially their overall influence on alumina NPs stability.

The positive charge of alumina, under acid and neutral-alkaline pHs, favours anion adsorption which, in turn, may result in a decrease of the net surface charge, promoting particle aggregation and the destabilisation

of the system. Results showed that, the higher the anion affinity for alumina surface and sites

occupancy, the higher the destabilisation of particles. The sorption selectivity observed in our study

was: SeO3 2- > SO42- > HCO3- > NO3- > ClO4-.

Upon anion adsorption, particles aggregation was evident, but a clear change in z-potential, was only

observed with very high surface occupancies. Surface complexation modelling has been shown to be useful supporting tool for stability studies.