RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The objective of this study is to assess, experimentally and theoretically, whether the addition of alumina (γ-Al₂O₃) nanoparticles (NPs) to smectite improves the retention of pollutants. The practical purpose is to design an optimised geochemical barrier against pollution.
Smectite clay was selected because it is widely used as geochemical or engineered barrier. Alumina NPs have large surface area and surface charge and then, high reactivity. The retention of four pollutants, with different speciation and sorption mechanisms, was analysed: selenium, strontium, cadmium and uranium.
Batch sorption experiments, complemented by spectroscopic measurements were carried out to analyse the retention of contaminants on smectite and alumina, and on their mixtures, under a wide range of conditions.
The possible interactions between smectite and alumina in mixtures, were deeply analysed by stability studies. It was observed that fast particle (hetero)aggregation is promoted in mixtures, strongly dependent on pH and on alumina content.
Results showed that the addition of alumina NPs to smectite improves the retention of anionic species, as selenium, under pH below the point of zero charge of alumina. Furthermore, the retention of cationic species is improved under the conditions where surface complexation is the dominant sorption mechanism. However, under conditions where cationic exchange dominates (acid pH and low ion strength), contaminant sorption decreases.
Surface complexation models were proposed to describe contaminant retention on smectite and alumina. Sorption on mixtures was satisfactorily simulated by the additive model in those cases where contaminant uptake in smectite is improved by alumina NPs incorporation. However, the additive model overestimates the sorption under conditions where cationic exchange is the main mechanism. The experimental decrease in sorption in the mixtures under acid pH, can be attributed to Al³⁺ ions coming from alumina dissolution, competing for sorption sites. However, effects due to (hetero)aggregation observed in mixtures at these pH, cannot be ruled out.