Adsorption of Antimonate by Gibbsite: Reversibility and the Competitive Effects of Phosphate and Sulfate

Abstract

Antimony (Sb) is a potential environmental contaminant of emerging concern that occurs in soils in the Sb(V) oxidation state as the antimonate species Sb(OH)6 -. In soils, metal oxyhydroxides play an important role in the immobilization of contaminants and in restricting bioaccessibility. One such mineral is gibbsite, which bears the reactive aluminol surface functional group. Both inner- and outer-sphere surface complexation mechanisms have been inferred from conflicting Sb(V) adsorption findings involving aluminol- bearing minerals. The objectives of this research are to characterize Sb(V) adsorption by gibbsite and to use the macroscopic findings to develop mechanistic adsorption models. Antimonate adsorption envelopes were developed using two equilibrium techniques: continuous pH titration and batch. The adsorption of Sb(V) decreases with increasing pH and increasing ionic strength, suggesting that outer-sphere surface complexation is an important adsorption mechanism. However, Sb(V) retention is irreversible at pH <6 but is reversible in pH >7 systems, suggesting that inner-sphere species may be significant in acidic environments. Adsorbed Sb(V) also generates a downward shift in the gibbsite isoelectric point, further supporting the formation of inner-sphere Sb(V) surface complexes. Both PO4 and SO4 decrease adsorbed Sb(V) concentrations: PO4 throughout the pH 3 to 10 range and SO4 in pH <7 systems. The triple-layer surface complexation model successfully predicts Sb(V) adsorption by using both outer-sphere and inner-sphere surface species. The triple-layer model also predicts ligand adsorption in the competitive systems without the need for reoptimization.