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Adsorption of Antimonate by Gibbsite: Reversibility and the Competitive Effects of Phosphate and Sulfate
Author(s) -
Essington Michael E.,
Stewart Melanie A.
Publication year - 2016
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2016.04.0129
Subject(s) - gibbsite , antimonate , adsorption , chemistry , isoelectric point , inorganic chemistry , outer sphere electron transfer , inner sphere electron transfer , ionic strength , phosphate , titration , antimony , kaolinite , mineralogy , ion , aqueous solution , organic chemistry , enzyme
Core Ideas Antimonate adsorption by gibbsite is pH and ionic‐strength dependent. Antimonate adsorption is reversible in alkaline but hysteretic in acidic systems. Adsorbed Sb(V) generates a downward shift in the gibbsite isoelectric point. Both phosphate and sulfate decrease adsorbed Sb(V) concentrations. The triple‐layer model predicts Sb(V) adsorption using inner and outer sphere surface complexes. 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 PO 4 and SO 4 decrease adsorbed Sb(V) concentrations: PO 4 throughout the pH 3 to 10 range and SO 4 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.