The Adsorption of Antimony(V) by Gibbsite.

Antimony (Sb) is a co-contaminant with lead (Pb) in shooting range soils at DoD installations throughout the United States. The in-situ immobilization of Pb in shooting range soils may be accomplished through the application of phosphate (PO₄). However, the impact of this treatment on the mobility and bioaccessibility of Sb is unknown. In soil, Sb commonly exists in the Sb(V) oxidation state as the hydroxyanion Sb(OH)₆^–, which is derived through the dissociation of a antimonic acid (Sb(OH)₅⁰, a weak acid). The objectives of this research were to elucidate the mechanisms and thermodynamics of Sb(V) adsorption by the hydrous Al hydroxide gibbsite, as a function of ionic environment, pH, temperature, and the competitive effects of PO₄ and SO₄. The adsorption of Sb(V) by gibbsite is pH-dependent. Maximum retention occurs in the pH 3 to 4 range, and decreases to a minimum in the pH 8 to 9 range. Adsorption is also independent of ionic strength when solution pH is < 7. However, at pH values > 7, Sb(V) adsorption increases with increasing ionic strength. The adsorption edge data were employed to develop a chemical model of Sb(V) adsorption using the constant capacitance surface complexation model. The data were satisfactorily described by imposing the ≡AlOSb(OH)₄⁰ and ≡AlOSb(OH)₅^– surface species. The chemical modeling activity will result in mechanistic parameters that describe Sb(V) retention and that are transferable; they can be used to predict Sb(V) fate and behavior in any soil environment (given that soil chemical information are available).