Objective

Antimony (Sb) is a co-contaminant with lead (Pb) in shooting range soils at Department of Defense (DoD) installations throughout the United States. The in situ immobilization of Pb in shooting range soils can be accomplished through the application of phosphate (PO₄); however, the impact of this treatment on the mobility and bioaccessibility of Sb is unknown. Further, the ability to predict Sb fate and behavior in contaminated soils, or as influenced by treatment technologies, has not been suitably developed. In soil, Sb commonly exists in the Sb(V) oxidation state as the hydroxyanion Sb(OH)₆^-. This anionic species is derived through the dissociation of an antimonic acid (Sb(OH)₅, a weak acid). As such, the principal mechanisms of retention in soils are anion exchange (weak adsorption) and ligand exchange (strong adsorption) by variable-charge soil minerals, such as iron (Fe) and aluminum (Al) oxyhydroxides. Available research findings suggest that Sb(V) is associated with Fe oxyhydroxides in soils and that PO₄ can enhance Sb(V) mobility and presumably bioaccessibility. This project seeks to address the following hypotheses: (1) the adsorption characteristics of Sb(V) by hydrous Fe and Al oxyhydroxides will establish the mechanisms and quantitative parameters needed to assess treatment strategies and predict bioaccessibility; (2) PO₄ and sulfate (SO₄) will compete with Sb(V) for adsorption sites on reactive soil minerals; the competitive effect will differ with competing ligand, initial saturation of the surfaces, and pH; and (3) chemical speciation models that employ surface complexation models (SCM) may be used to determine the distribution of Sb(V) between adsorbed and solution phases, thereby providing a molecular-level prediction of Sb fate and behavior in chemically complex environments.

The hypothesis-driven research objectives of this project are to (1) determine the mechanisms and thermodynamics of antimony adsorption by hydrous Fe and Al oxyhydroxides (goethite, gibbsite, and kaolinite) as a function of ionic environment, pH, temperature, and antimony concentration; (2) quantify the competitive effects of PO₄ and SO₄ on antimony adsorption; and (3) develop and evaluate the capability of chemical models to predict antimony adsorption within the holistic framework of a complex chemical environment.