CONTROLS ON OXIDATION AND MOBILIZATION OF METALLIC ANTIMONY IN AQUEOUS SYSTEMS WITH SIMULATED GROUND WATER

Antimony (Sb) is a contaminant of concern due to its toxic and, potentially, carcinogenic properties. One of the common sources of Sb in the environment is mobilization from spent Pb/Sb bullets used in army training and recreational shooting. Recent studies have shown that Sb in shooting range soils is present in either metallic Sb(0) form or as Sb(V) immobilized by Fe(III) oxides. Mobility of Sb depends on the oxidation state: Sb(V) is believed to be more mobile compared to Sb(III). The absence of Sb(III) in soils is indicative of fast kinetics of Sb(III) oxidation to Sb(V) under surface soil conditions; however in homogeneous aqueous systems, the oxidation of Sb(III) to Sb(V) by dissolved O₂ is extremely slow. In oxic aqueous systems, dissolved Fe(II) and several other cations (e.g. Pb²⁺) can increase the rate of Sb(III) oxidation to Sb(V). Currently, there is no clear understanding of major controls on the rate of Sb(0) oxidation and its mobility in surface soil.

For this project we performed multiple experiments designed to quantify the rate of Sb(0) dissolution as it is oxidized to Sb(III) and further to Sb(V), and determined the effect of variations in the aqueous matrix composition (simulated ground water). We also tested whether the addition of common cations (Na⁺ and Ca²⁺) at different concentrations has an effect on the rate of the oxidation reaction in homogeneous oxic systems with Sb(III). To test which oxidized Sb solid phase could potentially limit the mobility of Sb in the studied systems, we characterized the partially oxidized Sb(0) by means of x-ray absorption fine structure spectroscopy (XAFS).

Metallic Sb is mobilized readily through oxidation to Sb(III) and Sb(V) and dissolution. The amount of dissolved Sb and rate of Sb(III) oxidation to Sb(V) in deionized (DI) water was lower compared to the simulated ground water systems. This finding is confirmed by XAFS measurements – the fraction of oxidized Sb in the partially oxidized Sb(0) samples is higher in system with simulated ground water compared to the DI water. We found that in the presence of the common cations Na⁺ and Ca²⁺, the rate of Sb(III) oxidation increases with increasing ionic strength. These results suggest that shooting range soils with pore waters characterized by high ionic strength can promote oxidation and mobilization of antimony.