Oxidation and mobilization of metallic antimony in aqueous systems with simulated groundwater

Antimony (Sb) is a contaminant of concern that can be present in elevated concentrations in shooting range soils due to mobilization from spent lead/antimony bullets. Antimony in shooting range soils has been observed as either metallic Sb(0) or as Sb(V) immobilized by iron (hydr)oxides. The absence of Sb(III) in soils is indicative of rapid Sb(III) oxidation to Sb(V) under surface soil conditions. However, the major controls on antimony oxidation and mobility are poorly understood. To better understand these controls we performed multiple batch experiments under oxic conditions to quantify the oxidation and dissolution of antimony in systems where Sb(0) is oxidized to Sb(III) and further to Sb(V). We also tested how variations in the aqueous matrix composition and the presence of metallic lead (Pb) affect the dissolution, solid phase speciation, and oxidation of antimony. We monitored changes in the aqueous antimony speciation using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). To test which solid phases form as a result of Sb(0) oxidation, and therefore potentially limit the mobility of antimony in our studied systems, we characterized the partially oxidized Sb(0) powders by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and powder X-ray diffraction (XRD). The observed oxidation of Sb(0) to Sb(III) and mobilization to solution is rapid: after 5–15min of reaction the aqueous antimony concentration reached 50–600μM. The amount of dissolved antimony and the rate of Sb(III) oxidation to Sb(V) in deionized water is lower than what we measured in the simulated groundwater systems. Sénarmontite (Sb2O3), the primary crystalline oxidation product of Sb(0), was detected after one month from the beginning of Sb(0) oxidation. The maximum aqueous Sb(III) concentration is about 30 times larger than the predicted equilibrium concentration with respect to sénarmontite in the initial stages (