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 5-15 min of reaction the aqueous antimony concentration reached 50-600 mu M. 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. Senarmontite (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 senarmontite in the initial stages (<65 h) of our experiment. Concentrations reach equilibrium within 146-222 days. The maximum concentration of Sb(V) is controlled by cation availability for the precipitation of an antimonate. In the systems where sodium Na(I) exceeded 20 mM precipitation of mopungite is observed. No crystalline phases were detected in the systems with added lead, and the dissolved Sb(V) concentration is several orders of magnitude higher than would be expected in equilibrium with bindheimite (Pb2Sb2O7). The observed solubility of Sb(V) in the systems with Ca(II) is several orders of magnitude larger than the solubility reported for romeite (Ca2Sb2O7). The addition of Pb(0) lowered the extent of Sb(0) oxidation due to competitive oxidation or to the coupling of antimony and lead redox reactions. The results from our research can be used to identify substrates that promote precipitation of relatively insoluble antimony compounds in target berm soils and thus prevent the offsite migration of antimony from shooting range target berms. (C) 2014 Elsevier Ltd. All rights reserved.