Homogeneous oxidation of SbIII by aqueous O2: the effect of ionic strength, Pb2+ and EDTA

Abstract

Environmental contextWhy does antimony become mobile faster in shooting range soils than in laboratory settings? We used controlled experiments and found that increased salinity and the presence of lead (which occurs with antimony at shooting ranges because they are both used in bullets) accelerate the change of antimony to a form that moves more easily through the watershed. This work helps explain the behaviour of this important pollutant – antimony – in the environment. AbstractAntimony (Sb) is a recognised contaminant of concern. The oxidation state of antimony controls its mobility in soil and aqueous environments. The predominance of SbV (as compared with SbIII) in shooting range soils and outflows downstream from stibnite deposits indicates the fast oxidation of SbIII to SbV. To better understand chemical controls on SbIII oxidation to SbV, we performed batch experiments, testing the effect of ionic strength, the presence of a complexing agent—the disodium salt of EDTA (Na2EDTA), and the addition of lead (Pb2+), on the apparent oxidation rate of SbIII to SbV. We also tested whether aqueous FeII reduces SbV to SbIII. We found that the rate of SbIII oxidation increased with increasing ionic strength. We proposed that the reactive species is Sb(OH)2+. The rate of SbIII oxidation was also increased owing to the catalytic effect of Pb2+. The complexation by EDTA decreased the rate of SbIII oxidation to SbV; however, the catalytic effect of Pb2+ was also evident in the systems with EDTA. Although thermodynamically favourable, the reduction of SbV by FeII was not observed in our samples reacted for up to 2 months. The results further explain the absence of SbIII in the shooting range soils, where metallic bullets, containing lead and antimony, undergo oxidation. The potential chemical factors contributing to the fast oxidation observed in the field include the presence of high concentrations of Pb2+, which catalyses the oxidation of SbIII to SbV, and potentially high ionic strength of the soil pore waters.