Solubility and toxicity of antimony trioxide (Sb₂O₃) in soil

Abstract

Antimony trioxide (Sb₂O₃) is a widely used chemical that can be emitted to soil. The fate and toxicity of this poorly soluble compound in soil is insufficiently known. A silt-loam soil (pH 7.0, background 0.005 mmol Sb kg⁻¹) was amended with Sb₂O₃at various concentrations. More than 70% of Sb in soil solution was present as Sb(V) (antimonate) within 2 days. The soil solution Sb concentrations gradually increased between 2 and 35 days after Sb₂O₃amendment but were always below that of soils amended with the more soluble SbCl³ at the lower Sb concentrations. The soil solution Sb concentrations in freshly amended SbCl³ soils (7 days equilibration) were equivalent to those in Sb₂O₃-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled by a combination of sorption on the soil surface, Sb precipitation at the higher doses, and slow dissolution of Sb₂O₃, the latter being modeled with a half-life ranging between 50 and 250 days. Toxicity of Sb to plant growth (root elongation of barley, shoot biomass of lettuce) or to nitrification was found in soil equilibrated with Sb₂O₃(up to 82 mmol Sb kg⁻¹) for 31 weeks with 10% inhibition values at soil solution Sb concentrations of 110 µM Sb or above. These concentrations are equivalent to 4.2 mmol Sb per kg soil (510 mg Sb kg⁻¹) at complete dissolution of Sb₂O₃in this soil. No toxicity to plant growth or nitrification was evident in toxicity tests starting one week after soil amendment with Sb₂O₃, whereas clear toxicity was found in a similar test using SbCl³. However, these effects were confounded by a decrease in pH and an increase in salinity. It is concluded that the Sb(V) toxicity thresholds are over 100-fold larger than background concentrations in soil and that care must be taken to interpret toxicity data of soluble Sb(III) forms due to confounding factors.