Speciation and Toxicity of Antimonytroxide in Soil.

Antimony trioxide (Sb₂O₃) is a widely used Sb(III) chemical that can be emitted to soil. Here we report the gradual dissolution of Sb₂O₃ in soil by measuring concentration and redox speciation of Sb in soil solution with reference to that the more soluble reference form SbCl₃. A silt-loam soil (pH 7.0, background 0.6 mg Sb kg^-1) was amended with Sb₂O₃ or SbCl₃. More than 70% of Sb in soil solution was present as Sb(V) within 2 days as revealed by IC-ICP-MS analysis. Soil solution Sb concentrations in Sb₂O₃ amended soils gradually increased between 2 and 35 days. Soil solution Sb concentrations in SbCl₃-amended samples were stable beyond 2 days after spiking and 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 slow dissolution of Sb₂O₃ with a half-life ranging between 50 and 250 days, sorption on the soil surface and Sb precipitation as Ca-antimonate at the higher doses. Plant uptake of Sb from soils freshly spiked with SbCl₃ or Sb₂O₃ or from soils with 5 year aged Sb₂O₃ was correlated with soil solution Sb across all soils. However, toxicity to plant or soil nitrification was far larger in SbCl₃ amended soils than that Sb₂O₃ and was unrelated to soil solution Sb. This is because effects of SbCl₃ were confounded by a decrease in pH and an increase in salinity as proven with CaCl₂ reference treatments. The 10% effect concentration of Sb after complete Sb₂O₃ transformation is found at 500 mg Sb/kg and above suggesting that Sb is not toxic to soil organisms in the environmental relevant concentration range.