Quantitative antimony speciation in shooting-range soils by EXAFS spectroscopy

The Sb speciation in soil samples from Swiss shooting ranges was determined using Sb K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and advanced statistical data analysis methods (Iterative Transformation Factor Analysis, ITFA). The EXAFS analysis was supported by a spectral data base of 13 Sb minerals and 4 sorption complexes. In spite of a high variability in geology, pH (3.1 – 7.5), Sb concentrations (1000 – 17000 mg/kg) and range history, the spectra of all nine samples could be reconstructed with only two components, indicative of two Sb species. In the first species, Sb is surrounded solely by other Sb atoms at radial distances of 2.90, 3.3.5, 4.30 and 4.51 Å, which are typical for metallic Sb(0). The Sb(0) metal species prevailed in a very acidic (pH 3), organic matter-rich soil. Since no metallic Pb, but Pb2+ bound to soil organic matter was found in this soil by Pb LIII-edge EXAFS spectroscopy, the Pb component of the bullet PbSb alloy may have dissolved more rapidly than the Sb component, leaving metallic Sb behind. In the second species, Sb is coordinated to 6 O atoms at a distance of 1.98 Å, indicative of Sb(V). Iron atoms at radial distances of 3.10 and 3.56 Å are in line with edge-sharing and bidentate corner-sharing linkages between Sb(O,OH)6 and Fe(O,OH)6 octahedra, similar to structural units in the Fe antimonate tripuhyite. This local arrangement, the prevalence of Fe backscatterers and the small coordination numbers suggest formation of clusters of Fe-rich antimonate. This pentavalent Sb species was present in all samples, suggesting that it is the prevailing species after weathering of metallic Sb(0) in oxic soils.