The Sulfate Local Atomic Environment in Schwertmannite.

Schwertmannite (Fe₈O₈(OH)_6-x(SO₄)_x·nH₂O) is abundant and plays an important geochemical role in acidic iron- and sulfate-rich environment. Presumably, schwertmannite is a tunneled structure resembling akaganeite but with sulfate ions existing in the tunnels. Thus, sulfate acts as a template ion and stabilizes the tunnel structure. Understanding the role of sulfate requires the sulfate coordination environment, which however is still being debated. In this study, S K-edge EXAFS spectroscopy and X-ray atomic pair distribution function (PDF) analysis were used to determine the S local atomic environment of synthetic wet and air-dried schwertmannite samples. A selenate-version schwertmannite sample synthesized in the presence of sodium selenate was also examined. S EXAFS analysis clearly showed a S-Fe interatomic distance of 3.17 ± 0.05 Å for the air dried S-schwertmannite sample, suggesting that sulfate ions formed a bidentate-binuclear complex. This type of complexes exist in the tunnel of schwertmannite only when the tunnel has defects with missing FeO₆ octahedra. In contrast, no second coordination shells were observed for the wet sample, indicating sulfate was mainly put-sphere, probably H-bonded or existing as free sulfate ions under wet conditions. The PDF result shows neither S-Fe nor Se-Fe peaks for the air-dried samples, which may due to their low content and the S and Se binding sites, i.e., the defect sites, are disorderedly distributed in the structure. The results have important implication for understanding schwertmannite structure, formation mechanism, and crystal chemistry.