Structure of Sulfate Adsorption Complexes on Ferrihydrite

Sulfate adsorption on mineral surfaces is an important environmental chemical process. However, the structure of sulfate adsorption complexes has remained uncertain. In this study, we have determined the S−Fe distance of sulfate inner-sphere adsorption complexes on iron (Fe) oxyhydroxide (ferrihydrite) surfaces under air-dried conditions using sulfur K-edge extended X-ray absorption fine structure spectroscopy and differential X-ray pair distribution function analysis. Both approaches indicate that the S− Fe interatomic distance of the sulfate adsorption complexes is 3.22−3.25 Å, suggesting that sulfate forms bidentate−binuclear adsorption complexes on ferrihydrite surfaces. Outer-sphere complexes are also observed using infrared spectroscopic analysis. This finding clarifies the long-standing debate over the structure of sulfate adsorption complexes and has important implications for understanding the chemistry of sulfate at environmental interfaces.