Identification and Quantification of Sulfate Surface Complexes Formed on Ferrihydrite Surfaces Under Various Experimental Conditions.

Sulfate adsorption plays important role in many environmental geochemical processes. For example, in sulfate-enriched acidic soils and the environment impacted by acid mine drainage, sulfate complexation with dissolved Fe(III) or adsorption on mineral surfaces facilitates the formation of sulfate-bearing minerals, and promotes or inhibits adsorption of metals, radionuclides, anions and organic molecules on the surfaces. Sulfate adsorption also affects the behavior of aerosols because sulfate coatings on mineral dust aerosols could reduce their ice nucleation abilities. Due to such importance, sulfate adsorption has been extensively studied. However, the sulfate adsorption mechanisms still remain less understood partially due to the inadequacy of the widely-used vibrational spectroscopy in identifying and quantifying sulfate surface complexes. Here, we used recently available S K-edge EXAFS spectroscopy to determine the types of sulfate surface complexes on ferrihydrite and S K-edge XANES spectroscopy to quantify their proportions under various experimental conditions (pH, ionic strength, sulfate loading and hydration degrees). ATR-FTIR spectroscopy was used to provide complementary analyses. Results show that sulfate formed both inner- and outer-sphere complexes on ferrihydrite. The structure of the inner-sphere complexes was bidentate-binuclear, independent of the experimental conditions. The proportions of inner-sphere complexes of wet, air dried and freeze dried samples increased with decreasing pH, respectively, whereas at the same pH, they decreased in the order of increasing hydration degrees. Currently, the effects of ionic strength and sulfate loading are being examined. We are also trying to implement the bidentate-binuclear adsorption geometry into surface complexion modeling to quantify the proportions of the complexes. The quantification results will be compared to those obtained from the XANES analysis. This study provided a thorough quantitative analysis on sulfate complexation on ferrihydrite surfaces. The results solved some debates on sulfate adsorption and help improve the current understanding of the geochemical processes involving sulfate adsorption.