Spectroscopic Quantification of Inner- and Outer-Sphere Oxyanion Complexation Kinetics: Ionic Strength and Background Cation Effect on Sulfate Adsorption to Hematite

Sulfate adsorbs on Fe-oxide minerals by both inner- and outer-sphere modes; however, the time dependence of coexisting surface species is not clear. Using in situ attenuated total reflectance-Fourier transform infrared spectroscopy, peak fitting, and multivariate curve resolution analyses, we quantify adsorption and desorption kinetics of inner- and outer-sphere sulfate species on hematite at two ionic strengths (I = 0.01 and 0.10 M) and background cations (K+ and Ca2+) at pH 4.5. We experimentally observed inner-sphere, bidentate bridging and outer-sphere species kinetics congruent with the stepwise Eigen-Werner-Wilkins mechanism, in which a rapid formation of outer-sphere association precedes a slower conversion of outer-sphere to inner-sphere sulfate complexes. The rate limitation imposed by inner-sphere complex formation is likely linked to the displacement of protonated surface hydroxyl groups on the oxide surface by the adsorbing oxyanion. Outer-sphere complexes are responsible for rapid adsorption and desorption at I = 0.10 M seen in total sulfate, whereas inner-sphere species desorb more slowly. At I = 0.01 M, outer-sphere complexes similarly adsorb rapidly relative to inner-sphere complexes but desorb more slowly than inner-sphere complexes, possibly because of ionic strength effects on surface charge density. This work presents a novel, direct spectroscopic quantification of mixed surface species adsorption kinetics on a model mineral surface, which may be used to confirm proposed molecular mechanisms for other oxyanion adsorption to mineral surfaces. Our results enhance molecular-level understanding of oxyanion adsorption on soils and help predict their behavior in soils.