An analysis of Zinc sorption to amorphous versus crystalline iron oxides using XAS

This research probes the mechanisms of Zn adsorption on hydrated oxides of iron (HFO and goethite) using XAS. A systematic investigation reveals that Zn²⁺ upon sorption to HFO retains its hydration shell (N ∼6 oxygens, R ∼2.18 Å), irrespective of pH and adsorbate loading. Furthermore, the absence of second-shell contributions in combination with the temperature dependence of the structural parameters confirms outer-sphere adsorption complexes with HFO. In a coprecipitation study, the local coordination environment was consistent with Zn adsorption to HFO. On the other hand, Zn²⁺ strongly adsorbs to goethite forming a tetragonal structure (N ∼4 oxygens and R ∼1.97 Å). Evidence of two Fe³⁺ ions in the second shell at approximately 3.51 Å suggests an innersphere adsorption complex with goethite. Results demonstrate that even though the local structures of HFO and goethite are similar, the surface sites available to transition metals like zinc are vastly different. Overall, these spectroscopic analyses are consistent with macroscopic observations.