Facet-Dependent Atomic Distances Shape Vanadate Adsorption Complexes on Hematite Nanocrystals.

The environmental fate of vanadate (V(V)) is significantly influenced by iron oxide nanocrystals through adsorption. Nevertheless, the underlying driving force controlling V(V) adsorption on hematite (FeO) facets is poorly understood. Herein, V(V) adsorption on the {001}, {110}, and {214} FeO facets was explored using batch adsorption experiments, spectroscopic studies, and density functional theory (DFT) calculations. Adsorption experiments suggested that the order of V(V) adsorption capacity followed {001} > {110} > {214}. However, the affinity of V(V) to the {001} facet was the weakest, as evidenced by its least resistance to phosphate and sulfate competition. Our extended X-ray absorption fine structure (EXAFS) study indicated the formation of the inner-sphere monodentate mononuclear (V) complex on the {001} facet and bidentate corner-sharing (C) complexes on the {110} and {214} facets. Density functional theory (DFT) calculations showed the V complex is preferable when the adjacent Fe-Fe atomic distance is significantly larger than the O-O atomic distance of V(V). Otherwise, the C complex is formed if the distance is comparable. This determining factor in surface complex formation can be safely extended to other oxyanions that the compatibility in the atomic distance of Fe-Fe on FeO facets and O-O in oxyanions shapes the surface complex. The molecular-level understanding of the facet-dependent adsorption mechanism provides the basis for the design and application of oxyanion adsorbents.