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 (Fe2O3) facets is poorly understood. Herein, V(V) adsorption
on the {001}, {110}, and {214} Fe2O3 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 (1V) complex on the {001} facet and bidentate
corner-sharing (2C) complexes on the {110} and {214} facets.
Density functional theory (DFT) calculations showed the 1V complex is preferable when the adjacent Fe–Fe atomic distance
is significantly larger than the O–O atomic distance of V(V).
Otherwise, the 2C 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 Fe2O3 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.