Mechanistic Study for Antimony Adsorption and Precipitation on Hematite Facets.

Heterogeneous reactions at the mineral-water interface are of paramount importance in controlling the transport of contaminants. Herein, antimony (Sb) adsorption and subsequent precipitation on FeO facets were explored to understand its partitioning mechanisms by multiple complementary techniques. Our extended X-ray absorption fine structure spectroscopy and density functional theory results provided a consensus on the local coordination environment of Sb(III) and Sb(V) on FeO facets. We observed that Sb adsorption and the following precipitation are associated with both Sb concentrations and FeO facets, and a change in the Sb surface-binding mode from edge-sharing to corner-sharing is preferred in precipitation. FeO facets determine Sb binding structures, resulting in a facet-dependent transformation of adsorption to precipitation. The preferred corner-sharing complexes on the {001} facet facilitated the formation of SbO and NaSb(OH) precipitates at a lower Sb concentration compared with other two {110} and {214} facets. In addition, the facet-specific binding configuration renders a heterogeneous epitaxial growth of SbO. Our study provides a molecular understanding of facet effects on Sb adsorption and precipitation on minerals.