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 Fe 2 O 3 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 Fe 2 O 3 facets. We observed that Sb adsorption and the following precipitation are associated with both Sb concentrations and Fe 2 O 3 facets, and a change in the Sb surface-binding mode from edge-sharing to corner-sharing is preferred in precipitation. Fe 2 O 3 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 Sb 2 O 3 and NaSb(OH) 6 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 Sb 2 O 3 . Our study provides a molecular understanding of facet effects on Sb adsorption and precipitation on minerals.