In Situ Structural Study of Sb(V) Adsorption on Hematite (11̅02) Using X-ray Surface Scattering

Area of Science:

• Geochemistry
• Surface Science
• Mineralogy

Background:

• Understanding metal-ligand interactions on mineral surfaces is crucial for environmental remediation and geochemical processes.
• Hematite (iron oxide) is a ubiquitous mineral in soils and sediments, influencing the fate of contaminants.
• Antimony (Sb) is a metalloid of environmental concern, and its surface complexation behavior dictates its mobility and bioavailability.

Purpose of the Study:

• To determine the specific binding mechanism and geometry of pentavalent antimony (Sb(V)) on a single-crystal hematite (11̅02) surface.
• To elucidate the nature of the Sb(V)-hematite interaction at the atomic level.
• To provide insights into the protonation state of surface groups involved in Sb(V) adsorption.

Main Methods:

• In situ crystal truncation rod X-ray diffraction (CTR) was employed to study Sb(V) adsorption.
• Advanced CTR data analysis was used to model the surface complex structure.
• Bond valence analysis was performed to verify the proposed structure and deduce surface protonation.

Main Results:

• Sb(V) adsorbs to the hematite (11̅02) surface as an inner-sphere complex.
• A tridentate binding geometry was identified, involving both edge-sharing and corner-sharing Fe-O octahedra.
• The Sb-Fe distance was determined to be 3.09(4) Å, with an average Sb-O bond length of 2.08(5) Å.
• Bond valence analysis supported the proposed structure and indicated the release of one OH- group at pH 5.5.

Conclusions:

• The study reveals a specific tridentate binding mode for Sb(V) on hematite, contributing to a fundamental understanding of metalloid-surface interactions.
• The findings have implications for predicting Sb behavior in natural aquatic and soil environments.
• The deduced protonation scheme and reaction stoichiometry enhance geochemical models of contaminant sorption.