Antimony Isotope Fractionation during Adsorption on Iron (Oxyhydr)oxides

• Published in: Environmental science & technology 2024-01, Vol.58 (1), p.695-703
• Main Authors: Luo, Jiabei, Xie, Xianjun, Shi, Jianbo, Wang, Yanxin
• Format: Article
• Language: English
• Subjects: Adsorption; Antimony; Antimony isotopes; Bonding strength; Density functional theory; Fine structure; Fractionation; Free energy; Gibbs free energy; Goethite; Iron; Isotope fractionation; Isotopes; Occurrence, Fate, and Transport of Aquatic and Terrestrial Contaminants; Oxides; Ultrastructure; X ray absorption
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.3c05867

The fate of antimony (Sb) is strongly affected by adsorption, yet Sb isotope fractionation and the associated mechanism have not been widely reported. Here we experimentally investigated the process of Sb­(V) adsorption on iron (oxyhydr)­oxides and the associated isotope effects. Sb isotope fractionation occurs during adsorption (Δ123Sbsolution–mineral = 1.20 ± 0.02‰ for ferrihydrite and 2.35 ± 0.04‰ for goethite). Extended X-ray absorption fine structure (EXAFS) analysis shows that Sb­(V) adsorption on iron (oxyhydr)­oxides occurs via inner-sphere surface complexation, including mononuclear bidentate edge-sharing (2E) and binuclear bidentate corner-sharing (2C) complexes. A longer atom distance of Sb–Fe in ferrihydrite leads to less Sb isotope fractionation during Sb adsorption than in goethite. The Gibbs free energy and Mayer bond order were calculated based on density functional theory (DFT) and suggested that the strength of the bonding environment can be summarized as Sb­(OH)6 – > 2E > 2C. In turn, the bonding environment indicates the mechanism of Sb isotope fractionation during the process. This study reveals that Sb isotope fractionation occurs during Sb­(V) adsorption onto iron (oxyhydr)­oxides, providing a basis for the future study of Sb isotopes and further understanding of the fractionation mechanism.