Insights into Antimony Adsorption on {001} TiO₂: XAFS and DFT Study

Antimony (Sb) contamination poses an emerging environmental risk, whereas its removal remains a contemporary challenge due to the lack of knowledge in its surface chemistry and efficient adsorbent. In this study, self-assembly {001} TiO₂ was examined for its effectiveness in Sb removal, and the molecular level surface chemistry was studied with X-ray absorption spectroscopy and density functional theory calculations. The kinetics results show that Sb adsorption followed the pseudo-second order reaction, and the Langmuir adsorption capacity was 200 mg/g for Sb(III) and 156 mg/g for Sb(V). The PZC of TiO₂, which was 6.6 prior to the adsorption experiment, shifted to 4.8 and <0 after adsorption of Sb(III) and Sb(V), respectively, indicating the formation of negatively charged inner-sphere complexes. EXAFS results suggest that Sb(III/V) adsorption exhibited a bidentate binuclear surface complex. The orbital hybridizing of complexes was studied by XANES, molecular orbital theory (MO), and density of states (DOS) calculations. The change in orbital energy derived from orbital hybridizing of adsorbed Sb on surfaces is the driving force underlining the Sb surface chemistry. New bonds between Sb and TiO₂ surface were formed with matched orbital energies. Integrating the molecular and electronic structures into surface complexation modeling reveals the nature of macroscopic Sb adsorption behaviors.