Al-substitution-induced defect sites enhance adsorption of Pb2+ on hematite

Abstract

Al-substitution in hematite is ubiquitous in nature and strongly affects the environmental behaviors of hematite. However, the defect microstructure caused by Al-substitution and its inner relationship with the surface reactivity of hematite remain unclear. In this study, the crystal structure of Al-substituted hematite was characterized by high-resolution electron transmission microscopy (HRTEM) and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Acid–base titration and Pb²⁺ adsorption experiments were performed to investigate the surface reactivity of Al-substituted hematite. HRTEM images revealed that the proportion of (001) facets on hematite increased from 44.0 ± 3.8% to 84.8 ± 6.5% with increasing Al-substitution. HAADF-STEM images indicated that Al-substitution resulted in more vacancies of Fe atoms on hematite (001) facets. At these Fe defect sites, additional singly (FeOH^−0.5) and triply (Fe₃O^−0.5) coordinated hydroxyl sites were formed. The weight loss due to dehydrogenation was 1.7%, 3.3% and 6.2% with increasing Al-substitution from 0%, 5% to 10%. Besides, the relative percentage of surface oxygen atoms in the surface hydroxyl sites of Al-substituted hematite was consistent with the results of thermogravimetric analysis. At pH 5, the relative charge density of the hematite with 0%, 5% and 10% Al-substitution was 73, 94 and 241 mC m⁻², and their adsorption capacity for Pb²⁺ was 0.82, 1.23 and 3.51 μmol m⁻², respectively. These results indicate that the formation of more defect sites with increasing Al-substitution can significantly enhance the surface charge density and Pb²⁺ adsorption capacity of hematite.