Study on the difference of Cr（Ⅵ） adsorption on ｛001｝ and ｛012｝ facets of hematite

Hematite is the most thermally stable iron oxide, and its interaction (adsorption) with environmental pollutants has an important influence on the migration, transformation, and even environmental fate of pollutants. However, crude hematite was used to study its adsorption effect on pollutants, and the results obtained did not accurately indicate its mineral surface reactivity. In this study, hematite nanoparticles (HNPs and HNCs) with 001 and 012 facets were successfully synthesized by employing a hydrothermal method. On this basis, the kinetic and thermodynamic characteristics of Cr(Ⅵ) adsorption on different facets of hematite were investigated, and the influence of pH and ion strength on the adsorption of Cr(Ⅵ) on different facets of hematite was studied. The results showed that the adsorption processes of Cr(Ⅵ) on HNPs and HNCs were both described by the quasi-second-order kinetic model, which follows the Langmuir and Freundlich isothermal adsorption models, respectively. With the increase in temperature, the decrease in K_L and K_F as well as △H^ɵ<0 indicated that the adsorption processes were exothermic. The adsorption amounts of HNPs and HNCs to Cr(Ⅵ) decreased with increasing solution pH. Combined the results of the ionic strength experiments with the DFT calculations, the adsorption of Cr(Ⅵ) on HNPs was mainly dominated by surface complexation, and the adsorption configuration was monodentate mononuclear, while the adsorption of Cr(Ⅵ) on HNCs was mainly dominated by surface complexation and electrostatic adsorption, and the adsorption configuration was bidentate binuclear. The above results can be expected to provide a reference for the application of hematite in environmental governance.