Adsorption of hydrated Fe(OH)²⁺ on the kaolinite surface: A density functional theory study

The present study employed density functional theory (DFT) to analyze the adsorption configuration and mechanism of Fe(OH)²⁺ on the kaolinite (001) surface. The findings demonstrated that Fe(OH)₂(H₂O)⁴⁺ is the main type in which hydrated Fe(OH)²⁺ can be found in aqueous solution. On the surface of kaolinite, Fe(OH)₂(H₂O)⁴⁺ will be adsorbed. There are two forms of adsorption: outer-sphere and inner-sphere coordination (monodentate/bidentate) adsorption. Fe(OH)₂(H₂O)⁴⁺ has a moderate propensity to adsorb on the alumina octahedral sheet of kaolinite when the outer-sphere coordination adsorption takes place. In cases of inner-sphere coordination adsorption, Fe exhibits a tendency to form monodentate adsorption compounds in conjunction with Ou atoms. Additionally, it prefers to create bidentate adsorption compounds through coordination with both Ot and Ou atoms. The adsorption mechanism analysis results show that the ionic property of Fe atom decreases after outer-sphere coordination adsorption. After inner-sphere coordination adsorption, some electrons of Fe atom are transferred to the surface O atom. The presence of electrons between the Fe and O atoms enhances the formation of bonds, hence enhancing the covalent nature of the Fe-O bond. Theoretical FT-IR (Fourier transform infrared spectroscopy) calculations show that the formation of Fe-O chemical bonds. Because of the lower adsorption energy and more chemical bonds, hydrate Fe(OH)²⁺ is more likely to be bidentate adsorbed on the kaolinite surface.