Theoretical Investigation of Hydrated Y(III) Ion Adsorption on the Deprotonated Kaolinite (001) Surface: A Density Functional Study

Abstract

In ion adsorption-type deposits (IADs), the majority of hydrated rare earth element ions adsorb in an outer exchangeable form on the basal surface of kaolinite (001). However, the surface exhibits strong chemical reactivity, influenced by pH condition. It becomes negatively charged when the pH exceeds the isoelectric point of kaolinite (5–6). Under such conditions, the outer adsorption structure is disrupted, giving way to an inner adsorption structure. However, the structural details of inner-sphere adsorption have not been clearly elucidated. To explore this issue, based on confirmed outer-sphere adsorption, we established six inner-sphere adsorption models for hydrated Y(III) ions on deprotonated kaolinite (001) surfaces. Subsequently, these models were structurally optimized in the hope of obtaining plausible inner-sphere adsorption structures. These models consist of two monodentate (M1 and M2) and four bidentate types (B1-B4). Using density functional theory, we found that when considering monodentate adsorption, M2 is more stable, while in the case of bidentate site adsorption, B3 is more stable. During inner-sphere adsorption, yttrium forms ionic bonds with surface O− sites. Deprotonated upright hydroxyl groups (Ou) provide more intense ionic interactions with Y compared to tilted (Ol) groups. Projection state density analysis showed Y-O bonding is mainly due to Y-4d and O-2p orbitals interaction. Additionally, Y-4d and Ou-2p exhibit three sets of angular quantum number interactions, while Y-4d and Ol-2p have only two. This study enhances our atomic-level understanding of rare earth element adsorption on deprotonated kaolinite (001) surface.