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Graphical abstract Highlights • Y(OH) 3-n n+ ions are adsorbed on kaolinite mainly via Y atom and surface O atoms. • Ion hydrolysis and surface deprotonation affect the adsorption of Y(OH) 3-n n+ ions. • Adsorption energies increase with the increase of kaolinite deprotonation degree. • Adsorption energies decrease with the increase of the hydrolysis degree of Y3+ ion. Abstract The interaction of rare earth elements (REEs) ions with clay minerals is crucial for understanding the processing of mineralization, extraction and recovery of ion-adsorption rare earth. Theoretical studies based on density functional theory (DFT) were performed to investigate the adsorption of rare earth Y(OH) 3-n n+ (n = 1–3) ions on kaolinite surfaces of varying deprotonation degree depending on the solution pH. The Conductor-like Screening Model (COSMO) was used to simulate the water environment of adsorption. The calculated results showed that the surface electrostatic potential and frontier orbital are enhanced and increasingly localized around the exposed oxygen atoms with the increase of the deprotonation degree of kaolinite (0 0 1) surface hydroxyls. Y(OH) 3-n n+ ions are adsorbed mainly via the Y atom and the surface O atoms, and located over the center of Al-hexagonal ring on kaolinite (0 0 1) surface, and the center of Si-hexagonal ring on kaolinite (0 0 −1) surface. The adsorption energies increase with the increase of deprotonation degree of kaolinite (0 0 1) surface, and decrease with the increase of the hydrolysis degree of Y3+. The results of Mulliken bond length and population, atomic charge, electron density and density of states (DOS) revealed that Y(OH) 3-n n+ ions interact with kaolinite (0 0 1) surface through the combination of covalent and electrostatic bonding, while with (0 0 −1) surface mainly through electrostatic bonding. The above calculated results would provide some atomic-level insights for the interaction of rare earth ions with kaolintie surfaces in aqueous solution. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • F&Ti codoped heptazine/triazine g-C 3 N 4 heterostructure is explored by hybrid DFT. • F&Ti codoping induces a narrow bandgap of heptazine/triazine g-C 3 N 4 heterostructure. • The formed type-II heterostructure promotes the separation of electron-hole pairs. • F&Ti codoped heptazine/triazine g-C 3 N 4 has enough redox potential as photocatalyst. Abstract Graphitic carbon nitride (g-C 3 N 4) has been widely investigated as a metal-free photocatalyst for water splitting. However, the rapid recombination of photoexcited carriers and narrow visible-light response region substantially limit its performance. In this work, we have systematically studied the geometrical, electronic, optical, charge transfer and photocatalytic mechanism of (F, Ti) codoped heptazine/triazine based g-C 3 N 4 heterostructure using hybrid density functional approach. The interface interaction between heptazine and triazine g-C 3 N 4 shows heptazine and triazine g-C 3 N 4 form a Van Der Waals heterostructure. The bandgap of (F, Ti) codoped heptazine/triazine based g-C 3 N 4 heterostructure is narrow (2.39 eV), which enhances the absorption of visible light and leads to an obvious redshift of absorption edge. A type-II heterostructure is formed at the interface of (F, Ti) codoped heptazine/triazine based g-C 3 N 4 heterostructure, and leads to high photocatalytic activity. Furthermore, Bader charge and charge density difference indicate that the internal electric field promotes the separation of electron-hole pairs in the heptazine/triazine g-C 3 N 4 interface and inhibits carrier recombination. Meanwhile, electrons in the conduction band of triazine g-C 3 N 4 and holes in the valence band of (F, Ti) codoped heptazine g-C 3 N 4 have enough redox ability. This work is helpful in understanding the mechanism of photocatalytic water splitting and relevant experimental observations. [ABSTRACT FROM AUTHOR]