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The adsorption behavior of NH4+ and Mg2+ at kaolinite surfaces was investigated by using molecular dynamics (MD) simulations, considering the factors such as ion concentration, NH4+/Mg2+ mixing ratio, and layer charge of kaolinite. The results showed that the increase in ion concentration did not affect the adsorption modes of NH4+ and Mg2+ ions but promote the increase in the adsorption capacity. The total adsorption capacities of Mg2+ and NH4+ were 3.25 × 10−6 and 2.85 × 10−6 μmol·mm−2 at the ion concentration of 1.5 mol·L−1, respectively. When NH4+ and Mg2+ were co‐adsorbed, they could inhibit the adsorption of each other at the surface of kaolinite, except that the inner‐sphere (IS) adsorption of NH4+ at aluminum hydroxyl (Al–OH) surface could be enhanced by the presence of Mg2+. Both NH4+ and Mg2+ tended to adsorb at the siloxane (Si–O) surface of kaolinite rather than Al–OH surface. When layer charge occurred in kaolinite, a small number of Mg2+ began to adsorb in the IS complexes at 1.7 and 2.3 Å above the Al and O atoms of the lattice‐substituted tetrahedra of the Si–O surface, and at 1.7 Å above the hexahedra of the Al–OH surface. However, most of NH4+ were adsorbed in IS complexes at 1.7 Å above the center of the oxygen six‐membered ring of the Si–O surface and above the hexahedron of the Al–OH surface. The adsorption capacity of Mg2+ changed little with the increase of layer charge density, while the IS and total adsorption capacity of NH4+ increased significantly. [ABSTRACT FROM AUTHOR]

The ion-adsorbed rare earth is mainly concentrated in the in situ leaching process using NH4+ as the leaching agent. The adsorption mechanisms of Y3+ and NH4+ on kaolinite (001) and (00-1) surfaces were studied by density functional theory based on plane wave pseudopotential. The adsorption structures and energies, electrostatic potential, frontier orbital, Mulliken bond population and atomic charge, and electron density difference were calculated. The calculated results showed that the activity sites of kaolinite surfaces are located around the lying OH groups on (001) surface and above the center of the six-membered oxygen ring on (00-1) surface. Y3+ is adsorbed on kaolinite (001) surface primarily through covalent interaction while on kaolinite (00-1) surface primarily through Coulomb interaction. NH4+ is adsorbed on surface by electrostatic interaction and hydrogen bonding. The adsorption energy order on kaolinite (001) surface is Y3+ > NH4+ > H2O while NH4+ > Y3+ > H2O on kaolinite (00-1) surface. The presence of both the ligand water of cations and surface water can increase the Eads of NH4+/Y3+. The hydrated Y3+ can be still more strongly adsorbed on hydrated kaolinite surface than NH4+. When NH4+ is used as the leaching agent for ion-adsorption-type rare earth, it is necessary to increase the concentration of NH4+ to a certain value so that Y3+ could be effectively exchanged by NH4+. [ABSTRACT FROM AUTHOR]

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]