Adsorption of rare earth yttrium and ammonium ions on kaolinite surfaces: a DFT study

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 E-ads 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+.