The adsorption/desorption characteristics for light and heavy rare earth elements (REEs) on halloysite and illite (which are beneficial for the utilization of ion-adsorption RE ore) were systematically investigated and compared. Additionally, halloysite and illite were fully charactered by XRD, SEM, microscope, zeta potential, nitrogen adsorption–desorption isotherms and buffer pH to build the relationship between adsorption/desorption mechanisms and the minerals’ properties. The results of experiments show that the adsorption rate of halloysite is higher than illite, although they are both very fast and follow the pseudo-second-order kinetic model. The adsorption capacity of halloysite and illite increases with an increase in adsorption pH and remains constant when pH is higher than 4. Due to the narrow interlamellar spacing of halloysite and the fact that it is a nanotube, RE ions are adsorbed only through electrostatic attraction, whereas the adsorption and desorption pH have a significant effect on the recovery of RE ions from illite, because of the diverse adsorption mechanism. The results illustrated that the structure and surface properties of clays are also the key factors that affect RE ions leaching.

Kaolinite is the primary mineral in ion-adsorption rare earth ores, and improving our understanding of the adsorption state of REEs on kaolinite will benefit efforts to leach REEs from these ores. In order to explain why Eu³⁺ ions exhibit stronger pH-dependent desorption behavior than Lu³⁺, molecular dynamics simulations were applied to investigate the adsorption mechanism of Eu³⁺ and Lu³⁺ on a deprotonated kaolinite (001) surface. The simulation results show that the hydration numbers of Eu³⁺ and Lu³⁺ are comparable, whereas the ordering degree of water molecules around Eu³⁺ is higher than that of Lu³⁺, which is beneficial to the movement of Eu³⁺ to preferentially occupy favorable adsorption sites on the kaolinite surface, following which coordination bonds are formed between Eu³⁺ and the surface. As a result, the desorption rate of Eu³⁺ decreases sharply with the increase in leaching pH, while the desorption rate of Lu³⁺ is only slightly affected by pH.

Fe impurity is abundant in rare earth leaching solutions. The optimal hydrate structure of Fe(OH)²⁺ was calculated based on the quantum chemical in the water environment to investigate the microscopic occurrence of Fe impurity on kaolinite surfaces. The adsorption structure and bonding mechanism (including outer and inner layer) of hydrate Fe(OH)²⁺ on the kaolinite (001) surface were investigated. According to the results, the stable hydrate form of Fe(OH)²⁺ is [Fe(OH)(H₂O)₅]²⁺. Hydrated Fe(OH)²⁺ has a tendency to adhere to the Si-O surface in the form of outer layer adsorption. Adsorbate tends to adsorb to the O_u (deprotonated upright hydroxyl) site, where it generates a monodentate adsorption compound, and to the O_l and O_u (deprotonated lying and upright hydroxyl) sites, where it generates a bidentate adsorption compound if inner layer adsorption occurs. The Mulliken population and density of state analysis demonstrate that the ionic properties of Fe-O_s in the inner layer adsorption compound are reduced and Fe-O_s bonds are filled with strong bonding and weak antibonding orbitals. More chemical bonds are formed in the bidentate adsorption compound, and the bidentate adsorption has lower adsorption energy. Therefore, in the process of inner layer adsorption, bidentate adsorption is more likely to occur.