Investigation of protonation and deprotonation processes of kaolinite and its effect on the adsorption stability of rare earth elements

The surface hydroxyl groups of kaolinite can be protonated and deprotonated, which have an effect on the adsorption of rare earth elements (REEs). In this paper, the protonation and deprotonation processes of hydroxyl groups on kaolinite basal (001) and edge (010) surfaces were first investigated by density functional theory (DFT) calculations, and their effect on the adsorption stability of REEs was studied with experiments. The calculated adsorption energies of protonation and deprotonation on the (010) surface were − 129.51 and − 471.53 kJ/mol, respectively, which were higher than those on the (001) surface, moreover, the adsorption energies of deprotonation were much higher than those of protonation on both the (010) and (001) surfaces. The calculation results suggest that hydroxyl groups on the (010) surface were more reactive than (001) surface, and the surface hydroxyl groups were more favourable for deprotonation. The buffer pH of kaolinite was 4.1, and the exothermic quantity of kaolinite with pH = 12 solution was 81.7 mJ, which was much higher than 15.7 mJ with pH = 2 solution. These experimental results were consistent with the calculated adsorption energies of protonation and deprotonation. The adsorption stability results showed that REEs tended to adsorb on the deprotonated kaolinite surface and were hard to desorb.