Adsorption of La on kaolinite (001) surface in aqueous system: A combined simulation with an experimental verification

A systematic first-principles model study of La(III) adsorption on kaolinite (001) surface was studied using periodic DFT calculations. The effective coordination number, coordination geometry, preferred adsorption position and adsorption type were examined. Based on the binding energy, the form of [La(OH)(H2O)(8)](2+) would be more stable than [La(H2O)(10)](3+) in a high pH solution system. Outer-layer and inner-layer adsorption models were applied to account for the different bonding between La and the kaolinite surface. The inner-layer adsorption model was dominated by coordination bonds between La and surface oxygen, while the outer-layer adsorption model was related to hydrogen bonds. The inner-layer adsorption at the O-u site was the optimal adsorption site. The PDOS projections and Mulliken bond populations suggest that the bonding orbital combination of La 5d and O-u 2p was the dominant orbital contribution of L-a-O-u. Contrary to the stability of La ions in aqueous systems, [La(H2O)(10)](3+) would be more stable than [La(OH)(H2O)(8)](2+) after adsorption on the kaolinite (001) surface, which was verified by microcalorimetry experiments. (C) 2021 Elsevier B.V. All rights reserved.