Uranyl adsorption on (001) surfaces of kaolinite: A molecular dynamics study

The aim of the present paper is to investigate the adsorption of uranyl species (UO2)2 +(H2O)5 onto kaolinite (001) surfaces. To this end we have employed molecular dynamic simulations based on CLAYFF force field potential. Various types of surface model for inner-sphere adsorption complexes and one model for outer-sphere adsorption complexes were optimized. In order to have a neutral structure, the uranyl (UO2)2 +(H2O)5 or the kaolinite was deprotonated to form the outer-sphere or inner-sphere adsorption complexes. Both singly protonated and partially deprotonated states of the Al(0) kaolinite surface were considered for adsorption in the model of inner-sphere complexes. The first uranyl coordination shell exhibits pentagonal bi-pyramidal symmetry with the pentagonal formed by 5 water molecules. We show that the average U-OW distances are between 2.49 and 2.57 Å for water molecules. The bond of uranyl with deprotonated O− center is always short because of the charge attraction. The obtained results agree well with density functional calculations and EXAFS measurements, and show how and why the adsorption of uranyl appears on the surface of kaolinite.