Sorption of phenoxyacetic acid herbicides on the kaolinite mineral surface - an ab initio molecular dynamics simulation

Full atomic position optimizations and ab initio molecular dynamics simulations at room temperature were performed in a study of the adsorption of phenoxyacetic acid herbicides an the octahedral surface of a kaolinite layer formed from hydroxyl groups. The studied molecules are bound strongly to the surface hydroxyl groups via hydrogen bonds formed through the carboxyl group. Particularly strong bonding with the hydrogen atom approximately midway between the two adjacent oxygen atoms was observed between the OH group of the carboxyl group and one of the surface OH groups. Molecular dynamics simulations showed proton jumps within this strong hydrogen bond. Kaolinite surface hydroxyl groups are chemically active and able to act as proton donors or acceptors. Additional interactions were observed between atoms of the aromatic parts of the adsorbed molecules and the surface hydroxyl groups. The phenoxyacetic acids are significantly more strongly bound than acetic acid to the octahedral kaolinite surface. This strong bonding of the phenoxyacetic acids to the kaolinite layer surface indicates a strong retention capability in the soil matrix and a significant contribution of kaolinite clay minerals to the "bound residue" phenomenon.