Adsorption of Phosphate Ions on the Basal and Edge Surfaces of Kaolinite in Low Salt Aqueous Solutions Using Molecular Dynamics Simulations

Clay applications for phosphate export are not widespread, in part because the mechanisms of nutrient adsorption on clay particles are not fully understood. Molecular dynamics simulations are used to study the adsorption of two species of phosphates that prevail at neutral pH, the dihydrogen phosphate ion H2PO4- and the hydrogen phosphate ion HPO42- in a 1:1 ratio, on the three principal planes of kaolinite, which include the two basal surfaces, octahedral of gibbsite (001) and tetrahedral of siloxane (00 (1) over bar), and the edge sites (010), in the presence of small amounts of salt. In particular, the anchoring mechanisms of phosphate solvates on kaolinite and the role played by salt are studied. Adsorption on kaolinite occurs preferentially in large clusters through a few contacts. The hydrophilic surfaces (010) and (001) adsorb phosphates, HPO42- more than H2PO4-, whereas the hydrophobic surface (00 (1) over bar) does not. At kaolinite edges, sodium ions are adsorbed in large quantities, favoring the adsorption of phosphates through cationic bridges, which occur in practically the same proportion as H bonds. The adsorptive capacity of kaolinite, which compares well with the few available experimental results, increases with salt concentration, reinforcing that cationic bridges are relevant. Furthermore, the adsorption mechanisms show that both phosphates and kaolinite, despite their low lability, renounce their hydration to promote adsorption at the contact points. These results are expected to contribute to the use of clays that are common in nature in the control of phosphates in large bodies of water.