On glyphosate-kaolinite surface interactions. A molecular dynamic study

Glyphosate is an important and widely used herbicide, its environmental behaviour being of scientific and public interest. Computational models of clay minerals and their interactions with small organic molecules are valuable in studying adsorption processes at an atomistic resolution. We analysed the adsorption process of glyphosate on kaolinite, a clay mineral with a high abundance in several types of soils (e.g., of subtropical or tropical origin), in terms of the adsorption strength. The molecular interactions are characterized by monitoring the occurrence of hydrogen bonds, the orientation of the molecular dipole relative to the interface and the interaction energy. Two different ionic forms of glyphosate were considered: neutral and anionic (-1). It was shown that the main mechanism of the binding of both glyphosate forms to the aluminol surface of kaolinite is through multiple hydrogen bonds. The standard free energy of adsorption of neutral glyphosate from water solution to the basal octahedral surface of kaolinite was computed at -5 kJ mol^-1, whereas for the anionic form this quantity amounted to -14 kJ mol^-1. Our finding showed that kaolinite has an important contribution to overall adsorption capacity of soils for glyphosate, specifically in its anionic form.

The adsorption free energy of glyphosate on a kaolinite surface is quantifiedInteractions are computed by quantum mechanics and by classical force fieldMolecular interactions are characterized in terms of hydrogen bonds and orientationsThe effect of polarization of the medium on the calculations is analysed.