Glyphosate adsorption onto kaolinite and kaolinite-humic acid composites: Experimental and molecular dynamics studies

Glyphosate (PMG) has been the most widely used herbicide in the world, and its environmental mobility and fate are mainly controlled by interactions with mineral surfaces. In soil systems, kaolinite is typically associated with humic acids (HAs) in the form of mineral-HA complexes, and hence it is crucial to characterize the molecular-scale interactions that occur between PMG and kaolinite and kaolinite-HA complexes. Batch experiments, Fourier transform infrared spectrum (FTIR) and X-ray photoelectron spectroscopy (XPS), isothermal titration calorimetry (ITC), and molecular dynamics (MD) simulations were performed to decipher the molecular interactions between PMG and kaolinite and kaolinite-HA composites. Our results reveal that kaolinite-HA composites adsorb higher concentrations of PMG than does kaolinite alone, likely due to more adsorption sites existed on kaolinite-HA than on kaolinite. FTIR and XPS analysis reveal that the carboxyl, phosphonyl and amino groups of PMG interacted with kaolinite and kaolinite-humic acid via Hydrogen bonds. The ITC results and interaction energy calculations indicate that the adsorption of PMG onto the kaolinite-HA is more energetically favorable relative to that onto kaolinite. MD simulations suggest that the PMG molecule adsorbs parallel to the surface of kaolinite and the composites through hydrogen bonding. Humic acid increases the adsorption of PMG through the creation of H-bond networks between PMG, the kaolinite surface, and humic acid. The results from this study improve our molecular-level understanding of the interactions between PMG and two important components of soil systems, and hence yield valuable information for characterizing the fate and behavior of PMG in soil environments.