Glyphosate adsorption on montmorillonite: An experimental and theoretical study of surface complexes

In this study we perform X-ray diffraction and XPS analysis to study the adsorption of the herbicide glyphosate on montmorillonite. Structures of the surface complexes are proposed based on the XPS and XRD patterns and explored using molecular modeling techniques. Adsorption isotherms showed multistep profiles at all pHs studied, indicating the adsorption on aluminol and/or silanol groups of the external surface and in the interlayer space. PMG intercalation was indicated by the XRD patterns. The increase of the basal spacing as a function of the number of water molecules in the calculations indicated the appropriateness of the theoretical model to simulate macroscopic behaviors of the water/clay mineral interactions. Final conformations of the system determined by molecular mechanics indicated that the charged amino moiety was attracted by the negative surfaces of the interlayer space in absence or presence of water molecules, with marked pH dependence. Negative values of the estimated adsorption energies at all pH values studied suggested that glyphosate molecules were adsorbed via the amino moiety or through monodentate or bidentate complexes formed between the phosphonate moiety and the internal surfaces of the clay mineral, explaining the basal spacing changes at high concentrations of PMG.

Research Highlights
► The PMG adsorption isotherms on montmorillonite showed a multistep process at all pHs values.
► The adsorption occurs at least on two energetically different sites, the external surface and the interlayer space.
► Molecular mechanics is a useful tool in calculating the final conformations of glyphosate with montmorillonite in the interlayer spaces.
► The swelling of montmorillonite as a function of humidity was used to test the response of the molecular model.