Adsorption of Glyphosate on Goethite: Molecular Characterization of Surface Complexes

As a component of herbicides, the fate of glyphosate (PMG) in the environment is of significant interest. The nature of PMG adsorption on mineral surfaces plays a significant role in the degradation of PMG. The adsorption of PMG on goethite (alpha-FeOOH) has been studied as a function of pH and PMG concentration. Adsorption was investigated with batch experiments, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The N 1s line in XPS spectra showed deprotonation of the amine group of PMG (NH2+) with increasing pH. IR analyses showed no evidence for the interaction of PMG's carboxylate group with the goethite surface, while the phosphonate group formed inner-sphere complexes. There is evidence for intramolecular hydrogen bonding between NH2+ and both the carboxylate and the phosphonate groups at low pH. Intramolecular hydrogen bonding is lost when the amine group is deprotonated, and the trend in intramolecular hydrogen bonding between NH2+ and phosphonate shows that PMG adsorbs via predominantly monodentate complexation. A minor quantity of bidentate complexes is thought to form both at near-neutral pH and when the surface concentration of PMG is low. While the phosphonate group of PMG binds directly, the carboxylate group remains relatively "free" from complexation with goethite, leaving it subject to degradation and/or complexation with metal ions present in the environment.