Molecular Insights into Glyphosate Adsorption to Goethite Gained from ATR-FTIR, Two-Dimensional Correlation Spectroscopy, and DFT Study

Abstract

Glyphosate (PMG) complexation on iron (hydr)oxides impacts its fate and transport in the environment. To decipher the molecular-level interfacial configuration and reaction mechanism of PMG on iron (hydr)oxides, the PMG protonation process, which influences the chemical and physical properties of PMG, was first determined using ATR-FTIR spectroscopy. The FTIR results reveal that the deprotonation occurs at carboxylate oxygen when pK_a₁< pH < pK_a₂, at phosphonate oxygen when pK_a₂< pH < pK_a₃, and at amino nitrogen when pH > pK_a₃. PMG complexation on goethite was investigated using in situ flow-cell ATR-FTIR, two-dimensional correlation spectroscopy (2D-COS), and density functional theory (DFT) calculations. The results indicate that the phosphonate group on PMG interacts with goethite to form inner-sphere complexes with multiple configurations depending on pH: binuclear bidentate (BB) and mononuclear bidentate (MB) without proton under acidic conditions (pH 5), mononuclear monodentate (MM) with proton and BB without proton at pH 6-8, and MM without proton under alkaline conditions (pH 9). Phosphate competition significantly impacted the PMG adsorption capacity and its interfacial configurations. As a result, the stability of the adsorbed PMG was impaired, as evidenced by its elevated leachability. These results improve our understanding of PMG-mineral interactions at the molecular level and have significant implications for risk assessment for PMG and structural analog pollutants.