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

Yan, Wei, Jing, Chuanyong
Environmental science & technology 2018 v.52 no.4 pp. 1946-1953
Fourier transform infrared spectroscopy, adsorption, amino nitrogen, density functional theory, glyphosate, goethite, iron, oxides, oxygen, pH, phosphates, phosphonates, physical properties, pollutants, reaction mechanisms, risk assessment
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 pKₐ₁< pH < pKₐ₂, at phosphonate oxygen when pKₐ₂< pH < pKₐ₃, and at amino nitrogen when pH > pKₐ₃. 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.