The chemical mechanisms that govern glyphosate interactions with soil constituents are not fully understood. In studies of single soil constituents, iron and aluminum oxides have shown a particularly high affinity for glyphosate compared to other soil constituents. We used goethite and poorly crystalline boehmite (PCB) in this study as model sorbents to show the interactions of glyphosate on soil constituents. We identified glyphosate sorption on goethite and PCB using coupled sorption and Fourier-Transform Infrared (FTIR), high-performance liquid chromatography (HPLC), diffuse reflectance, and in situ attenuated total reflectance (ATR) FTIR measurements.

Glyphosate sorbed strongly to goethite and PCB. The sorption of glyphosate reached a maximum of 133 and 217 μmol/g glyphosate for goethite and PCB, respectively. As the loading of glyphosate increased, the solution pH also increased. The increase in pH suggests that hydroxyl groups were displaced from the sorbent surfaces, thus evidence of ligand exchange.

By using ATR-FTIR and Raman spectroscopy, we found that the phosphonate bands of glyphosate shifted with changes in pH. At pH 3 the protonated P-OH group displayed a band at 917 cm^-1, and the symmetric and asymmetric P-OH modes appear at 1080 and 1186 cm^-1, respectively. At higher pH values, the P-OH band disappears due to deprotonation of the P-O bond. The POH modes merge and form a single band at 1093 cm^-1, which represents the P-O asymmetric stretching mode.

We also compared the spectra of glyphosate in solution with those of glyphosate on the sorbents. We noticed that the sorbent perturbed the phosphonate group of glyphosate. In the presence of goethite and PCB, the phosphonate stretching bands appeared at 1137 and 1107 cm^-1, respectively. In solution, this band appeared at 1093 cm^-1. The shift of the P-O stretch to a higher frequency when in the presence of the sorbents is evidence for glyphosate binding via its phosphonate moiety.

In an effort to join batch sorption and spectroscopic studies, we also found that the coupled batch sorption and spectroscopic studies showed good agreement concerning the macroscopic sorption behavior of glyphosate on the sorbents. This information, linked with the aforementioned spectroscopic work is significant because it provides further information concerning the binding of glyphosate with the sorbents.