Glyphosate binding and speciation at the water-goethite interface: a surface complexation model consistent with IR spectroscopy and MO/DFT

Abstract

Binding of glyphosate (PMG) to metal (hydr)oxides controls its availability and mobility in natural waters and soils, and these minerals are often suggested for the removal of PMG from wastewaters. However, a solid mechanistic and quantitative description of the adsorption behavior and surface speciation on these surfaces is still lacking, while it is essential for understanding PMG behavior in aquatic and terrestrial systems. This study gives new insights through advanced surface complexation modeling of new and previously published adsorption data, supplemented with MO/DFT calculations of the geometry, thermochemistry and theoretical infrared (IR) spectra of the surface complexes. PMG complexation by goethite (FeOOH) was measured over a wide range of pH (∼4-10), solution concentration (∼10^-7-10^-3M), and surface loading (∼0.3-3.0 μmol m^-2). Mechanistical modeling using the charge distribution approach revealed the formation of both monodentate and bidentate PMG complexes, each in two protonation states. PMG adsorption is dominated (>60%) by the formation of a bidentate complex having a protonated amino group that deprotonates at high pH and low loading, aligning with previously published ATR-FTIR analyses. Monodentate complexes are less abundant and maintain a protonated amino group over the entire pH range. In addition, the phosphonate group becomes protonated at low pH and high loading. DFT calculations support the role of protons in the surface speciation. The obtained model was able to predict the solution concentration of PMG and its strong pH dependency over the full range in our experiments. Our study provides a new mechanistic and quantitative understanding of PMG binding to goethite, which enables improved predictions of the fate and transport of PMG in and towards natural waters, and provides a framework for optimizing the removal efficiency of PMG with metal (hydr)oxides.