Modeling of glyphosate and metal-glyphosate speciation in solution and at solution-mineral interfaces

Abstract: Glyphosate (N-(phosphonomethyl)glycine, PMG, H3L) is a widely used organophosphorous herbicide. It interacts with metal ions and mineral surfaces, which may affect its mobility, degradation and bioavailability in the environment. However, these interactions are far from fully understood. This thesis is a summary of five papers discussing the complexation of PMG with metal ions in aqueous solution and the adsorption of PMG and/or Cd(II) on different mineral surfaces.The complexation of PMG with the metals Cd(II) or Al(III) in aqueous solution was investigated with macroscopic and molecular scale techniques. Potentiometric titration data were combined with EXAFS, ATR-FTIR and NMR spectroscopic data to generate solution equilibrium models. In the PMG-Cd(II) system, only mononuclear complexes were formed, while both mono and binuclear complexes were observed in the PMG-Al(III) system.EXAFS, ATR-FTIR, and XPS measurements showed that PMG adsorbs to the surfaces of goethite (?-FeOOH), aged ?-alumina (?-Al2O3) and manganite (?-MnOOH) through one oxygen of its phosphonate group to singly-coordinated surface sites. Surface complexation models consistent with these spectroscopic results were fit to adsorption data using the 1pK reaction formalism. Electrostatic effects were accounted using either the Extended Constant Capacitance Model (ECCM) or the Basic Stern Model (BSM), and the charge of the surface complexes was distributed over the different planes. The formation of the surface complexes was described according to the following reactions:?MeOH(0.5-) + H3L <=> ?MeHL(1.5-) + H2O + H+?MeOH(0.5-) + H3L <=> ?MeL(2.5-) + H2O + 2H+The coadsorption of PMG and Cd(II) on the surfaces of goethite and manganite results in the formation of ternary mineral-PMG-Cd(II) surface complexes, as suggested from EXAFS results. Previous EXAFS measurements have also established the coordination geometries for the binary goethite-Cd(II) and manganite-Cd(II) surface complexes. In addition to the surface reactions in the binary mineral-Cd(II) and mineral-PMG systems, a single ternary complex with the stoichiometry ?MeLCd(OH)(1.5-) was sufficient to explain coadsorption data:?MeOH(0.5-) + H3L + Cd2+ <=> ?MeLCd(OH)(1.5-) + 3H+It was concluded that the affinity of PMG for the three mineral systems decreases within the series: goethite > aged ?-Al2O3 > manganite. The formation of the ternary surface complex is more significant on goethite surfaces than on manganite surfaces.