Glyphosate adsorption on montmorillonite: An experimental and theoretical study of surface complexes George A. Khoury a , Todd C. Gehris a , Lorena Tribe a , Rosa M. Torres Sánchez b , Maria dos Santos Afonso c, ⁎ a Division of Science, The Pennsylvania State University-Berks Campus, Tulpehocken Rd., Reading, PA 19610, United States b Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC), Cno. Centenario y 506, Gonnet 1897, Argentina c INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pabellón II 3er Piso, C1428EHA Buenos Aires, Argentina abstract article info Article history: Received 30 September 2009 Received in revised form 20 July 2010 Accepted 21 July 2010 Available online 30 July 2010 Keywords: Glyphosate Clay mineral Adsorption Herbicide Montmorillonite In this study we perform X-ray diffraction and XPS analysis to study the adsorption of the herbicide glyphosate on montmorillonite. Structures of the surface complexes are proposed based on the XPS and XRD patterns and explored using molecular modeling techniques. Adsorption isotherms showed multistep profiles at all pHs studied, indicating the adsorption on aluminol and/or silanol groups of the external surface and in the interlayer space. PMG intercalation was indicated by the XRD patterns. The increase of the basal spacing as a function of the number of water molecules in the calculations indicated the appropriateness of the theoretical model to simulate macroscopic behaviors of the water/clay mineral interactions. Final conformations of the system determined by molecular mechanics indicated that the charged amino moiety was attracted by the negative surfaces of the interlayer space in absence or presence of water molecules, with marked pH dependence. Negative values of the estimated adsorption energies at all pH values studied suggested that glyphosate molecules were adsorbed via the amino moiety or through monodentate or bidentate complexes formed between the phosphonate moiety and the internal surfaces of the clay mineral, explaining the basal spacing changes at high concentrations of PMG. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Glyphosate, N-(phosphonomethylglycine) (PMG), the active in- gredient in Roundup®, has become the most popular nonselective post-emergent herbicide in the world. This herbicide is used on many food and non-food crops as well as non-crop areas such as roadsides. When it is applied at low rates, it serves as a plant growth regulator. The most common uses include broadleaf weed control on grasses in hay/pasture, soybeans, field corn, ornamentals, lawns, turf, forest plantings, greenhouses and rights-of-way. The mechanism of action of glyphosate is the inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase which produces EPSP from shikimate-3-phosphate and phosphoenolpyruvate in the shikimic acid pathway. EPSP synthase inhibition leads to a depletion of the aromatic amino acids tryptophan, tyrosine and phenylalanine, all needed for protein synthesis or for biosynthetic pathways leading to growth (Schonbrunn et al., 2001). Due to the different application methods and weather conditions, a significant amount of herbicide reaches the soil. Herbicides that penetrate the soil may be bound in various ways to soil constituents. They may undergo transport from the contact area, and they may be degraded by different mechanisms (Araujo et al., 2003; Stenrød et al., 2005). Herbicides are generally adsorbed onto the soil components such as clay minerals, organic matter, (hydr)oxides and humic substances. Since the fate of herbicides in soil is generally mediated by water, the rates of transport processes are directly related with the water solubility, metal complexation and adsorption characteristics of the system. The chemical interactions of this herbicide with water and soils control its permanence in the environment. Upon contact with soils and clay minerals PMG is immobilized due to the formation of surface complexes with metal ions, mainly Fe 3+ , Al 3+ and Ca 2+ (Franz et al., 1997). The experimental evidence shows that the phosphonate group is responsible for the strong adsorption of glyphosate on iron oxides (Barja, 1999; Barja and dos Santos Afonso, 2005) and also of the metal complexation with free metals in aqueous solution (Barja and dos Santos Afonso, 1998; Barja et al., 2001). The carboxyl group of the molecule of PMG coordinates the Fe(III) ions in solid state to form a 1:1 metal to ligand ratio complex (Subramaniam and Hoggard, 1988), but seems to remain uncoordinated in aqueous solution (Mc Bride and Kung, 1989). Although some authors have reported the formation of surface complexes of PMG on goethite (Mc Bride and Kung, 1989; Barja and dos Santos Afonso, 2005), clay minerals (Shoval and Yariv, 1979; Glass, 1987; McConnell and Hossner, 1989; Morillo et al., 1997; Damonte et al., Applied Clay Science 50 (2010) 167–175 ⁎ Corresponding author. Tel.: + 54 11 4576 3380x125; fax: + 54 11 4576 3341. E-mail address: dosantos@qi.fcen.uba.ar (M. dos Santos Afonso). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.07.018 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay