Experimental and theoretical study of montmorillonite-glyphosate surface complexes.

L. TRIBE; R. M. TORRES SANCHEZ; M. DOS SANTOS AFONSO

Granada

Congreso; IV Internacional Conference Interfaces against pollution; 2006

Universidad de Granada

Glyphosate (N-phosphonomethylglycine), one of the most employed herbicides around the world, interacts strongly with many soil components. It forms strong complexes with many metals in solution, and it is adsorbed through inner-sphere complexation to iron- and aluminum oxides. The chemical group responsible for the adsorption processes is the phosphonic acid moiety. This phosphonic acid group through its very stable C-P bond makes glyphosate extremely resistant to chemical hydrolysis, thermal decomposition and photolysis Herbicides are generally adsorbed onto the soil components such as clay minerals, organic matter, metallic oxides and humic substances. In particular, expansive clays have important implications for agricultural, engineering and environmental purposes. Being montmorillonite the most common expansive clay found in soils. Montmorillonites have shown a great capacity of adsorption of both metallic cations and organic substances due to their small particle size (<2 µm), high surface area and isomorphic substitutions in their structure and because there have at least two types of sites available for adsorption process. Glyphosate adsorption isotherms were measured by ICP or CI.  Surface coverage was calculated by the difference between initial concentration and the concentration remained after 24 hs equilibrium. The adsorption dependence with pH was also measured and the thermodynamic adsorption parameters were calculated. The adsorption isotherm showed a multisite shape.  XRD and XPS were made on samples with glyphosate concentration correspondent to the different sections of isotherm and they were compare with those made without adsorbed glyphosate  XRD and XPS were also carried out at different pH values due to the influence of the pH on the montmorillonite structures and glyphosate molecule. The influence of the glyphosate content in the interlayer, was pointed out by a d(001) reflection parameter shift while XPS results indicated that the ratio of  [NH]/[NH2+] increase as the amino group of surface complexes is deprotonated.  The interactions of glyphosate molecules with montmorillonite were studied with theoretical methods ranging from ab initio to molecular mechanics calculations to explore the ways in which the molecule may bind to the interlayer surfaces of the mineral. To model the interaction of the fully protonated glyphosate molecule with  montmorillonite,  molecular modeling calculations were performed with the glyphosate molecule placed in the montmorillonite interlayer and allowed to move freely, without constraints. The structures of the surface complexes are proposed based on the XPS spectra, XRD, and theoretical molecular modeling techniques.  The role of the carboxylic acid and amino group in the structures of the surface complexes of the herbicide with montmorillonite is also discussed.