Saturday, 15 July 2006

Glyphosate Adsorption on Pure Soil Component (Montmorillonite): Mineral Structure Modifications View by XPS and DRX Techniques.

Guillermo Zampieri, Comisión Nacional de Energía Atómica Centro Atómico Bariloche, Av. Bustillo 9500, Bariloche, Argentina, Maria Dos Santos Afonso, INQUIMAE-FCEyN-UBA, Ciudad Universitaria Pabellón II 3to Piso, BUENOS AIRES, Argentina, and Rosa Maria Torres Sanchez, CETMIC, CC 49, M. B. Gonnet, Argentina.

The herbicide glyphosate (N-phosphonomethylglycine, PMG) interacts strongly with many soil components. It forms strong complexes with many metals in solution, and it is adsorbed through innersphere complexation to iron- and aluminium oxides. Glyphosate can also be adsorbed by clay minerals by forming complexes with interlayer cations or bounded to octahedral cations, specifically iron and aluminium, exposed on the edges of these minerals surface. Because of these interactions, glyphosate is strongly adsorbed in soils. To understand the PMG soils binding mechanism it is necessary firstly to know the interaction mechanism with pure minerals that compounds soils. Clay, sand and organic matter are the main soil components. Among the clays, the smectites develops the highest specific surface area where all the interfacial processes take place. Smectites have in common a relatively low layer charge which allows the individual layers to separate to large dimensions in water giving unique and dramatic swelling properties to this particular mineral group. Two types of sites available for adsorption processes made layered silicates particularly interesting for adsorption processes. Structural isomorphic substitutions in the montmorillonite crystal originate negative binding sites at the interlayer in a wide pH range, evaluated by the cation adsorption capacity of the clay. A second binding site is found in the ampholitic surface hydroxyls, responsible for metal surface complexation. The aims of this study is to elucidate the PMG adsorption on montmorillonite by exposing the clay to aqueous solutions at different pHs, PMG concentrations and ionic strengths. The structures of the surface complexes are proposed based on the XPS spectra, XRD techniques. To attain this task two bentonites from different origin were used. One of them was collected from Neuquén Province, Argentina (Nu) and the second one from Wyoming, USA (Wyo). The structural formula determined by chemical analysis were [(Si3.99Al0.01) (Al1.50Fe0.24Mg0.26)O10(OH)2]M+0.26 and [(Si3.94Al0.06)(Al1.56Fe0.18Mg0.16)O10(OH)2]M+0.32 for Nu and Wyo respectively. The XRD analysis shows an interlayer expansion (d(001)) with the increase of PMG concentration, due to the PMG come in to the clay interlayer. Different PMG arrangements were suggested to explain the dependence of the interlayer shift with the stabilization pH. XPS analysis showed a different formula ratio between NH/N (protonated/non protonated nitrogen) between the two samples at same stabilization pH and PMG concentration that could be assigned to differences on the structural octahedral substitution and cationic exchange.

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