Competition between glyphosate and phosphate for the surface of goethite. Adsorption and ligand exchange.

M. AVENA; C. WAIMAN; G. ZANINI; A. REGAZZONI

Nancy

Congreso; Interfaces Against Pollution; 2012

Interfaces Against Pollution

Glyphosate is a broad spectrum herbicide used in agriculture for the control of many different weeds. As a consequence of its widespread use, this herbicide has been found in surface waters, streams and other sites. The mobility and availability of glyphosate in the environment is strongly influenced by adsorption-desorption processes on minerals, particularly ferric oxides and hydrous ferric oxides. Glyphosate mainly adsorbs on these minerals by a ligand exchange process, resulting in the formation of inner-sphere surface complexes where the phosphonic group of the molecule binds Fe atoms at the surface displacing coordinated water molecules and/or hydroxyl ions. In natural environments, glyphosate adsorption on mineral surfaces is affected by pH, ionic strength and the presence of other substances such as other pesticides, ions, organic matter, fertilizers, surfactants, etc. Phosphate ions, for example, have a strong effect on glyphosate adsorption. Phosphate is a macronutrient for plants, and it is frequently applied to soils as a fertilizer. As it occurs with glyphosate, phosphate adsorbs on Fe and other mineral surfaces by forming inner-sphere complexes, and could compete with glyphosate for the same adsorption sites. In this work, the competition between glyphosate and phosphate for the surface of goethite is presented. More specifically, the desorption kinetics of glyphosate as induced by the presence of phosphate is discussed. The studies were performed by attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR), by first adsorbing glyphosate at different surface coverages and then adding phosphate to induce desorption. Data were complemented with individual glyphosate and phosphate adsorption isotherms and electrophoretic mobility measurements. The results show that both, glyphosate and phosphate adsorb strongly on goethite. ATR-FTIR is able to explore the goethite-solution interface at the molecular level, allowing the in-situ identification of the formed inner-sphere complexes. In desorption experiments, it was also possible to simultaneously quantify as a function of time the desorption of glyphosate complexes and the adsorption of phosphate complexes. When glyphosate is initially adsorbed at low surface coverages, added phosphate adsorbs mainly on free surface sites displacing a small amount of gyphosate. When the herbicide is initially adsorbed at high surface coverages, phosphate addition displaces significantly glyphosate from adsorption sites. Under these conditions, a phosphate-glyphosate ligand exchange can be observed. In view of the similarities in the structures of phosphate and the phosphonic moiety of glyphosate, it can be concluded that they are strong competitors for the goethite surface. Phosphate may thus play an important role in determining the bioavailability and leaching potential of glyphosate. CONICET, CONEA, ANPCYT, UNS and UNSAM are acknowledged.