INVESTIGADORES
ZANINI Graciela Pilar
congresos y reuniones científicas
Título:
COMPETITION BETWEEN GLYPHOSATE AND PHOSPHATE FOR THE SURFACE OF GOETHITE. ADSORPTION AND LIGAND EXCHANGE.
Autor/es:
MARCELO J. AVENA; CAROLINA V. WAIMAN; GRACIELA P. ZANINI; ALBERTO E. REGAZZONI
Lugar:
Nancy
Reunión:
Conferencia; 7th International Conference Interfaces Against Pollution; 2012
Resumen:
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.