“Glyphosate Adsorption on Soils

B. C. BARJA; R. C. PESSAGNO; H. R. TÉVEZ; M. DOS SANTOS AFONSO

New York, USA

Simposio; 226th ACS National Meeting, Environmental Division: Biogeochemistry of chelating agents.Vol 43 No2; 2003

ACS

Glyphosate (HOOC-CH2-NH-CH2-PO3H2) is a postemergent herbicide widely used around the world. It is a non-selective herbicide being increasingly used to control annual and perennial weeds. This herbicide is used on many foods and non-foods crops as well as non-crop areas such as roadsides. When it is applied at lower rates, it serves as a plant growth regulator. The most common uses include broadleaf weeds control and grasses in: hay/pasture, soybeans, field corn; ornamentals, lawns, turf, forest plantings, greenhouses, rights-of-way.  The chemical interactions of this herbicide with water and soils control their permanency on the environment.  Glyphosate is immobilized in soils due to the formation of surface complexes with the metal ions. The surface recovery and the adsorption isotherm of this herbicide on soil fractions from different provinces of Argentina (Santa Cruz, Misiones, Santiago del Estero and Corrientes) followed similar patterns to those of pure minerals that form this soils.           The experimental evidence shows that the phosphonate group is the responsible for the strong adsorption of glyphosate and that monodentated and bidentated structures should be proposed for the surface complexes.  The characterization of the surficial adsorption of the herbicide on the pure minerals that compounds those soils and the soils themselves was done by different analytical methods. This study shows that glyphosate adsorbs onto goethite, clays and soils at several pH values, the Langmuir adsorption isotherms are performed and their KL and Gmax., evaluated. The structures of the surfaces complexes are proposed based on the ATR-FTIR spectra of the aqueous suspensions and their electrophoretic mobilities.