Argentinean Soils and N-(Phosphonomethyl)Glycine. A Comparative Study of the Adsorption Process.
Hector R. Tevez, Facultad de Ciencias Forestales- Univ. Nac. de Santiago del Estero, Santiago del Estero, Santiago del Estero, Argentina, Romina C. Pessagno, INQUIMAE- FCEN-UBA, Ciudad Universitaria Pabellon II 3er Piso, Buenos Aires, 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.
Phosphonic acids and their derivatives are of interest due to their structural variety and great economic importance. Phosphonic acids derivatives are used as crop protection agents (weed control) in water treatment, in metal processing, and as flame proofing agents. Phosphonates are molecules containing one or more groups R-PO(OH)2. The dibasic phosphonic acids are mostly weaker acids than phosphoric acid. Significant amounts of herbicides may eventually reach the soil and remain in the soil or be transported to other areas before their ultimate decomposition due to the different application methods and the weather conditions. These compounds possess not only a very high ability to form strong complexes with transition metals in aqueous solution but also show a large affinity for the surface of aluminum and iron oxides. All these properties play a very important role in the fate and rate of transport of these compounds in the environment. N-phosphonomethylglycine (PMG) with an empirical molecular formula of C3H8NO5P is commonly known as glyphosate. Glyphosate is the active component of non-selective, postemergent and broad spectrum commercial herbicides widely used in agriculture. The phosphonic acid group with a very stable C-P bond makes glyphosate extremely resistant to chemical hydrolysis, thermal decomposition and photolysis. PMG in aqueous solution is present as a zwitterion and the three acid-base groups have the ability to dissociate with pKa2, pKa3 and pKa4 of 2.27; 5.57 and 10.25 for the carboxylic, phosphonic and amino groups respectively. It is known that PMG is immobilized upon contact with soils and clay minerals because of the formation of surface complexes with metal ions. The focus of this study is to explain the different PMG adsorption on the surface of Argentineans soils on the basis of the adsorption isotherms, XRD, zeta potential and surface speciation where the mineral composition of soils reveal to have a main importance. To attain this task soil samples from different provinces of Argentina were collected from horizon A. Ch, N, Az and Qui samples came from Chalten, Navaja, Cerro Azul and Quimili localities of Santa Cruz, Corrientes, Misiones and Santiago del Estero provinces respectively. These soils were typified as Typic Haplocryoll, Udult, Rhodudults and Haplustoll. PMG forms surface complexes on soils with similar maxima surface coverage and the extent of the complexation is dependent on the ligand concentration in solution and pH. The results suggest that the phosphonate moiety of PMG coordinates to the external surface site of soils components trough inner sphere surface complexes formation where the phosphonate group of PMG binding to the surface with the similar adsorption behavior observed for PMG onto goethite. These results are potentially important to provide a fundamental understanding of the degradability and bioavailability of PMG in soils and natural waters. The study of the properties of these soil and mineral surface complexes is of high importance in order to assess the implications for control of glyphosate contamination. Glyphosate is belongs to a unique class of strongly chelating agents and the adsorption process makes herbicide more persistent in soil.