INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
congresos y reuniones científicas
The adsorption of glyphosate and its degradation products on clays
DOS SANTOS AFONSO, M; TORRES SÁNCHEZ, ROSA M.; GENET, M
Monte Verità, Ascona, Switzerland
Workshop; Complexing Agents between Science, Industry, Authorities and Users; 2007
Glyphosate, phosphonomethylglycine, (PMG) is the most employed herbicide around the worldand is a non selective, post emergent and broad spectrum herbicide used for the eradication ofprevious crop vegetation and for broadleaf weeds control and grasses in: hay/pasture, soybeans,field corn; ornamentals, lawns, turf, forest plantings, greenhouses, rights-of-way. After thetreatment a significant amount of herbicide reaches the soil due to the different applicationmethods and weather conditions. As a consequence of that PMG is adsorbed onto the soilcomponents such as clay minerals, organic matter, metallic oxides and humic substances. Sincethe fate through the soil is generally mediated by water, the transport process is directly relatedwith the water solubility, metal complexation and adsorption characteristics of the system. PMGdegradation products before its ultimate mineralization are aminomethylphosphonic acid(AMPA), methylphosphonic acid (MPA) and sarcosine (SAR). Expansive clays have importantimplications for agricultural, engineering and environmental purposes. Montmorillonite (M), anexpansive clay that belong to the smectite family (2:1 layered silicate) develops high specificsurface and a net surface charge. Those properties and the at least two available types of surfacesites for adsorption studies made it particularly interesting for adsorption processes study. It iswell know that PMG forms inner sphere complexes with several metals such as Al+3 and Fe+3  and iron oxides surfaces . In this particular case, both PMG and M have negative net charge at all pH evaluated, so no electrostatic attractive phenomena is associated with PMG-M interaction consequently it seems reasonable to think that similar type of complexes are formed on M surfaces. Some parameters as particle morphology (SEM), superficial composition, point of zero charge, size particle (as function of pH and ionic strength), thermal gravimetry and surface area (using nitrogen and water as adsorbent) were measured to characterized the clay. Also,electrophoretic mobility and size particle of a treated sample with PMG was evaluated due to theenormous changes in M stability suspension in presence of PMG. The adsorption isotherms atseveral pH values and ionic strength were performed for all ligands. Multisite isotherms werefound for adsorption indicating that almost two types of specific adsorption sites are present inclay surface. DRX and XPS analysis of PMG, AMPA, MPA and SAR adsorbed onto M weremade to determine the interlayer role and the structure of surface complexes formed during theadsorption processes. In all the cases the d(001) parameter shifted to lower values of 2θ whichindicate the clay expansion due to ligand incoming into the interlayer space. The =NH2+, =NH and P2p surface speciation were calculated from XPS results. Physicochemical parameters related with adsorption process are reported for all sample at all measured conditions.