PH dependent adsorption isotherms of glyphosate

Abstract

The relationship between system pH and glyphosate adsorption on Al('+3), Ca('+2), and Na('+1) saturations of montmorillonite and nontronite, NaCl washed kaolinite, hematite, and goethite was investigated. The quantity of specific ionic species of glyphosate, a triprotic organic acid, were maximized by pH adjustment. These species are; neutral and zwitterionic at pH 2.0, negative one at pH 4.5, negative two at pH 7.0, and negative three at pH 11.5. Batch equilibria techniques were used to elucidate adsorption isotherms from pH buffered (2.0, 4.5, 7.0, and 11.5) clay mineral-glyphosate slurries. Direct injection of the supernatant liquid of the slurries into a high pressure liquid chromatograph (HPLC) was used to quantify the equilibrium concentration of glyphosate, and the glyphosate adsorbed to the clay. As the system pH was increased, glyphosate adsorption generally decreased. Exceptions to this were montmorillonite which had a low affinity for glyphosate at pH 2.0; both Al('+3) smectites which showed a greater adsorption at pH 11.5 than 7.0; and hematite and goethite which had a reversal of their pH 4.5 and 7.0 isotherm. X-Ray diffraction and infrared spectroscopy were used to discern the adsorption mechanisms of clay-glyphosate complexes. Glyphosate bands found in the spectra of treated minerals were shifted down scale. Shifts of this type indicate hydrogen bonding of glyphosate to the clay surface, interlayer water, or directly to the cation. Treated samples of kaolinite, hematite, goethite, and gibbsite exhibited no glyphosate bands. Glyphosate was held on these clays by anion exchange and removed during the washing step of preparation. X-Ray diffraction analysis of the layer silicate minerals indicated significant interlayer expansion occurred only for Al('+3) nontronite at pH 2.0, and Al('+3) montmorillonite at pH 2.0 and 4.5. The extremely acidic conditions at the clay surfaces induced by a bulk solution pH of 2.0 probably caused the glyphosate to accept an extra proton and become positively charged. Hydrolysis of the water associated with trivalent cations would readily act as the source of protons thereby inducing interlayer adsorption. This effect could not be verified by infrared spectroscopy due to overlapping glyphosate and smectite bands in the 1270 cm('-1) region.