Adsorption and Desorption of the Herbicide Glyphosate by Soils of Varying Organic Matter Content.

The adsorption and desorption of glyphosate (N-phosphonometylglycine) on three soils of different physicochemical properties was studied. The glyphosate adsorption and desorption data fit the Freundlich equation. The adsorption constant ranged from 0.813 to 1.5, while n varied from 2.2 to 3.8. The desorption constants ranged from 0.881 to 1.26, while n varied from 2.4, to 3.9. The influence of background electrolyte on the adsorption of glyphosate was also investigated using KC1 and CaCl₂. Glyphosate was adsorbed by Ca-soil to a much lesser extent than by K-soil. This appears to be related to the larger hydration sphere of Ca²⁺ which shrinks the effective size of the adsorption domains between exchangeable cations. The sorptive capacity of the soils was assessed at 25 and 45°C by the batch equilibrium procedure over a broad range of aqueous phase herbicide concentrations. The isosteric heats of adsorption, calculated from the desorption isotherms, were similar to those calculated from the adsorption isotherms indicating that there was no change in mechanisms between adsorption and desorption equilibrations and the isosteric heats of sorption were relatively small and exothermic, suggesting the involvement of entropy driven mechanism.. The thermodynamic parameters provided evidence that glyphosate adsorption on the three soils was reversible and that the hysteresis between adsorption and desorption isotherms was not due to an irreversible reaction (chemical or coulombic adsorption).