A mechanistic surface complexation model for glyphosate adsorption to ferrihydrite in competition with phosphate

The adsorption and mobility of glyphosate (PMG) in soils, sediments, natural waters, and wastewater treatment sludge are controlled by small-sized metal (hydr)oxides and affected by competition with phosphate. In this study, the adsorption of PMG to ferrihydrite, a ubiquitous nano-sized iron (hydr)oxide, is measured by batch adsorption experiments at a wide concentration (∼0.02 - 0.6 mM) and pH range (∼4-10), in the absence and presence of phosphate. The adsorption data were interpreted using the charge distribution and multisite ion complexation (CD-MUSIC) model for ferrihydrite. Including phosphate as a competitor induces electrostatic changes on the surface potential, independently of glyphosate adsorption, which allowed us to accurately distinguish between the chemical affinity and electrostatic effects contributing to PMG adsorption to ferrihydrite. PMG binds primarily as a binuclear bidentate complex, of which the amino group may protonate (logK_H=7.9). Only at low pH and high PMG surface loading, when binuclear bidentate binding sites become scarce, a monodentate complex with protonated amino and phosphonate groups becomes prominent. Phosphate effectively decreases PMG adsorption and contributes to its enhanced mobility. The resulting CD model provides a quantitative and mechanistic description of PMG adsorption to ferrihydrite, which can be used for improved predictions of the environmental fate of PMG and its water removal effectiveness.