The simultaneous presence of glyphosate and phosphate at the goethite surface as seen by XPS, ATR-FTIR and competitive adsorption isotherms

Abstract

The simultaneous adsorption of glyphosate and phosphate on goethite was studied by combining macroscopic (adsorption isotherms, electrophoresis) and spectroscopic (XPS, ATR-FTIR) techniques. Adsorption isotherms together with the Competitive Langmuir isotherm indirectly show that both substances coexist at the goethite surface. The adsorption maximum for phosphate was 2.50 μmol/m2 whereas for glyphosate it was 1.77 μmol/m2. The Langmuir adsorption constant was 0.15 μM-1 for phosphate and 0.01 μM-1 for glyphosate. The shape and position of the XPS signals of glyphosate did not change by the presence of phosphate at the surface and vice versa. Equivalent results were found with ATR-FTIR. Therefore, spectroscopic evidence indicates that the binding mode of glyphosate (type of inner-sphere complexes formed) to the goethite surface is not modified by adsorbing phosphate. This is valid for systems under equilibrium conditions and under dynamic conditions. The findings are important in environmental modeling, showing that surface complexation models can be used with confidence to predict speciation in double-ligand systems using adsorption parameters obtained with single-ligand systems.