Glufosinate, glyphosate and phosphate. Understanding their adsorption trends on a mineral surface

The USA, Brazil, and Argentina are the world's top producers of genetically modified (GM) crops, with China as their leading importer. These GM crops have been engineered to resist specific herbicides, primarily glyphosate (GLY) and glufosinate (GLU). The widespread use of these herbicides is controversial due to environmental concerns. Both GLY and GLU are organophosphorus compounds with their main reactive groups derived from phosphate, suggesting similarities in the interaction of these three compounds with mineral surfaces. This study investigates the adsorption of phosphate, GLY and GLU on goethite across a broad pH range (2-12), under identical conditions, using adsorption isotherms and ATR-FTIR spectroscopy. At any pH the trend in adsorption was phosphate > GLY >> GLU, with GLU exhibiting detectable adsorption only at pH 2 and negligible adsorption at all other pH values. Comparative analyses with other phosphorous-containing compounds measured here and reported in the literature reveal that molecules with the generalized structure PO(OH)₃ (e.g. phosphate) exhibit the highest adsorption, followed by PO(OH)₂R compounds (the GLY group), while PO(OH)RR´ compounds (the GLU group) show minimal adsorption. The adsorption behavior is primarily governed by the basicity and number of basic oxygens in PO groups, as evidenced by strong correlations between adsorption, the pKa values of the conjugated P-OH groups, and Mulliken charges obtained via DFT calculations. This generalization provides a fundamental framework for understanding the adsorption and environmental fate of various nutrients, pesticides, herbicides, and other phosphorus-containing molecules.