Mechanisms and reversibility of glyphosate and phosphorus ligands sorption on Al2O3: Experimental evidence and computational modeling

The environmental fate and risk of glyphosate (Gly) are critically influenced by its retention at soil-water interfaces. While aluminum (oxyhydr)oxides (Al₂O₃, Al(OH)₃, AlOOH) are abundant in soils, their adsorption mechanisms for Gly have received considerably less attention than iron (hydr)oxides in geochemical modeling. This work employs a combined experimental-surface complexation model (SCM)-density functional theory (DFT) approach to systematically investigate the complexation of Gly and various phosphorus ligands (individually and competitively) on Al₂O₃. Beyond the widely studied phosphate (PO₄), we also introduce the aminomethylphosphonic acid (AMPA) and phosphonic acid (HPO₃), both containing phosphonate groups (-PO₃), into the framework. For individual adsorptions, SCM and DFT results consistently reveal pH- and loading-dependent complexation structures, comparable to iron mineral studies while filling Al₂O₃-specific knowledge gaps. DFT provides a full-pH energy analysis for Gly and PO₄ on the Al₂O₃(110) surface, specifically indicating the superior stability of Gly via carboxylate (-COO) monodentate complexation under high pH. Crucially, our SCM model, as first applied in such competitive organic-inorganic systems, identifies the adsorption irreversibility for weakly adsorbed species (Gly, AMPA) and explicitly links monodentate coordination to irreversibility at high loadings. DFT results elucidate the thermodynamic competition mechanisms at low loadings. These insights pave the way in precisely predicting and elucidating the environmental adsorption behavior of Gly at aluminum-rich soil matrix.