Synergistic adsorption of glyphosate and calcium on ferrihydrite: Molecular insights into herbicide fate in soils

Hebicide glyphosate and calcium cation (Ca) commonly coexist in calcareous soils and aquatic environments, where their interfacial interactions with Fe (oxyhydr)oxide influence the environmental fate of glyphosate. However, the molecular mechanisms underlying their co-adsorption on mineral surfaces remain poorly understood. Here, we combined batch adsorption experiments, zeta potential analysis, in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and surface complexation modeling to investigate the co-adsorption behavior of glyphosate and Ca on ferrihydrite and to elucidate the underlying molecular mechanisms. The results demonstrated a synergistic co-adsorption of glyphosate and Ca on ferrihydrite across a wide range of glyphosate concentrations (0.0-2.0 mM) and pH values (4.0-10.0), which was driven by both electrostatic interactions and the formation of ternary surface complexes. Compared with the Ca-free system at pH 5.0 in the presence of 2.0 mM glyphosate, the addition of 1.0 mM Ca increased glyphosate adsorption by ∼20%. At pH 5.0, the Ca-induced enhancement in glyphosate adsorption followed a glyphosate/Ca stoichiometric ratio of ∼1.5. Spectroscopic, electrochemical, and modeling analyses further revealed that, at pH 5.0, glyphosate was predominantly adsorbed on ferrihydrite via bidentate and monodentate phosphonate-mediated Fe-O-P inner-sphere complexes, whereas Ca simultaneously coordinated with the phosphonate and carboxyl groups of one glyphosate molecule, thereby forming glyphosate-bridged ternary complexes. These findings provide direct molecular evidence for the interfacial configuration and cooperative adsorption of glyphosate and Ca on ferrihydrite, offering mechanistic insights into glyphosate retention, mobility, and environmental behavior in Fe- and Ca-rich soils.