摘要
Glyphosate and calcium ions (Ca 2+ ) commonly coexist in calcareous soils and aquatic environments, where their interfacial interactions with Fe (oxyhydr)oxide critically 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 2+ on ferrihydrite and to elucidate the underlying molecular mechanisms. The results demonstrated a synergistic co-adsorption of glyphosate and Ca 2+ 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 2+ -free system at pH 5.0 in the presence of 2.0 mM glyphosate, the addition of 1.0 mM Ca 2+ increased glyphosate adsorption by approximately 20%. At pH 5.0, the Ca 2+ -induced enhancement in glyphosate adsorption followed a glyphosate/Ca 2+ stoichiometric ratio of approximately 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 2+ simultaneously coordinated with the phosphonate and carboxyl groups of one glyphosate molecule, thereby forming glyphosate-bridged ternary complexes. These findings provide direct molecular-level evidence for the interfacial configuration and cooperative adsorption of glyphosate and Ca 2+ on ferrihydrite, offering mechanistic insights into glyphosate retention, mobility, and environmental behavior in Fe- and Ca-rich soils.
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