Modeling pH-Dependent Adsorption of Glyphosate on Iron Hydroxides: Competition with Phosphate and Influence of Fe ²⁺

Understanding and predicting the glyphosate (Gly) migration at the water–soil interface remains a critical challenge in environmental pollution control. Iron hydroxides, particularly goethite and ferrihydrite, serve as important sinks for pollutants and are significant for geochemical modeling. This study systematically investigates the pH-dependent competitive adsorption of Gly and phosphate (PO4) on these iron minerals, alongside the influence of ferrous ions (Fe2+) on Gly adsorption─a process whose mechanistic background remains inadequately explored in previous studies. Utilizing the Charge Distribution Multisite Surface Complexation (CD-MUSIC) model, we analyzed adsorption data to evaluate surface complexation constants (log K), binding mechanisms, and competitive effects. Results revealed that Gly and PO4 adsorption is primarily governed by pH, concentration, mineral type, and competitive interactions, with PO4 significantly suppressing Gly retention. Nevertheless, Gly maintains measurable adsorption even under strong competition, attributed to irreversible bidentate complexation. The model’s underestimation at higher loadings further points to the likely role of steric hindrance effects, a factor not accounted for in the current CD-MUSIC formulation. Fe2+ exhibits dual roles: it enhances Gly adsorption on ferrihydrite via coadsorption, potentially via electrostatic attraction and possible on distinct sorption sites, while its impact on goethite is negligible. These findings underscore the PO4 critical role in reducing Gly retention in iron-rich soils and highlight the context-dependent influence of Fe2+. Our work advances the application of CD-MUSIC modeling to complex organic–inorganic systems and provides insights for optimizing soil remediation to mitigate Gly leaching.