Modeling pH-dependent Adsorption of Glyphosate on Iron Hydroxides: Competition with Phosphate and Influence of Fe2+

The competitive adsorption of glyphosate and phosphate (PO₄^3-) on mineral surfaces mutually affects their mobility in the environment. Iron hydroxides, such as goethite and ferrihydrite, are abundant in soils and serve as important sinks for both glyphosate and phosphate. The adsorption of these compounds is modulated by pH which affects their surface complexation and mineral surface charge. Moreover, the release of ferrous ions (Fe²⁺) from the natural iron cycle may further impact glyphosate adsorption by altering surface complexation equilibria. Understanding these interactions is crucial for developing predictive models of glyphosate transport and retention in the environment.

In this study, we employed a surface complexation model (SCM) to evaluate adsorption data of glyphosate and PO₄^3- in aqueous suspensions of goethite and ferrihydrite, focusing on their pH-dependent processes, competitive interactions, and binding modes. Additionally, the influence of Fe²⁺ on glyphosate adsorption at pH 7 and the adsorption mechanism of Fe²⁺ on iron hydroxides were examined. Surface complexation constants (log K) for glyphosate, PO₄^3-, and Fe²⁺ were estimated, providing a robust thermodynamic basis for modeling interactions with the two iron minerals. The surface complexation of glyphosate and PO₄^3- varied with pH, concentration and competitive interactions. Despite the strong competition by PO₄^3-, complete desorption of glyphosate by PO₄^3- was only observed under alkaline conditions, indicating partial retention of glyphosate on iron hydroxides in most natural environments. Notably, Fe²⁺ and glyphosate mutually promote their adsorption on ferrihydrite at pH 7, indicating synergistic interactions or co-complexation, whereas on goethite Fe²⁺ has minimal influence on glyphosate adsorption. Structural modeling revealed that Fe²⁺ adsorption is dominated by monodentate complexes, highlighting the uniformity of adsorption mechanisms across these iron hydroxides.

Our findings underscore the significance of PO₄^3- in attenuating glyphosate retention in soils, while Fe²⁺ appears to play a dual role, enhancing glyphosate adsorption under specific conditions. This study contributes to a more comprehensive understanding of glyphosate dynamics in iron hydroxide-rich soils and provides directions for environmental management strategies aimed at mitigating glyphosate leaching and optimizing soil remediation practices.