Abstract:
Glyphosate (N-(phosphonomethyl)glycine, Gly), the world's most widely used herbicide, exhibits complex adsorption behaviors at mineral-water interfaces that govern its environmental fate and transport in soil systems. Understanding these interactions is crucial for predicting Gly retention, mobility, and bioavailability in diverse soil environments, ultimately informing risk assessment and remediation strategies. This dissertation systematically investigates the pH-dependent adsorption mechanisms of Gly on three major mineral systems through a comprehensive approach combining experimental batch studies, advanced surface complexation modeling (SCM), computational density functional theory (DFT) calculations, and pH-dependent equilibrium adsorption coefficient (Kd) models.
The first study provides mechanistic insights into Gly adsorption on aluminum oxides (Al2O3) by using integrated experimental-SCM-DFT approaches. We systematically investigated the complexation mechanisms of Gly and various phosphorus ligands including phosphate (PO4), aminomethylphosphonic acid (AMPA), and phosphonic acid (HPO3) on Al2O3 surfaces. SCM and DFT results revealed constant pH- and loading-dependent complexation structures, with DFT providing full-pH energy analysis for Gly on the Al2O3(110) plane. The superior stability of Gly via carboxylate (−COO) monodentate complexation under alkaline pH conditions was specifically identified. Crucially, the SCM model identified adsorption irreversibility for weakly adsorbed species and explicitly linked monodentate coordination to irreversibility at high loadings, representing the first such identification in a competitive organic-inorganic system.
The second investigation focused on the systematic analysis of pH-dependent competitive adsorption between Gly and PO4 on iron hydroxides (goethite and ferrihydrite) using the charge distribution multisite surface complexation (CD-MUSIC) model. Results demonstrated that Gly and PO4 adsorption are primarily governed by pH, concentration, mineral type, and competitive interactions, with PO4 significantly suppressing Gly retention. Despite this competition, Gly maintained partial adsorption through irreversible bidentate complexation under high-loading conditions. The dual role of ferrous ions (Fe2+) was elucidated: enhancing Gly adsorption on ferrihydrite via co-adsorption through electrostatic attraction and distinct sorption sites, while showing negligible impact on goethite. This work advanced the application of CD-MUSIC modeling to complex organic-inorganic systems and provided critical insights for understanding PO4's role in reducing Gly retention in iron-rich soils.
The third study employed adsorption edge experiments and pH-dependent Kd distribution models to evaluate species-specific adsorption contributions of Gly and PO4 on homo-ionic (K+, Ca2+) saturated kaolinite and montmorillonite. Modeling revealed H2Gly-1 as the dominant Gly species driving sorption on K-clays, while H2PO4-1 and PO4-3 governed PO4 adsorption. In contrast, both adsorbates exhibited Ca2+-bridged co-adsorption on Ca-clays, yielding enhanced adsorption capacities versus K-clays. The dual role of Ca2+ was identified: promoting Gly release during competition yet enhancing individual Gly adsorption. Comparative analysis with iron/aluminum oxides revealed preferential PO4 adsorption on oxides, while Gly showed comparable affinity on clays, validating previous kinetic competition analyses.
These findings collectively advance our fundamental understanding of Gly behavior in diverse soil mineral systems and provide critical parameters for environmental fate modeling. The identification of irreversible adsorption mechanisms and cation-mediated complexation offers new insights for predicting Gly persistence and mobility in heterogeneous soil environments. The pH-dependent complexation structures and competitive effects with naturally occurring PO4 have direct implications for assessing Gly bioavailability and potential groundwater contamination. Furthermore, the mechanistic understanding of surface complexation processes enables better prediction of Gly removal and biodegradation pathways, supporting the development of more effective soil remediation strategies. This comprehensive framework enhances our ability to assess environmental risks associated with Gly usage and informs evidence-based regulatory decisions for sustainable agricultural practices.
glyphosate