Effects of pH and cations on adsorption of glyphosate and phosphate at clay minerals

Although competitive interactions between glyphosate (Gly) and phosphate (PO 4 ) in soils were previously confirmed, systematic investigations of their behavior on homo-ionic phyllosilicate clay minerals remain limited. This study employed adsorption edge experiments at different pH conditions to derive solid/liquid distribution coefficients (K d ) and evaluated Gly and PO 4 species-specific adsorption contributions on potassium- and calcium-saturated homo-ionic kaolinite (K-, Ca-kao) and montmorillonite (K-, Ca-mont), alongside elucidating related adsorption mechanisms. The results identified H 2 Gly − as the dominant Gly species driving adsorption on K-clays. For PO 4 , H 2 PO 4 − governed adsorption on K-kao, while its enhanced uptake on K-mont under alkaline conditions was reflected in the model by PO 4 3− species. In contrast, both Gly and PO 4 exhibited Ca 2+ -bridged co-adsorption mechanisms on Ca-clays, yielding enhanced adsorption capacities compared to K-clays. At alkaline pH, removal of aqueous PO 4 was primarily attributed to Ca 2+ -PO 4 precipitation. Competitive adsorption data, combined with single-system results, resolved the interplay of surface complexation, interlayer adsorption, and hydrogen bonding under co-adsorption conditions. Notably, Ca 2+ played a dual role by promoting Gly release under competition yet enhancing Gly adsorption in single scenarios. The sorption comparison of Gly and PO 4 with Fe/Al oxides revealed preferential PO 4 adsorption at oxides, while Gly showed comparable adsorption affinity, validating previous kinetic competition analyses. This study elucidates the distinct roles of adsorbate speciation, cation type, and mineral structure, thereby providing a conceptual framework essential for advancing predictions of contaminant behavior in complex environmental systems. • Glyphosate/phosphate on K + homo-ionic clays occur distinct sorption-driven species. • Ca 2+ bridging co-sorption mediates glyphosate sorption on Ca 2+ homo-ionic clays. • Phosphate removal on Ca 2+ clays: Ca 2+ -P bridging (acidic), precipitation (basic). • Ca 2+ promotes glyphosate release in competition yet enhances individual sorption. • Clay vs. Fe/Al oxides: equilibrium data validates kinetic competition analysis.