Chemical Choreography at the Mn Oxide Interface: Protein Association Modulates Glyphosate Bonding Configurations and Favors the AMPA Oxidation Pathway

Glyphosate, the most commonly used and widely detected herbicide worldwide, adsorbs and degrades on environmental surfaces, with natural organic matter playing a significant role in these processes. This study provides mechanistic and kinetic insights into how organo-mineral associations influence glyphosate adsorption, desorption, bonding configurations, and abiotic oxidation pathways. We employed time-resolved in situ ATR-FTIR spectroscopy and microfluidic experiments coupled with LC-MS quantification to track glyphosate and its oxidation byproducts. A model protein (bovine serum albumin; BSA) and two Mn oxide minerals (K-birnessite and hausmannite) were used to represent environmentally relevant interfaces. Our results show that protein adsorption onto mineral surfaces (BSA-Mn oxide associations) favors the AMPA oxidation pathway over the glycine and sarcosine pathways, particularly under acidic pH. This shift in pathway selectivity correlates with changes in glyphosate coordination at the surface where the presence of protein promotes mononuclear monodentate binding via the phosphonate group. Additionally, protein association diminishes glyphosate oxidation by passivating Mn oxide active sites and forming glyphosate-protein complexes. Kinetic modeling further reveals that BSA association reduces glyphosate adsorption, desorption, and oxidation rate constants at both pH 4.6 and 7.2. Given the prevalence of organo-mineral associations in natural and engineered systems, the observed shift toward AMPA formation, a toxic and persistent byproduct, has important implications for glyphosate fate, mobility, and potential risks to human and ecosystem health.