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

Glyphosate, the most widely used herbicide, undergoes adsorption and abiotic degradation on environmental surfaces, with natural organic matter strongly influencing these processes. The role of organo-mineral associations in regulating glyphosate retention and transformation pathways, however, remains unclear. In this study, 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 oxides (K-birnessite and hausmannite) were used to represent environmentally relevant interfaces. Our results show that protein adsorption onto Mn oxide surfaces favors the AMPA oxidation pathway over the glycine and sarcosine pathways, particularly under acidic pH. This pathway selectivity correlates with altered surface coordination, where adsorbed protein promotes mononuclear monodentate binding via the phosphonate group. Additionally, protein association diminishes the extent and rate of glyphosate adsorption, desorption, and oxidation by passivating Mn oxide active sites and forming glyphosate-protein complexes at pH 4.6 and 7.2. Given the ubiquity of organo-mineral associations, the shift toward AMPA formation (a toxic and persistent byproduct) has critical implications for human and ecosystem health. These insights advance the mechanistic understanding of glyphosate behavior at redox-active organo-mineral interfaces and inform sustainable soil management and pollution mitigation strategies.