The (de)protonation of glyphosate significantly affects its interactions at water/solid interfaces. The predominant glyphosate species (>98%) at pH 4.6 and 7.2 are the monoanion [ - OOCCH2N + H2CH2-HPO3 -] and dianion [ -OOCCH2N + H2CH2-PO3 2-], respectively (Figure , SI). Despite being a tridentate ligand, evidence increasingly suggests glyphosate's carboxylate and amine groups do not directly interact with goethite (i.e., do not form inner-sphere (IS) complexes). but form H-bonds (i.e., outer-sphere (OS) complexes) with other glyphosate molecules at the goethite interface and with organic molecules in OMAs (e.g., with polysaccharide in polysaccharide-goethite associations). Representative in-situ ATR-FTIR spectra of glyphosate's retention dynamics at BSA-Mn oxide and Mn oxide interfaces highlight the progression of glyphosate adsorption and desorption through increases and decreases in intensity of the phosphonate (1200 -900 cm -1 ; ν(PO)) and carboxylate-amine (1690 -1515 cm -1 ; ν(CAc)) regions of the spectra (Figure ). Tentative peak assignments of adsorbed glyphosate at pH 4.6 and 7.2 are given in Table .