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Abstract
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 on how organo-mineral associations affect adsorption/desorption and abiotic oxidation of glyphosate. We use in-situ ATR-FTIR spectroscopy and microfluidic experiments coupled to LC-MS determination of glyphosate transformation by-products. A model protein (BSA) and Mn-oxide minerals (K-birnessite and hausmannite) are used. Results indicate the presence of protein adsorbed onto mineral surfaces (BSA-Mn oxide associations) favors the AMPA (+20%) over the glycine transformation pathway (-55%). The AMPA pathway is also favored in experiments conducted at circumneutral pH. The shift in pathway selectivity corresponds to changes in bonding of glyphosate with BSA-Mn oxide associations: we observe protein association enhances the contribution of the mononuclear monodentate conformation of glyphosate at the Mn oxide interface. In addition, protein association protects glyphosate from abiotic oxidation by diminishing Mn oxide catalytic activity (occupies surface active sites) and by forming organic-organic (i.e., glyphosate-protein) complexes. Furthermore, kinetic modeling shows protein association hinders the adsorption, desorption and transformation rate constants under circumneutral (pH 7.2) and acidic (pH 4.6) conditions. Since organo-mineral associations prevail in natural and engineer systems, the observed shifts in glyphosate transformation pathways that favor the formation of AMPA (a more toxic by-product) are relevant to human and ecosystem health.
Supplementary materials
Title
Supplementary Information
Description
Additional experimental and finding details, including Mn oxide characterization, microfluidic chamber with embedded Mn oxide chips, derivation of transformation and dissolution rate constants, instrumentation and experimental set-up for XPS and LC-MS, curve fitting and frequency assignments for glyphosate’s ν(PO) bands, formation and in-situ ATR-FTIR spectra of BSA-Mn oxide complexes, speciation diagram of glyphosate and associated FTIR spectra of glyphosate at pH 4.6 and 7.2, complementary breakthrough curves of glyphosate and its byproducts in effluents from microfluidic chamber at pH 7.2, 1H STD-NMR spectra of BSA-glyphosate complex, AMPA in-situ adsorption-desorption spectra at the surface of K-birnessite at pH 4.6 and 7.2, and high resolution XPS Mn 3p spectra and curve fitting results for Mn oxides are provided in the supplementary information file.
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