Degradation of glyphosate by Mn-oxides: mechanisms, pathways, and source tracking
Date
2018
Authors
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Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Glyphosate is a milestone product developed and introduced into the herbicidal industry by John Franz in the 1970s. It has been widely and heavily applied (e.g., ~280 million pounds in the U.S. in 2015) in agriculture and horticulture and drifted residues are found frequently in soils and other environments. Since the carcinogenicity of glyphosate has undergone intense debate and regulatory agencies have not yet reached consensus, studies on the fate of unreacted glyphosate in the environment are not only necessary but also urgent. ☐ Manganese oxides are of particular importance in abiotic degradation of glyphosate and some of its intermediate degradation products. In this dissertation research, birnessite, δ-MnO2, and ferrihydrite coated with δ-MnO2 were used to investigate degradation kinetics, preference in degradation pathways, and to identify source signature of glyphosate and its two products: aminomethylphosphonic acid (AMPA) and orthophosphate. Advanced instrumentation such as phosphate oxygen isotope ratios were used to determine the isotope signatures of parent and daughter products. Similarly, nuclear magnetic resonance (NMR) spectroscopy and high-performance liquid chromatography (HPLC) were utilized to identify and quantify degradation products, and density functional theory (DFT) calculations were used to investigate the bond critical point (BCP) properties of the C‒N bond in glyphosate and Mn(IV)-complexed glyphosate. ☐ The oxygen isotope ratios (δ18OP) of orthophosphate derived from glyphosate clearly demonstrated that the one external oxygen atom in each released orthophosphate was derived from ambient water, not from dissolved oxygen or minerals, with the other three oxygen atoms inherited from glyphosate. Orthophosphate derived from all commercial glyphosate studied had unusually light δ18OP values (3.91–9.30 ‰ VSMOW), which are distinct from many other sources of orthophosphates (varying between 16–24 ‰) known so far. These results provided a new and unsurpassed tool to track glyphosate and its degradation products in the environment by using naturally abundant isotopes in these compounds. Furthermore, this proxy allows distinguishing glyphosate-generated orthophosphate from other organophosphorus compounds in the environment. ☐ The abiotic degradation of glyphosate catalyzed by birnessite under aerobic and neutral pH conditions largely followed the glycine pathway generating glycine, formaldehyde, and orthophosphate. The other minor pathway was the AMPA pathway, forming AMPA and glyoxylic acid that ultimately degraded to form CO2, H2O, NH3, and orthophosphate. Sarcosine, the commonly recognized precursor to glycine, was not detected in any of the experiments performed despite its reasonably longer half-life (~13.6 h) than the sampling intervals. Preferential cleavage of the phosphonate adjacent C‒N bond to form glycine directly was also supported by the BCP analysis, which revealed that this C‒N bond was disproportionately affected by the interaction of glyphosate with Mn(IV). Overall, these results provide supportive evidence that glyphosate primarily degrades at the C–N bond position favoring the direct formation of glycine, which is less toxic than the major product of the other (AMPA) pathway. ☐ In natural soils, Mn-oxides frequently coexist with Fe-oxides with a relative molar ratio around 0.01. Therefore, Fe2O3/δ-MnO2 core-shell minerals with various Mn/Fe molar ratios are ideal candidates to test glyphosate degradation. The Mn/Fe ratio of 0.01, close to natural abundance, served as an important boundary because below this ratio glyphosate degradation was essentially absent. Present results highlighted the synergistic effects of the two oxides that are commonly present in soils on degradation and sorption of glyphosate and thus reducing the negative impacts of glyphosate in the environment. ☐ In summary, Mn-oxides with high Mn oxidation states (close to 4) are capable of efficient degradation of glyphosate with a short half-life. The C–N bond is preferentially cleaved to generate glycine and orthophosphate as major products. Stable isotope ratios of orthophosphate derived from glyphosate provide a reliable method to source track glyphosate. Furthermore, the preferential glycine pathway of degradation is more environmentally friendly because of the less toxic products than the AMPA pathway. These results provide useful insights to potential routes to attenuate negative effects of glyphosate in the environment.
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Keywords
Biological sciences, Glyphosate, Mechanisms, Mn-oxides, Pathways, Source tracking