Birnessite-Catalyzed Degradation of Glyphosate:A Mechanistic Study Aided by Kinetics Batch Studies and NMR Spectroscopy
2015
Paudel, Prajwal | Negusse, Awet | Jaisi, Deb P.
Glyphosate [N-(phosphonomethyl) glycine] is a broad- spectrum systemic herbicide. Because of the ever-increasing application of glyphosate in agricultural fields and detection of its presence in soils and environmental waters, there is a growing concern that it may potentially harm animal and human populations and the environment. In this study, we determined the mechanism and reaction pathway of birnessite-catalyzed degradation of glyphosate by using one-dimensional (¹H, ¹³C, and ³¹P) and two-dimensional homo- and hetero-nuclear NMR correlation spectroscopies. The NMR results showed the presence of several degradation products including orthophosphate, aminomethylphosphonic acid (AMPA), glycine, and sarcosine. Two-dimensional NMR and correlation spectroscopies confirmed the identity of products and contaminants in the reactions. Our results show that the glyphosate degradation reaction is very fast, with the majority of reaction products formed within 1 min of reaction. Degradation depends linearly on the birnessite concentration, indicating sorption as the prerequisite for glyphosate degradation. Based on the reaction kinetics and NMR data, we propose two glyphosate degradation pathways: one starts with C–P bond cleavage and the other with C–N bond cleavage. Interestingly, the dominance of one pathway over the other was found to vary with the birnessite/glyphosate ratio. This information could be exploited further to identify conditions under which glyphosate degradation occurs primarily under the C–P bond cleavage pathway because this pathway does not produce AMPA, a compound with a longer half-life than glyphosate and with known animal and plant toxicity.
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