Mechanism of Abiotic Degradation of Glyphosate: Results from Stable Isotopes, NMR, and DFT Calculations.

The N-(phosphonomethyl) glycine, commonly known as glyphosate, is a broad spectrum systemic herbicide. Widespread application of glyphosate in agriculture and its presence detected in soils and environmental waters and more importantly a recent classification of glyphosate as probable human carcinogen by the cancer research wing of United Nations has elevated the public concern on its impact on human and the environment. We studied the mechanism of glyphosate degradation using an array of advanced molecular and isotope techniques including 1- and 2-D ³¹P and ¹H-³¹P NMR, DFT molecular simulation, and phosphate oxygen stable isotope ratios (d¹⁸O_P). Our results show that the preference of C–P or C–N bond cleavage varies with changing glyphosate: Mn oxide ratio indicating sorption induced conformational change on mineral surface potentially playing a major role in the bond cleavage. DFT simulation results revealed that the C–P bond cleavage was more facile in the presence of hydroxyl radical.  Activation energy of C–N bond is comparable to that of C–P hydrolysis, suggesting the possibility of simultaneous cleavage of these two bonds and production of AMPA and sarcosine, a result consistent with NMR data. d¹⁸O_P values of orthophosphate produced from mineral- and UV-catalyzed degradation of glyphosate indicated similar extent of O atoms incorporation from water during C–P bond cleavage. The mechanisms of C–P and C–N bond cleavage are presumed to be different and it is likely that C–P bond cleavage occurs through nucleophilic substitution while C–N bond cleavage is promoted by radical and/or redox mediated reaction. These results highlight the distinction between C–P and C–N bond cleavage and corresponding formation of intermediate products. Findings from this study could be exploited further to identify conditions under which glyphosate degradation occurs under specific pathway that generates less toxic products.