Birnessite‐Catalyzed Degradation of Glyphosate: A Mechanistic Study Aided by Kinetics Batch Studies and NMR Spectroscopy

Abstract

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.

31P NMR Investigations on Roundup Degradation by AOP Procedures

“…This 31 P NMR technique has been applied for the analysis of PMG metabolites and degradation products in only a few earlier research works [26][27][28][29][30][31][32][33]. chemicals by the consumption of contaminated water [16][17][18][19][20][21][22][23][24][25][26]. Advanced oxidation processes (AOPs) are a key technology to solve pesticide contamination problems during both water and wastewater treatment (Table 1).…”

“…Advanced oxidation processes (AOPs) are a key technology to solve pesticide contamination problems during both water and wastewater treatment (Table 1). PMG Birnessite (Mn 4+ and Mn 3+ ) H3PO4 +Gly P + C-P x 31 P NMR [26] Birnessite (Na0.3Ca0.1K0.1)(Mn 4+ , Mn 3+ )2O4 × 1.5 H2O. Fe (III) (C2O4)n m− − Fe(C2O4)2 − and Fe(C2O4)3 3− .…”

“…These not always coherent results ( Figure 1) led us to investigate these process by using 31 P NMR monitoring of the PMG degradation processes. This 31 P NMR technique has been applied for the analysis of PMG metabolites and degradation products in only a few earlier research works [26][27][28][29][30][31][32][33].…”

Stochastic sensitivity analysis of glyphosate biochemical degradation

“…In fact, also AMPA has been found in the environment (Struger et al, 2015, Paris et al, 2013. Li et al (2015) and Paudel et al, (2015) have shown GLP and AMPA chemical degradation (P2R1c and P1R2c respectively, Figure 1) catalysed by Mn ions contained in birnessite mineral ((Na0.3Ca0.1K0.1)(Mn 3+ ,Mn 4+ ) 2 O 4 •1.5H 2 O).…”

Glyphosate and aminomethylphosphonic acid (AMPA) behavior in loess soils and off‐site transport risk assessment

“…This, though, also shows that glyphosate immediately degrades to AMPA within the first 2h after applying glyphosate (average time between sampling and freezing the samples). Paudel et al (2015) also reported that the majority of degradation products of glyphosate are formed within 1 min of reaction. Although the results on the formation fraction of AMPA (ffA) need to be treated with care for reasons already described in section 2.3.2, the percentage of glyphosate being degraded to AMPA also seems to be strongly influenced by temperature.…”