Oxidative Degradation of Glyphosate and Aminomethylphosphonate by Manganese Oxide

Abstract

Glyphosate (N-(phosphonomethyl)glycine), the most commonly used herbicide worldwide, degrades relatively rapidly in soils under most conditions, presumably by microbial processes. The most frequently detected degradation product in soil and water is AMPA (aminomethylphosphonic acid). We report the first evidence for an abiotic pathway of glyphosate and AMPA degradation under environmentally realistic conditions. Both glyphosate and AMPA degraded at 20 degrees C in dilute aqueous suspensions of birnessite, a manganese oxide common in soils, as evidenced by the accumulation of orthophosphate in solution over a period of several days. It is concluded that the abiotic degradation involved C-P bond cleavage at the Mn oxide surface, although evidence for C-N bond cleavage in the case of glyphosate and sarcosine, a likely degradation product of glyphosate, was found. The degradation of glyphosate was faster than that of AMPA, and higher temperature (50 degrees C) resulted in faster degradation of both glyphosate and AMPA. The addition of sulfate to the solution had no marked effect on the reaction rate, although Cu2+ addition inhibited degradation. As this metal ion complexes strongly with glyphosate, the inhibition can be attributed to the ability of Cu2+ to limit glyphosate coordination to reactive oxidation sites at the Mn oxide surface. Using a similar experimental design, we were unable to detect glyphosate degradation in an equimolar solution of MnCl2 (0.5 mM). However, we demonstrated that the oxidation of Mn2+ is enhanced both in solution and on an inert surface, in the presence of glyphosate (4:1 Mn-glyphosate molar ratio). This result suggests that the oxidative breakdown of glyphosate in the presence of Mn2+ may ultimately occur following the spontaneous oxygen-mediated oxidation of manganese.

Removal of glyphosate from water by electrochemically assisted MnO2 oxidation process

“…The continuous oxidation of glycine produced glycolic acid and NH 3 -N. Meanwhile, Mn 2+ ions were released into the aqueous solution. Similar intermediates (i.e., sarcosine, glycine, and formic acid) were observed in the degradation of glyphosate by using manganese oxide [14]. In the electro-MnO 2 process, sarcosine, glycine and PO 3À 4 were also identified in the initial phase of reaction.…”

“…It has been reported that both glyphosate and AMPA could be degraded in dilute aqueous suspensions of birnessite, a manganese oxide common in soils, as evidenced by the accumulation of orthophosphate in solution [14,15]. In the oxidation of glyphosate using MnO 2 , the oxidation efficiency was low and a large portion of Mn 2+ ions were released [16] …”

Pathogenic prion protein is degraded by a manganese oxide mineral found in soils

“…Well-drained soils may also contain these minerals due to previous wet conditions (McKenzie, 1989). Manganese III and IV minerals can mediate the transformation of numerous organic compounds, including herbicides (Barrett & McBride, 2005) and antibiotics (Zhang & Huang, 2005;Rubert & Pedersen, 2006).…”

Eletrochemical Oxidation of Herbicides

“…The efficiency of GH mineralization reached values around www.intechopen.com 60%. Barret & Mcbride (2005) have evaluated the oxidative degradation of GH on manganese oxide. There was no significant herbicide degradation (10 mg L -1 ), and much of the herbicide simply adsorbed onto the manganese oxide.…”