Oxidative Degradation of Glyphosate and Aminomethylphosphonate by Manganese Oxide

摘要

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 deg 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 thatthe abiotic degradation involved C-P bond cleavage atthe 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 deg 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 Cu~(2+)addition inhibited degradation.As this metal ion complexes strongly with glyphosate,the inhibition can be attributed to the ability of Cu~(2+)to limit glyphosate coordination to reactive oxidation sites atthe Mn oxide surface.Using a similar experimental design,we were unable to detect glyphosate degradation in an equimolar solution of MnCI_2(0.5 mM).However,we demonstrated that the oxidation of Mn~(2+)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 Mn~(2+)may ultimately occur following the spontaneous oxygen-mediated oxidation of manganese.