W8 AM Agrochemicals and Pesticides
Wednesday, 16 November 2005: 8:00 AM - 11:40 AM in 337-338

(MCB-1117-747303) Oxidative degradation of glyphosate and AMPA by manganese oxide.

Barrett , K. ¹, McBride , M.¹, ¹ Cornell University, Ithaca, NY, USA

ABSTRACT- Glyphosate (N-(phosphonomethyl)glycine), the most commonly used herbicide worldwide, degrades relatively rapidly in soils under most conditions, presumeably by microbial processes. The most commonly detected initial 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^oC in dilute aqueous suspensions of birnessite, a soil manganese oxide, as evidenced by the accumulation of orthophosphate in solution over a period of several days. The degradation of glyphosate was faster than that of AMPA, with the initial reaction rate somewhat faster at pH 5 than pH 6 or 7. Higher temperature (50^o C) resulted in faster degradation of both glyphosate and AMPA. The addition of sulfate to solution had no marked effect on reaction rate, although Cu²⁺ addition inhibited degradation. As this metal ion complexes strongly with glyphosate, the inhibition can be attributed to the ability of Cu to limit glyphosate coordination to the Mn oxide surface. Attempts to demonstrate the release of phosphate by abiotic degradation of glyphosate by incubation of glyphosate in soils were confounded by the background levels of phosphate present in these soils, and the slow abiotic degradation expected from the low concentrations of Mn oxides ( 500-1000 mg/kg total Mn) in these soils. Nevertheless, some excess phosphate appeared to be generated at 50^o C in a subsoil by degradation of glyphosate. It is concluded that the abiotic degradation involved C-P bond cleavage at the Mn oxide surface with the generation of orthophosphate and sarcosine as the decomposition products. Abiotic degradation may have little significance in topsoils where microbial decomposition is prevalent, but could assume greater importance in subsoils where microbial activity is low.

Key words: decomposition, birnessite, orthophosphate, soil