X-RAY ABSORPTION SPECTROSCOPIC STUDY OF CU-GLYPHOSATE ADSORBED BY MICROCRYSTALLINE GIBBSITE

Abstract

Glyphosate [N-(phosphonomethyl)glycine] is a broad-spectrum herbicide that is used widely in agriculture, forestry, and urban settings. It is one of the most heavily applied pesticides in the US, with an annual use greater than 12 million kg. When added to soil glyphosate is readily sorbed, losing both its efficacy as an herbicide and its potential for translocation. Despite such tenacious binding to soil colloids, the adsorbent surface offers little protection from biodegradation because the glyphosate decomposes so rapidly, with a half-life as short as several days. Attempts to elucidate the sorption mechanism of glyphosate, thereby providing a mechanistic explanation for this biolability, have thus far been unsuccessful. Therefore, the goal of this research was to derive the architecture of an important model system, the Cu-glyphosate-gibbsite surface complex, in which Cu serves as a probe of the local bonding environment and gibbsite serves as a representative soil mineral adsorbent. Copper K-edge extended X-ray absorption fine structure (EXAFS) spectra of Cu-glyphosate complexes in aqueous solution at pH 5.5 revealed Cu complexation by the phosphonate moiety. Introduction of microcrystalline gibbsite resulted in the loss of P from the coordination sphere of Cu and the formation of a ternary complex through chemisorption of the phosphonate group to the oxide surface. The EXAFS spectra of Cu in the Cu-glyphosate-gibbsite surface complex matched spectra for Cu-glycine and Cu-methylglycine complexes in aqueous solution, thereby providing evidence for Cu complexation by the carboxyl O and amine N. Despite the chemisorption of glyphosate to oxide surfaces via the phosphonate group, the distal end of the glyphosate molecule extends into aqueous solution where it may be biodegraded rapidly.