Complexation of Glyphosate and Related Ligands with Iron (III)

Abstract

The extent of chelation of aqueous Fe³⁺ by glyphosate [isopropylamine salt of N‐(phosphonomethyl)glycine] and the related ligands glycine, iminodiacetic acid and aminomethylphosphonic acid was estimated over a range of pH and ligand/Fe³⁺ ratios. Electron spin resonance (ESR) and UV‐visible spectroscopy were used to detect chelation, evidenced by changes in the inner‐sphere coordination environment of Fe³⁺. Generally, the spectroscopic methods confirmed the degree of chelation predicted from the known stability constants of these Fe³⁺complexes. The importance of the phosphonomoeity in conferring a high degree of stability to the complexes were confirmed and the Fe³⁺‐glyphosate soluble complex persisted in aqueous solution to pH 4 and higher. The Fe³⁺complexes without the phosphonate group failed to form or existed only in very acidic solution, since they were easily dissociated upon the hydrolysis of Fe³⁺ as the pH was raised. Infrared spectroscopy of glyphosate adsorbed on goethite confirmed direct coordination of the functional groups with surface Fe³⁺, and suggested that the strong adsorption of glyphosate on oxides is a consequence of the same chelation mechanism observed in solution.