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.

Adsorption of glyphosate on clays and soils from Paraná State: effect of pH and competitive adsorption of phosphate

“…On the other hand, at pH 6.00 even the ratio of 10.0 of phosphate/GPS did not change the amount of GPS adsorbed by montmorillonite in which it remained about 100% (data not shown). Mcbride and Kung (1989), Gimsing and Boggaard (2001) and Dion et al (2001) studied the competitive adsorption between GPS and phosphate on amorphous iron oxide, goethite and clays (illite, kaolinite), respectively. Gimsing and Borggaard (2001) observed that phosphate was able to desorb GPS, but GPS was unable to desorb phosphate.…”

“…Gimsing and Borggaard (2001) observed that phosphate was able to desorb GPS, but GPS was unable to desorb phosphate. McBride and Kung (1989) showed that GPS was completely displaced from an amorphous iron oxide by phosphate. Dion et al (2001) showed that for clays (illite, Kaolinite) there was preference to GPS by surface sites; however, GPS is displaced by the effect of the concentration of phosphate.…”

“…However, Glass (1987) showed that pH had no effect on the adsorption of GPS by ilite. The studies of competitive adsorption between phosphate and GPS on minerals, clays and soils have showed that GPS could be displaced by phosphate (McBride and Kung, 1989;Dion et al, 2001;Gimsing and Boggaard, 2001). The displacement of GPS by phosphate can occur easily in some minerals such as amorphous iron oxide and goethite (McBride and Kung, 1989;Gimsing and Boggaard, 2001), but the displacement of GPS by phosphate from clays or soils is not so easy (Dion et al, 2001).…”

“…The studies of competitive adsorption between phosphate and GPS on minerals, clays and soils have showed that GPS could be displaced by phosphate (McBride and Kung, 1989;Dion et al, 2001;Gimsing and Boggaard, 2001). The displacement of GPS by phosphate can occur easily in some minerals such as amorphous iron oxide and goethite (McBride and Kung, 1989;Gimsing and Boggaard, 2001), but the displacement of GPS by phosphate from clays or soils is not so easy (Dion et al, 2001). Desorption of GPS by phosphate can be considered a serious problem for the farmers, because of the phytotoxicity of GPS residues (Cornish, 1992).…”

Effect of KCl and CaCl2 as Background Electrolytes on the Competitive Adsorption of Glyphosate and Phosphate on Goethite

“…Competition studies suggest that the phosphate concentration is the most important factor in determining the amount of glyphosate sorbed, and that phosphate in some cases is able to completely desorb glyphosate (Nicholls and Evans, 1991;McBride and Kung, 1989;Hance, 1976;Sprankle et al, 1975a, b). Thus, phosphate may have a crucial influence on the environmental fate of glyphosate.…”