Abstract
Although, glyphosate (N-(phosphonomethyl) glycine) is one of the most widely used herbicides in the world, its interaction with poorly crystalline iron oxides, such as ferrihydrite, is not well studied. In this research, we examined the adsorption of glyphosate onto ferrihydrite using infrared spectroscopy (FT-IR), electron paramagnetic resonance spectroscopy (EPR), adsorption kinetic models and adsorption isotherm models. The effect of pH and sodium chloride concentration on the adsorption of glyphosate onto ferrihydrite as well as the effect of extractors (CaCl2 0.010 mol L−1 and Mehlich) on the desorption of glyphosate were also evaluated. There are two important findings described in this work. First, 84% of adsorbed glyphosate strongly interacted to ferrihydrite as an inner-sphere complex and phosphate and amine groups are involved in this interaction. Second, an increase of sodium chloride salt concentration increased the adsorption of glyphosate onto ferrihydrite. The non-linear Langmuir model and pseudo second order model showed a good agreement of theoretical limit of glyphosate adsorbed onto ferrihydrite, 54.88 µg mg−1 and 48.8 µg mg−1, respectively. The adsorption of glyphosate onto ferrihydrite decreased when the pH increased. Under the conditions used in this work, EPR spectra did not show dissolution of ferrihydrite. Surface area, pore volume and pHpzc of ferrihydrite decreased after adsorption of glyphosate.
Highlights
Herbicides are classified as micropolluting agents because of their low concentration in the environment
In the present work we studied, using infrared and electron paramagnetic resonance spectroscopies, the adsorption and desorption (CaCl2 0.010 mol L−1 and Mehlich extractors) of glyphosate on ferrihydrite and the interaction between glyphosate and ferrihydrite
Since, a low content of glyphosate (16%) was desorbed from ferrihydrite by Mehlich-1 extractor, most of glyphosate was strongly bonded to ferrihydrite as inner-sphere complex
Summary
Herbicides are classified as micropolluting agents because of their low concentration in the environment. Glyphosate [N-(phosphonomethyl) glycine-C3H8NO5P] is a non-selective, systemic and post emergent herbicide that was discovered at Monsanto in 1970 by a group of scientists led by Dr John Franz. It is among the most widely used herbicides in the world [1]. Glyphosate blocks the enzymatic synthesis of aromatic amino acids (phenylalanine, tyrosine, tryptophan) in plants. These amino acids are precursors in the synthesis of lignin, alkaloids, flavonoids, benzoic acids and vitamin K [5, 6]. In soil the principal decomposition product of glyphosate is aminomethylphosphonic acid [1, 2]
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