Zinc adsorption on goethite as affected by glyphosate

Cosorption of metals with herbicides on minerals affects their mobility and their environmental effect. Batch experiments were conducted to evaluate the interaction between Zn and glyphosate [N-(phosphonomethyl)glycine (GPS; H3L)] with regard to the effect of GPS on Zn adsorption on goethite. The herbicide GPS markedly affected Zn adsorption on goethite when they coexisted in a goethite suspension. When solution pH was not intentionally adjusted, addition of GPS decreased Zn adsorption on goethite, since the equilibrium solution pH was significantly decreased in the presence of GPS and correspondingly the negative surface charges of goethite decreased. Zinc adsorption on goethite in the presence and absence of GPS at different pH of the equilibrium solution was studied in order to know if pH was the only variable for Zn adsorption with coexisting GPS. At lower pH (pH<5), the presence of GPS increased the adsorption of Zn, because Zn adsorbed on the sites of goethite via GPS bridge. However, at higher pH (pH>5), the presence of GPS decreased the adsorption of Zn on goethite, because GPS reacted with solution Zn to form water-soluble complexes that had lower affinity to the goethite surface in comparison with Zn itself. Zeta potential of goethite significantly decreased after adsorption of GPS, suggesting a chemical bond occurred between GPS and the mineral. FTIRs also show that GPS adsorbs on goethite by coordinating through caboxylate group.