Adsorption Kinetics of Glyphosate and Copper(II) Alone and Together on Two Types of Soils

Glyphosate [N-(phosphonomethyl)glycine] is a nonselective, postemergence herbicide that contains multiple functional groups, which can form strong coordination with metal cations to give bidentate and tridentate complexes. The complexation of glyphosate with metal cations may affect their distribution and bioavailability in soils. Adsorption kinetics of glyphosate and Cu(II) alone and together were studied using a continuous flow experimental setup on two soils with different characteristics at pH5.5. Four kinetic models, i.e., the Lagergren first-order, pseudo-second-order, Elovich, and power function equations, were successfully used to describe their adsorption kinetics. Among the four models, the Lagergren first-order kinetic model fit the experimental data of glyphosate and Cu(II) adsorption the best. Glyphosate significantly increased the adsorption quantity of Cu(II) on the Red soil (a Hapludult or Udic Ferrosol), due to the fact that Cu(II) was adsorbed on the sites where glyphosate had been strongly adsorbed. Glyphosate decreased the adsorption of Cu(II) on the Wushan soil (a Haplaquept or Anthrosol), however, because adsorption of glyphosate on this soil was weak and the complex of glyphosate and Cu(II) tended to be highly soluble in water, thus preventing Cu(II) from exchanging with Ca 2+ and Mg 2+ ions on the soil surface. On the other hand, the presence of Cu(II) decreased the adsorption of glyphosate on both soils, which may be attributed to the lower affinity of the Cu(II)―glyphosate complex to the soils than glyphosate alone.