Adsorption of herbicide Glyphosate on metal-humic substances complexes

The interactionof glyphosate with a pure iron-humic acid (Fe-HA) complex was studied by constructing adsorption isotherms at different shaking times. The Freundlich equation better fitted the experimental data than the Langmuir equation. The maximum adsorption values (kf) showed that glyphosate is adsorbed on Fe-HA complex to an extent that is similar to that found for whole soils and even higher than that shown by other soil minerals. This suggests that HA complexes with polyvalent cations may represent a main binding substrate for glyphosate in soils. The S-type adsorption isotherms found through the application of the Freundlich equation, indicate that at low concentration glyphosate is adsorbed by a ligand exchange mechanism involving the phosphono group and the iron hydration sphere in the HA complex, whereas at higher concentration, more glyphosate may continue to be bound by a hydrogen bonding mechanism, to already sorbed molecules. The high strength of the glyphosate interaction with the Fe-HA complex is demonstrated by the lack of any appreciable desorption for any concentration and any time of contact applied. A rapid increase of the slope of the Freundlich adsorption isotherms at shaking times higher than 48 h is suggestive of some microbial degradation