Adsorption of Glyphosate on the Clay Mineral Montmorillonite: Effect of Cu(II) in Solution and Adsorbed on the Mineral

Adsorption-desorption of the herbicide glyphosate (GPS) onto the clay mineral montmorillonite has been studied in presence of copper, in order to clarify the effects that this strongly complexing metal could have on the availability and mobility of GPS in soil environment. In contrast with previous findings, GPS adsorption on montmorillonite decreases in the presence of Cu, in solution or adsorbed on the mineral, due to the formation of Cu-GPS complexes that have a lower tendency to be adsorbed on montmorillonite than the free GPS. GPS adsorption is higher at pH 4.2 than at pH 6.8, both in the presence and in the absence of Cu, but the decrease in GPS adsorption in the presence of Cu is lower at pH 4.2 than at pH 6.8, since at this latter pH the concentration of Cu-GPS complexes in relation to free GPS in solution is high. It is commonly believed that GPS remains almost permanently adsorbed onto soils and has very limited mobility, but GPS desorption experiments show that the amount of GPS desorbed from montmorillonite is not negligible and increases in the presence of Cu. These experiments lead to the conclusion that, in relation to GPS release from soil, it is necessary to take into account not only the type of soil to which it is applied (e.g., whether it has high clay minerals content) but also whether there is any element in the soil solution or adsorbed on the soil capable of forming strong complexes with GPS.