Sorption-desorption of atrazine and simazine by model soil colloidal components

Sorption-desorption of atrazine and simazine on montmorillonite, ferrihydrite, and humic acid was determined using the batch equilibration procedure. Sorption of atrazine and simazine was higher on humic acid than on montmorillonite. Ferrihydrite does not sorb triazine herbicides. Enhanced sorption of atrazine and simazine on montmorillonite was measured after increasing the surface acidity of the clay. Results indicate sorption of s-triazine herbicides on montmorillonite as protonated species must be preceded by sorption as molecular species on hydrophobic microsites of the clay unless the pH of the bulk solution is close to the pK(a) of the herbicide. In this latter case, cation exchange would be also operative. Sorption-desorption of atrazine and simazine was more hysteretic for humic acid than for montmorillonite, indicating that these herbicides desorb more difficultly from organic matter than from montmorillonite because of the contribution of hydrophobic interactions with humic acid. It is suggested that, besides organic matter, smectites of low charge density and high surface acidity should increase the retention of s-triazine herbicides in soils.Peer Reviewe