Adsorption-desorption of simazine on montmorillonite coated by hydroxy aluminum species

Adsorption-desorption of simazine on model systems which closely simulate those encountered in soils [i.e., pure montmorillonite and montmorillonite covered by different amounts of OH-Al species (chlorite-like complexes)] was investigated. The adsorption date were analyzed according to Langmuir and Freundlich equations. Results obtained indicate that the adsorption of simazine was strongly influenced by the charge characteristics of clay minerals and the pH of buffer (3.7 and 5.6). At pH 3.7, simazine was adsorbed in large amounts on montmorillonite, whereas adsorption significantly decreased with increasing amounts of Al(OH)(x) species on montmorillonite surfaces. When experiments were carried out at pH 5.6, the adsorption of the herbicide an montmorillonite drastically decreased, whereas no severe differences were observed with AM(18) (a montmorillonite loading 18 mequiv g(-1) M). Desorption tests showed that negligible and very little amounts of simazine were desorbed from montmorillonite and AM(18), respectively. Strong electrostatic interactions were involved in the adsorption process. Simazine molecules, arriving at support interfaces mostly as molecular species, dissociated as cations by the microenvironmental pH ("surface acidity") and were adsorbed by cation-exchange mechanism.