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

Adsorption-desorption of simazine on model systems which closely simulate t hose encountered in soils [i.e., pure montmorillonite and montmorillonite c overed 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 sima zine 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 la rge amounts on montmorillonite, whereas adsorption significantly decreased with increasing amounts of Al(OH)(x) species on montmorillonite surfaces. W hen experiments were carried out at pH 5.6, the adsorption of the herbicide an montmorillonite drastically decreased, whereas no severe differences we re observed with AM(18) (a montmorillonite loading 18 mequiv g(-1) M). Deso rption tests showed that negligible and very little amounts of simazine wer e desorbed from montmorillonite and AM(18), respectively. Strong electrosta tic interactions were involved in the adsorption process. Simazine molecule s, arriving at support interfaces mostly as molecular species, dissociated as cations by the microenvironmental pH ("surface acidity") and were adsorb ed by cation-exchange mechanism.