ADSORPTION OF ATRAZINE ONTO ZEOLITES AND ORGANOCLAYS, IN THE PRESENCEOF BACKGROUND ORGANICS

The adsorption behavior of atrazine in the presence of background orga nics present in river water has been evaluated by studying the adsorpt ion on two kinds of solids: zeolites and organoclays. The most hydroph obic solids such as zeolite ZSM-5 and organoclays adsorb atrazine bett er when background organics are present on the solid. On more hydrophi lic solids such as zeolite Y, the adsorbed amount of atrazine molecule s is lower and the isotherm presents the same behavior as background o rganics: the adsorption isotherm exhibits a maximum. The origin of suc h behavior is due to: (i) very high affinity of atrazine for backgroun d organics which can play the role of ''hosts'' for pesticides and (ii ) the evolution of the conformation of the background organic molecule s along the isotherm. The conformation depends on the Ca2+/background organics ratio. At a high ratio, the molecules have a coil-like confor mation known to be favorable for adsorption. At a low ratio, a rod-lik e conformation is the reason for the low adsorption level. The same be havior seems to occur when tannic acids are used for models of backgro und organic. Polysaccharides like dextran do not exhibit such behavior . Taking into account the amount of background organic adsorbed, the r elative affinity constants, K-oc of atrazine for zeolite ZSM-5 and org anoclays have been calculated. K-oc is 550 on ZSM-5 and 3000 on organo clays. On this last solid, the affinity is 6 times larger than with ZS M-5.