Adsorption of atrazine onto zeolites and organoclays, in the presence of background organics

The adsorption behavior of atrazine in the presence of background organics present in river water has been evaluated by studying the adsorption on two kinds of solids: zeolites and organoclays. The most hydrophobic solids such as zeolite ZSM-5 and organoclays adsorb atrazine better when background organics are present on the solid. On more hydrophilic solids such as zeolite Y, the adsorbed amount of atrazine molecules is lower and the isotherm presents the same behavior as background organics: the adsorption isotherm exhibits a maximum. The origin of such behavior is due to: (i) very high affinity of atrazine for background organics which can play the role of “hosts” for pesticides and (ii) the evolution of the conformation of the background organic molecules along the isotherm. The conformation depends on the Ca 2+/background organics ratio. At a high ratio, the molecules have a coil-like conformation known to be favorable for adsorption. At a low ratio, a rod-like conformation is the reason for the low adsorption level. The same behavior seems to occur when tannic acids are used for models of background organic. Polysaccharides like dextran do not exhibit such behavior. Taking into account the amount of background organic adsorbed, the relative affinity constants, K co of atrazine for zeolite ZSM-5 and organoclays have been calculated. K oc is 550 on ZSM-5 and 3000 on organoclays. On this last solid, the affinity is 6 times larger than with ZSM-5.