Factors influencing the adsorption of atrazine on montmorillonitic and kaolinitic clays

The herbicide atrazine, a member of the S-triazine family was reacted with a montmorillonitic and a kaolinitic clay, two soil constituents of different mineralogical composition and with different adsorption properties. The clays were characterized by determination of the cation exchange capacity (CEC), the total and internal surface area and the particle size distribution. X-ray diffraction studies were made to acquire the mineralogical composition of the two clays. The influence of the initial pesticide concentration, the pH and the solid/liquid ratio on the adsorption of atrazine was tested in a miniaturized approach, yielding less than 5 ml of suspension per sampling. Freundlich adsorption isotherms were set up to determine the Freundlich k and l/n values. Heat of adsorption ΔH and the velocity of the establishment of a steady state were determined. Furthermore, investigations on the desorbability of the pesticide adsorbed were made. The montmorillonitic clay adsorbed a great amount of the pesticide in contrast to the kaolinitic one. Montmorillonite reached apparent adsorption equilibrium within a few minutes while establishment of apparent equilibrium with kaolinite took some days. ΔH varied between 10 and 50 kJ mol^-1. Adsorption increased with higher herbicide concentrations and decreased with rise in temperature. Larger solid/liquid ratios diminished adsoprtion coefficients. All experiments were carried out with dry as well as with water-saturated montmorillonitic clay. Desorption experiments showed that only part of the herbicide adsorbed was desorbable by several subsequent water treatment steps. For the first time immunoassay technology in the form of an ELISA (enzyme-linked immunosorbent assay) selective for atrazine was applied to detect low concentrations of the analyte in the few milliliters of eluent obtained in this miniaturized approach.