Physicochemical interactions between atrazine and clay minerals

The aim of this work was to study the sorption behaviour of atrazine on clay minerals at low environmentally relevant concentrations. Adsorption and desorption isotherms of atrazine were determined on different clay minerals using the 14C tracer technique. The adsorption isotherms are linear at pH 5.8 in the low concentration range studied. The adsorption constant Kd is proportional to the external surface in Na+ layer silicates, such as kaolinite, illite and montmorillonite. This implies that atrazine molecules do not intercalate even in swelling Na+ clay minerals. The experiments with homoionic montmorillonites (Na+, Ca2+, Ni2+, Cu2+ and Fe3+) indicate a correlation between the adsorption constant and the hydrolysis constant of the exchangeable cation. This suggests a participation of the protonated atrazine molecules in sorption due to electrostatic interactions. It is assumed that adsorption shifts the chemical equilibrium to the side of the protonated form for Men+ montmorillonites with a low hydrolysis constant of Men+. In contrast, protonation clearly dominates in Fe3+ montmorillonite because of the high hydrolysis constant of the Fe(III) ion and the adsorption isotherm obtained is not linear. The desorption isotherms show a hysteresis on all the Men+ montmorillonites examined for the time interval of 3.5 weeks. It is suggested that only that fraction of the bound atrazine, which is adsorbed due to the relatively weak physical forces, can be desorbed. The larger the fraction of protonated atrazine molecules on the surface, the less is remobilized.