Adsorption of Pesticides by Clay Minerals

The rate and equilibrium of adsorptions of DDT, dieldrin, and heptachlor by illite, kaolinite, and montmorillonite from aqueous pesticidal solutions are evaluated using the batch technique under laboratory conditions. The adsorption of each of these three pesticides at an initial concentration of 100 μg/l by kaolinite and illite is nearly instantaneous, with the molecules of pesticides being deposited on the clays' surfaces. As montmorillonite is an expansible clay, DDT and heptachlor are not only able to concentrate on the clay's surface but can diffuse gradually into the clay's interlamellar spaces. The adsorptive capacity of each clay is not related to its ion exchange capacities or specific surface area, yet dependent upon a specific pesticide-clay combination. The adsorption of DDT and heptachlor is relatively irreversible; however, a certain extent of dieldrin desorption occurs. It is concluded that the primary mechanisms of the adsorption of these pesticides by clays are through the formation of hydrogen bondings and some other strong forces of interaction. Only in the case of dieldrin do the van der Waals forces contribute to a significant extent to the overall adsorption reaction.