Adsorption and Desorption of Carbamate Pesticides by Montmorillonite and Humic Acid-Clay Complexes

In this study adsorption and desorption of three carbamate pesticides namely, oxamyl (I); S-Ethyl-N-(methyl carbamoyl) oxythioacetimidate (II); and N-Phenyl (ethyl carbamoyl) propylcarbamate (III) by Na⁺-, Ca²⁺-montmorillonite, and humic acid-montomorillonite, was investigated. The adsorption of pesticides was in the order III > I > II; adsorption for Na-montmorillonite was more than that for Ca-saturated clay. The presence of humic acid enhanced the pesticide adsorption. A composite model for estimating pesticide adsorption, which assumes mineral and organic matter functioning individually as adsorbent phases, predicted sorption within a factor of 0.75–1.4. The desorption data indicated that humic acid retains more pesticide than the clay mineral. The XRD patterns showed that humic acid is restricted to the external surfaces of clay tactoids, denoting that clay mineral fractions in soils including those with organic coatings play an important role in the retention of polar carbamate pesticides.