Thermodynamics of Carbamate Pesticides Adsorption by Clay Minerals and Humic Acid-Clay Complexes

The adsorption of three carbamate pesticides Oxamyl (I); S-Ethyl-N-(methylcarbamoyl) oxythioacetimidate (II) and N-Phenyl (ethyl carbamoyl) propylcarbamate (III) used as nematicide and herbicides has been determined at two different temperatures from dilute solutions of montmorillonite; humic and humic acid-montmorillonite complex saturated with Na⁺, Ca²⁺ and Al³⁺ cations. The presence of humic acid enhanced the pesticide adsorption while rise in temperature decreases the adsorption. Adsorption isotherms were of ‘L’ type. The adsorption of pesticides was in order II > I > III. The adsorption was maximum for Al-saturated complexes and followed the order Al- > Na- > Ca. The standard free energy changes (ΔG°) for the adsorption reactions were negative, signifying a spontaneous reaction. The enthalpy changes (ΔH°) calculated from the temperature coefficient of the equilibrium constant showed that process is exothermic. The adsorption of pesticides on adsorbents give rise to an entropy loss as pesticide molecules adsorbed on surface had fewer translational and rotational degrees of freedom than pesticide molecules in solution. Various studies e.g. IR and X-ray, as well as thermodynamic parameters and Freundlich adsorption isotherms indicated the existence of protonation or coordination (or both) between exchangeable cation and oxygen of >C=O group of pesticides. The values of thermodynamic parameters indicated towards a partial physical adsorption.