NOTE ON THE ADSORPTION OF ORGANIC-MOLECULES ON CLAYS

Results of adsorption studies of several pesticides on soils and clays show that the application of the reduced concentration concept to adsorption can either reduce the temperature effect on the isotherms, eliminate it altogether, increase the temperature dependence or even reverse the temperature dependence of the isotherms. Literature and experimental data for the adsorption of parathion by Ca-attapulgite and by attapulgite with organic exchangeable cations of different sizes demonstrate that for many surface interactions the terms organophilic or hydrophilic are misleading. Organic compounds which are insoluble in water and soluble in apolar solvents will not necessarily adsorb preferentially on ″organophilic″ surfaces. The specific interactions between adsorbate and adsorbent and steric considerations will determine the partition between the adsorbed and the solution phase. An outstanding example is the order of adsorption of parathion or attapulgite. The order is neither directly nor inversely related to the ″organophilic″ nature of the surfaces. Refs.