Phorate and Terbufos adsorption onto four tropical soils

Adsorption of Phorate and Terbufos onto four tropical soils was investigated in this study. It was found that the adsorption kinetics was fast and that the equilibrium was established within 6 h. Adsorption isothermal data could be well described by the Freundlich equation. It was demonstrated that the soils were more favorable for the adsorption of Terbufos than Phorate, which was due to the higher hydrophobicity of Terbufos (and its lower water solubility). The presence of organic compounds in soils played an important role. A higher organic content caused higher adsorption. A new term of K _oc′, the Freundlich based organic content-normalized partition coefficient K_oc′=K/f_oc was defined. In the above equation, K and f_oc are the Freundlich constant and the fraction by weight of organic content in the soils, respectively. It was demonstrated that the K_oc′ was independent of the types of soils (or organic content). The pH effect was found to be insignificant for the adsorption of both pesticides. Finally, a competitive study demonstrated that the presence of the more strongly adsorbed Terbufos played a more important role than that of the more weakly adsorbed Phorate.