The adsorption of myo-inositol hexaphosphate onto kaolinite and its effect on cadmium retention

Abstract Anions are known to affect the adsorption of metal ions to soil minerals, with the change in adsorption depending on the nature of the anion. Organic phosphates, like inositol hexaphosphate (IP 6 ), are present in significant amounts in most soil systems but little is known about their effect on metal ion adsorption. Since strong complexes are formed between organic phosphates and metal ions, their presence in soils should significantly affect metal ion adsorption. Adsorption edge and isotherm experiments were performed for binary IP 6 –kaolinite, Cd(II)–kaolinite and IP 6 –Cd(II) systems, and the ternary IP 6 –Cd(II)–kaolinite system. All results were modeled with Extended Constant Capacitance surface complexation models. The presence of IP 6 significantly increased the adsorption of Cd(II) to kaolinite in the pH range from 4 to 8.5 with a small decrease in adsorption at higher pH values. There was evidence of Cd(II)–IP 6 precipitate formation at higher concentrations. Modeling indicated that IP 6 was bound to the surface as both inner- and outer-sphere complexes while two extra ternary complexes were required to fit the IP 6 –Cd(II)–kaolinite data. Suppression of Cd(II) adsorption at higher pH most probably resulted from the formation of soluble Cd(II)–IP 6 complexes.