Competitive adsorption behavior of heavy metals on kaolinite

Abstract

Polluted and contaminated soils can often contain more than one heavy metal species. It is possible that the behavior of a particular metal species in a soil system will be affected by the presence of other metals. In this study we have investigated the adsorption of Cd(II), Cu(II), Pb(II), and Zn(II) onto kaolinite in single- and multi-element systems as a function of pH and concentration, in a background solution of 0.01 M NaNO 3. In adsorption edge experiments, the pH was varied from 3.5 to 10.0 with total metal concentration 133.3 μM in the single-element system and 33.3 μM each of Cd(II), Cu(II), Pb(II), and Zn(II) in the multi-element system. The value of pH 50 (the pH at which 50% adsorption occurs) was found to follow the sequence Cu < Zn < Pb < Cd in single-element systems, but Pb < Cu < Zn < Cd in the multi-element system. Adsorption isotherms at pH 6.0 in the multi-element systems showed that there is competition among various metals for adsorption sites on kaolinite. The adsorption and potentiometric titrations data for various kaolinite–metal systems were modeled using an extended constant-capacitance surface complexation model that assumed an ion-exchange process below pH 7.0 and the formation of inner-sphere surface complexes at higher pH. Inner-sphere complexation was more dominant for the Cu(II) and Pb(II) systems.