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 NaNO3. In adsorption edge experiments, the pH was varied from 3.5 to 10.0 with total metal concentration 133.3 mu M in the single-element system and 33.3 mu M 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. (c) 2005 Elsevier Inc. All rights reserved.