Competitive adsorption of zinc, cadmium, copper, and lead in three highly-weathered Brazilian soils

Equilibrium adsorption experiments on zinc (Zn), cadmium (Cd), copper (Cu), and lead (Pb) were conducted in three horizons of two Ultisols and one Oxi sol with and without liming, from Vicosa-MG (Brazil). Equilibrium solutions were applied as a "cocktail" containing 700 mg L-1 of Zn, 20 mg L-1 of Cd, 200 mg L-1 of Cu, and 300 mg L-1 of Pb and its dilutions of 1:5 and 1:20. After shaking, the mixture was centrifuged, the supernatant collected and t he pH and the concentrations of metals in the mixture were determined. Soil order, soil horizon, and liming had significant effects on the metal adsor ption. Some important changes in the adsorption characteristics of the meta ls, especially in Zn and Cd, were observed due to competition between the d ifferent cations present in the solution. Also, desorption of Zn and Cd was observed with an increasing concentration of the solution. The adsorption data for Zn and Cd did not fit the linear, Langmuir, Freundlich, and Temkin isotherm. equations for most situations, as these equations do not conside r the possibility of a decrease in the amount of metal adsorbed with increa sing metal competition for the adsorption sites. Due to the competition wit h other metals, the equations, which offered the best fit for Zn and Cd, we re quadratic polynomial models. On the other hand, for Cu and ph, the equat ions, which showed the best fit were linear, Langmuir, and Temkin, for diff erent situations. The reasons for this behavior were related to the strong competitive forces for the adsorption sites presented by these two metals.