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 Oxisol 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 the pH and the concentrations of metals in the mixture were determined. Soil order, soil horizon, and liming had significant effects on the metal adsorption. Some important changes in the adsorption characteristics of the metals, especially in Zn and Cd, were observed due to competition between the different 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 consider the possibility of a decrease in the amount of metal adsorbed with increasing metal competition for the adsorption sites. Due to the competition with other metals, the equations, which offered the best fit for Zn and Cd, were quadratic polynomial models. On the other hand, for Cu and ph, the equations, which showed the best fit were linear, Langmuir, and Temkin, for different situations. The reasons for this behavior were related to the strong competitive forces for the adsorption sites presented by these two metals.