Simultaneous competitive adsorption of heavy metals by the mineral matrix of tropical soils

Disposal of sewage sludge, composts and industrial wastes onto agricultural lands is commonly and increasingly used in Brazil, potentially adding multi-component heavy metal solutions. Soil organic matter was removed from representative Brazilian soils and the simultaneous competitive adsorption on the mineral matrix of Cr, Ni, Cu, Zn, Cd and Pb was determined. The competition affected the behavior of heavy metal cations in such a way that the soils adsorbed less Ni, Zn and Cd and more Cr, Cu and Pb with increasing total concentrations, regardless of the molar concentration applied. On the other hand, the occupancy of the adsorption complex by the heavy metals was proportional to the relative concentration applied. As the applied concentrations increased Cr, Cu and Pb adsorption increased, while Ni, Zn and Cd adsorption decreased. The maximum adsorption capacity for the heavy metal cations was calculated, with the soils differing markedly. Ultisol and Alfisol soils showed the highest maximum adsorption values, in the order of 50.76 and 64.52 mmol kg−1, whereas some Oxisols showed the lowest values, in the order of 23.92 and 30.86 mmol kg−1. The results suggest that the first two soil types are capable of receiving and holding higher heavy metal loadings compared to the Oxisols, which are more vulnerable to heavy metal contaminant inputs.