Selectivity sequence and competitive adsorption of heavy metals by Brazilian soils

Heavy-metal cations can be introduced into agricultural soils by application of fertilizers, liming materials, sewage sludge, composts, and other industrial and urban waste materials. Therefore, heavy-metal adsorption reactions, in a competitive system, are important to determine heavy-metal availability to plants and their mobility throughout the soil. This study was conducted to evaluate the selectivity sequence and estimate the competitive adsorption of several heavy metals in seven soils with different chemical and mineralogical characteristics. Distribution coefficients (K-d), which represent the sorption affinity of metals for the solid phase, were obtained for each soil and heavy-metal cation. On the basis of these K-d, the selectivity sequence was evaluated. The most common sequences were Cr > Ph > Cu > Cd > Zn > Ni and Ph > Cr > Cu > Cd > Ni > Zn. Chromium, Ph, and Cu were the heavy-metal cations most strongly adsorbed by all soils, whereas Cd, Ni, and Zn were the least adsorbed, in the competitive situation. Selectivity sequences related to valence for the trivalent Cr. For metals of the same valence, sequences did not exactly follow the order of electronegativity. For individual elements, the Misono softness parameter and hydrolysis properties of the heavy-metal cations may have influenced the sequences. Correlation analysis showed that soil characteristics that may have affected the heavy-metals adsorption, represented by the distribution coefficients, were pH and cation-exchange capacity (CEC) for Cd and Cr; organic carbon, clay, and gibbsite contents for Cu; pH and CEC for Ni and Ph.