The relative adsorption selectivities of Pb, Cu, Zn, Cd and Ni by soils developed on shale in New Valley, Egypt

Reactions of heavy metals with soils are important in determining the bioavailability and the ultimate fate of these metals in the environment. To assess the competitive sorption and the selectivity sequence of Pb, Cu, Zn, Cd and Ni, batch equilibrium experiments were performed using a mixture of metal solutions and surface samples of six Egyptian soils that were developed on shale and differed in their physicochemical and mineralogical properties. The results indicated that the Langmuir isotherm could well describe the adsorption of all studied heavy metals. The sorption behaviours of Cd and Ni were similar to each other in the most studied soils, while those of Pb, Cu and Zn were different. Metal solid/liquid distribution coefficients (Kd), which represent the adsorption affinity of the metallic cations in solution for the solid phase, were calculated the overall initial added concentrations of heavy metals. The highest Kd values were found for Pb (0.29–1647 L kg− 1) and followed by Cu (0.21–813 L kg− 1). However, low Kd values were shown for Zn (0.17–118 L kg− 1), Ni (0.08–163 L kg− 1) and Cd (0.04–45 L kg− 1). On the basis of the Kd values, the most common selectivity sequence of the metal adsorption is Pb > Cu > Zn > Ni > Cd, which it is related to the first hydrolysis equilibrium constant. Generally, the simple correlation and the multiple regression analysis suggest that the ability of the soil to adsorb heavy metals depends upon the type (mainly, smectites) and the amount of clay, as well as the CEC.