Sorption and lability of cadmium and lead in different soils from Egypt and Greece

Reactions of heavy metals with soils are important in determining their availability and fate in the environment. Mono-metal sorption and lability of sorbed cadmium (Cd) and lead (Pb) in different representative soils from Egypt and Greece as influenced by their properties were investigated in this study. For this purpose eleven surface soil samples varying widely in their origin and properties were selected. Four of them were from Egypt representing the main soil orders i.e., Entisols and Aridisols and the rest, seven, from different sites of Greece belonging to the orders Entisols, Alfisols, Vertisols, Mollisols, and Histosols. In these samples sorption isotherms were developed from which sorption parameters, and distribution coefficient (Kd) of Cd and Pb were determined. In addition lability of these metals was estimated by DTPA extraction following their sorption.The results showed that Freundlich model described satisfactorily sorption of both metals. In all the soils studied Kd values of Pb were higher than that of Cd indicating that this was retained by the soils stronger than Cd. Sorption parameters (kf, n) and Kd values of Egyptian Entisol developed on lacustrine deposits showed higher affinity for Pb, Greek Histosol for Cd while acidic Alfisols showed the lowest affinity for both metals studied. Permanent charge clayey soils with relatively low Fe, Al and Mn oxides content sorbed more Cd and Pb than the variable charge red soils with higher content of these oxides. In variable charge red soils with similar sesquioxides content, Pb and Cd sorption was pH dependent. However, in variable charge soils with similar soil pH, no significant differences were recorded for Pb sorption, while Cd sorption capacities differed significantly depending on the active ratios of Fe and Al oxides. Sorption parameters of Cd were correlated to clay content, cation exchange capacity, organic matter content, total free and amorphous aluminum oxides, amorphous iron oxides and CaCO3 content while Pb sorption parameters were correlated with clay content, total free and amorphous silica oxides, and amorphous aluminum oxides content as well as cation exchange capacity (CEC).Lability of the adsorbed Cd was higher than Pb in all the studied soils and may pose more threats to the ground water and plants. The Greek acidic Alfisols (Rhodoxeralf) showed the lowest lability of sorbed Cd, while the alkaline one exhibited the highest Cd lability. The lowest lability of sorbed Pb was in Greek Histosols and the highest in acidic Alfisol (Typic Haploxeralf). Labile Cd was negatively correlated only with CaCO3 content in all the soils studied while labile Pb was negatively correlated with CEC, clay, organic matter, total free aluminum oxides and the amorphous iron and aluminum oxides content.