Abstract
We aimed to investigate the distribution coefficients (K-d) of Cd and Zn in different soils in mono-metal (MS) and competitive sorption (CS) systems. Eleven surface soil samples varying widely in their origin and properties were collected in Egypt and Greece. Sorption isotherms were determined and K-d derived. Clayey Entisols showed a higher affinity for Zn, while a Histosol showed a higher affinity for Cd. In MS the K-d of Zn was higher than the K-d of Cd in the studied soils, except for an Aridisol and acidic Alfisols. In case of CS the K-d of Zn was higher than the K-d of Cd in the soils. The K-d of Cd decreased in CS compared to MS. The decreasing percentage of K-d-Cd ranged from 6.5% in the Histosol to 81.8% in the sandy Entisols. K-d-Zn in CS decreased in the Entisols, Alfisols, and Histosol, but increased in the Aridisol, Vertisol, and Mollisol compared to MS. This suggests that upon co-addition of the two metals to the soil Zn became preferentially sorbed at the expense of Cd. Permanent charge clayey soils with relatively low Fe-, Al- and Mn-oxides content sorbed more Cd and Zn than the variable charge red soils with a higher content of these oxides. Cation exchange capacity (CEC), soil organic matter, Al-d, Fe-d, Fe-o, Al-o and total carbonates are important for the sorption of Cd, whereas clay, CEC, Si-d, Si-o and Al-o content determine the sorption of Zn.