Competitive Sorption of Nickel and Cadmium in Different Soils

Competing ions strongly affect heavy-metal retention and release in soils. In this study, we evaluated the sorption of Ni and Cd in single and binary Ni-Cd systems for one neutral and 2 acidic soils. Ni and Cd sorption isotherms were obtained using batch techniques over a wide range of concentrations. Both Ni and Cd adsorption isotherms exhibited strong nonlinear behavior with similar overall patterns for all soils. Sorption of Ni and Cd correlated well the cation exchange capacity for all soils. Moreover, Cd sorption by the two acidic soils was greater than Ni, whereas for the neutral soil, Ni sorption was greater than Cd. The Freundlich model was utilized to describe the family of Ni and Cd isotherms in the presence of competing ions. For both Ni and Cd isotherms, the Freundlich distribution coefficient K decreased with increasing concentration of the competing ion. The dimensionless parameter n was within a narrow range of 0.50 to 0.64 for both Ni and Cd and was not affected by the competing ion. Competitive sorption was described using the Sheindorf-Rebhun-Sheintuch (SRS) equation, which is based on multicomponent Freundlich-type models. Estimates of SRS competitive sorption coefficients indicated that for the neutral soil, Ni sorption was least affected by Cd. The SRS equation was also capable of describing the overall competitive Ni and Cd sorption patterns for all soils. However, SRS model modifications are needed to improve sorption prediction for the highest competing concentrations.