Cadmium Sorption Behavior in Some Highly Calcareous Soils of Iran

Cadmium (Cd) sorption by selected calcareous soils with a wide range of soil physical and chemical properties was investigated. Two-gram samples of each soil were equilibrated for 24 h with 40 mL 0.01 M CaCl2 containing 5 to 500 mg Cd L−1. The Cd sorption data well fits to Freundlich, two-surface Langmuir and Temkin adsorption isotherms. However, the fit to Langmuir adsorption model was closer than that of Freundlich or Temkin. The Freundlich coefficient (k) was significantly correlated with cation exchange capacity (CEC), calcium carbonate equivalent (CCE), and clay content (Clay). The slope in the first surface was greater than that of the second surface, indicating a higher rate of Cd sorption at low Cd equilibrium concentrations and a proportionally smaller increase in Cd retention at higher initial Cd solution concentrations. Stepwise regression analysis indicated that 67% of the variation in the adsorption maxima of the second surface (b 2) was due to the combined effect of CEC and CCE. Temkin adsorption coefficients, namely k 1 was significantly related to CEC and b 2 was positively correlated with CEC, CCE, and clay content.