Comparison of Cadmium Adsorption Behavior in Selected Calcareous and Acid Soils

Cadmium (Cd) concentration in soil solution and its bioavailability is controlled by sorption-desorption reactions. The objective of this research was to compare Cd adsorption behavior in six calcareous and four acid soils. Soil samples were equilibrated with 0.01 M Ca(NO3)2 containing 25 to 3200 mg Cd L−1. Results showed that the tendency for adsorption was high at low Cd concentrations (0-400 mg L-1), but decreased as the Cd increased (400-3200 mg L-1). Among five equations evaluated, Freundlich, Langmuir, and Gunary equations best described Cd adsorption in both calcareous and acid soils as indicated by high values for coefficient of determination (R2) and low values for standard error of estimate (SE). Stepwise regression equations between constants of the best-fitted models and soil properties revealed that cation exchange capacity (CEC) and clay content were the most important soil properties affecting Cd adsorption behavior in calcareous soils, whereas in acid soils Cd adsorption was mainly affected by soil pH. Constants of the best fitted models showed good correlations with pH, clay, and calcium carbonate equivalent (CCE) in combined calcareous and acid soils. Our tentative conclusion is that higher pH in calcareous soils resulted in greater negative exchange sites available for Cd sorption, therefore Cd adsorption in these soils is greater compared to that of acid soils.