Effect of the cationic composition of sorption solution on the quantification of sorption–desorption parameters of heavy metals in soils

We obtained the sorption isotherms of Cd, Cu, Pb and Zn in clay, clay saline and organic soils. The distribution coefficients (Kd) were determined in 0.02 eq l−1 CaCl2 and in a solution that simulated the soil solution cationic composition. The Kd values greatly varied with the composition of the sorption solution and the initial metal concentration. The sorption experiments were complemented with the quantification of the extractable metal, to estimate the reversibility of metal sorption. The extraction yields depended on the metal–soil combination, and the initial metal concentration, showing no correlation with previous Kd values. The effect of the solution composition in mobility predictions was estimated through a Retention Factor, defined as the ratio of the Kd versus the extraction yield. Results showed that risk was over- or underestimated using the CaCl2 medium in soils with a markedly different soil solution composition.