ADSORPTION AND MOBILITY OF CADMIUM IN NATURAL, UNCULTIVATED SOILS

The influence of soil properties on the adsorption and mobility of Cd was studied in 22 natural, uncultivated soils from Salamanca province to understand the environmental characteristics of Cd as a pollutant. Cadmium adsorption was studied by means of adsorption isotherms, using low Cd concentrations that were similar to those potentially found in polluted soils, and Cd mobility was investigated by using soil thin layer chromatography. The Freundlich adsorption constant (K) was not found to be related to any soil properties studied. In contrast, the distribution coefficients (K(d)) at 15 and 30 mug L-1 equilibrium concentrations were found to be related to the cation exchange capacity (CEC) of the soil (r = 0.43 and r = 0.44, respectively, p < 0.05). The adsorption (Q), amount adsorbed at high concentrations (C(i) = 1000/mug L-1), was found to be significantly related to the CEC, the exchangeable Ca2+ content and the sum of bases. In addition, R(f) values, which are indicative of the Cd mobility in soil, bore a highly significant (p < 0.001) negative correlation with some soil parameters such as pH, the exchangeable Ca2+ and Mg2+ contents and the sum of bases, as well as a less significant (p < 0.05), negative correlation with the CEC and clay content of the soil, but no correlation whatsoever with the adsorption constants. The type of organic matter present in the soil must be more influential on Cd mobility than was its content. The above results confirm that a CEC of a soil is highly influential on Cd adsorption and mobility.