INFLUENCE OF ELECTROLYTE ON CADMIUM INTERACTION WITH SELECTED ANDISOLS AND ALFISOLS

The adsorption-desorption of cadmium (Cd) as a function of pH, ionic s trength, and composition of the electrolyte was examined for four vari able charge soils: two Alfisols from Zimbabwe (Mazowe and Bulawayo) an d two Andisols from Spain (Realejos-1 and Realejos-2), At the natural soil pH, Cd adsorption measured in 0.01 mol dm(-3) NaNO3 was related t o the net negative surface charge of the soils, Most of the Cd adsorbe d in the four soils was strongly bonded, Increasing pH increased the s urface negative charge of all soils, Adsorption of Cd also increased c ontinuously with rising pH, and part of the variation in adsorption wi th pH was explained by the variation in the negative surface charge, D esorption decreased as the pH increased and became a trace at pH > 6. Differences in extractability between the two groups of soils were con sidered to be related to differences in the characteristics of the soi l matrix, Increasing the concentration of the electrolyte from 0.001 t o 0.1 mol dm(-3) NaNO3 caused a decrease in Cd adsorption and enhanced the amount of sorbed metal that could subsequently be desorbed, Varia tions in the initial pH of the soil suspensions, however, accounted fo r part of the effect, This pH-ionic strength interaction was even more evident when the most dilute electrolyte was used, Adsorption was red uced in the Ca-electrolyte compared with the Na-electrolyte, Calcium, on the contrary. increased Cd desorption.