CADMIUM SPECIATION IN SOIL SOLUTIONS

Total Cd concentration was measured in the saturation extract of three unpolluted arid-zone forest soils to which Cd was added as Cd(NO3)2 solution (0.1 mg Cd kg-1 soil). The ratio of free to total Cd (Cd2+/Cd(t)) in the extracted soil solution was measured using a Cd selective electrode by an addition method. A chemical equilibrium model including the major complexation reactions of Cd with anionic ligands was used to estimate Cd speciation in solution and was validated by the agreement between the calculated and measured Cd2+/Cd(t) ratios. Free Cd2+ and CdHCO3+ were each estimated to constitute 35% to 45%, respectively, of total soluble Cd in the calcareous soil solutions studied, which had pH values between 7.5 and 8.5. Agreement between measured and calculated Cd2+/Cd(t) ratios improved slightly after destruction of organic matter by UV irradiation of the extracted solutions. Organo-Cd complexes were shown to be present in relatively minor amounts in these soils.