CADMIUM SPECIATION AND COMPLEXATION BY NATURAL ORGANIC-LIGANDS IN FRESH-WATER

Free Cd2+ concentration and Cd complexation by natural organic ligands in fresh water were determined by a technique involving ethylene diam ine (EN,1,2-diaminoethane) ligand exchange and measurement by differen tial pulse anodic stripping voltammetry. The validity of the ligand ex change approach was demonstrated in a synthetic solution containing ED TA as a model ligand. The technique was applied to water samples colle cted from the lakes Greifen. Sempach and Lucerne with different trophi c states, a polluted lake (Orta), a polluted river (Glatt) and groundw ater at an infiltration site of this river. The dissolved Cd concentra tions range from 0.03 to 0.09 nM, the free Cd2+ ion concentrations exp ressed as pCd=-log[Cd2+]=11.5-12.2, and the conditional stability cons tants of stable organic complexes are log K=10.2+/-0.1 (pH 7.8-8), wit h ligand concentrations of 1-6 nM in the lakes Greifen, Sempach and Lu cerne. 80% of the dissolved Cd is free Cd2+ in Lake Orta with lower pH (<7.3) and higher dissolved Cd (0.7 nM). In the Glatt river and the i nfiltrated groundwaters, polluted by effluents from sewage treatment p lants, dissolved Cd concentrations of 0.2-2 nM, pCd=10.5+/-0.2, log K= 9.4+/-0.1 (pH 7.9-8) and ligand concentrations of 2.8-3.8 nM are obser ved. The reliability of the technique and relevant implications of the results in these waters are discussed. The extent of Cd complexation is linked to the biological primary productivity in the lakes studied, suggesting biological sources for the organic ligands. (C) 1998 Elsev ier Science B.V.