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 diamine (EN,1,2-diaminoethane) ligand exchange and measurement by differential pulse anodic stripping voltammetry. The validity of the ligand exchange approach was demonstrated in a synthetic solution containing EDTA as a model ligand. The technique was applied to water samples collected from the lakes Greifen, Sempach and Lucerne with different trophic states, a polluted lake (Orta), a polluted river (Glatt) and groundwater at an infiltration site of this river. The dissolved Cd concentrations range from 0.03 to 0.09 nM, the free Cd2+ ion concentrations expressed as pCd=−log[Cd2+]=11.5–12.2, and the conditional stability constants of stable organic complexes are log K=10.2±0.1 (pH 7.8–8), with ligand concentrations of 1–6 nM in the lakes Greifen, Sempach and Lucerne. 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 infiltrated groundwaters, polluted by effluents from sewage treatment plants, 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 observed. 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.