COMPETITION OF COPPER AND ZINC FOR STRONG LIGANDS IN A EUTROPHIC LAKE

Free aquo copper ion and zinc ion concentrations were determined in water samples from eutrophic Lake Greifen by means of ligand exchange with catechol and cathodic stripping voltametry (for Cu) and ligand exchange with EDTA and anodic stripping voltametry (for Zn). The ratios of total dissolved Zn (10-40 nM) to dissolved Cu (7-20 nM) were [Zn]:[Cu] = 0.5-3 in samples taken at different seasons of a year; the ratios of the free aquo ion concentrations [Zn2+]:[Cu2+] were similar to 10(6). pCu was in the range of 14.5-15.9 and pZn 8.6-9.5 at different times and depths. The release of Zn from electrochemically inert complexes upon addition of Cu suggested direct competition of Cu and Zn for ligands in all lake-water samples examined. The selectivity of the natural ligands for Cu over Zn was evaluated as the conditional constant for the reaction Cu2+ + ZnL(1) = Zn2+ + CuL(1); K = (1.4 +/- 0.9) x 10(6) (average in the euphotic zone; pH 8). These highly selective ligands are probably of biological origin.