Labile species of Pb, Zn and Cd determined by anodic stripping staircase voltammetry and their toxicity to tetrahymena

The amounts of free ions and labile complexes of Pb, Zn and Cd have been determined in a complex organic growth medium by use of anodic stripping staircase voltammetry, ASSV. It was possible to determine the labile fractions of metals and to obtain good correlation with data on the toxicity of the metals to Tetrahymena when using a specific reduction potential (E(s)) for each element, - 600, - 800 and - 1200 mV vs. SCE for Pb, Zn and Cd, respectively. The labile fractions in the organic growth medium were less than 1% for Pb (Pb precipitated), 15% for Zn (as labile complexes) and 30-40% for Cd (as free ions) for total heavy metal concentrations of 0.5mM-2mM. The toxicity to Tetrahymena decreased in the order Cd Zn Pb. The effect of Pb was greater than predicted by ASSV, probably because Tetrahymena additionally ingests the lead-containing precipitate and therefore is exposed to concentrations of Pb exceeding those of the soluble species found in the medium. The results stress the importance of using different specific reduction potentials when different elements are compared, instead of one potential common to all elements. The use of a fixed potential may lead to erroneous conclusions regarding the concentrations of labile species in solution.