INORGANIC AND ORGANIC LIGAND-BINDING OF LEAD AND CADMIUM AND RESULTANT IMPLICATIONS FOR BIOAVAILABILITY

Interactions between humic acids, carbonate ions and the cations Pb, Cd and Ca were examined in an effort to provide a basis for predicting Cd and Pb bioavailability. Percentages of total Pb and Cd which were labile or present as complexes less than 1000 Daltons in size were determined using differential pulse polarography and equilibrium dialysis techniques respectively. Measurements on model systems, using concentrations similar to those found in regional lakes, indicate that Pb is largely bound by humic acids and to a lesser extent by carbonates but that high Ca concentrations can increase labile Pb through competition for binding sites. Cd binding to humic acids is much lower than that of Pb. These results suggest that Cd should be more bioavailable than Pb. An examination of concentration factors in the mussel Elliptio complanata from 10 lakes in Southern Ontario, Canada, indicates the significance of humic materials in controlling bioavailability. However, the results also point out the importance of hydrogen ions in the bioavailability of these trace metals.