Competition of Al~(3+) in Complexation of Humic Matter with Pb~(2+): A Comparative Study with Other Ions

摘要

Humic matter (HM) presents a high affinity to bind lead ions, but complexation is affected by the presence of major ions such as Ca~(2+), Mg~(2+), and Al~(3+), which might have severe consequences in the bioavailability of lead. In this work the influence of aluminum(Ⅲ) in complexation of HM with Pb~(2+) will be studied and compared to the competition of Ca~(2+) in complexation of humic matter with Cd~(2+) or Zn~(2+). The competition between Al~(3+) and Pb~(2+) for the complexing groups of HM was studied from the titration of humic samples with lead, in the presence and absence of aluminum, at pH 4.5 and 0.01 M KNO_3, followed by anodic stripping voltammetry. For the concentration ranges used, it was found that aluminum influences the coordination of lead to HM through an electrostatic effect expressed by an exponential factor, which is a function of aluminum and humic matter concentrations. This behavior is different from the competition of Ca~(2+) in complexation of HM with Cd~(2+) (or Zn~(2+)), where Ca~(2+) decreases the number of HM sites available for the complexation but no electrostatic effect is observed. It is important to stress that the model presented can be applied to the complexation of lead with DOC in acidic soils containing gibbsite, where Al~(3+) concentrations may be high.