Competition of Al3+ in complexation of humic matter with Pb2+: A comparative study with other ions

Humic matter (HM) presents a high affinity to bind lead ions, but complexation is affected by the presence of major ions such as Ca2+, Mg2+, and Al3+, which might have severe consequences in the bioavailability of lead. In this work the influence of aluminum(lll) in complexation of HM with Pb2+ will be studied and compared to the competition of Ca2+ in complexation of humic matter with Cd2+ or Zn2+. The competition between Al3+ and Pb2+ 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 KNO3, 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 Ca2+ in complexation of HM with Cd2+ (or Zn2+), where Ca2+ 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 Al3+ concentrations may be high.