Voltammetric analysis of the competition between calcium and heavy metals for complexation by humic material

The competition between calcium and the heavy metals zinc(II) and cadmium(II) in their interaction with humic substances has been studied by differential pulse anodic stripping voltammetry at constant ionic strength. Two experimental methodologies were employed: (1) addition of humic acid to a solution of heavy metal; and (2) addition of heavy metals to a humic acid solution, both using various concentrations of calcium. The former method leads to complexation curves from which an overall mean stability for the heavy metal complex can be obtained. In the latter procedure, mean stability values are obtained at each titration point from the fractions of free and bound heavy metal ions. In the presence of competing ions like calcium, the association of heavy metals with the humic material can be well understood by incorporation of an additional equilibrium reaction for calcium with the humate polyanion. Hence, we are able to obtain information successfully on heavy metal speciation in more complex systems like natural waters, which contain competing cations such as calcium.