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 cadm ium(II) in their interaction with humic substances has been studied by differential pulse anodic stripping voltammetry at constant ionic str ength. Two experimental methodologies were employed: (1) addition of h umic acid to a solution of heavy metal; and (2) addition of heavy meta ls to a humic acid solution, both using various concentrations of calc ium. The former method leads to complexation curves from which an over all mean stability for the heavy metal complex can be obtained. In the latter procedure, mean stability values are obtained at each titratio n point from the fractions of free and bound heavy metal ions. In the presence of competing ions like calcium, the association of heavy meta ls with the humic material can be well understood by incorporation of an additional equilibrium reaction for calcium with the humate polyani on. Hence, we are able to obtain information successfully on heavy met al speciation in more complex systems like natural waters, which conta in competing cations such as calcium.