Complexation of Cd(II) and Pb(II) with humic acids studied by anodic stripping voltammetry using differential equilibrium functions and discrete site models

Binding of Cd(II) and Pb(II) to two humic acids was studied by differential pulse anodic stripping voltammetry using continuous differential equilibrium functions and linear Scatchard plots. One of the humic acids was a commercial sample from Aldrich (AHA) and the other was a sedimentary humic acid isolated from a Brazilian water reservoir (BBHA). The complexation study was performed at pH between 5 and 6 in 0.020 mol l−1 KNO3. The complexes were considered fully labile in the range of the metal to ligand concentration ratios studied, having diffusion coefficients (DML=1.3×10−7 and 2.7×10−7 cm2 s−1, for BBHA and AHA, respectively) smaller than the free cation [DM=8×10−6 and 7×10−6 cm2 s−1 for Cd(II) and Pb(II), respectively]. Differential equilibrium functions and the discrete site model led to similar interpretations of the experimental data. Binding of Pb(II) to both HA samples was stronger than for Cd(II), with a greater heterogeneity of binding sites. This was verified by the heterogeneity parameter, Γ, obtained from the differential equilibrium functions, as well as from the number of classes of binding sites used to fit the titration data to Scatchard plots.