Three binding models for determining conditional stability constants of Cd(II) and Cu(II) with Humic Materials were evaluated using experimental data obtained from the measurement of free Cu(II), Cd(II) with ion selective electrodes. Three binding models were 1:1 complex model, Scatchard-plot model, Continuous Distribution model based on the Scatchard-plot. A Continuous Distribution model based on the Scatchard-plot showed the greatest promise. This is because this mode has taken into consideration variations in binding energies, without regard to the manner in which metal ion is bound, and variations in binding properties of metal ion - humate complex. So using this method, complexation of Cd(II) and Cu(II) with humic materials was investigated at differing pH, humic material concentrations and ionic strengths (I). In the case of Cd-CoHA complex, at pH = 4, average value of $logK_i(\mu)$ decreased with increasing I : values were 4.51, 4.47 and 4.27 at I values of 0.005M, 0.025M and 0.05M, respectively. Values for μ at I = 0.1M and pH of 4, 5 and 6 were 5.29, 5.42 and 5.70, respectively for the Cu-SFA complex. Decreasing SFA total concentration increased binding strength of Cd-SFA complex. But binding strength of Cu-SFA complex was not relevant to variation of SFA total concentration.
The geometry of ligand concentration of Cd(II) bound on SHA and PMAA was investigated with EXAFS at room temperature. The results of EXAFS data analysis indicated that the ligand coordination of Cd(II) in SHA was very similar to that of $Cd(H_2O)_6^{2+}$ whereas that in PMAA was different from that of $Cd(H_2O)_6^{2+}$.
