| Freshwaters are recognized as dynamic systems that may be far removed from equilibrium. The purpose of this research was to investigate the chemical speciation of Zn(II), Cd(II), Pb(II) and Cu(II) in model solutions of humic acid using three independent, non-equilibrium techniques. (i) pseudopolarography, (ii) Diffusive Gradients in Thin films, and (iii) a kinetic approach based on a Competing Ligand Exchange Method. Speciation parameters such as lability, mobility, and heterogeneity of the metal humic acid complexes were investigated. The results indicate that Zn(II) and Cd(II) tend to form more labile and more mobile complexes than Pb(II) or Cu(II). In addition, Zn(II) and Cd(II) were found to form relatively homogeneous complexes whereas Pb(II) and Cu(II) formed relatively heterogeneous complexes with humic acid. The detection window was identified to be the crucial parameter in defining metal species measured by a given analytical technique. The conditional stability constants of Zn(II), Cd(II) and Pb(II) were found to decrease with the ionic potential of the metal. This was accompanied by a corresponding increase in the dissociation rate constants, suggesting that the binding between trace metals and humic acid has significant covalent component. Furthermore, the markedly slow kinetics of Cu(II)-HA species suggests that the usual equilibrium assumption may not be valid in freshwaters. |
