Complexation equilibria of humic substances in aqueous solution (Review)

Thermodynamic and spectroscopic studies on the complexation equilibria of humic and fulvic acids in aqueous solutions are reviewed. Two complicating factors, i.e., polyelectrolytic and functional group heterogeneity effects inherent in the ion-binding equilibria of these naturally occurring polyacids, have been treated separately. Applicability of a Donnan model to the analysis of the proton and metal ion binding equilibria of fulvic acid standard samples and polyacrylic acid has been examined. It has been revealed that electrostatic binding as well as specific binding such as chelate and monodentate ligand complex formation play important roles in the complexation equilibria of these polyion systems.