THE BINDING CONSTANTS OF HEAVY METAL-HUMATE AND FULVATE COMPLEXES

A technique based on the combined use of potentiometric titration and spectrophotometry is developed for studying the binding of divalent heavy- metal cations (Cd^2+, Pb^2+, Hg^2+ and Zn^2+) to humic and fulvic acids. The binding constants were determined at concentrations of 10^-4 and 10^-3 M for the investigated metal cations, and at pH s of 4.5, 5.5 and 6.5. Two parameters, β1 and β2, were needed to fit the binding as a function of carboxylate concentration; the metal ions bound to the carboxylate sites in humic and fulvic acids forming 1:1 and 1:2 complexes. The results indicated that fulvic acid has lower binding capacity than humic acid. The binding constants increased with increasing pH and with decreasing metal ion concentration. They also increase as the ionic potential of the metal ions increase, especially in case of fulvic acid. The order of increasing the binding constants was found: Cd^2+ Hg^2+ Pb^2+ Zn^2+, for humic acid and Pb2+ Hg^2+ Cd^2+ Zn^2+, for fulvic acid.