STUDY OF CADMIUM INTERACTION WITH HUMIC-ACID AT THE MERCURY WATER INTERFACE

The influence of humic acid on the cathodic (reduction) and the anodic (oxidation) waves of cadmium was examined by electrochemical methods. At low pH values (pH = 1.6) the decrease of both the cathodic and anodic waves of cadmium follow the adsorption isotherm of humic acid (HA) studied, because the adsorbed layer of the protonated HA represents a barrier for the interaction of cadmium with the electrode surface. At higher pH values (pH = 6.7 and 9.0) the influence of HA on the cathodic and anodic wave of cadmium is completely different. While the anodic wave of cadmium became gradually suppressed in the presence of humic acid the cathodic wave increased high above the initial height of the cadmium reduction wave without the presence of humic acid. It is supposed that before the formation of the Cd-HA complex in solution the binding of cadmium on the electrode surface occurs which leads to this increase in relative height of the cadmium reduction wave. This feature is dependent on the pH of solution, ionic strength and the presence of major ions (chloride, calcium, and magnesium ions).