METAL SPECIATION IN A HUMIC SURFACE-WATER SYSTEM POLLUTED BY ACIDIC LEACHATES FROM A MINE DEPOSIT IN SWEDEN

A speciation study of some metals (Al, Fe, Cu, Zn and Cd) was performed in the surface water downstream from a deposit with sulphidic mine waste where acidic leachates were neutralized by mixing with well buffered groundwater and surface run-off. The water was rich in humic and fulvic acids (HFA). The chemical forms of the metals were characterized with respect to their cationic and anionic exchange properties. In the present pH range (5-8) Al and Fe precipitated as solids that were partly retained by both cation and anion exchangers. Zinc and cadmium were dominated by cationic forms but for cadmium anionic forms were always present in minor amounts under the conditions of the study. Copper was predominantly anionic (as HFA-complex) except at low pH and high concentrations of Al and Fe, which evidently occupied all available complexing sites on the HFA. Conditional stability constants were estimated for the corresponding metal-HFA complex entirely from field data. The values obtained with this empirical method (logbeta* at pH 7; 4.7 Cu; 3.2 Zn; 4.0 Cd) are in reasonable agreement with measurements in controlled model systems.