Cadmium removal from diluted aqueous solutions by electrowinning in a flow-by cell

Mineral processing and metal finishing industries produce large amounts of effluents containing a host of heavy metals - cadmium, lead, nickel and chromium - with cadmium as a heavy hazard, since, even in mild concentrations, it may give rise to serious environmental problems. Effluents from electroplating industries and from zinc ore hydrometallurgical processing may contain this metal in the range of 15 to 100 ppm. Cadmium harm to all living systems is aggravated by its strong cumulative effect, because of the long number of years required for its elimination. The electrolytic waste treatment has shown advantages over the classical chemical precipitations by alkalis or carbonates, because it does not produce bulky sludges, which require land for its accumulation, and which may have a retarded pollution effect, due to the redissolution by rain; a great advantage comes from the possibility of the metal recovery in a commercially pure form. In this paper, a laboratory scale electrolytic flow-by cell with a reticulated vitreous carbon (RVC) cathode, developed in our laboratory, is described, which permitted the purification of aqueous solutions containing cadmium ion on the usual order of concentration of metallurgical industries effluents; with this cell, operating in a mass transport regime, it has been possible to decrease cadmium concentration of solutions of 2 liters volume from 200 to 0.1 ppm, with current efficiencies up to 40%, in a matter of 85 minutes.