Electrochemical conversion of chromium from tannery effluents for potential reuse in industrial applications

Electrochemical oxidation of trivalent chromium from leather tanning mud waste leachates (containing ca 6 g^.L^-1 Cr(III)) to its hexavalent form was carried out using a PbO_x/Pb anode electrode in a prototype small (0.4 L) cylindrical batch electrochemical reactor. The PbO_x/Pb anode was prepared by electrochemical anodization at constant current (75 mA cm^-2 for 30 min) in a sulfuric acid solution and characterized by the cyclic voltammetry technique to investigate the effect of pH on the process. It was found that at pH = 3, Cr(III) oxidation prevails over the competing water oxidation-oxygen evolution reaction (OER), hence increasing the efficiency of the process. A detailed study of pH (0-3), current density (12-24 mA cm^-2), and cell type (divided-undivided) effects on bulk electrolysis of Cr(III) leachates in the batch prototype reactor resulted in process optimization. At pH = 3, 12 mA cm^-2 and a cathode inserted in a porous diaphragm envelope, nearly 70% conversion was achieved at a nearly 60% current efficiency, among the highest in the previously reported literature. The method (further optimized with an ion-selective membrane separator) could offer an attractive route for tannery Cr(III) conversion to Cr(VI) for reuse as an etchant or electroplating agent.