Towards Scalable Electrochemical Reduction Cells for Hexavalent Chromium

Abstract

The ability of electrochemical cells with carbon-based anodes and cathodes to reduce hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) was investigated over a range of applied potentials and feed pH values. These cells reduced >99% of the 200 ppm Cr(VI) present with a residence time below one minute when applying at least 0.75 V to solutions with a pH below 2.5. Loss of reduction at higher pH values is associated with the predicted formation of solid chromium compounds on the cathode surface due to local pH shifts during operation. A long-term breakthrough study was performed comparing the all-carbon cell to one with a mixed-metal oxide (MMO) anode. Near total reduction of Cr(VI) to Cr(III) was maintained by the carbon cell for 4 hours of operation at 1.5 V before higher effluent Cr(VI) concentrations were seen. While the MMO cell did not suffer a reduction in performance, cost savings with carbon electrodes versus MMO could be quite substantial. Postmortem analysis of the carbon anodes shows increased resistivity, which correlates with the loss of reductive capacity. Based on these results, commercial projections were possible, which shows that this type of electrochemical cell may be an environmentally and economically sound alternative to sulfite addition when managing process streams containing hexavalent chromium.