Electrochemical removal of Cr(VI) from aqueous media using iron and aluminum as electrode materials: Towards a better understanding of the involved phenomena

In the present work, electrocoagulation process with iron and aluminum electrodes was investigated to deepen the understanding of the mechanism of hexavalent chromium (Cr(VI)) removal. Electrocoagulation treatment efficiency was studied with regards to the abatement of Cr(VI) and the resulting species-namely Cr(III), Fe(II) and/or Fe(III). Unlike iron, aluminum electrodes were found to be unsatisfactory for Cr(VI) removal. To elucidate the removal mechanism of hexavalent chromium, different anode/cathode materials and configurations (Fe/Fe, Pt Ti (platinized titanium)/Fe. Al/Al and Pt Ti/Al) were considered. At nearly neutral pH and considering aluminum electrodes, both electrochemical reduction (Cr(VI) to Cr(Ill)) at the cathode surface and adsorption on AI(OH)(3) floc mechanisms were responsible for Cr(VI) exhaustion. However, the contribution of the two mechanisms to Cr(VI) removal was not discriminated. On the other hand, in the case of iron electrodes, even though electrochemical reduction may contribute to chromium removal, its influence seemed to be minor since the effect was confined to less than 5% of the removal efficiency. Hence, there was essentially one real root for the reduction of Cr(VI) by electrocoagulation with iron electrodes, and it was proven to be the chemical reduction by Fe(II) anodically generated. Moreover, the resulting Cr(III) was quickly removed from solution, via efficient precipitation as Cr(OH)(3) hydroxides. Besides, the electrodissolved iron remained at low level owing to the precipitation of Fe(OH)(2) and/or Fe(OH)(3). Although chemical reduction by Fe(II) predominantly governed the removal of Cr(VI), acidic pH media was found to promote electrochemical reduction of hexavalent chromium at the cathode and accordingly to enhance Cr(VI) removal efficiency. In contrast, a delay of Cr(III) precipitation and a contamination of the electrolyte by electrodissolved iron were observed under acidic conditions. The effect of supporting electrolyte (Na2SO4 and NaCl) on chromium removal was also studied. No conspicuous disparity in the treatment efficiency was noted under the electrocoagulation conditions used. (C) 2009 Elsevier B.V. All rights reserved.