Electrochemical Cr(VI) reduction using a sacrificial Fe anode: Impacts of solution chemistry and stoichiometry

A systematic investigation of Cr(VI) reduction using electrochemical reduction revealed that the Cr(VI) reduction was extremely fast with reaction kinetics limited by the anodic generation of Fe(II). The Cr(VI) reduction rate increased with decreasing pH at the initial stage of reaction but the time to reach complete Cr(VI) reduction is pH independent. The amount of Fe(II) generated per mole of Cr(VI) reduced was calculated and compared with the stoichiometric value, i.e., 3 mole of Fe(II) needed per mole of Cr(VI) reduced. The values are 11.1% higher than the stoichiometric value for pH 7 and 9, but are 32.0% less for pH 3 and 5. The spontaneous reduction of Cr(VI) by Fe⁰ and adsorption of Cr(VI) to Fe(OH)₃ precipitates might contribute the additional Cr(VI) removal. Effect of DO was investigated under various mixing schemes. Under N₂ purging, Fe(II) generated for one mole of Cr(VI) reduced is 3.67% higher than the stoichiometric value, while mechanic mixing and aeration mixing show 15% and 19%, respectively, higher than stoichiometric value, indicating that DO does impact Cr(VI) reduction. The electrochemical Cr(VI) reduction process was also employed to treat electroplating wastewater with and without pH pre-adjustment, achieving 100% total Cr and Ni removal for both cases. ORP can be used as a controlling parameter when electrochemical reduction is implemented for Cr(VI) reduction.