Electrochemical Treatment of Acidic Aqueous Ferrous Sulfate and Copper Sulfate as Models for Acid Mine Drainage

Abstract

Acid mine drainage (AMD) is a serious environmental problem in the mining industry. The present work describes electrolytic reduction of solutions of synthetic AMD, comprising FeSO 4/H 2SO 4 and CuSO 4/H 2SO 4, in flow-through cells whose anode and cathode compartments were separated using ion exchange membranes. In the case of FeSO 4/H 2SO 4 at constant flow rate, the pH of the effluent from the catholyte increased progressively with current at a variety of cathodes, due to electrolytic reduction of H + ions to elemental hydrogen. Near-quantitative removal of iron was achieved by sparging air into the catholyte effluent, thereby precipitating iron outside the electrochemical cell, and avoiding fouling of the electrodes. The anode reaction was the oxidation of water to O 2, a proton-releasing process. Using cation exchange membranes and sodium sulfate as the supporting electrolyte in the anode compartment, the efficiency of the process was compromised at high currents by transport of H + competitively with Na + from the anode to the cathode compartments. Higher efficiencies were obtained when anion exchange membranes were used, and in this case no additional supporting electrolyte other than dilute H 2SO 4 was needed, the net reaction being the electrochemically driven transfer of the elements of H 2SO 4 from the cathode to the anode compartments. Current efficiencies ∼50% were achieved, the loss of efficiency being accounted for by ohmic heating of the solutions. In the case of CuSO 4/H 2SO 4 and anion exchange membranes at high currents, reduction of Cu 2+ and H + ions and transport of SO 4 2− ions out of the catholyte caused unacceptably high potentials to be generated.