Elimination of Heavy Metals from Leachates by Membrane Electrolysis
Authors: Fischer R. , Seidel H. , Rahner D. , Morgenstern P. , Löser C. 
Published: October 2004

The elimination of heavy metals from bioleaching process waters (leachates) by electrolysis was studied in the anode and cathode region of a membrane electrolysis cell at current densities of 5–20 mA/cm2 using various electrode materials. The leaching waters containing a wide range of dissolved heavy metals, were high in sulfate, and had pH values of approx. 3. In preliminary tests using a rotating disc electrode the current density‐potential curve (CPK) was recorded at a rotation velocity of 0, 1000 and 2000 rpm and a scan rate of 10 mV/s in order to collect information on the influence of transport processes on the electrochemical processes taking place at the electrodes. The electrochemical deposition‐dissolution processes at the cathode are strongly dependent on the hydrodynamics. Detailed examination of the anodic oxidation of dissolved Mn(II) indicated that the manganese dioxide which formed adhered well to the electrode surface but in the cathodic return run it was again reduced. Electrode pairs of high‐grade steel, lead and coal as well as material combinations were used to investigate heavy metal elimination in a membrane electrolysis cell. Using high‐grade steel, lead and carbon electrode pairs, the reduction and deposition of Cu, Zn, Cr, Ni and some Cd in metallic or hydroxide form were observed in an order of 10–40 % in the cathode chamber. The dominant process in the anode chamber was the precipitation of manganese dioxide owing to the oxidation of dissolved Mn(II). Large amounts of heavy metals were co‐precipitated by adsorption onto the insoluble MnO2. High‐grade steel and to some extent lead anodes were dissolved and hence were proven unsuitable as an anode material. These findings were largely confirmed by experiments using combination electrodes of coal and platinized titanium as an anode material and steel as a cathode material. With both electrode combinations and current densities of 5 or 10 mA/cm2, in the cathode region low depositions of 10–20 % Cd, 2–10% Mn, 5–20 % Zn, 1–20 % Co and 5–15 % Ni were measured. By contrast, the elimination of other metals was substantially larger: Fe 40 –60 %, Cu 20–40 %, and Cr 40–60 %. In the anode region the removal of heavy metals was in the order of 30–50%, with Mn being as high as 80 %. The anode materials exhibit good resistance at the current densities tested. The precipitates deposited in both electrode regions contained as main components Al with 10–20 %, Mg with approximately 10 %, and SO4 with 5–20 %. The solid material in the cathode chamber consisted of relatively high proportions of Zn and Mn. Calcium in the solids indicated the co‐precipitation of calcium sulfate. The main components in the solids of the anode chamber were Mn in the form of pyrolusite, Al as basic sulfate, and Mg. The results indicate that electrochemical metal separation in the membrane electrolysis cell can represent a practical alternative to the metal separation by alkalization. Regarding the main heavy metals Zn, Mn and Ni in the process water, combination electrodes using steel as a cathode material and coal or platinized titanium as an anode material proved to be suitable for eliminating the heavy metals from the aqueous phase. However, for practical application, further work is necessary to improve the efficiency, applicability and costs of the process.