Removal of Multiple Metallic Species from Sludge by Electromigration

The treatment of runoff waters from a lignite mine and a power plant generated a sludge heavily polluted with metals. Electromigration laboratory experiments were conducted to evaluate the feasibility of this technology for the removal of metals from the sludge. The experimental results proved that the mobilization of heavy metals could be achieved by the acidification of the sludge. The electro-oxidation of water in the anode can be used as a source of ${\mathrm{H}}^{+}$ to mobilize the metals transported toward the cathode. The suppression of ${\mathrm{OH}}^{-}$ generation at the cathode by the controlled addition of nitric or sulfuric acid increased the electric conductivity of the sludge specimen and the electric current intensity. The increase in the current intensity in the electrokinetic cell resulted in more ${\mathrm{H}}^{+}$ ions electrogenerated at the anode, favoring mobilization of the metals by the direct attack of the ${\mathrm{H}}^{+}$ ions. The electrokinetic tests proved that acid pH in the cathode resulted in higher electric current intensity and faster metal mobilization and removal, but the energy consumption sharply increased. A trade-off occurred among metal removal, energy consumption, and the pH in the catholyte. The best results were achieved by using sulfuric acid to control the pH in the catholyte at 4. The average metal removal was: Fe 27%, Ca 68%, Mg 24%, Na 25%, Mn 68%, Sr 33%, Zn 60%, and Cu 60%. Other elements such as Al, K, Ti, and Zr associated with the crystalline structure of minerals were not affected by the electrokinetic treatment.