Abstract
Wastewater containing toxic metal ions, such as cadmium, copper, gold, lead, nickel, silver, and zinc ions, are generated in large quantities in the mining, metal finishing, microelectronic fabrication, and photographic film processing industries. The heavy metal ions in wastewater can be recovered in their metallic form by electrodeposition at the cathode of an electrolytic cell. The concentration limits for safe discharge of metal ions to waste streams are typically less than 1 ppm. At these low concentrations, the current efficiency for electrodeposition becomes extremely low, which results into high energy consumption. The electrodialysis method, on the other hand, is able to reduce the metal ion concentrations to a low level by permeating the metal ions through an ionexchange membrane with low energy requirement. The method, however, produces a byproduct of concentrated solution containing mixed metal ions and impurities, which requires further purification or treatment with other processes. In this work, the electrolytic and electrodialytic process were integrated into a single cell. An integrated electrolytic-electrodialytic cell was designed and constructed. The functionality of the cell was tested with a wastewater containing 1000 ppm of Cu+2 ions. The integrated process could reduce the copper ions concentration from 1000 ppm to few ppms. The effect of some parameters like applied potentials; distance between membranes and concentration of sulfuric acid in the side compartments on the apparent reaction rate constant, energy consumption and current efficiency were discussed. The apparent reaction rate constants could be enhanced by reducing the distance between membranes and the concentration of sulfuric acid in the side compartments. At a cell potential of 8.0 V and side concentration of sulfuric acid of 0.5, the apparent reaction rate constant is 8.9780 × 10-5 sec-1 at a membrane distance of 7.1 cm while it is 2.4410× 10-4 sec-1 at a membrane distance of 5.0 cm. Similarly, at the same cell potential and membrane spacing of 7.1 cm, the apparent reaction rate constant is 8.9780 × 10-5 sec-1 at a side concentration of sulfuric acid of 0.5 M while it is 1.2150 × 10-4 sec-1 at a side concentration of sulfuric acid of 0.5 M. The current and energy requirement for the electrodeposition were calculated. The current efficiency increases as the copper ions concentration is high and the cell potential increases. However, the energy consumption decreases as the potential increases, passes through a minimum then it increases.
