ELECTROCHEMICAL TREATMENT OF WASTEWATERS DRIVEN BY REVERSE ELECTRODIALYSIS PROCESSES

Abstract

Wastewater treatment technology is undergoing a transformation due to more restrictive regulations governing the dischar ge and disposal of hazardous pollutants. Electrochemical based technologies are very promising methods for treating wastewaters containing organic and inorganic pollutants resistant to biological processes or toxic for microorganisms. These methods present numerous advantages including the utilisation of a green reagent such as the electron, very high removal of numerous recalcitrant pollutants, efficient disinfection, high flexibility and no necessity to transport or stock chemical oxidants or reducents. O n the other hand, a wide utilisation of such methods is likely to be limited by: (i) the cost of electric energy necessary to drive electrode reactions; (ii) the cost of the supporting electrolyte for waste waters with no adequate conductibility and (iii), for some applications, by the cost of electrodic materials. In order to overcome some of these drawbacks, some innovative solutions were proposed in the last years such as the utilization of micro reactors to avoid the utilization of supporting electrolyt es and to increase the current efficiencies for electrochemical processes controlled by mass transport stages such as direct oxidation processes or electro - Fenton (EF). To avoid the supply of electric energy to the system also the utilization of microbial fuel cells or reverse electrodialysis processes was proposed. Reverse electrodialysis is a clean, renewable energy with large global potential since the electricity is produced from supplies of water with different salt concentrations. In reverse electrod ialysis (RED), cation and anion conductive membranes are placed in an alternating way in order to produce dilute and concentrate compartments. The salt concentration difference (salt gradient) between both compartments in the cell pair creates a Nernst pot ential across the cell pair which causes an electrical current to flow through the electrical load connected to the electrodes. The electrochemical treatment of two different kinds of waste waters contaminated by Cr(VI) and a model dye, the Acid Orange 7 (AO7), respectively, driven by reverse electrodialysis processes was studied. It was shown that in both cases salinity gradients can be used to obtain electric energy and to successfully treat contaminated waste waters.