Abstract
A three-pathway reaction mechanism is proposed to describe the electrochemical oxidation for decomposition of organic molecules. On the basis of the mechanism, experiments were conducted as direct and indirect treatments to distinguish the contribution of each pathway on color removal and chemical oxygen demand (COD) reduction. For indirect treatment, efforts were made to investigate the interaction among pH, oxidation reduction potential, and the formation of OCl-, the major oxidizing agent in the electrolysis of NaCl aqueous solution. With 30 min electrolysis under 10 V on a solution of 1 g/L NaCl, the current was able to produce enough oxidizing agent to completely remove the color in a solution of 50 mg/L Reactive Blue 19. It took 2 g/L NaCl under 20 V in 30 min to generate enough oxidizing agent to reduce >99% of the COD in 20 min. In direct electrochemical treatment, dye molecules are oxidized by oxychlorine compounds and secondary oxidizing agents generated in electrolysis as well as by direct oxidation on the surface of the anode. Synergetic effects between oxidizing agents and surface oxidation dramatically increase the reaction rate constant for color reduction and shorten the treatment time for COD removal. With 2 g/L NaCl in the solution, color removal was almost instantaneously and 99% COD removal was achieved in 10 min. The three-pathway mechanism adequately explains why freshly generated hypochlorite solutions are more effective than commercial bleach for color reduction. It also explains the difference in COD reduction between direct and indirect electrochemical treatment processes. Quantitative data from direct and indirect treatments were collected and presented to assist reactor and process design. This work also lays a foundation for future research on system optimization.