Degradation of 4-chlorophenol by anodic oxidation, electro-Fenton, photoelectro-Fenton, and peroxi-coagulation processes

The degradation of 4-chlorophenol in acidic solution of pH similar to 3.5 has been studied by different electrochemical methods involving H2O2 electrogeneration from an O-2-diffusion cathode. While the solution is slowly mineralized by anodic oxidation in the presence of H2O2, the rate for organic carbon removal increases notably by electro-Fenton, photoelectro-Fenton, and peroxi-coagulation, where Fe2+ acts as catalyst to produce oxidizing OH. from electrogenerated H2O2. A complete mineralization was only reached in the photoelectro-Fenton process. For peroxi-coagulation, the removal of organic carbon in solution is mainly due to the coagulation of dechlorinated intermediates with the Fe(OH)(3) precipitate formed. The decay for substrate concentration is faster by electro-Fenton and photoelectro-Fenton than by peroxi-coagulation. In all methods, the initial hydroxylated intermediate is 4-chloro-1,2-dihydroxybenzene, which is further oxidized with loss of chloride ion to yield maleic and fumaric acids. Fe3+ complexes are produced in the processes using iron ions. These complexes are slowly mineralized by electro-Fenton and rapidly photodecomposed to CO2 by photoelectro-Fenton processes. The apparent current efficiencies for the mineralization processes have been determined. A general pathway for the degradation of 4-chlorophenol by the different methods studied is proposed.