Electrochemical oxidation of several chlorophenols on diamond electrodes:: Part II.: Influence of waste characteristics and operating conditions

The electrochemical treatment of wastes containing several chlorophenols ( 4- chlorophenol, 2,4- dichlorophenol and 2,4,6- trichlorophenol) using boron- doped diamond electrodes is described. Both direct and indirect processes are involved in the oxidation of the organics, indirect processes being mediated by oxidising agents ( such as hypochlorite or peroxodisulphate) generated on the surface of the anode. The influence of the waste characteristics ( initial concentration, pH and supporting media) is reported. The presence of reversible redox reagents, like the sulphate/ peroxodisulphate redox couple, plays an important role in determining the global oxidation rate. Hypochlorite formation depends only on the organochlorinated compound and not on the presence of other reversible redox reagents in the waste. Alkaline pH favours the accumulation of carboxylic acid intermediates since, under these conditions, the oxidation rate of such compounds is low. The influence of the operating conditions ( temperature and current density) is also discussed. The results show that high temperatures improve the rate of the mediated reactions and that high current density values decrease the efficiency of the direct electrochemical processes.