Anodic mineralization of pentachlorophenol in alkaline media

In the present work the electrochemical incineration of pentachlorophenate anion has been studied, in consideration of the stability towards chemical attack and of its pronounced bio-refractory character. Experiments have been carried out at PbO2, RuO2, IrO2 and Pt electrodes, in order to assess the influence of the electrode material on the process. Constant current electrolyses have been carried out at a Ti/Pt electrode for solutions of pentachlorophenol (100 ppm) in 0.1 M NaOH. The experiments have also been carried out in 0.01 M NaOH solutions, keeping the ionic strength constant to a value of 0.1 by addition of suitable amounts of NaCl, Na2SO4, NaClO4, Na2HPO4 respectively. The concentration of PCP in solution has been followed by UV-visible spectroscopy. An important feature that could established feature, is certainly the high sensitivity of the PCP anodic oxidation to the nature of the anion of the supporting electrolyte. The effect on the reaction mechanism is also evident from a qualitative exam of the UV-visible spectra, which show changes in shape, in the presence of the different anions studied in this work. Anions like Cl- and SO4= can take part in the overall mineralization process, giving themselves origin to strong oxidants under the chosen polarization conditions. In fact, species like ClO-, ClO2, ClO2- can be produced by anodic oxidation of Cl-, and S2O8= by anodic oxidation of SO4=. This role can be understood both, in terms of surface reactions involving adsorbed oxidizing radicals and adsorbed pentachlorophenate itself, and of bulk reactions between chlorine dioxide and persulphate respectively with the substrate. Basing the attempt of explanation on an inhibition of the mineralization due to anion adsorption it is more difficult to find out some systematic trend. On the other hand, anions that give origin to some extent of charge transfer upon electrosorption, eventually leading to their oxidation, would play the role of partially inhibiting the oxygen evolution reaction. As shown elsewhere, for the case of glucose mineralization in the presence of chloride ions, this can cause, in turn, an increase in the yield of the oxidation of the organic substrate. Following this approach, we can reach the conclusion that anions that can be anodically converted into strong oxidants, can play their mineralizing action both through partial inhibition of the parasitic oxygen evolution and through the direct oxidation of the organic substrate, at the electrode surface and in the near-surface-region of the solution bulk