The in-cell and ex-cell Fenton treatment of phenol, 4-chlorophenol and aniline

A comparative study of phenol, 4-chlorophenol and aniline degradation with the electro-generation of H2O2 at gas-diffusion electrodes was carried out under three different conditions: electro-Fenton (R) stop treatment in an undivided cell; electro-Fenton treatment in the catholyte of a membrane cell divided by a proton-exchange membrane (in-cell electro-Fenton membrane process); and a treatment of polluted solution in the cathode space of a membrane cell with the generation of H2O2, followed by the addition of Fe(II) salt in the other reactor (ex-cell electro-Fenton process). An optimized cell design with no gap between the membrane and the anode, along with the appropriate choice of supporting electrolytes, ensured a voltage reduction with a membrane cell in comparison with that of an undivided cell. The accumulation of hydrogen peroxide in concentrations sufficient for the almost complete destruction (90-98%) of aromatic organic pollutants was achieved in all cases but the ex-cell process with the preparative electrolysis in the pilot scale membrane reactor separated by the proton-exchange membrane MK-40 showed higher treatment efficiency and lower specific energy consumption in comparison with known technologies. Damage of the gas-diffusion layer was observed in some tests which could be caused by alkaline conditions in the pores of the gas-diffusion cathode (GDE). The pH indicator paper showed a color specific for alkaline media in contact with the GDE treated in the solution with pH 3 in the bulk. A possible explanation could be that even in acid media, hydrogen peroxide generation in pores of the gas diffusion layer proceeds with formation of HO2- which is common for alkaline media and consecutive protonation occurs at the interface with the acid solution.