Catalytic effect of Fe²⁺, Cu²⁺and UVA light on the electrochemical degradation of nitrobenzene using an oxygen-diffusion cathode

The electrochemical treatment of 100 mg L−1 nitrobenzene solutions in 0.05 M Na2SO4 in the pH range 2.0–4.0 at 25 °C has been comparatively studied in the presence of Fe2+, Cu2+ and/or UVA light as catalysts using an undivided cell with a Pt anode and an O2-diffusion cathode able to electrogenerate H2O2. A quite slow degradation is found for the solution with 1 mM Cu2+ due to the low production of oxidizing hydroxyl radical (OH˙) from anodic oxidation of water and the action of the Cu2+/Cu+ system. Electro-Fenton treatment with 1 mM Fe2+ leads to a high concentration of OH˙ in the medium from Fenton's reaction, but less than 70% of the nitrobenzene is mineralized since stable complexes of Fe3+ with products are formed. These complexes are quickly photodecomposed in the photoelectro-Fenton treatment of the same solution under UVA irradiation, leading to overall depollution at low currents. Complete degradation is also feasible using 1 mM Cu2+ and 1 mM Fe2+ at high current because OH˙ can slowly destroy the complexes between Cu2+ and intermediates. The positive synergetic effect of all catalysts allows the quickest nitrobenzene mineralization using 1 mM Cu2+ and 1 mM Fe2+ under UVA irradiation. The nitrobenzene decay always follows a pseudo-first-order reaction. Aromatic products such as o-nitrophenol, m-nitrophenol, p-nitrophenol and 4-nitrocatechol have been followed by reverse-phase chromatography. Nitrate ions are formed from oxidation of nitroaromatic products. Generated carboxylic acids such as maleic, fumaric and oxalic have been detected by ion-exclusion chromatography. The different evolution of complexes of oxalic with Cu2+ and Fe3+ explains the influence of the catalysts on nitrobenzene degradation.