Electro-Fenton and photoelectro-Fenton degradations of the drug beta-blocker propranolol using a Pt anode: Identification and evolution of oxidation products

The beta-blocker propranolol hydrochloride has been degraded by electrochemical advanced oxidation processes like electro-Fenton (EF) and photoelectro-Fenton (PEF) using a single cell with a Pt anode and an air diffusion cathode (ADE) for H2O2 electrogeneration and a combined system containing the above Pt/ADE pair coupled in parallel to a Pt/carbon-felt (CF) cell. Organics are mainly oxidized with hydroxyl radical (·OH) formed from Fenton's reaction between added Fe2+ and electrogenerated H2O2. The PEF treatment in Pt/ADE-Pt/CF system yields almost total mineralization because ·OH production is enhanced by Fe2+ regeneration from Fe3+ reduction at the CF cathode and Fe(III) complexes with generated carboxylic acids are rapidly photodecarboxylated under UVA irradiation. Lower mineralization degree is found for PEF in Pt/ADE cell due to the little influence of UVA light on Fe2+ regeneration. The homologous EF processes are much less potent as a result of the persistence of Fe(III)-carboxylate complexes. Aromatic intermediates such as 1-naphthol, 1,4-naphthoquinone and phthalic acid and generated carboxylic acids such as pyruvic, glycolic, malonic, maleic, oxamic, oxalic and formic are identified. While chloride ion remains stable, NH4+ and NO3- ions are released to the medium. A reaction sequence for propranolol hydrochloride mineralization is proposed.