Mineralization of Metoprolol by Electro-Fenton and Photoelectro-Fenton Processes

Solutions of about 0.25 mM of the β-blocker metoprolol tartrate (100 mg L−1 total organic carbon) with 0.5 mM Fe2+ in the presence and absence of 0.1 mM Cu2+ of pH 3.0 have been comparatively degraded under electro-Fenton (EF) and photoelectro-Fenton (PEF) conditions. The electrolyses were carried out with two systems: (i) a single cell with a boron-doped diamond (BDD) anode and an air-diffusion cathode (ADE) for H2O2 electrogeneration and (ii) a combined cell with a BDD/ADE pair coupled with a Pt/carbon felt (CF) cell. Overall mineralization was reached in all PEF treatments using both systems due to the efficient production of hydroxyl radical (•OH) from Fenton's reaction induced by UVA light and the quick photolysis of Fe(III) carboxylate complexes formed. In EF, the combined cell was much more potent than the single one by the larger •OH generation from the continuous Fe2+ regeneration at the CF cathode, accelerating the oxidation of organics. However, almost total mineralization in EF was feasible using the combined cell in the presence of 0.1 mM Cu2+, because of the parallel quick oxidation of Cu(II) carboxylate complexes by •OH. Metoprolol decay always followed a pseudo-first-order reaction. Aromatic products related to consecutive hydroxylation/oxidation reactions of metoprolol were detected by gas chromatography−mass spectrometry. The evolution of the aromatic 4-(2-methoxyethyl)phenol and generated carboxylic acids was followed by HPLC. The degradation rate and mineralization degree of metoprolol tartrate were limited by the removal of Fe(III) and Cu(II) complexes of ultimate carboxylic acids such as formic, oxalic, and oxamic. NH4+ ion and to a lesser extent NO3− ion were released in all treatments, being quantified by ionic chromatography.