Solar photoelectro-Fenton degradation of the antibiotic metronidazole using a flow plant with a Pt/air-diffusion cell and a CPC photoreactor

The degradation of 10 dm³ of solutions of the heterocyclic antibiotic metronidazole in 0.10 mol dm^-3 Na₂SO₄ of pH 3.0 has been comparatively studied by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). Experiments were performed in a solar flow plant equipped with a Pt/air-diffusion cell and coupled to a compound parabolic collector (CPC) photoreactor. A very weak mineralization was found for the EF process in the dark, indicating a large recalcitrance of heterocyclic compounds to be destroyed by hydroxyl radicals formed at the Pt anode from water oxidation and mainly in the bulk from Fenton's reaction between added Fe²⁺ and cathodically generated H₂O₂. The quick photolysis of intermediates by UV radiation from sunlight enhanced largely the mineralization process by SPEF. The effect of applied current density and Fe²⁺ and drug contents on the SPEF treatment was examined. The best process was found for 1.39 mmol dm^-3 metronidazole with 0.50 mmol dm^-3 Fe²⁺ at 55.4 mA cm^-2 giving 53% mineralization, 36% mineralization current efficiency and 0.339 kWh (g DOC)^-1 in 300 min. Metrodinazole was completely removed and its decay obeyed a pseudo-first-order kinetics. LC-MS analysis allowed identifying five heterocyclic products and twelve hydroxylated derivatives. Ion-exclusion HPLC analysis revealed that final oxalic and oxamic acids were practically removed at the end of electrolysis due to the efficient photolysis of their Fe(III) complexes by sunlight. The initial N of metronidazole gave NO₃^- ion as main inorganic ion. A large proportion of initial N remained in solution as unidentified N-products and their major part was lost as N-volatile species. Based on the detected products, a reaction sequence for metronidazole mineralization by SPEF is proposed.