Advanced oxidation of real sulfamethoxazole + trimethoprim formulations using different anodes and electrolytes

A commercial sulfamethoxazole + trimethoprim formulation has been degraded in 0.050 M Na₂SO₄ at pH 3.0 by electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂), electro-Fenton (EF), photoelectro-Fenton with a 6-W UVA lamp (PEF) and solar photoelectro-Fenton (SPEF). The tests were performed in an undivided cell with an IrO₂-based, Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode for H₂O₂ electrogeneration. The anode material had little effect on the accumulated H₂O₂ concentration. Both drugs always obeyed a pseudo-first-order decay with low apparent rate constant in EO-H₂O₂. Much higher values were found in EF, PEF and SPEF, showing no difference because the main oxidant was always OH formed from Fenton's reaction between H₂O₂ and added Fe²⁺. The solution mineralization increased in the sequence EO-H₂O₂ < EF < PEF < SPEF regardless of the anode. The IrO₂-based and Pt anodes behaved similarly but BDD was always more powerful. In SPEF, similar mineralization profiles were found for all anodes because of the rapid removal of photoactive intermediates by sunlight. About 87% mineralization was obtained as maximum for the powerful SPEF with BDD anode. Addition of Cl^- enhanced the decay of both drugs due to their quicker reaction with generated active chlorine, but the formation of persistent chloroderivatives decelerated the mineralization process. Final carboxylic acids like oxalic and oxamic were detected, yielding Fe(III) complexes that remained stable in EF with BDD but were rapidly photolyzed in SPEF with BDD, explaining its superior mineralization ability.