Degradation of the antibiotic trimethoprim by electrochemical advanced oxidation processes using a carbon-PTFE air-diffusion cathode and a boron-doped diamond or platinum anode

The degradation of 20.0mgL^-1 of trimethoprim (TMP), an antibiotic commonly detected in wastewaters, in an aqueous solution with 7.0gL^-1 Na₂SO₄ was accomplished by electrochemical advanced oxidation processes (EAOPs) such as anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF), as well as by the classical Fenton and photo-Fenton processes. All experiments were performed in a novel 2.2L lab-scale flow plant equipped with compound parabolic collectors (CPCs) and an electrochemical filter-press cell with a BDD or Pt anode and a carbon-PTFE air-diffusion cathode to electrogenerate H₂O₂. The effect of initial Fe²⁺ concentration, current density and pH on the PEF method with the BDD anode (PEF-BDD) was firstly assessed by means of TMP and dissolved organic carbon (DOC) decays, aiming to establish a treatment process using minimal iron concentration, adequate current density/H₂O₂ production and maximal pH. This treatment was efficiently performed using a low Fe²⁺ dose of 2.0mgL^-1, a low current density of 5mAcm^-2 and pH of 3.5 without iron precipitation. The relative oxidation ability of EAOPs using the BDD/air-diffusion cell increased in the order: AO-H₂O₂<EF<PEF<SPEF. The EF-BDD and PEF-BDD processes were more effective than the comparable Fenton and photo-Fenton ones. The PEF-BDD process exhibited slightly faster TMP degradation than the PEF-Pt one, whereas in SPEF the influence of the anode was almost negligible. After ca. 37kJL^-1 UV energy, 77 and 73% mineralization with 30 and 26% current efficiency and 1.2 and 0.9kWhm^-3 energy cost were obtained, respectively. It was found a slow and partial TMP mineralization mainly linked to the formation of a high content of hardly oxidizable N-derivatives, containing the major part of N. Up to 18 aromatic products and 19 hydroxylated derivatives were detected by LC-MS during TMP degradation by PEF-Pt. An additional SPEF-Pt experiment using a real wastewater matrix spiked with TMP attained slower TMP and DOC decays.