Comparative electro-Fenton and UVA photoelectro-Fenton degradation of the antibiotic sulfanilamide using a stirred BDD/air-diffusion tank reactor

• Total mineralization of sulfanilamide by photoelectro-Fenton with a BDD/air-diffusion cell.
• Large, but slower, mineralization by comparative electro-Fenton.
• Oxalic and oxamic acids as the most persistent final carboxylic acids.
• Release of sulfate and ammonium ions, as well as nitrate ion in much lesser extent.

Here, the degradation of 100 mL of sulfanilamide solutions of pH 3.0 has been comparatively studied by electro-Fenton (EF) and photoelectro-Fenton (PEF) using a stirred tank reactor equipped with a 3 cm2 boron-doped diamond (BDD) anode and a 3 cm2 air-diffusion cathode. In both treatments, organics are destroyed by hydroxyl radicals formed at the BDD surface and in the solution bulk from Fenton’s reaction between cathodically generated H2O2 and initially added Fe2+, whereas in PEF, additional photolysis of intermediates under UVA radiation is also feasible. The influence of applied current density and substrate concentration on antibiotic decay, degradation rate and mineralization degree was examined. The sulfanilamide decay always followed a pseudo-first-order reaction, as found by reversed-phase liquid chromatographic monitoring, being faster for PEF. The EF process led to large mineralization of sulfanilamide concentrations up to 2390 mg L−1. Nevertheless, the PEF process was much more potent and allowed achieving total mineralization (>99% of dissolved organic carbon removal) of all the solutions tested. Results showed that the application of this treatment is preferable at low current densities and high substrate concentrations to obtain the best mineralization current efficiencies. Ion-exclusion liquid chromatography revealed that oxalic and oxamic acids were the most persistent generated carboxylic acids. Their Fe(III) complexes were slowly removed by hydroxyl radical in EF and much more quickly photolyzed by UVA light in PEF. Ion chromatography confirmed the main release of NH4+ and SO42- ions, and NO3- ion in relatively smaller extent, during all the mineralization processes.