Degradation of sulfanilamide in acidic medium by anodic oxidation with a boron-doped diamond anode

Abstract

Solutions of the antimicrobial sulfanilamide have been degraded in acidic medium by anodic oxidation (AO) using an undivided or divided cell with a boron-doped diamond (BDD) anode and a stainless steel cathode at constant current density. Almost total mineralization was attained in solutions with 0.50M Na2SO4 at regulated pH 3.0 contained in the anodic compartment of the divided cell due to the efficient destruction of organics with hydroxyl radicals generated at the BDD anode from water oxidation. This cell had higher oxidation ability than the undivided one using 0.05M Na2SO4, which yielded slower degradation and lower efficiency. In each cell, a similar degradation rate was found at a pH interval 2.0–6.0, indicating the existence of the same electroactive species for sulfanilamide. The mineralization current efficiency was always enhanced at lower current density and higher initial substrate concentration. The sulfanilamide decay was followed by reversed-phase HPLC, always showing a pseudo-first-order kinetics that was faster in the divided cell. 1,2-Benzenediol and 1,3-benzenediol were detected as aromatic intermediates in the undivided cell by gas chromatography–mass spectrometry, while p-benzoquinone formed in both cells was identified by reversed-phase HPLC. Generated carboxylic acids like maleic, fumaric, acetic, formic, oxalic and oxamic were quantified by ion-exclusion HPLC. The initial N content of sulfanilamide was mainly released as NH4+ ion and in much smaller proportion as NO3- ion. A reaction sequence for sulfanilamide mineralization in acidic medium by AO with a BDD anode considering all detected intermediates is finally proposed.