Enhanced transformation of sulfonamide antibiotics by manganese(IV) oxide in the presence of model humic constituents

In this study, a manganese(IV) oxide-mediator (MnO₂-mediator) system for the abatement of sulfonamide antibiotics was evaluated. Two simple model humic constituents, syringaldehyde (SA) and acetosyringone (AS), could promote the transformation of sulfonamides at pH 5–8. Two additional potential mediators, tannic acid and 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), had negligible enhancement on the transformation of sulfonamides by MnO₂. The enhancing effect was attributed to the reaction of the oxidized mediator (i.e., phenoxy radical or benzoquinone-like compounds) produced from the oxidation of the mediators by MnO₂ with SMX. Thereby cross-coupling products from sulfamethoxazole (SMX) with oxidized SA were formed in the MnO₂-SA system, which was confirmed by liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry. Coexisting metal ions (i.e., Ca(II), Mg(II) and Mn(II)) showed inhibitory effects in the order of Mn(II)> Ca(II)> Mg(II). For repetitive runs of the MnO₂-SA-SMX system, MnO₂ lost its oxidative capacity due to the sorption of Mn(II) on the reactive sites of the MnO₂ surface. A full regeneration of partially deactivated MnO₂ by oxidation of the sorbed Mn(II) with Mn(VII) could be achieved.