Sulfate radical-based oxidation of antibiotics sulfamethazine, sulfapyridine, sulfadiazine, sulfadimethoxine, and sulfachloropyridazine: Formation of SO₂ extrusion products and effects of natural organic matter

The widespread occurrence of sulfonamide antibiotics in the environment has raised great concerns about their potential to proliferate antibacterial resistance. Sulfate radical (SO₄^{• -}) based advanced oxidation processes (SR-AOPs) are promising in-situ chemical oxidation (ISCO) technologies for remediation of soil and groundwater contaminated by antibiotics. The present study reported that thermally activated persulfate oxidation of sulfonamides (SAs) bearing six-membered heterocyclic rings, e.g., sulfamethazine (SMZ), sulfapyridine (SPD), sulfadiazine (SDZ), sulfadimethoxine (SDM), and sulfachloropyridazine (SCP), all produced SO₂ extrusion products (SEPs), a phenomenon that is of potential importance, but not systematically studied. As an electrophilic oxidant, SO₄^{• -} tends to attack the aniline moiety, the reactive site of SAs, via electro-transfer mechanism. The resulting anilinyl radical cations are subjected to further intermolecular Smiles-type rearrangement to produce SEPs. Formation of SEPs is expected to occur in other SR-AOPs as well. The temperature-dependent evolution pattern of SEP of SMZ, 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl)aniline, can be well fitted by kinetic modeling concerning sequential formation and transformation of intermediate product. The presence of natural organic matter (NOM) influenced the evolution patterns of 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl)aniline significantly. Toxicological effects of SEPs on ecosystem and human health remain largely unknown, thus, further monitoring studies are highly desirable. - Highlights: • Sulfonamides bearing six-membered heterocyclic rings exhibit similar degradation kinetics. • SO₄^{• -} oxidize sulfonamides to generate SO₂ extrusion products via Smiles-type rearrangement. • SO₄^{• -} tends to attack the aniline moieties of sulfonamides via electron-transfer mechanism. • The evolution of SO₂ extrusion product can be well fitted by sequential reaction kinetic model.