New insight into linear substituents influencing electrooxidation treatment of sulfonamide antibiotics: Linking kinetics, pathways, toxicity, and active species with density functional theory

Linear substituents, despite their simpler structures compared to heterocyclic ones, exhibit distinct chemical behaviors. Using sulfacetamide (SAM) and sulfaguanidine (SGD) as model compounds, we assessed the impact of these substituents on degradation efficiency, active species identification, reaction pathways, and intermediate toxicity during electrooxidation in water. Through density functional theory, we elucidated the mechanisms, focusing on electronic structural changes and interactions with active species. Notably, the acetyl group in SAM (0.1016) acquired more electrons than the guanidyl group in SGD (0.0281), resulting in SAM having a higher free energy change ($\Delta G=15.06\text{kcal/mol}$) compared to SGD ($\Delta G=9.59\text{kcal/mol}$). This difference makes SAM less likely to undergo direct electron transfer and less reactive towards hydroxyl radical addition, leading to slower degradation rates. The applied potential notably increased SAM’s sensitivity to hydroxyl radicals. Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were contributed by the parent fragment, facilitating electrophilic reactions mainly on the aniline part. Seventeen intermediate products and three major transformation pathways were identified, emphasizing aniline group destruction before discharge. This research enhances understanding of the degradation and environmental fate of sulfonamides, providing valuable insights for optimizing pollutant degradation and discharge reduction.