Contrasting Sonodegradation and Anodic Oxidation of Sulfonamides in Water: Degradation routes, Matrix Effects, and Theoretical Study

Mid-high-frequency ultrasound (375 kHz) and anodic oxidation at low current intensity (<50 mA, NaCl as the supporting electrolyte) were employed to treat sulfonamide antibiotics (sulfamethoxazole—SMX and sulfacetamide—SAM). The sonodegradation involved HO•, while electrogenerated HClO was mainly responsible for the antibiotics’ elimination in the electrochemical process. A comparison of the processes evidenced that the degradation of SMX by ultrasound was faster due to its higher hydrophobicity. In contrast, in the electrochemical system, the SAM degradation was more efficient, which was associated with a higher reactivity of its acetamide moiety toward HClO. Interestingly, SMX was selectively sonodegraded in synthetic hospital wastewater and seawater, whereas the matrix components strongly accelerated the electrochemical degradation but affected the process performance in the hospital wastewater. On the other hand, theoretical analyses of atomic charge indicated that the central S-N bond, the N and aromatic ring in the aniline moiety, the C=C bond, and methyl groups in the isoxazole groups on SMX are the most susceptible moieties to the attacks by HO• and HClO. Furthermore, for the typical byproducts, calculations of the probability of being active against bacteria were slightly lower than that of the parent pharmaceutical, even being much lower for the byproducts from the electrochemical treatment.