Oxidation of sulfonamide antibiotics by chlorine dioxide in water: Kinetics and reaction pathways

- Sulfonamides (SAs) were effectively oxidized by ClO2 following second-order kinetics.
- The reactivity of SAs with ClO2 showed a strong dependence on pH and temperature.
- Breakage of SN and CS bonds and hydroxylation of aniline group induced SAs removal.
- Antibacterial functional moieties of SAs were disrupted by ClO2.
- Effective removal of SAs can be expected under practical water treatment conditions.

Sulfonamides (SAs), commonly used as human and veterinary antibiotics, are of great concerns because of their frequent detections in aquatic environment. This study investigated the oxidation of six SAs (i.e., sulfamethoxazole (SMX), sulfamethizole, sulfamethazine, sulfadimethoxine, sulfamerazine, and sulfathiazole) by chlorine dioxide (ClO2). The results indicate that the reactions followed the second-order kinetic model, with rate constants ranging from 3.85 × 103 to 2.59 × 104 M−1 s−1 at pH 7.0 and 20 °C. For each SA, the rate constant increased by 1.6-2.2 orders of magnitude as the solution pH increased from 4.0 to 9.5. The activation energies of the selected SAs ranged from 31.6 to 39.8 kJ mol−1. In addition, SMX was selected as a model compound to explore the degradation pathways during ClO2 oxidation. The reactivity of SMX toward ClO2 was strongly related to the ionization equilibrium of the amido-nitrogen in SMX molecule. The cleavage of SN and CS bonds and the hydroxylation of aniline moiety in the SMX molecule constituted the major degradation pathways. ClO2 oxidation was likely to decrease the antibacterial activity of SMX solution because of the destruction of p-aminobenzenesulfonamide moiety. The obtained rate constants could well predict the fate of SAs during ClO2 oxidation in a surface water, where an effective removal of SAs by ClO2 can be expected under practical water treatment conditions.

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