Enhanced degradation of sulfonamide antibiotics by soluble Mn(III) via polyphenol interactions: Key role of quinone intermediates rather than phenoxy radicals

• Naturally occurring catechols greatly enhanced sulfonamides degradation by Mn(III) • Mn(III) stoichiometrically oxidizes catechol to quinone rather than radical coupling • Sulfonamides are removed via 1,4-nucleophilic addition with quinone intermediates • Covalent adducts markedly reduces the toxicity of sulfonamides Soluble Mn(III) has been recently recognized as prevalent relevant manganese species in natural waters. However, its potential role as an oxidant in the transformation of emerging contaminants has largely overlooked. In this study, we found that widespread naturally occurring polyphenols (e.g., catechol and its derivatives) interestingly and effectively enhanced Mn(III)-mediated oxidation of sulfonamides, contaminants of major concern to microbial ecology and public health. In contrast, neither soluble Mn(III) nor polyphenols alone exhibited appreciable reactivity toward sulfonamides. Mechanistic investigations revealed that Mn(III) initially oxidized catechol to phenoxy radicals. It was noteworthy that phenoxy radicals subjected to Mn(III) oxidation preferentially underwent conversion to ortho -benzoquinone ( o -BQ) through phenoxenium ion formation rather than radical-radical coupling. In other words, catechol was stoichiometrically converted to o -BQ via two-electron transfer. o -BQ, rather than phenoxy radicals was identified as the key intermediate driving enhanced sulfonamides degradation, as supported by a series of experiments such as quenching tests, o -BQ formation and extensive products identification experiments. Specifically, o -BQ reacted with the anilinic nitrogen of sulfonamides via 1,4-nucleophilic addition rather than 1,2-nucleophilic addition, forming covalent adducts that significantly reduced both acute and chronic toxicity. In addition, electron-withdrawing substituents were found to enhance this transformation by increasing the electrophilicity of quinone intermediates and facilitating nucleophilic attack, which was demonstrated by QSARs and density functional theory calculations. Overall, these findings for the first time reveal a previously underappreciated pathway in which polyphenols mediate soluble Mn(III) reactivity, offering new mechanistic insights into Mn(III) redox chemistry and its implications for contaminant transformation in aquatic environments.