Conversion of carbonaceous materials into solid acids for tylosin mitigation: effect of preprocessing methods on the reactivity of sulfonation reaction.

Carbon-based solid acids have been successfully employed as acidic catalysts for pollutant mitigation in wastewater. To fully tap the potentials of commercially viable carbons for the preparation of solid acids and enhance their catalytic performances is a challenging problem. In this work, three commercialized carbons including biochar, activated carbon and graphite were preprocessed (ball-milling, Hummer exfoliation, HNO3 soaking, and microwave heating in HNO3, etc.), sulfonated, and evaluated as solid-acid catalysts for tylosin mitigation. Graphite-originated solid acid performed the best through a balling-milling preprocess, while biochar-originated solid acids behaved well under all preprocessing treatments, in which 40 mg L−1 of tylosin was mitigated within 8 min by 1 g L−1 of biochar-originated solid acids. The biochar solid acid through the ball-milling preprocess presented high total acidity and large amounts of –SO3H groups, due to dramatically increased surface area and the rise of activation groups (hydroxyl, alkyl and alkoxy groups, etc.) facilitating electrophilic reaction. In addition, decreased particle size and aromaticity and increased structural defects also contributed. Theoretical calculation of average local ionization energy (ALIE) of condensed aromatic model molecules with substituted activation groups confirmed the promoting effects on sulfonation from strong to weak were 8.40–9.06 eV. These findings have deepened the knowledge in tuning carbon surface chemistry for better sulfonation, thus strengthening catalytic degradation of tylosin. The value of this study is in pulling a clear thread for maneuvering solid-acid catalysts using carbons, which holds a novel promise for rationally functionalizing biochar-based catalysts for the remediation of macrolide antibiotics in polluted water. Highlights: The cooperative effects of the properties and four preprocesses on sulfonation of three carbon materials were revealed. Ball milling increases surface area, structural defects and activation groups thus facilitating sulfonation electrophilic reaction. Activation groups facilitating electrophilic substitution enhanced the reactivity of sulfonation for ball-milled biochar. [ABSTRACT FROM AUTHOR]