MnO₂ Crystal Phases Mediate o-Semiquinone Radical Generation for Selective Aniline Contaminant Oxidation

Traditional advanced oxidation processes (AOPs) often face significant challenges in contaminant degradation due to strong interference from complex water matrices. In this study, o-semiquinone radicals (o-SQ^•-)-driven AOP was established by MnO₂-mediated catechol oxidation, achieving selective degradation of aniline contaminants (e.g., sulfamethoxazole (SMX)) in real water matrices. Four MnO₂ crystal phases (α-, β-, γ-, and δ-MnO₂) were evaluated, and the degradation efficiency of SMX followed the order γ > α > β > δ-MnO₂. Both MnO₂ surface-bound o-SQ^•- and aqueous-phase Mn(II)-o-SQ^•- contributed to the SMX degradation. Crystal phases dictated o-SQ^•- generation─α-, β-, and γ-MnO₂─favored the MnO₂ solids surface binding of o-SQ^•-, while δ-MnO₂ promoted the interaction of o-SQ^•- with Mn(II) in the aqueous phase. Higher MnO₂ redox potentials and Mn(IV) content correlated with enhanced o-SQ^•- generation and faster SMX degradation. Mechanistic studies revealed that o-SQ^•- attacks SMX through radical addition, forming low-toxicity products. Given that dihydroxyphenyl is a prevalent component of natural aquatic environments, this work advances the design of selective, eco-friendly AOPs with anti-interference capabilities.