Sulfamethoxazole degradation by alpha-MnO2/periodate oxidative system: Role of MnO2 crystalline and reactive oxygen species

Pollutant degradation via periodate (IO4-) and transitional metal oxides provides an economical, energy-efficient way for chemical oxidation process in environmental remediation. However, catalytic activation of periodate by manganese dioxide and the associated mechanism were barely investigated. In this study, four MnO2 polymorphs (alpha-, beta-, gamma- and beta-MnO2) were synthesized and tested to activate IO4- for the degradation of sulfamethoxazole (SMX). The reactivity of different MnO2 structures followed the order of alpha-MnO2 > beta-MnO2 > gamma-MnO2 > delta-MnO2, suggesting that the particular crystalline structure in alpha-MnO2 would exhibit higher activities via IO4- activation. Herein, in alpha-MnO2/IO4- system, 91.1% of SMX was eliminated within 30 min with degradation rate constant of 0.0649 min(-1), and the neutral pH exhibited higher efficiency in SMX degradation compared with acidic and alkaline conditions. Singlet oxygen (O-1(2)) was unveiled to be the dominant ROS according to the results of electron paramagnetic resonance, chemical probes and radical quenching experiments, whereas O-2(center dot-) and center dot OH were mainly acted as a free-radical precursor. Six oxidation products were identified by LC-MS, and the elimination of sulfonamide bond, hydroxylation and direct oxidation were found to be the important oxidation pathways. The study dedicates to the mechanistic study into periodate activation over alpha-MnO2 and provides a novel catalytic activation for selective removal in aqueous contaminants.

Automated summary

This study investigated the activation of IO4- for the degradation of SMX using different MnO2 structures. Results showed that α-MnO2 exhibited higher activity, mainly through the generation of singlet oxygen for oxidation reactions. Under different pH conditions, neutral pH demonstrated higher efficiency in SMX degradation.