Mn₂O₃ as an Electron Shuttle between Peroxymonosulfate and Organic Pollutants: The Dominant Role of Surface Reactive Mn(IV) Species

Abstract

The environmentally benign Mn oxides play a crucial role in the transformation of organic contaminants, either through catalytically decomposing oxidants, e.g., peroxymonosulfate (PMS), or through directly oxidizing the target pollutants. Because of their dual roles and the complex surface chemical reactions, the mechanism involved in Mn oxide-catalyzed PMS activation processes remains obscure. Here, we clearly elucidate the mechanism involved in the Mn2O3 catalyzed PMS activation process by means of separating the PMS activation and the pollutant oxidation process. Mn2O3 acts as a shuttle that mediates the electron transfer from organic substrates to PMS, accompanied by the redox cycle of surface Mn­(IV)/Mn­(III). Multiple experimental results indicate that PMS is bound to the surface of Mn2O3 to form an inner-sphere complex, which then decomposes to form long-lived surface reactive Mn­(IV) species, without the generation of sulfate radicals (SO4 •–) and hydroxyl radicals (HO•). The surface reactive Mn­(IV) species are proposed to be responsible for the degradation of organic contaminants (e.g., phenol) and the formation of singlet oxygen (1O2), followed by the regeneration of the surface Mn­(III) sites on Mn2O3. This study advances the fundamental understanding of the underlying mechanism involved in transition metal oxide-catalyzed PMS activation processes.

Origin of the Excellent Activity and Selectivity of a Single-Atom Copper Catalyst with Unsaturated Cu-N2 Sites via Peroxydisulfate Activation: Cu(III) as a Dominant Oxidizing Species

“…Owing to the generation of powerful oxidizing species such as hydroxyl and sulfate radicals ( • OH E 0 = +1.9 to +2.7 V NHE and SO 4 •– E 0 = +2.5 to +3.1 V NHE ), peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation-based advanced oxidation processes (AOPs) are considered an efficient technology for the degradation of refractory organic pollutants in water/wastewater. , However, radicals with rather limited selectivity are inevitably consumed by coexisting organics (e.g., natural organic matter) and inorganic substrates (e.g., HCO 3 – and Cl – ), which hinders targeted pollutant degradation and increases the toxicity caused by halogenated byproducts. , Seeking for efficient approaches to selectively degrade refractory organic pollutants under a complex background matrix in water/wastewater is extremely urgent for water purification …”

CNT encapsulated MnOx for an enhanced flow-through electro-Fenton process: the involvement of Mn(iv)

“…S18,† the MnO x -in-CNT system still achieved high BPA removal efficiencies of 90.5% (tap water), 86.1% (lake water), 85.4% (secondary effluent), and 82.6% (pharmaceutical wastewater). The outstanding anti-interference capacity could benefit from the selective oxidation capability of Mn( iv ) and extended lifetime in water, 47 outperforming traditional radical-based oxidation systems. Remarkably, compared to the CNT-confined Fe 2 O 3 composite catalysts previously reported in our group, the active Mn( iv ) species generated in the MnO x -in-CNT system have a much longer lifetime than aqueous reactive species 1 O 2 (3.5 μs) generated in the Fe 2 O 3 -in-CNT system.…”

“…Such moderate inhibition (<7.4%) might be assigned to the change in the MnO x surface charge with solution pH. 47,48 The zeta-potential of MnO x is determined to be about 4.2 (Fig. S10 †), and thus, the MnO x surface is positively charged at pH < 4.2.…”

“…A recent study has also reported that reactive Mn(IV) species with an extended lifetime (on the order of hours) may play a dominant role in the oxidation of electron-rich organic compounds, e.g., BPA. 47…”

Self-Accelerating Interfacial Catalytic Elimination of Gaseous Sulfur-Containing Volatile Organic Compounds as Microbubbles in a Facet-Engineered Three-Dimensional BiOCl Sponge Fenton-Like Process

“…Moreover, the existence of 20 mM NO 3 − almost completely inhibited the catalytic oxidation of CH 3 SH (Figure 5b), thus revealing that electron transfer is the main pathway to generate active species in the 1BiOCl-001 sponge/H 2 O 2 system. 54 The process of self-accelerating interfacial electron-transfer in the 3D-BiOCl sponge/MiB/DO system was further revealed by analyzing the radicals produced by DMPO/TEMPcaptured. In Figure 5c − in electron-rich Vo centers.…”