Identifying the Persistent Free Radicals (PFRs) Formed as Crucial Metastable Intermediates during Peroxymonosulfate (PMS) Activation by N-Doped Carbonaceous Materials

Abstract

A nonradical mechanism involved in peroxymonosulfate (PMS) activation in carbonaceous materials (CMs) is still controversial. In this study, we prepared N-doped CMs, including hollow carbon spheres (NHCSs) and carbon nanotubes (N-CNTs), to probe the crucial intermediates during PMS activation. The results suggested that the higher efficiency and lower activation energy (13.72 kJ mol–1) toward phenol (PN) degradation in an NHCS/PMS system than PMS alone (∼24.07 kJ mol–1) depended on a typical nonradical reaction. Persistent free radicals (PFRs) with a g factor of 2.0033–2.0045, formed as crucial metastable intermediates on NHCS or N-CNT in the presence of PMS, contribute largely to the organic degradation (∼73.4%). Solid evidence suggested that the formation of PFRs relied on the attack of surface-bonded •OH and SO4 •– or peroxides in PMS, among which surface-bonded SO4 •– was most thermodynamically favorable based on theoretical calculations. Electron holes within PFRs on NHCSs shifted the Fermi level to the positive energy with the valance band increasing from 1.18 to 1.98 eV, promoting the reactivity toward nucleophilic substances. The degradation intermediates of aromatic compounds (e.g., PN) and electron rearrangement triggered the evolution of PFRs from oxygen-centered to carbon-centered radicals. Moreover, due to the specific electron configuration, graphitic N on NHCS was critical for stabilizing the PFRs. This study provides insightful understanding of the fate of organic contaminants and the structure–activity relationship of reactivity of CMs toward PMS activation.

Bibliometric analysis of biochar research in 2021: a critical review for development, hotspots and trend directions

“…The contents of VOCs in biochar decreased with increasing HTTs (Buss et al 2015). Numerous studies have reported the stable and large molecular PFRs in biochar during the biomass carbonization process (Liao et al 2014;Lieke et al 2018;Yang et al 2016;Zhen et al 2021). It was found that the electron paramagnetic resonance (EPR) signals in lignin-derived biochar were higher than those in cellulose-derived biochar.…”

Elucidating the Mechanistic Origin of a Spin State-Dependent FeNx–C Catalyst toward Organic Contaminant Oxidation via Peroxymonosulfate Activation

“…The extensive use of antibiotics, pharmaceuticals, organic dyes, and industrial chemicals has released increasing amounts of organic contaminants in the environment, posing a great threat to the ecological environment and human beings . Reactive oxygen species-based advanced oxidation processes (AOPs) have exhibited significant potential in the broadband abatement of refractory pollutants in aqueous systems. , Among these, persulfate-mediated Fenton-like processes including peroxymonosulfate (PMS) and peroxydisulfate are attracting enormous interest owing to the generated highly oxidative oxygen species such as sulfate radicals, hydroxyl radicals, and superoxide radicals . By cleaving O–O bonds in PMS molecules via electron and energy transfer processes, the most commonly reactive species of sulfate radicals [E 0 (SO 4 •–/SO 4 2– ) = 2.60 ∼ 3.10 V NHE ] and hydroxyl radicals [E 0 (•OH/OH – ) = 1.90 ∼ 2.70 V NHE ] can be formed and further attack organic compounds .…”

Hydrothermal converting quinonimine dye into pyrrolic-N for sustainable activated carbon regeneration

“…After reacting with Cr(VI) at pH 3.0-9.0, the binding energy peak of pyrrolic-N shifted from 399.8 to 400.0 eV (Fig. 5d-g), meaning only pyrrolic-N acted as electron donor for Cr(VI) reduction 42 . The proportion of pyrrolic-N after reaction at pH 3.0-9.0 decreased to 40.52-43.8% (Fig.…”