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

  • Yufei Zhen
    Yufei Zhen
    State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China
    More by Yufei Zhen
  • Shishu Zhu
    Shishu Zhu
    School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China
    Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, P. R. China
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  • Zhiqiang Sun*
    Zhiqiang Sun
    State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China
    *Email: [email protected]. Tel/Fax: +86-451-86283010.
    More by Zhiqiang Sun
  • Yu Tian
    Yu Tian
    State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China
    More by Yu Tian
  • Zeng Li
    Zeng Li
    State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China
    More by Zeng Li
  • Chen Yang
    Chen Yang
    State Key Laboratory of Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China
    More by Chen Yang
  • , and 
  • Jun Ma*
    Jun Ma
    State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China
    *Email: [email protected]. Tel: +86-451-86283010. Fax: +86-451-82368074.
    More by Jun Ma
Cite this: Environ. Sci. Technol. 2021, 55, 13, 9293–9304
Publication Date (Web):June 17, 2021
https://doi.org/10.1021/acs.est.1c01974
Copyright © 2021 American Chemical Society
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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.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.1c01974.

  • Details on synthesis of in situ N-doped NHCS, analytical methods, electrochemical analysis, and discussion about the properties of prepared NHCSs (Text S1, S2, S3, S4); elemental contents of NHCS samples (Table S1); the decomposition of PMS using PBS as buffer in the presence or the absence of PN (Figure S1); characterization of the prepared NHCSs: (a) SEM and (b) TEM images of NHCS-8, (c) N2 adsorption–desorption isotherms, (d) pore distributions of NHCSs, and (e) XPS survey of NHCSs (Figure S2); XPS curves and deconvoluted peaks of N 1s for (a) NHCS-6, (b) NHCS-8, and (c) NHCS-10 (Figure S3); (a) adsorptive removal of PN on NHCS-6, NHCS-8, and NHCS-10 and PN degradation by PMS alone in the 60 min reaction, (b) PN degradation during PMS activation using different NHCSs, and (c) different organic degradation in the NHCS-8/PMS system (Figure S4); degradation of PN in the (a) NHCS-8/PMS system and (b) PMS alone, under different reaction temperatures from 5 to 45 °C and the correlation of 1/T with Lnk (Figure S5); fitting first-order kinetic model for the NHCS-8/PMS system under (a) different NHCS-8 concentrations and (b) different PMS concentrations (Figure S6); (a) PMS activation by NHCS-8 for PN degradation under four cycles and (b) TOC removal efficiencies of the NHCS-8/PMS system under four cycles (Figure S7); identifying the possible ROS: EPR signals representing (a) DMPO-OH and DMPO-SO4, (b) TEMP-1O2 in the NHCS-8/PMS/PN system, and (c) EPR signal in the presence of DMPO in the NHCS/PMS/MeOH system (Figure S8); the decomposition of PMS in the presence of different scavengers with different concentrations: (a) TBA, (b) FFA, (c) MeOH, and (d) DMSO (Figure S9); the impact of different quenching agents on the PMS decomposition on NHCS in the presence of PN (Figure S10); the quenching effects for PN degradation using different alcohols with different concentrations (Figure S11); electron transfer pathway: (a) linear-sweep voltammograms of NHCS-8 alone, NHCS-8/PMS, and NHCS-8/PMS/PN systems and (b) cyclic voltammetry curves of abovementioned different systems (Figure S12); EPR signal of N-doped CNT representing PFRs under 0.325 mM PMS (Figure S13); characteristic peaks for in situ Raman spectra of NHCS-8 under different reaction times (Figure S14); degradation products of PN in the PMS/NHCS system (Figure S15); and DFT calculations: the binding energies of molecular PMS, OH, and SO4•– with (a) pyridinic N, (b) graphitic N, and (c) pyrrolic N structures in NHCS, as well as their spin densities of different atoms (Figure S16) (PDF)

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