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Fe(III)-Doped g-C3N4 Mediated Activation of Peroxymonosulfate for Selective Degradation of Phenolic Compounds via High valent Iron-oxo Species Hongchao Li, Chao Shan, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05563 • Publication Date (Web): 26 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018
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Fe(III)-Doped g-C3N4 Mediated Peroxymonosulfate Activation for Selective
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Degradation of Phenolic Compounds via High-valent Iron-oxo Species
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Hongchao Li1, Chao Shan1,2 and Bingcai Pan1,2*
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1. State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China 2. Research Center for Environmental Nanotechnology (ReCENT), Nanjing University, Nanjing 210023, China
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*To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +86-25-8968-0390.
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ABSTRACT
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Herein, we proposed a new peroxymonosulfate (PMS) activation system employing
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the Fe(III) doped g-C3N4 (CNF) as catalyst. Quite different from traditional sulfate
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radical-based advanced oxidation processes (SR-AOPs), the PMS/CNF system was
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capable of selectively degrading phenolic compounds (e.g., p-chlorophenol, 4-CP) in
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a wide pH range (3-9) via nonradical pathway. The generated singlet oxygen (1O2) in
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the PMS/CNF3 (3.46 wt% Fe) system played negligible role in removing 4-CP, and
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high-valent iron-oxo species fixated in the nitrogen pots of g-C3N4 (≡FeV=O) was
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proposed as the dominant reactive species by using dimethyl sulfoxide as a probe
25
compound. The mechanism was hypothesized that PMS was firstly bound to the
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Fe(III)-N moieties to generate ≡FeV=O, which effectively reacted with 4-CP via
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electron
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1,4-benzoquinone were the major intermediates, which could be further degraded to
29
carboxylates. The kinetic results suggested that the formation of ≡FeV=O was
30
proportional to the dosages of PMS and CNF3 under the experimental conditions.
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Also, the PMS/CNF3 system exhibited satisfactory removal of 4-CP in the presence
32
of inorganic anions and natural organic matters. We believe that this study will
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provide a new routine for effective PMS activation by heterogeneous iron-complexed
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catalysts to efficiently degrade organic contaminants via nonradical pathway.
transfer.
GC-MS
analysis
indicated
that
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4-chlorocatechol
and
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Introduction
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Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) have
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recently received increasing interest for decontamination of organic pollutants from
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water. Various Fe-based catalysts (e.g. Fe2+,1 Fe2O3,2 and Fe3O43) have been widely
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exploited for PMS activation due to their environmental benignity and cost
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effectiveness. In such catalytic processes, the activation of PMS usually involves the
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redox cycle of Fe(III)/Fe(II), through which sulfate radicals (SO4•−) are continuously
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produced via homolytic cleavage of the peroxide bond (O-O) (eqs. 1 and 2).4
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→ Fe(III) + SO4•− + HO− Fe(II) + HSO5−
(1)
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→ Fe(II) + SO5•− + H+ Fe(III) + HSO5−
(2)
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However, the homogeneous PMS/Fe(II) process is limited to acidic pHs and results in
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undesirable accumulation of iron oxy-hydroxides sludge.5 Fe oxides such as Fe2O3
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and Fe3O4 are alternative catalysts of PMS activation, while the slow rate of Fe(III)
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transformation back to Fe(II) significantly reduces their catalytic activity for PMS
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decomposition.6 Though the assistance of reducing agents (e.g., hydroxylamine5,7 and
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epigallocatechin-3-gallate8), UV,9 and electrochemistry10 could accelerate such step
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during PMS activation, the external addition of chemical regents or energy input is
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inevitable for such methods. Moreover, the nonselective property and robust reactivity
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of SO4•− towards organic/inorganic coexisting substrates (e.g., NOM and Cl−) would
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result in a variety of competitive reactions that negatively affect the removal
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efficiency of target pollutants.6
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Recently, Fe coordination complexes with N-based ligands have been utilized to 3 ACS Paragon Plus Environment
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activate PMS via a nonradical pathway due to the unique redox properties of the
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centered Fe atom.11,12 For example, Wang et al.11 reported that light-stimulated iron(II)
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hexadecachlorophthalocyanine (FePcCl16) could activate PMS to generate the
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powerful FeIV=O intermediate for oxidation of carbamazepine. The high-valent
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iron-oxo species such as FeIV=O and FeV=O are noted for their high reactivity and
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selectivity for oxidation of refractory organic contaminants over a wide pH range.13-17
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However, complicated macrocyclic N-based ligands and the light stimulation were
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usually required in the current studies.11,12 Hence, it is still imperative to develop
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highly efficient and cost effective catalysts for PMS activation.
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Graphitic carbon nitride (g-C3N4) is a novel heterogeneous nitrogen-rich material of
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stable structure and cost effectiveness for water treatment processes.18-20 Moreover,
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g-C3N4 is capable of complex Fe(II) or Fe(III) with its six lone-pair electrons in the
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nitrogen pots.21-23 The Fe-N interaction in such complexes has been interpreted based
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on density functional theory (DFT)24 and experimentally evidenced by X-ray
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photoelectron spectroscopy (XPS).25 Considering the specific Fe-N interaction, we
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speculate that Fe-doped g-C3N4 could activate PMS via a nonradical approach to
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generate high-valent iron-oxo species. However, to the best of our knowledge, such
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nonradical PMS activation process based on Fe-doped g-C3N4 has never been
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investigated.
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The main objective of this study was to provide a novel nonradical approach to
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activate PMS with Fe(III) doped g-C3N4 (CNF) for highly efficient and selective
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degradation of organic pollutants. Several CNFs of different Fe(III) loadings were 4 ACS Paragon Plus Environment
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synthesized via one-pot polymerization and characterized by multiple techniques. The
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performance of PMS/CNF system towards several aromatic pollutants was examined
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in consideration of the widespread concern of their carcinogenicity, teratogenicity and
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bio-refractory nature.26 The underlying mechanism for the PMS/CNF-involved
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degradation of p-chlorophenol (4-CP) was particularly concerned. Also, the
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degradation kinetics, the effect of solution chemistry (pH, anions, and nature organic
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matters (NOM)) on the performance of the system, as well as the reusability of CNF
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were considered.
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MATERIALS AND METHODS
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Chemicals.
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Dicyandiamide (DCD) and ferric chloride hexahydrate (FeCl3·6H2O) were
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purchased from Sinopharm Chemical Reagent Co., Ltd., China. Potassium
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peroxymonosulfate
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p-hydroxylbenzoic acid (PHBA), bisphenol A (BPA), ethanol (EtOH), furfuryl alcohol
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(FFA), sodium azide (NaN3), sodium citrate, sodium oxalate, dimethyl sulfoxide
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(DMSO), nitro blue tetrazolium (NBT), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO),
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and 2,2,6,6-tetramethylpiperidinyloxyl (TEMP) were obtained from Sigma-Aldrich
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Chemical Co., Ltd. NOM from Suwannee River was purchased from International
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Humic Substances Society (IHSS). All the chemicals were of analytical grade or
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better and used without further purification. Deionized (DI) water with a resistivity of
(PMS),
4-CP,
benzoic
acid
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18.2 MΩ/cm was used throughout the experiments.
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Preparation of Hybrid CNF
(BA),
nitrobenzene
(NB),
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Fe(III) doped g-C3N4 (CNF) catalysts were synthesized according to a slightly
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modified method.25 DCD of 5 g was dissolved in 20 mL deionized water, where a
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certain amount of FeCl3·6H2O was added. The suspensions were continually stirred at
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80 ℃ until the water was completely evaporated. After being dried at 100 ℃
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overnight, the mixtures were ground and heated to 575℃ at 3 ℃/min and kept for 4 h
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in air. The resultant products were ground and washed with HCl (0.10 M) and DI 6 ACS Paragon Plus Environment
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water for several times to remove the impurities. Using different dosages of
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FeCl3·6H2O (0.25 mmol, 0.5 mmol, 0.75 mmol, and 1.0 mmol), we obtained four
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Fe(III) modified g-C3N4 denoted as CNF1, CNF2, CNF3, CNF4, respectively. The
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pristine g-C3N4, denoted as CN, was also prepared by the same procedure except in
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FeCl3·6H2O free solution.
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Experimental Procedures.
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All the experiments were carried out in a 100 mL triangular flasks at 20 °C under
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magnetic stirring. Predetermined volumes of target organic compounds (e.g., 4-CP)
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and PMS stock solutions were first added to the reactor with/without substrates (e.g.,
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Cl−, NOM). Afterwards, the experiments were triggered by adding the desired dosage
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of catalysts (e.g., CNF3). The pH values were adjusted by sodium hydroxide or
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perchloric acid using automatic acid-base titrating apparatus (T50, Mettler Toledo,
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Switzerland) equipped with a glass pH electrode (DGi115-SC, Mettler Toledo,
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Switzerland). During the experiments, pH was kept at the preset values within the
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deviation of ±0.2. Samples were withdrawn at different intervals from the reactor,
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filtered through a 0.22 μm PTFE filter (ANPEL, China), and quenched with excessive
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NaNO2 (0.5 M) before analysis. The batch experiments were conducted in duplicates
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at least, and the average values with standard deviations were presented.
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Analytical Methods.
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The specific surface area and pore size distribution of CN and CNF were
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determined on the basis of N2 adsorption at 77 K using surface and pore analyzer
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(Nova 3000, Quantachrome, Boynton Beach, FL). The abundances of C, N, and H
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were determined by an automatic elemental analyzer (CHN-O-Rapid, Heraeus,
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Germany). The contents of Fe loaded within CNF were determined by an atomic
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absorption spectrophotometer (AA-7000, Shimadzu, Japan) after acidic digestion
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using the binary HCl-HNO3 solution at the molar ratio of 1:1. The morphology was
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characterized with scanning electron microscope (SEM, S-3400 II, Hitachi, Japan),
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transmission electron microscopy (TEM, JEM-200CX, JEOL, Japan), and high
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resolution transmission electron microscope (HR-TEM, Tecnai G2 F20 S, FEI, USA).
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X-ray diffraction (XRD, X’TRA, ARL, Switzerland) was used to investigate the
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mineralogy of the catalysts with Cu Kα radiation. The functional groups were
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characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo
140
Scientific, United States) equipped with attenuated total reflectance (ATR, iD5,
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Thermo Scientific, United States) accessory in the spectral range of 4000-500 cm-1.
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X-ray photoelectron spectroscopy (XPS, ESCALAB-2, Great Britain) with an Al Kα
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anode radiation as the excitation source was employed to detect the chemical states of
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different elements of CN and CNF. All the binding energies were referenced to the
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C1s peak at 284.8 eV.
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Ultra-high performance liquid chromatography (UHPLC, UltiMate 3000, Thermo
147
Scientific, United States) equipped with a diode array detector was used to determine
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the concentration of substrates. C18 reverse phase column (50 mm×2.1 mm, 1.9 μm
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particle size, Thermo Scientific, United States) was employed to detect 4-CP, BPA, 8 ACS Paragon Plus Environment
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BA, NB, PHBA, and FFA, while H reverse phase column (300 mm×7.7 mm, 8 μm
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particle size, Agilent, United States) was used for determination of oxalate and citrate.
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The degradation products were analyzed by gas chromatography-mass spectrum
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(GC-MS, TRACE1310/ISQ LT, Thermo Scientific, United States) with a TG-5 MS
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column (30 m × 0.25 mm × 0.25 μm, Thermo Scientific, United States). Detailed
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conditions were listed in Table S1 and Text S1 (in Supporting Information). The PMS
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concentration was measured by the spectrophotometric method proposed elsewhere.27
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Radicals (SO4•− and HO•) and singlet oxygen (1O2) were detected by employing
158
electron paramagnetic resonance (EPR, EMX-10/12, Bruker, Germany) spectroscopy
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with 60.0 mM of DMPO and TEMP as the spin-trapping agent, respectively. The
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production of superoxide radicals (O2•−) was examined by NBT method.28
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RESULTS AND DISCUSSION
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Characterization of CN and CNF
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The basic properties of CN and CNF were summarized in Table 1 and Figure S1.
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As the Fe dosage increased, the Brunauer-Emmett-Teller (BET) surface area of CNF
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was gradually increased from 28.2 to 62.2 m2/g along with the average pore size
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diameter drop from 20.0 to 12.8 nm. This was possibly because the added Fe inhibited
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the polymerization of DCD.25 In addition, CN and CNF exhibited similar C/N ratio
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(0.6) lower than the theoretical value (0.75). SEM images showed that all the samples
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consisted of irregular particles with aggregated structures (Figure S2). TEM images in
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Figure S3 showed that a typical layered and platelet-like g-C3N4 structure was formed
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for all the catalysts, and the EDS element mapping results demonstrated that the
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elements of C, N, O and Fe were uniformly distributed in CNF3 (Figure S4). In
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addition, no visible particles or obvious crystallite structure in the HR-TEM images of
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CNF3 (Figure S5) implied that the elemental Fe was mostly bound by CNF3 instead
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of Fe oxides.29
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The XRD pattern in Figure 1(a) showed a classical g-C3N4 phase for the
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as-prepared catalysts. For the CN, the diffraction peak at 27.8° arose from the
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interlayer stacking of the conjugated aromatic rings, i.e., the (002) facet with a
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spacing of 0.336 nm. The weaker diffraction peak at 12.9° was indexed as the (100)
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facet with a distance of 0.672 nm, which corresponded to the in-plane reflection. Both
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peaks were in good agreement with g-C3N4 reported elsewhere.30 However, a slight 10 ACS Paragon Plus Environment
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shift to higher diffraction angle as well as a weak intensity of the main peak for all the
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CNF over the CN were observed, indicating that Fe was successfully invaded into the
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framework of g-C3N4.22,25 Interestingly, no XRD peak of Fe oxides, such as Fe2O3,
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Fe3O4 or FeOOH was observed, suggesting that Fe were chemically coordinated to
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the host of g-C3N4, probably in the form of Fe-N bonds.
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The ATR-FTIR spectra of all the obtained materials were illustrated in Figure 1(b).
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No obvious vibrational spectra change was observed between CN and CNF. The
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bands at 3247–3078 cm-1 could be assigned to the stretching modes of secondary and
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primary amines.30 The bands in the range of 1700-1000 cm-1 corresponded to the
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typical stretching modes of C-N heterocycles.31 Specially, the peaks at 1311 and 1226
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cm-1 occurred due to the stretching vibrations of the connected units of N-(C)3 and
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C-NH-C, respectively. In addition, the peak at 809 cm-1 resulted from the bending
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mode of C-N heterocycles.
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XPS studies were performed to give insights into the chemical states of the main
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element. As shown in Figure S6, the signals of C, N, and O were observed in the
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spectra survey of all the catalysts. The trace oxygen element (< 5%) was attributed to
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the absorbed O2/H2O.22 A weak Fe signal was observed in the catalysts of CNF3 and
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CNF4 due to the low Fe loadings. As shown in Figure 1(c), no chemical shift occurred
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for C 1s after Fe doping. The peak at 284.8 eV was attributed to either adventitious
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carbon adsorbed on the surface or sp3 graphitic carbon formed during the
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polymerization process.32 The binding energy at 288.3 eV belonged to the sp2
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hybridized carbon in N-C=N.31 In N 1s region of CN (Figure 1(d)), the peak at 398.7 11 ACS Paragon Plus Environment
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eV was assigned to the hybridized aromatic nitrogen atoms (C-N=C).22 The peak at
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400.1 eV represented the tertiary nitrogen (N-(C)3 or C-NH-C) groups.32 In addition, a
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broad peak at 404.7 eV resulted from π-π* excitations between the stacking
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interlayers.32 However, an obvious shift to higher binding energy was observed for
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CNF3, suggesting the chemical state of hybridized aromatic N changed after Fe
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modification. To be specific, the six lone-pair electrons in the nitrogen pots of g-C3N4
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occupied the free orbital of the central Fe to form Fe-N moieties, resulting in lower
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electron density as well as higher binding energy of N atom accordingly.22,25 The
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valent state of the central Fe was also probed, and the results were depicted in Figure
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S7. The peaks at 724.2 and 710.7 eV belonged to Fe 2p1/2 and Fe 2p3/2, respectively.
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The binding energy of Fe 2p3/2 higher than that of Fe(II) phthalocyanine (709.2 eV)
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fell in the range of Fe(III) valence state (710.3–711.8 eV).25 The above results
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demonstrated that Fe(III) was successfully coordinated with N atom to form Fe(III)-N
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moieties, which was in good agreement with the XRD analysis.
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Catalytic oxidation of the PMS/CNF3 system
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Several aromatic pollutants, including 4-CP, BA, NB, PHBA, BPA were selected as
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the target pollutants to preliminarily probe the catalytic oxidation properties of the
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PMS/CNF3 system. As shown in Figure 2, no noticeable removal of BA was observed,
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and