Fe(III)-Doped g-C3N4 Mediated Activation of Peroxymonosulfate for

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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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Environmental Science & Technology

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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

19

radical-based advanced oxidation processes (SR-AOPs), the PMS/CNF system was

20

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

26

Fe(III)-N moieties to generate ≡FeV=O, which effectively reacted with 4-CP via

27

electron

28

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.

31

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

33

provide a new routine for effective PMS activation by heterogeneous iron-complexed

34

catalysts to efficiently degrade organic contaminants via nonradical pathway.

transfer.

GC-MS

analysis

indicated

that


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

39

exploited for PMS activation due to their environmental benignity and cost

40

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

115

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

129

(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

132

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

136

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

160

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

192

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

Fe(III)-Doped g-C3N4 Mediated Activation of Peroxymonosulfate for

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