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Article Cite This: Environ. Sci. Technol. 2018, 52, 14371−14380

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Electronic Structure Modulation of Graphitic Carbon Nitride by Oxygen Doping for Enhanced Catalytic Degradation of Organic Pollutants through Peroxymonosulfate Activation Yaowen Gao, Yue Zhu, Lai Lyu, Qingyi Zeng, Xueci Xing, and Chun Hu* Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou 510006, China Environ. Sci. Technol. 2018.52:14371-14380. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/13/19. For personal use only.

S Supporting Information *

ABSTRACT: Oxygen-doped graphitic carbon nitride (O−CN) was fabricated via a facile thermal polymerization method using urea and oxalic acid dihydrate as the graphitic carbon nitride precursor and oxygen source, respectively. Experimental and theoretical results revealed that oxygen doping preferentially occurred on the two-coordinated nitrogen positions, which create the formation of low and high electron density areas resulting in the electronic structure modulation of O−CN. As a result, the resultant O−CN exhibits enhanced catalytic activity and excellent long-term stability for peroxymonosulfate (PMS) activation toward the degradation of organic pollutants. The O−CN with modulated electronic structure enables PMS oxidation over the electron-deficient C atoms for the generation of singlet oxygen (1O2) and PMS reduction around the electron-rich O dopants for the formation of hydroxyl radical (•OH) and sulfate radical (SO4•−), in which 1O2 is the major reactive oxygen species, contributing to the selective reactivity of the O−CN/PMS system. Our findings not only propose a novel PMS activation mechanism in terms of simultaneous PMS oxidation and reduction for the production of nonradical and radical species but also provide a valuable insight for the development of efficient metal-free catalysts through nonmetal doping toward the persulfatebased environmental cleanup.

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INTRODUCTION In recent decades, environmental pollution by refractory organics from aqueous medium has become a serious problem facing humanity.1 Many recalcitrant organics with toxicity and nonbiodegradability are difficult to be remove by conventional treatment methods. In this regard, the development of advanced treatment techniques offering prominent treatment efficiency is of paramount significance and highly desired. Among those techniques, persulfate-based advanced oxidation processes have raised tremendous interest owing to their capability and adaptability in the oxidation of recalcitrant organic contaminants.2−5 Most commonly, the activation of persulfate (peroxymonosulfate (PMS, HSO5−) or peroxydisulfate (PDS, S2O82−)) involves the generation of radical species such as hydroxyl radical (•OH) and sulfate radical (SO4•−). Transition-metal-based catalysts have been demonstrated to possess high catalytic activity toward persulfate activation for the production of such reactive radicals, but the relatively poor stability or the requirement of reactivation after catalytic reactions impedes their practical application in water decontamination.6−9 Therefore, considerable efforts have been devoted to developing efficient and durable catalysts for persulfate activation. In the search for high quality and green persulfate activators, carbon-based materials have elicited ripples of © 2018 American Chemical Society

excitement in the research communities as alternative and sustainable catalysts due to their metal-free and abundant nature, great tolerance to acid and base and tunable electronic structure. 10 To date, carbonaceous materials such as graphene,11 carbon nanotubes,12 and nanodiamonds13 have been extensively investigated for the activation of persulfate toward oxidative degradation of organic pollutants. Moreover, the catalytic performance of such carbonaceous materials can be enhanced by virtue of doping with heteroatoms, such as nitrogen (N).14,15 The enhancement of catalytic activity of these N-doped carbon materials is attributed to the higher electronegativity of nitrogen (3.04) relative to that of carbon (2.55), which can tailor the charge distribution of carbon atoms adjacent to the substitutional N dopants, expediting the electron transport between the catalyst and persulfate.10 Although the impressive catalytic performance of N-doped carbon-based catalysts in the activation of persulfate is known, these catalysts still suffer from the problem of unsatisfactory long-term durability, hindering their large-scale application. Additionally, the reaction mechanism in terms of radicalReceived: Revised: Accepted: Published: 14371

September 17, 2018 November 6, 2018 November 14, 2018 November 14, 2018 DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

Article

Environmental Science & Technology

Adamas Reagent Co., Ltd. Ciprofloxacin (≥98%, CIP) and tert-butyl alcohol (≥99.5%, TBA) were purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd. and Aladdin Industrial Corporation, respectively. Other chemicals such as urea, oxalic acid dihydrate, sodium persulfate (PDS), ethanol, and so forth were of analytical grade and used as received without further purification. The molecular structures of various organic pollutants are shown in Figure S1 of the Supporting Information (SI). Ultrapure water was used through the experiments. Preparation of Catalysts. Oxygen-doped g-C3N4 was synthesized according to a method described in the literature,24 but with some modification. Urea (10 g) and oxalic acid dihydrate (OA, 4 g) were mixed and grinded in a mortar for 30 min. Then the mixture was calcined at 550 °C for 4 h with a ramping rate of 2 °C min−1 in air atmosphere. The obtained sample was milled into powders and denoted as O−CN. As a reference sample, pristine g-C3N4 was prepared similarly without adding oxalic acid dihydrate. Characterizations. The morphology and microstructure of the obtained samples were observed on a FEI Quanta FEG 250 scanning electron microscopy (SEM) and a FEI Tecnai G2 F20 transmission electron microscopy (TEM). The distribution of C, N and O elements in samples was examined using energy dispersive spectroscopy (EDS) attached to the SEM (FEI Quanta FEG 250). The contents of C, N, and O in samples were analyzed via an elemental analyzer (Vario EL III, German). The specific surface area was measured via N2 adsorption−desorption measurements at 77 K on a Micrometrics ASAP 2460 apparatus. The crystal structure of samples was characterized by an X-ray diffractometer (XRD, Bruker D8 Advance, German) with Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared (FTIR) spectra were performed using a Nicolet iS10 spectrometer with samples dispersed in KBr at a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was conducted by a Thermo ESCALAB 250 Xi instrument using monochromatic Al Kα radiation. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker A300 spectrometer at room temperature. Electrochemical measurements were carried out in a standard three-electrode cell system (the as-synthesized samples, Pt foil and Ag/AgCl electrode as working electrode, counter electrode and reference electrode, respectively and 0.5 M sodium sulfate as an electrolyte) on a CHI 700E electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was measured under the frequency range of 1 Hz to 100 kHz with 5 mV amplitude. Linear sweep voltammetry (LSV) was performed in the potential region from −1.0 to 0.2 V (vs Ag/AgCl) at a scan rate of 10 mV s−1. Experimental Procedure and Analyses. Bisphenol A (BPA), a representative refractory organic pollutant, was selected to assess the catalytic activity of catalysts. In a typical experiment, an aqueous solution of BPA (30 mL, 0.05 mM) with an initial pH of 6.4 was poured into a 50 mL beaker, followed by the addition of a certain amount of PMS (10 mM) and catalyst (0.03 g). The temperature of reaction solution was maintained at 30 °C using a water bath. During the reaction, samples were withdrawn using a 1 mL syringe at predetermined time intervals and were filtered immediately through a Millipore filter (0.45 μm) for analysis. For the cycling test, the catalyst was recycled after each run via filtration and rinsed thoroughly with adequate ultrapure water. The concentration of BPA was analyzed by a 1260 Infinity

mediated oxidative pathway or nonradical oxidation process behind persulfate activation over the metal-free catalysts is not fully understood, which needs to be further elucidated. Graphitic carbon nitride (g-C3N4), a metal-free conjugated polymer, has attracted intensive attention as a very promising material that finds widespread application in the fields of hydrogenation, photocatalysis, sensing, and separation owing to its facile synthesis, low cost, and good stability.16−18 Nonetheless, the catalytic activity of pristine g-C3N4 in activating persulfate is greatly restricted by its poor electron transport ability. To realize g-C3N4 being active for persulfate activation, visible light irradiation19,20 or support (e.g., activated carbon)21 was commonly introduced. Unfortunately, energy input or preparation of g-C3N4-supported materials was not cost-effective, which was disadvantageous to practical application. At this point, it is essential to design low-cost gC3N4-based catalysts with appealing electron mobility for efficient persulfate activation. There are multifarious modification strategies concerning the optimization of electronic structure of g-C3N4. As a matter of fact, nonmetal doing is a feasible approach to introduce alien atoms into the g-C3N4 matrix to modulate its electronic structure, meanwhile preserve the metal-free characteristic. Recent studies manifested that doping of g-C3N4 with the oxygen (O) atom can regulate the electronic structure of g-C3N4 since an O atom contains one more valence electron than a N atom, leading to the delocalization of π-electrons in the polymeric framework of oxygen-doped g-C3N4.22−24 On account of electronic structure modification of g-C3N4 upon O doping, it is acceptable to assume that the oxygen-doped g-C3N4 can stand for a potential arternate for the activation of persulfate, which has not yet been reported so far. In this work, we demonstrate a facile one-step synthesis of oxygen-doped g-C3N4 (O−CN) via thermal polymerization of urea and oxalic acid dihydrate, in which urea performs as the gC3N4 precursor and oxalic acid dihydrate serves as the oxygen source, respectively. As compared with pristine g-C3N4, O doping enables electronic structure modulation of O−CN, endowing it with enhanced catalytic activity in PMS activation for the degradation of organic pollutants in a broad pH range. The morphology, crystal structure, chemical composition, and electronic structure of O−CN were characterized in detail to clarify the enhancement of catalytic activity. The activation of PMS by O−CN involved the generation of •OH, SO4•−, and singlet oxygen (1O2), in which 1O2 was the major reactive oxygen species (ROS), affording the selectivity of the O−CN/ PMS system in the oxidation of organic contaminants. More importantly, different from the previously reported production of ROS in the persulfate-based catalytic systems, a novel PMS activation mechanism in terms of PMS oxidation over the electron-deficient C atoms for the generation of 1O2 and PMS reduction around the electron-rich O dopants for the formation of •OH and SO4•− is proposed.

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EXPERIMENTAL SECTION Chemicals. Potassium monopersulfate triple salt (KHSO5·1/2KHSO4·1/2K2SO4, 42.8−46%, PMS), methanol (high performance liquid chromatography (HPLC) ultra), bisphenol A (≥98%, BPA), 2-chlorophenol (99%, 2-CP), 2,4dichlorophenoxyacetic acid (≥99%, 2,4-D), diphenhydramine (99%, DP), benzoquinone (99%, BQ), L-histidine (98%), 5,5dimethyl-1-pyrroline N-oxide (≥98%, DMPO), 2,2,6,6-tetramethyl-4-piperidinol (≥98%, TMP) were obtained from 14372

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

Article

Environmental Science & Technology

Figure 1. SEM images of (a) g-C3N4 and (b) O−CN. TEM images of (c) g-C3N4 and (d) O−CN. EDS mapping images of (e) g-C3N4 and (f) O− CN.

DMPO as the spin-trapping agent in the aqueous and methanol media, respectively. Singlet oxygen was detected by EPR spectrometry with the spin-trapping agent of TMP in aqueous solution. Theoretical Computation. The first-principles calculations in the framework of density functional theory, including structural and electronic performances, were carried out based on the Cambridge Sequential Total Energy Package known as CASTEP.25 The Perdew−Burke−Ernzerhof generalized gradient approximation (GGA)26 was adopted for exchangecorrelation potential with a cutoff energy of 750 eV and a kpoint sampling set of 5 × 5 × 1. A force tolerance of 0.01 eV Å−1, energy tolerance of 5.0 × 10−7 eV per atom and maximum

HPLC (Agilent, U.S.A.) equipped with a UV detector and a Poroshell 120 EC-C18 column (4.6 × 100 mm2, 2.7 μm). The mobile phases and detection wavelengths were set as follows: methanol/water (70:30, v/v) with λ = 225 nm for BPA; methanol/water (60:40, v/v) with λ = 275 nm for 2-CP; acetonitrile/water (0.08% phosphoric acid) (20:80, v/v) with λ = 278 nm for CIP; acetonitrile/water (0.08% phosphoric acid) (60:40, v/v) with λ = 220 nm for 2,4-D; acetonitrile/water (0.08% phosphoric acid) (50:50, v/v) with λ = 271 nm for DP. The flowing rate of mobile phase was 1 mL min−1. Total organic carbon (TOC) in the solution was measured by a TOC-L analyzer (Shimadzu, Japan). EPR measurements for in situ detection of •OH/SO4•− and O2•− were conducted using 14373

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Environmental Science & Technology

Figure 2. (a) N2 adsorption−desorption isotherms, (b) XRD patterns, and (c) FTIR spectra of g-C3N4 and O−CN. High resolution XPS spectra in (d) C 1s, (e) N 1s, and (f) O 1s of g-C3N4 and O−CN.

displacement of 5.0 × 10−4 Å were considered. Each atom in the storage models was allowed to relax to the minimum in the enthalpy without any constraints. The formation energy (E) was defined as follows: E = EO−CN − Eg − C3N4 + μ N − μO

CN sample presented an apparent enhancement in the O content compared with pristine g-C3N4. Table S1 shows the alteration in the elemental composition of g-C3N4 before and after O doping as determined by elemental analysis (EA). Upon O doping, the O content in O− CN was significantly increased, while the content of N in O− CN underwent an evident decrease. However, the C content in O−CN was almost unchanged compared to that in pristine gC3N4. This result was in line with the observation of EDS mapping and implied the substitution of N atoms with O atoms during the synthesis of O−CN. The N2 adsorption− desorption isotherms for O−CN displayed a typical type IV isotherm because of the increased N2 adsorption at high relative pressure (Figure 2a), indicative of mesoporous structure of O−CN. The pore size distribution curve (inset of Figure 2a) verified the existence of mesopores in O−CN, which was in agreement with SEM and TEM results. As expected, the BET surface areas and pore volumes of O−CN were markedly higher than those of pristine g-C3N4 (Table S1). Figure 2b illustrates X-ray diffraction (XRD) patterns of gC3N4 and O−CN. Obviously, pristine g-C3N4 showed two diffraction peaks at 13.0° and 27.4°, which were indexed as (100) and (002) planes of g-C3N4, referring the in-plane

(1)

where Eg‑C3N4 and EO−CN denoted the total energy of g-C3N4 before and after O doping, and μN and μO were the chemical potentials of N and O atoms.

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RESULTS AND DISCUSSION Characterization of Catalysts. Figure 1a depicts the scanning electron microscopy (SEM) image of pristine g-C3N4, which exhibited an evident sheet-like structure with smooth surface. After O doping, a distinct change in morphology of O−CN was observed, as manifested by the appearance of flourishing porous structure in O−CN (Figure 1b). The transmission electron microscopy (TEM) images in Figure 1c, d confirms the transformation from thick g-C3N4 into thin and porous O−CN. The energy-dispersive X-ray spectroscopy (EDS) mapping images shown in Figure 1e, f reveal the main components of C, N, and O and the uniform distributions of these three elements in g-C3N4 and O−CN. Notably, the O− 14374

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Environmental Science & Technology

Figure 3. (a) Calculated formation energy (E) of O−CN by substituting the N atom with the O atom. Two-dimensional valence-electron density color-filled maps of (b) g-C3N4 and (c) O−CN. (C, N, O, and H are presented by gray, blue, red, and white spheres, respectively.) (d) Roomtemperature EPR spectra and (e) EIS spectra of g-C3N4 and O−CN.

peak at 807 cm−1 from the out-of-plane bending vibration of tri-s-triazine units, which originated from the weakened interaction between the “nitrogen pots” in O−CN owing to the introduction of O atoms.28 Further, the peak at 1235 cm−1 in the spectrum of g-C3N4, which referred to the stretching vibration of C−N bond, exhibited a small shift to higher frequency (1255 cm−1) in the O−CN spectrum. This change indicated the partial transformation of C−N into C−O considering the appearance of stretching vibration of C−O bond at higher frequency because of the strong electronattracting effect of O atoms. The XPS survey spectra (Figure S2) testified g-C3N4 and O−CN were mainly composed of C, N, and O elements. The

repeating units of continuous heptazine framework and the stacking of conjugated aromatic structure, respectively.27 In regard to O−CN, both the diffraction peaks were broadened with attenuated intensities, reflecting disturbance of the packing of the singlet layers caused by the incorporation of O dopant.24 Moreover, the (002) reflection peak shifted toward lower angle indicating larger interlayer distance of O− CN. Fourier transform infrared (FTIR) spectra of g-C3N4 and O−CN in Figure 2c present similar characteristic vibration modes, meaning the maintenance of the tri-s-triazine-based structure of g-C3N4 in O−CN. Compared to pristine g-C3N4, two alterations in the FTIR spectrum of O−CN could be detectable. The first was the attenuation in the intensity of 14375

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Figure 4. (a) Degradation of BPA in various systems. (b) Effect of initial pH on the removal of BPA in the O−CN/PMS system. (c) Stability test of O−CN. (d) Degradation of various organic contaminants in the O−CN/PMS system. Reaction conditions: (a), (c) [BPA] = 0.05 mM; [O− CN] = 1.0 g L−1; [PMS] = 10 mM; [Temp] = 30 °C; initial pH = 6.4. (b) [BPA] = 0.05 mM; [O−CN] = 1.0 g L−1; [PMS] = 10 mM; [Temp] = 30 °C; and (d) [Contaminant] = 0.05 mM; [O−CN] = 1.0 g L−1; [PMS] = 10 mM; [Temp] = 30 °C.

lowest formation energy, the substitution of N atoms in twocoordinated positions (N2 site) was much more feasible than other sites, demonstrating that O atoms preferred to substitute the two-coordinated N atoms. Taking into account that the electronegativity of O atoms is higher than that of N atoms, the substitution of N atoms by O atoms shall alter the electronic structure of O−CN, which could be evidenced by the change in two-dimensional valence-electron density color-filled maps of g-C3N4 and O−CN (Figure 3b, c). Clearly, the electrons of C atoms preferentially flowed toward the O dopant, resulting in the O atom with more negative charge and the nearby C atom with lower electron density (the white dashed circle in Figure 3c). Consequently, the electronic structure of O−CN was modulated, contributing to the formation of unpaired electrons and the promotion of electron transfer. The electron paramagnetic resonance (EPR) spectra (Figure 3d) showed that both g-C3N4 and O−CN exhibited the paramagnetic signal with a g value of 2.003, which was attributable to unpaired electrons within the π-conjugated aromatic rings.29 Compared with pristine g-C3N4, this EPR signal in O−CN was significantly strengthened, suggesting higher concentration of unpaired electrons. In addition, the electrochemical impedance spectroscopy (EIS) spectra of g-C3N4 and O−CN (Figure 3e) disclosed more efficient electron transfer in O−CN due to the much smaller arc radius of O−CN relative to that of parent gC3N4. Apparently, the O doping enables electronic structure modulation of O−CN, evidently improving its electron mobility, and thus an enhanced catalytic performance can be highly anticipated. Enhanced Degradation of Organic Pollutants. The catalytic activity of O−CN was assessed by the activation of PMS toward degradation of bisphenol A (BPA) from aqueous solution. As illustrated in Figure 4a, PMS alone was unable to oxidize BPA directly. In the absence of PMS, the removal of BPA onto g-C3N4 or O−CN was inappreciable, signifying the

change trend in the elemental contents before and after O doping was in accordance with results presented by EDS mapping and EA. High-resolution C 1s spectrum for pristine gC3N4 (Figure 2d) displayed three peaks centered at the binding energies of 284.8, 288.2, and 293.4 eV, representing C−C/CC, sp2-hybridized carbon (NC−N) and π excitation, respectively. For O−CN, the intensity and concentration of dominant peak corresponding to sp2hybridized carbon observably decreased (Figure 2d, Table S2), and a new peak at 286.7 eV associated with C−O bond appeared.23 The N 1s spectra for g-C3N4 and O−CN (Figure 2e) both contained four components located at 398.7, 400.0, 401.1, and 403.9 eV, which were ascribed to two-coordinated nitrogen (C−NC), three-coordinated nitrogen (N−(C)3), terminal amino groups and charging effects in triazine-based rings, respectively. Interestingly, as shown in Table S3, the value of area ratio of two-coordinated nitrogen to threecoordinated nitrogen and terminal amino groups for O−CN (1.36) was found to be lower than that of pristine g-C3N4 (1.61), suggesting the reduced content of C−NC groups in O−CN. In the O 1s spectra (Figure 2f), besides the prime peak corresponding to surface adsorbed water (532.2 eV), a novel peak at 531.4 eV attributing to C−O species was noticeable for O−CN.23 On the basis of the results of above characterizations, it can be deduced that the substitution of N atoms with O atoms indeed took place in O−CN during the thermal polymerization. Moreover, the XPS results suggested that O atoms were prone to be bonded to the sp2-hybridized carbon by replacing the two-coordinated N atoms. Density functional theory (DFT) computation further confirmed the specific substitution position of O atoms. Figure 3a shows three possible doping positions (numbered as N1, N2, and N3) in a unit of g-C3N4. The formation energy (E) of a substitutional O atom in N1, N2, and N3 sites was calculated to be 1.89, −0.71, and 0.15 eV, respectively. In view of the 14376

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Environmental Science & Technology

Figure 5. Spin-trapping EPR spectra for (a) •OH/SO4•−, (b) O2•−, and (c) 1O2 in various systems. (d) Spin-trapping EPR spectra for ROS with and without BPA or DP. (e) LSV curves of O−CN with and without PMS and BPA or DP. (f) Room-temperature EPR spectra of O−CN after various reactions. Reaction conditions: (a)-(d) [O−CN] = 1.0 g L−1; [PMS] = 10 mM; [DMPO] = 10 mM; [TMP] = 10 mM; [BPA] = [DP] = 0.05 mM; [Temp] = 30 °C.

still achieved after five successive cycles, suggesting the robust stability of O−CN, which was superior to that of N-doped carbon catalysts. The unobvious change in the XRD pattern of the used O−CN after catalytic reactions (Figure S3c) was also highly supportive of the excellent durability of O−CN. Besides being active for PMS activation, O−CN was found to be capable of activating PDS, though only 40% BPA removal in the O−CN/PDS system was fulfilled under identical conditions (Figure S 3d). The inferior performance of PDS might be ascribed to its symmetric molecular structure, making it more difficult to be activated.30 To further evaluate the catalytic reactivity of the O−CN/PMS system, the degradation of four other organic pollutants such as ciprofloxacin (CIP), 2chlorophenol (2-CP), 2,4-dichlorophenoxyacetic acid (2,4-D) and diphenhydramine (DP) was investigated. The diversity in the oxidation of various organic pollutants (Figure 4d) demonstrated the selective reactivity of the O−CN/PMS system. Specifically, BPA, CIP, and 2-CP were effectively eliminated, while the oxidative degradation of 2,4-D and DP was less effective. The above merits in terms of resistance of pH variation, tolerance to background inorganic anions and admirable stability as well as selectivity in the degradation of organic pollutants rendered O−CN a potential metal-free

negligible contribution of adsorption. In the g-C3N4/PMS system, a removal efficiency of 16% was achieved in 60 min, indicating modest catalytic performance of pristine g-C3N4 in activating PMS. However, in the simultaneous presence of O− CN and PMS, a significantly enhanced BPA removal was observed in the O−CN/PMS system, as demonstrated by complete degradation of BPA within 45 min. Moreover, 53% of total organic carbon (TOC) was removed after 60 min reaction (Figure S3a), again highlighting the excellent catalytic activity of O−CN toward PMS activation. Figure 4b illustrates that the initial pH exerted no influence on BPA degradation, suggesting that O−CN was effective for PMS activation in a wide pH range. Additionally, the exposure to the common inorganic anions such as Cl−, HCO3−, and CO32− showed insignificant impact on the degradation of BPA (Figure S3b). In addition to the high catalytic activity, the long-lasting stability is another crucial parameter for an advanced catalyst. However, the stability of the aforementioned N-doped carbon materials was unsatisfactory, of which the catalytic activity decreased dramatically after several consecutive cycles.14,15 As such, the cycling experiments were carried out to test the stability of O−CN. From Figure 4c, as high as 94% BPA removal over O−CN via PMS activation was 14377

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Environmental Science & Technology

O−CN/PMS system, the formation of ROS could be illustrated as follows:

catalyst for the persulfate-mediated environmental remediation. Mechanistic Insights into PMS Activation. The FTIR spectra of O−CN after different reactions in Figure S4a indicated the close interactions of O−CN, PMS, and BPA. In particular, the presence of a new absorption peak at 617 cm−1 (asymmetric angular vibration of SO42− in PMS molecular structure) in the FTIR spectrum of used O−CN after reaction with PMS suggested the intimate interaction between O−CN and PMS. Upon the addition of BPA, the intensity of this peak slightly increased in the spectrum of used O−CN after reaction with PMS and BPA, which was indicative of PMS activation by O−CN resulting in the formation of sulfate ion. The active radicals generated in the O−CN/PMS system was detected by electron paramagnetic resonance (EPR) technique using 5,5-dimethyl-pyrroline N-oxide (DMPO) as the spin-trapping agent. In the presence of DMPO, the specific EPR signals of DMPO−•OH (aN = aH = 14.9 G) and DMPO− SO4•− (aN = 13.8 G, aH = 10.1 G, aH = 1.42 G, aH = 0.83 G) adducts31 appeared in the O−CN/PMS system (Figure 5a), suggesting the generation of •OH and SO4•−. Figure 5b shows the DMPO-trapped EPR spectra for O2•− in various systems. The intensity of DMPO−O2•− signal was insignificantly changed in the systems of PMS alone and O−CN/PMS, which ruled out the production of O2•− during PMS activation by O−CN. Besides •OH and SO4•−, 1O2 was found to be formed in the O−CN/PMS system, as evident by the observation of triplet EPR spectrum with equal intensities32 when 2,2,6,6-tetramethyl-4-piperidinol (TMP) was present (Figure 5c). Seeing that the signal intensity of 1O2 was much stronger than that of •OH and SO4•−, it can be inferred that 1 O2 was the major ROS in the O−CN/PMS system. This can be confirmed by the results of quenching experiments in Figure S4b. In particular, the addition of L-histidine significantly inhibited the degradation of BPA but the presence of TBA or ethanol showed slight inhibition on BPA oxidation. When BQ was added, nearly no suppression of BPA degradation was noted. Moreover, the unsuppressed oxidation of BPA in the N2-saturated O−CN/PMS system (Figure S4c) verified (i) no formation of O2•− and (ii) the origin of 1O2 generation from PMS activation rather than from the conversion of dissolved oxygen in the reaction solution. Considering the modulation of electronic structure respecting the formation of electron-rich O atoms and nearby electron-poor C atoms in O−CN due to O doping, the generation pathway of •OH, SO4•− and 1O2 over O−CN via PMS activation was likely to be disparate. In the course of PMS activation by O−CN, the electron-rich O atoms with high electron density could act as electron donor delivering electrons to reduce PMS into •OH and SO4•−. However, the electron-poor C atoms adjacent to O dopants served as electron acceptor and PMS engaged as electron donor because of the presence of peroxy bond in its molecule,33 thus permitting the electron transfer from PMS to the electrondeficient C atoms (PMS oxidation) for the generation of PMS anion radical (SO5•−). Subsequently, the SO5•− radical reacted with water molecule to produce 1O2. The generation mechanism of 1O2 in terms of PMS oxidation over the electron-poor C atoms was fairly distinct from the previous reports claiming that the carbonyl groups of carbonaceous materials were active functional groups responsible for persulfate activation toward 1O2 production.1,12 In the present

The evolution of ROS in the presence of organic pollutants was further explored and the corresponding results were displayed in Figure 5d. After the injection of BPA in the O− CN/PMS system, the intensities of •OH and SO4•− signals were attenuated to some extent, suggesting the degradation of BPA by •OH and SO4•−. On the contrary, the 1O2 signal intensity for the catalytic system increased slightly upon adding BPA. This was because the degradation of organic pollutant consumed •OH and SO4•− decreasing the electron density of the O dopants, which resulted in more electrons being transferred from PMS to O−CN due to the strong electronwithdrawing effect of O atoms, thus facilitating the formation of 1O2. The similar alteration trend for the ROS evolution was noticed when DP was added into the O−CN/PMS system, however, the decrease in the signal intensities of •OH and SO4•− and the increase in the intensity of 1O2 signal were much more significant. This result together with the difference in the degradation efficiency of BPA and DP in the O−CN/ PMS system (Figure 4d) indicated that a relatively small amount of •OH and SO4•− were held responsible for the partial degradation of DP. Nonetheless, besides •OH and SO4•−, 1O2 also contributed to BPA oxidation via electrophilic attack due to the existence of electron-donating group (i.e., phenolic hydroxyl group) in the BPA molecule, which was absent in the molecule of DP (Figure S1). Similarly, CIP and 2-CP, which contained fluoric group and phenolic hydroxyl group with high electron densities, were effectively degraded. In contrast, the degradation of 2,4-D was less effective owing to the absence of such groups in its molecule. The selective degradation of specific organic compounds in the O−CN/ PMS system strongly demonstrated the primary involvement of 1O2. The electron transfer from PMS to O−CN was evidenced by the distinctly increased current for the O−CN electrode in the presence of PMS in the linear sweep voltammetry (LSV) analysis (Figure 5e). Subsequently, upon the addition of BPA or DP, the current of the O−CN electrode was further enhanced, which corroborated that more electrons were transported from PMS to O−CN, in accordance with the literature.34 This promoted PMS oxidation to boost the generation of 1O2. Moreover, the significantly strengthened EPR signal corresponding to the unpaired electrons of used O−CN after 60 min reaction with PMS relative to pristine O− CN in Figure 5f provided additional evidence for electron transfer from PMS to O−CN. After complete degradation of BPA in the O−CN/PMS system (reaction for 60 min), a further reinforced EPR signal of the used O−CN was observed, suggesting that the generated organic radical intermediates (R•) from BPA degradation donated extra electrons to O−CN, which was consistent with the previous report on the degradation of BPA by the heterogeneous 14378

DOI: 10.1021/acs.est.8b05246 Environ. Sci. Technol. 2018, 52, 14371−14380

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Environmental Science & Technology Fenton reaction.35 The electron transfer from PMS to O−CN suggested that PMS oxidation over the electron-poor C atoms was the main reaction in the course of PMS activation by O− CN, which rendered 1O2 the major ROS in the O−CN/PMS system, endowing such catalytic system with selectivity in the degradation of specific organic pollutants (such as BPA, CIP, and 2-CP) that contained electron-donating groups. Environmental Implications. This study demonstrates the electronic structure modulation of g-C3N4 via O doping, as confirmed by DFT calculations and electron paramagnetic resonance measurements. The resultant O−CN exhibited impressively enhanced catalytic activity and excellent stability for PMS activation toward the degradation of organic pollutants because of creation of the electron-poor and electron-rich areas. Unlike the conventional radical-generating persulfate-based catalytic systems, the PMS activation by O− CN involved the formation of 1O2 resulted from PMS oxidation over the electron-poor C atoms and the production of •OH and SO4•− due to PMS reduction around the electronrich O atoms, in which 1O2 was the major ROS enabling the O−CN/PMS system selective reactivity toward the particular organic contaminants with electron-donating groups. The findings in this work have significant implications for the persulfate-mediated environmental processes in the aspect of organic pollutant elimination. This work provided a facile, lowcost, and green approach to synthesis novel advanced g-C3N4based materials for the activation of persulfate. Moreover, the study expanded the scope of persulfate activators and deepened the mechanistic insights into persulfate activation in terms of peroxymonosulfate oxidation and reduction at the electron-deficient and electron-rich sites, respectively, for the simultaneous generation of nonradical species (1O2) and radicals (•OH and SO4•−). This consolidated the fundamental theories of persulfate-based catalytic reactions. Further, the role of organic pollutants in the catalytic degradation processes was clearly elucidated, in which the organic contaminants containing electron-donating groups were oxidized more efficiently in the 1O2-dominating catalytic systems. Most importantly, the capability of nonmetal doping to modulate the electronic structure of O−CN paves a new avenue for the development of highly efficient metal-free catalysts toward the persulfate-mediated environmental remediation.

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ORCID

Lai Lyu: 0000-0002-5624-961X Chun Hu: 0000-0003-3217-7671 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Plan (2016YFA0203200), the National Natural Science Foundation of China (51538013, 51808140, 51808142, 51808143), the Natural Science Foundation of Guangdong Province (2018A030313367, 2018A030313487), the Science Starting Foundation of Guangzhou University (6918ZX10299, 2700050315, 2700050302, and 69-18ZX10298), the Young Innovative Talent Project in Higher Education of Guangdong Province (2017KQNCX150), and the Special Funds for the Cultivation of Guangdong College Students’ Scientific and Technological Innovation (“Climbing Program” Special Funds PDJHA0393).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b05246. Tables showing element contents, BET surface areas and pore volumes as well as XPS spectra results; figures showing molecular structures of various organic pollutants, XPS survey spectra, TOC removal, effect of common inorganic anions, XRD patterns of fresh and used O−CN, activation of persulfate by O−CN, FTIR spectra of O−CN after different reactions, and effect of scavengers and degradation of BPA in N2-saturated atmosphere (PDF)

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REFERENCES

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