Graphitic Carbon Nitride from Burial to Re-emergence on

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Graphitic Carbon Nitride from Burial to Re-emergence on Polyethylene Terephthalate Nanofibers as an Easily Recycled Photocatalyst for Degrading Antibiotics under Solar Irradiation Dandan Qin, Wangyang Lu, Xiyi Wang, Nan Li, Xia Chen, Zhexin Zhu, and Wenxing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07680 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Graphitic Carbon Nitride from Burial to Reemergence

on

Polyethylene

Terephthalate

Nanofibers as an Easily Recycled Photocatalyst for Degrading Antibiotics under Solar Irradiation Dandan Qin, Wangyang Lu*, Xiyi Wang, Nan Li, Xia Chen, Zhexin Zhu and Wenxing Chen* National Engineering Lab for Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China. KEYWORDS: g-C3N4, PET, nanofiber, photocatalytic antibiotics degradation, solar irradiation

ABSTRACT: For powder catalysts to be recycled easily and to be applied in practical wastewater treatment, it is imperative to search suitable carriers that can be applied to support catalytic particles. Herein, we highlight a facile route to synthesize an easily recycled photocatalyst using polyethylene terephthalate (PET) to disperse graphitic carbon nitride (gC3N4) via electrospinning and subsequent hydrothermal treatment. The resultant nanofiber is labeled T-g-C3N4/PET. The design concept is to expose the g-C3N4 on the PET surface and convert it from inactivation to relive. g-C3N4 is embedded into the PET, which avoids the reunion and unrecyclable deficiencies of powder catalysts. T-g-C3N4/PET was characterized by field-emission scanning electronic microscopy, transmission electron microscopy, UV-vis diffuse reflectance spectra, two-dimensional X-ray diffraction, Fourier-transform infrared spectroscopy and thermogravimetric analysis technologies. T-g-C3N4/PET showed a high

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photocatalytic activity for the degradation of antibiotics such as sulfaquinoxaline and sulfadiazine under solar irradiation, and the activity was almost unaffected in a high background. The as-obtained catalysts could be reused several times with no loss in performance in cycling photodegradation tests. Finally, a possible pathway and mechanism for degrading sulfaquinoxaline with T-g-C3N4/PET were proposed, respectively, in which holes and the superoxide radical were the predominant active species, and resulted in the oxidative degradation of antibiotics. These results demonstrate that the preparation method may provide a novel idea for supporting nanoscale catalysts for reuse.

1. INTRODUCTION Increasing attention is being given to global environmental purification and protection, and there is a pressing requirement to dispose of wastewater discharge from various factories at a lower cost and energy consumption. Over recent decades, massive methods have been applied to deal with water decontamination such as physical adsorption,1,2 biochemical methods3 and advanced oxidation processes (AOPs)4. However, deficiencies exist in physical adsorption, for instance, heavy sludge makes transportation fairly difficult, and disposal costs are relatively high. Although biodegradation is of interest for pollutant elimination, it is time-consuming. In this regard, AOPs, including ionizing radiation treatment, ultraviolet (UV)/H2O2,5 O3/H2O2,6-9 photo/Fenton10,11 and Fenton,12 are popular because of their effective and rapid mineralization of organic pollution. Unfortunately, these systems usually cannot perform well in the absence of additional oxidants, such as H2O2 and O3. In addition, tedious, multi-step preparation processes and harsh experimental conditions are required to obtain the target catalysts, which commonly have a low yield. Furthermore, metal introduction destroys the environment and is accompanied


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with high material costs. Many catalysts that are used to remove pollutants contain noble metals13-15 and heavy metals,16 which adversely affect ecosystems and human health. Currently, photocatalysis has attracted wide notice for degrading pollutants over inexhaustible sunlight,17-20 and accordingly, the catalysts for water treatment are very important.21-23 Moreover, since the discovery of graphitic carbon nitride polymer (g-C3N4) with photocatalytic activity for water splitting in 2009,24 g-C3N4 has been touted as one of the most promising alternatives compared with traditional materials for degrading micro-pollutants. g-C3N4 is a non-metallic, conjugated semiconductor, and is the most stable allotrope of carbon nitride.25 It differs from the photocatalyst TiO2, which opposes photo-corrosion and has a narrower band gap 2.7 eV. Hence, it has motivated a surge of research interest in its exceptional photoelectric properties.26,27 Unfortunately, some restrictions still exist in the application of bare g-C3N4, including a wide band gap, a fast charge recombination and aggregation. Tremendous endeavours have been undertaken to overcome the above limitations, such as doping with noble metals,28,29 nonmetallic materials,30-32 constructing heterojunctions33-37 and loading g-C3N4 on some carriers.38 These material modifications are still insufficient for commercial and practical applications, thus, catalyst-support technology is required to improve the catalytic activity, dispersion, recyclability and stability of catalysts.39,40 More importantly, in general, most photocatalysts with high activities and a high specific surface area exist as nanoscale powder materials.28, 41,42 Eventually, they settle to the bottom of a container so that they are difficult to collect from wastewater and reuse. Limited research has addressed this issue. For instance, Maciej et al. synthesized a floating TiO2 photocatalyst deposited on expanded perlite.43,44 Furthermore, significant attempts have focused on fascinating polymers45 used as supports to achieve easily recycled catalysts for wastewater

treatment.

For

example,

Xu

et

al.

synthesized

supported

g-


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C3N4/polyacrylonitrile(PAN) catalyst nanofibers over electrospinning, which dispersed g-C3N4 in PAN.46 Herein, we adopted a relatively simple solution to fabricate an effective hybrid nanofiber in which g-C3N4 was loaded on the polyethylene terephthalate (PET) to serve as a solar light-driven photocatalyst for water purification. The powder catalyst g-C3N4 and the support PET were blended uniformly using an electrospinning process. Thereafter, an alkaline hydrothermal treatment was carried out to expose the catalyst on the fiber surface. In other words, the PET was etched by alkaline solutions, which resulted in the reduction of the nanofibers diameter and the g-C3N4 was exposed on the PET surface accordingly. The hydrothermal treatment covered the gC3N4 from inactivation to relive in the obtained nanofibers. The competent catalytic active sites generated were generated resulting from the uniform dispersion and exposure of catalysts in the PET substrate. Polymers are not used commonly as photocatalysts because of their insulating properties, which leads to poor electron transfer,47 so limited attention was given to the polymers. However, the polymers display intriguing advantages such as their inertness, low-cost, lightness, mass production. When floating PET fibers are located at the air/water interface, they can contact solar light directly, which avoids the shield of solution for light energy. More importantly, they can be collected readily from water. Therefore, we chose PET as a catalyst support. In addition, we investigated the photocatalytic performance of the resultant photocataysts over solar light irradiation. Sulfaquinoxaline (SQX) that is often used to prevent and treat coccidiosis, a severe livestock disease, and was selected as the template to evaluate the photocatalytic performance.48 The incorrect use of SQX will form organic micro-pollutants, which may lead to an increase in bacterial resistance.49,50 Furthermore, a degradation pathway of SQX was proposed and the removal rate of the other antibiotics was also studied. Finally, possible mechanisms for


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degrading antibiotics in this photocatalytic system and the high stability of the obtained photocatalysts were demonstrated. To some extent, these results may provide a novel and rational method for the design of low-cost and easily recyclable supported photocatalysts for use in water purification and energy conversion. Additional, antibiotic decomposition to biodegradable products provides the opportunity for pollutant conversion into energy. 2. EXPERIMENTAL SECTION Materials. PET was from Zhejiang Guxiandao Industrial Fiber Co., Ltd. 1,1,1,3,3,3Hexafluoro-2-propanol (HFIP) was from Fluorochem, Ltd. Urea, sodium hydroxide (NaOH), cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfonate (SDS), polyethylene glycol (PEG), SQX, sulfadiazine (SD), sulfamerazine (SMZ), sulfachloropyridazine (SCP), sulfamethoxazole

(SMX),

sulfamonomethoxine

(SMM),

sulfamethoxydiazine

(SMD),

sulfaguanidine (SGD) and p-benzoquinone (BQ) were from Aladdin Chemical Co., Ltd. Isopropanol (IPA) and potassium iodide (KI) were from Tianjin Wing Tai Chemical Co., Ltd. and Hangzhou Gao Jing Fine Chemical Co., Ltd., respectively. Acetonitrile (Sigma–Aldrich), methanol (Merck) and formic acid (J&K chemical Inc.) were of chromatographic grade for ultraperformance liquid chromatography (UPLC). All other chemicals were of analytical grade and were used without further purification. Ultrapure water used in all experiments was from Milli-Q Advantage A10 (Millipore). Photocatalyst Preparation. g-C3N4 was synthesized by thermal decomposition of urea.51 Urea (50 g) in a sealed crucible was heated to 550°C in a tune furnace at 2.5°C/min, and was kept at temperature for 3 h. After cooling, the faint yellow powder that was obtained was g-C3N4. The thickness of as-prepared g-C3N4 is about 3.7 nm according the AFM image (Figure S1). PET


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(1.7730 g) and g-C3N4 (0.1773 g) were each dissolved in 5 mL HFIP. After ultrasonic dispersion and having achieved uniform dissolution, these two solutions were mixed together, followed by stirring for 5 h. The electrospinning process was carried out using the mixture solution and a uniform spinning process. A fixed voltage of 15 kV and a settled flow rate of 1.1 mL/h were applied, and fibers were collected on an aluminium foil plate at a distance of 25 cm. The obtained nanofibers were termed U-g-C3N4/PET. After treating the U-g-C3N4/PET with NaOH (4 g/L) and CTAB (0.4 g/L) at 65°C in aqueous solution for 1.5 h, the resultant nanofibers were labelled T-g-C3N4/PET. The mass loss of the U-g-C3N4/PET was ~40%. For comparison, PET nanofibers were fabricated by electrospinning 10 wt% PET solutions under the same conditions. The PET fibers after treatment were labelled T-PET. Characterization. Atomic force microscopy (AFM) was operated by Asylum Research MFP-3D (Oxford Instruments, USA). Field emission scanning electronic microscopy (FESEM, ULTRA-55) and transmission electron microscopy (TEM, JEOL JEM-2010) were used to study the morphology of the as-prepared photocatalysts. Fourier-transform infrared spectroscopy (FTIR) spectra were recorded on a Thermo Nicolet 5700 FTIR spectrometer from 400 to 4000 cm-1. The UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrometer (1J1-0015, HITACHI). Two-dimensional X-ray diffraction (2D-XRD) measurements were carried out using a Bruker D8 Discover X-ray diffractometer with Cu Kα radiation at 40 kV and 40 mA. The surface area analyses of the as-obtained nanofibers were conducted using the Brunauer-Emmett-Teller (BET) method with a nitrogen adsorption at a Surface area and porosity instrument (TRISTAR II 3020, micromeritics). Thermogravimetric analysis (TGA) was conducted on a TGA 1 (Mettler Toledo, Switzerland) from 25 to 800°C at 10°C /min in air.


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Photocatalytic Experiments. We chose SQX as the main model antibiotic drug to investigate the photocatalytic performance of T-g-C3N4/PET. A stock solution of SQX (2 × 10-5 mol/L) was prepared with ultrapure water. The solution pH was adjusted by H2SO4 or NaOH addition. The T-g-C3N4/PET photocatalytic experiments were conducted in a glass sample beaker under solar irradiation provided by a SUN-Q-Light photoreactor (Xe-1-BC, USA). The spectral power distribution (SPD) of Q-SUN Xe-1 with Daylight compared with Noon Summer Sunlight is shown in Figure S2. In a typical photocatalytic experiment, 30 mL as-prepared SQX solution that contained T-g-C3N4/PET (0.05 g) was shaken in the dark for 2.5 h to reach SQX adsorption/desorption equilibrium on the T-g-C3N4/PET surface. The solution was collected at designated time intervals and was filtered using membrane filters (0.22 µm pore size). The SQX concentration was determined by UPLC (Acquity BEH C18 column, 1.7 µm, 2.1 × 50 mm, Waters). Cycling experiments were carried out to evaluate the stability of the T-g-C3N4/PET. After each cycle, samples were washed with deionized water and were added to fresh SQX solution, followed by another cycle. Comparative photocatalysis experiments were also conducted to study the catalytic mechanism, in which BQ, IPA and KI were each added to SQX solution with the other conditions unchanged. 3. RESULTS AND DISCUSSION Characterization. The FE-SEM images of PET, U-g-C3N4/PET and T-g-C3N4/PET are shown in Figure 1. The pure PET fiber surfaces are relatively clean and the U-g-C3N4/PET nanofiber surfaces are also smooth with some irregularities caused by electrostatic spinning.


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Because the U-g-C3N4/PET was treated with heated NaOH solution, many small pits and grooves formed on the T-g-C3N4/PET surface, because of the ester linkage hydrolysis.45 Some sheet protuberances exist on the surface (Figure 1F), which are assumed to the result of the exposed gC3N4. The T-g-C3N4/PET diameter is smaller than that of the U-g-C3N4/PET, which indicates that U-g-C3N4/PET was etched through the hydrothermal process and it is possible that g-C3N4 reemerged and was exposed on the nanofiber surface, which lead to sufficient contact of contaminants and the formation of catalytic active sites. Figure 2 shows TEM images of T-g-C3N4/PET and U-g-C3N4/PET. The T-g-C3N4/PET surface is rougher than that of U-g-C3N4/PET clearly, while some protuberances can be observed on the surface. Furthermore, Figure 2C displays obvious lattice fringes with a stacking distance of 0.326 nm,52 which proves that the protuberance in Figure 2B was g-C3N4 with a high partial crystallinity. From the Figure 2B, it can be seen that g-C3N4 was inlaid into the fibers, which is consistent with the FE-SEM Figures. Moreover, the T-g-C3N4/PET diameter is distinctly smaller than that of U-g-C3N4/PET. According to the TEM images, the ratio (0.798) of diameters of the T-g-C3N4/PET versus U-g-C3N4/PET agrees with the theoretical value (0.775) calculated from the mass loss of U-g-C3N4/PET. UV-vis diffuse reflectance spectra of T-g-C3N4/PET and the reference samples such as U-g-C3N4/PET, g-C3N4 and PET are compared in Figure S3. The PET absorbs UV light below 300 nm only. Characteristic absorption peaks at 230 and 300 nm originate from the PET benzene ring structure.45 The curves show that g-C3N4 existed on the Tg-C3N4/PET and U-g-C3N4/PET surface compared with pure g-C3N4. As seen in curves of the Tg-C3N4/PET, the absorption edge exhibits a slightly shift versus that U-g-C3N4/PET, which is attributed to the increase of exposed g-C3N4 on the PET surface after treatment.45 The absorbance intensity of T-g-C3N4/PET is increased because the alkaline hydrothermal treatment


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can lead the surface area of g-C3N4 to increase.53 These results confirm that T-g-C3N4/PET could absorb visible light energy and exhibited good photocatalytic activity. Further evidence for the existence of g-C3N4 in the T-g-C3N4/PET and U-g-C3N4/PET is given in the FTIR spectra (Figure 3). The bands at 3100 cm-1, 1200–1600 cm-1 and 810 cm-1 belong to primary and secondary amines, aromatic carbon and nitrogen heterocycles, and the striazine ring, respectively.41, 54 The absorption band at ~1730 cm-1 contributed to the stretching vibration of C=O. The bands at ~2960 cm-1 originated from asymmetrical stretching of -CH2and the peak at 730 cm-1 is the in-plane bending vibration of C-H. Moreover, the ratio of absorption intensity between g-C3N4 and PET increased significantly from the curve of T-gC3N4/PET to U-g-C3N4/PET, which is responsible for the PET mass loss that results from ester linkage hydrolysis during post-treatment of U-g-C3N4/PET. Figure 4(A–E) presents 2D-XRD patterns of the PET, T-PET, U-g-C3N4/PET, T-gC3N4/PET and g-C3N4 powders with the integral of their XRD patterns shown in Figure 4F. The XRD peaks of PET and U-g-C3N4/PET both have an expansion, which indicates the poor crystal quality of PET. However, after hydrothermal treatment, T-PET and T-g-C3N4/PET both exhibit an isotropic diffraction pattern in Figure 4(B, D). This suggests that the T-PET crystallinity increases and T-PET shows three preferred ((100), (-110) and (010)) orientations, which correspond to peaks with 2θ values of 17.6°, 23.0° and 25.6°, respectively.55 The obvious isotropic diffraction pattern preferred (002)52 orientation is visible in Figure 4(C, D), which indicates that g-C3N4 nucleation is distributed randomly in the U-g-C3N4/PET and T-g-C3N4/PET samples. The characteristic peak of T-g-C3N4/PET with 2θ value of 27.4° corresponds to an interlayer distance of 0.326 nm,52 which is coincident with the TEM image results. Moreover, from Figure 4F, the intensity of the characteristic peak (2θ value of 27.4°) became strong in


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contrast with that of U-g-C3N4/PET, because the mass ratio of g-C3N4 and PET became large in the T-g-C3N4/PET. The results illustrate that g-C3N4 was dispersed in PET nanofibers when electrospinning. The specific surface area was derived from nitrogen adsorption/desorption isotherm by using Brunauer-Emmett-Teller (BET) method, as shown in Figure S4 It can be seen that the surface area of T-g-C3N4/PET is twice as large as that of the U-g-C3N4/PET, which further illustrates that the former surface area is increased because of the hydrothermal process, and it would has a higher photocatalytic activity than that of the latter. The TGA curves of PET, U-gC3N4/PET, T-g-C3N4/PET and g-C3N4 are shown in Figure S5. The PET curve shows a mass loss at 410°C, owing to thermal decomposition,56 and g-C3N4 has a favourable thermal stability because its decomposition temperature exceeds 500°C. U-g-C3N4/PET and T-g-C3N4/PET display two mass-loss steps on the TG curves, which are ascribed to the doping of g-C3N4. In summary, the thermal stability of T-g-C3N4/PET is good. Based on the FESEM, TEM, FTIR, DRS and 2D-XRD results for the as-prepared samples, it can be concluded that T-g-C3N4/PET is composed of g-C3N4 and PET, and that the g-C3N4 were inlaid into the PET nanofibers successfully. The formation of T-g-C3N4/PET exhibits an effective synergistic effect between the two components, which results in an enhanced photocatalyst stability and reusability. Furthermore the as-prepared photocatalysts possess high photocatalytic performance. Photocatalytic Results and Analysis. The photocatalytic activities of T-g-C3N4/PET, U-gC3N4/PET and PET were evaluated by the degradation of SQX solution (pH 5) under sunlight radiation. Firstly, we studied photolysis for SQX and the adsorption of T-g-C3N4/PET for SQX.


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Figure 5 shows that in the dark, ~5% SQX was adsorbed by T-g-C3N4/PET. Additionally, ~20% SQX was degraded under sunlight radiation. Meanwhile the photolysis performance of SQX was nearly equal to the removal rate of SQX by PET and U-g-C3N4/PET under solar irradiation for 2.5 h, which suggests that PET and U-g-C3N4/PET had almost no photocatalytic activity for degrading SQX. Interestingly, SQX was degraded completely by the T-g-C3N4/PET. According to the above results, T-g-C3N4/PET possesses a high photocatalytic activity for degrading SQX, which proves that the hydrothermal process for U-g-C3N4/PET was essential in order to expose g-C3N4 on the PET surface, and the surface area is increased, competent catalytic active sites are also generated. We also compared the photocatalytic activities between T-g-C3N4/PET and pure g-C3N4 under static and shaking conditions, respectively. Figure S6A shows that the activities of pure g-C3N4 are inhibited significantly under static conditions versus T-g-C3N4/PET, which positively demonstrates T-g-C3N4/PET can distribute g-C3N4 effectively and avoid aggregation successfully. Besides，we can apparently observe T-g-C3N4/PET can float on the SQX solution, whereas bare g-C3N4 eventually settles at the bottom of the container in the Figure S6B, which leads to the deficient absorbance of solar light energy, and thus the photocatalytic activity decreases. pH is an important factor in organic pollutant degradation in an aquatic environment. Thus, we studied the photocatalytic performance of SQX under different pH conditions (Figure S7). A weak acidic condition is preferred above neutral and alkaline conditions, which is ascribed to the high performance of g-C3N4 under acidic conditions.57 Therefore pH 5 was applied for all photocatalytic degradation experiments in this work. T-g-C3N4/PET exhibited a fairly good photocatalytic activity under acidic, neutral and alkaline conditions with complete degradation of SQX under sunlight irradiation for 5 h. It is suggested that the synergistic effect of PET fibers


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and g-C3N4 is conductive to pollutant elimination in practical wastewater with a wide pH range. Additionally, the effect of initial SQX concentration was determined, as displayed in the Figure S8. With increase in initial SQX solution concentration, the SQX removal rate was more rapid. Contrast experiments were conducted to assess the influence of various compounds that exist in wastewater. In Figure 6, Na2CO3 and NaHCO3 show a slight inhibition for the T-gC3N4/PET activities, whereas NaCl, CaSO4 and MgSO4 barely influence the performance. Compared with most AOPs,58 the presence of HCO3- and Cl- has a negative effect on the photocatalytic degradation of micro-pollutants, which indicates that common radicals such as hydroxyl radical are not dominant active species in this photocatalytic system. Furthermore, the photocatalytic performance of T-g-C3N4/PET are showed in Figure 6(B) in the presence of organic compounds SDS (0.5 mM), urea (10 mM) and PEG (10 mM) in the reaction system. Although SDS and urea would inhibit the activity slightly, the SQX removal rate reached 90% after 2.5 h. In general, no matter containing inorganic or organic compounds in the system, there are no obvious interferences noticed for SQX degradation, presumably because short volatile fatty acids are generated, such as malonic acid, during the degradation of SQX, which decreases the pH and concentration of the addition agents. In summary, the above results imply that T-gC3N4/PET has an excellent photocatalytic performance and high stability in the high background. Stability and Reusability of the Photocatalysts. A T-g-C3N4/PET recycling experiment was conducted (Figure 7) because the synthesis of green and easily recyclable catalysts is of great importance. After repeating this experiment 15 times, the SQX removal rate reached up to 100%. Interestingly, the activity improved, which may result from that the fibers became fluffy after manifold cycles and the intervals among fibers became large, which increased the bare gC3N4 spots. In order to evaluate the stability of the as-obtained photocatalysts, the XRD and


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FESEM of as-prepared photocatalysts after photocatalytic reactions have also been measured, as shown in Figure S9 and Figure S10. Obviously, XRD image points out the structure and degree of crystallinity of the used photocatalysts had almost no change. Meanwhile, In the Figure S10, the photocatalysts surface is still rough with some pits and grooves. The surface morphology of the photocatalysts was well maintained after photocatalytic reactions, which indicates the excellent fastness of g-C3N4 inlayed into the PET fibers.59-60 The results substantiate that PET fibers play a positive role for the recycle of g-C3N4, and T-g-C3N4/PET possesses potential application prospects for water decontamination. Photocatalytic Activities for Other Antibiotics. We also investigated the photocatalytic degradation of other antibiotics by T-g-C3N4/PET under the same experimental conditions, as shown in Table S1. The removal rate of antibiotics such as SD and SMZ reached ~98% for different solar irradiation times in the presence of T-g-C3N4/PET. According the Table S1, it can be speculated that a higher substrate electron-donating ability makes oxidation by T-g-C3N4/PET easier. Furthermore, the antibiotic removal rate may be related to their hydrophobicity. As the substrate hydrophobicity increased, the removal rate increased. The above findings indicate that T-g-C3N4/PET can degrade many antibiotics with conjugated structure. Mechanisms and Degradation Pathway. To demonstrate the mechanism for antibiotic photocatalytic degradation by T-g-C3N4/PET, the scavengers BQ, KI and IPA were added to quench the possible active species •O2-, hole (h+) and •OH, respectively.61,62 As shown in Figure 8, when IPA was added, the removal rate of SQX increased, resulting from that IPA could enhance the catalyst adsorption for SQX, as shown in Figure S11. The result implies that •OH is not the dominant active species. However, the SQX removal rate dramatically occurred to decline in the existence of BQ and KI, which revealing that •O2- and h+ are the main active


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species. It is generally known that the redox potential of O2/•O2- (-0.33 eV) is more negative than the CB of g-C3N4 (-1.21 eV), so the photogenerated electrons in the CB of g-C3N4 are favourable to react with O2. However, the photogenerated holes in the VB of g-C3N4 can directly oxidise the SQX instead of the generation of •OH radicals, because the redox potential of VB of g-C3N4 (1.49 eV)63 is lower positive versus the redox potential of •OH/OH- (1.99 eV).37, 64 It further demonstrates that •OH is not the main active species and •O2- radicals is the dominant active species. The results are consistent with that of g-C3N4.65 The EPR spectrum in Figure 9(A) shows the generation of DMPO-•O2- adducts, because •O2- were trapped by DMPO in the above photocatalytic system, which further confirms the production of •O2- radicals. Figure 9(B) illustrates that •OH radicals were not detected, which coincides with the results in Figure 8. Sunlight irradiation excites the exposed g-C3N4 on the PET nanofiber surface to generate electron-hole pairs. The formed h+ plays a vital role in antibiotic degradation. Furthermore, the photogenerated electrons can react further with dissolved oxygen to form •O2-, which can degrade the target contaminant. Figure S12 shows the EPR spectrum of T-g-C3N4/PET. A Lorentzian line centering at g=2.0034 can be observed, which indicates that an unpaired electron on π-conjugated CN aromatic rings generated.66,53 Under visible light (> 400 nm) or UV + visible light irradiation, the EPR signals of T-g-C3N4/PET are enhanced significantly, which indicates the efficient generation of photochemical radical pairs in the photocatalysts. These conduction electrons are in favor of the photocatalytic degradation. Based on the above results, we proposed the preparation of T-g-C3N4/PET and the mechanistic diagram of antibiotics degradation in the presence of T-g-C3N4/PET under solar radiation, as shown in Figure 10. Furthermore, the intermediate degradation products were detected via UPLC Synapt G2-S HDMS in positive and negative ion modes for different reaction processes. We proposed a


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possible degradation pathway, as shown in Figure 11, and the intermediate products are listed in Table S1 and Table S2. One of the degradation pathways was the hydroxylation of the quinoxalin moiety of SQX at first,67 forming product B (SQX-OH), and then the S-C5 and SN68,69 bonds were destroyed. Another was that SO2 was probably extruded from SQX,70 thus we propose that product C was N1-(quinoxalin-2yl) benzene-1,4-diamine, therefore, it could be inferred that N-C bond cleavage led to SQX decomposition. Finally, four biodegradable small molecular acids (P1–P4) were obtained, including adipic, maleic, malic and malonic acids, which is ascribed to aromatic ring-opening oxidation. 4. CONCLUSIONS In summary, T-g-C3N4/PET nanofibers had been successfully prepared via an easily accessible method. FESEM, TEM, DRS, FT-IR, 2D-XRD, BET and TGA analysis revealed the morphology, structure and composition of the obtained T-g-C3N4/PET nanofibers. The T-g-C3N4/PET nanofibers exhibited a high photocatalytic performance and stability in the degradation of antibiotics such as SQX and SD. The photocatalysts induced a 100% degradation rate of SQX under solar irradiation over 2.5 h, which is equivalent to pure g-C3N4. Additionally, the photocatalytic activity of T-g-C3N4/PET was unaffected in a high background. The significant enhancement in photocatalytic stability and reusability of the T-g-C3N4/PET nanofibers may be ascribed to the dispersion and recycling functions of the support. These results provide a novel idea for the design of low-cost and easily-recycled supported photocatalysts for practical water purification and energy conversion.


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FIGURES

Figure 1. SEM images of PET (A, B), U-g-C3N4/PET (C, D), T-g-C3N4/PET (E, F).


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Figure 2. TEM images of (A) U-g-C3N4/PET, (B) T-g-C3N4/PET, (C) a part of T-g-C3N4/PET.


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Figure 3. FTIR spectrum of g-C3N4, PET, U-g-C3N4/PET, T-g-C3N4/PET.


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Figure 4. 2D-XRD patterns of different samples: (A) PET, (B) T-PET, (C) U- g-C3N4/PET, (D) T- g-C3N4/PET, (E) g-C3N4, (F) integral of the XRD patterns of above mentioned samples.


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Figure 5. Photocatalytic degradation of SQX (2 × 10−5 mol/L) under different conditions, pH 5, 200 rpm.


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Figure 6. (A) Effect of various inorganic compounds (10mM), (B) organic compounds, SDS (0.5 mM), PEG (10 mM) and Urea (10 mM) on photocatalytic degradation of SQX (2 × 10−5 mol/L) by T-g-C3N4/PET under solar irradiation, pH 5, 200 rpm.


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Figure 7. Cyclic photocatalytic degradation of SQX (2 × 10-5 mol/L) by T-g-C3N4/PET under solar irradiation for 2.5 h.

Figure 8. Effect of trapping agents on photocatalytic degradation of SQX (2 × 10−5 mol/L) by Tg-C3N4/PET under solar irradiation, pH 5.


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Figure 9. DMPO spin-trapping EPR spectra in aqueous or methyl alcohol solutions in the presence of T -g-C3N4/PET under solar irradiation, (A) aqueous solution, (B) methanol solution. [DMPO]=10 mM, pH 5.


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Figure 10. Preparation of T-g-C3N4/PET nanofibers and proposed mechanism of degrading antibiotics in presence of T-g-C3N4/PET over solar irradiation.

Figure 11. Proposed possible degradation pathway of SQX by T-g-C3N4/PET over solar irradiation, pH 5.


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ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The spectral power distribution (SPD) of Noon Summer Sunlight compared to the Q-SUN Xe-1 with Daylight, UV–vis diffuse reflectance absorption spectrum and thermogravimetric analysis of the samples, photocatalytic performance of T-g-C3N4/PET compared with g-C3N4, effect of initial pH and concentration of SQX solution, The XRD and FESEM of as-prepared photocatalysts after photocatalytic reactions. EPR spectra of T-g-C3N4/PET samples, Photocatalytic activities for various antibiotics, degradation intermediates of SQX examined by UPLC Synapt G2-S HDMS in the positive and negative ion mode. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest.


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ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 51133006 and 51103133), Zhejiang Provincial Natural Science Foundation of China (No. LY14E030013), and the Public Welfare Technology Application Research Project of Zhejiang Province (No. 2015C33018 and 2016C33019). We are specially grateful to Professor Gangqiang Wang, Zhejiang Sci-Tech University, China, for the valuable suggestions and technical discussions. REFERENCES 1. Alsbaiee, A.; Smith, B. J.; Xiao, L.; Ling, Y.; Helbling, D. E.; Dichtel, W. R., Rapid Removal of Organic Micropollutants from Water by a Porous Beta-Cyclodextrin Polymer. Nature 2016, 529 (7585), 190-194. 2. Putra, E. K.; Pranowo, R.; Sunarso, J.; Indraswati, N.; Ismadji, S., Performance of Activated Carbon and Bentonite for Adsorption of Amoxicillin from Wastewater: Mechanisms, Isotherms and Kinetics. Water Res. 2009, 43 (9), 2419-2430. 3. Hazen, T. C.; Prince, R. C.; Mahmoudi, N., Marine Oil Biodegradation. Environ. Sci. Technol. 2016, 50 (5), 2121-2129. 4. Yang, Y.; Pignatello, J. J.; Ma, J.; Mitch, W. A., Comparison of Halide Impacts on the Efficiency of Contaminant Degradation by Sulfate and Hydroxyl Radical-Based Advanced Oxidation Processes (AOPs). Environ. Sci. Technol. 2014, 48 (4), 2344-2351. 5. Liao, Q.-N.; Ji, F.; Li, J.-C.; Zhan, X.; Hu, Z.-H., Decomposition and Mineralization of Sulfaquinoxaline Sodium During UV/H2O2 Oxidation Processes. Chem. Eng. J. 2016, 284, 494502. 6. Rodayan, A.; Roy, R.; Yargeau, V., Oxidation Products of Sulfamethoxazole in Ozonated Secondary Effluent. J. Hazard. Mater. 2010, 177 (1-3), 237-243. 7. Lin, A. Y.; Lin, C. F.; Chiou, J. M.; Hong, P. K., O3 and O3/H2O2 Treatment of Sulfonamide and Macrolide Antibiotics in Wastewater. J. Hazard. Mater. 2009, 171 (1-3), 452458. 8. Lee, Y.; Gerrity, D.; Lee, M.; Gamage, S.; Pisarenko, A.; Trenholm, R. A.; Canonica, S.; Snyder, S. A.; von Gunten, U., Organic Contaminant Abatement in Reclaimed Water by UV/H2O2 and a Combined Process Consisting of O3/H2O2 Followed by UV/H2O2: Prediction of Abatement Efficiency, Energy Consumption, and Byproduct Formation. Environ. Sci. Technol. 2016, 50 (7), 3809-3819. 9. Beltrán, F. J.; Pocostales, P.; Álvarez, P. M.; López-Piñeiro, F., Catalysts to Improve the Abatement of Sulfamethoxazole and the Resulting Organic Carbon in Water During Ozonation. Appl. Catal., B 2009, 92 (3-4), 262-270. 10. Gonzalez, O.; Sans, C.; Esplugas, S., Sulfamethoxazole Abatement by Photo-Fenton Toxicity, Inhibition and Biodegradability Assessment of Intermediates. J. Hazard. Mater. 2007, 146 (3), 459-464.


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Graphitic Carbon Nitride from Burial to Re-emergence on

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