graphene under visible light irradiation for Trimethoprim degradation

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Activation of peroxymonosulfate by Fe doped g-C3N4 /graphene under visible light irradiation for Trimethoprim degradation Ruobai Lia, Jiashu Huanga, Meixuan Caia, Jiaxing Huanga, Zhijie Xiea, Qianxin Zhanga, Yang Liub, Haijin Liuc, Wenying Lva,⁎, Guoguang Liua,⁎ a

School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China b Faculty of Environmental & Biological Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China c School of Environment, Henan Normal University, Henan Key laboratory for Environmental Pollution Control, Xinxiang 453007, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

Fe-doped g-C3N4 / graphene (rGO) composites were investigated as catalysts for the activation of peroxymonosulfate (PMS) to degrade Trimethoprim (TMP) under visible light irradiation. The rapid recombination of photogenerated electron-hole pairs in g-C3N4 may be suppressed by doping with Fe and incorporating rGO. The TMP degradation efficiency using 0.2% Fe-g-C3N4/2 wt% rGO/PMS was 3.8 times than that of g-C3N4/PMS. The degradation efficiency of TMP increased with higher catalyst dosages and PMS concentrations. Acidic condition (pH = 3) was observed to significantly enhance the TMP degradation efficiency from 61.4% at pH = 6 to nearly 100%. By quenching experiments and electron spin resonance (ESR), O2%− was found to play an important role for the activation of PMS to accelerate the generation of reactive radicals for the TMP degradation. A total of 8 intermediates derived from hydroxylation, demethoxylation and carbonylation were identified through theoretical calculations and the HRAM/LC–MS-MS technique, and transformation pathways of TMP oxidation were proposed. TOC removal rate of TMP increased as reaction time was prolonged. Acute toxicity estimation by quantitative structure-active relationship analysis indicated that most of the less toxic intermediates were generated. The aim of this study was to elucidate and validate the functionality of a promising polymeric catalyst for the environmental remediation of organic contaminants.

Keywords: Fe-doped graphitic carbon nitride Graphene Peroxymonosulfate Reactive species Transformation pathway

⁎

Corresponding authors. E-mail addresses: [email protected] (W. Lv), [email protected] (G. Liu).

https://doi.org/10.1016/j.jhazmat.2019.121435 Received 13 July 2019; Received in revised form 7 October 2019; Accepted 7 October 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Ruobai Li, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121435

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the destruction of recalcitrant organics, in contrast to %OH (E0 (%OH/ OH−) = 1.9–2.7 VNHE) (Buxton et al., 1988). SO4%− reacts with organic compounds primarily through an electron transfer mechanism due to its high selectivity. Jo et al. found that the introduction of PMS can be activated by photogenerated electrons to produce SO4%−; thus, enhancing the photocatalytic performance of TiO2 in a TiO2/PMS/vis system (Jo et al., 2018). For this study, the Fe-g-C3N4/rGO composite mediated activation of PMS for the oxidative degradation of TMP under visible light irradiation was investigated. A number of Fe-g-C3N4 composites with different rGO loadings were synthesized via one-pot polymerization and elucidated through multiple characterization methods. The effects of various reaction parameters such as catalyst dosage, PMS concentration, and pH were also investigated. The reactive species derived from catalytic processes were determined by quenching experiments and electron spin resonance (ESR) measurements. Subsequently, a plausible mechanism and degradation pathway were tentatively proposed through the identification of intermediates.

1. Introduction Trimethoprim (TMP) is one of the most commonly used antibiotics for the treatment of various types of bacterial infections and as a growth promoter (Oros-Ruiz et al., 2013; Paula et al., 2008). Following ingestion, only approximately 22.5% TMP can be metabolized by the human body (Rieder, 1973). And once TMP is discharged into wastewater, it cannot be effectively removed through traditional wastewater treatment technologies. Due to its continuously increasing clinical use, TMP has been frequently detected in ambient waterways and wastewater streams at levels ranging from ng/L to μg/L (Göbel et al., 2007). Thus, it is considered to be a major threat to human health and ecosystems due to its adverse biological effects and the potential risk of bacterial resistance (Kolar et al., 2014; Yang et al., 2018). It is therefore imperative to develop environmentally compatible and cost-effective treatment techniques for the eradication of TMP. Semiconductor-based photocatalysis technologies are regarded as a promising strategy for addressing environmental contamination issues through the utilization of solar energy. However, the most widely employed semiconductor photocatalyst (TiO2) possesses a rather wide band gap (3.2 eV) that is responsive only in the UV region, which inhibits its effective utility and practical application under visible light (Nakata and Fujishima, 2012). Tremendous efforts have been devoted to enhancing the visible-light response of TiO2 and its charge carrier lifetimes through chemical modifications (Pelaez et al., 2012; Rawal et al., 2013; Jo and Natarajan, 2015). There is an increasingly urgent need to develop novel visible-light activated photocatalysts with high quantum efficiency, which can generate abundant quantities of reactive species for the oxidation of organic contaminants. Graphitic carbon nitride (g-C3N4) exhibits reliable chemical and thermal stability; thus, it has strong potential to serve as a photocatalyst for myriad applications. Its unique optical and electronic properties, along with a band gap of 2.7 eV, endow it with the capacity to harvest solar energy (W. X et al., 2009). Nevertheless, g-C3N4 suffers from a narrow range of visible-light response, low specific surface area, rapid charge recombination, and slow charge transport, which inhibits its photocatalytic activity (Cao et al., 2015). Various strategies have been explored to overcome these drawbacks, including doping with metal and nonmetal elements (Wang et al., 2010), vacancies (Li et al., 2018a; Cao et al., 2019a), morphological control (Zheng et al., 2015), the construction of heterojunctions with other semiconductors (Hou et al., 2013a; Cao et al., 2019b), and combining with carbon materials (Dong et al., 2012; Fang et al., 2016). Structurally, g-C3N4 consists of heptazine rings with six nitrogen lone pair electrons, which are capable of complexing Fe (II) and Fe (III). Furthermore, it has been demonstrated that carbon materials coupled with g-C3N4 can increase the population of surface active sites and accelerate charge transfer. Graphene comprises a single layer of sp2-conjugated carbon atoms that are packed within a honeycomb lattice, which has garnered tremendous attention due to its high conductivity, superior electron mobility, and high specific surface area (Xiang et al., 2012). It has been considered as an alternative functional material that serves as a photogenerated electron carrier for semiconductors under visible light irradiation (Zhu et al., 2012; Yang et al., 2016; Pan et al., 2019). Although the photocatalytic performance of g-C3N4 has been somewhat improved by the aforementioned modifications to some extent, g-C3N4-based photocatalysts with regard to visible light activity remain unsatisfactory for practical applications. Photocatalytic performance maybe significantly accelerated via the introduction of oxidants (e.g., H2O2, peroxydisulfate (PDS), peroxymonsulfate (PMS)) to yield reactive species (e.g., hydroxyl radical (%OH), sulfate radical (SO4%−)) (Duan et al., 2015; Gao et al., 2017). As a simple peroxide, PMS has the capacity to serve as a radical precursor. Electrons transferred from the surfaces of g-C3N4 complexes may initiate the generation of SO4%− radicals. As a highly reactive species, SO4%− (E0 (SO4%−/SO42−) = 2.5–3.1VNHE) holds a great advantage for

2. Experimental 2.1. Materials Trimethoprim (99%) (see Fig.S1 for molecular structure) was purchased from Sigma–Aldrich (USA). Other chemicals, suppliers and purities are listed in the Supplementary Data, Text S1. 2.2. Synthesis of Fe-g-C3N4/rGO samples Fe-g-C3N4 was synthesized based on a method described in a previous study (Liu et al., 2017). Typically, 4 g dicyandiamide and a certain amount of FeCl3·6H2O were introduced into a ceramic crucible that contained 30 mL distilled water. This was followed by adjusting to pH = 2 using HCl and placing in an 80 °C water bath to remove the water. The obtained product was placed in a muffle furnace to 550 °C at a heating rate of 5 °C min−1 for 4 h. The Fe-g-C3N4/rGO composites were prepared following the same process as Fe-g-C3N4 by adding different molar ratios of the rGO samples (0.1, 0.2, 0.5, 1, 2 and 2.5 wt% rGO/Fe-g-C3N4) into the mixture that contained dicyandiamide (4 g) and 0.2 wt% of FeCl3·6H2O. 2.3. Characterization Scanning electron microscopy (SEM) was performed using a Hitachi S4800 to investigate the surface morphology of the samples. The crystal planes and fringes of the samples were characterized through transmission electron microscopy (TEM) with a JEM-2100 F. The crystallographic structures of the samples were determined by X-ray diffraction (XRD) using a Rigaku Ultimate IV diffractometer under Cu Kα radiation. To identify the functional groups of the samples, Fourier transform infrared (FT-IR) spectra were carried out using a Nicolet FTIR 6700 spectrometer. The chemical states of the different elements of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Scientific). The UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded with a UV-2550, Shimadzu UV–vis spectrometer. A transient steady-state (FLS980) fluorescence spectrometer was employed to measure the photoluminescence (PL) spectrum and fluorescence lifetime of the samples. Electrochemical measurements including transient photocurrent, electrochemical impedance spectroscopy (EIS) and Mott–Schottky plots were performed using a CHI660E electrochemical analyzer with a three-electrode system. The operational details are presented in the Supplementary data, Text S2. Zeta potential measurements were obtained by dynamic light-scattering analysis using a Zetasizer Nano ZS (Malvern Instruments). The electron spin resonance (ESR) signals of the radicals were captured using a Bruker model ESR JES-FA200 spectrometer with 5, 5-dimethyl-12

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pyrroline-N-oxide (DMPO) for the detection of %OH, SO4%−, and O2%−, and 4-oxo-2, 2, 6, 6-tetramethylpiperidine (TEMP) to detect 1O2.

C3N4, 0.2 %Fe-g-C3N4, rGO, 0.2 % Fe-g-C3N4/rGO composites. The diffraction peak at 27.8° was attributed to the interlayer stacking of the conjugated aromatic rings, which were indexed to the (002) planes (Lin et al., 2015). The weaker diffraction peak at 12.8° was assigned to the in-plane reflection, corresponding to the (100) diffraction planes (Zheng et al., 2015), which were in accordance with the g-C3N4 reported in previous studies. A weaker intensity of the main peak of the 0.2% Fe-g-C3N4 compared with g-C3N4, which revealed no other Fe oxide peak. This suggested that the Fe was successfully integrated into the g-C3N4 structure. The FT-IR spectra of the synthesized materials are illustrated in Fig. 2b. The peak at 3448 cm−1 in the rGO spectra corresponded to the stretching mode of the structural OeH group. The peak at 1636 cm−1 was ascribed to the C]O stretching mode of the COOH group. The peaks at 1266 cm−1 and 1028 cm−1 were attributed to C–O in the C–OH and CeOeC groups. There were no obvious changes of vibrational spectra observed between g-C3N4 and 0.2% Fe-gC3N4. The absorption bands in the range of from 1200-1700 cm-1 were derived from the stretching of the C–N heterocycles. The peaks positioned at 1409 cm−1 and 1240 cm−1 were assigned to the trigonal (N(C)3) and bridging CeNHeC units (Lu et al., 2014). The peak ranging from 3000 to 3800 cm−1 corresponded to the stretching mode of the free amino groups. The sharp band at 809 cm-1 arose from the vibration of tri-s-triazine units (Liu et al., 2016). The 0.2 % Fe-g-C3N4/rGO sample exhibited almost the identical characteristic features as g-C3N4, which confirmed that the structural integrity of the g-C3N4 remained intact following the incorporation of rGO. To investigate the chemical composition and chemical state of the constituent elements of the g-C3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO samples, X-ray photoelectron spectroscopy (XPS) measurements were performed (Fig. 3). It was observed that the XPS spectrum of C1 s was deconvoluted into two peaks occurring at 284.8 and 288.4 eV for all samples. A stronger peak at 284.8 eV (g-C3N4 83.8%, Fe-g-C3N4 84.4%, Fe-g-C3N4/rGO 72.4%) was assigned to the sp2 CeC bonds of graphitic carbon with the incorporation of rGO (Ma et al., 2014). The peaks at 288.4 eV (g-C3N4 16.2%, Fe-g-C3N4 15.6%, Fe-g-C3N4/rGO 21.4%) were attributed to sp2 hybridized carbon in N-containing rings. For the Fe-g-C3N4/rGO sample, the peak at 285.7 eV were due to the existence of CeOH (6.2%). This result was consistent with the high resolution O1 s spectrum of the Fe-g-C3N4/rGO, wherein the peaks centered at 532.3 eV corresponded to C–O (Wang et al., 2013a). In the N1 s region of the deconvoluted peaks at 398.9 eV were ascribed to hybridized aromatic N atoms bound to C atoms in the triazine units (C–N = C, gC3N4 68.5%, Fe-g-C3N4 76.2%, Fe-g-C3N4/rGO 77.2%). The peak at 400.5 eV was derived from a tertiary nitrogen (N-(C)3, g-C3N4 31.5%, Fe-g-C3N4 23.8%, Fe-g-C3N4/rGO 22.8%) (Shao et al., 2017). A slight shift between the binding energies of the Fe-g-C3N4 and Fe-g-C3N4/rGO composites inferred that the chemical state of hybridized N changed after the decoration of Fe. The six lone-pair electrons at the N site of the g-C3N4 occupied the orbital of the central Fe, resulting in the decreased electron density of N though the formation of Fe-N moieties (Hu et al., 2017). In the high resolution XPS spectrum of the Fe 2p (Fig. S3), the peaks at 723.8 and 710.7 eV belonged to Fe 2p1/2 and Fe 2p3/2, respectively. The above results confirmed that Fe (III) was coordinated with the N atom in the formation of the Fe-N bonds, and that g-C3N4 was successfully hybridized with rGO.

2.4. Catalytic activity tests The degradation of TMP was performed in a photocatalytic reaction system (XPA-7, Nanjing XUJ Co. Ltd.) displayed in Fig. S2. A certain number of the Fe-g-C3N4/rGO samples were dispersed in 50 mL of a 0.02 mM TMP aqueous solution under magnetic stirring for 30 min to achieve adsorption equilibrium prior to irradiation. Subsequently, a certain quantity of the PMS was introduced into the photoreactor and illuminated with a Xenon lamp (350 W, with a 420 nm cut-off filter). Afterwards, 1 mL samples were periodically extracted from the suspension and filtered through a 0.22 μm Millipore filter. The concentration of TMP was determined by high performance liquid chromatography (HPLC) analysis. All of the experiments were performed in triplicate to assure accurate data acquisition and the error bars represent standard deviations (with 95% confidence intervals). 2.5. Analytical methods High-performance liquid chromatography (HPLC, Waters, USA) with a Hypersil C18 column (250 × 4.6 mm, 5 μm) was employed to measure the concentrations of the TMP at their maximum absorption wavelength of 239 nm. The mobile phase for TMP was acetonitrile: 0.2 % formic acid solution (35:65) at a flow rate of 1 mL·min−1. The degradation intermediates were detected with a HRAM LC/MS/ MS. Detailed information on the intermediate detection are presented in Text S3 and Table S1-S3. The total organic carbon (TOC) was measured via a TOC-VCPH Analyzer (Shimadzu, Japan). QSAR analysis calculated by the Ecological Structure–Activity Relationship Model (ECOSAR) program was carried out to estimate the acute toxicity for fish, daphnid, and green algae (Kuang et al., 2013). 2.6. Theoretical calculation method Molecular orbital calculations were conducted via the B3LYP hybrid functional with the Grimme's dispersion-correction D3 and the double split valence basis set 6-31+G (d, p). The solvent effects were taken into consideration using the integral equation formalism polarizable continuum model (IEFPCM). 3. Results and discussion 3.1. Characterization The morphologies of the as-prepared g-C3N4, Fe-g-C3N4, and Fe-gC3N4/rGO were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1a–d display SEM images of the g-C3N4, Fe-g-C3N4, rGO, and Fe-g-C3N4/rGO composites. It may be seen that the g-C3N4 and Fe-g-C3N4 exhibited a layered structure, while rGO displayed a two-dimensional network structure. The Fe-g-C3N4 samples were combined with the 2D structure of rGO to form a well-dispersed layered sheet structure. Fig. 1e–f depict TEM images of the Fe-g-C3N4/rGO. The Fe-g-C3N4/rGO samples revealed an overlapping layered structure (Fig. 1e). Otherwise, the lattice fringes with an interplanar distance of 0.326 nm corresponding to the (002) plane of the Fe-g-C3N4/rGO composites were observed (Fig. 1f). The above results demonstrated good interfacial contact between the Fedoped g-C3N4 and rGO. The inset of Fig. 1d depicts the selected area electron diffraction (SAED) patterns of the Fe-g-C3N4/rGO composites. As can be seen, the SAED pattern of Fe-g-C3N4/rGO exhibited a diffuse ring, which was indexed to the characteristic (002) planes of Fe-gC3N4/rGO (Hou et al., 2013b). Fig. 2a reveals the X-ray diffraction patterns of the synthesized g-

3.2. Catalytic activity The catalytic activity of the Fe-g-C3N4/rGO composites with different rGO loadings were investigated via the elimination of TMP under visible light irradiation. It can be seen that g-C3N4, 0.2 % Fe-g-C3N4, 0.2 % Fe-g-C3N4/2 wt% rGO with and without PMS composites had almost no ability for the degradation of TMP without visible light irradiation (Fig. S4). Fig. 4a reveals the variations in TMP concentrations under various photocatalytic processes under visible light irradiation. The negligible removal of TMP was observed in the vis/PMS process. Only 3

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Fig. 1. SEM images of (a) g-C3N4; (b) Fe-g-C3N4; (c) rGO; (d) Fe-g-C3N4/rGO; and TEM images of (e)-(f) Fe-g-C3N4/rGO.

˜18% of the TMP was removed under vis/g-C3N4 and vis/Fe-g-C3N4 processes. Compared to the g-C3N4, the Fe-g-C3N4 composite demonstrated a potential capacity for the activation of PMS under visible light irradiation. This suggested that Fe species can result in the formation of

Fe-N bonds, which may serve as electron trapping sites for the activation of PMS. However, the TMP removal efficiency was still unsatisfactory. The incorporation of rGO significantly facilitated the catalytic performance of the Fe-g-C3N4 mediated PMS system. The

Fig. 2. (a) XRD patterns; (b) FT-IR spectra of g-C3N4, Fe-C3N4, rGO and Fe-g-C3N4/rGO composites. 4

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Fig. 3. (a) XPS survey spectra and high resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) O 1s for the nanocomposites.

apparent that higher catalyst dosages accelerated the degradation of TMP. When the dosage ranged from 0.25 to 1 g L−1, the degradation rate increased accordingly from 43.4% to 75.1%. This enhancement might have been caused by higher populations of active surface resident binding sites for TMP molecules, which facilitated their exposure to reactive species (Deng et al., 2017). The effect of the PMS concentration on the degradation of TMP is illustrated in Fig. 4c. The degradation rate improved rapidly as the PMS concentration increased from 0.1 to 0.2 mM. A slight enhancement

degradation of TMP by the Fe-g-C3N4/rGO samples were enhanced with an increased rGO content from 0.1 wt% to 2 wt% in the presence of PMS. It can be also observed that the degradation of TMP was only slightly increased when the amount of RGO was up to 2.5 wt%. The higher rGO content in the Fe-g-C3N4/rGO samples provided adequate electron storage sites to inhibit the recombination of electron-hole pairs and to promote the generation of reactive species. Fig. 4b reveals the effects of the catalyst dosage on the degradation of TMP in Fe-doped g-C3N4/rGO-mediated PMS systems. It was

Fig. 4. (a) Photocatalytic activities of TMP degradation under various processes (catalysts =0.5 g L−1, [PMS] =0.2 mM); (b) Effects of catalyst dose; (c) Effects of PMS concentration; (d) Effects of pH. (Experimental conditions: [TMP] =0.02 mM, pH = 6.0.) Uncertainties represent one standard deviation from triplicate experiments.

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The deactivation of excited g-C3N4 was analyzed by monitoring its emission decay. Fig. 6b depicts the emission decay of g-C3N4, Fe-gC3N4, and Fe-g-C3N4/rGO. The pure g-C3N4 exhibited emission decays with average lifetimes of 2.27 ns. The averaged lifetime of the Fe-gC3N4 and Fe-g-C3N4/rGO decreased to 1.03 ns and 0.86 ns, respectively. The decreased lifetimes of the charge carriers might be relevant to charge transfer, and the charge-transfer rate constant could be estimated by Eq. (1) (Kongkanand et al., 2008; Gong et al., 2018; Zhang et al., 2018):

occurred when the PMS concentration continued to increase up to 0.3 mM. On one hand, the higher the PMS concentration, the more reactive radicals were generated for the removal of the TMP target compound; on the other hand, excessive PMS concentrations were detrimental to the sustainable generation of the active radicals responsible for the reaction, due to its self-quenching effects (Das, 2017). The role of pH is complex for the oxidation of TMP under Fe-doped g-C3N4/rGO- mediated PMS systems. The surface charge of the sample, as well as the speciation of the TMP may be altered due to changes in pH. Fig. 4d presents the effects of the initial pH on the degradation of TMP. As revealed, the optimal TMP degradation rate occurred under acidic conditions. The zeta potential of the Fe-doped g-C3N4/rGO composite was investigated to clarify the influence of pH on the degradation of TMP. As presented in Fig. S5b, the pHpzc of 0.2 % Fe-doped g-C3N4/1 wt% rGO was 3.2, which meant that negative charges existed on the surface of the composite at pH > 3.2. Hence, the high positive charge on the surface of the Fe-doped g-C3N4/rGO composite likely attracted negatively charged PMS due to HSO5− being the dominant PMS species under acidic conditions (Guan et al., 2011). This might have facilitated electrostatic interactions between negative radicals and protonated TMP (pKa = 3.2, pKa = 7.1, Fig. S5a) (Hu et al., 2010). Under basic conditions, most of the TMP were presented as anionic species, where the repulsion interactions between the Fe-doped g-C3N4/ rGO composites and TMP resulted in the unsatisfactory degradation of TMP.

ket =

1 1 − τ (Fe − g − C3 N4 orFe − g − C3 N4 /rGO ) g − C3 N4

(1)

The obtained electron-transfer rate constants were 5.30 × 108 s−1 and 7.22 × 108 s-1 for Fe-g-C3N4 and Fe-g-C3N4/rGO, which suggested efficient interfacial charge transfer in both composites. Photoelectric current and electrochemical impedance spectroscopy (EIS) were also performed to investigate the separation efficiency of the photogenerated electron-hole pairs of the as-prepared composites. Fig. 6c depicts the transient photocurrent responses of g-C3N4, Fe-gC3N4, and Fe-g-C3N4/rGO. Under visible light irradiation, the generated photocurrent for the Fe-g-C3N4/rGO composite was higher than for gC3N4 and Fe-g-C3N4. This indicated that the Fe-g-C3N4/rGO composite possessed a more potent capacity for the separation of electron-hole pairs. Fig. 3c shows the EIS changes of g-C3N4, Fe-g-C3N4, Fe-g-C3N4/ rGO, and Fe-g-C3N4/rGO/PMS. The smaller arc in the EIS Nyquist plot represents a smaller charge-transfer resistance (Huang et al., 2017a). It may be seen that the relative arc sizes of the four electrodes were Fe-gC3N4/rGO/PMS > Fe-g-C3N4/rGO > Fe-g-C3N4 > g-C3N4, which suggested that the introduction of PMS enhanced the efficacy of electron-hole separation. The electronic band structures of the samples were further determined by Mott-Schottky plots. As shown in Fig. 6d, the positive slopes in the Mott-Schottky plots at 0.5 and 1 kHz frequencies exhibited the n-type semiconductor features of the three samples. The calculated that the VFB from the x-intercept of the Mott-Schottky plots for the gC3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO were -1.08, -1.11, and -1.18 V (vs.NHE), respectively. For n-type semiconductors, the flat-band potential was 0-0.1 V more positive than the conduction band potential (Huang et al., 2016), which meant that the conduction band potentials of g-C3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO were negatively shifted to -1.18, -1.21, and -1.28 eV. Consequently, the VCB of g-C3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO were more negative than the redox potential of SO4%−/HSO5−, which satisfied the thermodynamic requirements for the activation of PMS by photogenerated electrons to produce SO4%− (Gao et al., 2016). Combined with the band gap energy obtained from the UV–vis DRS spectra, the band gap of the three samples were 2.70 eV. Based on the empirical formula of Eg = EVB-ECB, the valance band position (EVB) of the g-C3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO were calculated to be 1.42, 1.39, and 1.32 eV, respectively.

3.3. Optical and photoelectrochemical properties The optical properties of the g-C3N4, Fe-g-C3N4, and Fe-g-C3N4/rGO samples were investigated through UV–vis diffuse reflectance spectra (DRS) measurements. As shown in Fig. 5a, an absorption edge appeared at ˜450 nm across all samples. With the doping of Fe and the introduction of rGO, the Fe-g-C3N4 and Fe-g-C3N4/rGO composites exhibited a similar absorption edge, relative to that of g-C3N4. Fe-doped gC3N4 incorporated with rGO extended to the broader visible light region. The band gap energy is obtained by plotting (ahv)2 vs. hv, where a is the absorption co-efficient and hv is the photon energy (Tauc, 1970). The band gap energies (Fig. 5b) estimated from the intercept of the tangent to the plot were 2.70 eV for all samples. PL was employed to elucidate the transfer efficiency of photogenerated charge carriers to examine the doping of Fe and the incorporation of rGO into g-C3N4 during the photocatalytic process. As shown in Fig. 6a, there was found to be a dramatic decrease in the PL intensity of Fe-g-C3N4/rGO in contrast to g-C3N4 and Fe-C3N4, while the PL intensity of Fe-g-C3N4 was weaker than for g-C3N4. The weaker PL intensity indicated a lower probability for the recombination of photoinduced electrons-holes (Chen et al., 2012; Wang et al., 2013b). Therefore, the doping of Fe and incorporation of rGO could effectively inhibit the recombination of photogenerated electrons-holes, which facilitated the photocatalytic performance of g-C3N4.

Fig. 5. (a) UV–vis DRS spectra of the as-prepared composites; (b) the corresponding Tauc plot. 6

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Fig. 6. (a) PL spectra; (b) Time-resolved transient PL decay; (c) photocurrent responses; (d) EIS Nyquist plots; Mott-Schottky plots (e) g-C3N4, (f) Fe-C3N4 and (g) Feg-C3N4/rGO.

ESR techniques were also performed to further demonstrate the reactive species in the Fe-g-C3N4/rGO/vis mediated PMS system. As depicted in Fig. 7b, the characteristic peaks with a relative intensity of 1:1:1:1 were indexed to the DMPO-O2%− adducts (Chen et al., 2017). It may be observed that the intensity of DMPO-O2%− adducts in the Fe-gC3N4/rGO/vis mediated PMS system was stronger compared to the Feg-C3N4/PMS/vis system. For 0.2 % Fe-g-C3N4/1 wt % rGO /PMS, the intensity of DMPO-%OH and DMPO-SO4%− adducts gradually increased from 5 min to 10 min (Fig. 7c). A typical peak of the TEMP-1O2 adducts with an intensity ratio of 1:1:1 (Liu et al., 2017) were observed in both the Fe-g-C3N4 and Fe-g-C3N4/rGO mediated PMS systems. As shown in Fig. 7d, the intensity of TMPO-1O2 adducts in the Fe-g-C3N4/rGO mediated PMS system were stronger than that of the Fe-g-C3N4/rGO system. It was established that 1O2 could be generated by the self-decomposition of PMS (Eq. (2)). Otherwise, O2%− might also be attributed to the production of 1O2 during the photocatalytic and PMS activation processes (Eq. (3)) (Zhou et al., 2015; Qi et al., 2016).

3.4. Investigation of reactive species and mechanism To identify the reactive species involved in the photocatalytic degradation of TMP under the Fe-g-C3N4/rGO/vis mediated PMS system, quenching experiments were conducted. Ethanol (EtOH, 100 mM) was employed to capture both %OH (k = 1.2–2.8 ×109 M−1s−1) and SO4%− (k = 1.6–7.7 ×107 M−1s−1), whereas tert-butyl alcohol (TBA, 100 mM) was used to capture %OH (k = 3.8–7.6 ×109 M−1s−1), as TBA % reacts with OH over 1000-fold faster than SO4%− (k = 4.0–9.1 ×105 M−1s−1) (Anipsitakis and Dionysiou, 2004; Ji et al., 2015). Further, p-benzoquinone (p-BQ, 1 mM) and Na2C2O4 (10 mM) were employed as a superoxide radical (O2%−) and hole (h+) (Li et al., 2018b), respectively. N2 purge was also performed to investigate the effect of dissolved oxygen. As shown in Fig. 7a, it was observed that the degradation of TMP was significantly inhibited when purged N2 to remove the dissolved O2 in the reaction system. Otherwise, the inhibition for the degradation of TMP ranked in the sequence of p-BQ > EtOH > Na2C2O4 > TBA, while the addition of TBA also inhibited the degradation of TMP to some extent, which suggested that O2%− was the major reactive species responsible for the degradation, while %OH, h+, SO4%− had a relatively reduced effect on the degradation of TMP. 7

HSO5− + SO52 − → SO42 − + HSO4− +1 O2

(2)

2O2⋅− + 2H2 O → H2 O2 + 2HO− +1 O2

(3)

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Fig. 7. (a) Effects of different reactive species (c) scavengers;(b) DMPO-O2%−adducts; DMPO-%OH and DMPO-SO4%− adducts; (d) TEMP-1O2 adducts (Experimental conditions: catalysts =0.75 g L−1, [PMS] =0.2 mM, initial pH = 6, [DMPO] =20 mM, [TEMP] =20 mM.).

Fig. 8. Schematic illustration of proposed photocatalytic mechanism for the Fe-g-C3N4/rGO/PMS system.

more positive than the valence band position of (OH−/%OH=2.29 eV vs. NHE) (Li et al., 2016). %OH radicals maybe transformed through a series of reactions as described by Eqs. (3) and (8) (Huang et al., 2017b). Furthermore, SO4%− might also transform to %OH through interconversion reactions (Eq. (9)). In summary, O2%− played the most important role, while h+, 1O2, SO4%−, and %OH could also be accountable for the accelerated photocatalytic degradation of TMP in the Fe-gC3N4/rGO/PMS/Vis process.

On the basis of the above discussion, a plausible mechanism diagram for the degradation of TMP under the Fe-g-C3N4/rGO/vis mediated PMS process was proposed (Fig. 8). Fe-doped g-C3N4 coupled with rGO was excited via the absorption of visible light, where after photoinduced electrons and holes were generated accordingly (Eq. (4)). Photogenerated electrons could be trapped by O2 to produce O2%−(O2/ O2%− = −0.33 eV vs. NHE) (Zhang et al., 2015). Subsequently, PMS could react with O2%− and photogenerated electrons to generate SO4%− (Eqs. (5)–(6)). The formation of Fe-N bonds provided additional active sites and served as the active center for the activation of PMS. Meanwhile, the O2%− radicals present in aqueous solution might result in the generation of 1O2, which were also derived from the self-decomposition of PMS (Eqs. (2)–(3)). Further, h+ possesses a potent capacity for the oxidization of organic compounds. However, h+ cannot directly oxidize OH− to form %OH, due to the redox potential of %OH/OH−, which is 8

− + Fe − g − C3 N4 /rGO + hv → eCB + hVB

(4)

− O2 + eCB → O2⋅−

(5)

HSO5− + O2⋅− → SO4⋅− + OH− + O2

(6)

− HSO5− + eCB → SO4⋅− + OH−

(7)

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Fig. 9. Potential degradation pathways of the vis/Fe-g-C3N4/rGO/PMS system.

H2 O2 + e− → ⋅OH + OH−

(8)

SO4⋅− + OH− → ⋅OH + SO42 −

(9)

O2⋅− / SO4⋅− / ⋅OH / h+/1O2 + TMP → products

trimethoxybenzene ring, and bridging methylene group, resulting in the formation of three different intermediates, which was consistent with previous reports (Hu et al., 2011). Radical oxidants, such as SO4%− and O2%− reacted with the methylene that bridged the trimethoxybenzene ring, which gave rise to a ketone through various pathways (Zhang et al., 2016). Based on the structural analyses of intermediates, a TMP degradation pathway was tentatively proposed (Fig. 9). The SO4%− degraded organic contaminants primarily through an electron transfer mechanism (Anipsitakis et al., 2006; Ahmed et al., 2012). The SO4%− was inclined to attack the diaminopyrimidine ring to form a diaminopyrimidine radical. Subsequently, the radical cation lost a proton from the bridging methylene carbon to generate a carbon-centered radical via the adjacent diaminopyrimidine and trimethoxybenzene rings through resonance (Anquandah et al., 2011). The carbon-centered radical cation reacted with H2O to yield hydroxylated TMP m/z 307 (TMP-PII), which could be further oxidized to m/z305 (TMP-PVIII) and transformed to m/z 277 (TMP-PV) through demethylation. The nitrogen-centered radical cation derived from the SO4%− attack transformed to m/z309 (TMP-PI). The O2%− reacted with TMP to form a superoxide intermediate, which transformed to a carbonyl moiety to produce TMPPVIII. The ensuing formation of additional intermediate compounds, appearing at m/z 336 (TMP -P VII), was attributed to hydroxylation. The addition of the electrophilic %OH to the aromatic ring resulted in the formation of a resonance-stabilized carbon-centered radical, which underwent the subsequent addition of oxygen and elimination of a hydroperoxyl radical (HO2%) to produce phenolic products (Luo et al., 2012). The formation of hydroxylated TMP was also likely derived from a direct %OH attack on the diaminopyrimidine ring to generate m/z 309 (TMP-P III). TMP-P III underwent the addition of a hydroxyl to produce dihydroxylated m/z 325 (TMP -P IV), which with further oxidation resulted in the cleavage of the pyrimidine ring to yield m/z 323 (TMP-

(10)

3.5. Theoretical calculations and transformation pathways Frontier molecular orbital theory has been successfully employed to investigate the oxidative degradation processes of organic compounds. By visualizing the HOMO and LUMO frontier orbital of TMP, it is easier to elucidate the areas of electron density and potential target sites (Figs. S6a-b). Radical reactions often occur at sites with higher frontier electron density fr = ∑i (CriHOMO )2 + ∑i (CriLUMO )2 , where in the coefficient of each atomic orbital, r is the number of carbon atoms in i:2 s, 2px, 2py, and 2pz. Further, in an electrophilic reaction, the electrons can be more easily extracted from atoms with higher values of 2 ∑i (CriHOMO )2 (Fukui et al., 1952, 1954; Lee et al., 2001). The frontier electron densities of TMP were calculated to predict the potential reaction sites for radical attack and electron extraction (Table S4). Higher ∑i (CriHOMO )2 + ∑i (CriLUMO )2 values for the TMP molecule occurred at the C (27) atom, which indicated that it was the most reasonable target for attack by hydroxyl radicals. The formation of the TMP-III transformation product could thus be assigned to the attack of hydroxyl radicals at the C (27) atom. Moreover, the N (31) and C (27) atoms possessed the higher value of 2 ∑i (CriHOMO )2 , which suggested that these positions were potential sites for SO4%− attack to extricate electrons. To elucidate the intermediates involved in the oxidation of TMP under the vis/Fe-g-C3N4/rGO/PMS system, the LC/MS/MS technique was performed. Eight intermediates are illustrated in Table S5, with the ion mass spectra of the products presented in Figs. S7(a)-(i). Hydroxylation occurred at the diaminopyrimidine ring, 9

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Declaration of Competing Interest The manuscript is approved by all authors for publication. The authours declare that thay have no competing financial interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21677040), Guangzhou Municipal Science and Technology Project (No. 201903010080). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121435. References

Fig. 10. TOC removal rate on TMP degradation. (Experimental conditions: [TMP] =0.02 mM, catalysts =0.75 g L−1, [PMS] =0.2 mM, pH = 3.0).

Ahmed, M.M., Barbati, S., Doumenq, P., Chiron, S., 2012. Sulfate radical anion oxidation of diclofenac and sulfamethoxazole for water decontamination. Chem. Eng. J. 197, 440–447. Anipsitakis, G.P., Dionysiou, D.D., 2004. Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 38, 3705. Anipsitakis, G.P., Dionysiou, D.D., Gonzalez, M.A., 2006. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ. Sci. Technol. 40, 1000–1007. Anquandah, G.A., Sharma, V.K., Knight, D.A., Batchu, S.R., Gardinali, P.R., 2011. Oxidation of trimethoprim by ferrate(VI): kinetics, products, and antibacterial activity. Environ. Sci. Technol. 45, 10575–10581. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅O− in Aqueous Solution. J. Phys. Chem. Ref. Data 17, 513–886. Cao, S., Low, J., Yu, J., Jaroniec, M., 2015. Polymeric photocatalysts based on graphitic carbon nitride. Adv. Mater. 27, 2150–2176. Cao, J., Nie, W., Huang, L., Ding, Y., Lv, K., Tang, H., 2019a. Photocatalytic activation of sulfite by nitrogen vacancy modified graphitic carbon nitride for efficient degradation of carbamazepine. Appl. Catal. B 241, 18–27. Cao, J., Pan, C., Ding, Y., Li, W., Lv, K., Tang, H., 2019b. Constructing nitrogen vacancy introduced g-C3N4 p-n homojunction for enhanced photocatalytic activity. J. Environ. Chem. Eng. 7, 102984. Chen, L., Zhang, W., Feng, C., Yang, Z., Yang, Y., 2012. Replacement/Etching route to ZnSe nanotube arrays and their enhanced photocatalytic activities. Ind. Eng. Chem. Res. 51, 4208–4214. Chen, P., Wang, F., Chen, Z.F., Zhang, Q., Su, Y., Shen, L., Yao, K., Liu, Y., Cai, Z., Lv, W., 2017. Study on the photocatalytic mechanism and detoxicity of gemfibrozil by a sunlight-driven TiO 2 /carbon dots photocatalyst: the significant roles of reactive oxygen species. Appl. Catal. B 204, 250–259. Das, T.N., 2017. Reactivity and role of SO5•- radical in aqueous medium chain oxidation of sulfite to sulfate and atmospheric sulfuric acid generation. J. Phys. Chem. A 105, 9142–9155. Deng, J., Feng, S.F., Zhang, K., Li, J., Wang, H., Zhang, T., Ma, X., 2017. Heterogeneous activation of peroxymonosulfate using ordered mesoporous Co 3 O 4 for the degradation of chloramphenicol at neutral pH. Chem. Eng. J. 308, 505–515. Dong, G., Zhao, K., Zhang, L., 2012. Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3N4. Chem. Commun. 48, 6178–6180. Duan, L., Sun, B., Wei, M., Luo, S., Pan, F., Xu, A., Li, X., 2015. Catalytic degradation of Acid Orange 7 by manganese oxide octahedral molecular sieves with peroxymonosulfate under visible light irradiation. J. Hazard. Mater. 285, 356–365. Fang, S., Xia, Y., Lv, K., Li, Q., Sun, J., Li, M., 2016. Effect of carbon-dots modification on the structure and photocatalytic activity of g-C3N4. Appl. Catal. B 185, 225–232. Fukui, K., Yonezawa, T., Shingu, H., 1952. A molecular orbital theory of reactivity in aromatic hydrocarbons. J. Chem. Phys. 20, 1653. Fukui, K., Yonezawa, T., Nagata, C., Shingu, H., 1954. Molecular orbital theory of orientation in aromatic, Heteroaromatic, and other conjugated molecules. J. Chem. Phys. 22, 1433–1442. Gao, Y., Li, S., Li, Y., Yao, L., Zhang, H., 2017. Accelerated photocatalytic degradation of organic pollutant over metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate. Appl. Catal. B 202, 165–174. Gao, Y., Zhang, Z., Li, S., Jin, L., Yao, L., Li, Y., Hui, Z., 2016. Insights into the mechanism of heterogeneous activation of persulfate with a clay/iron-based catalyst under visible LED light irradiation. Appl. Catal. B 185, 22–30. Göbel, A., Mcardell, C.S., Joss, A., Siegrist, H., Giger, W., 2007. Fate of sulfonamides, macrolides, and trimethoprim in different wastewater treatment technologies. Sci. Total Environ. 372, 361. Gong, Y., Zhao, X., Zhang, H., Yang, B., Xiao, K., Guo, T., Zhang, J., Shao, H., Wang, Y., Yu, G., 2018. MOF-derived nitrogen doped carbon modified g-C 3 N 4 heterostructure composite with enhanced photocatalytic activity for bisphenol A degradation with peroxymonosulfate under visible light irradiation. Appl. Catal. B. Guan, Y.H., Ma, J., Li, X.C., Fang, J.Y., Chen, L.W., 2011. Influence of pH on the formation of sulfate and hydroxyl radicals in the UV/peroxymonosulfate system.

PVI). This can be supported by the fact that the oxidation of the C]C double bonds generated hydroxyl aldehyde, ketone, and carboxylic acid products (Hu et al., 2011). 3.6. Mineralization and toxicity assessment of TPs by ECOSAR The TOC removal rate was performed to investigate the mineralization of TMP in the vis/Fe-g-C3N4/rGO/PMS system. As illustrated in Fig. 10, the TOC removal rate of TMP increased as the reaction time was prolonged, indicating that extended reaction time might be beneficial for obtaining an improved TOC removal rate of TMP. To demonstrate the different responses toward TMP and its transformation products (TPs) from various species, a QSAR analysis via the ECOSAR program was employed to estimate acute toxicity of fish, daphnids, and green algae (Table S6). As shown, the LC50, LC50 and EC50 obtained for TMP for fish, daphnid and green algae were 319.370 mg/L, 2.134 and 6.630 mg/L, respectively. The LC50 and EC50 values for fish and green algae for all of the transformation products determined exhibited lower toxicity than those of TMP. As for LC50 values for daphnid, only the product TMP -PⅤshowed higher toxicity than TMP. Based on the results of using different species for toxicity assessment, a decreased toxicity for TMP was observed. Considering the results of mineralization and toxicity assessment, the vis/Fe-g-C3N4/ rGO/PMS system demonstrated excellent potential applications for TMP degradation in the future. 4. Conclusion A newly developed polymeric Fe-g-C3N4/rGO catalyst was prepared and employed for the degradation of the organic contaminant TMP, with PMS as a radical precursor. Under acidic condition (pH = 3), the almost complete removal of TMP was achieved in a 0.2 % Fe-g-C3N4/ 1 wt% rGO mediated PMS system. The doping of Fe and incorporation of rGO accelerated the photocatalytic performance of g-C3N4 in the presence of PMS. Increases in the quantity of the catalyst and PMS concentration promoted the degradation efficiency of TMP. ESR detection and quenching experiments revealed that O2%− were the major oxidizing species, while h+, SO4%−, and %OH, 1O2 also contributed to the reaction. PMS could be activated by primarily O2%− and photogenerated electrons to yield SO4%−. The intermediates elucidated by mass spectrometry analysis and frontier electron density calculations revealed that the primary TMP degradation pathways were hydroxylation, carbonylation, and demethoxylation. During the degradation of TMP, most of the by-products was less toxic than the initial TMP compound, with an increased TOC removal rate. 10

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