g-C3N4 composite combined with persulfate for the enhanced photocatalytic degradation of norfloxacin under visible light

Ecotoxicology and Environmental Safety 190 (2020) 110062

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Shuttle-like CeO2/g-C3N4 composite combined with persulfate for the enhanced photocatalytic degradation of norfloxacin under visible light

T

Wei Liua,b, Jiabin Zhoua,b,∗, Jun Yaoa a b

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, China School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, China

ARTICLE INFO

ABSTRACT

Keywords: g-C3N4 nanosheets CeO2 Persulfate Visible-light photocatalysis Norfloxacin degradation

In this work, the shuttle-like CeO2 modified g-C3N4 composite was synthesized and was combined with persulfate (PS) for the efficient photocatalytic degradation of norfloxacin (NOR) under visible light. Scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) emission spectra were used to characterize the structural and optical properties of the as-prepared catalysts. Active species trapping experiments demonstrated that additional sulfate radicals (·SO4-) formed upon the addition of PS which could cooperate with superoxide radicals (%O2-), holes (h+) and hydroxyl radicals (%OH) to decompose NOR. Singlet oxygen (1O2) was also formed during the reaction and acted as an important active species. The degradation products of NOR were also identified and analyzed by using LC-MS technology, and the possible degradation mechanism and pathways were proposed and discussed. This work indicated that the shuttle-like CeO2 modified g-C3N4 coupled with PS displayed promising applications in the field of pharmaceutical wastewater purification.

1. Introduction The rapid development of industrialization has led to serious environmental pollution problems. The discharge of industrial and pharmaceutical wastewater has become a major threat to the sustainable development of society (Jiang et al., 2017). Fluoroquinolones (FQs) are a class of antibiotics with a broad spectrum of antibiotic activity and are commonly used for bacterial infections in humans and animals (Osorio et al., 2019). As the second-generation of FQs, norfloxacin (NOR), has high activity against gram-negative and gram-positive bacteria and is extensively used in the treatment of infectious diseases (Zong et al., 2019). In spite of FQs have positive effects on the treatment of human health, antimicrobial resistance could emerge with prolonged exposure to these antibiotics (Huang et al., 2019). The accumulation of FQs in the environment is a potential threat to ecology and human health. Therefore, many water treatment technologies have been developed and applied. The traditional wastewater treatment technologies are generally adsorption, extraction and ozone oxidation. These techniques have the problems of incomplete removal and the production of unwanted byproducts (Luo et al., 2016; Raziq et al., 2017). In recent decades, metal oxide semiconductor photocatalytic materials represented by TiO2 have been received enormous attention because of its utilization of light energy. Semiconductor photocatalytic technology as ∗

a green, convenient energy conversion technology has great development potential. It has been found that with irradiation, these semiconductor photocatalysts can be used for hydrogen production (Bagherzadeh et al., 2017; Li et al., 2017c; Ravishankar et al., 2018; Seadira et al., 2018), carbon dioxide reduction (Cheng et al., 2017; Li et al., 2017b; Xin et al., 2017; Xiong et al., 2017), organic pollutant degradation (Appavu et al., 2018; Chen et al., 2019a; Wei et al., 2019; Zhou et al., 2019), disinfection (Chen et al., 2016; Yu et al., 2016) and so on. However, most metal oxide semiconductor photocatalysts only display their high activities under ultraviolet irradiation owing to the large bandgaps. For example, the bandgaps of TiO2, SnO2, and ZnO are approximately 3.3 eV (Naraginti and Yong, 2019), 3.6 eV (Tao and Yan, 2016) and 3.3 eV (Renganathan et al., 2019), respectively, which can only respond to UV light irradiation. Nevertheless, UV light only accounts for no more than 4% of the solar spectrum, the wide bandgaps of them greatly limit their practical photocatalytic application. Therefore, there are growing demands to develop visible light response photocatalysts for highly efficient utilization of solar (Lam et al., 2016). In recent years, inorganic non-metallic polymer material, graphitic carbon nitride (g-C3N4), has attracted great attention. Its characteristics of simple synthesis, low cost, good stability, and more importantly, the narrow bandgap (2.7 eV) make it an attractive candidate for harvesting solar energy. Among various photocatalytic applications, g-C3N4 has

Corresponding author. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, China. E-mail address: [email protected] (J. Zhou).

https://doi.org/10.1016/j.ecoenv.2019.110062 Received 12 October 2019; Received in revised form 22 November 2019; Accepted 6 December 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

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been widely used for pollutant degradation (Pang et al., 2019), hydrogen production and CO2 reduction (Huang et al., 2017; Liu et al., 2017b; Xu et al., 2017). Nevertheless, there are still demerits such as poor conductivity and fast photogenerated electron-hole recombination that limit the effective application of g-C3N4 in environmental purification. To solve these issues, many strategies, including doping with metals, non-metals, or combining with other materials such as Bi2O3 (Liu et al., 2018), SnO2 (Yang et al., 2018), Co3O4 (Shao et al., 2017), ZIF-8 (Chen et al., 2019b) and fullerene (Ouyang et al., 2017), have been employed to enhance the photocatalytic activity of g-C3N4. As a rare earth semiconductor material, CeO2 semiconductor photocatalyst has attracted considerable attention in recent years because it is non-toxic, stable, and has strong oxidizing properties. There are abundant oxygen vacancy defects in CeO2, and it has high oxygen storage capacity and the ability to uptake and release oxygen via the transformation between Ce3+ and Ce4+ (Hu and Niu, 2017). Thus CeO2 has been widely used for synthetic natural gas (Atzori et al., 2018) and supercapacitor (Luo et al., 2017). However, due to its wide bandgap and low utilization of sunlight, the application of CeO2 in wastewater treatment has been limited. In order to overcome this shortcoming and expand its application, a lot of efforts have been made to improve the performance of CeO2. Doping (Ravi and Winfred Shashikanth, 2017) and composing it with other semiconductors (Feng et al., 2019; Moradi et al., 2018) are the main effective methods. Binary materials always exhibit better performance than single material, thus, combining CeO2 with g-C3N4 is a viable method of increasing solar utilization. It has been reported that the combination of the two material has enhanced photocatalytic properties (Liu et al., 2019). However, traditional photocatalytic technology still cannot meet the demand for the rapid degradation of pollutants. Recently, the combination of photocatalysis and active oxidants to efficiently and rapidly degrade pollutants has become an emerging research (Hassanshahi and Karimi-Jashni, 2018). Advanced oxidation processes (AOPs) based on persulfate (PS, S2O82−) are often used in water treatment because sulfate radicals (.SO4−) with strong oxidizing properties can be produced by activating PS (Yang et al., 2019a). Recently, researches on the high efficient degradation of contaminants by combining PS and photocatalysts have been developed (Kim et al., 2018; Yan et al., 2019). For instance, the degradation activity of g-C3N4 on bisphenol A was enhanced by the assistance of PS (Liu et al., 2017a). It has also been reported that by adding PS to the Co3O4/CeO2 photocatalytic system, the degradation efficiency of tetracycline was effectively improved (Guan et al., 2019). In addition, the CuO/CeO2 photocatalytic system obtained enhanced degradation ability for Rhodamine B with the aid of peroxymonosulfate (Li et al., 2019). The added persulfate or peroxymonosulfate eventually formed .SO4− which could assist the photocatalytic degradation. While, at present, research on PS-assisted CeO2/gC3N4 system for drug degradation is rare. Therefore, in this work, we propose to combine CeO2 modified gC3N4 with PS to achieve the purpose of efficient and rapid degradation of norfloxacin under visible light irradiation. The degradation products of norfloxacin were identified by LC-MS and the degradation pathway was also proposed. Through a series of characterization of the synthesized photocatalysts, it was proved that the composite photocatalyst could effectively separate photo-generated carriers, and the participation of PS in the photocatalytic process enhanced the photocatalytic effect. Finally, a reasonable degradation mechanism was proposed. This work provides a simple and effective solution for the purification and treatment of pharmaceutical wastewater.

Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Norfloxacin (NOR, purity ≥ 98%) was purchased from Aladdin Biotechnology Co., Ltd. (Shanghai, China). 2.1.1. Preparation of CeO2 CeO2 was synthesized via a two-step self-assembly procedure. Briefly, 0.88 g Ce(NO3)3·6H2O and 0.28 g urea were dissolved in 25 mL deionized (DI) water under vigorous magnetic stirring for 30 min, respectively. Then the urea aqueous solution was added to the cerium nitrate aqueous solution under stirring. After stirring for another 30 min, the mixed solution was transferred into a 100 mL Teflon-lined autoclave and heated at 150 °C for 24 h. After the solution cooling to room temperature, the white precipitates were collected and washed with DI water and ethanol three times in turn, and then dried them at 80 °C overnight. The slight yellow CeO2 was obtained by calcining the dried precipitates at 600 °C for 2 h. 2.1.2. Preparation of g-C3N4 nanosheets g-C3N4 was obtained by directly calcining 10 g urea in air at 530 °C for 2 h, with the heating rate of 10 °C/min. Then the as-prepared yellow solid was grounded into fine powders. 2.1.3. Preparation of CeO2/g-C3N4 composite To fabricate CeO2/g-C3N4 composite, for each sample, 0.4 g g-C3N4 powder and different mass ratio percentages (1, 5, 10 and 15%) of CeO2 were taken and added into a mixed solution of 5 mL DI water and 15 mL ethanol. The mixture was kept vigorous stirring for 3 h. After that, the mixture was dried in oven at 80 °C and then heated at 400 °C for 2 h. Then, SC/CN composites with different CeO2 content were obtained, and they are abbreviated as 1%-SC/CN, 5%-SC/CN, 10%-SC/CN, and 15%-SC/CN, respectively. And the schematic diagram of sample synthesis is shown in Fig. S1. 2.2. Catalyst characterization An X-ray diffractometer (XRD, RU-200B/D/MAX-RB, Rigaku, Japan) was used for the detection of the crystal phases of all samples. The field emission scanning electron microscope (FESEM, Ultra Plus, Zeiss, Germany) and field emission high-resolution transmission electron microscope (FHTEM, JEM-2100F, JEOL, Japan) was employed for the investigation of the microscopic morphologies and the structures of the samples. The Fourier transform infrared spectrometer (FTIR, Nicolet 6700, Thermo Fisher Scientific, USA) was utilized to detect the functional group of all the samples. The UV–visible/near-infrared spectrophotometer (UV–vis, Lambda 750 S, PerkinElmer, USA) was an instrument for studying the optical absorption properties of all the as-synthesized materials. An X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Thermo Fisher Scientific, USA) was used to determine the surface element composition and valence information of the samples. The fluorescence spectrophotometer (PL, F-380, Gangdong Sci. & Tech., China) equipped with a 300 W Xe arc lamp was employed for the photoluminescence tests of the samples at the excitation wavelength of 320 nm. The content of Ce was measured by the inductively coupled plasma optical emission spectrometry (ICP-OES, Prodigy 7, Leeman Labs, USA). Total organic carbon (TOC) was detected by a TOC detector (TOC-L, Shimadzu, Japan). Zeta potential of the composite sample was carried out on a particle potential analyzer (Zetasizer Nano, Malvern, UK). An ESR spectrometer (Bruker A300, Germany) was utilized to detect the singlet oxygen formed by the composite sample during the photocatalytic process.

2. Experimental

2.3. Catalytic performance tests

2.1. Catalyst preparation

The photocatalytic degradation experiment for NOR was carried out in a 100 mL glass beaker and the radiation source was a 150 W highpressure Xenon lamp (CEAuLight, China) with a cut-off filter of 420 nm. In each experiment, 0.05 g of photocatalyst was taken and mixed with

All the reagents were of analytical grade purity and used as received without further purification. Urea, cerium nitrate hexahydrate (Ce (NO3)3·6H2O) and sodium persulfate (PS) were purchased from 2

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Fig. 1. XRD patterns of shuttle-like CeO2, g-C3N4, and x%-SC/CN composites.

50 mL of NOR aqueous solution (10 mg/L). After stirring for 30 min in dark to reach adsorption equilibrium, a certain amount of PS was added to the above solution to make the concentration of PS was 5 mM. Then the solution was irradiated under visible light for photocatalytic degradation. During this process, every 10 min, 3 mL solution was taken out until 60 min, and a 0.22 μm filter membrane was used to filter the sample to obtain a clarified solution, then it was analyzed by a spectrophotometer (UV-mini 1240, Shimadzu, Japan) at 278 nm to get the NOR concentration. Subsequently, NOR degradation products were analyzed by a liquid chromatography-mass spectrometry (LC-MS, 6120, Agilent, USA), and the specific experimental conditions and spectra are shown in the Support Information. 3. Results and discussion 3.1. X-ray diffraction analysis The phase structures of as-prepared CeO2, g-C3N4, and SC/CN composites were characterized by XRD. As shown in Fig. 1, the XRD pattern of as-prepared CeO2 shows characteristic strong and sharp diffraction peaks that are located at 2θ = 28.40°, 33.00°, 47.36°, 56.28°, 59.14°, 69.42°, 76.68°, and 79.14°, corresponding to the (111), (200), (220), (311), (222), (400), (331) and (420) plane of the cubic fluorite CeO2 (JCPDS No. 34–0394), respectively (Li et al., 2017a). No impurity peaks are observed, indicating the synthesized CeO2 is pure. The XRD pattern of pure g-C3N4 shows the typical diffraction peaks at 13.1° and 27.3°, matching the (100) and (002) planes of g-C3N4, respectively (Wang et al., 2017). It also can be observed that all the SC/ CN composites display strong diffraction peaks of g-C3N4, while as the CeO2 content increased from 1% to 15%, the diffraction peaks of CeO2 gradually increased and became obvious.

Fig. 2. SEM images of the obtained shuttle-like CeO2 (a and b), g-C3N4 (c) and 5%-SC/CN composite (d) and TEM images of the obtained g-C3N4 (e), 5%-SC/ CN composite (f and g), and high-resolution TEM image of shuttle-like CeO2 (h).

large number of nanosheets. The morphology 5%-SC/CN composite is shown in Fig. 2(d), we can see that the shuttle-like CeO2 is wrapped in the layers of g-C3N4. TEM was also used for further exploration of the structures of the samples. As shown in Fig. 2(e), g-C3N4 shows a graphite-like extended sheet structure. The structure of 5%-SC/CN can be seen from Fig. 2(f) and (g), the shuttle-shaped CeO2 wrapped by the sheets of g-C3N4 can be observed. From the high-resolution TEM image of CeO2 exhibited in Fig. 2(h), the lattice fringe with the lattice spacing of 0.312 nm can be observed, which can be corresponded to the (111) crystal plane of CeO2.

3.2. Morphology

3.3. XPS analysis

The surface morphologies of the as-prepared CeO2, g-C3N4, and 5%SC/CN composite were investigated by SEM images. As seen in Fig. 2(a), CeO2 exhibits a uniform shuttle-like shape which is a threeplane prismatic rod with sharp angles formed at both ends. From Fig. 2(b), it can be observed that there are many scattered nanoparticles around the surface of shuttle-like CeO2, thus, the shuttle-like CeO2 can be attributed to the assembling or overlapping of CeO2 nanoparticles. Fig. 2(c) shows the morphology of g-C3N4, which is overlapped by a

XPS was employed to analyze the surface composition and chemical states of the as-prepared materials. The XPS survey spectra of CeO2, gC3N4 and 5%-SC/CN are shown in Fig. 3(a), it can be seen that the 5%SC/CN composite is composed of Ce, O, N and C elements. The highresolution XPS spectra of Ce 3d of CeO2 and 5%-SC/CN composite are shown in Fig. 3(b). It is clear that the Ce 3d spectra of both CeO2 and 5%SC/CN samples are divided into 8 peaks and Ce is present in mixed3

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Fig. 3. XPS survey spectra (a) of CeO2, g-C3N4 and 5%-SC/CN composite, high resolution XPS spectra of Ce 3d (b) and O 1s (c) of CeO2 and 5%-SC/CN samples and C 1s (d), N 1s (e) of g-C3N4 and 5%-SC/CN samples.

valence states of both Ce3+ and Ce4+. The peaks at 916.4 eV, 907.6 eV, 900.3 eV, 897.6 eV, 889.1 eV and 882.2 eV present characteristic of Ce4+ 3d state, whereas the peaks at 903.8 eV and 885.5 eV are indicative of Ce3+ 3d state in CeO2 sample (Li et al., 2017a; She et al., 2015). The coexistence of Ce3+ and Ce4+ oxidation states indicating that there are some oxygen vacancies and defects produced on the surface of the composite (Li et al., 2017a). The above peaks of Ce 3d in 5%-SC/CN sample shift to 915.4 eV, 906.7 eV, 899.8 eV, 897.3 eV, 889.4 eV, 881.5 eV, 902.5 eV and 886.0 eV, respectively, indicating that there are some interactions between CeO2 and g-C3N4. The XPS spectra of O 1s of CeO2 and 5%-SC/CN samples are shown in Fig. 3(c), there are two peaks

at 529.2 eV and 530.7 eV in the samples, which are attributed to lattice oxygen in CeO2 and the adsorbed oxygen (e.g. oxygen in hydroxyl groups) (Zou et al., 2017). The two characteristic peaks of C1s of both gC3N4 and 5%-SC/CN samples in Fig. 3(d) located at 284.8 eV and 287.6 eV are attributed to adventitious carbon from the instrument and typical sp2 C atoms bonded to N atoms in an aromatic ring (Huang et al., 2017; Li et al., 2016; Zou et al., 2017), respectively. The N 1s peaks of gC3N4 and 5%-SC/CN samples in Fig. 3(e) can be divided into three peaks at 398.3 eV, 399.6 eV, and 400.8 eV, respectively. The main peak at 398.3 eV is attributed to sp2-hybridized nitrogen (C]N–C) (Tian et al., 2015), while the peaks at 399.6 and 400.8 eV are attributed to tertiary 4

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nitrogen (N-(C)3) and amino functional groups (C2–N–H) (Qiu et al., 2017). The XPS results confirmed the interactions between CeO2 and gC3N4, which would improve the charge transfer and separation, thus enhance the photocatalytic activity. 3.4. FTIR analysis In order to study the functional group information of the samples, FTIR detection was performed and the results are shown in Fig. S2. For the FTIR spectrum of CeO2, the absorption peak at 846 cm-1 in the fingerprint region is attributed to the stretching vibration of the Ce–O bond (Negi et al., 2019). The FTIR spectrum of g-C3N4 shows strong absorption at 810 cm-1, which represents the out-of-plane vibration of the triazine unit in the g-C3N4 structure and is the characteristic absorption peak of gC3N4 (Jiang et al., 2019). The plurality of absorption peaks in the range of 1208–1637 cm-1 are attributed to the stretching vibrations of C–N and C]N (Jia et al., 2019), and another absorption peak at 3179 cm-1 belongs to the stretching vibration of the N–H bond in the amino group in the g-C3N4 structure (Yang et al., 2019c). The FTIR spectra of all SC/CN composite samples show the same absorption peaks as pure g-C3N4. This is because, on the one hand, the CeO2 content in the composite is low, and on the other hand, the characteristic absorption peak intensity of CeO2 is much weaker than that of g-C3N4. 3.5. Optical properties The optical properties of pure CeO2, g-C3N4, and SC/CN composites were characterized by the UV–vis spectroscopy technique. As depicted in Fig. S3, CeO2 showed an absorption band edge at around 440 nm and the absorption edge of g-C3N4 located at about 445 nm. All the SC/CN composites displayed similar absorption edges to g-C3N4. The absorption edges of all the as-synthesized samples were in visible light region, indicating their visible light absorption capacity. The bandgaps of as-prepared CeO2 and g-C3N4 were calculated by using the following formula (Wen et al., 2017):

Eg = 1240/

max

(1)

where Eg and λmax represent the bandgap and maximum absorption wavelength of the samples, respectively. And the bandgaps of CeO2 and g-C3N4 were calculated to be 2.82 eV and 2.79 eV, respectively.

Fig. 4. NOR degradation curves by the samples with and without the addition of PS (a) and the corresponding first-order kinetics fitting plots (b).

3.6. Photocatalytic activity

photocatalytic efficiency of NOR was reduced from 88.6% to 75.6%. The reason for the above photocatalytic results was that the photocatalytic efficiency of pure g-C3N4 and CeO2 was limited by the rapid recombination of photogenerated carriers, and the active species could not be fully involved in the photocatalytic reaction, resulting in their limited degradation efficiency for NOR. When the proper amount of CeO2 was combined with g-C3N4, due to their different bandgap structure, photogenerated carriers can be effectively separated, which could increase the photocatalytic activity of the composite material. When PS was added into the system, the photocatalytic performances of the samples were all improved. This is because, PS could give S2O82ions, which could react with photogenerated electrons to produce %SO4with strong oxidizing ability, and rapid degradation of NOR in a short time was achieved by simultaneous action of multiple active species. Furthermore, Langmuir-Hinshelwood pseudo-first-order kinetics model was used to evaluate the efficiency of the as-prepared photocatalysts. The equation was shown as below:

Photodegradation of NOR under visible light was used to evaluate the photocatalytic performance of g-C3N4, CeO2, and SC/CN composite catalysts. In order to study the effect of PS on the NOR degradation in this SC/CN photocatalytic system, the NOR degradation experiments by adding PS into the system were also carried out. The ratio of C/C0 is used to represent the photocatalytic efficiency of the different catalysts. Wherein C is the concentration of NOR at different photocatalytic times, and C0 is the concentration of NOR before photocatalysis. As shown in Fig. 4(a), almost no NOR decomposition was observed by using only PS in 60 min. The NOR degradation efficiency was also very poor within 60 min, which was only 24.3% and 13.8% when only gC3N4 and CeO2 was used. The 5%-SC/CN composite showed a better NOR degradation ability than pure g-C3N4 and CeO2 with 46.1% NOR was degraded within 60 min. It can be clearly seen that when 5 mM PS was added into the system, the degradation efficiency of NOR was significantly improved. Within 60 min, in g-C3N4 + PS system, 60.9% NOR was degraded, and in CeO2 + PS system, 52.7% NOR was degraded. The SC/CN composites + PS systems showed stronger photocatalytic properties than that of pure g-C3N4 and CeO2. When the mass ratio of shuttle-like CeO2 to g-C3N4 increased from 1% to 5%, the photocatalytic activity of the composite was enhanced, and the NOR degradation efficiency increased from 73.9% to 88.6%. When the mass ratio of CeO2 to g-C3N4 continued to increase from 5% to 15%, the

ln(C / C0 ) =

kt

(2)

Where C0 is the concentration of NOR before irradiation, and C is the concentration of NOR at the irradiation time t, and k is the apparent reaction rate constant. Fig. 4(b) showed the corresponding kinetic curves of the photodegradation of NOR under visible light irradiation 5

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by the as-prepared samples. It can be seen from Fig. 4(b) that in the absence of PS, the degradation rate of NOR by 5%-SC/CN composite (k = 0.01013 min−1) is higher than that of pure g-C3N4 (k = 0.00736 min−1) and CeO2 (k = 0.00316 min−1). While after the addition of PS, the degradation rate of NOR by the catalysts were greatly improved. The apparent rate constant of 5%-SC/CN + PS system (k = 0.03573 min−1) for NOR degradation was the highest, which is 2.37 and 2.88 times of g-C3N4 + PS (k = 0.0151 min−1) and CeO2 + PS (k = 0.0124 min−1), respectively, and is 4.85 and 11.31 times of g-C3N4 and CeO2 without PS. The results indicated that 5%-SC/CN catalyst was the best performing sample and the introduction of PS could increase the photocatalytic degradation rate of the catalysts. The PS residual and the leaching of Ce were also measured by spectrophotometry (Liang et al., 2008) and ICP-OES, respectively. It was determined that after the photocatalytic reaction, the PS concentration was only 0.42 mM, indicating that most of the PS was consumed by photogenerated electrons during the photocatalytic reaction for NOR degradation. And the leaching of Ce of 5%-SC/CN was measured to be only 0.533 mg/L, due to the high stability of g-C3N4 and CeO2 in the composite. This indicates that the as-synthesized SC/CN composite photocatalyst could maintain a stable state and could degrade NOR efficiently. To investigate the mineralization of NOR, the TOC of NOR degradation by using g-C3N4, CeO2, and 5%-SC/CN in the presence of 5 mM PS was determined. Mineralization efficiency was expressed as 1TOC/TOC0 and the results are shown in Fig. S4. It can be seen that compared to the low mineralization efficiency by using g-C3N4 (39.4%) and CeO2 (31.8%), when using the 5%-SC/CN composite, the mineralization efficiency of NOR was the highest, reaching 63.8% at 60 min. This further illustrates that the composite photocatalyst possesses enhanced photocatalytic activity and has the ability to decompose NOR.

degradation efficiency was similar within 60 min. When the pH value increased to 10, the NOR degradation was inhibited and the degradation efficiency was poor. This is because, the pKa1 and pKa2 values of NOR are 6.22 and 8.51, respectively (Yang et al., 2019b), and the pH of zero charges point (pHzpc) of 5%-SC/CN sample was measured to be 3.4 (Fig. S5). When NOR existed in the environment with pH < 8.5, the dissociation of it was inhibited and NOR was mainly present in the molecular state in the solution. When pH < 3.4 or 3.4 < pH < 8.5, the composite catalyst was positive or negative charged, the charged catalyst could attract and degrade the molecular NOR due to the electrostatic attraction. When in the alkaline environment with pH > 8.5, the NOR was completely dissociated and was negatively charged, and the catalyst was also negatively charged. Thus, the degradation of NOR was hindered due to the electrostatic repulsion of the same negative charge. Besides, in the strongly alkaline environment, SO42− ions would be formed instead of %SO4−, thereby also suppressing the degradation process of NOR. Therefore, in a word, NOR could be effectively degraded in acidic, neutral and weakly alkaline environments and its degradation would be greatly inhibited in the strongly alkaline environment. 3.7. PL analysis To investigate the photogenerated charge recombination properties of as-prepared samples, Photoluminescence (PL) emission spectra were measured at the excitation wavelength of 320 nm (Han et al., 2017). As shown in Fig. S6, g-C3N4 exhibits a strong PL signal centering at about 450 nm, after combing with different amount CeO2, all the SC/CN composites show lower PL signals than g-C3N4, and the 5%SC/CN composite exhibits the lowest PL signal, indicating that the combination with CeO2 could impede the fluorescence of g-C3N4. The PL attribute is closely related to the recombination of photoinduced carriers in semiconductors (Shen et al., 2017). It is well known that the stronger is the PL signal, the higher is the photogenerated charge recombination. Therefore, the PL spectra results confirmed the photogenerated carriers transfer between the semiconductor materials of CeO2 and g-C3N4.

3.6.1. Effect of PS concentration PS could promote the reaction rate and improve the photocatalytic efficiency. Therefore, the effect of PS concentration (1 mM, 3 mM, 5 mM, 7 mM, and 10 mM) on the degradation efficiency of NOR was investigated in the 5%-SC/CN system. As the results shown in Fig. 5(a), it is obvious that with the concentration of PS increased from 1 mM to 5 mM, the degradation efficiency of NOR increased significantly from 52.7% to 88.6%. When the concentration of PS continued to increase from 5 mM to 10 mM, the degradation efficiency of NOR was not significantly increased. This is because the 5 mM PS has been enough to sufficiently react with the photocatalyst in the system. Therefore, 5 mM PS was selected to use in the 5%-SC/CN system.

3.8. Cycling test For practical applications of the catalysts, regeneration capacity and stability are important factors for the photocatalyst. Herein, 5%-SC/CN composite was selected for evaluating the cycling photocatalytic performance under visible light by a 3-run cycling test. The experiments were carried out in the presence of 5 mM PS in the system. After each run, the 5%-SC/CN composite photocatalyst was collected by centrifugation and was washed by DI water for several times. After dried at 80 °C, the photocatalyst was reused in a fresh NOR solution. Cyclic experiment results are shown in Fig. S7. As shown in Fig. S7, the photocatalytic activity of 5%-SC/CN for NOR degradation did not appear significant decline after three recycles. The slight decrease in catalytic performance may due to the loss of the catalyst during the recovery process. The cycling test of 5%-SC/CN photocatalyst showed that the as-prepared CeO2/g-C3N4 composite is a stable photocatalyst and could be used for wastewater treatment. Further, the photocatalytic degradation performance of different photocatalysts on NOR are summarized and shown in Table S1. It can be seen from Table S1 that compared to other previous works, the rate constant of NOR degradation in this work is the highest under the same photocatalyst dosage and NOR concentration, and the degradation efficiency of NOR is also the highest at 60 min by using the minimum power xenon lamp (150 W). Therefore, it can be proved that this CeO2/g-C3N4+PS system has a satisfactory effect on NOR degradation and can be considered as a promising candidate for photocatalytic pharmaceutical wastewater degradation.

3.6.2. Effect of photocatalyst dosage In order to evaluate the effect of the amount of photocatalyst on NOR degradation in the system, NOR degradation experiments were performed by using different doses of the photocatalyst (0.5 g/L, 1 g/L, 1.5 g/L and 2 g/L), and the results are shown in Fig. 5(b). It can be seen that as the amount of photocatalyst increased from 0.5 g/L to 1 g/L, the degradation efficiency of NOR gradually increases rapidly from 61.7% to 88.6%. When the amount of catalyst was increased from 1 g/L to 2 g/ L, the degradation efficiency of NOR increased slowly. Because when the amount of photocatalyst was too small, the number of active species involved in the NOR oxidation process was small, resulting in lower photocatalytic degradation efficiency. When the amount of the photocatalyst was too large, there was not enough PS reacted with it to accelerate the reaction. Therefore, in this system, the amount of catalyst was chosen to be 1 g/L. 3.6.3. Effect of initial pH values To investigate the effect of pH on NOR degradation efficiency in the system, NOR degradation was carried out at different initial solution pH values (from pH = 2 to pH = 10). As can be seen in the results presented in Fig. 5(c), when the pH value was in the range of 2–8, the NOR degradation exhibited a good trend and the NOR 6

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Fig. 5. NOR degradation curves under different concentration of PS (a), different catalyst dosage (b) and different pH (c).

3.9. Degradation products and pathway analysis

hydroxyl radical (%OH), respectively (Wang et al., 2015). Methanol (MeOH) was used as the scavenger for both ·OH and %SO4− (Yang et al., 2019a). It has been reported that in the %SO4− involved photocatalytic process, singlet oxygen (1O2) may form and participate in the oxidation of the target (Ahmadi and Ghanbari, 2018; Huang et al., 2020). Therefore, L-histidine was also added to the system as a scavenger for 1 O2 (Zhang et al., 2018). The trapping results are shown in Fig. 7(a). It can be found in Fig. 7(a) that when N2 and L-histidine were introduced into the 5%-SC/CN + PS system, the degradation efficiency of NOR was greatly suppressed, from 88.6% decreased to 12.8% and 16.8%, respectively. When TEOA and TBA were added, the degradation efficiency of NOR was reduced to 22.2% and 70.1%, respectively. This indicated that %O2- and 1O2 were the dominant active species, followed by h+, and %OH contributed the least to NOR degradation. When MeOH was added, the degradation efficiency of NOR was reduced to 31.8%. That proved that the addition of PS in the photocatalytic system would produce %SO4−, which played an important role in the degradation of NOR. 1O2 can be formed by the recombination of%O2- and the reaction of %O2- and ·OH (Ahmadi and Ghanbari, 2018). The presence of %O2that plays a dominant role in the photocatalytic system has prompted 1 O2 to become a major active species. From the results of ESR detection of 1O2 shown in Fig. 7(b), it can be seen that the characteristic triple peak signal was obtained, which proved that 1O2 existed in the system, and was consistent with the trapping experiment results. As discussed above, the bandgaps of g-C3N4 and CeO2 are estimated to be 2.79 eV and 2.82 eV, respectively. To understand their bandgap structure, the conduction band (CB) and valence band (VB) edge position should be calculated. The VB potential could be estimated by Mulliken electronegativity theory (Chen et al., 2017), as shown in the equation:

The degradation products (P1–P10) of NOR by 5%-SC/CN sample were identified by LC-MS, and the possible degradation pathways were inferred by analysis of the structure of them. The Mass spectra of them are shown from Figs. S8–S18 in the Support Information and the possible pathway is exhibited in Fig. 6. As shown in Fig. 6, P1 (m/z = 318) was obtained by substituting F on the NOR parent ion (m/z = 320) by a hydroxyl group. P2 (m/z = 336) was produced because the parent ion was attacked by %O2-, and P2 lost one methyl group to get P3 (m/ z = 322). Then, P4 (m/z = 294) was produced by losing one formaldehyde molecule from P3. The F group can be substituted by a hydroxyl group to form P5 (m/z = 292). Also, P4 can be attacked by % O2- to produce P6 (m/z = 279). Then P8 (m/z = 249) was obtained by losing formaldehyde molecule from P6. P8 decomposed the amino or hydroxyl group to obtain P9 (m/z = 236) and P10 (m/z = 233), respectively. And then, P11 (m/z = 160) was obtained by losing one carboxyl group and one ethyl group from P10. It can be seen that the NOR molecule was gradually decomposed by losing the groups and eventually can be mineralized into CO2 and H2O. 3.10. Possible mechanism It is well known that when the semiconductor catalyst is irradiated by light, photogenerated carriers are separated, electrons are introduced on the conduction band, and holes are formed in the valence band. Electrons have a strong ability of reduction, and holes have a strong ability of oxidation. In order to probe the main active species generated during the photocatalytic degradation process, the active species trapping experiments were carried out by using 5%-SC/CN composite in the presence of 5 mM PS. Nitrogen (N2), triethanolamine (TEOA) and tertiary butyl alcohol (TBA) were employed as the scavengers for superoxide radical (%O2−), photo-generated holes (h+), and

EVB = X

(3)

EC + 0.5Eg C

where X is the electronegativity of the semiconductor, E represents the 7

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Fig. 6. The possible pathways in NOR degradation process.

energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the bandgap of the semiconductor. The X value of CeO2 and g-C3N4 are 5.56 eV and 4.72 eV, respectively (Luo et al., 2015). The calculated VB and CB of CeO2 are −0.35 eV and 2.47 eV, respectively. In the case of g-C3N4, the top of the VB is 1.62 eV and the bottom of CB is −1.17 eV. According to the VB and CB values of CeO2 and g-C3N4 calculated above, the possible degradation mechanism for NOR degradation was proposed and shown in Fig. S19. It can be seen that the VB edge of g-C3N4 is higher than that of CeO2, whereas the CB edge of g-C3N4 is lower than that of CeO2. Therefore, in the contact surface between these two materials, the photogenerated electrons on the g-C3N4 surface can easily transfer to

h+ (CeO2) → h+ (g-C3N4)

(7) (8)

h

+

(CeO2) + OH

−

→ %OH

(9) (10) (11) (12) (13)

(14) 4. Conclusion

the CB of CeO2, while the holes on the VB of CeO2 can migrate to gC3N4. The charge transfer effectively inhibits the recombination of electron-hole pairs in the two semiconductors and thereby improves the photocatalytic efficiency. In addition, both g-C3N4 and CeO2 could use oxygen (O2) to generate %O2- because their CB potentials were greater than 0.33 eV, then formed 1O2. And because the VB potential of CeO2 is higher than 1.99 eV (Kumar et al., 2019), it could use the hydroxide in water to produce ·OH. Furthermore, with the addition of PS, S2O82− could react with photogenerated electrons and H2O to generate %SO4− and ·OH respectively. Therefore, in the whole system, %O2-, 1O2, h+, % SO4− and ·OH worked together to degrade NOR, which greatly reduced the degradation time. And the main reaction process can be described as follows: g-C3N4 + hv → e− (g-C3N4) + h+ (g-C3N4) CeO2 + hv → e e

−

−

(g-C3N4) → e

(CeO2) + h

−

(CeO2)

+

(CeO2)

This work presents a method of preparing a composite of shuttlelike CeO2 and g-C3N4 nanosheets and explores the process by using this composite photocatalyst in combination with PS to degrade NOR. The obtained composite photocatalyst exhibit enhanced degradation efficiency for NOR than pure CeO2 and g-C3N4. In addition, the addition of PS further improved the photocatalytic efficiency of norfloxacin. LC-MS analysis of degradation fragments of norfloxacin showed that norfloxacin was degraded into small molecular products by gradual shedding of functional groups, which were then oxidized to CO2 and H2O. The acceleration of NOR degradation was due to the formation of strong oxidative ·SO4- and 1O2 in the presence of PS, which worked together with ·O2-, h+ and ·OH to degrade the NOR. This work provides an effective antibiotic degradation strategy based on the CeO2 modified gC3N4 + PS system.

(4) (5) (6) 8

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References Ahmadi, M., Ghanbari, F., 2018. Degradation of organic pollutants by photoelectro-peroxone/ZVI process: synergistic, kinetic and feasibility studies. J. Environ. Manag. 228, 32–39. Appavu, B., Thiripuranthagan, S., Ranganathan, S., Erusappan, E., Kannan, K., 2018. BiVO4/N-rGO nano composites as highly efficient visible active photocatalyst for the degradation of dyes and antibiotics in eco system. Ecotoxicol. Environ. Saf. 151, 118–126. Atzori, L., Cutrufello, M.G., Meloni, D., Cannas, C., Gazzoli, D., Monaci, R., Sini, M.F., Rombi, E., 2018. Highly active NiO-CeO2 catalysts for synthetic natural gas production by CO2 methanation. Catal. Today 299, 183–192. Bagherzadeh, S.B., Haghighi, M., Rahemi, N., 2017. Novel oxalate gel coprecipitation synthesis of ZrO2-CeO2-promoted CuO-ZnO-Al2O3 nanocatalyst for fuel cell-grade hydrogen production from methanol: influence of ceria-zirconia loading. Energy Convers. Manag. 134, 88–102. Chen, C.R., Zeng, H.Y., Yi, M.Y., Xiao, G.F., Zhu, R.L., Cao, X.J., Shen, S.G., Peng, J.W., 2019a. Fabrication of Ag2O/Ag decorated ZnAl-layered double hydroxide with enhanced visible light photocatalytic activity for tetracycline degradation. Ecotoxicol. Environ. Saf. 172, 423–431. Chen, L., He, F., Huang, Y., Meng, Y., Guo, R., 2016. Hydrogenated nanoporous TiO2 film on Ti25Nb3Mo2Sn3Zr alloy with enhanced photocatalytic and sterilization activities driven by visible light. J. Alloy. Comp. 678, 5–11. Chen, W., Hua, Y.X., Wang, Y., Huang, T., Liu, T.Y., Liu, X.H., 2017. Two-dimensional mesoporous g-C3N4 nanosheet-supported MgIn2S4 nanoplates as visible-light-active heterostructures for enhanced photocatalytic activity. J. Catal. 349, 8–18. Chen, W., Kong, S., Wang, J., Du, L., Cai, W., Wu, C., 2019b. Enhanced fluorescent effect of graphitic C3N4@ZIF-8 nanocomposite contribute to its improved sensing capabilities. RSC Adv. 9, 3734–3739. Cheng, M., Yang, S., Chen, R., Zhu, X., Liao, Q., Huang, Y., 2017. Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2. Int. J. Hydrogen Energy 42, 9722–9732. Feng, K., Song, B., Li, X., Liao, F., Gong, J., 2019. Enhanced photocatalytic performance of magnetic multi-walled carbon nanotubes/cerium dioxide nanocomposite. Ecotoxicol. Environ. Saf. 171, 587–593. Guan, R., Yuan, X., Wu, Z., Jiang, L., Zhang, J., Li, Y., Zeng, G., Mo, D., 2019. Efficient degradation of tetracycline by heterogeneous cobalt oxide/cerium oxide composites mediated with persulfate. Separ. Purif. Technol. 212, 223–232. Han, Z., Yu, Y., Zheng, W., Cao, Y., 2017. The band structure and photocatalytic mechanism for a CeO2-modified C3N4 photocatalyst. New J. Chem. 41, 9724–9730. Hassanshahi, N., Karimi-Jashni, A., 2018. Comparison of photo-Fenton, O3/H2O2/UV and photocatalytic processes for the treatment of gray water. Ecotoxicol. Environ. Saf. 161, 683–690. Hu, H., Niu, X., 2017. A facile hydrothermal synthesis of large-scale skull-like CeO2 nanostructures with many holes and their optical performances. J. Mater. Sci. Mater. Electron. 28, 11306–11310. Huang, C., Wang, Y., Gong, M., Wang, W., Mu, Y., Hu, Z.H., 2020. α-MnO2/Palygorskite composite as an effective catalyst for heterogeneous activation of peroxymonosulfate (PMS) for the degradation of Rhodamine B. Separ. Purif. Technol. 230, 115877. Huang, H., Xiao, K., Tian, N., Dong, F., Zhang, T., Du, X., Zhang, Y., 2017. Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity. J. Mater. Chem. A. 5, 17452–17463. Huang, W., Qiu, Q., Chen, M., Shi, J., Huang, X., Kong, Q., Long, D., Chen, Z., Yan, S., 2019. Determination of 18 antibiotics in urine using LC-QqQ-MS/MS. J. Chromatogr. B 1105, 176–183. Jia, J., Jiang, C., Zhang, X., Li, P., Xiong, J., Zhang, Z., Wu, T., Wang, Y., 2019. Ureamodified carbon quantum dots as electron mediator decorated g-C3N4/WO3 with enhanced visible-light photocatalytic activity and mechanism insight. Appl. Surf. Sci. 495, 143524. Jiang, L., Yuan, X., Pan, Y., Liang, J., Zeng, G., Wu, Z., Wang, H., 2017. Doping of graphitic carbon nitride for photocatalysis: a reveiw. Appl. Catal. B Environ. 217, 388–406. Jiang, X., Lai, S., Xu, W., Fang, J., Chen, X., Beiyuan, J., Zhou, X., Lin, K., Liu, J., Guan, G., 2019. Novel ternary BiOI/g-C3N4/CeO2 catalysts for enhanced photocatalytic degradation of tetracycline under visible-light radiation via double charge transfer process. J. Alloy. Comp. 809, 151804. Kim, J.G., Park, S.M., Lee, M.E., Kwon, E.E., Baek, K., 2018. Photocatalytic co-oxidation of As(III) and Orange G using urea-derived g-C3N4 and persulfate. Chemosphere 212, 193–199. Kumar, A., Sharma, S.K., Sharma, G., Al-Muhtaseb, A.H., Naushad, M., Ghfar, A.A., Stadler, F.J., 2019. Wide spectral degradation of Norfloxacin by Ag@BiPO4/BiOBr/ BiFeO3 nano-assembly: elucidating the photocatalytic mechanism under different light sources. J. Hazard Mater. 364, 429–440. Lam, S.-M., Sin, J.-C., Mohamed, A.R., 2016. A review on photocatalytic application of gC3N4/semiconductor (CNS) nanocomposites towards the erasure of dyeing wastewater. Mater. Sci. Semicond. Process. 47, 62–84. Li, H., Meng, F., Gong, J., Fan, Z., Qin, R., 2017a. Structural, morphological and optical properties of shuttle-like CeO2 synthesized by a facile hydrothermal method. J. Alloy. Comp. 722, 489–498. Li, K., Zeng, X., Gao, S., Ma, L., Wang, Q., Xu, H., Wang, Z., Huang, B., Dai, Y., Lu, J., 2016. Ultrasonic-assisted pyrolyzation fabrication of reduced SnO2-x/g-C3N4 heterojunctions: enhance photoelectrochemical and photocatalytic activity under visible LED light irradiation. Nano. Res. 9, 1969–1982. Li, N., Zou, X., Liu, M., Wei, L., Shen, Q., Bibi, R., Xu, C., Ma, Q., Zhou, J., 2017b. Enhanced visible light photocatalytic hydrogenation of CO2 into methane over a Pd/

Fig. 7. NOR degradation curves by 5%-SC/CN composite with different scavengers under the addition of PS (a) and ESR spectrum of 1O2.

Acknowledgement This work was supported by the Sichuan Science and Technology Program (2019YFS0495) and the Fundamental Research Funds for the Central Universities, China (WUT: 195208001). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecoenv.2019.110062. Author contribution statement Wei Liu: Investigation, Original Draft. Jiabin Zhou: Conceptualization, Supervision, Funding acquisition, Reviewing. Jun Yao: Visualization, Editing. Declaration of interest statement The author declares that there is no conflict of interest. 9

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W. Liu, et al. Ce-TiO2 nanocomposition. J. Phys. Chem. C 121, 25795–25804. Li, Y., Li, F., Wang, X., Zhao, J., Wei, J., Hao, Y., Liu, Y., 2017c. Z-scheme electronic transfer of quantum-sized α-Fe2O3 modified g-C3N4 hybrids for enhanced photocatalytic hydrogen production. Int. J. Hydrogen Energy 42, 28327–28336. Li, Z., Liu, D., Zhao, Y., Li, S., Wei, X., Meng, F., Huang, W., Lei, Z., 2019. Singlet oxygen dominated peroxymonosulfate activation by CuO-CeO2 for organic pollutants degradation: performance and mechanism. Chemosphere 233, 549–558. Liang, C., Huang, C.F., Mohanty, N., Kurakalva, R.M., 2008. A rapid spectrophotometric determination of persulfate anion in ISCO. Chemosphere 73, 1540–1543. Liu, B., Qiao, M., Wang, Y., Wang, L., Gong, Y., Guo, T., Zhao, X., 2017a. Persulfate enhanced photocatalytic degradation of bisphenol A by g-C3N4 nanosheets under visible light irradiation. Chemosphere 189, 115–122. Liu, W., Zhou, J., Hu, Z., 2019. Nano-sized g-C3N4 thin layer @ CeO2 sphere core-shell photocatalyst combined with H2O2 to degrade doxycycline in water under visible light irradiation. Separ. Purif. Technol. 227, 115665. Liu, W., Zhou, J., Hu, Z., Zhou, J., Cai, W., 2018. In situ facile fabrication of Z-scheme leaf-like β-Bi2O3/g-C3N4 nanosheets composites with enhanced visible light photoactivity. J. Mater. Sci. Mater. Electron. 29, 14906–14917. Liu, X., Yang, D., Zhou, Y., Zhang, J., Luo, L., Meng, S., Chen, S., Tan, M., Li, Z., Tang, L., 2017b. Electrocatalytic properties of N-doped graphite felt in electro-Fenton process and degradation mechanism of levofloxacin. Chemosphere 182, 306–315. Luo, J., Zhou, X., Ma, L., Xu, X., 2015. Enhancing visible-light photocatalytic activity of gC3N4 by doping phosphorus and coupling with CeO2 for the degradation of methyl orange under visible light irradiation. RSC Adv. 5, 68728–68735. Luo, L., Cai, W., Zhou, J., Li, Y., 2016. Facile synthesis of boehmite/PVA composite membrane with enhanced adsorption performance towards Cr(VI). J. Hazard Mater. 318, 452–459. Luo, Y., Yang, T., Zhao, Q., Zhang, M., 2017. CeO2/CNTs hybrid with high performance as electrode materials for supercapacitor. J. Alloy. Comp. 729, 64–70. Moradi, B., Nabiyouni, G., Ghanbari, D., 2018. Rapid photo-degradation of toxic dye pollutants: green synthesis of mono-disperse Fe3O4-CeO2 nanocomposites in the presence of lemon extract. J. Mater. Sci. Mater. Electron. 29, 11065–11080. Naraginti, S., Yong, Y.C., 2019. Enhanced detoxification of p-bromophenol by novel Zr/ Ag-TiO2@rGO ternary composite: degradation kinetics and phytotoxicity evolution studies. Ecotoxicol. Environ. Saf. 170, 355–362. Negi, K., Umar, A., Chauhan, M.S., Akhtar, M.S., 2019. Ag/CeO2 nanostructured materials for enhanced photocatalytic and antibacterial applications. Ceram. Int. 45, 20509–20517. Osorio, A., Toledo-Neira, C., Bravo, M.A., 2019. Critical evaluation of third-order advantage with highly overlapped spectral signals. Determination of fluoroquinolones in fish-farming waters by fluorescence spectroscopy coupled to multivariate calibration. Talanta 204, 438–445. Ouyang, K., Dai, K., Chen, H., Huang, Q., Gao, C., Cai, P., 2017. Metal-free inactivation of E. coli O157:H7 by fullerene/C3N4 hybrid under visible light irradiation. Ecotoxicol. Environ. Saf. 136, 40–45. Pang, N., Lin, H., Hu, J., 2019. Photodegradation of fluazaindolizine in aqueous solution with graphitic carbon nitride nanosheets under simulated sunlight illumination. Ecotoxicol. Environ. Saf. 170, 33–38. Qiu, P., Xu, C., Chen, H., Jiang, F., Wang, X., Lu, R., Zhang, X., 2017. One step synthesis of oxygen doped porous graphitic carbon nitride with remarkable improvement of photo-oxidation activity: role of oxygen on visible light photocatalytic activity. Appl. Catal. B Environ. 206, 319–327. Ravi, S., Winfred Shashikanth, F., 2017. Preparation of Mn doped CeO2 nanoparticles with enhanced ferromagnetism. Mater. Chem. Phys. 194, 37–41. Ravishankar, T.N., de O Vaz, M., Ramakrishnappa, T., Teixeira, S.R., Dupont, J., Pai, R.K., Banuprakash, G., 2018. The heterojunction effect of Pd on TiO2 for visible light photocatalytic hydrogen generation via water splitting reaction and photodecolorization of trypan blue dye. J. Mater. Sci. Mater. Electron. 29, 11132–11143. Raziq, F., Qu, Y., Humayun, M., Zada, A., Yu, H., Jing, L., 2017. Synthesis of SnO2/B-P codoped g-C3N4 nanocomposites as efficient cocatalyst-free visible-light photocatalysts for CO2 conversion and pollutant degradation. Appl. Catal. B Environ. 201, 486–494. Renganathan, V., Balaji, R., Chen, S.M., Kogularasu, S., Akilarasan, M., 2019. Bifunctional bimetallic heterojunction material based on Al2O3/ZnO micro flowers for electrochemical sensing and catalysis. Ecotoxicol. Environ. Saf. 176, 250–257. Seadira, T.W.P., Sadanandam, G., Ntho, T., Masuku, C.M., Scurrell, M.S., 2018. Preparation and characterization of metals supported on nanostructured TiO2 hollow spheres for production of hydrogen via photocatalytic reforming of glycerol. Appl. Catal. B Environ. 222, 133–145. Shao, H., Zhao, X., Wang, Y., Mao, R., Wang, Y., Qiao, M., Zhao, S., Zhu, Y., 2017. Synergetic activation of peroxymonosulfate by Co3O4 modified g-C3N4 for enhanced

degradation of diclofenac sodium under visible light irradiation. Appl. Catal. B Environ. 218, 810–818. She, X., Xu, H., Wang, H., Xia, J., Song, Y., Yan, J., Xu, Y., Zhang, Q., Du, D., Li, H., 2015. Controllable synthesis of CeO2/g-C3N4 composites and their applications in the environment. Dalton Trans. 44, 7021–7031. Shen, H., Zhao, X., Duan, L., Liu, R., Li, H., 2017. Enhanced visible light photocatalytic activity in SnO2@g-C3N4 core-shell structures. Mater. Sci. Eng. B 218, 23–30. Tao, B., Yan, Z., 2016. In-situ synthesis of highly efficient visible light driven stannic oxide/graphitic carbon nitride heterostructured photocatalysts. J. Colloid Interface Sci. 480, 118–125. Tian, N., Huang, H., Liu, C., Dong, F., Zhang, T., Du, X., Yu, S., Zhang, Y., 2015. In situ copyrolysis fabrication of CeO2/g-C3N4 n-n type heterojunction for synchronously promoting photo-induced oxidation and reduction properties. J. Mater. Chem. A. 3, 17120–17129. Wang, J.C., Cui, C.X., Li, Y., Liu, L., Zhang, Y.P., Shi, W.N., 2017. Porous Mn doped gC3N4 photocatalysts for enhanced synergetic degradation under visible-light illumination. J. Hazard Mater. 339, 43–53. Wang, L., Ding, J., Chai, Y., Liu, Q., Ren, J., Liu, X., Dai, W.L., 2015. CeO2 nanorod/gC3N4/N-rGO composite: enhanced visible-light-driven photocatalytic performance and the role of N-rGO as electronic transfer media. Dalton Trans. 44, 11223–11234. Wei, Z.S., He, Y.M., Huang, Z.S., Xiao, X.L., Li, B.L., Ming, S., Cheng, X.L., 2019. Photocatalytic membrane combined with biodegradation for toluene oxidation. Ecotoxicol. Environ. Saf. 184, 109618. Wen, X.J., Niu, C.G., Huang, D.W., Zhang, L., Liang, C., Zeng, G.M., 2017. Study of the photocatalytic degradation pathway of norfloxacin and mineralization activity using a novel ternary Ag/AgCl-CeO2 photocatalyst. J. Catal. 355, 73–86. Xin, C., Hu, M., Wang, K., Wang, X., 2017. Significant enhancement of photocatalytic reduction of CO2 with H2O over ZnO by the formation of basic zinc carbonate. Langmuir 33, 6667–6676. Xiong, Z., Lei, Z., Xu, Z., Chen, X., Gong, B., Zhao, Y., Zhao, H., Zhang, J., Zheng, C., 2017. Flame spray pyrolysis synthesized ZnO/CeO2 nanocomposites for enhanced CO2 photocatalytic reduction under UV-Vis light irradiation. J. CO2. Util. 18, 53–61. Xu, J., Wang, Z., Zhu, Y., 2017. Enhanced visible-light-driven photocatalytic disinfection performance and organic pollutant degradation activity of porous g-C3N4 nanosheets. ACS Appl. Mater. Interfaces 9, 27727–27735. Yan, S., Shi, Y., Tao, Y., Zhang, H., 2019. Enhanced persulfate-mediated photocatalytic oxidation of bisphenol A using bioelectricity and a g-C3N4/Fe2O3 heterojunction. Chem. Eng. J. 359, 933–943. Yang, L., Bai, X., Shi, J., Du, X., Xu, L., Jin, P., 2019a. Quasi-full-visible-light absorption by D35-TiO2/g-C3N4 for synergistic persulfate activation towards efficient photodegradation of micropollutants. Appl. Catal. B Environ. 256, 117759. Yang, L.Q., Huang, J.F., Shi, L., Cao, L.Y., Liu, H.M., Liu, Y.Y., Li, Y.X., Song, H., Jie, Y.N., Ye, J.H., 2018. Sb doped SnO2-decorated porous g-C3N4 nanosheet heterostructures with enhanced photocatalytic activities under visible light irradiation. Appl. Catal. B Environ. 221, 670–680. Yang, X., Zhang, X., Wang, Z., Li, S., Zhao, J., Liang, G., Xie, X., 2019b. Mechanistic insights into removal of norfloxacin from water using different natural iron orebiochar composites: more rich free radicals derived from natural pyrite-biochar composites than hematite-biochar composites. Appl. Catal. B Environ. 255, 117752. Yang, Y., Mao, B., Gong, G., Li, D., Liu, Y., Cao, W., Xing, L., Zeng, J., Shi, W., Yuan, S., 2019c. In-situ growth of Zn-AgIn5S8 quantum dots on g-C3N4 towards 0D/2D heterostructured photocatalysts with enhanced hydrogen production. Int. J. Hydrogen Energy 44, 15882–15891. Yu, H., Song, L., Hao, Y., Lu, N., Quan, X., Chen, S., Zhang, Y., Feng, Y., 2016. Fabrication of pilot-scale photocatalytic disinfection device by installing TiO2 coated helical support into UV annular reactor for strengthening sterilization. Chem. Eng. J. 283, 1506–1513. Zhang, J., Zhao, X., Wang, Y., Gong, Y., Cao, D., Qiao, M., 2018. Peroxymonosulfateenhanced visible light photocatalytic degradation of bisphenol A by perylene imidemodified g-C3N4. Appl. Catal. B Environ. 237, 976–985. Zhou, Q., Wang, M., Tong, Y., Wang, H., Zhou, X., Sheng, X., Sun, Y., Chen, C., 2019. Improved photoelectrocatalytic degradation of tetrabromobisphenol A with silver and reduced graphene oxide-modified TiO2 nanotube arrays under simulated sunlight. Ecotoxicol. Environ. Saf. 182, 109472. Zong, L., Jiao, Y., Guo, X., Zhu, C., Gao, L., Han, Y., Li, L., Zhang, C., Liu, Z., Liu, J., Ju, Q., Yu, H.D., Huang, W., 2019. Paper-based fluorescent immunoassay for highly sensitive and selective detection of norfloxacin in milk at picogram level. Talanta 195, 333–338. Zou, W., Shao, Y., Pu, Y., Luo, Y., Sun, J., Ma, K., Tang, C., Gao, F., Dong, L., 2017. Enhanced visible light photocatalytic hydrogen evolution via cubic CeO2 hybridized g-C3N4 composite. Appl. Catal. B Environ. 218, 51–59.

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