English

• 0.   Introduction

  The release of antibiotics at sublethal concentrations from clinical facilities and maricultural operations contributes to an increased likelihood of antibiotic resistance gene (ARG) formation in pathogenic bacteria. This selective pressure encourages the assimilation and dissemination of ARGs within marine ecosystems, posing a significant threat to public health. Oxytetracycline (OTC), among various antibiotics, is commonly employed as a prophylactic agent in mariculture ponds due to its broad-spectrum efficacy and affordability^[1]. However, its discharge into water bodies can result in severe ecological damage to offshore environments and potential harm to human well-being^[2-3]. Photocatalysis has garnered significant interest as an innovative and environmentally friendly purification technology when compared to conventional membrane filtration^[4], adsorption^[5], electrochemical oxidation^[6], Fenton reaction^[7], and ozonation^[8]. This is primarily because the degradation of pollutants occurs on a semiconductor photocatalyst, which can be regenerated concurrently under light irradiation, such as solar light. Among the various semiconductor photocatalysts, graphite carbon nitride (g-C₃N₄) is regarded as a particularly promising candidate, due to its combination of unique properties including suitable band gap, non-toxicity, and photochemical stability^[9-11]. Nevertheless, the primitive g-C₃N₄ remains plagued by problems such as rapid charge recombination, high band gap, low surface area, and unsatisfactory visible light harvesting, which limits its large-scale applications. To overcome these shortcomings approaches such as controlled nanomorphology, non-metallic doping, metal loading, carrier coupling, and heterostructure construction have been applied to further optimize the structure and composition of g-C₃N₄ to achieve improved photocatalytic efficiencies^[12-15]. Of these, the increase in specific surface area was considered to be the simpler and more practical method of modifying g-C₃N₄. Liu et al. fabricated mesoporous g-C₃N₄ by a molten salt-assisted silica aerogel template method, which achieved 90.9% efficiency in photocatalytic degradation of RhB^[16]. Jin et al. used a simple two-step condensation method to prepare g-C₃N₄ nanotubes with high specific surface area and defects, which enhanced the photocatalytic activity of RhB under visible light irradiation^[17].

  In addition to these solutions, there has been a recent increase in attention to modulating g-C₃N₄ with carbon materials to create g-C₃N₄/carbon hybrids or heterojunctions due to their significantly enhanced features on g-C₃N₄ in transporting reagents in bulk, quantity of active sites, optical and electric properties^[18-19]. The carbon-based materials currently developed for composite semiconductor photocatalysts include carbon fibers, carbon nanotubes, reduced graphene oxide, mesoporous carbon, etc. Liu and Li et al. proposed a graphene oxide (GO) and reduced graphene oxide (rGO)/g-C₃N₄ structure that significantly improved the efficiency of photogenerated electron-hole separation^[20-21]. The metal-free two-dimensional (2D) g-C₃N₄/graphdiyne heterojunction designed by Lu et al. can shorten the transfer distance of photogenerated carriers and increase the hole mobility of g-C₃N₄ due to the high π-conjugated structure of graphdiyne^[22]. Even though some desirable improvements in photocatalytic activities have been achieved, most of them require preparation with difficult synthetic steps, high cost of raw materials, extensive time and energy consumption, and cannot be produced at scale. In contrast, biomass-derived carbons from renewable wastes have been recognized as attractive candidates for the fabrication of functional carbon materials with superior properties due to their greenness, reproducibility, low cost, versatility, and electrical conductivity. For this reason, biomass-derived carbons have been applied in wastewater treatment as efficient and appropriate catalyst supports. For instance, Li et al. synthesized Fe₃O₄/BiOBr/biomass-derived carbon heterojunctions stacked on reed straw-derived biochar via a one-step hydrolysis method, and the introduction of biochar significantly increased the open pathway to more available active sites and promoted the electronegativity of the separated photogenerated electron-hole pairs^[23]. He et al. prepared chitin-based carbon/g-C₃N₄ heterojunctions by in situ calcination of a mixture of chitin and urea and utilized the electron density imbalance of urea and the terminal group of chitin to modulate the microstructure of g-C₃N₄ during calcination, resulting in an approximately 10-fold increase in the specific surface area of g-C₃N₄ with a stronger ability to inhibit charge recombination^[24]. Despite these advances, the exploration of diverse biomass-derived carbons to assist in enhancing the photocatalytic properties of g-C₃N₄ remains extremely attractive.

  In this work, biomass-derived carbon (BC)-modified porous g-C₃N₄ (pg-C₃N₄) heterojunctions were fabricated from banana peel (BP) and urea by a simple one-pot thermal polycondensation process. The photocatalytic properties of pg-C₃N₄/BC in different ratios were determined by photodegradation of OTC in the artificial seawater under visible light irradiation, which exhibited significantly enhanced photocatalytic activity in comparison with pure pg-C₃N₄. A preliminary proposal for a possible visible-light-driven photodegradation mechanism in the pg-C₃N₄/BC system was made considering the directional behavior of charge migration. In particular, the objective of this study is to use biochar derived from massive biomass waste for the remediation of pollutants in industrial wastewater.

  1.   Experimental

  1.1   Chemicals and reagents

  Banana was purchased from local supermarket in Changzhou, China. Urea, OTC, isopropyl alcohol (IPA), ethylene diamine tetraacetic acid (EDTA), and p-benzoquinone (BQ) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used without further purification.

  1.2   Synthesis of pg-C₃N₄/BC composites

  The specific synthetic procedures are schematically illustrated in Fig. 1 and the details are described as follows. Firstly, the BP was chopped into bits and allowed to dry in an oven at 60 ℃ for 24 h, and then powdered in a grinder. Subsequently, 13.16 g of urea was solved in 20 mL of distilled water. Then, a dedicated quantity of BP powder was admitted to the aqueous urea solution with vigorous stirring for 30 min followed by sonication for 30 min. After evaporating the water by baking the mixture at 60 ℃ overnight, it was cooled at room temperature to allow urea to recrystallize. In the following step, the sample was calcined under N₂ flow for 2 h under 550 ℃ with a 5 ℃·min^-1 heating rate. Similar procedures were replicated by altering the BP addition amounts to 0.040, 0.116, 0.193, and 0.386 g, respectively. The pg-C₃N₄/BC composite was labeled as pg-C₃N₄/BC-X (X=1, 3, 5, 10), where X% refers to the theoretical load percentage of BC. For comparison, pure pg-C₃N₄ and BC were produced by calcination of urea and BP at 550 ℃ for 2 h, respectively.

  Figure 1

  

  Figure 1.  Schematic diagram of the synthesis of pg-C₃N₄/BC

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  1.3   Characterizations

  The morphology of the samples was investigated by scanning electron microscopy (Sigma 500) at the acceleration voltage of 5 kV. The crystalline phases were identified using the SmartLab diffractometer (Rigaku) at 40 kV and 30 mA with Cu Kα X-ray radiation source, a nickel filter (λ=0.154 nm), and a 2θ range of 10°-60°. Transmission electron microscopy (TEM, Hitachi-9000) images was obtained at an accelerating voltage of 200 keV. Elements contents and their chemical states were obtained by X-ray photoelectron spectrometers (XPS, ESCALAB 250xi, Thermo Fisher Scientific, USA) equipped with an Al Kα monochromatic X-ray source (hν=1 486.7 eV) with a line width of 0.20 eV in an analysis chamber. The FTIR spectra were measured on a Nicolet iS50 spectrophotometer from 4 000 to 500 cm^-1 at room temperature using KBr as a diluting agent. The BET (Brunauer-Emmett-Teller) surface area was calculated based on the adsorption isotherm (ASAP2020HD88). Optical properties were analyzed by a UV-Vis spectrophotometer (UV-2600, Shimadzu). The electron spin resonance (ESR) spectra were acquired by the spectrometer (FA200, JEOL, Japan) with 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO). Photoluminescence (PL) spectra were recorded on an F-7000 spectrometer (Hitachi, Japan) with an excitation wavelength of 372 nm. Photoelectric properties of the sample were performed in a three-electrode chemical cell using a CHI 660E electrochemical analyzer with a Pt foil counter electrode and an Ag/AgCl reference electrode, and the solution of Na₂SO₄ (0.5 mol·L^-1) was used as the electrolyte. The working electrode was prepared as follows: 2 mg of the photocatalyst powder was placed in 2 mL of ethanol containing 20 μL of a 5% Nafion solution to prepare slurry. Then, the powder was uniformly dispersed by grinding. 500 μL of the solution was deposited on an indium tin oxide (ITO) glass substrate (1 cm×1 cm). The obtained electrode was dried at 353 K for 30 min. The light source was a Xe lamp (300 W).

  1.4   Static photocatalytic degradation

  To evaluate the photocatalytic efficiencies of the fabricated photocatalysts, OTC was degraded in a quartz-jacketed photoreactor at room temperature with water circulation. In detail, 20 mg of the as-synthesized photocatalyst was dispersed in 100 mL of artificial seawater (25 g·L^-1 of NaCl, 11 g·L^-1 of MgCl₂, 4 g·L^-1 of Na₂SO₄, and 1.6 g·L^-1 of CaCl₂ in deionized water) containing 10 g·L^-1 of OTC. After being shaken in the dark for 60 min to achieve adsorption-desorption equilibrium, the resulting suspension was illuminated under visible light by a Xenon lamp (300 W) equipped with an ultraviolet filter (λ≥400 nm) at a distance of approximately 10 cm. Samples of the suspension were taken at 10-minute intervals and centrifuged to separate the photocatalyst and the absorbance of the reaction solution was measured with UV-Vis at 353 nm.

  1.5   Photocatalysis in a continuous flow system

  In Fig. 2, a 300 W Xenon lamp in conjunction with a 400 nm cut-off filter was positioned above the reactor at a distance of 15 cm to serve as the light source. A quantity of 0.125 g of catalyst was applied and sintered onto a Ni mesh measuring 3.8 cm×1.8 cm, which was subsequently placed within a custom-made reactor (length: 4 cm, width: 2 cm, height: 1.5 cm). The antibiotics-contaminated artificial seawater and the reactor were connected using a peristaltic pump operating at a flow rate of 2 mL·min^-1. The concentrations of antibiotics within the reactor were measured over time using a UV-Vis spectrophotometer.

  Figure 2

  

  Figure 2.  Scheme illustration of the continuous flow system

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  2.   Results and discussion

  The XRD patterns of the as-synthesized pure pg-C₃N₄ and pg-C₃N₄/BC-X samples are illustrated in Fig. 3a. It was observed that the pure g-C₃N₄ had two distinct diffraction peaks. The strong peak around 27.4° can be indexed as the (002) plane, which may be attributed to the characteristic interplanar stacking peak of the conjugated aromatic system. The weak diffraction peak around 13.1° is assigned to the (100) plane which corresponds to the in-plane structural packing motifs of tri-s-triazine^[25]. In comparison with pristine pg-C₃N₄, the characteristic peaks of the pg-C₃N₄/BC-X composites shifted from 27.4° to 26.8° as the BC content increased, demonstrating an increase in the distance between the pg-C₃N₄ layers^[26]. Such phenomena may be attributed to the confined carbon construction in the pg-C₃N₄ layer via the strong π-π stacking interactions among the carbon and the pg-C₃N₄ matrix coming from the aromatic character^[27]. The FTIR analysis (Fig. 3b) provided further confirmation of the structural characteristics of both pure pg-C₃N₄ and pg-C₃N₄/BC-X. The pristine pg-C₃N₄ exhibited a peak at 1 638 cm^-1, which can be attributed to C—N stretching, and four peaks at 1 568, 1 421, 1 325, and 1 247 cm^-1, indicative of aromatic C—N stretching vibrations^[28-29]. Additionally, the peak at 809 cm^-1 is associated with the triazine ring modes, while the broad peaks in the range of 3 000-3 500 cm^-1 were attributed to terminal amino groups and surface-adsorbed hydroxyl species^[30-31]. As anticipated, virtually all characteristic pg-C₃N₄-related peaks were reflected in pg-C₃N₄/BC-X, providing straightforward proof of heterotopic binding of pg-C₃N₄ and BC.

  Figure 3

  

  Figure 3.  XRD patterns (a) and FTIR spectra (b) of pg-C₃N₄ and pg-C₃N₄/BC-X

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  The morphology and microstructures of the pg-C₃N₄, BC, and pg-C₃N₄/BC-5 samples were further investigated through the utilization of SEM and TEM techniques. The SEM and TEM images of the pg-C₃N₄ synthesized using urea as a precursor (Fig. 4a, 4d) revealed a 2D structure composed of small, flat sheets with wrinkles, as well as a three-dimensional porous structure with minimal aggregation. Fig. 4b and 4e exhibit the SEM and TEM images of BC, revealing its composition as a disordered arrangement of carbon sheets. These amorphous sheets exhibited localized regions of crystallinity, characterized by stacked planes of carbon structures at the nanometer scale. In contrast, Fig. 4c and 4f present the SEM and TEM images of the pg-C₃N₄/BC-5 sample, illustrating the successful integration of carbon sheets with pg-C₃N₄ nanosheets, resulting in the formation of a 2D-2D structure. Furthermore, it is noteworthy that the pg-C₃N₄/BC-5 sample exhibited a wrinkled and porous lamellar structure. This porous morphology significantly augments the surface area of the pg-C₃N₄/BC-5 sample, thereby facilitating enhanced diffusion and transport of reactant substrates. Consequently, this structural characteristic potentially contributes to the improved efficiency of photocatalysis.

  Figure 4

  

  Figure 4.  SEM images of pg-C₃N₄ (a), BC (b), and pg-C₃N₄/BC-5 (c); TEM images of pg-C₃N₄ (d), BC (e), and pg-C₃N₄/BC-5 (f)

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  XPS was applied to investigate the surface chemical state of the as-prepared sample. The XPS survey spectra (Fig. 5a) show the majority of elements of carbon, nitrogen, and oxygen in pg-C₃N₄ and pg-C₃N₄/BC-5. The high-resolution XPS spectra of C1s and N1s for g-C₃N₄ and pg-C₃N₄/BC-5 samples are shown in Fig. 5b and 5c. The C1s spectrum for pg-C₃N₄ was deconvoluted into three peaks centered around 284.80, 286.38, and 288.18 eV, corresponding to C—C bands, C—O species, and N=C—N coordination bond^[32-34], respectively. The peak of sp² hybridized carbon in the pg-C₃N₄/BC-5 sample shift to higher binding energy, which should be attributed to spontaneous self-assembly electron redistribution between pg-C₃N₄ and BC, demonstrating the formation of strong interaction between pg-C₃N₄ and BC. The N1s spectrum of pg-C₃N₄ can be well fitted with three deconvolution peaks centered at 398.29, 400.06, and 403.66 eV, which are attributed to the triazine rings of sp²-hybridized aromatic nitrogen (C=N—C), tertiary nitrogen coupled to three carbon (N—(C)₃), and amino function groups (C—N—H)^[35-37]. Interestingly, the peak shifting phenomenon also occurred in the XPS spectra of N1s which is similar to the high-resolution XPS spectra of C1s and has the same shifting direction for pg-C₃N₄/BC-5 compared with pg-C₃N₄.

  Figure 5

  

  Figure 5.  High-resolution XPS spectra of (a) survey, (b) C1s, and (c) N1s spectra of pg-C₃N₄ and pg-C₃N₄/BC-5

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  The N₂ adsorption-desorption isotherms were used to determine the BET specific surface areas (S_BET) of the pg-C₃N₄ and pg-C₃N₄/BC-X samples. Fig. 6 illustrates that all the samples exhibited a type-Ⅳ isotherm model with a hysteresis loop within the p/p₀ range of 0.8-1.0, indicating the presence of mesopores in both the pg-C₃N₄ and pg-C₃N₄/BC-X samples^[38-39]. By analyzing the desorption curve, the BET surface area of the pg-C₃N₄/BC-5 sample was calculated to be 117 m²·g^-1, surpassing that of the pg-C₃N₄ sample. This phenomenon suggests that the introduction of BC probably prevents pg-C₃N₄ from restacking^[40]. This phenomenon implies that the incorporation of BC likely inhibits the restacking of pg-C₃N₄. It is widely acknowledged that a high concentration of surface-active sites is advantageous for the adsorption and transportation of pollutant molecules within the interconnected porous structure, thereby promoting enhanced photocatalytic performance. Furthermore, the specific surface area exhibited an initial increase with the rising of BC content, followed by a subsequent decrease as the BC content continued to rise, owing to the potential aggregation induced by excessive BC, consequently leading to a reduction in the specific surface area (as depicted in the inset of Fig. 6).

  Figure 6

  

  Figure 6.  N₂ adsorption-desorption isotherms and BET surface areas (Inset) of pg-C₃N₄ and pg-C₃N₄/BC-X

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  The optical absorption properties of materials were analyzed using UV-Vis diffuse reflectance absorption spectroscopy (DRS). Fig. 7a illustrates that pristine pg-C₃N₄ demonstrates a typical light-responsive capability within the visible light range, with an absorption edge extending up to 460 nm. In contrast, the pg-C₃N₄/BC-X samples exhibited enhanced light absorption across the entire UV-visible light spectrum, accompanied by an increase in adsorption intensity, when compared to pristine pg-C₃N₄. The narrow gap of the sp² carbon cluster embedded in BC is widely recognized for its exceptional light absorption across a broad range of wavelengths^[41]. Consequently, the incorporation of BC material in pg-C₃N₄/BC-X composites can significantly enhance their light harvesting efficiency. The findings indicate that these composites can capture a greater amount of light, leading to the generation of more electron-hole pairs and ultimately augmenting their photocatalytic activity under visible light irradiation. Moreover, the conversion of DRS spectra into Tauc plots were employed to examine the energy-band structure of pg-C₃N₄ and pg-C₃N₄/BC-5 (Fig. 7b). The calculated band gaps for pg-C₃N₄ and pg-C₃N₄/BC-5 were determined to be 2.59 and 1.27 eV, respectively. Additionally, the observed reduction in band gap width suggests that electron transitions can occur with lower energy, thereby enhancing the photocatalytic capacity^[42]. It is widely recognized that the investigation of energy band positions is crucial for comprehending the redox mechanism of the photocatalyst. Fig. 7c shows the XPS-valence band (VB) spectra of pg-C₃N₄ and pg-C₃N₄/BC-5. The VB positions of pg-C₃N₄ and pg-C₃N₄/BC-5 were determined to be located at approximately 1.65 and 0.39 eV by linear extrapolation, respectively. So the E_CB (conduction band energy) of pg-C₃N₄ and pg-C₃N₄/BC-5 were equal to -0.94 and -0.88 eV, respectively. The above measurements and calculation helped to estimate the band structures in Fig. 7d. Compared with pg-C₃N₄, pg-C₃N₄/BC-5 had a lower conduction band position and VB position, which may be caused by the introduction of BC with rich electron-absorbing groups^[43]. The more negative the conduction band, the stronger its reducing ability, and the more conducive to the generation of free radicals^[44], which should be evidence that pg-C₃N₄/BC-5 had a stronger catalytic ability.

  Figure 7

  

  Figure 7.  (a) UV-Vis DRS spectra of BC, pg-C₃N₄, and pg-C₃N₄/BC-X; (b) Tauc plots converted from DRS spectra, (c) XPS-VB spectra, and (d) energy band structure of pg-C₃N₄ and pg-C₃N₄/BC-5

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  PL experiments are performed to examine the efficacy of charge carrier separation in the photocatalysts that have been prepared. Fig. 8 illustrates the PL spectra of the original pg-C₃N₄ and pg-C₃N₄/BC-X samples, with an excitation wavelength of 325 nm. It is evident that the emission spectrum of pure pg-C₃N₄ displayed heightened intensity. In contrast, it was observed that the intensities in the pg-C₃N₄/BC-X composites noticeably decreased with an increase in the BC content. This decrease in PL intensity indicates a more effective separation of photo-excited charge carriers at the interface between BC and pg-C₃N₄, implying a lower rate of recombination. As a result, the incorporation of BC in the composites can significantly prolong the lifetime of photogenerated electrons and holes during the transfer process, thereby enhancing the quantum efficiency and improving the photocatalytic efficiency.

  Figure 8

  

  Figure 8.  PL emission spectra of the prepared samples

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  The transfer kinetics of photogenerated carriers were further analyzed by transient photocurrent response and electrochemical impedance spectroscopy. As shown in Fig. 9a, pg-C₃N₄/BC-5 exhibited the highest photocurrent density far surpassing those of pg-C₃N₄ and pristine BC, indicating that BC introduction effectively promoted the electron-hole pairs separation. To provide further support for the enhanced interfacial charge separation in the binary heterojunction system, electrochemical impedance spectroscopy (EIS) was performed (Fig. 9b). It was observed that the arc radius of the Nyquist circle for pg-C₃N₄/BC-5 was smaller than that of pg-C₃N₄, indicating a reduced resistance for carrier transfer. This suggests that the efficient migration of photogenerated electrons and holes within the components effectively inhibits their recombination, allowing a greater number of free charges to participate in the photocatalytic degradation process.

  Figure 9

  

  Figure 9.  (a) Photoelectrochemical response and (b) EIS plots of BC, pg-C₃N₄, and pg-C₃N₄/BC-5

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  To examine the photocatalytic efficacy of the samples, OTC was selected as the indicator for organic contaminants in artificial seawater. As evidenced in Fig. 10a, the adsorption equilibrium was attained within a 60-minute duration for all catalysts under dark conditions. After reaching the adsorption equilibrium, Fig. 10a illustrates the alteration in OTC concentration (ρ/ρ₀) with varying illumination durations for all photocatalysts. An experiment was conducted in which no photocatalyst was utilized, and the findings indicate that there was no significant reduction in OTC under visible light exposure. This implied that OTC remained stable and the occurrence of self-photolysis can be dismissed. The photocatalytic degradation of OTC using BC was slightly increased from 24.1% (under dark) to 26.3% (under light irradiation). It suggests that BC only acts as an adsorbent and does not have any photocatalytic properties due to the stability of BC under light. In contrast, the utilization of pure pg-C₃N₄ as the catalyst led to a gradual reduction in the concentration of OTC, resulting in a degradation efficiency of 45.3% after 70 min of irradiation. Nevertheless, the incorporation of BC significantly improved the photocatalytic degradation performance of the pg-C₃N₄/BC-X samples. The pg-C₃N₄/BC-5 sample achieved a degradation efficiency of 96.6% after undergoing 70 min of irradiation in the same reaction. However, the introduction of higher BC content in pg-C₃N₄/BC-X composites (pg-C₃N₄/BC-10) resulted in a significant decline in photocatalytic activity. This observation highlights the significance of maintaining an appropriate BC content to attain optimal photocatalytic performance. This phenomenon can be elucidated in the following manner: the excessive presence of residual BC leads to the formation of agglomerates that cover the surface of pg-C₃N₄, as evidenced by the BET results presented in Fig. 6. Consequently, this coverage partially obstructs the absorption of light, thereby causing a reduction in the efficiency of OTC photocatalytic degradation. In addition, it is well known that carbon can adsorb OTC molecules onto surfaces of the composites, and the adsorption capacity increased with the increase of carbon contents. Thus, when the carbon content was above that in pg-C₃N₄/BC-5, the excessive adsorbed OTC molecules on the surfaces of pg-C₃N₄/BC-10 partly decelerate the interfacial charge transfer, resulting in a decrease of OTC photodegradation efficiency^[45]. The results of the kinetic study indicate that the photocatalytic degradation of OTC using these catalysts follows pseudo-first-order kinetics, as shown in Fig. 10b and 10c. The experimental results reveal that the photocatalytic degradation efficiency was significantly improved by incorporating BC into pg-C₃N₄, as evidenced by the notably higher reaction rate constant (k) of 0.047 min^-1 for pg-C₃N₄/BC-5 compared to the pristine pg-C₃N₄ with k of 0.005 min^-1. This enhancement was estimated to be approximately 8.4 times greater, indicating the effective role of BC in enhancing photocatalytic performance. Furthermore, to facilitate the practical implementation of photocatalysts for water purification, they must exhibit not only high efficiency but also exceptional stability to enable multiple uses. Consequently, the stability and reusability of the pg-C₃N₄/BC-5 photocatalyst were assessed through five successive reaction cycles. As depicted in Fig. 10d, the photocatalytic activity of the pg-C₃N₄/BC-X samples remained largely unaffected even after undergoing five cycles. Although a slight decline in activity was observed, it is plausible to attribute this to the inevitable loss of catalyst during the recycling process. This finding provides additional evidence of the exceptional reusability and stability exhibited by the pg-C₃N₄/BC-X photocatalysts.

  Figure 10

  

  Figure 10.  (a) Photocatalytic degradation efficiency of OTC under visible-light irradiation over pg-C₃N₄, BC, and pg-C₃N₄/BC-X, and corresponding (b) degradation kinetic curves and (c) k values, and (d) cyclic stability test for degradation of OTC over pg-C₃N₄/BC-X

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  To assess the potential of pg-C₃N₄/BC-5 as a viable technology, a continuous flow reaction system (Fig. 2) was employed to investigate the likelihood of photocatalytic degradation of OTC in an artificial seawater solution. The results depicted in Fig. 11 demonstrated a rapid decline in the concentration of OTC within the cell, occurring within a span of 40 min. This can be attributed to the ongoing transfer of OTC molecules from the solution to the surface of pg-C₃N₄ and pg-C₃N₄/BC-5. Despite demonstrating a 45.3% degradation efficiency of OTC in seawater (Fig. 10a), the efficacy of pg-C₃N₄ in the dynamic system was deemed unsatisfactory at 9.7%. Photocatalysts exhibiting superior adsorption capabilities performed more effectively in the dynamic system. Notably, the pg-C₃N₄/BC-5 photocatalyst achieved a removal rate of 39.1% over an extended duration, indicating the synergistic effect of adsorption and photocatalysis in facilitating OTC removal in a dynamic system.

  Figure 11

  

  Figure 11.  Photocatalysis performance of OTC in the artificial seawater in continuous flow test

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  Experiments were conducted to evaluate the functions of the active species in the photocatalysis process, employing radical capturing techniques and utilizing the pg-C₃N₄/BC-5 composites. IPA, EDTA, and BQ were employed as scavengers for ·OH, hole (h⁺) and ·O₂^-, respectively. According to Fig. 12a, the photocatalytic efficiency was moderately influenced, resulting in a degradation rate reduction of 82.3% when IPA (1 mmol·L^-1) was introduced into the OTC solution. This suggests that ·OH had minimal impact on the degradation reaction. However, the addition of EDTA (1 mmol·L^-1) and BQ (1 mmol·L^-1) resulted in a notable decrease in the degradation rate of OTC, with reductions of 26.4% and 8.2% respectively. This suggests that the presence of ·O₂^- and h⁺ is crucial in facilitating the photodegradation of OTC over pg-C₃N₄/BC-5. The ESR spectra were conducted to further analyze the roles of ·OH and ·O₂^-. As shown in Fig. 12b and 12c, no signal peaks of DMPO-·OH and DMPO-·O₂^- adducts appeared in dark conditions, indicating that the light irradiation was necessary to generate ·OH and ·O₂^-. The signal peaks of DMPO-·OH and DMPO-·O₂^- were detected under visible-light irradiation, which demonstrated that ·OH and ·O₂^- could be produced by pg-C₃N₄ and pg-C₃N₄/BC-5. Besides, a typical weak quadruple peak spectral characteristic signal of DMPO-·OH with an intensity ratio of 1∶2∶2∶1 was detected in the pg-C₃N₄/BC-5 photocatalytic system^[44], demonstrating that low activity ·OH occurs in the degradation of OTC. At the same time, a typical strong six-peak characteristic signal of DMPO-·O₂^- can be observed^[46]. The above conclusions indicate that ·O₂^- is the main dominant active free radical in the pg-C₃N₄/BC-5 photocatalytic system. Based on the aforementioned findings and subsequent analysis, a plausible mechanism for the degradation of organic pollutants through a visible-light-driven photocatalytic reaction in the presence of pg-C₃N₄/BC-5 sample is proposed in Fig. 12d. The incorporation of BC into the pg-C₃N₄/BC-5 sample has resulted in an augmented specific surface area, thereby facilitating the availability of more reactive sites. Furthermore, owing to the exceptional optical characteristics of the pg-C₃N₄/BC-5 composites, a substantial number of charge carriers are generated upon exposure to visible light irradiation. The BC exhibits conductive properties and functions as a transient electron acceptor. Consequently, the photogenerated electrons originating from the CB (conduction band) of pg-C₃N₄ can be transferred to BC via the contact surface between pg-C₃N₄ and BC. This facilitates the efficient separation of photogenerated electrons and holes, thereby diminishing the recombination rate of photogenerated electrons (e^-) and h⁺ in pg-C₃N₄/BC-5. Simultaneously, the transferred electrons engage in a reaction with free O₂ in the solution, resulting in the production of superoxide radicals (·O₂^-). The valence band (VB) energy of pg-C₃N₄/BC-5 (0.39 eV) exhibited a more negative value compared to the standard redox potential of OH^-/·OH (1.99 eV), resulting in the ineffectiveness of h⁺ in converting OH^- to ·OH^[47]. However, ·O₂^- can react further with H₂O and electrons, leading to the formation of ·OH^[48]. Consequently, the generated ·O₂^-, ·OH, and h⁺ can all serve as active species in engaging in redox reactions with phenanthrene adsorbed on the catalyst surface, thereby facilitating the mineralization and decomposition of pollutants. The degradation mechanism of OTC can be explained by the following equations:

  $ \mathrm{pg}-\mathrm{C}_3 \mathrm{N}_4 / \mathrm{BC}-5+h \nu \rightarrow \mathrm{e}^{-}+\mathrm{h}^{+} $

  (1)

  $ \mathrm{O}_2+\mathrm{e}^{-} \rightarrow \cdot \mathrm{O}_2^{-} $

  (2)

  $ \mathrm{O}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OOH}+\mathrm{OH}^{-} $

  (3)

  $ \cdot \mathrm{OOH}+\mathrm{H}_2 \mathrm{O}+2 \mathrm{e}^{-} \rightarrow \cdot \mathrm{OH}+2 \mathrm{OH}^{-} $

  (4)

  $ \begin{array}{l} \cdot {\rm{OH}} + \cdot {\rm{O}}_2^ - + {{\rm{h}}^ + } + {\rm{OTC}} \to \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{Degradation products}} \end{array} $

  (5)

  Figure 12

  

  Figure 12.  Trapping experiments of the active species during the photocatalytic degradation of OTC for pg-C₃N₄/BC-5 (a); ESR spectra of (b) DMPO-·OH and (c) DMPO-·O₂^- adducts; Mechanism for the enhanced photocatalytic activity (d)

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  3.   Conclusions

  In conclusion, the pg-C₃N₄/BC composites were effectively synthesized via a straightforward one-step calcination method utilizing a precursor mixture of BP and urea. The present study comprehensively evaluated the photocatalytic efficiency of the as-synthesized photocatalysts under visible-light conditions for the degradation of OTC, a representative organic pollutant in seawater. The results indicate that the pg-C₃N₄/BC composites exhibited enhanced photocatalytic performance in the removal of OTC compared to the pristine pg-C₃N₄. Furthermore, the pg-C₃N₄/BC composites with an optimal BC content demonstrate superior efficiency compared to all other prepared photocatalysts. The study determined that the reaction rate constant of pg-C₃N₄/BC exhibited a notable increase of approximately 8.4 times compared to that of pg-C₃N₄ during the degradation of OTC. This significant augmentation in photocatalytic activity can be ascribed to the amplified specific surface area, enhanced light-harvesting capability, and efficient separation of photo-generated charge carriers. Furthermore, the pg-C₃N₄/BC-5 composites exhibited favorable stability, as evidenced by their negligible decline in activity following five consecutive reactions. This study has successfully showcased the potential of the synthesized pg-C₃N₄/BC composites as efficient photocatalysts under visible light irradiation for effective environmental remediation.

  
  Acknowledgments: This work was financially supported by the Natural Science Foundation of Jiangsu Province (Grant No.BK20181048) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No.SJCX22_1457), and the authors express their gratitude to the Jiangsu University of Technology (JSUT), located in Changzhou, China, for its substantial resources and support.
• 1. [1]

     Wang J Z, Zang L L, Wang L B, Tian Y T, Yang Z Y, Yue Y M, Sun L G. Magnetic cobalt ferrite/reduced graphene oxide (CF/rGO) porous balls for efficient photocatalytic degradation of oxytetracycline[J]. J. Environ. Chem. Eng., 2022, 10(5):  108259. doi: 10.1016/j.jece.2022.108259

  2. [2]

     Brumovský M, Bečanová J, Kohoutek J, Borghini M, Nizzetto L. Contaminants of emerging concern in the open sea waters of the Western Mediterranean[J]. Environ. Pollut., 2017, 229:  976-983. doi: 10.1016/j.envpol.2017.07.082

  3. [3]

     Zhang R J, Yu K F, Li A, Wang Y H, Huang X Y. Antibiotics in corals of the South China Sea: Occurrence, distribution, bioaccumulation, and considerable role of coral mucus[J]. Environ. Pollut., 2019, 250:  503-510. doi: 10.1016/j.envpol.2019.04.036

  4. [4]

     Wang S, Ma X X, Liu Y L, Yi X S, Du G C, Li J. Fate of antibiotics, antibiotic-resistant bacteria, and cell-free antibiotic-resistant genes in full‑scale membrane bioreactor wastewater treatment plants[J]. Bioresour. Technol., 2020, 302:  122825. doi: 10.1016/j.biortech.2020.122825

  5. [5]

     Lu D W, Xu S, Qiu W, Sun Y, Liu X B, Yang J J, Ma J. Adsorption and desorption behaviors of antibiotic ciprofloxacin on functionalized spherical MCM-41 for water treatment[J]. J. Cleaner. Prod., 2020, 264:  121644. doi: 10.1016/j.jclepro.2020.121644

  6. [6]

     Aseman-Bashiz E, Rezaee A, Moussavi G. Ciprofloxacin removal from aqueous solutions using modified electrochemical Fenton processes with iron green catalysts[J]. J. Mol. Liq., 2021, 324:  114694. doi: 10.1016/j.molliq.2020.114694

  7. [7]

     Zhang N Q, Xue C J, Wang K, Fang Z Q. Efficient oxidative degradation of fluconazole by a heterogeneous Fenton process with Cu-Ⅴ bimetallic catalysts[J]. Chem. Eng. J., 2020, 380:  122516. doi: 10.1016/j.cej.2019.122516

  8. [8]

     Li S Y, Wang J, Ye Y Y, Tang Y M, Li X K, Gu F L, Li L S. Composite Si-O-metal network catalysts with uneven electron distribution: Enhanced activity and electron transfer for catalytic ozonation of carbamazepine[J]. Appl. Catal. B-Environ., 2020, 263:  118311. doi: 10.1016/j.apcatb.2019.118311

  9. [9]

     Chen Y X, Cheng M, Lai C, Wei Z, Zhang G X, Li L, Tang C S, Du L, Wang G F, Liu H D. The collision between g-C₃N₄ and QDs in the fields of energy and environment: Synergistic effects for efficient photocatalysis[J]. Small, 2023, 19(14):  2205902. doi: 10.1002/smll.202205902

  10. [10]

      Wang J L, Wang S Z. A critical review on graphitic carbon nitride (g-C₃N₄)-based materials: Preparation, modification and environmental application[J]. Coord. Chem. Rev., 2022, 453:  214338. doi: 10.1016/j.ccr.2021.214338

  11. [11]

      Chin J Y, Ahmad A L, Low S C. Graphitic carbon nitride photocatalyst for the degradation of oxytetracycline hydrochloride in water[J]. Mater. Chem. Phys., 2023, 301:  127626. doi: 10.1016/j.matchemphys.2023.127626

  12. [12]

      Alhanash A M, Al-Namshah K S, Mohamed S K, Hamdy M S. One-pot synthesis of the visible light sensitive C-doped ZnO@g-C₃N₄ for high photocatalytic activity through Z-scheme mechanism[J]. Optik, 2019, 186:  34-40. doi: 10.1016/j.ijleo.2019.04.084

  13. [13]

      Li L L, Ma D K, Xu Q L, Huang S M. Constructing hierarchical ZnIn₂S₄/g-C₃N₄ S-scheme heterojunction for boosted CO₂ photoreduction performance[J]. Chem. Eng. J., 2022, 437:  135153. doi: 10.1016/j.cej.2022.135153

  14. [14]

      Zhang C, Qin D Y, Zhou Y, Qin F Z, Wang H, Wang W J, Yang Y, Zeng G M. Dual optimization approach to Mo single atom dispersed g-C₃N₄ photocatalyst: Morphology and defect evolution[J]. Appl. Catal. B-Environ., 2022, 303:  120904. doi: 10.1016/j.apcatb.2021.120904

  15. [15]

      Nasri A, Jaleh B, Nezafat Z, Nasrollahzadeh M, Azizian S, Jang H W, Shokouhimehr M. Fabrication of g-C₃N₄/Au nanocomposite using laser ablation and its application as an effective catalyst in the reduction of organic pollutants in water[J]. Ceram. Int., 2021, 47(3):  3565-3572. doi: 10.1016/j.ceramint.2020.09.204

  16. [16]

      Liu Z, He X, Yang X J, Ding H K, Wang D B, Ma D C, Feng Q G. Synthesis of mesoporous carbon nitride by molten salt-assisted silica aerogel for rhodamine B adsorption and photocatalytic degradation[J]. J. Mater. Sci., 2021, 56:  11248-11265. doi: 10.1007/s10853-021-05994-z

  17. [17]

      Jin Z Y, Zhang Q T, Yuan S S, Ohno T. Synthesis high specific surface area nanotube g-C₃N₄ with two-step condensation treatment of melamine to enhance photocatalysis properties[J]. RSC Adv., 2015, 5(6):  4026-4029. doi: 10.1039/C4RA13355B

  18. [18]

      Asadzadeh-Khaneghah S, Habibi-Yangjeh A. g-C₃N₄/carbon dot-based nanocomposites serve as efficacious photocatalysts for environmental purification and energy generation: A review[J]. J. Cleaner Prod., 2020, 276:  124319. doi: 10.1016/j.jclepro.2020.124319

  19. [19]

      Bharagav U, Reddy N R, Rao V N K, Ravi P, Sathish M, Rangappa D, Prathap K, Chakra C S, Shankar M, Appels L, Aminabhavi T M, Kakarla R R, Kumari M M. Bifunctional g-C₃N₄/carbon nanotubes/WO₃ ternary nanohybrids for photocatalytic energy and environmental applications[J]. Chemosphere, 2023, 311:  137030. doi: 10.1016/j.chemosphere.2022.137030

  20. [20]

      Dai K, Lu L H, Liu Q, Zhu G P, Wei X Q, Bai J, Xuan L L, Wang H. Sonication assisted preparation of graphene oxide/graphitic‑CN nanosheet hybrid with reinforced photocurrent for photocatalyst applications[J]. Dalton Trans., 2014, 43(17):  6295-6299. doi: 10.1039/c3dt53106f

  21. [21]

      Li Y B, Zhang H M, Liu P R, Wang D, Li Y, Zhao H J. Cross-linked g‑C₃N₄/rGO nanocomposites with tunable band structure and enhanced visible light photocatalytic activity[J]. Small, 2013, 9(19):  3336-3344. doi: 10.1002/smll.201203135

  22. [22]

      Han Y Y, Lu X L, Tang S F, Yin X P, Wei Z W, Lu T B. Water splitting: Metal-free 2D/2D heterojunction of graphitic carbon nitride/graphdiyne for improving the hole mobility of graphitic carbon nitride[J]. Adv. Energy Mater., 2018, 8(16):  1870077. doi: 10.1002/aenm.201870077

  23. [23]

      Li S, Wang Z W, Zhao X T, Yang X, Liang G W, Xie X Y. Insight into enhanced carbamazepine photodegradation over biochar-based magnetic photocatalyst Fe₃O₄/BiOBr/BC under visible LED light irradiation[J]. Chem. Eng. J., 2019, 360:  600-611. doi: 10.1016/j.cej.2018.12.002

  24. [24]

      He B, Feng M, Chen X Y, Zhao D W, Sun J. One-pot construction of chitin-derived carbon/g-C₃N₄ heterojunction for the improvement of visible-light photocatalysis[J]. Appl. Surf. Sci., 2020, 527(15):  146737.

  25. [25]

      Zhang Y W, Liu J H, Wu G, Chen W. Porous graphitic carbon nitride synthesized via direct polymerization of urea for efficient sunlight-driven photocatalytic hydrogen production[J]. Nanoscale, 2012, 4(17):  5300-5303. doi: 10.1039/c2nr30948c

  26. [26]

      Mohamed M A, Zain M, Minggu L J, Kassim M B, Amin N A S, Salleh W, Salehmin M N I, Nasir M F M, Hir Z A M. Constructing bio-templated 3D porous microtubular C-doped g-C₃N₄ with tunable band structure and enhanced charge carrier separation[J]. Appl. Catal. B-Environ., 2018, 236:  265-279. doi: 10.1016/j.apcatb.2018.05.037

  27. [27]

      Xu Q L, Jiang C J, Cheng B, Yu J G. Enhanced visible-light photocatalytic H₂-generation activity of carbon/g-C₃N₄ nanocomposites prepared by two-step thermal treatment[J]. Dalton Trans., 2017, 46(32):  10611-10619. doi: 10.1039/C7DT00629B

  28. [28]

      Fu J W, Zhu B C, Jiang C J, Cheng B, You W, Yu J G. Hierarchical porous O-doped g-C₃N₄ with enhanced photocatalytic CO₂ reduction activity[J]. Small, 2017, 13(15):  1603938. doi: 10.1002/smll.201603938

  29. [29]

      Tong Z W, Yang D, Sun Y Y, Nan Y H, Jiang Z Y. Tubular g-C₃N₄ isotype heterojunction: Enhanced visible-light photocatalytic activity through cooperative manipulation of oriented electron and hole transfer[J]. Small, 2016, 12(30):  4093-4101. doi: 10.1002/smll.201601660

  30. [30]

      Liu H, Xu Z Z, Zhang Z, Ao D. Highly efficient photocatalytic H₂ evolution from water over CdLa₂S₄/mesoporous g-C₃N₄ hybrids under visible light irradiation[J]. Dalton Trans., 2016, 192:  234-241.

  31. [31]

      Wu X Y, Li S M, Wang B, Liu J H, Yu M. From biomass chitin to mesoporous nanosheets assembled loofa sponge-like N-doped carbon/g-C₃N₄ 3D network architectures as ultralow-cost bifunctional oxygen catalysts[J]. Mesoporous Mater., 2017, 240:  216-226. doi: 10.1016/j.micromeso.2016.11.022

  32. [32]

      Liu X L, Wang P, Zhai H S, Zhang Q Q, Huang B B, Wang Z Y, Liu Y Y, Dai Y, Qin X Y, Zhang X Y. Synthesis of synergetic phosphorus and cyano groups (CN) modified g-C₃N₄ for enhanced photocatalytic H₂ production and CO₂ reduction under visible light irradiation[J]. Appl. Catal. B-Enviton., 2018, 232:  521-530. doi: 10.1016/j.apcatb.2018.03.094

  33. [33]

      Lin K W, Wang Z F, Hu Z C, Luo P, Yang X Y, Zhang X, Rafiq M, Huang F, Cao Y. Amino-functionalised conjugated porous polymers for improved photocatalytic hydrogen evolution[J]. J. Mater. Chem. A, 2019, 7(32):  19087-19093. doi: 10.1039/C9TA06219J

  34. [34]

      Guo F, Huang X L, Chen Z H, Cao L W, Cheng X F, Chen L Z, Shi W L. Construction of Cu₃P-ZnSnO₃-g-C₃N₄ pnn heterojunction with multiple built-in electric fields for effectively boosting visible-light photocatalytic degradation of broad-spectrum antibiotics[J]. Sep. Purif. Technol., 2021, 265:  118477. doi: 10.1016/j.seppur.2021.118477

  35. [35]

      Xue Y J, Guo Y C, Liang Z Q, Cui H Z, Tian J. Porous g-C₃N₄ with nitrogen defects and cyano groups for excellent photocatalytic nitrogen fixation without co-catalysts[J]. J. Colloid Interface Sci., 2019, 556:  206-213. doi: 10.1016/j.jcis.2019.08.067

  36. [36]

      Zhang W Q, Shi W L, Sun H R, Shi Y X, Luo H, Jing S R, Fan Y Q, Guo F, Lu C Y. Fabrication of ternary CoO/g-C₃N₄/Co₃O₄ nanocomposite with p-n-p type heterojunction for boosted visible-light photocatalytic performance[J]. J. Chem. Technol. Biotechnol., 2021, 96(7):  1854-1863. doi: 10.1002/jctb.6703

  37. [37]

      Shi Y X, Li L L, Xu Z, Sun H R, Amin S, Guo F, Shi W L, Li Y. Engineering of 2D/3D architectures type Ⅱ heterojunction with high-crystalline g-C₃N₄ nanosheets on yolk-shell ZnFe₂O₄ for enhanced photocatalytic tetracycline degradation[J]. Mater. Res. Bull., 2022, 150:  111789. doi: 10.1016/j.materresbull.2022.111789

  38. [38]

      Lian Y R, Zhu W K, Yao W T, Yi H, Hu Z W, Duan T, Cheng W C, Wei X F, Hu G Z. A biomass carbon mass coated with modified TiO₂ nanotube/graphene for photocatalysis[J]. New J. Chem., 2017, 41(10):  4212-4219. doi: 10.1039/C6NJ04005E

  39. [39]

      Devi T B, Mohanta D. Ahmaruzzaman M, Biomass derived activated carbon loaded silver nanoparticles: An effective nanocomposites for enhanced solar photocatalysis and antimicrobial activities[J]. J. Ind. Eng. Chem., 2019, 76:  160-172. doi: 10.1016/j.jiec.2019.03.032

  40. [40]

      Wu G, Hu Y, Liu Y, Zhao J J, Chen X L, Whoehling V, Plesse C, Nguyen G T, Vidal F, Chen W. Graphitic carbon nitride nanosheet electrode-based high-performance ionic actuator[J]. Nat. Commun., 2015, 6(1):  7258. doi: 10.1038/ncomms8258

  41. [41]

      Xia X Y, Deng N, Cui G W, Xie J F, Shi X F, Zhao Y Q, Wang Q, Wang W, Tang B. NIR light induced H₂ evolution by a metal-free photocatalyst[J]. Chem. Commun., 2015, 51(54):  10899-10902. doi: 10.1039/C5CC02589C

  42. [42]

      Wang C T, Dang Y C, Pang X X, Zhang L, Bian Y J, Duan W, Yang C J, Zhen Y Z, Fu F. A novel S-scheme heterojunction based on 0D/3D CeO₂/Bi₂O₂CO₃ for the photocatalytic degradation of organic pollutants[J]. New J. Chem., 2022, 46(33):  15987-15998. doi: 10.1039/D2NJ03192B

  43. [43]

      Bai X J, Wang L, Wang Y J, Yao W Q, Zhu Y F. Enhanced oxidation ability of g-C₃N₄ photocatalyst via C₆₀ modification[J]. Appl. Catal. B-Environ., 2014, 152:  262-270.

  44. [44]

      Qiu P X, Xu C M, Chen H, Jiang F, Wang X, Lu R M, Zhang X R. 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[J]. Appl. Catal. B-Environ., 2017, 206:  319-327. doi: 10.1016/j.apcatb.2017.01.058

  45. [45]

      Patnaik S, Martha S, Acharya S, Parida K. An overview of the modification of g-C₃N₄ with high carbon containing materials for photocatalytic applications[J]. Inorg. Chem. Front., 2016, 3(3):  336-347. doi: 10.1039/C5QI00255A

  46. [46]

      Wu Y T, Li X, Liu C Q, Chang X J, Hu X F. Studies on the active radicals in synthesized inverse opal photonic crystal and its effect on photocatalytic removal of organic pollutants[J]. J. Mol. Liq., 2020, 320:  114414. doi: 10.1016/j.molliq.2020.114414

  47. [47]

      Wang F L, Wang Y F, Feng Y P, Zeng Y Q, Xie Z J, Zhang Q X, Su Y H, Chen P, Liu Y, Yao K, Lv W Y, Liu G G. Novel ternary photocatalyst of single atom-dispersed silver and carbon quantum dots co-loaded with ultrathin g-C₃N₄ for broad spectrum photocatalytic degradation of naproxen[J]. Appl. Catal. B-Environ., 2018, 221:  510-520. doi: 10.1016/j.apcatb.2017.09.055

  48. [48]

      Wu Y L, Wang F L, Jin X Y, Zheng X S, Wang Y F, Wei D D, Zhang Q X, Feng Y P, Xie Z J, Chen P, Liu H J, Liu G G. Highly active metal-free carbon dots/g-C₃N₄ hollow porous nanospheres for solar-light-driven PPCPs remediation: Mechanism insights, kinetics and effects of natural water matrices[J]. Water Res., 2020, 172:  115492. doi: 10.1016/j.watres.2020.115492

• 

  Figure 1  Schematic diagram of the synthesis of pg-C₃N₄/BC

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  Figure 2  Scheme illustration of the continuous flow system

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  Figure 3  XRD patterns (a) and FTIR spectra (b) of pg-C₃N₄ and pg-C₃N₄/BC-X

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  Figure 4  SEM images of pg-C₃N₄ (a), BC (b), and pg-C₃N₄/BC-5 (c); TEM images of pg-C₃N₄ (d), BC (e), and pg-C₃N₄/BC-5 (f)

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  Figure 5  High-resolution XPS spectra of (a) survey, (b) C1s, and (c) N1s spectra of pg-C₃N₄ and pg-C₃N₄/BC-5

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  Figure 6  N₂ adsorption-desorption isotherms and BET surface areas (Inset) of pg-C₃N₄ and pg-C₃N₄/BC-X

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  Figure 7  (a) UV-Vis DRS spectra of BC, pg-C₃N₄, and pg-C₃N₄/BC-X; (b) Tauc plots converted from DRS spectra, (c) XPS-VB spectra, and (d) energy band structure of pg-C₃N₄ and pg-C₃N₄/BC-5

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  Figure 8  PL emission spectra of the prepared samples

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  Figure 9  (a) Photoelectrochemical response and (b) EIS plots of BC, pg-C₃N₄, and pg-C₃N₄/BC-5

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  Figure 10  (a) Photocatalytic degradation efficiency of OTC under visible-light irradiation over pg-C₃N₄, BC, and pg-C₃N₄/BC-X, and corresponding (b) degradation kinetic curves and (c) k values, and (d) cyclic stability test for degradation of OTC over pg-C₃N₄/BC-X

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  Figure 11  Photocatalysis performance of OTC in the artificial seawater in continuous flow test

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  Figure 12  Trapping experiments of the active species during the photocatalytic degradation of OTC for pg-C₃N₄/BC-5 (a); ESR spectra of (b) DMPO-·OH and (c) DMPO-·O₂^- adducts; Mechanism for the enhanced photocatalytic activity (d)

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•