Synthesis of Biochar-Supported K-doped g-C3N4 Photocatalyst for Enhancing the Polycyclic Aromatic Hydrocarbon Degradation Activity

The development of novel and green photocatalysts have attracted considerable attentions due to their excellent performance for environmental remediation, especially for the degradation of persistent pollutants. In this work, the biochar-supported K-doped g-C3N4 composites with the high photocatalytic activity under visible light irradiation was prepared by the calcination-impregnation method. The crystal structure, apparent morphology and functional group composition of the as-prepared photocatalytic materials were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscope (FTIR). Moreover, the characterization of UV-Vis diffuse reflectance spectra (UV-Vis DRS) and photoluminescence technique (PL) verified the good optical properties of resultant samples. Naphthalene was selected as the representative compound to evaluate the photocatalytic performance of the prepared photocatalysts under visible light irradiation. The evaluation results showed that the biochar-supported K-doped g-C3N4 composites exhibited excellent photocatalytic activity (82.19%). Moreover, the photocatalytic degradation rate basically remained unchanged after five cycles, indicating the good stability of the prepared photocatalysts. In addition, a possible mechanism for the photodegradation process was proposed on the basis of the main intermediates detected by gas chromatography-mass spectrometer (GC-MS). This study may provide a promising approach for the polycyclic aromatic hydrocarbon degradation by waste utilization of agricultural biomass and increasing the photocatalytic performance of pure g-C3N4.


Introduction
The pollution of petroleum hydrocarbons is one of the environmental problems in the petrochemical industry in recent years [1]. Petroleum hydrocarbons are complex mixtures composed of alkanes, olefins and polycyclic aromatic hydrocarbons (PAHs). Petroleum hydrocarbon leakage may cause serious and persistent water and soil pollution. For example, due to petroleum hydrocarbon contamination, the quality and visibility of water body decrease, and the fertility and water-holding capacity of soil are also reduced [2]. In particular, PAHs have the characteristics of high toxicity, mutagenicity and carcinogenicity [3]. Unfortunately, PAHs deposited in soil and groundwater pose a tremendous threat to the entire ecosystem. Naphthalene, as a representative substance of polycyclic aromatic hydrocarbons (PAHs), has been classified as a priority control pollutant by the Environmental Protection Agency of the United States and has been identified to be hazardous to the health of humans and properties of photocatalysts were characterized with X-ray diffractometer (XRD), scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscope, UV-visible diffuse reflectance spectroscope (DRS) and photoluminescence (PL) spectroscope. On the basis of the above analyses, the photocatalytic activity and stability for the degradation of naphthalene were investigated under visible light irradiation. The possible mechanism of naphthalene degradation was also discussed based on the intermediates determined with gas chromatography-mass spectrometer (GC-MS).

Synthesis of Photocatalysts
Dicyandiamide, potassium hydroxide, isopropanol, ascorbic acid, ethylenediamine tetraacetic acid and naphthalene were purchased from Sinopharm Chemical Reagent Co., Ltd., Shenyang, China and directly used without further purification.
Biochar (corn stalk biomass waste), pure g-C 3 N 4 and K-doped g-C 3 N 4 were prepared according to previous methods [27,30,31] with slight modifications. Potassium-doped g-C 3 N 4 was synthesized with the calcination method according to the doping ratio of 1.40%. Biochar-supported K-doped g-C 3 N 4 photocatalyst was fabricated with the calcination-impregnation method. The typical preparation procedure of biochar-supported K-doped g-C 3 N 4 photocatalysts was described as follows. A certain proportion of corn straw biochar and potassium-doped g-C 3 N 4 powder was dissolved in ultrapure water to obtain a suspension, which was dispersed uniformly by ultrasonic vibration, stirred overnight on a magnetic stirrer, evaporated to dryness in a water bath and then dried in an oven for a period. The dried materials were calcined at a high temperature again, cooled to room temperature and fully ground to obtain biochar-supported K-doped g-C 3 N 4 composite photocatalysts (the mass ratios of biochar to K-doped g-C 3 N 4 were respectively 1:0.8, 1:1 and 1:1.2).

Characterization of Photocatalysts
The crystallinity of the prepared samples was recorded by an X-ray diffractometer (XRD) on a Rigaku D/max-2400 instrument with Cu-Kα radiation. The morphologies and structures of the samples were collected by using a scanning electron microscope (S-4200, Hitachi, Tokyo, Japan) with an accelerating voltage of 15 kV. Chemical-bonding status was assessed by Fourier transform infrared (FT-IR) spectra (Nicolet 6700 IR spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) in the range of 400 to 4000 cm −1 . UV-Vis diffuse reflectance spectral measurement was carried out on a JASCO V-550 model UV-Vis spectrophotometer (JASCO, Tokyo, Japan) to determine the optical band gap of the photocatalysts. Photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (FP-6300, JASCO, Tokyo, Japan) using a Xe lamp as an excitation source under the excitation wavelength of 380 nm.

Photocatalytic Experiments
Naphthalene was selected as the representative substrate to evaluate the photocatalytic performance of prepared photocatalysts under visible light irradiation. Considering that the prepared photocatalyst has a certain adsorption potential, we adopted two protocols to evaluate its photocatalytic activity. In the first protocol, prior to illumination, 50 mg of photocatalysts were dispersed in an aqueous solution of naphthalene (100 mL and 20 mg L −1 ), and the suspension was stirred for 30 min in the dark to achieve the absorption-desorption equilibrium. For the photocatalytic experiments, the mixture was exposed to a 200-W high-pressure sodium lamp with the main emission in the range of 400~800 nm (the UV light portion of the sodium lamp was filtered by 0.5-mol L −1 NaNO 2 solution) [32]. The experimental temperature was maintained at 30 • C with water circulation, and air was bubbled through the solution at a flow rate of 80 mL min −1 . According to certain time intervals, 5 mL of the suspension was taken with a syringe and immediately centrifuged to remove the particles. The concentrations of naphthalene were measured by UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan) at a wavelength of 221 nm. In the second protocol, the suspension was directly exposed to visible light for adsorption and photo-degradation. The active species-trapping experiments were performed according to the same procedure to the photocatalytic experiments, except that additional isopropanol, ascorbic acid and ethylenediamine tetraacetic acid were added during the photocatalytic degradation process.
The stability of the photocatalysts were investigated by recycling experiments. The photocatalytic degradation of the recycled photocatalysts was carried out under visible light irradiation. After each cycle, the photocatalysts were collected through filtration, washed and dried before reuse.

Determination of Intermediates
The degradation intermediates of naphthalene were analyzed with gas chromatography (GC) coupled with a mass spectrometer (MS) technique (Agilent 7890A/5975C, Agilent, Santa Clara, CA, USA). The gas chromatograph was fitted with an HP-5MS column (30 m × 0.25 mm × 0.25 µm). The column temperature was linearly programmed to increase from 35 to 300 • C at a rate of 5 • C min −1 . The mass spectrometric data were acquired at 70 eV in electron ionization mode. Afterwards, the possible degradation pathway of naphthalene was proposed according to the intermediates.

XRD Patterns
The XRD patterns of biochar, g-C 3 N 4 , K-doped g-C 3 N 4 and biochar-supported K-doped g-C 3 N 4 are shown in Figure 1. The corn straw biochar was amorphous carbon, and its XRD pattern did not show characteristic peaks. A weaker diffraction peak at 13.1 • and a stronger one at 27.5 • were observed in the XRD pattern of g-C 3 N 4 . The peak at 13.1 • was ascribed to an in-plane structural repeating motif of tri-s-triazine units and indexed as the (100) peak with the d value of 0.675 nm. The peak at 27.5 • was assigned to interlayer stacking of the conjugated aromatic segments with a distance of 0.332 nm and indexed as the (002) peak of the stacked aromatic system [33]. However, the diffraction peak intensity of K-doped g-C 3 N 4 was largely weakened, indicating that the K incorporation resulted in the decrease in the crystallinity of g-C 3 N 4 , because the increased dopants inhibited crystal growth. Moreover, its diffraction peak positions showed a downshift toward a lower 2θ value, indicating the shorter-range order and larger distance of the interlayer stacking of aromatic segments [34]. In the XRD pattern of the biochar-supported K-doped g-C 3 N 4 , the diffraction peaks at 13.1 • almost disappeared, indicating that the inner structure of carbon nitride was destroyed to a certain degree. the particles. The concentrations of naphthalene were measured by UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan) at a wavelength of 221 nm. In the second protocol, the suspension was directly exposed to visible light for adsorption and photo-degradation. The active species-trapping experiments were performed according to the same procedure to the photocatalytic experiments, except that additional isopropanol, ascorbic acid and ethylenediamine tetraacetic acid were added during the photocatalytic degradation process. The stability of the photocatalysts were investigated by recycling experiments. The photocatalytic degradation of the recycled photocatalysts was carried out under visible light irradiation. After each cycle, the photocatalysts were collected through filtration, washed and dried before reuse.

Determination of Intermediates
The degradation intermediates of naphthalene were analyzed with gas chromatography (GC) coupled with a mass spectrometer (MS) technique (Agilent 7890A/5975C, Agilent, Santa Clara, CA, USA). The gas chromatograph was fitted with an HP-5MS column (30 m × 0.25 mm × 0.25 μm). The column temperature was linearly programmed to increase from 35 to 300 °C at a rate of 5 °C min −1 . The mass spectrometric data were acquired at 70 eV in electron ionization mode. Afterwards, the possible degradation pathway of naphthalene was proposed according to the intermediates.

XRD Patterns
The XRD patterns of biochar, g-C3N4, K-doped g-C3N4 and biochar-supported K-doped g-C3N4 are shown in Figure 1. The corn straw biochar was amorphous carbon, and its XRD pattern did not show characteristic peaks. A weaker diffraction peak at 13.1° and a stronger one at 27.5° were observed in the XRD pattern of g-C3N4. The peak at 13.1° was ascribed to an in-plane structural repeating motif of tri-s-triazine units and indexed as the (100) peak with the d value of 0.675 nm. The peak at 27.5° was assigned to interlayer stacking of the conjugated aromatic segments with a distance of 0.332 nm and indexed as the (002) peak of the stacked aromatic system [33]. However, the diffraction peak intensity of K-doped g-C3N4 was largely weakened, indicating that the K incorporation resulted in the decrease in the crystallinity of g-C3N4, because the increased dopants inhibited crystal growth. Moreover, its diffraction peak positions showed a downshift toward a lower 2θ value, indicating the shorter-range order and larger distance of the interlayer stacking of aromatic segments [34]. In the XRD pattern of the biochar-supported K-doped g-C3N4, the diffraction peaks at 13.1° almost disappeared, indicating that the inner structure of carbon nitride was destroyed to a certain degree. Figure 1. X-ray diffraction (XRD) patterns of biochar, g-C 3 N 4 , K-doped g-C 3 N 4 and biochar-supported K-doped g-C 3 N 4 .

SEM Images
The morphologies of biochar and photocatalyst powder were examined with SEM analysis (Figure 2). Similar to the analogue graphite, g-C 3 N 4 displayed a large particle shape with the layered structure ( Figure 2a). K-doped g-C 3 N 4 exhibited much smaller polymer particles than that of g-C 3 N 4 , demonstrating that the structure of the catalyst particles was changed to some degree by the dopant atoms with large diameters, as confirmed by XRD results. Figure 2c shows the morphology of the biochar synthesized by calcination at 550 • C, and its main body is a keel structure with a large number of pores on the top. The K-doped g-C 3 N 4 particles were dispersed nonuniformly on the surface of the biochar, and no apparent agglomeration was observed. A large contact area between the photocatalyst and organic pollutant largely increased the reactive sites and facilitated the photocatalytic activity.

SEM Images
The morphologies of biochar and photocatalyst powder were examined with SEM analysis (Figure 2). Similar to the analogue graphite, g-C3N4 displayed a large particle shape with the layered structure ( Figure 2a). K-doped g-C3N4 exhibited much smaller polymer particles than that of g-C3N4, demonstrating that the structure of the catalyst particles was changed to some degree by the dopant atoms with large diameters, as confirmed by XRD results. Figure 2c shows the morphology of the biochar synthesized by calcination at 550 °C , and its main body is a keel structure with a large number of pores on the top. The K-doped g-C3N4 particles were dispersed nonuniformly on the surface of the biochar, and no apparent agglomeration was observed. A large contact area between the photocatalyst and organic pollutant largely increased the reactive sites and facilitated the photocatalytic activity.

FT-IR Spectra
FTIR spectra of as-prepared biochar, g-C3N4, K-doped g-C3N4 and biochar-supported K-doped g-C3N4 are shown in Figure 3. In the spectra of the biochar, the peak at 3600 cm −1 is attributed to the -OH stretching of the cellulosic component in the biochar; the peaks at 2990 cm −1 and 2894 cm −1 are attributed to -CH2-symmetric stretching of an alkyl moiety on the biochar surface; the absorption peaks at 1400 cm −1 and 1054 cm −1 correspond to C=O and C-O-C stretching vibrations of a phenolic moiety and polysaccharides components of the biochar, respectively [35]. The broad absorption peak of g-C3N4 in the region of 3102~3373 cm −1 corresponds to the N-H stretching of uncondensed amine groups [36]; a series of peaks in the region of 1016~1727 cm −1 are attributed to the C-N and C=N vibrations of aromatic CN heterocycles; the peak at 806 cm −1 can be assigned to the particular bending vibration mode of the s-triazine rings of g-C3N4; the bands at 889 cm −1 can be assigned to the bending mode of the heptazine unit and terminal amine groups [37]. Furthermore, the spectra of K-doped g-C3N4 exhibits the same bands as pure g-C3N4, except a band at 2200 cm −1 originated from the defects of a cyanogen group stretch caused by the incomplete polymerization or amino groups dropped off from the surface of g-C3N4 [38]. The FTIR spectra of biochar-supported K-doped g-C3N4 are similar to those of g-C3N4, indicating that the biochar did not destruct the chemical structure of g-C3N4, but abundant functional groups on the surface of the biochar effectively promoted the dispersion of the composite photocatalysts in polar solvents.

FT-IR Spectra
FTIR spectra of as-prepared biochar, g-C 3 N 4 , K-doped g-C 3 N 4 and biochar-supported K-doped g-C 3 N 4 are shown in Figure 3. In the spectra of the biochar, the peak at 3600 cm −1 is attributed to the -OH stretching of the cellulosic component in the biochar; the peaks at 2990 cm −1 and 2894 cm −1 are attributed to -CH 2 -symmetric stretching of an alkyl moiety on the biochar surface; the absorption peaks at 1400 cm −1 and 1054 cm −1 correspond to C=O and C-O-C stretching vibrations of a phenolic moiety and polysaccharides components of the biochar, respectively [35]. The broad absorption peak of g-C 3 N 4 in the region of 3102~3373 cm −1 corresponds to the N-H stretching of uncondensed amine groups [36]; a series of peaks in the region of 1016~1727 cm −1 are attributed to the C-N and C=N vibrations of aromatic CN heterocycles; the peak at 806 cm −1 can be assigned to the particular bending vibration mode of the s-triazine rings of g-C 3 N 4 ; the bands at 889 cm −1 can be assigned to the bending mode of the heptazine unit and terminal amine groups [37]. Furthermore, the spectra of K-doped g-C 3 N 4 exhibits the same bands as pure g-C 3 N 4 , except a band at 2200 cm −1 originated from the defects of a cyanogen group stretch caused by the incomplete polymerization or amino groups dropped off from the surface of g-C 3 N 4 [38]. The FTIR spectra of biochar-supported K-doped g-C 3 N 4 are similar to those of g-C 3 N 4 , indicating that the biochar did not destruct the chemical structure of g-C 3 N 4 , but abundant functional groups on the surface of the biochar effectively promoted the dispersion of the composite photocatalysts in polar solvents.  3.1.4. UV-vis Diffuse Reflectance Spectra UV-vis diffuse reflectance spectra of the prepared samples are shown in Figure 4. The intersection point of the tangent of the curves and the abscissa are used to estimate the absorption edge. The biochar shows the strongest adsorption to the light of all wavelengths. Pure g-C3N4 exhibits an absorption edge around 460 nm, corresponding to a band gap of 2.69 eV [39]. The intrinsic absorption edge of K-doped g-C3N4 is red-shifted compared with that of pure g-C3N4, confirming that K-doping could modulate its electronic structure, reduce the band gap and enhance the absorption of visible light [40]. In the spectra of biochar-supported K-doped g-C3N4, the absorption edges show the obvious red shifts, and the absorption intensity remarkably increases, suggesting that the introduction of amorphous carbon sources effectively promoted the separation of photogenerated electron-hole pairs and enhanced their response range to visible light. In Figure 4b, compared with pure g-C3N4, the energy gap of biochar-supported K-doped g-C3N4 is significantly narrowed; the narrower energy gap of biochar-supported K-doped g-C3N4 is able to make full use of the visible light, further resulting in a higher photocatalytic performance. 3.1.5. PL Spectra Figure 5 illustrates the PL emission spectra of biochar, g-C3N4, K-doped g-C3N4 and biocharsupported K-doped g-C3N4. PL is a highly sensitive technique used to determine the photocatalytic performance and reveal charge separation/recombination rates of photo-excited carriers [41]. In general, the higher PL intensity corresponds to the higher recombination rates of photogenerated

UV-vis Diffuse Reflectance Spectra
UV-vis diffuse reflectance spectra of the prepared samples are shown in Figure 4. The intersection point of the tangent of the curves and the abscissa are used to estimate the absorption edge. The biochar shows the strongest adsorption to the light of all wavelengths. Pure g-C 3 N 4 exhibits an absorption edge around 460 nm, corresponding to a band gap of 2.69 eV [39]. The intrinsic absorption edge of K-doped g-C 3 N 4 is red-shifted compared with that of pure g-C 3 N 4 , confirming that K-doping could modulate its electronic structure, reduce the band gap and enhance the absorption of visible light [40]. In the spectra of biochar-supported K-doped g-C 3 N 4 , the absorption edges show the obvious red shifts, and the absorption intensity remarkably increases, suggesting that the introduction of amorphous carbon sources effectively promoted the separation of photogenerated electron-hole pairs and enhanced their response range to visible light. In Figure 4b, compared with pure g-C 3 N 4 , the energy gap of biochar-supported K-doped g-C 3 N 4 is significantly narrowed; the narrower energy gap of biochar-supported K-doped g-C 3 N 4 is able to make full use of the visible light, further resulting in a higher photocatalytic performance.

UV-vis Diffuse Reflectance Spectra
UV-vis diffuse reflectance spectra of the prepared samples are shown in Figure 4. The intersection point of the tangent of the curves and the abscissa are used to estimate the absorption edge. The biochar shows the strongest adsorption to the light of all wavelengths. Pure g-C3N4 exhibits an absorption edge around 460 nm, corresponding to a band gap of 2.69 eV [39]. The intrinsic absorption edge of K-doped g-C3N4 is red-shifted compared with that of pure g-C3N4, confirming that K-doping could modulate its electronic structure, reduce the band gap and enhance the absorption of visible light [40]. In the spectra of biochar-supported K-doped g-C3N4, the absorption edges show the obvious red shifts, and the absorption intensity remarkably increases, suggesting that the introduction of amorphous carbon sources effectively promoted the separation of photogenerated electron-hole pairs and enhanced their response range to visible light. In Figure 4b, compared with pure g-C3N4, the energy gap of biochar-supported K-doped g-C3N4 is significantly narrowed; the narrower energy gap of biochar-supported K-doped g-C3N4 is able to make full use of the visible light, further resulting in a higher photocatalytic performance. 3.1.5. PL Spectra Figure 5 illustrates the PL emission spectra of biochar, g-C3N4, K-doped g-C3N4 and biocharsupported K-doped g-C3N4. PL is a highly sensitive technique used to determine the photocatalytic performance and reveal charge separation/recombination rates of photo-excited carriers [41]. In general, the higher PL intensity corresponds to the higher recombination rates of photogenerated  Figure 5 illustrates the PL emission spectra of biochar, g-C 3 N 4 , K-doped g-C 3 N 4 and biochar-supported K-doped g-C 3 N 4 . PL is a highly sensitive technique used to determine the photocatalytic performance and reveal charge separation/recombination rates of photo-excited carriers [41]. In general, the higher PL intensity corresponds to the higher recombination rates of photogenerated electron-hole pairs and the lower photocatalytic activity. The biochar possesses the strong electron transport capacity, as indicated by the lowest PL intensity in the prepared materials.

PL Spectra
Pure g-C 3 N 4 shows a strong peak at around 460 nm, and this is consistent with the absorption edge in the UV-vis diffuse reflectance spectra [42]. PL emission spectra of K-doped g-C 3 N 4 exhibit similar signals to those of pure g-C 3 N 4 , but the intensity of the PL spectra is affected, indicating that doping K reduced the recombination of electrons and holes and enhanced the separation efficiency [43]. Besides, the PL intensity of biochar-supported K-doped g-C 3 N 4 is further weakened, since the biochar can serve as efficient electron-transfer channels and acceptors to inhibit the recombination of photogenerated electron-hole pairs. Int. J. Environ. Res. Public Health 2020, 17, x 7 of 15 electron-hole pairs and the lower photocatalytic activity. The biochar possesses the strong electron transport capacity, as indicated by the lowest PL intensity in the prepared materials. Pure g-C3N4 shows a strong peak at around 460 nm, and this is consistent with the absorption edge in the UV-vis diffuse reflectance spectra [42]. PL emission spectra of K-doped g-C3N4 exhibit similar signals to those of pure g-C3N4, but the intensity of the PL spectra is affected, indicating that doping K reduced the recombination of electrons and holes and enhanced the separation efficiency [43]. Besides, the PL intensity of biochar-supported K-doped g-C3N4 is further weakened, since the biochar can serve as efficient electron-transfer channels and acceptors to inhibit the recombination of photogenerated electron-hole pairs.

Photocatalytic Activity
The photocatalytic performances of as-prepared samples were investigated by the degradation experiments with 20-mg L −1 naphthalene solution under visible light illumination. Figure 6a shows the results of the first protocol. For the purpose of comparison, in the absence of photocatalysts, naphthalene degradation experiments were performed under visible light irradiation, and degradation of naphthalene was not observed, indicating that a photocatalyst was an essential factor in the photocatalytic process [44]. It is generally believed that a biochar plays an important role in the adsorption of organic pollutants, since a biochar has a large specific surface area and sufficient pore geometry [45]. Unsurprisingly, pure g-C3N4 displayed a lower degradation rate because of the high electron-hole recombination rate. For K-modified g-C3N4 showed the obviously improved naphthalene degradation rate, because doping K reduced the recombination of electrons and holes and improved the photocatalytic activity. A series of biochar-supported K-doped g-C3N4 compound materials showed the much higher activity than K-modified g-C3N4, since the biochar acted as a cocatalyst to provide abundant catalytic sites. Moreover, the biochar had a strong affinity to organic pollutants, and naphthalene dissolved in the reaction system could be enriched at the surface of the biochar. Particularly, biochar-supported K-doped g-C3N4 compound materials prepared with two mass ratios (1:0.8 and 1:1) showed much higher activity than those prepared with other mass ratios, because excessive K-doped g-C3N4 powder was aggregated or crystallized on the surface of the biochar and blocked some active sites.

Photocatalytic Activity
The photocatalytic performances of as-prepared samples were investigated by the degradation experiments with 20-mg L −1 naphthalene solution under visible light illumination. Figure 6a shows the results of the first protocol. For the purpose of comparison, in the absence of photocatalysts, naphthalene degradation experiments were performed under visible light irradiation, and degradation of naphthalene was not observed, indicating that a photocatalyst was an essential factor in the photocatalytic process [44]. It is generally believed that a biochar plays an important role in the adsorption of organic pollutants, since a biochar has a large specific surface area and sufficient pore geometry [45]. Unsurprisingly, pure g-C 3 N 4 displayed a lower degradation rate because of the high electron-hole recombination rate. For K-modified g-C 3 N 4 showed the obviously improved naphthalene degradation rate, because doping K reduced the recombination of electrons and holes and improved the photocatalytic activity. A series of biochar-supported K-doped g-C 3 N 4 compound materials showed the much higher activity than K-modified g-C 3 N 4 , since the biochar acted as a cocatalyst to provide abundant catalytic sites. Moreover, the biochar had a strong affinity to organic pollutants, and naphthalene dissolved in the reaction system could be enriched at the surface of the biochar. Particularly, biochar-supported K-doped g-C 3 N 4 compound materials prepared with two mass ratios (1:0.8 and 1:1) showed much higher activity than those prepared with other mass ratios, because excessive K-doped g-C 3 N 4 powder was aggregated or crystallized on the surface of the biochar and blocked some active sites.  Figure 6c shows the degradation performances of naphthalene under the second protocol. Kdoped g-C3N4 and different proportions of biochar-supported K-doped g-C3N4 showed better results under direct light exposure experiments. The results might be interpreted as follows. After the adsorption in the suspension for 30 min in the dark, active sites of the catalyst were occupied by naphthalene, thus restricting the exposure of the photocatalyst to the light source. However, when the suspension was directly illuminated, adsorption and photodegradation were carried out simultaneously. Naphthalene was degraded once it was adsorbed, thus effectively increasing the degradation rate of pollutants. The result was consistent with the previous report [46].
In addition, the photodegradation process of naphthalene with the photocatalysts appeared to follow the pseudo-first-order kinetics reaction. For the first protocol, the relative rate constants were calculated to be 0.0011, 0.0028, 0.0038, 0.0053, 0.0051 and 0.0046 min −1 for the biochar, g-C3N4, K-doped g-C3N4, biochar-supported K-doped g-C3N4 (1:0.8), biochar-supported K-doped g-C3N4 (1:1) and biochar-supported K-doped g-C3N4 (1:1.2), respectively (Figure 6b). For the second protocol, the apparent rate constants of K-doped g-C3N4, biochar-supported K-doped g-C3N4 (1:0.8), biocharsupported K-doped g-C3N4 (1:1) and biochar-supported K-doped g-C3N4 (1:1.2) samples were 0.0046, 0.0104, 0.0098 and 0.0082 min −1 , respectively (Figure 6d).When the suspension is directly exposed to light, the relative rate constants of all K-doped g-C3N4 and biochar-supported K-doped g-C3N4 photocatalysts were higher than those of the first condition. It was observed that the biocharsupported K-doped g-C3N4 (1:0.8) performs best under visible light. The rate constant of optimal photocatalysts under second condition is about two times higher than that of the first condition.  Figure 6c shows the degradation performances of naphthalene under the second protocol. K-doped g-C 3 N 4 and different proportions of biochar-supported K-doped g-C 3 N 4 showed better results under direct light exposure experiments. The results might be interpreted as follows. After the adsorption in the suspension for 30 min in the dark, active sites of the catalyst were occupied by naphthalene, thus restricting the exposure of the photocatalyst to the light source. However, when the suspension was directly illuminated, adsorption and photodegradation were carried out simultaneously. Naphthalene was degraded once it was adsorbed, thus effectively increasing the degradation rate of pollutants. The result was consistent with the previous report [46].
In addition, the photodegradation process of naphthalene with the photocatalysts appeared to follow the pseudo-first-order kinetics reaction. For the first protocol, the relative rate constants were calculated to be 0.0011, 0.0028, 0.0038, 0.0053, 0.0051 and 0.0046 min −1 for the biochar, g-C 3 N 4 , K-doped g-C 3 N 4 , biochar-supported K-doped g-C 3 N 4 (1:0.8), biochar-supported K-doped g-C 3 N 4 (1:1) and biochar-supported K-doped g-C 3 N 4 (1:1.2), respectively (Figure 6b). For the second protocol, the apparent rate constants of K-doped g-C 3 N 4 , biochar-supported K-doped g-C 3 N 4 (1:0.8), biochar-supported K-doped g-C 3 N 4 (1:1) and biochar-supported K-doped g-C 3 N 4 (1:1.2) samples were 0.0046, 0.0104, 0.0098 and 0.0082 min −1 , respectively (Figure 6d).When the suspension is directly exposed to light, the relative rate constants of all K-doped g-C 3 N 4 and biochar-supported K-doped g-C 3 N 4 photocatalysts were higher than those of the first condition. It was observed that the biochar-supported K-doped g-C 3 N 4 (1:0.8) performs best under visible light. The rate constant of optimal photocatalysts under second condition is about two times higher than that of the first condition.

Photostability
The stability and reusability of the prepared photocatalysts largely determine their application. In order to investigate the stability of the catalyst, the photocatalytic degradation of recycled biochar-supported K-doped g-C 3 N 4 (1:0.8) was carried out according to the second protocol. The photocatalytic activity of the recovered photocatalyst showed no noticeable decrease after five consecutive cycles (Figure 7). The comparison of the FTIR spectra of the photocatalysts before and after the reaction indicated that the chemical structure was not changed. Therefore, it was confirmed that the biochar-supported K-doped g-C 3 N 4 photocatalyst was not photo-corroded and had high stability during the photocatalytic degradation process. The high stability might be ascribed to the biochar, which acted as a cocatalyst and was well-connected with carbon nitride.

Photostability
The stability and reusability of the prepared photocatalysts largely determine their application. In order to investigate the stability of the catalyst, the photocatalytic degradation of recycled biocharsupported K-doped g-C3N4 (1:0.8) was carried out according to the second protocol. The photocatalytic activity of the recovered photocatalyst showed no noticeable decrease after five consecutive cycles (Figure 7). The comparison of the FTIR spectra of the photocatalysts before and after the reaction indicated that the chemical structure was not changed. Therefore, it was confirmed that the biochar-supported K-doped g-C3N4 photocatalyst was not photo-corroded and had high stability during the photocatalytic degradation process. The high stability might be ascribed to the biochar, which acted as a cocatalyst and was well-connected with carbon nitride.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪OH), superoxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (

Photostability
The stability and reusability of the prepared photocatalysts largely determine their application. In order to investigate the stability of the catalyst, the photocatalytic degradation of recycled biocharsupported K-doped g-C3N4 (1:0.8) was carried out according to the second protocol. The photocatalytic activity of the recovered photocatalyst showed no noticeable decrease after five consecutive cycles (Figure 7). The comparison of the FTIR spectra of the photocatalysts before and after the reaction indicated that the chemical structure was not changed. Therefore, it was confirmed that the biochar-supported K-doped g-C3N4 photocatalyst was not photo-corroded and had high stability during the photocatalytic degradation process. The high stability might be ascribed to the biochar, which acted as a cocatalyst and was well-connected with carbon nitride.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), superoxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species. OH), superoxide radical anion ( 9 of 15 photocatalysts largely determine their application. the photocatalytic degradation of recycled biochard out according to the second protocol. The talyst showed no noticeable decrease after five the FTIR spectra of the photocatalysts before and ture was not changed. Therefore, it was confirmed tocatalyst was not photo-corroded and had high rocess. The high stability might be ascribed to the -connected with carbon nitride. n of naphthalene over biochar-supported K-doped (b) FTIR spectra of biochar-supported K-doped gcatalytic reaction. alene tive Species s of naphthalene degradation in the photocatalytic cted. In this work, isopropanol (IPA), ascorbic acid ) were employed as the hydroxyl radical (▪ OH), ncher, respectively [46]. The results are shown in of ▪OH, was used, the photodegradation rate of ayed an important role in the photodegradation of re respectively added, the degradation rates were h + were also the dominant active species. O 2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of lth 2020, 17, x 9 of 15 eusability of the prepared photocatalysts largely determine their application. he stability of the catalyst, the photocatalytic degradation of recycled biochar--C3N4 (1:0.8) was carried out according to the second protocol. The of the recovered photocatalyst showed no noticeable decrease after five ure 7). The comparison of the FTIR spectra of the photocatalysts before and ted that the chemical structure was not changed. Therefore, it was confirmed rted K-doped g-C3N4 photocatalyst was not photo-corroded and had high otocatalytic degradation process. The high stability might be ascribed to the a cocatalyst and was well-connected with carbon nitride.
ive photocatalytic degradation of naphthalene over biochar-supported K-doped 1:0.8) under visible light and (b) FTIR spectra of biochar-supported K-doped g-.8) before and after the photocatalytic reaction.
dation Mechanism of Naphthalene ents and Formation of Active Species elucidate the active species of naphthalene degradation in the photocatalytic ng experiments were conducted. In this work, isopropanol (IPA), ascorbic acid ine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), on (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in A, the typical scavenger of ▪OH, was used, the photodegradation rate of 5%, indicating that ▪OH played an important role in the photodegradation of r, when AA and EDTA were respectively added, the degradation rates were 4%, indicating that ▪O2 − and h + were also the dominant active species.
OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that 9 of 15 f the prepared photocatalysts largely determine their application. of the catalyst, the photocatalytic degradation of recycled biochar-.8) was carried out according to the second protocol. The overed photocatalyst showed no noticeable decrease after five comparison of the FTIR spectra of the photocatalysts before and chemical structure was not changed. Therefore, it was confirmed ed g-C3N4 photocatalyst was not photo-corroded and had high degradation process. The high stability might be ascribed to the t and was well-connected with carbon nitride. alytic degradation of naphthalene over biochar-supported K-doped visible light and (b) FTIR spectra of biochar-supported K-doped gd after the photocatalytic reaction.
anism of Naphthalene ormation of Active Species he active species of naphthalene degradation in the photocatalytic nts were conducted. In this work, isopropanol (IPA), ascorbic acid etic acid (EDTA) were employed as the hydroxyl radical (▪ OH), d hole (h + ) quencher, respectively [46]. The results are shown in ical scavenger of ▪OH, was used, the photodegradation rate of ng that ▪OH played an important role in the photodegradation of and EDTA were respectively added, the degradation rates were ng that ▪O2 − and h + were also the dominant active species.
OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that 9 of 15 f the prepared photocatalysts largely determine their application. of the catalyst, the photocatalytic degradation of recycled biochar-.8) was carried out according to the second protocol. The vered photocatalyst showed no noticeable decrease after five comparison of the FTIR spectra of the photocatalysts before and chemical structure was not changed. Therefore, it was confirmed ed g-C3N4 photocatalyst was not photo-corroded and had high degradation process. The high stability might be ascribed to the t and was well-connected with carbon nitride. lytic degradation of naphthalene over biochar-supported K-doped isible light and (b) FTIR spectra of biochar-supported K-doped gd after the photocatalytic reaction.
anism of Naphthalene ormation of Active Species e active species of naphthalene degradation in the photocatalytic nts were conducted. In this work, isopropanol (IPA), ascorbic acid etic acid (EDTA) were employed as the hydroxyl radical (▪ OH), d hole (h + ) quencher, respectively [46]. The results are shown in cal scavenger of ▪OH, was used, the photodegradation rate of ng that ▪OH played an important role in the photodegradation of and EDTA were respectively added, the degradation rates were ng that ▪O2 − and h + were also the dominant active species. Under visible light irradiation, g-C3N4 is activated to move electrons from the valence band (VB) to the conduction band (CB), followed by the formation of electron hole pairs [47]. The electrons on CB rapidly transfer to the biochar, because the biochar has the good electronic transmission ability. Under visible light irradiation, g-C 3 N 4 is activated to move electrons from the valence band (VB) to the conduction band (CB), followed by the formation of electron hole pairs [47]. The electrons on CB rapidly transfer to the biochar, because the biochar has the good electronic transmission ability. Then, a part of the electrons on the CB by the biochar transfer directly and serve as the terminal electron acceptor. The lifetime of photogenerated electrons is prolonged, thus effectively promoting charge separation and facilitating the photocatalytic reaction. Photogenerated electrons transferred to the biochar surface can react with O 2 dissolved in the reaction solution to form superoxide radical anion ( biochar-supported K-doped ochar-supported K-doped ggradation in the photocatalytic opropanol (IPA), ascorbic acid s the hydroxyl radical (▪ OH), [46]. The results are shown in the photodegradation rate of ole in the photodegradation of ed, the degradation rates were minant active species.
lene degradation in the photocatalytic work, isopropanol (IPA), ascorbic acid loyed as the hydroxyl radical (▪ OH), ctively [46]. The results are shown in used, the photodegradation rate of ortant role in the photodegradation of ly added, the degradation rates were o the dominant active species. m of Naphthalene ation of Active Species ctive species of naphthalene degradation in the photocatalytic were conducted. In this work, isopropanol (IPA), ascorbic acid acid (EDTA) were employed as the hydroxyl radical (▪ OH), ole (h + ) quencher, respectively [46]. The results are shown in scavenger of ▪OH, was used, the photodegradation rate of hat ▪OH played an important role in the photodegradation of EDTA were respectively added, the degradation rates were hat ▪O2 − and h + were also the dominant active species.
otocatalytic Degradation Mechanism of Naphthalene rapping Experiments and Formation of Active Species order to further elucidate the active species of naphthalene degradation in the photocatalytic n system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid nd ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), xide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of alene reached 65%, indicating that ▪OH played an important role in the photodegradation of alene. Moreover, when AA and EDTA were respectively added, the degradation rates were tively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), superoxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), superoxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.
OH can oxidize naphthalene. In addition, holes (h + ) can also be used as an active species to undergo the oxidation reaction with naphthalene, ultimately producing CO 2 , H 2 O and other small molecules (Figure 9) [49]. Under visible light irradiation, g-C3N4 is activated to move electrons from the valence band (VB) to the conduction band (CB), followed by the formation of electron hole pairs [47]. The electrons on CB rapidly transfer to the biochar, because the biochar has the good electronic transmission ability. Then, a part of the electrons on the CB by the biochar transfer directly and serve as the terminal electron acceptor. The lifetime of photogenerated electrons is prolonged, thus effectively promoting charge separation and facilitating the photocatalytic reaction. Photogenerated electrons transferred to the biochar surface can react with O2 dissolved in the reaction solution to form superoxide radical anion (▪O2 − ). ▪O2 − can also generate ▪OH through a series of chemical reactions [48]. Meanwhile, photogenerated holes can react with H2O to form hydroxyl radicals (▪OH). Both ▪O2 − and ▪OH can oxidize naphthalene. In addition, holes (h + ) can also be used as an active species to undergo the oxidation reaction with naphthalene, ultimately producing CO2, H2O and other small molecules (Figure 9) [49].

Intermediates of Naphthalene Degradation
The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol The intermediates of the photocatalytic degradation of naphthalene were determined by the GC-MS technique. The main intermediate products are listed in Table 1. Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by ▪OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O2 and subsequently eliminate HO2▪ to generate 1-naphthol (8) and 2-naphthol 2-formylcinnamaldehyde

Reaction Pathway
Based on the detection results of the main intermediates, the reaction pathway of naphthalene degradation is proposed ( Figure 10). Firstly, naphthalene (1) is attacked by Figure 7. (a) Repetitive photocatalytic degradation of naphthalene over biochar-supported K-doped g-C3N4 (mass ratio, 1:0.8) under visible light and (b) FTIR spectra of biochar-supported K-doped g-C3N4 (mass ratio, 1:0.8) before and after the photocatalytic reaction.
Photocatalytic Degradation Mechanism of Naphthalene

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic tion system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid ) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), eroxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in re 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of hthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of hthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were ectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.
OH either at the α position or the β position to generate three different hydroxylated naphthalene radicals (2,3 and 4), which can react with O 2 and subsequently eliminate HO 2 otocatalytic degradation of naphthalene over biochar-supported K-doped nder visible light and (b) FTIR spectra of biochar-supported K-doped gore and after the photocatalytic reaction.
gradation Mechanism of Naphthalene riments and Formation of Active Species her elucidate the active species of naphthalene degradation in the photocatalytic ping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid iamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of d 65%, indicating that ▪OH played an important role in the photodegradation of ver, when AA and EDTA were respectively added, the degradation rates were d 44%, indicating that ▪O2 − and h + were also the dominant active species.

Trapping Experiments and Formation of Active Species
In order to further elucidate the active species of naphthalene degradation in the photocatalytic reaction system, trapping experiments were conducted. In this work, isopropanol (IPA), ascorbic acid (AA) and ethylenediamine tetraacetic acid (EDTA) were employed as the hydroxyl radical (▪ OH), superoxide radical anion (▪O2 − ) and hole (h + ) quencher, respectively [46]. The results are shown in Figure 8. When the IPA, the typical scavenger of ▪OH, was used, the photodegradation rate of naphthalene reached 65%, indicating that ▪OH played an important role in the photodegradation of naphthalene. Moreover, when AA and EDTA were respectively added, the degradation rates were respectively 32% and 44%, indicating that ▪O2 − and h + were also the dominant active species.

Conclusions
In summary, the novel visible-light-driven biochar-supported K-doped g-C3N4 photocatalyst was synthesized by a facile calcination-impregnation method. The properties of the biocharsupported K-doped g-C3N4 photocatalyst, such as optical and catalytic properties and absorption capacity, were improved by the introduction of potassium and biochar. The biochar-supported Kdoped g-C3N4 composites with the mass ratio of 1:0.8 exhibited the highest naphthalene degradation efficiency of 82.19% within 180 min under visible light irradiation due to the synergetic effects between K-doped g-C3N4 and biochar. The introduction of potassium effectively inhibited the growth of g-C3N4 grains, reduced the band gap energy and enlarged the response range to visible light. In addition, the biochar, as electron transfer channels and acceptors, effectively inhibited the recombination of photogenerated electron-hole pairs and had a large number of surface hydrophilic functional groups, which could effectively improve the dispersion of the prepared composite photocatalytic materials in polar solvents such as water and further improve the photocatalytic performance. Meanwhile, the biochar-supported K-doped g-C3N4 photocatalyst possessed good In addition to 9 of 15 atalysts largely determine their application. tocatalytic degradation of recycled biocharaccording to the second protocol. The showed no noticeable decrease after five IR spectra of the photocatalysts before and as not changed. Therefore, it was confirmed yst was not photo-corroded and had high The high stability might be ascribed to the ted with carbon nitride. hthalene over biochar-supported K-doped spectra of biochar-supported K-doped greaction.
ecies phthalene degradation in the photocatalytic n this work, isopropanol (IPA), ascorbic acid employed as the hydroxyl radical (▪ OH), respectively [46]. The results are shown in , was used, the photodegradation rate of important role in the photodegradation of ectively added, the degradation rates were re also the dominant active species.
OH, h + and 9 of 15 epared photocatalysts largely determine their application. talyst, the photocatalytic degradation of recycled biocharcarried out according to the second protocol. The photocatalyst showed no noticeable decrease after five ison of the FTIR spectra of the photocatalysts before and al structure was not changed. Therefore, it was confirmed N4 photocatalyst was not photo-corroded and had high ation process. The high stability might be ascribed to the as well-connected with carbon nitride. radation of naphthalene over biochar-supported K-doped ght and (b) FTIR spectra of biochar-supported K-doped ge photocatalytic reaction.
Naphthalene n of Active Species species of naphthalene degradation in the photocatalytic e conducted. In this work, isopropanol (IPA), ascorbic acid (EDTA) were employed as the hydroxyl radical (▪ OH), h + ) quencher, respectively [46]. The results are shown in enger of ▪OH, was used, the photodegradation rate of ▪OH played an important role in the photodegradation of TA were respectively added, the degradation rates were ▪O2 − and h + were also the dominant active species.

O 2
− are also important active species. Naphthalene (1) and a photogenerated hole undergo an electron transfer reaction to generate a naphthalene radical cation (19), which can react with the superoxide anion to form the endoperoxide (20). The endoperoxide (20) continues to decay into a diradicalic species (21) under illumination, which forms 1,2-dicarboxybenzene (22) through an elimination reaction and oxidation reaction. Naphthalene radical cation (19) can also react with a superoxide anion to form a peroxide species (23), which is then dissociated into 2-formylcinnamaldehyde (25). The main intermediates and the proposed mechanism were in agreement with previous studies [50].

Conclusions
In summary, the novel visible-light-driven biochar-supported K-doped g-C 3 N 4 photocatalyst was synthesized by a facile calcination-impregnation method. The properties of the biochar-supported K-doped g-C 3 N 4 photocatalyst, such as optical and catalytic properties and absorption capacity, were improved by the introduction of potassium and biochar. The biochar-supported K-doped g-C 3 N 4 composites with the mass ratio of 1:0.8 exhibited the highest naphthalene degradation efficiency of 82.19% within 180 min under visible light irradiation due to the synergetic effects between K-doped g-C 3 N 4 and biochar. The introduction of potassium effectively inhibited the growth of g-C 3 N 4 grains, reduced the band gap energy and enlarged the response range to visible light. In addition, the biochar, as electron transfer channels and acceptors, effectively inhibited the recombination of photogenerated electron-hole pairs and had a large number of surface hydrophilic functional groups, which could effectively improve the dispersion of the prepared composite photocatalytic materials in polar solvents such as water and further improve the photocatalytic performance. Meanwhile, the biochar-supported K-doped g-C 3 N 4 photocatalyst possessed good stability without a significant activity loss after five degradation cycles. Hence, the biochar-supported K-doped g-C 3 N 4 photocatalyst could be considered a promising material for the degradation of organic pollutants.