Efficient Activation of Peroxymonosulfate by V-Doped Graphitic Carbon Nitride for Organic Contamination Remediation

Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) activation have been developed as an ideal pathway for completely eradication of recalcitrant organic pollutants from water environment. Herein, the V-doped graphitic carbon nitride (g-C3N4) is rationally fabricated by one-step thermal polymerization method to activate PMS for contamination decontamination. The results demonstrate the V atoms are successfully integrated into the framework of g-C3N4, which can effectively improve light absorption intensity and enhance charge separation. The V-doped g-C3N4 displays superior catalytic performance for PMS activation. Moreover, the doping content has a great influence on the activation performances. The radical quenching experiments confirm •O2−, SO4•−, and h+ are the significant species in the catalytic reaction. This work would provide a feasible strategy to exploit efficient g-C3N4-based material for PMS activation.


Introduction
The organic contamination in groundwater and wastewater has become a challenging issue in the development of a sustainable society [1][2][3]. A number of toxic and persistent organic pollutants such as endocrine disrupting chemicals, antibiotics, and petroleum hydrocarbon, are frequently found in groundwater [4][5][6]. These pollutants are difficult to be biodegraded because of their chemical stability and environmental risk [7,8]. To date, various treatment techniques have been developed for organic contamination remediation, such as membrane separation, anaerobic technologies, and adsorption [3,5]. Unfortunately, their application are dramatically impeded by the low removal efficiency [3,9,10]. It is imperative but still remains challenges to develop cost-effective treatment methods for addressing organic contamination issues.
In recent years, advanced oxidation processes (AOPs) have been developed as an ideal pathway for completely eradication of recalcitrant organic pollutants from water environment [3,9,11]. In this process, the structure of organic contaminants can be destroyed by the highly reactive oxygen species [3,12,13]. Among various active radicals, sulfate radicals (SO 4 •− ) generated from peroxymonosulfate (PMS) activation have been widely exploited due to their stronger oxidative capacity and longer lifetime [14][15][16]. More importantly, SO 4 •− is generally reacted with target compounds via an electron transfer pathway due to its high selectivity [12,17]. Therefore, many approaches have been extensively applied to activate PMS into SO 4 •− , such as electrochemistry, transition metal ions, UV, and photocatalysis [18,19]. It is recognized that the photocatalysis is a promising tool for PMS activation owing to its low cost and environmentally compatible [20][21][22]. Nevertheless, it

Synthesis of Materials
The V-doped g-C 3 N 4 was synthesized via the thermal polymerization method. In a typical procedure, 5 g of melamine was thoroughly ground with an appropriate amount of NH 4 VO 3 . The collected mixture was placed into a crucible with a cover, and calcined at 550 • C for 4 h. The product was washed with NaOH solution (1 M), ethanol and water, and dried at 60 • C. A series of V-doped g-C 3 N 4 were obtained by adjusting the weight ratio of NH 4 VO 3 and melamine (0.1, 0.2, 0.5, and 0.8%) and denoted as V-g-C 3 N 4 -0.1, V-g-C 3 N 4 -0.2, V-g-C 3 N 4 -0.5, and V-g-C 3 N 4 -0.8.

Activation Performance Tests
The photodegradation of BPA was selected as a model reaction to study the activation performance of the V-doped g-C 3 N 4 . In a typical catalytic experiment, the sample (50 mg) was put in 50 mL BPA solution (10 mg/L), and stirred in the dark for 30 min. The 300 W Xe lamp with a 400 nm cut-off filter (CEL-HXF300, Aulight, Beijing, China) was immediately turned on after a specific amount of PMS was added. The experiments were conducted at 25 • C. The pH was not controlled during the experiment. At specified time intervals, the sample was taken out from the solution and filtered with a PTFE filter. Then, 0.1 mL of methanol was added immediately to quench radicals. The amount of BPA was analyzed by high performance liquid chromatography (HPLC, Agilent 1260, Santa Clara, CA, USA) with an Agilent C-18 column and UV detection wavelength of 276 nm. The mixture solutions of 30% water and 70% methanol were used as the mobile phase, and the flow rate was 1.0 mL·min −1 .
For the quenching experiment, BQ, EDTA, IPA, and MeOH were added as quenchers for different radicals.

Electrochemical Measurements
The Mott-Schottky plots and electrochemical impedance spectroscopy (EIS) were obtained on a CHI-660E electrochemical analyzer (CH Instruments, Shanghai, China) in a standard three electrode configuration. The Ag/AgCl and Pt sheet were used as the reference electrode and the counter electrode, respectively. The working electrode was prepared on indium doped tin oxide (ITO) substrates. The 1 mg sample was dispersed in a mixture containing 0.98 mL of anhydrous ethanol and 0.02 mL of Nafion solution by ultrasonication for 30 min. The obtained slurry was dropped onto the surface of the ITO glass with a size of 1 × 1 cm 2 , and then dried at room temperature. 0.5 M Na 2 SO 4 was employed as the electrolyte. The Mott-Schottky plot was obtained through measurement of the capacitance as a function of potentials at 1 kHz. The EIS measurements were carried out in the frequency range of 10 −2 to 10 5 Hz with a 10 mV amplitude.

Characterizations of Materials
The structure and morphology of the g-C 3 N 4 and V-g-C 3 N 4 -0.5 samples were characterized by SEM and TEM. From Figure 1a, pure g-C 3 N 4 shows a typical 2D nanosheetstacked structure with a micrometer scale lateral size. Figure 1b further confirms that the 2D sheet structure has a loose and flats surface. Figure 1c,d reveals the V-g-C 3 N 4 -0.5 displays a similar layered structure and morphology with that of g-C 3 N 4 . TEM was further utilized to reveal the detailed structure of V-doped g-C 3 N 4 . The layer sheet morphology with some porous structure is clearly observed in V-g-C 3 N 4 -0.5 (Figure 1e), which may be generated by the release of gas during the thermal decomposition of precursors [34][35][36]. The HRTEM image (Figure 1f) reveals that no crystalline phases are seen on the surface of g-C 3 N 4 , which is consistent with the results of SEM [31,37]. The homogeneous distribution of N, C, and V elements on the entire sample are shown in the element mapping ( Figure 1g). Thus, the V atoms are successfully introduced into the framework of g-C 3 N 4 .
of N, C, and V elements on the entire sample are shown in the element mapping ( Figure  1g). Thus, the V atoms are successfully introduced into the framework of g-C3N4. The chemical structures of the materials were investigated by XRD patterns. As shown in Figure 2, the g-C3N4 shows two distinct peaks at 12.8 and 27.6°, corresponding to the extended distance of the interplanar repeat unit in the (1 0 0) diffraction plane and the stacking of the π-π conjugated aromatic rings in the (0 0 2) diffraction plane, which is in accordance with previous studies [29,37,38]. The peak related to the (0 0 2) facets of g-C3N4 is clearly noticed in V-doped g-C3N4. However, the weaker diffraction peak at 12.8° has become indistinct gradually with increasing ratios of NH4VO3, suggesting its low crystallinity and disordered structure. Interestingly, no peaks of V oxides are found in Vdoped g-C3N4. Thus, the V atoms are integrated into the framework of g-C3N4. The chemical structures of the materials were investigated by XRD patterns. As shown in Figure 2, the g-C 3 N 4 shows two distinct peaks at 12.8 and 27.6 • , corresponding to the extended distance of the interplanar repeat unit in the (1 0 0) diffraction plane and the stacking of the π-π conjugated aromatic rings in the (0 0 2) diffraction plane, which is in accordance with previous studies [29,37,38]. The peak related to the (0 0 2) facets of g-C 3 N 4 is clearly noticed in V-doped g-C 3 N 4 . However, the weaker diffraction peak at 12.8 • has become indistinct gradually with increasing ratios of NH 4 VO 3 , suggesting its low crystallinity and disordered structure. Interestingly, no peaks of V oxides are found in V-doped g-C 3 N 4 . Thus, the V atoms are integrated into the framework of g-C 3 N 4 . The FT-IR spectra of the synthesized materials are illustrated in Figure 3. The absorption bands at 1200~1700 cm −1 are attributed to the stretching of the C-N heterocycle [27,39]. Specifically, the peaks positioned at 1314 and 1238 cm −1 are corresponded to the stretching vibrations of the connected units of N−(C)3 and C−NH−C, respectively. The broad band ranging from 3000 to 3650 cm −1 is assigned to the stretching vibrations of surface hydroxyl groups (OH-) and terminal and residual amino-groups (NH2-and NH-) [31,40]. The strong peak of 808 cm −1 is corresponded to the bending mode of C−N heterocycles [41,42]. Comparatively, the FT-IR spectra of the V-doped g-C3N4 are similar to that of pure g-C3N4. In addition, no characteristic absorption peaks of V-O are detected, which can be due to the low content of V atoms. These findings confirm the doping V atoms does not significantly alter the main structure and functional groups of g-C3N4.  The FT-IR spectra of the synthesized materials are illustrated in Figure 3. The absorption bands at 1200~1700 cm −1 are attributed to the stretching of the C-N heterocycle [27,39]. Specifically, the peaks positioned at 1314 and 1238 cm −1 are corresponded to the stretching vibrations of the connected units of N−(C) 3 and C−NH−C, respectively. The broad band ranging from 3000 to 3650 cm −1 is assigned to the stretching vibrations of surface hydroxyl groups (OH-) and terminal and residual amino-groups (NH 2 -and NH-) [31,40]. The strong peak of 808 cm −1 is corresponded to the bending mode of C−N heterocycles [41,42]. Comparatively, the FT-IR spectra of the V-doped g-C 3 N 4 are similar to that of pure g-C 3 N 4 . In addition, no characteristic absorption peaks of V-O are detected, which can be due to the low content of V atoms. These findings confirm the doping V atoms does not significantly alter the main structure and functional groups of g-C 3 N 4 . The FT-IR spectra of the synthesized materials are illustrated in Figure 3. The absorption bands at 1200~1700 cm −1 are attributed to the stretching of the C-N heterocycle [27,39]. Specifically, the peaks positioned at 1314 and 1238 cm −1 are corresponded to the stretching vibrations of the connected units of N−(C)3 and C−NH−C, respectively. The broad band ranging from 3000 to 3650 cm −1 is assigned to the stretching vibrations of surface hydroxyl groups (OH-) and terminal and residual amino-groups (NH2-and NH-) [31,40]. The strong peak of 808 cm −1 is corresponded to the bending mode of C−N heterocycles [41,42]. Comparatively, the FT-IR spectra of the V-doped g-C3N4 are similar to that of pure g-C3N4. In addition, no characteristic absorption peaks of V-O are detected, which can be due to the low content of V atoms. These findings confirm the doping V atoms does not significantly alter the main structure and functional groups of g-C3N4.   The valence states of the V-doped g-C 3 N 4 sample were further identified by XPS. The XPS survey spectra (Figure 4a) prove the coexistence of N, C, and V elements in the V-g-C 3 N 4 -0.5 sample. The C 1s spectrum in Figure 4b exhibits two characteristic peaks at 288.5 and 284.8 eV, ascribing to the sp2 hybridized carbon (N−C=N) and graphitic carbons (C=C), respectively [38,42]. The N 1s spectrum in Figure 4c shows two peaks at binding energies of 398.9 and 400.5 eV, which are ascribed to the hybridized aromatic N atoms bound to C atoms in the triazine units (C−N=C) and tertiary nitrogen (N-(C) 3 , respectively [37,38,43]. Furthermore, two major peaks at binding energies of 517.2 and 524.4 eV are observed from the V 2p spectrum (Figure 4d), which are belonged to the V 2p 3/2 and V 2p 1/2 of V-O bond of V 5+ species, respectively [34]. These results further confirm the V atoms are successfully implanted in the framework of the g-C 3 N 4 .
The valence states of the V-doped g-C3N4 sample were further identified b XPS survey spectra (Figure 4a) prove the coexistence of N, C, and V elements i C3N4-0.5 sample. The C 1s spectrum in Figure 4b exhibits two characteristic pea and 284.8 eV, ascribing to the sp2 hybridized carbon (N−C=N) and graphit (C=C), respectively [38,42]. The N 1s spectrum in Figure 4c shows two peaks energies of 398.9 and 400.5 eV, which are ascribed to the hybridized aromati bound to C atoms in the triazine units (C−N=C) and tertiary nitrogen (N-(C)3, re [37,38,43]. Furthermore, two major peaks at binding energies of 517.2 and 52 observed from the V 2p spectrum (Figure 4d), which are belonged to the V 2 2p1/2 of V-O bond of V 5+ species, respectively [34]. These results further confirm oms are successfully implanted in the framework of the g-C3N4. The DRS was performed to gain insights into the influence of V doping on property of g-C3N4. As shown in Figure 5, the g-C3N4 possesses a distinct visibl sorption feature with an absorption edge of about 450 nm, which is consistent w ous studies [28,43]. As observed, the V-doped g-C3N4 samples show wide and sorption in the visible light range. Moreover, a slight redshift of the absorption be observed for the V-doped g-C3N4 compared with that of g-C3N4. The visibl sorption abilities of V-doped g-C3N4 enhances gradually with increasing the d centration, revealing superior visible light utilization [29,44]. Therefore, the do oms could effectively strengthen light absorption intensity, which would boos The DRS was performed to gain insights into the influence of V doping on the optical property of g-C 3 N 4 . As shown in Figure 5, the g-C 3 N 4 possesses a distinct visible-light absorption feature with an absorption edge of about 450 nm, which is consistent with previous studies [28,43]. As observed, the V-doped g-C 3 N 4 samples show wide and strong absorption in the visible light range. Moreover, a slight redshift of the absorption edge can be observed for the V-doped g-C 3 N 4 compared with that of g-C 3 N 4 . The visible-light absorption abilities of V-doped g-C 3 N 4 enhances gradually with increasing the dopant concentration, revealing superior visible light utilization [29,44]. Therefore, the doping V atoms could effectively strengthen light absorption intensity, which would boost the catalytic performance of g-C 3 N 4 .

Activation Performance
The BPA was selected as the target organic pollutant to preliminarily probe t lytic properties of the V-doped g-C3N4. As shown in Figure 6, the BPA concentra mains almost the same without any catalyst, suggestting that BPA is stable unde light irradiation. Only 24.7% of BPA is removed by pure g-C3N4, indicating that t lytic capability of g-C3N4 is considerably low. This result is similar to the previous [29,31]. Obviously, the V-doped g-C3N4 samples show relatively high catalytic compared with g-C3N4, which can be assigned to the doping V atoms into the netw g-C3N4. Moreover, the catalytic capability of V-doped g-C3N4 are firstly increased content increased. When the content of V is further increased, the degradation ef of BPA is slightly decreased. These results manifest that the dopant content plays a role in the catalytic capability of V-doped g-C3N4.

Activation Performance
The BPA was selected as the target organic pollutant to preliminarily probe the catalytic properties of the V-doped g-C 3 N 4 . As shown in Figure 6, the BPA concentration remains almost the same without any catalyst, suggestting that BPA is stable under visible light irradiation. Only 24.7% of BPA is removed by pure g-C 3 N 4 , indicating that the catalytic capability of g-C 3 N 4 is considerably low. This result is similar to the previous studies [29,31]. Obviously, the V-doped g-C 3 N 4 samples show relatively high catalytic activity compared with g-C 3 N 4 , which can be assigned to the doping V atoms into the networks of g-C 3 N 4 . Moreover, the catalytic capability of V-doped g-C 3 N 4 are firstly increased as the V content increased. When the content of V is further increased, the degradation efficiency of BPA is slightly decreased. These results manifest that the dopant content plays a crucial role in the catalytic capability of V-doped g-C 3 N 4 .

Activation Performance
The BPA was selected as the target organic pollutant to preliminarily probe the catalytic properties of the V-doped g-C3N4. As shown in Figure 6, the BPA concentration remains almost the same without any catalyst, suggestting that BPA is stable under visible light irradiation. Only 24.7% of BPA is removed by pure g-C3N4, indicating that the catalytic capability of g-C3N4 is considerably low. This result is similar to the previous studies [29,31]. Obviously, the V-doped g-C3N4 samples show relatively high catalytic activity compared with g-C3N4, which can be assigned to the doping V atoms into the networks of g-C3N4. Moreover, the catalytic capability of V-doped g-C3N4 are firstly increased as the V content increased. When the content of V is further increased, the degradation efficiency of BPA is slightly decreased. These results manifest that the dopant content plays a crucial role in the catalytic capability of V-doped g-C3N4. The PMS activation performance of the prepared samples for organic pollutions degradation was further verified. As shown in Figure 7, the negligible removal of BPA is observed with the addition of PMS, suggesting the oxidation of PMS can be negligible. The  The PMS activation performance of the prepared samples for organic pollutions degradation was further verified. As shown in Figure 7, the negligible removal of BPA is observed with the addition of PMS, suggesting the oxidation of PMS can be negligible. The pure g-C 3 N 4 shows obvious catalytic activity in PMS activation under visible light irradiation for BPA degradation, owing to its unique electronic structure and moderate band gap. Furthermore, the V-doped g-C 3 N 4 possesses excellent catalytic performance in PMS activation. Meanwhile, the catalytic performance of the V-doped g-C 3 N 4 is increased firstly and then decreased with the increase of doping content, in which the BPA can be completely removed within 25 min over V-g-C 3 N 4 -0.5. In addition, the control experiment illustrates the visible light irradiation also plays the key role in PMS activation for organic pollutions degradation. It should be pointed out that the trend is consistent with the photocatalytic degradation of BPA in absence of PMS, further confirming the catalytic performances for PMS activation comes from the intrinsic photocatalytic capability of V-doped g-C 3 N 4 . This could be explained by the fact that the doping V atoms in the networks of g-C 3 N 4 would decrease band gap energy and improve carrier utilization, resulting in the enhanced degradation efficiency. Nevertheless, an excess of V atoms might conduct recombination sites for charge carriers, which resulted in abatement of catalytic performance. The results illustrate that the doping content has great effects on the catalytic performances of V-doped g-C 3 N 4 , and the V-g-C 3 N 4 -0.5 has the best performance.
Materials 2022, 15, x FOR PEER REVIEW pure g-C3N4 shows obvious catalytic activity in PMS activation under visible ligh ation for BPA degradation, owing to its unique electronic structure and modera gap. Furthermore, the V-doped g-C3N4 possesses excellent catalytic performance activation. Meanwhile, the catalytic performance of the V-doped g-C3N4 is in firstly and then decreased with the increase of doping content, in which the BPA completely removed within 25 min over V-g-C3N4-0.5. In addition, the control exp illustrates the visible light irradiation also plays the key role in PMS activation for pollutions degradation. It should be pointed out that the trend is consistent with t tocatalytic degradation of BPA in absence of PMS, further confirming the catalytic mances for PMS activation comes from the intrinsic photocatalytic capability of V g-C3N4. This could be explained by the fact that the doping V atoms in the networ C3N4 would decrease band gap energy and improve carrier utilization, resulting enhanced degradation efficiency. Nevertheless, an excess of V atoms might cond combination sites for charge carriers, which resulted in abatement of catalytic mance. The results illustrate that the doping content has great effects on the cataly formances of V-doped g-C3N4, and the V-g-C3N4-0.5 has the best performance. In order to further clarify the performance of V-doped g-C3N4 for catalytic ac of PMS, HPLC was used to study the degradation process of BPA. As shown in F the peak of BPA is declining with the increase of reaction time, and disappeared min. A weak peak is increased and then dropped during the reaction process, whic be ascribed to the generation intermediates from the degradation reaction of BP thermore, it is worth noting that this peak is disappeared completely after the com of the catalytic reaction. These results indicates that BPA can be successfully mine to CO2 and H2O. The similar result is also observed in previous literatures [22,45] dition, this system was also compared with previously reported literatures, and the are listed in Table 1. These result reveal that the V-doped g-C3N4 performs superi lytic capability for PMS activation under visible light. In order to further clarify the performance of V-doped g-C 3 N 4 for catalytic activation of PMS, HPLC was used to study the degradation process of BPA. As shown in Figure 8, the peak of BPA is declining with the increase of reaction time, and disappeared after 25 min. A weak peak is increased and then dropped during the reaction process, which might be ascribed to the generation intermediates from the degradation reaction of BPA. Furthermore, it is worth noting that this peak is disappeared completely after the completion of the catalytic reaction. These results indicates that BPA can be successfully mineralized to CO 2 and H 2 O. The similar result is also observed in previous literatures [22,45]. In addition, this system was also compared with previously reported literatures, and the results are listed in Table 1. These result reveal that the V-doped g-C 3 N 4 performs superior catalytic capability for PMS activation under visible light.

Activation Mechanism
The surface area and microstructures of the g-C3N4 and V-doped g-C were determined by the N2 sorption isotherms measurement. As presented i of them show a similar shape of type IV isotherms and H2 hysteresis loops, i presence of some mesopores. The specific surface areas of the g-C3N4, V-g-C C3N4-0.2, V-g-C3N4-0.5, and V-g-C3N4-0.8 were calculated to be ca. 16.95, 17.63 and 25.56 m 2 /g, respectively. It can be seen that the doping V atoms into the g-C3N4 may slightly ameliorate the specific surface areas. Thus, the specific has little impact on the catalytic activity.

Activation Mechanism
The surface area and microstructures of the g-C 3 N 4 and V-doped g-C 3 N 4 samples were determined by the N 2 sorption isotherms measurement. As presented in Figure 9, all of them show a similar shape of type IV isotherms and H2 hysteresis loops, indicating the presence of some mesopores. The specific surface areas of the g-C 3 N 4 , V-g-C 3 N 4 -0.1, V-g-C 3 N 4 -0.2, V-g-C 3 N 4 -0.5, and V-g-C 3 N 4 -0.8 were calculated to be ca. 16.95, 17.63, 19.69, 22.63, and 25.56 m 2 /g, respectively. It can be seen that the doping V atoms into the framework of g-C 3 N 4 may slightly ameliorate the specific surface areas. Thus, the specific surface areas has little impact on the catalytic activity. For an in-depth investigation of the degradation mechanism, the quenching ex ments were performed to identify the reactive species in this catalytic reaction sy (Figure 10a). After the addition of BQ and EDTA, the degradation rate is remarkabl creased, implying the •O2 − and h + •have pivotal roles in BPA degradation [20,21]. How the degradation rate is not obviously changed by the addition of IPA, suggesting plays a minor role in this degradation reaction. Meanwhile, the degradation reacti BPA is markedly inhibited by MeOH, indicating that the SO4 •− plays a significant Therefore, •O2 − , SO4 •− , and h + are the important species in the catalytic destruction of under the visible light. For an in-depth investigation of the degradation mechanism, the quenching experiments were performed to identify the reactive species in this catalytic reaction system (Figure 10a). After the addition of BQ and EDTA, the degradation rate is remarkably decreased, implying the •O 2 − and h + ·have pivotal roles in BPA degradation [20,21]. However, the degradation rate is not obviously changed by the addition of IPA, suggesting •OH plays a minor role in this degradation reaction. Meanwhile, the degradation reaction of BPA is markedly inhibited by MeOH, indicating that the SO 4 •− plays a significant role. For an in-depth investigation of the degradation mechanism, the quenching experiments were performed to identify the reactive species in this catalytic reaction system (Figure 10a). After the addition of BQ and EDTA, the degradation rate is remarkably decreased, implying the •O2 − and h + •have pivotal roles in BPA degradation [20,21]. However, the degradation rate is not obviously changed by the addition of IPA, suggesting •OH plays a minor role in this degradation reaction. Meanwhile, the degradation reaction of BPA is markedly inhibited by MeOH, indicating that the SO4 •− plays a significant role. Therefore, •O2 − , SO4 •− , and h + are the important species in the catalytic destruction of BPA under the visible light.  In principle, the charge transfer exhibits a positive relationship with the catalytic performance of PMS activation [51]. Thus, the separation efficiency of generated carriers over V-g-C 3 N 4 -0.5 and g-C 3 N 4 was investigated by EIS. From the EIS Nyquist plot (Figure 10b), the V-g-C 3 N 4 -0.5 shows a smaller semi-circular radius than pure g-C 3 N 4 , indicating a high charge mobility and a low interfacial resistance. Therefore, the doping V atoms can effectively ameliorate the electronic conductivity, enhance charge separation, and decrease the charge carrier recombination rate.
The electronic structure of prepared samples was further explored by the Tauc plot and Mott-Schottky analysis to understand the charge transfer mechanism [30,[52][53][54]. It is widely known that the band gap energy (Eg) of semiconductors can be estimated according to the Kubelka-Munk method [52,[55][56][57]. As illustrated in Figure 10c, the Eg of g-C 3 N 4 and V-g-C 3 N 4 -0.5 are determined as 2.72 and 2.66 eV, respectively. This result reveals that doping V atoms can narrow the band gap of g-C 3 N 4 . Furthermore, the CB potential of g-C 3 N 4 and V-g-C 3 N 4 -0.5 were analyzed from Mott-Schottky curves. As exhibited in Figure 10d, the g-C 3 N 4 and V-g-C 3 N 4 -0.5 shows a positive slope, verifying they are n-type semiconductors. Meanwhile, the flat-band potentials of g-C 3 N 4 and V-g-C 3 N 4 -0.5 are −1.38 and −1.31 V (Ag/AgCl, pH = 7). Accordingly, the CB position of g-C 3 N 4 and V-g-C 3 N 4 -0.5 are determined to be −1.16 and −1.09 eV (NHE, pH = 7), respectively. Furthermore, the VB position of g-C 3 N 4 and V-g-C 3 N 4 -0.5 are 1.56 and 1.57 eV (NHE, pH = 7), indicating that the doping V atoms has no significant influence on the VB position of g-C 3 N 4 .
Based on the above results, a schematic diagram of the mechanism for the PMS activation over the V-doped g-C 3 N 4 under light irradiation is proposed in Figure 11. The V-doped g-C 3 N 4 are excited to produce electrons and holes by visible light irradiation. These electrons and holes would shift onto the VB and CB edges, respectively. Afterwards, these excited electrons can activate PMS to generate SO 4 •− . Additionally, these photoinduced electrons would capture O 2 molecules to generate •O 2 − . The h + accumulated on the CB edges possesses strong oxidation activity to mineralize organic compounds. It should be noted that the •OH cannot be directly formed by oxidization of h + due to the thermodynamic limitations [29,34] In principle, the charge transfer exhibits a positive relationship with the catalytic performance of PMS activation [51]. Thus, the separation efficiency of generated carriers over V-g-C3N4-0.5 and g-C3N4 was investigated by EIS. From the EIS Nyquist plot (Figure 10b), the V-g-C3N4-0.5 shows a smaller semi-circular radius than pure g-C3N4, indicating a high charge mobility and a low interfacial resistance. Therefore, the doping V atoms can effectively ameliorate the electronic conductivity, enhance charge separation, and decrease the charge carrier recombination rate.
The electronic structure of prepared samples was further explored by the Tauc plot and Mott-Schottky analysis to understand the charge transfer mechanism [30,[52][53][54]. It is widely known that the band gap energy (Eg) of semiconductors can be estimated according to the Kubelka-Munk method [52,[55][56][57]. As illustrated in Figure 10c, the Eg of g-C3N4 and V-g-C3N4-0.5 are determined as 2.72 and 2.66 eV, respectively. This result reveals that doping V atoms can narrow the band gap of g-C3N4. Furthermore, the CB potential of g-C3N4 and V-g-C3N4-0.5 were analyzed from Mott-Schottky curves. As exhibited in Figure  10d, the g-C3N4 and V-g-C3N4-0.5 shows a positive slope, verifying they are n-type semiconductors. Meanwhile, the flat-band potentials of g-C3N4 and V-g-C3N4-0.5 are −1.38 and −1.31 V (Ag/AgCl, pH = 7). Accordingly, the CB position of g-C3N4 and V-g-C3N4-0.5 are determined to be −1.16 and −1.09 eV (NHE, pH = 7), respectively. Furthermore, the VB position of g-C3N4 and V-g-C3N4-0.5 are 1.56 and 1.57 eV (NHE, pH = 7), indicating that the doping V atoms has no significant influence on the VB position of g-C3N4.
Based on the above results, a schematic diagram of the mechanism for the PMS activation over the V-doped g-C3N4 under light irradiation is proposed in Figure 11. The Vdoped g-C3N4 are excited to produce electrons and holes by visible light irradiation. These electrons and holes would shift onto the VB and CB edges, respectively. Afterwards, these excited electrons can activate PMS to generate SO4 •− . Additionally, these photoinduced electrons would capture O2 molecules to generate •O2 − . The h + accumulated on the CB edges possesses strong oxidation activity to mineralize organic compounds. It should be noted that the •OH cannot be directly formed by oxidization of h + due to the thermodynamic limitations [29,34]. Besides, the •OH could be formed via interconversion reactions of •O2 − and SO4 •− [15,37]. Therefore, the BPA molecules can be mineralized under the synergistic effect of •O2 − , SO4 •− , h + , and •OH. Figure 11. The proposed catalytic mechanism of PMS activation on the V-doped g-C3N4. Figure 11. The proposed catalytic mechanism of PMS activation on the V-doped g-C 3 N 4 .

Conclusions
In summary, the V-doped g-C 3 N 4 was successfully designed and fabricated via onestep thermal polymerization method to activate PMS toward organic contamination degradation. The results reveal that the V atoms are successfully integrated into the structure of g-C 3 N 4 , which can effectively increase visible light absorption intensity and enhance charge separation. The obtained V-doped g-C 3 N 4 shows superior catalytic performance for PMS activation, in which the BPA can be completely removed within 25 min. Moreover, doping content has a great influence on the activation performances. The radical quenching experiments confirm the •O 2 − , SO 4 •− , and h + are significant species in degradation reactions. Finally, a possible degradation mechanism is discussed and proposed. This work provides a feasible strategy to optimize the performance of novel materials for environmental remediation. Funding: This research was funded by a special project for performance incentive and guidance of Chongqing institutions (cstc2021jxjl20014 and cstc2021jxjl20024) and the Natural Science Foundation of Chongqing (cstc2020jcyj-msxmX0610 and cstc2021jcyj-msxmX1194).