1. Introduction Globally, the large amounts of anthropogenic contaminants in the aquatic environment, including azo dyes, Cr(VI) ions and phenols, have hindered the development of human society.[1] Over the past years, the semiconductor-photocatalysis has been emerged as an efficient, economical and eco-friendly technology for the waste water purification.[2] The semiconductor catalysts play a vital role for the photo-degradation.[1, 2] Among diversified photocatalysts, graphitic carbon nitrides(g-C3N4), as an ideal visible-light-driven semiconductor, exhibits excellent photocatalytic performance and photo-stability.[3, 4] Nonetheless, the fast electron-hole recombination limits its photocatalytic activity.[5] Various strategies, including band-structured engineering,[6-8] heterostructured constructing,[9-12] and nano-structuralization,[12, 13] has been developed to solve that problem.[14] Particularly for the g-C3N4 material with the abundant kinds of active surface groups on the layer edge, the surface-group variation of g-C3N4 photocatalysts could play a significant role in the transfer of photo-induced electron-holes at the semiconductor interface and influences of the acidity of the photocatalytic system. [15-18] For example, Zhang et al. introduced the positive surface changes on g-C3N4 photocatalysts by the acid treatment, which improved the photocatalytic activities of Cr(VI) reduction.[19] Dong et al. proven that porous g-C3N4 nanosheet, which was synthesized by the multiple thermal treatment, could effectively enhance the photocatalytic activity and oxidation.[20] She et al. reported that the electrophilic groups (C-O, C=O and -COOH) could inhibit electron-hole recombination and improve redox ability of gC3N4.[21] Yet, instability of modified groups on the photocatalysts surface was existed, because of various kinds of oxygen radicals in the process of pollutant degradation. Besides, for the g-C3N4 with 2D layer structure, the melem (C6N7(NH2)3) triangular plate building blocks sharing their corners assemble into one multi-membered ring with a diameter of ∼4.88 Å on (0 0 1) crystal face, resulting that the interstitial spaces are easy to accommodate guest molecule.[14] The atomic arrangement could be influenced on the intrinsic properties of the edged groups. The metal-ion doping, as one way of band-structure engineering to extend the absorption active range of sunlight and enhance the separation of photo-induced carrier, always affect the distortion of the atom arrangement to lead the change of the electron density of semiconductor.[4] For instance, Wang et al. reported that the photocatalytic activity of the hydroxylation of benzene to phenol using H2O2 was improved by the metal-ion doping modification of g-C3N4 photocatalyst, including Fe, Cu, Co, Zn and Mn.[22] Lu. et al. has proven that the doped Cu and Fe ions effectively promoted the photocatalytic performance of g-C3N4 for H2 generation and H2O2 disproportionation under visible light.[23] Pati et al. demonstrated that the electron density of Mn-g-C3N4/graphene photocatalysts increased as compared with that of the no-doped material.[24] Therefore, the combined effect of the ion doping and surface modification of g-C3N4 semiconductor would be of significance to promote the photocatalytic performance. In addition, most of researchers paid attention to the separated photocatalysis of dye degradation and Cr(VI) reduction.[2, 25] Since most of the inorganic and organic pollutants are considered to be co-existed in industrial wastewaters, simultaneous photocatalysis of mixed-pollutants degradation has been an active field of intensive research.[26-29] Shang et al.[30] reported that the gC3N4/Ag2CrO4 composite exhibited the highest photocatalytic activity for degrading multiple organics in single system, and about 99.2% and 99.1% of RhB and MB were removed after 90

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and 120 min visible-light irradiation, respectively. Liu et al.[31] introduced that g-C3N4/Ti-SBA15 composite showed the good photocatalytic activity of Cr (VI) reduction and phenol degradation which could reach about 90% and 65% in acidic mixed system (pH 2.3) after 120 min illumination. Zhang et. al.[32] proven that the simultaneously photoelectrocatalytic activity for reduction of Cr(VI) and degradation of phenol using g-C3N4/TiO2-NTs exhibited higher than that using naked TiO2-NTs under UV-visible light irradiation, and the synergistic effect of Cr(VI) reduction and organics degradation were also observed in the Cr(VI)/benzyl alcohol system, and Cr(VI)/dyes system.[33, 34] Hence, it is crucial to build the stable photocatalytic system using highly efficient and stable photocatalysts for the multi-pollutants degradation. Herein, the porous Mn doped g-C3N4 was got by the calcination-refluxing method and exhibited the remarkable photocatalytic performance. The affection of the Mn doping and surface modification of the g-C3N4 on the catalytic activity of Cr(VI) reduction and RhB degradation was investigated under visible light irradiation (> 400 nm). Furthermore, synergistic photocatalysis of Cr(VI) reduction and organics degradation was evaluated over porous Mn doped g-C3N4 sample. In order to further understand the influence of the doped Mn ion and edged group on the inherent property of g-C3N4 catalysts, the highest occupied molecular orbital and lowest unoccupied molecular orbital of the as-prepared samples were explored by computational modeling. Besides, the active oxygen species in the process of the photocatalytic degradation were detected experimentally and the mechanism of the synergistic photocatalysis was thus proposed.

2. Experimental

2.1. Sample preparation The g-C3N4 sample was synthesized by direct calcination using recrystallized melamine, as starting material. The g-C3N4-Mn-H sample was produced by the calcination-refluxing method. In a typical test, the melamine powder (1.232 g) added in the 100 mL of HCl solution (0.01 mol/L) with the 0.198 g MnCl2·4H2O and the mixed solution was heated at 95 oC for 2.0 h after 0.5 h of ultrasonic treatment. After fast cooling to room temperature, the precipitate was acquired by the filtration. The obtained powder was kept at 550 oC for 1.0 h at a heating rate of 10.0 o C/min. Then, the light-yellow-colored powder, denoted as g-C3N4-Mn, was got after washing by water and HNO3 solution (1.0 mol/L). The mixed solution of g-C3N4-Mn powder (0.50 g) and 100 mL of HNO3 solution (8.0 mol/L) was heated at 95 oC for 2.0 h. Finally, the powder was centrifuged and then washed with distilled water to remove the residual acid. The final powder (g-C3N4-Mn-H) was dried at 70 oC for 12 h. To study the role of the Mn cation, the g-C3N4-H sample was synthesized by the above method of g-C3N4-Mn-H expect with all solution without the addition of MnCl2·4H2O.

2.2. Catalyst characterization The X-ray diffraction (XRD) patterns of the samples were measured on an X'Pert PRO X-ray powder diffractometer with Cu-Kα radiation (λ =1.5418 Å). Contacts of the Mn element in sample were obtained by 2600T ICP-AES (Skyray Instrument, China). The real C/N/O composition evaluated from the automatic element analyses(EA) for the g-C3N4-H samples was got by the Thermo Flash EA 1112 spectrometry. N2 adsorption-desorption isotherms were performed on a NOVA 1000e surface area and porosity analyzer (Quantachrome, USA) after the sample had been degassed in the flow of N2 at room temperature. The BET surface area was

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estimated using desorption data. The temperature-programmed desorption (TPD-NH3) was measured by Automated Gas Sorption Analyzer (Autosorb-IQ, Quantachrome, USA). Scanning electron microscope (SEM) was carried out by Quanta FEG 250 electron microscope (FEI, USA). Transmission electron microscopy (TEM) was performed on a Tecnai G2 20 S-TWIN electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS, AXISULTRA DLD-600W, Shimdzu) were recorded using Al-Kα irradiation source. The samples were subsequently analyzed for carbon, nitrogen, hydrogen and oxygen on a CE Instruments Flash EA 1112 HT analyzer(Thermo, USA) with a sequence of standards and blanks. UV-Vis diffuse reflectance spectra (DRS) was obtained using an UV-vis spectrometer (UV-3600, Shimdzu).

2.3. Photocatalytic test A 300 W Xe arc lamp (CEL-HXF300, Beijing CEAULIGHT Co., Ltd.) equipped with an ultraviolet cutoff filter (> 400 nm) was applied as the visible-light source. The photocatalytic experiments were measured with the sample powder (50 mg) suspended in the 100 mL aqueous solution (pollutant concentration: RhB 10 mg/L, MB 10 mg/L, 4-CP 10 mg/L and K2CrO4: 15 mg/L) with constant stirring for 1.5 h in the dark. It could establish the adsorption-desorption equilibrium. At given time (10 min) intervals under light irradiation, about 3 mL of the suspension was taken to ensure further analysis after the centrifugation. Content of organic dyes including RhB and MB was analyzed directly by UV-vis spectroscopy.[35] The concentration of 4-CP was estimated by measuring the absorbance at a wavelength of 505 nm using the colorimetric method of 4-aminoantipyrine with K3Fe(CN)6, as color agent. Meanwhile, the concentration of Cr(VI) was analyzed by the indirect UV-vis spectroscopy with given wavelength (353 nm) light.[36] The degradation efficiency of the as-prepared sample was calculated by the average results of three tests. The detailed process of cycling experiment was described in the supporting information.

3. Results and Discussion

3.1. Composition, Structure and Morphology The XRD spectra of the as-prepared g-C3N4 samples were shown in Figure 1. Two distinct peaks at about 13.0 and 27.3 o were indexed as the (0 0 2) and (1 0 0) crystal planes of graphite phase carbon nitride, which was corresponding to the interplanar stacking of the conjugated aromatic systems and the inplane ordering of tri-s-triazine units, respectively.[37] Although the Mn atom was existed in g-C3N4-Mn and g-C3N4-Mn-H samples, depending on ICP-AES results(Table S1), all the samples exhibit no other impurity phase. There is only a small shift of the peak position of the (1 0 0) crystal plane between Mn doped and no-doped g-C3N4 samples. It illustrated that the Mn ion maybe has doped into the center-hole of the tri-heptazine units. Besides, the peak intensity of the (0 0 2) crystal plane regularly weakens after the acidic treatment. Due to the C and O contents in as-prepared samples(Table S1), a large amount of the oxygen-containing group was formed on the g-C3N4 surface by acidity-etching method, leading to the dislocation enhancement of the crystal structure. To accurately determine the morphology of the as-prepared samples, the TEM and SEM measurements were carried out. As shown in Figure 2a, the g-C3N4 sample exhibited a typical layer structure. According to Figure 2b, Mn doping has little effect on the material morphology. It was notable that the distinct crack was present on the layer surface of g-C3N4-H sample (Figure 2c), which was caused by surface etching through acidic oxidation. In Figure 2d, the

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layer structure of g-C3N4-Mn-H was obviously destroyed simultaneously by Mn doping and the acidic oxidation. The pure g-C3N4 shows a crumpled layered structure in Figure 3a. As presented in Figure 3b, the g-C3N4-Mn sample exhibits layered structure without obvious crumple and no nanoparticle appeared on the g-C3N4-Mn surface. Only evident lattice fringe of the (0 0 2) crystal plane of gC3N4 was discovered in the HRTEM image(the inset of Figure 3b). Morphology of g-C3N4-H sample reveals a very thin flake structure with abundant pores, and the average pore diameter of the continued layer reaches about 43.2 nm. Similarly, the large amount of pore structure on the surface clearly appeared in the g-C3N4-Mn-H samples(Figure 3c). According to the results of N2 absorption-desorption measurement(Figure S1), the surface area of the treated samples all was higher than that of the pristine g-C3N4 and a new peak of the larger pore sizes was appeared in the pore size distribution of g-C3N4 samples after the acid-refluxing. It may be the reason that the g-C3N4 surface was etched to form the pore structure through the acid-oxidation. To explore the detailed elemental distributions over the surface of the g-C3N4-Mn-H sample, energy mappings of N and Mn elements in green box of Figure 3d were constructed and displayed in the Figure S2. Both of the two elements were uniformly distributed on the sample surface. By combining with TEM and XRD results of the g-C3N4-Mn-H, it is demonstrated that the Mn cation could be successfully doped into the g-C3N4 materials. X-ray photoelectron spectroscopy (XPS) measurement was used to investigate the surface chemical compositions and the valence states of C, N, O and Mn atoms. The main element ratios of the as-prepared g-C3N4 samples were summarized in Table S2. The change of element ratios indicates that the abundant oxygen-containing groups were existed on the surface of g-C3N4-H and g-C3N4-Mn-H samples. To further prove this point, the adsorption type and strength of molecular ammonia on the surface of the three samples were investigated by Temperatureprogrammed desorption (TPD). As shown in Figure S3, the desorption peak at around 140 oC was attributed to the ammonia absorption of the carboxyl group on the surface.[21] The peak area of g-C3N4-H and g-C3N4-Mn-H sample all exhibited higher than that of the as-prepared g-C3N4 without acidic refluxing, which revealed that more acidic groups were formed on the surface of acidic-treated g-C3N4 samples. For the purpose of acquiring more insights into the chemical bonding between the carbon, nitrogen, oxygen and manganese atoms in the as-prepared samples, the high resolution XPS spectra of C, N, O and Mn were deconvoluted by the Gaussian-Lorenzian method.[38] In the highresolution C 1s spectrum of g-C3N4 sample (Figure 4a), three fitted peaks were observed and located at around 284.7, 285.9 and 287.8 eV, which can be regarded as the sp2 carbon of the C-C bonds, the carbon in C-NH2 species and the hybridized carbon in N-containing aromatic ring (NC=N), respectively.[38, 39] Notably, two new peaks in the g-C3N4-H and g-C3N4-Mn-H were detected at 288.5 and 289.0 eV, which was assigned to the C=O and C-O species on the g-C3N4 surface.[21, 40] As shown in Figure 4b, the XPS spectra of O 1s for the g-C3N4 sample can be fitted with two peaks centered at 531.8 and 533.3 eV, which was regarded as the absorbed H2O molecule and the hydroxyl group on the surface, respectively.[39, 41] Apart from the above O 1s peaks, two other peaks at 532.0 and 531.4 eV were obtained in the spectra of acid-treated g-C3N4 sample, which was attributed to the C=O and N-C-O groups, respectively.[21] Then, according to the TPD results and the pH value of dispersion solution (Table S1), the carboxyl group was able to be formed after the acid-refluxing. Figure 4c displays the high-resolution XPS spectrum of Mn 2p. Except for the g-C3N4-Mn-H and g-C3N4-Mn sample, the Mn element was not found in other samples. Two distinct peaks at 640.1 and 652.0 eV with the spin-orbital splitting of 11.9 eV were

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observed in the Mn 2p core level spectrum of g-C3N4-Mn sample, which corresponds well to the Mn 2p3/2 and Mn 2p1/2 of the Mn(II) state.[42, 43] As shown in the N 1s core level spectrum(Figure 4d), four peaks at 398.1, 399.0, 400.3 and 403.9 eV are observed, which can be attributed to terminal amino functions (C=N-H), the tertiary nitrogen (N-(C)3), the nitrogen of the triazine rings(C-N=C) and π-excitations.[44] Besides, the N 1s peaks shift of g-C3N4 sample with Mn doping and/or carboxyl modification was appeared, because the electron density of the N atom could be changed by these extra O and doping Mn atoms.[41] Based on the above results, the Mn cation and the carboxyl group simultaneously existed on the g-C3N4-Mn-H sample. For purpose of the study of the optical absorption of the as-prepared g-C3N4 based sample, the UV-Vis absorption spectra was shown in Figure 5. The light absorption edges of the g-C3N4-H, g-C3N4-Mn and g-C3N4-Mn-H samples extended slightly to the long wavelength range as compared with the pristine g-C3N4. The band-gap energies of the as-prepared samples were estimated from the intercept of the tangent in the plots of (αhν)1/2 versus Energy band (the inset of the Figure 5). The apparent Eg values of the as-prepared g-C3N4-H, g-C3N4-Mn and g-C3N4Mn-H samples reached 2.65, 2.62, 2.60 and 2.61 eV, respectively. The light absorption range of g-C3N4 materials could be slightly widened by the Mn doping and carboxylation method.

3.2. Photocatalytic performance of Cr(VI) reduction and organics degradation The photocatalytic performance over the as-prepared samples was investigated in the single Cr(VI) and RhB solutions. Figure 6a shows the photocatalytic activities of Cr(VI) reduction under visible-light illumination. The degradation efficiencies of the modified g-C3N4 samples were all better than that of pristine g-C3N4, and g-C3N4-Mn-H sample displayed the highest photocatalytic activity of Cr(VI) reduction, which agreed basically with the change of their surface area (Figure S1). It was noted that the catalytic suspensions of g-C3N4-H and g-C3N4Mn-H samples were all acidic, because of the formation of a carboxyl group on their surface. The thermodynamic driving force for Cr(VI) reduction increases by 79 mV with a decrease of per pH unit in catalytic system.[45, 46] The influence of the acidity on the photocatalytic efficiency is investigated, keeping all other parameters constant over g-C3N4 photocatalyst (Figure S4 and S5). The photocatalytic activities of Cr(VI) reduction over g-C3N4 were lower than that over modified g-C3N4 photocatalysts when the catalytic solutions kept the same acidity. The result illustrated that the acidic groups on the host layer not only enhanced the acidity in catalytic system, but also promoted the carriers separation and electron transfers at interface. As illustrated in Figure 6b of RhB degradation, the g-C3N4 catalysts with acidic refluxing also exhibited higher than that over pristine g-C3N4 sample, which suggests that the acidic groups on the host layer promotes the photocatalytic activity. the impact of the acidity of catalytic suspension should never be ignored. According to the GC-MS and UV-Vis measurement in different acidity (see in Figure S6~S8 and Table S3), it could be explained that two different mechanisms of the RhB photodegradation in acidic system were simultaneously involved, the successive deethylation of the four ethyl groups and the direct degradation of the chromophoric system.[47] In order to quantitatively understand the reaction kinetics of the RhB degradation and Cr(VI) reduction over the g-C3N4 photocatalysts, the pseudo-first order mode given by Eq. 1 was used for the photocatalytic degradation process.[30, 48] ln(C0/C)=kt (1)

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where k, C0 and C are the pseudo-first order rate constant and the concentrations of pollutant in single solution at time 0 and t, respectively. According to the above Eq. 1, the the pseudo-first order rate constant k was calculated and listed in Table 1. A rather good correlation to the pseudo-first-order reaction kinetics was found, and the kRhB and kCr(VI) values of g-C3N4-Mn-H sample were more 4 times that of the pristine g-C3N4. The Mn doped g-C3N4 all exhibited the higher activity than that of g-C3N4 without Mn element, and the g-C3N4-Mn-H was the best sample. Hence, the photocatalytic results suggested that both Mn doping and carboxyl modification could cause the enhancement of photocatalytic activity of Cr(VI) reduction and RhB degradation under visible-light illumination. As presented in Figure 7, the photocatalyst exhibited the good photodegadation activities of Cr(VI) and organics in co-existed solution using g-C3N4-Mn-H, as photocatalyst. The degradation efficiency in the mixed system all exhibited the higher than that in single. Especially, the photocatalytic activities of Cr(VI) reduction and RhB degradation in the coexisted system was increased significantly to 76.5% and 88.9%, compared with 31.5% and 51.4% in the single pollutant, respectively. As displayed in Figure S9, no apparent photocatalytic activity was found in the control experiment in organics and Cr(VI) mixed systems without catalyst, indicating that the self-photolysis of targeted pollutant could be ignored. Compared with the need for an extra acid or sacrificial agent to promote photocatalytic efficiency of Cr(VI) reduction, the simultaneous photocatalysis is imperative for wastewater purification. As displayed in Figure 8, the photocatalytic efficiency of Cr(VI) reduction and RhB degradation in the 7th cycling could reach more than 65.0% and 70.0%, respectively. Besides, no change of the crystal structure could be found, on the basis of the XRD results of catalysts before/after the cycling test(Figure S10). However, according to the XPS measurement(Figure S11), the partial C=O and C-O groups on the catalyst surface was lost and the residual Cr(VI) and Cr(III) were discovered on the g-C3N4 photocatalysts after the 7th cycling.[38] Hence, the porous Mn doped gC3N4 has the good activity and the cycling stability of the synergetic photocatalysis under visible light.

3.3 Computational modeling of the carboxyl functional group and doping Mn cation According to the above degradation experiments, the g-C3N4-Mn-H sample exhibited the good photocatalytic performance. In order to explore the role of the carboxyl functional group and doping Mn cation, our g-C3N4 monolayer model was built on the bridged heptazine units with a hexagonal unit cell. The model contained a large size void, as shown in Figure 9a. Yet, all C and N atoms in one hypothetical unit never stay at one same plane, due to the difference between the electronegativity of N and C atoms.[24, 49] Although the appearance of the carboxyl group could increase the acidic property of the surface, it never brought the vastly obvious change of the atomic arrangement in g-C3N4-H sample. Compared with the pristine g-C3N4, the spacing between N atoms around the central hole of g-C3N4-H only has a slight decreasing, as summered in Table S4. Interestingly, there was a major change of the N atoms spacing between the pristine g-C3N4 and g-C3N4-Mn. As shown in Figure 9c, the arrangement of C and N atoms tends to the conplane structure, owing to the doped Mn atom. Analogously, atom construction of g-C3N4Mn-H presented the conplane framework, according to the optimized geometry. The result distinctly illustrates that the Mn doping modification could availably influence the arrangement of C and N atoms in g-C3N4. The change of atom framework for semiconductors frequently affects the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of as-prepared

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photocatalysts. The HOMO and LUMO of as-prepared g-C3N4 samples were plotted in Figure 10. In pristine g-C3N4 materials, the HOMO is uniformly distributed in all region of g-C3N4. The location of the HOMO mainly tended to one tri-s-triazine unit.[48] In the presence of carboxyl group, the HOMO gradually distributed to two bridged heptazine units without the carboxyl group, while the LUMO of g-C3N4-H sample predominantly distributed on other heptazine unit. Compared with the pristine g-C3N4 material, the distribution of HOMO in g-C3N4-Mn was located at around the angle of the calculated units, while the location of LUMO exhibited the reserved areas, as shown Figure 10c. The distributive position of HOMO and LUMO was distinctly presented in the different regions, resulting in the beneficial separation of photoinduced carriers.[50] Hence, the Mn doping method may improve the separation of photo-included carriers in g-C3N4 materials. In the g-C3N4-Mn-H samples (Figure 10d), the locations of HOMO and LUMO were segregated in the hypothetical units. Based on the structure calculations, the Mn doping and carboxyl modification may affect the distribution of the photo-included electron and holes in the g-C3N4 materials. To further confirm the affection of the Mn doping and surface modification on the photo-induced charge transfer property of g-C3N4, the photoelectric currents of sample during on-off illumination were carried out in Figure 11. The photoelectric currents of modified g-C3N4 samples all exhibit better than that of pristine g-C3N4, while the transient photocurrent of the gC3N4-Mn-H is about 2 and 3 times higher than that of g-C3N4-Mn and g-C3N4-H, respectively. The results indicated that the Mn doping and the carboxyl modification enhance the photoinduced carrier separation of the photogenerated carriers and the electron-transfer at interface,[51] and the synergistic effect may be existed between two modification, leading to increasing the photocatalytic activities of Cr(VI) reduction and RhB degradation.

3.4. Possible photocatalytic mechanism For the purpose of exploring the function of photo-induced electron-holes in the process of photocatalysis, the controlled experiments with the different scavengers have been carried out over g-C3N4-Mn-H photocatalyst(Figure S12). The results indicated that the photocatalysis of the Cr(VI) reduction and the RhB degradation could rapidly consumption of photo-generated electron and holes, respectively. Furthermore, various kinds of active species from dissolved oxygen can affect the RhB degradation and the Cr(VI) reduction.[46, 52] As presented in the UVvis spectra(Figure S13) using the o-tolidine, as the trapping agent,[37] the formation of H2O2 was verified after illumination and the contents of H2O2 in the existence of RhB and/or Cr(VI) molecules reveals lower that in water. It was the reason that the H2O2 molecules could be consumed by the oxidation of the RhB molecule and the chromium species with low valance states. To testify about the effect of the active radical, the ESR experiment was employed after 10 min irradiation in different systems and the results were presented in Figure 12. Except for three peaks of the DMPO oxidation in blank test, the characteristic signals of the DMPO-•OH and DMPO-O2-• adducts were observed in water under air atmosphere, on the basis of an obvious quartet peaks and a six-fold peaks, respectively.[48] However, no signal of any radical could be found under N2 atmosphere, because of the formation of the •OH and O2-• radicals from the dissolved oxygen. Intensity of DMPO-O2-• signal in the Cr(VI) solution never has significant change, compared with that in water. Yet, the DMPO-•OH and DMPO-O2-• signals disappeared in single and co-existed RhB solution. Hence, the oxidizing radicals, including O2-• and •OH could be effectively consume with the RhB degradation.

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Based on the above results, the photogenerated electrons and holes under light illumination could transfer to the surface of catalysts. The photogenerated hole on the surface could join in the oxidation of the adsorbed RhB molecule. Concurrently, the photogenerated electrons not only directly transfer to Cr(VI) to form lower valence states chromium, but also could be scavenged by dissolved oxygen converting to O2−•. After combination with H+ ions, the O2−• radical could produce •OH and H2O2. They all participated in the organics oxidation. Unfortunately, the coexisted H2O2 could unavoidably oxide low valance states of chromium to Cr(VI). It is notable that the photogenerated holes and various active oxidations including H2O2, •OH and O2−• could be effectively consumed by the RhB degradation, according to the ESR analysis and the catalytic experiments. In addition, according to the Figure S14 of the controlled test about the dye photosensitization in the degradation process, the result indicates that the RhB photosensitization plays a positive and significant role of synergistic photocatalysis in co-existed system.[47] The probable mechanism thus was proposed and illustrated in Figure 13.

4. Conclusions In summary, the porous Mn doped g-C3N4 was successfully prepared by the simple calcinationrefluxing method. The g-C3N4-Mn-H sample shows outstanding photocatalytic performance for Cr(VI) reduction and organics degradation under visible light irradiation. In Cr(VI)/RhB coexisted system, the photocatalytic ratio of Cr(VI) reduction is increased from 9.5% to 76.5% while that of RhB degradation is promoted from 15.3% to 88.9% after 1 h irradiation. Fortunately, the g-C3N4-Mn-H photocatalyst also exhibits the good synergetic photocatalytic performance in the 7th cycling. Analysis of computational modeling indicates that the Mn doping and carboxyl modification could affect the atomic arrangement and molecular orbital distribution of g-C3N4 photocatalysts, resulting in the significant enhancement of photo-induced carrier separation. Additionally, the RhB molecule not only promotes utilization of H2O2, O2−•, •OH and hole, inhibiting the oxidation of low valance states chromium in co-existed systems, but also validly supplies the photogenerated electron of the photocatalyst by the photosensitization effect. The gained knowledge may provide some insights into the synergistic effect of Cr(VI) reduction and RhB oxidation over g-C3N4 catalyst and the photocatalytic mechanism is finally proposed. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21307026), the Education Department of Henan Province (No. 2013GGJS-138, 14B150019), China Postdoctoral Science Foundation Funded Project (No. 2017M612391), the Innovation Team in Henan Province (No. C20150020), the Key Scientific Research Project Funding Scheme of Colleges and Universities of Henan Province (16A150007), the Landmark Innovation Project of Henan Institute of Science and Technology (No. 2015BZ02) and the Research Foundation for Advanced Talents of Henan Institute of Science and Technology (No. 2016035). References [1] D. Spasiano, R. Marotta, S. Malato, P. Fernandez-Ibañez, I. Di Somma. Solar Photocatalysis: Materials, Reactors,;1; Some Commercial, and Pre-industrialized Applications. A Comprehensive Approach, Appl. Catal., B, 170-171 (2015) 90-123. [2] R.L. House, N.Y.M. Iha, R.L. Coppo, L. Alibabaei, B.D. Sherman, P. Kang, M.K. Brennaman, P.G. Hoertz, T.J. Meyer, Artificial Photosynthesis:;1; Where are We now Where can We Go, J. Photochem. Photobio. C, 25 (2015) 32-45.

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Figure 1. XRD spectra of g-C3N4, g-C3N4-Mn, g-C3N4-H and g-C3N4-Mn-H samples.

Figure 2. SEM images of g-C3N4 (a), g-C3N4-Mn (b), g-C3N4-H (c) and g-C3N4-Mn-H (d).

Figure 3. TEM images of g-C3N4 (a), g-C3N4-Mn (b), g-C3N4-H (c) and g-C3N4-Mn-H (d).

Figure 4. XPS spectra for C 1s (a), O 1s (b), Mn 2p (c) and N 1s (d) of g-C3N4, g-C3N4Mn, g-C3N4-H and g-C3N4-Mn-H samples.

Figure 5. UV-vis absorption spectrum and the plot of (αhν)1/2 versus energy band (inset) results of g-C3N4, g-C3N4-Mn, g-C3N4-H and g-C3N4-Mn-H.

Figure 6. Photocatalytic activity of Cr(VI) reduction (100 mL, 15 mg/L) (a) and RhB degradation (100 mL, 10 mg/L) (b) under visible-light irradiation over different samples.

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Figure 7. Photocatalytic Cr(VI) reduction and organics degradation over g-C3N4-Mn-H catalysts (50 mg) after 1 h visible light irradiation in single and co-existed solution.

Figure 8. Photocatalytic activity with g-C3N4-Mn-H catalysts under irradiation above 400 nm in the Cr(VI)/RhB (15: 10) mixed solution.

Figure 9. Atom arrangements of the g-C3N4, g-C3N4-H, g-C3N4-Mn and g-C3N4-Mn-H samples.

Figure 10. HOMO and LUMO distributions of g-C3N4, g-C3N4-H, g-C3N4-Mn and gC3N4-Mn-H samples

Figure 11. Photocurrents of the g-C3N4, g-C3N4-H, g-C3N4-Mn and g-C3N4-Mn-H electrodes with visible light on/off test.

Figure 12. ESR spectra for the DMPO-containing aqueous suspensions of g-C3N4-MnH at different solutions after 10 min visible light irradiation (in air atmosphere: blank test (a), pure water (b), single Cr(VI) solution (d), single RhB solution (e) and co-existed system (f). in N2 atmosphere: pure water (c)).

Figure 13. Simplified pathways for synergistic photocatalysis of Cr(VI) reduction and RhB degradation using g-C3N4-Mn-H catalysts. Table 1 Degradation rate constants of various as-prepared photocatalysts in decomposing Cr(VI) and RhB. Cr(VI) Reduction RhB Degradation Sample g-C3N4

k Cr(VI) (×10-2 min-1) 0.144

0.9897

k RhB (×10-2 min-1) 0.301

0.9939

g-C3N4-Mn

0.480

0.9879

0.682

0.9932

g-C3N4-H

0.385

0.9851

1.023

0.9944

g-C3N4-Mn-H

0.626

0.9921

1.304

0.9963

R*

R*

*Correlation coefficient TDENDOFDOCTD

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