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Article

Petal-like g-C3N4 Enhances the Photocatalyst Removal of Hexavalent Chromium

by
Huijuan Yu
1,2,
Qiang Ma
3,
Cuiping Gao
1,
Shaohua Liao
1,
Yingjie Zhang
1,2,*,
Hong Quan
1,2 and
Ruiqi Zhai
1,2
1
College of Agriculture and Biological Science, Dali University, Dali 671000, China
2
Key Laboratory of Ecological Microbial Remediation Technology of Yunnan Higher Education Institutes, Dali University, Dali 671000, China
3
School of Architecture and Civil Engineering, Chengdu University, Chengdu 610106, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(3), 641; https://doi.org/10.3390/catal13030641
Submission received: 28 February 2023 / Revised: 19 March 2023 / Accepted: 20 March 2023 / Published: 22 March 2023
(This article belongs to the Special Issue Synthesis and In-Depth Characterization of Supported and Highly Dispersed Catalysts)
Browse Figures
Versions Notes

Abstract

:
The rapid progress of modern industry not only brings convenience to people’s lives, but also brings negative effects. Industrial development produces a large amount of waste metal, which brings harm to the environment and human health. Carbon nitride (g-C3N4) was successfully prepared using the thermal-polymerization method and petal-like g-C3N4 (CA-g-C3N4) was impregnated with citric acid (CA). Compared with g-C3N4, CA-g-C3N4 showed extremely high photocatalytic activity because the petal-like g-C3N4 (CA-g-C3N4) had a larger specific surface area, which increased the active sites on the surface of the photocatalyst and improved the photocatalytic activity. After citric acid treatment, the removal of hexavalent chromium (Cr(VI)) by g-C3N4 increased from 48% to 93%. The photocatalytic materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) and UV-vis diffuse reflectance spectra (UV-vis). In summary, this study confirmed that citric acid can improve the photocatalytic activity of g-C3N4 by increasing its specific surface area and the active site of the photocatalytic material so as to achieve the purpose of removing hexavalent chromium from water.

Graphical Abstract

1. Introduction

In the process of modernization, mining, metallurgy and electroplating have also been developed and applied extensively, becoming an important support for economic development in many places. While they promote the development of local economies, the problem of disposing of waste pollutants generated by factories has also arisen. Heavy metal pollution has attracted a great deal of attention as one of the most concerning types of pollution [1,2,3], and it is harmful to human health and ecosystems in different ways [4,5]. Chromium pollution is a type of heavy metal pollution, and the main source is the electroplating industry. As a surface treatment in major manufacturing industries, the electroplating industry has become an important carrier for the development of modern manufacturing industry. Depending on the different types of processed products, the types of wastewater and treatment methods can be very different. Electroplating wastewater is divided into chromium wastewater, cyanide wastewater and other wastewater in the process of treatment. Cr(VI) and Cr(III) are the main forms of chromium in wastewater [6]. Cr(VI) has strong oxidation and can enter the human body through breathing and contact. In severe cases, it can cause cancer and harm human health [7]. Cr(III) is considered a trace element that our body needs [8]. In order to reduce the harm of Cr(VI) in water systems and human health, various scholars have studied its related removal methods. At present, Cr(VI) is mainly removed using physical [9,10,11], chemical [12,13,14] and photocatalytic [15,16,17] processes. In the process of Cr(VI) removal by physical methods, the equipment used is expensive and the technological process is complex, so the removal effect of low-concentration pollutants is not ideal. In the chemical removal of Cr(VI), chemical agents are often added to achieve the purpose of reducing Cr(VI) in water. This method can effectively remove Cr(VI). However, due to the difficulty of controlling the dosage of chemical agents, other pollutants will be produced in the process of removing Cr(VI), causing pollution of the environment.
As an emerging industry, photocatalysis is an economical, efficient, non-secondary-polluting, high-removal-efficiency and reusable pollutant-removal technology, which can effectively solve the drawbacks of the original treatment technology. Photocatalysis is widely used in pollution removal [18,19,20], new energy development [21,22,23] and other areas. g-C3N4 is often used to degrade pollutants in water, such as Rhodamine B, ofloxacin and heavy metals such as arsenic. Photocatalysis technology can also be applied in the removal of Cr(VI) [24,25] because of its advantages, such as good thermal stability and low preparation cost [26,27,28]. It can be used to remove heavy metal contaminants and is easy to prepare. At present, the most commonly used preparation method of g-C3N4 is the thermal-polymerization method [29,30]. In the process of preparing g-C3N4 by the calcination method, however, the g-C3N4 prepared by these methods has the disadvantages of structural accumulation, easy recombination of photogenerated electron–hole pairs, small specific surface area and low photocatalytic activity [31,32].
In order to improve these shortcomings, many scholars have conducted many studies. g-C3N4 has many modification methods, such as element doping [33,34], morphology regulation [35,36], heterojunction construction [37,38] etc. By modifying g-C3N4, photocatalysts with high photocatalytic activity can be obtained. Yang et al. [39] used melamine as a precursor to prepare it using the thermal-polymerization method under air atmosphere and obtained oxygen-doped carbon nitride (OCN). The structure of nanoparticles and O-doping have an effect on the structure of graphite. The doped O atom replaces the original N atom in the catalyst, which makes the catalyst have higher electronegativity, thus forming a good superposition effect and strengthening the interaction between catalyst layers. O-doping increases the specific surface area of the catalyst and the active site on the catalyst surface, thus increasing the photocatalytic activity of the photocatalyst. Michela Sturiniet et al. [40] obtained g-C3N4 by polymerizing dicyandiamide, which greatly improved the removal effect of the catalyst on the antibiotic ofloxacin in water. Mitra Mousavi et al. [41] improved the reduction ability of g-C3N4 to Cr(VI), and successfully prepared g-C3N4-nanosheet/ZnMoO4 heterojunction materials by using the calcining-hydrothermal method. This method improves the absorption of visible light by photocatalytic materials and inhibits the recombination of electric charge, thus achieving the purpose of improving the reduction of Cr(VI) by photocatalytic materials. The reducing capacity of the g-C3N4-nanosheet/ZnMoO4 for Cr(VI) is 3.77 times that of g-C3N4 nanosheet. Wang et al. [42] prepared petal-like g-C3N4 by changing the proportion of chlorine doping in order to change the disadvantages of small specific surface area and low light utilization, which increased the specific surface area of the catalyst and effectively improved the photocatalytic activity of g-C3N4. Wang et al. [43] increased the reaction temperature in the process of preparing graphitic carbon nitride, released more gas in the process, and obtained modified graphitic carbon nitride with a porous network structure. The catalyst not only retained the original complete lamellar structure, but also produced a gauze-like porous structure, which increased the specific surface area of the catalyst surface, thus adsorbing more pollutants and achieving the purpose of improving the removal rate of pollutants. g-C3N4 can be modified in different ways so as to improve its specific surface area, increase the active sites on the surface of the photocatalyst and improve the photocatalytic activity. In conclusion, the main ways to improve the photocatalytic activity of g-C3N4 include increasing the specific surface area of the g-C3N4, increasing the active sites on the catalyst surface and inhibiting the recombination of photogenerated electron–hole pairs.
Given this background, we prepared g-C3N4 by thermal polycondensation with urea as the precursor system, and successfully prepared flower-like g-C3N4 with large specific surface area by adding citric acid, achieving the purpose of the efficient removal of Cr(VI) from water. At the same time, we used XRD, SEM, XPS, BET and UV-vis characterization methods to explore the reaction mechanism. This study provides technical support for the remediation of heavy-metal-polluted water by photocatalysis in the future.

2. Results and Discussion

2.1. Characterization

2.1.1. XRD Analysis

The XRD patterns of g-C3N4 and CA-g-C3N4 are shown in Figure 1. g-C3N4 (PDF#87-1526) has two diffraction peaks with 2θ angles of 13.1° and 27.5°, which correspond to crystal planes (100) and (002), respectively. The prepared g-C3N4 corresponds to the peak value of g-C3N4 in theory, indicating that g-C3N4 was successfully synthesized by the calcination method. The XRD pattern of CA-g-C3N4 (PDF# 87-1526) has two diffraction peaks at 13.1° and 27.5°, corresponding to crystal planes (100) and (002). The position and intensity of the diffraction peaks of g-C3N4 after citric acid treatment are the same with g-C3N4, which indicates that the phase composition of g-C3N4 did not change after citric acid treatment. Furthermore, we confirmed that the increase of the photocatalytic activity of the catalyst in SEM analysis was due to the increase of its specific surface.

2.1.2. SEM Analysis

The SEM images of g-C3N4 and CA-g-C3N4 after the reaction of CA-g-C3N4 and the EDS element mapping of post-reaction CA-g-C3N4 (d–h) are shown in Figure 1. In Figure 2a, the pure g-C3N4 exhibits layers with curved structure at the edges and micropores on some layers. However, the specific surface area of the g-C3N4 is small because its structure was destroyed under high-temperature calcination, forming the accumulation structure. The SEM image of Figure 2b reflects CA-g-C3N4 as a chelating agent. Citric acid has a certain dispersion ability. After the addition of citric acid, g-C3N4 shows a smooth surface structure, and the multiple layered structures form a petal shape, which is conducive to the exposure of materials. At the same time, the image shows that a larger specific surface area provides more active sites, and can effectively improve the photocatalytic activity of the catalyst. Figure 2c displays CA-g-C3N4 after the reaction. Compared with before the reaction, the layered structure is completely eliminated and the specific surface area is greatly reduced. The reason may be that, during the removal of Cr(VI), a portion of the Cr(VI) accumulated on the surface of the catalyst to form the displayed structure. Figure 2d–g show the mapping of samples after the reaction of CA-g-C3N4; chromium element was detected in the catalyst. This confirms that the accumulation on the catalyst surface is due to chromium element accumulation. This indicates that citric acid could improve the morphology and structure of g-C3N4 and enhance the photocatalytic activity.

2.1.3. XPS Analysis

In order to determine the elemental composition and chemical bond of g-C3N4 and CA-g-C3N4, g-C3N4 and CA-g-C3N4 were characterized by XPS, and the results are shown in Figure 3. Figure 3a is a survey of g-C3N4 and CA-g-C3N4. In the figure, the composition of g-C3N4 and CA-g-C3N4 is the same, which indicates that the addition of citric acid did not change the composition of g-C3N4. Figure 3b is the C 1s of g-C3N4 and CA-g-C3N4. The two fitting peaks of g-C3N4 are 287.54 and 284.80 eV, respectively. The two fitting peaks of CA-g-C3N4 are 288.11 and 284.80 eV, respectively, which correspond to N–C=N and C–C in the triazine ring. Figure 3c is the N 1s picture of g-C3N4 and CA-g-C3N4. G-C3N4 has four peaks, which are 398.06, 399.48, 400.52 and 404.00 eV, respectively. CA-g-C3N4 corresponds to 398.61, 400.14, 401.21 and 404.18 eV, respectively, and C–N=C, N–C3, C–N–H and Π-excitation [44]. Figure 3d shows the O 1s of g-C3N4 and CA-g-C3N4, corresponding to 531.89 and 532.50 eV. It can be seen from the XPS results that the addition of citric acid had no effect on the composition and chemical bonds of g-C3N4.

2.1.4. BET Analysis

The specific surface area, pore volume and pore size of the g-C3N4, CA-g-C3N4, post-reaction g-C3N4 and post-reaction CA-g-C3N4 are shown in Table 1. The specific surface areas of g-C3N4, CA-g-C3N4, post-reaction g-C3N4 and post-reaction CA-g-C3N4 were 62.2, 104.0, 48.4 m2g−1 and 58.7 m2g−1, respectively. Pore volumes were 0.3, 0.5, 0.2 and 0.3 cm3g−1, and pore sizes of the three were 18.3, 20.3, 18.3 and 20.2 nm, respectively. It can be found from these data that the specific surface area of g-C3N4 modified by citric acid was significantly higher than that of pure g-C3N4, indicating that the addition of citric acid effectively improved the specific surface area and pore size of g-C3N4. The reason is that the increase of citric acid caused the structure of g-C3N4 to evacuate and form a larger specific surface area. During the reaction, the photocatalyst can reduce Cr(VI) to Cr(III), it can form Cr(OH)3 colloids in solution and the specific surface area is reduced by deposition on the surface of the catalyst. We found chrominum on the surface of the material after the reaction through SEM characterization.

2.1.5. UV-Vis Analysis

Figure 4 shows the test results of UV-vis characterization of g-C3N4 and CA-g-C3N4. g-C3N4 and CA-g-C3N4 obtained by this method had excellent light absorption properties in the ultraviolet region. The light absorption range of CA-g-C3N4 photocatalytic material obtained after citric acid modification of g-C3N4 was the same as that of the unmodified photocatalytic material. This result indicates that the addition of citric acid had no effect on the structure and chemical composition of g-C3N4, and the addition of citric acid did not change the light absorption range of g-C3N4. This proves that the fundamental reason for the increase in the removal rate of Cr(VI) in water by CA-g-C3N4 is that the addition of citric acid, as an organic acid, effectively changes the microstructure of g-C3N4, thus increasing the specific surface area of the photocatalyst and the active sites on the catalyst surface, thus improving the photocatalytic activity of the material.

2.2. g-C3N4 and CA-g-C3N4 Performances

Figure 5 shows the removal rates of Cr(VI) in water by g-C3N4 and CA-g-C3N4 under ultraviolet irradiation. The removal rate of Cr(VI) was set under the condition of no light and no catalyst. The catalyst was added to the Cr(VI) solution, and the adsorption–desorption equilibrium was reached by stirring in the dark for 30 min. In this case, the adsorption effect of g-C3N4 on Cr(VI) was slightly higher than that of CA-g-C3N4 on Cr(VI). However, after ultraviolet irradiation, the removal rate of Cr(VI) by g-C3N4 was only 48%, while the removal efficiency of Cr(VI) by CA-g-C3N4 was up to 93%, showing a higher photocatalytic activity, and the double-beam UV-visible spectrophotometry results for before and after the removal of Cr(VI) are provided in Table 2. The k values of CA-g-C3N4 and g-C3N4 were calculated as 0.53753 min−1 and 0.38984 min−1. In order to more intuitively present the color change of the solution before and after the reaction, we display the solution before and after the reaction. From left to right the color of the solution before the reaction and that of the solution after g-C3N4 and CA-g-C3N4 removal can be seen. The solutions were filtered using membranes. The main reason is that the addition of citric acid effectively dispersed g-C3N4, increased the specific surface area of the material, improved the active site on the surface of the photocatalyst and thus improved the photocatalytic activity of the photocatalyst.
In order to test the repeatability of the performance of CA-g-C3N4, we conducted three experiments on the material under the same conditions. The removal rate of Cr(VI) was 93% during the first removal process and 87% during the second and third use processes, indicating that the material has good reusability, as shown in Figure 5d.
In order to further explore the mechanism of increasing photocatalytic activity of CA-g-C3N4, the main reactive species affecting the photocatalytic reduction of Cr(VI) in CA-g-C3N4 was identified. 1,4-Benzoquinone (BQ) and Potassium persulfate (K2S2O8) were used as scavengers for superoxide radicals (·O2−) and photo-induced electrons (e−). When BQ was added, the removal rate of Cr(VI) on CA-g-C3N4 decreased significantly, indicating that e− was the main active substance in the reaction process. When K2S2O8 was added, the removal rate did not decrease significantly, indicating that ·O2− was not the main active substance, as shown in Figure 6.

2.3. Mechanism of Photocatalysis

On the basis of the experimental results, characterization data and previous reference materials, we conclude that the reaction mechanism that may be involved in the process of Cr(VI) removal from water by CA-g-C3N4 is as follows: When the pH value is 1, the main form of chromium is H2CrO4; when the pH value is 2–6, the form of chromium is HCrO4−, Cr2O72−; when pH > 8, the form of chromium is CrO42−. Studies have found that when the solution is acidic, the redox potential of Cr(VI) is higher than that of alkaline conditions, and the catalyst surface is positive, which is conducive to the adsorption of Cr(VI) by the catalyst [26,45]. Under the irradiation of ultraviolet light on the catalyst surface, the CA-g-C3N4 surface is excited to produce photogenerated electrons (e−) and holes (h+) As shown in Equation (1), the photogenerated electron (e−) reduces Cr(VI) to Cr(III). As shown in Equation (2), the hole (h+) reacts with H2O to form ·OH and O2. As shown in Equation (3), under acidic conditions, the surface of the catalyst is positively charged, which is conducive to the catalyst reduction of Cr(VI), and the redox potential under this condition is higher than that under alkaline conditions. The toxicity of Cr(VI) is greatly reduced by reducing Cr(III), and thus the purpose of degrading heavy metal pollution is achieved. The reaction mechanism is shown in Figure 7.
CA-g-C3N4 + hv → h+ + e−
CrO42− + 8H++ 3e− → Cr3+ + 4H2O
h+ + H2O →·OH+ O2

3. Materials and Methods

3.1. Materials

Urea (H2NCONH2) and Citric Acid (C6H8O7·H2O) were purchased from ZhiYuan chemical reagent company (Tianjin, China). Both were analytical reagents. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were bought from Xilong Co., Ltd. (Guangzhou, China), where HCl was the guarantee reagent and NaOH was the analytical reagent.

3.2. Preparation of g-C3N4

In this study, urea was used as the precursor to prepare carbon nitride (g-C3N4) by thermal polymerization. The specific preparation methods are as follows. An amount of 20 g urea is placed in a covered crucible and sealed with tin foil to achieve an anoxic environment, and then placed in a Muffle furnace. The Muffle furnace temperature is heated to 550 °C at the heating rate of 5 °C/min, and the urea is calcined at the changing temperature for 4 h. Then, the urea is cooled slowly until the temperature reaches room temperature. Then, the material is taken out and a yellow material with low quality is obtained. This yellow material is finely ground in an agate mortar to produce g-C3N4 powder, which is bagged for future use.

3.3. Preparation of CA/g-C3N4

CA-g-C3N4 is prepared by modifying g-C3N4 by soaking it with citric acid. The specific preparation method is as follows. First, 2 g citric acid (CA) is dissolved in 50 mL deionized water, which is thoroughly mixed to obtain a certain concentration of citric acid solution. Then, g-C3N4 (2 g) is dissolved in 20 mL citric acid and stirred to fully dissolve g- C3N4 and citric acid into solution, followed by soaking at room temperature for 24 h to obtain modified g-C3N4. The obtained g-C3N4 soaked in citric acid is washed with deionized water and filtered until the pH value of the filtered solution is neutral. The solid parts left after filtering are dried at 60 °C for 12 h to obtain blocky materials, which are fully ground with an agate mortar to obtain yellow powdery CA-g-C3N4, which is bagged for use.

3.4. Characterization

X-ray diffraction (XRD, Smart Lab SE, Rigaku Corp., Tokyo, Japan) was used to analyze and characterize the phase composition and lattice parameters of g-C3N4 and CA-g-C3N4. The test conditions were as follows: Cu target was used, the acceleration voltage was 40 kV, and the current was 40 mA.
A scanning electron microscope (SEM, TESCAN MIRA LMS, TESCAN CHINA, Ltd., Shangai, China) was used to characterize the microstructure of the catalysts. A narrow focused high-energy electron beam was used to scan the sample, and physical information was excited through the interaction between the beam and the substance. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific Co., Ltd., Shanghai, China) was used to characterize the elemental composition, content and valence states of the catalyst. The light source used was Al Kα. The Brunauer-Emmett-Teller (BET, Micromeritics ASAP2460, Micromeritics Corp., Norcross, GA, USA) N2 adsorption–desorption method was used to characterize the specific surface area and pore size of the samples. UV-vis diffuse reflectance spectra (UV-vis DRS, UV-3600i Plus, Shimadzu Corp., Kyoto, Japan) were used to test the light absorption properties of the samples.

3.5. Photocatalytic Tests

The performance of photocatalyst g-C3N4 and CA-g-C3N4 was tested using UV-visible spectrophotometry to measure the absorbance of Cr(VI) before and after the reaction, and the removal effect of Cr(VI) was judged by calculation. The test method of this experiment was as follows: 100 mL Cr(VI) solution with a concentration of 20 mg·L−1 and a pH value of 2 was prepared in advance and placed in a quartz reactor; 50 mg photocatalyst g-C3N4 or CA-g-C3N4 was added; a rotor was added into the solution and it was stirred through magnetic force to make it evenly mixed. At room temperature, the solution was stirred in the dark for 30 min to achieve adsorption–desorption equilibrium between the solution and the photocatalyst so as to facilitate the subsequent judgment of the photocatalytic activity of the photocatalytic materials. After dark reaction for 30 min, the UV light source was turned on to irradiate the solution, and the water circulation was turned on to control the reaction temperature. When the total irradiation time of the UV lamp was 20, 40, 60, 80, 100 and 120 min, respectively, 2 mL of the reaction solution was taken out, filtered with a biofiltration membrane and 0.5 mL of the filtered solution was taken out for absorbance testing. The solution was not put back each time after removal. According to the absorbance of the solutions obtained at different time periods, the corresponding solution concentration could be calculated, and the above solution concentration and the original solution concentration could be calculated to obtain the removal effect of Cr(VI) in water at each time period by the photocatalyst.

4. Conclusions

In this study, the preparation methods of g-C3N4 and CA-g-C3N4 photocatalytic material modified by citric acid were studied, and the reaction mechanism involved in the removal of Cr(VI) from water by photocatalyst was explored. g-C3N4 was prepared by thermal-polymerization and modified by citric acid solution. After modification, CA-g-C3N4 was obtained. As an organic acid, citric acid has a dispersive effect. After citric acid is added to g-C3N4, it can be effectively dispersed, which solves the deficiency of the structure accumulation of g-C3N4 prepared by thermal condensation. By studying the removal effects of different photocatalytic materials on Cr(VI) in water, it was determined that the photocatalytic activity of g-C3N4 modified by citric acid was effectively improved. The phase composition of g-C3N4 before and after citric acid modification was determined by XRD, XPS and UV-vis. The crystal structure and light absorption range did not change, indicating that the addition of citric acid did not change the chemical properties of g-C3N4. However, SEM and BET showed that, compared with g-C3N4, CA-g-C3N4 obtained after citric acid modification had a more porous microstructure with significantly larger specific surface area and pore size, which proved that the addition of citric acid effectively dispersed g-C3N4 and changed its morphology and structure. By increasing its specific surface area, the number of active sites on the material surface was increased. The contact area between Cr(VI) and the photocatalyst was larger, and the removal rate was increased. The ability of g-C3N4 modified with citric acid to efficiently remove Cr(VI) from water fully proves that morphology control can improve the photocatalytic activity of materials, and provides a new idea for the removal of heavy metals and other pollutants by photocatalytic technology in the future.

Author Contributions

All authors contributed to the study conception and design. Methodology and writing—original draft preparation were performed by H.Y.; Writing—Reviewing, editing, and conceptualization were performed by Q.M.; Data curation was performed by C.G.; Validations were performed by S.L.; Investigation and funding acquisition were performed by Y.Z.; Writing—Reviewing and editing were performed by H.Q.; Data curation was performed by R.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Basic Research Project of Yunnan Province Science and Technology Department (grant number: 202201AU070004), the Yunnan Province Education Department Scientific Research Fund Project (grant number: 2023J0959; 2023Y1049), and the Research Program of Yunnan Provincial Science and Technology Department (grant number: 202101BA070001-079).

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00641 g001 550
Figure 1. XRD patterns of g-C3N4 and CA-g-C3N4.
Figure 1. XRD patterns of g-C3N4 and CA-g-C3N4.
Catalysts 13 00641 g001
Catalysts 13 00641 g002 550
Figure 2. SEM images of (a) g-C3N4, (b) CA-g-C3N4 and (c) after the reaction of CA-g-C3N4. EDS element mapping (d–h) of after the reaction of CA-g-C3N4.
Figure 2. SEM images of (a) g-C3N4, (b) CA-g-C3N4 and (c) after the reaction of CA-g-C3N4. EDS element mapping (d–h) of after the reaction of CA-g-C3N4.
Catalysts 13 00641 g002
Catalysts 13 00641 g003a 550Catalysts 13 00641 g003b 550
Figure 3. XPS of g-C3N4 and CA-g-C3N4: (a) survey, (b) C 1s, (c) N 1s and (d) O 1s.
Figure 3. XPS of g-C3N4 and CA-g-C3N4: (a) survey, (b) C 1s, (c) N 1s and (d) O 1s.
Catalysts 13 00641 g003aCatalysts 13 00641 g003b
Catalysts 13 00641 g004 550
Figure 4. UV-vis of g-C3N4 and CA-g-C3N4.
Figure 4. UV-vis of g-C3N4 and CA-g-C3N4.
Catalysts 13 00641 g004
Catalysts 13 00641 g005a 550Catalysts 13 00641 g005b 550
Figure 5. Removal rate of g-C3N4 and CA-g-C3N4 on Cr(VI) (a), the color changes before and after the reaction (b), the kinetic constant k of different samples (c), the cyclic removal curve of CA-g-C3N4 (d).
Figure 5. Removal rate of g-C3N4 and CA-g-C3N4 on Cr(VI) (a), the color changes before and after the reaction (b), the kinetic constant k of different samples (c), the cyclic removal curve of CA-g-C3N4 (d).
Catalysts 13 00641 g005aCatalysts 13 00641 g005b
Catalysts 13 00641 g006 550
Figure 6. Photocatalytic reduction of Cr(VI) experiments with the addition of radical scavenger.
Figure 6. Photocatalytic reduction of Cr(VI) experiments with the addition of radical scavenger.
Catalysts 13 00641 g006
Catalysts 13 00641 g007 550
Figure 7. The mechanism of photocatalytic CA-g-C3N4 removal of Cr(VI).
Figure 7. The mechanism of photocatalytic CA-g-C3N4 removal of Cr(VI).
Catalysts 13 00641 g007
Table
Table 1. Comparison of specific surface area, pore volumes and pore sizes.
Table 1. Comparison of specific surface area, pore volumes and pore sizes.
SampleSBET (m2g−1)Vpore (cm3g−1)dpore (nm)
g-C3N462.20.318.3
CA-g-C3N4104.00.520.3
Post-reaction g-C3N448.40.218.3
Post-reaction CA-g-C3N458.70.320.2
Table
Table 2. The double beam UV-visible spectrophotometer for before and after removal of chromium (VI).
Table 2. The double beam UV-visible spectrophotometer for before and after removal of chromium (VI).
CatalyticCA-g-C3N4g-C3N4DarkBlank
−30 minA (nm)0.1450.1120.1460.150
removal efficiency (%)6.428.945.70
20 minA (nm)0.0740.1060.1460.149
removal efficiency (%)54.8733.045.70
40 minA (nm)0.0510.1000.1450.150
removal efficiency (%)70.5837.136.40
60 minA (nm)0.0380.1000.1440.150
removal efficiency (%)79.4537.137.10
80 minA (nm)0.0290.1000.1440.151
removal efficiency (%)85.6037.137.10
100 minA (nm)0.0220.0980.1430.152
removal efficiency (%)90.3738.507.7%0
120 minA (nm)0.0190.0980.1430.150
removal efficiency (%)92.4238.507.7%0
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Yu, H.; Ma, Q.; Gao, C.; Liao, S.; Zhang, Y.; Quan, H.; Zhai, R. Petal-like g-C3N4 Enhances the Photocatalyst Removal of Hexavalent Chromium. Catalysts 2023, 13, 641. https://doi.org/10.3390/catal13030641

AMA Style

Yu H, Ma Q, Gao C, Liao S, Zhang Y, Quan H, Zhai R. Petal-like g-C3N4 Enhances the Photocatalyst Removal of Hexavalent Chromium. Catalysts. 2023; 13(3):641. https://doi.org/10.3390/catal13030641

Chicago/Turabian Style

Yu, Huijuan, Qiang Ma, Cuiping Gao, Shaohua Liao, Yingjie Zhang, Hong Quan, and Ruiqi Zhai. 2023. "Petal-like g-C3N4 Enhances the Photocatalyst Removal of Hexavalent Chromium" Catalysts 13, no. 3: 641. https://doi.org/10.3390/catal13030641

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