A porous carbon quantum dot grafted g-c3n4 catalyst, a preparation method and application thereof

Abstract

This invention belongs to the field of wastewater treatment technology, and relates to a porous carbon quantum dot-grafted g-C3N4 catalyst, its preparation method, and its application. The preparation method includes: 1) transferring urea into a muffle furnace and heating it at 5°C·min. ‑1 1) The temperature of the muffle furnace was raised to 550℃ and calcined for 120 min to obtain CN; 2) CN was uniformly dispersed in deionized water and sonicated for 10 min to form a suspension; 3) Citric acid was added to the suspension and continuously stirred in an 80℃ constant temperature water bath magnetic stirrer until the water was completely evaporated, and the remaining solid mixture was ground into fine powder, and then stirred at 5℃·min ‑1 The heating rate was increased to 550℃ and calcined for 120 min; 4) After calcination and cooling to room temperature, the resulting black mixture was placed in a mortar and ground evenly to obtain porous CQDs / CN. Its preparation is simple, low-cost, and highly efficient in removal, providing a win-win strategy for the current treatment of antibiotic-containing wastewater pollution.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and relates to a carbon quantum dot-grafted g-C3N4 catalyst, its preparation method and application, and particularly to a porous carbon quantum dot-grafted g-C3N4 catalyst, its preparation method and application. Background Technology

[0002] As an emerging organic pollutant, fluoroquinolones (FQs) are a class of synthetic antibacterial antibiotics widely used to treat infectious diseases in humans and other organisms. Among them, levofloxacin (LFX), a typical FQ antibiotic, suffers from antibiotic residues in various aquatic environments such as drinking water, groundwater, and surface water due to overuse and incomplete metabolism, posing a serious threat to human health and the growth and development of plants, animals, and microorganisms in ecosystems. Therefore, finding efficient methods to remove LFX from water bodies has become an important environmental problem that urgently needs to be solved.

[0003] Antibiotics containing sulfonylureas (LFX) work by inhibiting the activity of bacterial DNA helicases, thus preventing bacterial DNA synthesis and replication and leading to bacterial death. This mechanism has led to their widespread use in treating respiratory, gastrointestinal, and other systemic infections caused by susceptible bacteria. In recent years, LFX accumulated in water bodies has been found to be easily absorbed by certain crops and accumulate in plants, thus affecting plant growth. Simultaneously, untreated LFX-containing wastewater entering the ecological environment harms farmland soil, surface and groundwater, and various organisms in the ecosystem, inducing and spreading various antibiotic-resistant pathogens, posing a serious threat to human health and becoming a significant challenge to water resource reuse. Exploring efficient methods for removing LFX from water bodies is of paramount importance for the remediation of antibiotic residue pollution.

[0004] Currently, wastewater treatment solutions for this type of wastewater are mainly divided into source control and end-of-pipe treatment. The former refers to controlling wastewater discharge standards and volumes to reduce the threat to the aquatic environment at the source; the latter refers to treating discharged water using various treatment methods, which can be categorized into physical methods (adsorption, etc.), biological methods (aerobic or anaerobic biological treatment), and chemical methods (electrochemical oxidation, photocatalysis, etc.) based on their principles. However, the "stubbornness" of LFX's physicochemical properties, along with the high efficiency, low cost, and simplicity of its treatment methods, has led to the emergence of advanced oxidation processes mediated by persulfate (PDS). These processes utilize the generated strong oxidizing free radicals to selectively mineralize and degrade recalcitrant pollutants over a wide pH range, making them stand out among numerous methods.

[0005] The activation performance of persulfate is mainly influenced by the catalyst structure. Metal-based catalysts often suffer from metal leaching, posing a potential risk of secondary pollution. Therefore, developing non-metallic catalysts with excellent structures for efficient pollutant removal through persulfate activation is a current research focus. Carbon-based materials, due to their simple synthesis and low cost, are widely studied for their application in water environment remediation. Among them, g-C3N4 has attracted great interest from researchers due to its good stability and unique electronic structure, and has been widely used in advanced persulfate oxidation processes. However, the lack of sufficient persulfate activation sites and poor electron transfer capacity during the reaction severely limits the removal activity of g-C3N4 during activation.

[0006] To leverage the advantages of carbon-based materials in advanced persulfate oxidation processes, this study further investigated non-metallic nanoscale carbon supported on g-C3N4 catalysts, offering promising prospects for activating persulfate to remove pollutants. Carbon quantum dots (CQDs), a class of zero-dimensional carbon nanomaterials, possess unique physicochemical properties such as extremely small size (2-10 nm), strong chemical inertness, high water solubility, and environmental friendliness. The coupling of CQDs with g-C3N4 increases the specific surface area of ​​the catalyst and improves charge transfer efficiency, further enhancing the removal efficiency of pollutants from water. However, current research faces two challenges: firstly, most studies utilize a two-step method to achieve CQDs-g-C3N4 coupling; secondly, external auxiliary energy (such as light energy) is required to achieve efficient activation of PDS by CQDs / g-C3N4 for antibiotic degradation. These factors undoubtedly lead to high energy consumption and complex equipment, hindering practical production applications.

[0007] Therefore, it is extremely important to design a simple, green, and efficient catalyst and combine it with the persulfate advanced oxidation process to provide a theoretical basis and technical support for the treatment of antibiotic-containing wastewater. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a porous carbon quantum dot-grafted g-C3N4 catalyst, its preparation method and application. It is easy to prepare, low in cost and highly efficient in removal, and can provide a win-win strategy for the treatment of antibiotic-containing wastewater pollution.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A method for preparing a porous carbon quantum dot-grafted g-C3N4 catalyst, characterized by comprising the following steps:

[0011] 1) Transfer the urea to a covered alumina crucible and then place it in a muffle furnace at 5°C·min. -1The heating rate was increased to 550°C in the muffle furnace, and the urea was directly calcined at this temperature for 120 min. The resulting yellow mixture was named CN.

[0012] 2) Disperse CN evenly in deionized water and sonicate for 10 minutes to form a suspension;

[0013] 3) Add citric acid to the above suspension and stir continuously in a heated constant temperature water bath magnetic stirrer at 80°C until all water evaporates. Grind the remaining solid mixture into a fine powder, and then stir at 5°C·min. -1 The temperature was increased to 550°C at a heating rate, and the fine powder was directly calcined at this temperature for 120 minutes.

[0014] 4) After calcination and cooling to room temperature, the resulting black mixture is placed in a mortar and ground evenly to obtain a powdered porous carbon quantum dot grafted g-C3N4 catalyst, namely, porous CQDs / CN.

[0015] Preferably, in step 1), the amount of urea is 20g.

[0016] Preferably, in step 2), the amount of CN is 0.3g and the amount of deionized water is 10mL.

[0017] Preferably, in step 3), the amount of citric acid is 0.3g.

[0018] Furthermore, the present invention also provides a porous carbon quantum dot-grafted g-C3N4 catalyst, characterized in that it is prepared by the above-described preparation method.

[0019] Furthermore, this invention also provides the application of the above-mentioned porous carbon quantum dot-grafted g-C3N4 catalyst in removing levofloxacin from water.

[0020] Preferably, the above application is characterized by comprising the following steps:

[0021] 1) Pour the persulfate into the prepared levofloxacin solution and place it in a heat-collecting constant-temperature magnetic stirrer to stir evenly.

[0022] 2) After stirring evenly, add the porous carbon quantum dot-grafted g-C3N4 catalyst to carry out the reaction.

[0023] Preferably, in step 1), the volume of the levofloxacin solution is 50 mL and the concentration is 10 mg·L⁻¹. -1 The pH is 7.0, and the amount of persulfate is 0.04 g.

[0024] Preferably, in step 1), the temperature of the heat-collecting constant-temperature magnetic stirrer is adjusted to 25°C.

[0025] Preferably, in step 2), the amount of the porous carbon quantum dot-grafted g-C3N4 catalyst is 0.025 g and the reaction time is 60 min.

[0026] Compared with the prior art, the porous carbon quantum dot-grafted g-C3N4 catalyst, its preparation method, and its application of the present invention have one or more of the following beneficial technical effects:

[0027] 1. This invention uses an in-situ uniform loading method to synthesize porous carbon quantum dots (CQDs) grafted with g-C3N4. Compared with the existing two-step method, its preparation method is simple, low-cost and low-energy consumption.

[0028] 2. In the porous carbon quantum dots (CQDs) grafted onto g-C3N4 prepared in this invention, the oxygen-containing functional groups on the surface of CQDs combine with the terminal amino groups of g-C3N4. CQDs form a tight interfacial contact with g-C3N4 in the form of CO or C=O bonds to form a porous catalyst CQDs / CN. Thanks to its high specific surface area, high hydrophilicity, and easily tunable electronic structure, porous CQDs / CN can activate PDS to achieve efficient removal of LFX from water without the need for external auxiliary energy (such as light energy).

[0029] 3. Experimental results show that CQDs / CN / PDS exhibit rapid LFX removal performance, up to 83%, which is about 7.55 times that of CN-activated PDS in removing LFX.

[0030] 4. Thanks to its simple synthesis method, low cost and high efficiency removal performance, the porous CQDs / CN of this invention provides a theoretical reference for carbon-based materials in the field of advanced persulfate oxidation process, and at the same time provides a win-win strategy for the treatment of wastewater pollution containing FQs-type antibiotics. Attached Figure Description

[0031] Figure 1 (a) and Figure 1 (b) are SEM images of CN and CQDs / CN, respectively, where the inset is a corresponding physical image; Figure 1 (c) is the TEM image of CQDs / CN; Figure 1 (d) is the HRTEM plot of CQDs / CN, where the inset is a size distribution plot of CQDs; Figure 1 (eh) is the TEM-EDS analysis plot of CQDs / CN.

[0032] Figure 2 (a) shows the XRD patterns of CN and CQDs / CN; Figure 2 (b) is the infrared spectrum of CN and CQDs / CN; Figure 2 (c) and Figure 2(d) shows the nitrogen adsorption-desorption isotherms and pore size distributions of CN and CQDs / CN, respectively.

[0033] Figure 3 (a) shows the removal performance of LFX in different systems; Figure 3 (b) shows the corresponding pseudo-first-order dynamics plot.

[0034] Figure 4 The stability test results for CQDs / CN are shown. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments. The content of the embodiments is not intended to limit the scope of protection of the present invention.

[0036] The coupling of CQDs with g-C3N4 increases the specific surface area of ​​the catalyst and improves charge transfer efficiency, further enhancing the removal efficiency of pollutants in water. However, current research faces two challenges: firstly, it requires a two-step method to achieve the coupling of CQDs and g-C3N4; secondly, it necessitates external auxiliary energy (such as light energy) to achieve efficient activation of PDS by CQDs / g-C3N4 for antibiotic degradation. These factors undoubtedly lead to high energy consumption and complex equipment, hindering practical production applications. Therefore, this invention provides a porous carbon quantum dot-grafted g-C3N4 catalyst, which, when combined with the persulfate advanced oxidation process, provides a theoretical basis and technical support for the treatment of antibiotic-containing wastewater.

[0037] The preparation method of the porous carbon quantum dot-grafted g-C3N4 catalyst of the present invention includes the following preparation steps:

[0038] First, transfer 20g of urea to a covered alumina crucible and then place it in a muffle furnace. Set the parameters to 550℃ and calcine directly for 120min with a heating rate of 5℃·min. -1 The resulting yellow mixture was named CN.

[0039] Next, 0.3g CN was evenly dispersed in 10mL of deionized water and subjected to strong sonication for 10min to form a suspension.

[0040] Next, 0.3g of citric acid (CA) was added to the above suspension and continuously stirred in a heat-collecting constant temperature water bath magnetic stirrer at 80°C until the water was completely evaporated. The remaining solid mixture was then ground into a fine powder.

[0041] Then, it was calcined at 550℃ for 120 min, with a heating rate of 5℃·min. -1 .

[0042] After the pyrolysis reaction is completed and cooled to room temperature, the resulting black mixture is placed in a mortar and ground evenly to obtain powdered porous CQDs / CN, i.e., porous carbon quantum dot grafted g-C3N4 catalyst.

[0043] This invention uses an in-situ uniform loading method to synthesize porous carbon quantum dots (CQDs) grafted with g-C3N4. Compared with the existing two-step method, its preparation method is simple, low-cost and low-energy consumption.

[0044] To demonstrate the effectiveness of the porous CQDs / CN, i.e., the porous carbon quantum dot-grafted g-C3N4 catalyst prepared in this invention, experimental operations were conducted.

[0045] The experimental procedure is as follows:

[0046] 1. Use the prepared raw CN and CQDs / CN-activated PDS to remove LFX from water.

[0047] This experiment was conducted under air conditions and did not require external auxiliary energy (such as light energy). First, the prepared LFX solution with an initial pH of 7.0 (50 mL, 10 mg / L) was... -1 Place the solution in a 100 mL beaker, add 0.04 g of PDS, and place the beaker in a heated magnetic stirrer (set to 25°C) to stir until homogeneous. Then add 0.025 g of CQDs / CN powder. During the reaction, take samples at predetermined time intervals using a 2 mL syringe (0, 5, 10, 20, 30, 40, 50, 60 min) and immediately filter through a 0.22 μm microporous filter.

[0048] 2. Samples at different time intervals were analyzed and measured using a UV-Vis spectrophotometer, and the concentration of pollutants in the samples at each time interval was calculated. The characteristic absorbance wavelength was 293 nm.

[0049] 3. The removal experiment for the reuse performance test is the same as above. After the reaction is complete, filter the solution after the reaction, rinse it thoroughly with a large amount of ultrapure water, and then put it in an oven to dry for 4-5 hours. Repeat the above operation 4 times to complete the catalyst reuse performance test.

[0050] The pollutant removal rate (C / C0) is calculated using the following formula:

[0051]

[0052] In the formula, C0(mg L) -1 ) and C (mg L -1 ( ) represent the initial and final concentrations of pollutant LFX, respectively.

[0053] The experimental results are as follows:

[0054] The morphology and structure of the catalysts before and after modification were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1 (a) is a SEM image of the original g-C3N4(CN), which has a smooth surface and exhibits a thick, layered structure with curled edges. Figure 1 As shown in (b), after grafting CQDs, a significant change in the surface morphology of CQDs / CN was observed, with CQDs / CN exhibiting a robust porous structure. Figure 1 The TEM image in (c) shows that the CQDs / CN grafted with CQDs transforms into a thin and porous morphology. Meanwhile, Figure 1 The HRTEM image in (d) reveals a large number of carbon quantum dots on the CN nanosheets. The inset also shows that CQDs with an average size of ~2.60 nm are uniformly dispersed on the surface of the CN nanosheets, further confirming the successful grafting of CQDs and the close interfacial contact between the grafted CQDs and CN.

[0055] It is worth noting that, such as Figure 1 As shown in the illustrations (a, b), after grafting CQDs, the catalyst color also changed from the original pale yellow (CN) to black (CQDs / CN), similar to the color of the CQDs themselves. Figure 1 As shown in (eh), the EDS scan image shows the three main elements C, N, and O, and their uniform distribution in CQDs / CN. It can be seen from the figure that the content of O element in CQDs / CN is significantly increased compared with that in ungrafted CN.

[0056] Figure 2 (a) shows the XRD patterns of CN and CQDs / CN. Clearly, CN exhibits two characteristic diffraction peaks at ~13.1° and ~27.9°, located on the (100) and (002) crystal planes of g-C3N4, respectively, originating from the repeating units of the in-plane continuous heptazine framework and the stacking of conjugated aromatic structures. For CQDs / CN, both characteristic diffraction peaks are significantly weakened, indicating that grafting CQDs causes the collapse of the in-plane tris-triazine unit structure and changes in the interlayer stacking. Simultaneously, by observing... Figure 2 As can be seen in the inset of (a), the characteristic diffraction peak of the (002) crystal plane of CQDs / CN shifts to a lower angle (from 27.9° of CN to 27.7° of CQDs / CN), indicating that the interlayer distance of CQDs / CN increases. This may be due to the introduction of oxygen-containing groups after grafting CQDs or the breaking of CN bonds during the surface functionalization process.

[0057] Figure 2(b) The FTIR spectra of CN and CQDs / CN show similar characteristic vibrational modes, indicating that CQDs / CN still retains a typical g-C3N4-like heptaazine ring structure. This is mainly reflected in the 812 cm⁻¹... -1 The strong characteristic peak at [location] represents the out-of-plane bending vibration of the tri-s-triazine ring, confirming the formation of the basic methyl group after bonding with the -NH / NH2 group; the stretching vibrations of the CN / C=N covalent bonds in the aromatic heterocyclic skeleton are distributed in the range of 900–1800 cm⁻¹. -1 Five characteristic absorption peaks; 3000–3600 cm⁻¹ -1 The broad absorption band characteristic peaks belong to the stretching vibrations of the OH and NH bonds of H2O molecules adsorbed on the catalyst surface. Compared with the original CN, the infrared spectrum of CQDs / CN shows three changes. The first is at a wavenumber of 812 cm⁻¹. -1 The weakening of the characteristic peak intensity is due to the possible disruption of the original CN skeleton after grafting CQDs or the interaction caused by the introduction of oxygen-containing groups. Secondly, in the CA sample, the intensity at 1350 cm⁻¹... -1 The stretching vibrations of CH disappear in CQDs / CN, indicating that the CA adsorbed on the CN surface undergoes carbonization, forming CQDs uniformly distributed on the CN nanosheets. Furthermore, it is noteworthy that, compared to CN, CQDs / CN exhibit a higher ratio at ~1225 cm⁻¹. -1 and 1715cm -1 Two new peaks identical to those in CA appeared, which are attributed to the vibrations of CO and C=O, respectively. This change confirms that the successful grafting of CQDs is achieved by breaking CN bonds and forming CO or C=O bonds, resulting in a close interfacial contact with CN.

[0058] like Figure 2 As shown in (c, d), the adsorption capacity of N2 increases with increasing relative pressure. The N2 adsorption-desorption isotherms of both CN and CQDs / CN exhibit typical type IV isotherms and type H3 hysteresis loops, indicating that both CN and CQDs / CN possess a fractured porous structure resulting from layered aggregation. The pore size distribution curves are shown in... Figure 2 The insets (c, d) confirm the presence of mesopores in the CQDs / CN, which is consistent with... Figure 1 The SEM and TEM results were consistent. As expected, the BET specific surface area and pore volume of CQDs / CN were significantly increased, reaching 224.70 m². 2 ·g -1 and 0.90cm 3 ·g -1 Compared to the original CN (74.17m) 2 ·g -1 0.30cm 3 ·g -1The specific surface area and pore volume are increased by approximately three times. The increased specific surface area and pore volume help expose more reactive sites during the removal reaction, while reducing the transport resistance of pollutants, thereby achieving the goal of rapid and efficient pollutant removal. In summary, the synthesized catalyst has a higher specific surface area and more abundant pores than g-C3N4, and possesses the unique hydrophilicity of CQDs, significantly enhancing its ability to activate PDS, thus achieving the goal of highly efficient pollutant removal.

[0059] like Figure 3 As shown in (a), PDS alone cannot directly oxidize LFX. Without PDS, CN alone has little effect on LFX removal, while CQDs / CN alone achieves a removal efficiency of 40% for LFX within 60 minutes, indicating that CQDs / CN has a certain adsorption effect on LFX. In the CN / PDS system, the removal rate of LFX within 60 minutes is only 11%, indicating that ungrafted CN can activate PDS, but the activation efficiency is low. However, when CQDs / CN and PDS are present simultaneously, a significantly enhanced removal effect of LFX is observed in the CQDs / CN / PDS system, reaching as high as 83% within 60 minutes. In the above reaction systems, the LFX removal efficiency conforms to pseudo-first-order kinetics. Figure 3 As shown in (b), the removal rate constant (k) of LFX by the CQDs / CN / PDS system is 0.048 min. -1 It is significantly superior to CN, PDS, CN / PDS and CQDs / CN (0.0005, 0.002, 0.003, 0.014 min). -1 These four reaction systems.

[0060] Besides high removal efficiency, long-term stability and reusability are also important parameters for evaluating highly efficient catalysts. Therefore, this invention conducted repeated experiments to test the stability of CQDs / CN. From Figure 4 As can be seen, after 5 consecutive cycles, the removal efficiency of CQDs / CN is still as high as 83%, indicating that CQDs / CN has strong stability and is reusable in the process of removing LFX from water.

[0061] This invention employs an in-situ uniform loading method to synthesize carbon quantum dots (CQDs) grafted onto g-C3N4. The oxygen-containing functional groups on the surface of CQDs bind to the terminal amino groups of g-C3N4. In the CQDs / CN method, CQDs form a close interfacial contact with g-C3N4 via CO or C=O bonds, creating a catalyst CQDs / CN. Benefiting from its high specific surface area, high hydrophilicity, and easily tunable electronic structure, CQDs / CN-activated PDS achieves highly efficient removal of LFX from water. Experimental results show that CQDs / CN / PDS exhibits rapid LFX removal performance, reaching up to 83%, which is approximately 7.55 times that of CN-activated PDS.

[0062] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. An application of a porous carbon quantum dot-grafted g-C3N4 catalyst for removing levofloxacin from water, wherein the preparation method of the porous carbon quantum dot-grafted g-C3N4 catalyst includes the following steps: 1) Transfer the urea to a covered alumina crucible and then place it in a muffle furnace at 5 °C·min. -1 The muffle furnace was heated to 550 °C at a heating rate of 100 °C, and the urea was directly calcined at this temperature for 120 min. The resulting yellow mixture was named CN. 2) Disperse CN evenly in deionized water and sonicate for 10 min to form a suspension; 3) Add citric acid to the above suspension and stir continuously in a heated constant temperature water bath magnetic stirrer at 80 °C until all water evaporates. Grind the remaining solid mixture into a fine powder, and then stir at 5 °C·min. -1 The temperature was increased to 550 °C at a heating rate, and the fine powder was directly calcined at this temperature for 120 min. 4) After calcination and cooling to room temperature, the resulting black mixture is placed in a mortar and ground evenly to obtain a powdered porous carbon quantum dot-grafted g-C3N4 catalyst.

2. The application according to claim 1, characterized in that, In step 1), the amount of urea is 20g.

3. The application according to claim 2, characterized in that, In step 2), the amount of CN is 0.3 g and the amount of deionized water is 10 mL.

4. The application according to claim 3, characterized in that, In step 3), the amount of citric acid is 0.3 g.

5. The application according to claim 1, characterized in that, It includes the following steps: S1) Pour persulfate into the prepared levofloxacin solution and place it in a heat-collecting constant-temperature magnetic stirrer to stir evenly. S2) After stirring evenly, add the porous carbon quantum dot-grafted g-C3N4 catalyst to carry out the reaction.

6. The application according to claim 5, characterized in that, In step S1), the volume of the levofloxacin solution is 50 mL and the concentration is 10 mg·L⁻¹. -1 pH = 7.0, and the amount of persulfate is 0.04 g.

7. The application according to claim 6, characterized in that, In step S1), the temperature of the heat-collecting constant-temperature magnetic stirrer is adjusted to 25 ℃.

8. The application according to claim 7, characterized in that, In step S2), the amount of the porous carbon quantum dot-grafted g-C3N4 catalyst is 0.025 g and the reaction time is 60 min.

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