Method for preparing carbon dots by electrochemical coupling of oxidants and applications thereof

Abstract

The application discloses a method for preparing carbon dots by electrochemical coupling of an oxidant and application thereof. The method is to prepare carbon quantum dots by electrolyzing a graphite rod in an electrolyte solution containing hydrogen peroxide. The carbon dots synthesized by the method have the characteristics of simple and economical synthesis process, high yield, good performance, high content of surface functional groups and the like. When the carbon dots are applied to a Fenton-like process, better phenol degradation effect can be achieved in a wider pH range.

Description

Technical Field

[0001] This invention relates to methods for preparing nanomaterials and their applications, specifically to a method and application for preparing carbon dots using an electrochemical coupling agent. Background Technology

[0002] With the rapid development of society and the economy, the wastewater treatment industry faces increasing pressure, and the degradation of pollutants in water bodies has always been a major focus of researchers. Advanced oxidation processes (AOPs), as a highly efficient treatment method, are widely used in the wastewater treatment industry to reduce recalcitrant organic matter in wastewater and improve its biodegradability. Fenton's advanced oxidation process, discovered in 1893 by chemist Fenton HJ, has become the most widely used advanced oxidation technology. This process utilizes Fe... 2+ Activated H2O2 produces highly oxidizing ·OH, which oxidizes pollutants.

[0003] Fe 2+ +H₂O₂=Fe 3+ +OH - +·OH

[0004] However, this process has problems such as a narrow pH range, high iron sludge production, and low hydrogen peroxide utilization.

[0005] To overcome the aforementioned shortcomings, researchers have made various attempts, such as using heterogeneous catalysts to address the problem of large iron sludge production and using chelating agents to complex iron ions to broaden the pH application range. Qian Xufang et al. (Environ. Sci. Technol. 2022, 56, 4, 2617-2625) applied carbon quantum dots prepared by electrolysis to the Fenton process. They utilized the carboxyl groups on the surface of carbon quantum dots to complex with iron ions through a dual-coordination mechanism to form a CQDs-Fe(III) structure. In this nanostructure, iron ions act as hole donors and form hydrogen bonds with phenol, generating intermediates for electron transport and accelerating the Fe(III) / Fe(II) cycle, thereby improving the utilization rate of hydrogen peroxide and broadening the pH application range of the Fenton reaction. However, the amount of oxygen-containing functional groups on the surface of the carbon dots obtained by this method is limited, and its Fenton-like degradation effect needs to be optimized. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a method and application for preparing carbon dots using an electrochemical coupling oxidant. This invention uses an electrolyte solution with added hydrogen peroxide instead of a conventional electrolyte solution for electrochemical exfoliation to prepare carbon quantum dots; it successfully obtains carbon quantum dots with a higher degree of oxidation, which are then applied to the CQDs-Fe(III) / H2O2 reaction, achieving better phenol degradation under higher pH conditions compared to existing preparation methods.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] This invention relates to a method for preparing carbon dots by electrochemically coupling an oxidant, wherein carbon dots are prepared by electrolyzing a graphite rod in an electrolyte solution containing hydrogen peroxide.

[0009] As one embodiment of the present invention, the method includes the following steps:

[0010] Electrolysis was performed in an aqueous NaCl solution containing H2O2, using graphite rods as both the cathode and anode.

[0011] The solution obtained by electrolysis is filtered, the filtrate is collected, and the solution is freeze-dried (at -80°C for 12 to 24 hours) to obtain the carbon dots.

[0012] As one embodiment of the present invention, the NaCl concentration in the NaCl aqueous solution containing H2O2 is 0.1-10 mmol / L.

[0013] In one embodiment of the present invention, the concentration of H2O2 in the NaCl aqueous solution containing H2O2 is 1-10 mmol / L.

[0014] As one embodiment of the present invention, the electrode spacing is maintained at 4-8 cm during the electrolysis process.

[0015] In one embodiment of the present invention, the electrolysis (voltage 30V, room temperature) lasts for 12-72 hours. Preferably, the electrolysis time is 48 hours, and the hydrogen peroxide concentration is 5 mmol / L.

[0016] In some embodiments, the method specifically includes the following steps:

[0017] (1) Use two identical graphite rods as the anode and cathode, keep the electrode spacing 4-8 cm, and electrolyze in H2O2 solution with 0.1-10 mmol / L sodium chloride added;

[0018] (2) Filter the reaction product obtained in step (1) using a 0.22 μm PTFE filter membrane;

[0019] (3) Collect the filtrate obtained in step (2) and freeze-dry it.

[0020] The present invention also relates to the use of carbon dots prepared by the aforementioned method in the degradation of phenolic pollutants in a Fenton-like process.

[0021] As one embodiment of the present invention, the phenol-containing water sample includes phenol-containing water sample and / or phenol-containing wastewater.

[0022] In one embodiment of the present invention, the carbon dots are used to treat phenol-containing water samples in a Fenton-like system. The process is as follows: ferric nitrate nonahydrate is mixed with carbon dots, then added to the phenol-containing water sample. The pH is adjusted, hydrogen peroxide is added, and samples are taken at intervals. The change in phenol content in the water sample is determined by high-performance liquid chromatography (HPLC). Preferably, the concentration of ferric nitrate nonahydrate is 12 μmol / L, the concentration of carbon dots is 22 mg / L, the concentration of hydrogen peroxide is 6.4 mmol / L, the concentration of phenol is 10 mg / L, the pH is adjusted to 6, and the reaction time is 1 hour.

[0023] In one embodiment of the present invention, the carbon dots are used in a Fenton-like system to treat phenolic resin wastewater. The treatment process is as follows: ferric nitrate nonahydrate is mixed with carbon dots in a specific ratio, then added to diluted phenolic resin wastewater (diluted to a COD concentration ≤300, e.g., 150 mg / L). After adjusting the pH, hydrogen peroxide is added. Samples are taken (after 5 hours), and changes in pollutants in the water are detected by three-dimensional fluorescence. Preferably, the concentration of ferric nitrate nonahydrate is 30 μmol / L, the concentration of carbon dots is 22 mg / L, the concentration of hydrogen peroxide is 6.4 mmol / L, the initial COD of the phenolic resin wastewater is 150 mg / L, the pH is adjusted to 4.5, and the reaction time is 5 hours.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1) The carbon dots prepared by this invention have a higher degree of oxidation and a greater number of oxygen-containing functional groups on their surface;

[0026] 2) The carbon dot preparation method involved in this invention is simple and easy to implement, avoids a long-term high-speed centrifugation process, and the added oxidant is green and clean, with no secondary pollution;

[0027] 3) The carbon dots prepared by this invention can achieve a 93.7% removal of 10 mg / L phenol in 1 hour under pH 6 conditions when applied to a Fenton-like process, without producing iron sludge. In addition, the carbon dots can also effectively degrade pollutants in phenolic resin wastewater under pH 4.5 conditions when applied to a Fenton-like system. Attached Figure Description

[0028] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0029] Figure 1 Transmission electron microscopy (TEM) image of carbon dots;

[0030] Figure 2 High-resolution transmission electron microscopy image of carbon dots;

[0031] Figure 3 This is a schematic diagram of the carbon dot particle size distribution;

[0032] Figure 4 This is a schematic diagram showing the zeta potential detection results for carbon dots.

[0033] Figure 5 The fluorescence emission spectrum of carbon dots;

[0034] Figure 6 The fluorescence excitation spectrum of carbon dots;

[0035] Figure 7 Here is the FTIR infrared spectrum of carbon dots;

[0036] Figure 8 Graphs showing the degradation effect of phenol by carbon dots-Fenton-like process prepared with different electrolysis times;

[0037] Figure 9 Graphs showing the degradation effect of phenol by carbon dots-Fenton-like process prepared with different hydrogen peroxide concentrations;

[0038] Figure 10 Graphs showing the degradation effect of phenol by carbon dot-Fenton-like process prepared by electrolysis with 5mM H2O2 for 48h under different pH conditions;

[0039] Figure 11 The carbon dots prepared by electrochemical stripping carbon dot preparation process using H2O2 (5mM) / KMnO4 (2mM) / NaClO3 (1.67mM) as oxidants are shown in the figure. The effect of CQDs-Fe(III) / H2O2 system on phenol degradation is shown.

[0040] Figure 12 The three-dimensional fluorescence spectrum of the degradation effect of phenolic resin wastewater is shown. The left figure (a) is the test result of the raw water sample of phenolic resin wastewater with COD=300mg / L; the right figure (b) is the test result of the water sample after the phenolic resin wastewater underwent a CQDs-Fe(III) / H2O2-type Fenton reaction for 5h. Detailed Implementation

[0041] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0042] Example 1: Preparation of carbon dots

[0043] 0.0256 g of sodium chloride and 255 μL of 30% hydrogen peroxide were added to 500 mL of ultrapure water. Two graphite rods with a diameter of 5 mm and a length of 100 mm were used as the anode and cathode, respectively, with an electrode spacing of 6 cm and a voltage of 30 V. Electrolysis was carried out at room temperature for 48 h. The resulting solution was filtered through a 0.22 μm PTFE membrane, and the filtrate was freeze-dried at -80 °C for 24 h to obtain carbon dot powder.

[0044] Example 2: Preparation of carbon dots in systems with different electrolysis times and different hydrogen peroxide concentrations

[0045] The methods for preparing carbon dots with different electrolysis times (12 / 24 / 48 / 72h) and different hydrogen peroxide concentrations (0 / 1 / 5 / 10mM) are the same as those in Example 1, except that the hydrogen peroxide concentration and electrolysis time are different. The sodium chloride dosage, ultrapure water volume, anode and cathode materials, electrode spacing and filter membrane pore size are the same.

[0046] Example 3: Preparation of carbon dots in electrolyte solutions with different oxidizing agents

[0047] The method for preparing carbon dots using different oxidant H2O2 (5mM) / KMnO4 (2mM) / NaClO3 (1.67mM) electrolyte solutions differs from Example 1 in the type and concentration of oxidant, but the amount of sodium chloride added, the volume of ultrapure water, the anode and cathode materials, the electrode spacing, and the filter membrane pore size are all the same.

[0048] Example 4: Transmission electron microscopy characterization of carbon dots

[0049] The carbon dots prepared in Example 1 were diluted with deionized water to 10 mg / L, and 10 μL was dropped onto an ultrathin carbon film for transmission electron microscopy characterization. Figure 1 This is a transmission electron microscope (TEM) image of carbon dots. Figure 3 Based on Figure 1 The statistical distribution chart of carbon dot size obtained from the data is as follows: Figure 1 and Figure 3 It can be seen that the carbon dots prepared according to Example 1 have a particle size of about 3 nm and a uniform particle size distribution.

[0050] Example 5: Measurement of the lattice size of carbon dots

[0051] Figure 2 The image shows a high-resolution transmission electron microscope (TEM) image of the carbon dots prepared in Example 1. As can be seen from the image, the lattice line width prepared in Example 1 is 0.21 nm, which is consistent with the lattice spacing of graphite crystal (100).

[0052] Example 6: Detection of zeta potential of carbon dots

[0053] Figure 4The graph shows the zeta potential of the carbon dots prepared according to Example 1 under different pH conditions. The carbon dots prepared in Example 1 were diluted and placed in a zeta-nanoparticle size analyzer to measure their zeta potentials. The results are as follows. Figure 4 As shown. By Figure 4 It can be seen that the carbon dots prepared in Example 1 are stable under a wide pH range. Compared with the carbon dots obtained by high-speed centrifugation after electrolysis in ultrapure water for 72 hours, the carbon dots prepared in Example 1 are more stable under alkaline conditions.

[0054] Example 7: Fluorescence Spectroscopy Scanning of Carbon Dots

[0055] The carbon dots prepared in Example 1 were diluted and placed in a three-dimensional fluorescence spectrometer for three-dimensional fluorescence spectroscopy scanning. The excitation wavelength of the carbon dots in Example 1 was 324 nm and the emission wavelength was 475 nm. At an excitation wavelength of 324 nm, the emission spectrum of the carbon dot solution in Example 1 was scanned, and the results were as follows: Figure 5 The excitation spectrum of the carbon dot solution was scanned at an emission wavelength of 475 nm, and the results were as follows: Figure 6 .Depend on Figure 5 , 6 It can be seen that under a specific excitation wavelength, the fluorescence intensity of the CQDs obtained in Example 1 is stronger and the fluorescence characteristics are better than those of the carbon dots obtained by electrolysis for 72 hours, filtration, and high-speed centrifugation.

[0056] Example 8: Carbon dot FTIR characterization

[0057] Figure 7 To characterize and analyze the carbon dots obtained in Example 1 by freeze-drying and then performing total reflectance infrared spectroscopy, Figure 7 It can be seen that the carbon dots prepared in Example 1 have functional groups such as C=O and CO on their surface, and their absorption intensity is stronger and the content of functional groups is greater than that of carbon dots prepared by the prior art.

[0058] Example 9: Quantitative Detection of Oxygen-Containing Functional Groups on Carbon Dot Surface

[0059] The carbon dot solutions obtained in Examples 1 and 2 were diluted to 50 mg / L, and then 0.1 mol / L NaOH, NaHCO3, and Na2CO3 were added respectively. After reacting for 12 h, potentiometric titration was performed with 0.1 mol / L HCl. Quantitative analysis of the functional groups on the carbon dot surface was performed by utilizing the properties that NaOH, Na2CO3, and NaHCO3 can react with hydroxyl, carboxyl, and ester-like groups, respectively, on the carbon dot surface. Wherein:

[0060] n(-OH)=ΔNaOH-ΔNa2CO3

[0061] n(-COOH)=ΔNaHCO3

[0062] n(-COOR)=ΔNa2CO3-ΔNaHCO3

[0063] Example 10: Evaluation of Phenol Removal Efficiency of Carbon Dot-Fenton-like Agents

[0064] The carbon dots prepared in Examples 1 and 2 (22 mg / L) were mixed with 12 μmol / L ferric nitrate nonahydrate and added to an aqueous sample containing 10 mg / L phenol. 56 μL of 30% hydrogen peroxide was added, and the pH was adjusted to 6 with NaOH solution. The reaction was carried out at 25°C for 1 h. The degradation of phenol in the system was determined by liquid chromatography. The results are as follows: Figure 8 , Figure 9 As shown.

[0065] Depend on Figure 8 It can be seen that the carbon dots prepared in Example 1 can achieve 93% removal of phenol under the above reaction conditions, and the carbon dots prepared in Example 2 can all achieve phenol degradation of more than 60% within 1 hour; existing technology (carbon dots obtained by electrolysis for 72 hours followed by filtration and high-speed centrifugation) removes less than 10% of phenol under the same conditions. Figure 9 It can be seen that the carbon dots prepared in Example 2 can achieve more than 60% degradation of 10 mg / L phenol under the conditions of 22 mg / L, pH 6, Fe(III) 12 μM, H2O2 6.4 mM, and pH 6.

[0066] Figure 10 The graph shows the effect of the carbon dots obtained in Example 1 on the degradation of 10 mg / L phenol under pH conditions of 3 / 4 / 5 / 6 / 7 / 8 / 9. As can be seen from the graph, the carbon dots prepared in Example 1 can achieve a degradation of more than 40% of 10 mg / L phenol under pH conditions of 3 / 4 / 5 / 6 / 7 / 8.

[0067] Figure 11 Different carbon dots obtained in Example 3 were mixed with 12 μmol / L ferric nitrate nonahydrate and added to a water sample containing 10 mg / L phenol. 56 μL of 30% hydrogen peroxide was added, and the pH was adjusted to 6 with NaOH solution. The reaction was carried out at 25°C for 1 h. The resulting graph shows the phenol degradation effect. The graph indicates that the addition of different oxidants did not promote the degradation of phenol by the CQDs-Fe(III) / H2O2 system. Compared to the blank control group without added oxidant, the carbon dots prepared with the addition of 2 mM KMnO4 showed poorer phenol degradation performance; while the carbon dots prepared with the addition of 1.67 mM KMnO4 and 5 mM H2O2 both showed enhanced phenol degradation performance, with the 5 mM H2O2 group showing the best degradation effect.

[0068] Example 11: Evaluation of the effect of carbon dot-like Fenton on the treatment of phenolic resin wastewater

[0069] Figure 12 The images show the three-dimensional fluorescence spectra of the carbon dot-Fenton-like system before (a) and after (b) degradation of phenolic resin wastewater. 22 mg / L of the carbon dots prepared in Example 1 was mixed with 30 μmol / L ferric nitrate nonahydrate, added to diluted phenolic resin wastewater with a chemical oxygen demand (COD) of 300 mg / L, and 56 μL of 30% hydrogen peroxide was added. The reaction was carried out at pH 4.5 and 25°C for 5 h. Figure 12 It is evident that the addition of carbon dots in Example 1 can effectively degrade pollutants in phenolic resin wastewater.

[0070] Table 1. Statistics on the surface functional group content of CQDs obtained in Examples 1 and 2.

[0071] Electrolysis time (h) <![CDATA[H2O2 concentration mM]]> hydroxyl mmol / g Carboxyl group mmol / g 48 0 11.17 16.56 48 1 17.81 11.25 48 5 16.72 22.66 48 10 18.98 17.50 72 0 2.82 6.22 12 5 25.28 18.37 24 5 41.92 8.21 72 5 27.22 15.09

[0072] Table 1 summarizes the Boehm titration results of the carbon dots described in Examples 1 and 2. As shown in Table 1, the main functional group formed on the surface of the carbon dots prepared by the method in Example 1 is carboxyl groups, with a concentration reaching 22.66 mmol / L. This is significantly higher than the content of carboxyl and hydroxyl groups in the carbon dots obtained from the control group.

[0073] Table 2. Statistics on the surface functional group content of CQDs obtained in Example 3.

[0074] Electrolysis time (h) Types of oxidants Oxidant concentration (mM) Carboxyl group mmol / g hydroxyl mmol / g 48 <![CDATA[KMnO4]]> 2 10.0 5.0 48 <![CDATA[NaClO3]]> 1.67 12.5 7.5 48 <![CDATA[H2O2]]> 5 22.7 16.7 48 / / 16.6 11.2

[0075] Table 2 summarizes the calculation results of potentiometric titration of the carbon dots described in Example 3. As shown in Table 2, the addition of different oxidants in Example 3 leads to significant changes in the content of oxygen-containing functional groups on the carbon dot surface. Specifically, the carbon dots prepared with NaClO3 and NaClO3-containing electrolyte groups exhibit reduced surface functional group content, and the corresponding Fenton-like degradation effect is weaker than that of the 5mMH2O2-added group.

[0076] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for electrochemically coupling an oxidant to produce carbon dots, the method comprising: providing a solution comprising a carbon source and an oxidant; and applying an electric field to the solution to produce carbon dots. The carbon dots are prepared by electrolysis of a graphite rod in an electrolyte solution containing hydrogen peroxide; the method comprises the following steps: electrolysis is carried out in an aqueous NaCl solution containing H2O2, wherein the concentration of NaCl is 0.1-10 mmol / L, and the concentration of H2O2 is 5-10 mmol / L; the electrolysis time is 12-48 h; The solution obtained by electrolysis is subjected to suction filtration, and the filtrate is collected and freeze-dried to obtain the carbon dots. 2.The method of claim 1, wherein, The electrode distance is kept at 4-8 cm during electrolysis.

3. Use of the carbon dots prepared by the method of claim 1 in the degradation of phenolic pollutants in a Fenton-like process.

4. Use according to claim 3, characterized in that, The phenol-containing water sample includes a phenol-containing water sample and / or a phenol-containing wastewater.

5. Use according to claim 4, characterized in that, The carbon dots are used in a Fenton-like system to treat a phenol-containing water sample, and the process is as follows: after mixing ferric nitrate nonahydrate with the carbon dots, the phenol-containing water sample is added, hydrogen peroxide is added after adjusting the pH, and the sample is taken at intervals, and the change in the phenol content in the water sample is determined by high-performance liquid chromatography.

6. Use according to claim 4, characterized in that, The carbon dots are used in a Fenton-like system to treat a phenol-containing water sample, and the process is as follows: after mixing ferric nitrate nonahydrate with the carbon dots, the phenol-containing water sample is added, hydrogen peroxide is added after adjusting the pH, and the sample is taken at intervals, and the change in the phenol content in the water sample is determined by high-performance liquid chromatography. The carbon dots are used in a Fenton-like system to treat a phenol-containing water sample, and the process is as follows: after mixing ferric nitrate nonahydrate with the carbon dots, the phenol-containing water sample is added, hydrogen peroxide is added after adjusting the pH, and the sample is taken at intervals, and the change in the phenol content in the water sample is determined by high-performance liquid chromatography.

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