Sulfur-nitrogen co-doped graphene hollow nanospheres and their preparation and application

Abstract

The present invention discloses a sulfur-nitrogen co-doped graphene hollow nanosphere and its preparation and application, belonging to the technical field of graphene nanomaterials. During the preparation, a mixture of thiourea and zinc powder is first calcined under the protection of an inert gas, and then H2O and HCl are added to the calcined product and heated for reaction, followed by post-processing to obtain the target product. The resulting product is a sulfur-nitrogen co-doped graphene material, which presents a hollow spherical morphology. This material has a large specific surface area and pore volume, and both the nitrogen and sulfur doping amounts have reached a high level. It has been verified that the obtained high-sulfur-nitrogen co-doped graphene hollow nanospheres have unique graphite nitrogen-pyridine nitrogen-thiophene sulfur synergistic active sites, which have high atomic utilization and high stability, and exhibit the best performance of activating persulfate to degrade organic matter. It can continuously and rapidly separate organic pollutants in actual water bodies and exhibit excellent stability.

Description

Technical Field

[0001] The invention belongs to the technical field of graphene nanomaterials, and particularly relates to sulfur-nitrogen co-doped graphene hollow nanospheres and preparation and application thereof. Background Art

[0002] The research and development of efficient water purification technology has always been a topic of unremitting exploration by researchers in related fields. Common methods for removing organic pollutants from water bodies mainly include three categories of methods: physical, biological and chemical methods. Among them, the physical method mainly uses physical means to remove organic pollutants from water bodies. Although this method is simple to operate and low in cost, it cannot fundamentally remove organic pollutants. It requires the introduction of additional post-processing operations and may cause secondary pollution problems. The biological method mainly uses microorganisms to degrade and metabolize specific organic pollutants into CO2 and H2O. Although the biological method has the advantages of low energy consumption, simple operation and low demand for chemical reagents, the treatment cycle of this method is long, and the specificity of the microorganisms is high. They are very easy to die in highly toxic sewage, which will cause the method to fail.

[0003] Compared to physical and biological methods, chemical methods are considered more effective for degrading highly toxic and recalcitrant organic pollutants. Advanced oxidation technologies (ADOs) are widely used in water treatment due to their high efficiency, thoroughness, and cost-effectiveness in removing organic pollutants. In recent years, persulfate-activated ADOs have garnered significant attention due to their robust oxidation capacity and high degradation efficiency.

[0004] High-temperature heating, ultraviolet light, transition metals, and carbon materials can all be used to activate persulfate to produce reactive oxygen species, which are then used to degrade organic pollutants. Compared with other activation methods, carbon materials are considered the most promising class of persulfate activation materials due to their low cost, low energy consumption, high efficiency, and lack of metal ion contamination. Nanocarbons (such as carbon nanotubes, graphene oxide, and graphene) have also been widely used to replace metal-based catalysts for persulfate activation.

[0005] Two-dimensional graphene, due to its unique physical, chemical, and mechanical properties, has important applications in electronics, sensors, supercapacitors, batteries, catalysis, and other fields. To further improve the activation efficiency of graphene, heteroatom doping has proven to be a very effective method. Experimental and theoretical results have demonstrated that the doping of heteroatoms such as nitrogen, boron, phosphorus, and halogen elements can regulate and modify the catalytic activity and chemical stability of graphene, further expanding the potential application range of graphene materials. Among the many heteroatoms, nitrogen doping has the most significant effect on improving graphene performance. Since Wang Shaobin's research group first reported in 2017 that nitrogen-doped graphene could efficiently activate persulfate to degrade organic pollutants, researchers have used hydrothermal or calcination methods to prepare nitrogen-doped graphene and began to study its persulfate activation properties.

[0006] During persulfate activation, pyridinic and pyrrolic nitrogens on nitrogen-doped graphene are believed to be the primary free radical active sites, while graphitic nitrogen is a non-radical active site. Although these active sites coexist in nitrogen-doped graphene, typically only one type of active site is active during persulfate activation. This limits the number of active sites, resulting in reduced nitrogen atom utilization during activation, which in turn hinders the improvement of the catalytic performance of such materials. Furthermore, the nitrogen active sites are easily oxidized during persulfate activation, resulting in poor cyclic stability of such materials and making them difficult to reuse.

[0007] Currently, research on doped graphene is primarily focused on theoretical studies of nitrogen- or boron-doped graphene. However, recent studies have shown that sulfur (electronegativity: 2.58) has a similar electronegativity to carbon (electronegativity: 2.55), suggesting that sulfur is also a promising doping element. Sulfur-doped graphene could offer a path to increasing graphene's band gap, potentially offering applications in microelectronics, gas sensors, and biomedicine.

[0008] Chinese patent CN 109967111 A discloses a nitrogen-sulfur co-doped graphene with activated persulfate and details the specific preparation process. It uses industrial graphene to prepare nitrogen-sulfur co-doped graphene (IrGO-NS) material through subsequent heat treatment. The prepared IrGO-NS has a large specific surface area (576m 2 g -1), the document points out that the nitrogen doped in the co-doped sample (IrGO-NS) prepared by this method mainly exists in the form of pyridinic N, pyrrolic N and graphitic N, and the sulfur mainly exists in the form of thiophenic sulfur. It is used in the field of water treatment and used in combination with potassium persulfate (PMS) as an oxidant. Under the premise of low dosage, it can effectively remove new pollutants such as preservative methylparaben, sunscreen BP-4 and conventional pollutant phenol in water. However, the action time of the co-doped graphene material prepared by the method disclosed in this document is relatively long, and it generally takes five minutes to achieve an 80% removal effect. This may be due to the low utilization rate of nitrogen atoms and not all types of active sites can play a role in the persulfate activation process. In addition, the removal efficiency of this type of catalytic material for tetracycline, a common pollutant in water, is unknown, and it is also unknown whether it has a good cyclic stability effect. In addition, it is still unknown whether this material can continuously and quickly remove organic pollutants in different actual water bodies.

[0009] Therefore, the design and preparation of nitrogen-sulfur co-doped graphene materials with high stability and high nitrogen atom utilization efficiency remain issues that technicians in this field need to continue to pay attention to. The research and development of corresponding materials has important theoretical significance and practical value for the removal of organic pollutants in actual environments. Summary of the Invention

[0010] The purpose of the present invention is to solve the problems of low nitrogen active site atomic utilization and instability in the conventional nitrogen-doped graphene persulfate activation process in the prior art, and to provide a sulfur-nitrogen co-doped graphene nanosphere material having graphitic nitrogen-pyridinic nitrogen-thiophene sulfur synergistic active sites and a preparation method thereof. Such a material can be used for the continuous and rapid removal of organic pollutants in actual water bodies.

[0011] The technical solution of the present invention is: a method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres, the preparation steps are as follows:

[0012] 1) Thoroughly mix thiourea and zinc powder and place them in a calcination device;

[0013] 2) calcining the mixture under inert gas protection;

[0014] 3) adding H2O and HCl to the calcined product and heating to react;

[0015] 4) The reaction product of step 3) is centrifuged, and the precipitate is washed and dried to obtain sulfur-nitrogen co-doped graphene hollow nanospheres.

[0016] Furthermore, in step 1), the mass ratio of thiourea to zinc powder is 1:5 to 2:1.

[0017] Furthermore, in step 2), the calcination conditions are: a heating rate of 1 to 20° C. / min, heating to 800 to 1000° C. and calcining for 1 to 3 hours.

[0018] Furthermore, in step 3), the volume of H2O added is 100-300 mL, and the volume of HCl is 10-30 mL.

[0019] Furthermore, in step 3), the concentration of HCl is 8 to 12 mol / L.

[0020] Furthermore, in step 3), the heating reaction temperature is 60-100° C., and the heating reaction time is 24-36 hours.

[0021] The sulfur-nitrogen co-doped graphene hollow nanospheres prepared by the above method have unique graphitic nitrogen-pyridinic nitrogen-thiophene sulfur synergistic active sites, which can efficiently remove organic pollutants in actual water bodies. When removing tetracycline at a concentration of 20 mg / L in actual water bodies, the removal rate can reach 90% within 2 minutes.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The sulfur-nitrogen co-doped graphene hollow nanosphere material prepared by the method disclosed in this application has a nitrogen doping content of up to 13.93 at%, and a sulfur doping content of up to 1.12 at%. This material not only has high nitrogen and sulfur doping amounts, but also has synergistic active sites of graphitic nitrogen, pyridinic nitrogen, and thiophene sulfur. These different active sites work together during the persulfate activation process, solving the problems of low nitrogen active site atomic utilization and instability in the traditional nitrogen-doped graphene persulfate activation process, thereby helping to further improve the catalytic performance of the material.

[0024] 2. The sulfur-nitrogen co-doped hollow graphene nanospheres disclosed in this application were packed into a separation column to continuously and rapidly remove organic pollutants from actual water bodies, verifying their feasibility in practical application. This provides new ideas for the design and preparation of heteroatom-doped graphene materials and has important theoretical significance and practical value for the practical application of such materials.

[0025] 3. Since the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in this application have a large specific surface area, their reaction activity is significantly improved. They can be used to continuously and quickly separate organic pollutants in actual water bodies. In addition, they show excellent cyclic stability in column separation experiments and exhibit optimal performance in activating persulfate to degrade organic matter, indicating that such nanospheres can be used repeatedly, which helps to improve economic benefits.

[0026] 4. The preparation process of the sulfur-nitrogen co-doped graphene hollow nanospheres disclosed in this application is simple, cost-controllable, and easy to operate and implement, which will help to expand the application of such materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The process flow chart for preparing sulfur-nitrogen co-doped graphene nanospheres;

[0028] Figure 2 This is a scanning electron microscope photograph of sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1;

[0029] Figure 3 This is the X-ray spectrum of the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1;

[0030] Figure 4 This is the Raman spectrum of the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1;

[0031] Figure 5 This is a transmission electron microscope photograph of sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1;

[0032] Figure 6 The scanning transmission electron micrograph of the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1 and the corresponding element scanning photographs thereof;

[0033] Figure 7 The isothermal adsorption and pore size distribution curves of sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1 are shown;

[0034] Figure 8 This is the X-ray photoelectron spectrum of the sulfur-nitrogen co-doped hollow graphene nanospheres prepared in Example 1;

[0035] Figure 9 This is a graph showing the experimental results of the degradation of tetracycline by persulfate activated by sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1;

[0036] Figure 10 The nitrogen 1s X-ray photoelectron spectra of sulfur-nitrogen co-doped graphene hollow nanospheres in different states;

[0037] Figure 11 This is a scanning electron microscope photograph of the nitrogen-doped graphene nanosheets prepared in Comparative Example 1;

[0038] Figure 12 This is the efficiency diagram of removing tetracycline by persulfate activated by sulfur-nitrogen co-doped graphene hollow nanospheres measured in an application example;

[0039] Figure 13The figure shows the efficiency of sulfur-nitrogen co-doped graphene hollow nanospheres in activating persulfate to remove tetracycline in different water bodies;

[0040] Figure 14 Schematic diagram of the structure of sulfur-nitrogen co-doped graphene hollow nanospheres loaded in a separation column for continuous and rapid removal of organic pollutants in actual water bodies;

[0041] Figure 15 This is a diagram showing the effect of using a chromatographic column to continuously and rapidly remove tetracycline from the Yangtze River water;

[0042] Figure 16 This is a diagram showing the cyclic stability effect of tetracycline degradation by activating persulfate in a deionized water system using sulfur-nitrogen co-doped graphene hollow nanospheres;

[0043] Figure 17 This is a comparison of the performance of nitrogen-doped graphene sheets and high-sulfur-nitrogen co-doped graphene hollow nanospheres in activating persulfate to degrade tetracycline;

[0044] Figure 18 A comparison of the performance of nitrogen-doped graphene sheets in degrading tetracycline in deionized water and Yangtze River water;

[0045] Figure 19 This is a comparison of the performance of sulfur-nitrogen co-doped graphene hollow nanospheres in removing tetracycline in Yangtze River water and simulated Yangtze River water;

[0046] Figure 20 This is the performance graph of removing tetracycline alone from simulated Yangtze River water with added metal ions;

[0047] Figure 21 Sulfur 2p X-ray photoelectron spectra of sulfur-nitrogen co-doped graphene hollow nanospheres in different states. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0049] Example 1. Preparation of sulfur-nitrogen co-doped graphene nanospheres

[0050] See the process flow Figure 1 , the specific preparation steps are as follows:

[0051] 2g of thiourea and 4g of zinc powder were added to an agate mortar and ground thoroughly. The mixture of thiourea and zinc powder was loaded into a ceramic ark. The ceramic ark containing the raw materials was placed in a tube furnace and nitrogen was introduced until the tube furnace was filled with inert gas. The calcination conditions were set to a target temperature of 900℃, a heating rate of 2℃ / min, and a calcination time of 2 hours. The sample was calcined under these conditions. After calcination, the calcined product was poured into a 250mL single-necked flask, and 100mL of H2O and 20mL of HCl (10mol / L) were added in sequence. After mixing, the mixture was reacted at 60℃ for 24h. After the reaction was completed, the solution was centrifuged and the obtained precipitate was repeatedly washed with deionized water until the pH value of the supernatant was 7. Finally, the precipitate was placed in a vacuum drying oven and dried to obtain sulfur-nitrogen co-doped graphene hollow nanospheres (actual product weight: 500mg).

[0052] Related performance tests

[0053] 1. Figure 2 This is a scanning electron microscope photograph of the obtained sulfur-nitrogen co-doped graphene nanomaterial. It can be seen from the figure that the obtained material has a spherical morphology, and from individual broken spheres it can be observed that the sphere has a hollow structure.

[0054] 2. Characterize the physical structure of the sulfur-nitrogen co-doped graphene material prepared in this example. Figure 3 This is the X-ray spectrum of the obtained sulfur-nitrogen co-doped graphene material. It can be seen from the figure that the material has two characteristic peaks at 26.6° and 43.4°, which are consistent with the X-ray diffraction peaks of standard carbon materials, proving that the material is a carbon material.

[0055] 3. Figure 4 The Raman spectrum of the obtained sulfur-nitrogen co-doped graphene material shows two characteristic peaks specific to carbon materials, namely, at 1327 cm -1 The D band and 1586 cm -1 The peak at 37° belongs to the G band, which further confirms that the material is a carbon material.

[0056] 4. Figure 5 This is a transmission electron microscope photograph of the obtained sulfur-nitrogen co-doped graphene material. It can be seen from the figure that the material has a hollow nanosphere structure, and the interplanar spacing of the material is 0.34nm, further proving that it is a graphene structure.

[0057] 5. Figure 6 This is a scanning transmission electron microscope photo of the obtained sulfur-nitrogen co-doped graphene material and its corresponding element scanning photos. From the photo, it can be found that sulfur and nitrogen elements are evenly distributed on the surface of the material, indicating that the material is a sulfur-nitrogen co-doped graphene hollow nanosphere material.

[0058] 6. Figure 7The isothermal adsorption and pore size distribution curves of the obtained sulfur-nitrogen co-doped graphene material are shown in the figure. The specific surface area of ​​the material is 144.72 m 2 / g, pore volume 1.0782m 3 This value is much larger than the corresponding parameter of bulk carbon material, which suggests that the obtained carbon material may be graphene material obtained by exfoliation of carbon material.

[0059] 7. Measure the X-ray photoelectron spectrum of the obtained sulfur-nitrogen co-doped graphene hollow nanospheres, such as Figure 8 As shown in the figure, X-ray photoelectron spectroscopy results show that the obtained material is composed of four elements: carbon, nitrogen, oxygen, and sulfur, demonstrating that this method can successfully prepare sulfur-nitrogen co-doped carbon materials. The nitrogen doping content is 13.93 at%, mainly graphitic nitrogen and pyridinic nitrogen; the sulfur doping content is 1.12 at%, mainly thiophenic sulfur. The nitrogen and sulfur doping levels of the obtained hollow graphene nanospheres are significantly higher than those of sulfur-nitrogen co-doped graphene materials prepared using traditional methods.

[0060] 8. Synergistic site demonstration experiment

[0061] 1) In order to prove that the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in this example activate the active sites of persulfate degradation of tetracycline, a quenching experiment was first performed to verify the generation of active oxygen species in the reaction system. The results are shown in FIG. Figure 9 As shown in the figure, the addition of ethanol and p-benzoquinone has little inhibitory effect on the reaction, indicating the absence of free radicals in the reaction system. When furfuryl alcohol is added to the system, the reaction is almost completely quenched, indicating that the reactive oxygen species generated in the reaction system is singlet oxygen. Furthermore, the addition of potassium iodide, an electron quencher, also fails to quench the reaction, indicating that there is no electron-specific pathway for the oxidation of organic matter in this system.

[0062] 2) Generally, graphitic nitrogen in nitrogen-doped graphene can activate persulfate to produce singlet oxygen. In order to further prove that the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1 activate the active sites of persulfate to degrade tetracycline, the nitrogen 1s X-ray photoelectron spectroscopy of fresh, used and regenerated sulfur-nitrogen co-doped graphene hollow nanospheres was characterized ( Figure 10). Unlike other nitrogen-doped graphene, the proportion of graphitic nitrogen in sulfur-nitrogen co-doped graphene hollow nanospheres increased slightly from 39.96% to 48.40% after use and recovered to 36.90% after regeneration, which means that there are other active sites in the material in addition to graphitic nitrogen. We observed that the proportion of pyridinic nitrogen decreased significantly from 29.78% to 20.27% after use and recovered to 35.87% after regeneration. These results indicate that in addition to graphitic nitrogen, pyridinic nitrogen also contributes to the activation of persulfate to produce singlet oxygen. In contrast, we observed that the proportion of pyrrolic nitrogen in the material gradually decreased from 25.33% to 17.68% during the use and regeneration cycles. This indicates that pyrrolic nitrogen is not an active site. Therefore, the above experiments show that both graphitic nitrogen and pyridinic nitrogen are active sites for the reaction.

[0063] Comparative Example 1: Preparation of nitrogen-doped graphene material based on melamine

[0064] 2g of melamine and 4g of zinc powder were added to an agate mortar and thoroughly mixed. The melamine-zinc powder mixture was then placed in a ceramic ark, which was then placed in a tube furnace. Nitrogen was introduced until the furnace was filled with inert gas. The sample was calcined under these conditions: a target temperature of 900°C, a heating rate of 2°C / min, and a calcination time of 2 hours. After calcination, the product was poured into a 250mL single-necked flask, and 100mL of H2O and 20mL of 10mol / L HCl were added, followed by reaction at 60°C for 24 hours. After the reaction, the solution was centrifuged, and the resulting precipitate was repeatedly washed with deionized water until the pH of the supernatant reached 7. Finally, the precipitate was dried in a vacuum oven to obtain nitrogen-doped graphene nanomaterials.

[0065] Figure 11 This is a scanning electron microscope image of the prepared nitrogen-doped graphene nanomaterial. As can be seen from the image, using melamine instead of thiourea as the raw material under the same conditions results in a nanosheet-like structure rather than a hollow nanosphere. This indicates that the sulfur in thiourea promotes the curling of the generated graphene sheets into a hollow nanosphere structure.

[0066] Application Example 1: Tetracycline removal experiment based on the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1

[0067] 1. The high sulfur and nitrogen co-doped graphene hollow nanospheres prepared in Example 1 were used to activate persulfate to remove tetracycline.

[0068] 1) Weigh 40 mg of tetracycline into a beaker, dissolve in deionized water, transfer to a 2 L volumetric flask, and finally adjust the volume to a 20 mg / L tetracycline solution.

[0069] 2) Weigh 10 mg of the high-sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1 and place them in a 200 mL beaker. Then, add the tetracycline solution prepared in step 1) and stir for 5 minutes to achieve adsorption equilibrium.

[0070] 3) Weigh 24 mg of potassium persulfate monopotassium and dissolve it in the tetracycline solution to initiate the degradation reaction. Stir continuously, starting the timer from the moment of addition. At regular intervals, aspirate 3 mL of the reaction solution with a syringe and filter through a 0.22 μm filter to remove the catalyst. Measure the concentration of residual tetracycline in the filtrate by UV spectroscopy.

[0071] Figure 12 This figure shows the efficiency of high-sulfur and nitrogen co-doped graphene hollow nanospheres in activating persulfate to remove tetracycline. A control experiment was conducted by adding persulfate alone to a tetracycline solution of the same concentration. As can be seen from the figure, when persulfate alone was added to a 20 mg / L tetracycline solution, only approximately 20% of the tetracycline was removed within 30 minutes. However, when persulfate and high-sulfur and nitrogen co-doped graphene hollow nanospheres were added simultaneously to the tetracycline solution, approximately 83% of the tetracycline was removed within 30 minutes. This indicates that high-sulfur and nitrogen co-doped graphene hollow nanospheres can activate persulfate to degrade high concentrations of tetracycline.

[0072] 2. Experimental study on the removal of organic pollutants in actual water using the high sulfur and nitrogen co-doped graphene hollow nanospheres prepared in Example 1

[0073] 1) Tap water from Nanjing, Xianlin Lake, Xuanwu Lake, and Yangtze River were selected as representatives. Except for tap water, all other water samples were filtered to remove sediment.

[0074] 2) Weigh 40 mg of tetracycline into four different beakers, dissolve each in the four different waters, transfer the solution to a 2 L volumetric flask, and finally adjust the volume to obtain a tetracycline solution with a concentration of 20 mg / L.

[0075] 3) Weighing 10 mg of high sulfur and nitrogen co-doped graphene nanospheres into four 200 mL beakers, and then adding the tetracycline solution prepared in step 2) into each beaker and stirring for 5 minutes to reach adsorption equilibrium;

[0076] 4) Weigh 24 mg of potassium persulfate monobasic solution and dissolve it in different tetracycline solutions to initiate degradation reactions. Stir continuously, starting the timer from the moment of addition. At regular intervals, aspirate 3 mL of the reaction solution with a syringe and filter through a 0.22 μm filter to remove the catalyst. Measure the concentration of residual tetracycline in the filtrate by UV spectroscopy.

[0077] Figure 13The figure shows the efficiency of high sulfur and nitrogen co-doped graphene hollow nanospheres in activating persulfate to degrade tetracycline in different water bodies. It can be seen from the figure that high sulfur and nitrogen co-doped graphene nanospheres show high tetracycline removal performance in actual water bodies, with a removal rate of more than 90.0% within 2 minutes, which is significantly improved compared to the removal efficiency in deionized water system. The conversion frequency of the catalytic reaction is 6.24min. -1 , which is the best performance reported so far.

[0078] 3. The high sulfur and nitrogen co-doped graphene hollow nanospheres prepared in Example 1 were loaded into a chromatographic column to conduct a cyclic stability experiment on the removal of organic pollutants in actual water bodies.

[0079] 1) Weigh 200 mg of high sulfur and nitrogen co-doped graphene hollow nanospheres and spread them evenly in the chromatographic column (see Figure 14 );

[0080] 2) Weigh 40 mg of tetracycline and dissolve it in 2 L of Yangtze River water (filtered to remove sediment) to obtain a 20 mg / L tetracycline solution.

[0081] 3) Weighing 480 mg of potassium persulfate and dissolving it in the tetracycline solution prepared in step 2), taking 50 mL of the tetracycline solution containing potassium persulfate and pouring it into the chromatographic column containing the sulfur-nitrogen co-doped hollow graphene nanospheres loaded in step 1), applying pressure above the solution to allow the tetracycline solution to quickly pass through the sulfur-nitrogen co-doped hollow graphene nanospheres, thereby achieving rapid removal of the tetracycline;

[0082] 4) Repeat step 3) for about 40 times to achieve continuous and rapid removal of tetracycline.

[0083] Figure 15 The following diagram illustrates the effectiveness of using a chromatographic column to rapidly and continuously remove tetracycline from Yangtze River water. To assess the contribution of adsorption to tetracycline removal, adsorption experiments were first conducted without the addition of persulfate. The results show that relying solely on the material's adsorption for tetracycline removal is very limited. After the 20th reaction cycle, the efficiency of tetracycline removal by adsorption dropped from 83.6% to approximately 5.0%. In stark contrast, when persulfate was added, the material was able to rapidly and efficiently activate persulfate for tetracycline degradation, maintaining an efficiency of over 90.0% even after 40 consecutive cycles. These results confirm that the highly sulfur-nitrogen co-doped graphene hollow nanospheres obtained in Example 1 exhibit high catalytic performance and reusability for tetracycline degradation in real water.

[0084] 4. The high sulfur and nitrogen co-doped graphene hollow nanospheres prepared in Example 1 were loaded into a chromatographic column to conduct a cyclic stability experiment on the removal of organic pollutants in deionized water.

[0085] 1) Weigh 40 mg of tetracycline into a beaker, dissolve in deionized water, transfer to a 2 L volumetric flask, and finally dilute to a 20 mg / L tetracycline solution.

[0086] 2) Weigh 10 mg of sulfur-nitrogen co-doped graphene hollow nanospheres into a 200 mL beaker, then add the tetracycline solution prepared in step 1) and stir for 5 minutes to achieve adsorption equilibrium;

[0087] 3) Weighing 24 mg of potassium persulfate monopotassium and dissolving it in the tetracycline solution prepared in step 2) to initiate a degradation reaction, stirring continuously. Timing was started from the moment of addition. 3 mL of the reaction solution was aspirated at regular intervals using a syringe and filtered through a 0.22 μm filter to remove the catalyst. The concentration of residual tetracycline in the filtrate was measured by ultraviolet spectroscopy.

[0088] 4) After the reaction time reaches 30 minutes, the catalyst is immediately separated from the reaction solution by centrifugation at a speed of 8000-10000 rpm / min for 5-10 minutes; after the centrifugation, the supernatant is removed and the mixture is washed with deionized water three times, and the centrifugation operation is repeated. Finally, the catalyst is dried at 60° C. to obtain single-use sulfur-nitrogen co-doped graphene hollow nanospheres;

[0089] 5) Weigh 10 mg of the sulfur-nitrogen co-doped graphene hollow nanospheres after single use, place them in a 200 mL beaker, add the tetracycline solution prepared in step 1), and stir for 5 minutes to achieve adsorption equilibrium;

[0090] 6) Weighing 24 mg of potassium persulfate monopotassium and dissolving it in the tetracycline solution obtained in step 5) to initiate a second degradation reaction, stirring continuously. Timing was started from the moment of addition. 3 mL of the reaction solution was aspirated at regular intervals using a syringe and filtered through a 0.22 μm filter to remove the catalyst. The concentration of residual tetracycline in the filtrate was measured by UV spectroscopy.

[0091] 7) After the reaction time reaches 30 minutes, the catalyst is immediately separated from the reaction solution by centrifugation at a speed of 8000-10000 rpm / min for 5-10 minutes; after the centrifugation, the supernatant is removed and the mixture is washed with deionized water three times, and the centrifugation operation is repeated. Finally, the catalyst is dried at 60° C. to obtain sulfur-nitrogen co-doped hollow graphene nanospheres for secondary use;

[0092] 8) Repeat steps 2)-7) to complete the third and fourth cycle experiments. If the remaining material after a single reaction is less than 10 mg, that is, not enough for the next cycle experiment, repeat the process multiple times and collect the material to make up the amount required for the next cycle experiment;

[0093] 9) Placing more than 10 mg of the sulfur-nitrogen co-doped hollow graphene nanospheres, which had been recycled four times and then centrifuged, washed, and dried, into a ceramic ark. The ceramic ark containing the material was placed in a tube furnace and heated from room temperature to 900°C at a heating rate of 2°C / min for 2 hours. After the reaction, regenerated sulfur-nitrogen co-doped hollow graphene nanospheres were obtained.

[0094] 10) Weigh 10 mg of regenerated sulfur-nitrogen co-doped graphene hollow nanospheres and place them in a 200 mL beaker. Then, add the tetracycline solution prepared in step 1) and stir for 5 minutes to achieve adsorption equilibrium.

[0095] 11) Weighing 24 mg of monopotassium persulfate and dissolving it in the tetracycline solution in step 10) to initiate a degradation reaction, stirring continuously, and starting the timer with the moment of addition as the origin. At regular intervals, 3 mL of the reaction solution was drawn out with a syringe and filtered through a 0.22 μm filter to remove the catalyst. The concentration of residual tetracycline in the filtrate was measured by ultraviolet spectroscopy to obtain the performance of the regenerated sulfur-nitrogen co-doped graphene hollow nanospheres in activating persulfate to degrade organic matter.

[0096] Figure 16 This is the cyclic performance diagram of sulfur-nitrogen co-doped graphene hollow nanospheres in a deionized water system. Considering that after four operations in deionized water, the tetracycline removal rate dropped from 82.3% to 60.7%, but after 40 consecutive cycles in the Yangtze River water, it can still reach a removal rate of more than 90% and maintain stability. This indicates that there are special active sites in the sulfur-nitrogen co-doped graphene hollow nanospheres, which enable them to activate persulfate to degrade organic matter more efficiently and stably in actual water bodies.

[0097] Application Example 2: Experiment on the removal of organic pollutants in water based on the nitrogen-doped graphene nanosheets prepared in Comparative Example 1

[0098] 1. Using the nitrogen-doped graphene nanosheets prepared in Comparative Example 1 to activate persulfate and remove tetracycline in deionized water

[0099] 1) Weigh 40 mg of tetracycline into a beaker, dissolve in deionized water, transfer to a 2 L volumetric flask, and finally adjust the volume to a 20 mg / L tetracycline solution.

[0100] 2) Weigh 10 mg of nitrogen-doped graphene nanosheets and place them in a 200 mL beaker. Then, add the tetracycline solution prepared in step 1) and stir for 5 minutes to achieve adsorption equilibrium.

[0101] 3) Weigh 24 mg of potassium persulfate monopotassium and dissolve it in the tetracycline solution to initiate the degradation reaction. Stir continuously, starting the timer from the moment of addition. At regular intervals, aspirate 3 mL of the reaction solution with a syringe and filter through a 0.22 μm filter to remove the catalyst. Measure the concentration of residual tetracycline in the filtrate by UV spectroscopy.

[0102] Figure 17 This is a performance comparison chart of nitrogen-doped graphene sheets and high-sulfur and nitrogen co-doped graphene hollow nanospheres (prepared in Example 1) in activating persulfate to degrade tetracycline. As can be seen from the figure, the nitrogen-doped graphene sheets prepared based on melamine have a removal rate approximately 10% lower than that of high-sulfur and nitrogen co-doped graphene hollow nanospheres prepared under the same conditions.

[0103] 2. Experimental study on removal of organic pollutants in actual water using the nitrogen-doped graphene nanosheets prepared in Comparative Example 1

[0104] 1) Weigh 40 mg of tetracycline into a beaker, dissolve it in pretreated Yangtze River water and deionized water, transfer it to a 2 L volumetric flask, and finally adjust the volume to a 20 mg / L tetracycline solution.

[0105] 2) Weigh 10 mg of nitrogen-doped graphene nanosheets and place them in a 200 mL beaker. Add the tetracycline solution prepared in step 1) and stir for 5 minutes to achieve adsorption equilibrium.

[0106] 4) Weigh 24 mg of potassium persulfate monobasic and dissolve it in the tetracycline solution to initiate the degradation reaction. Stir continuously, starting the timer from the moment of addition. At regular intervals, aspirate 3 mL of the reaction solution with a syringe and filter through a 0.22 μm filter to remove the catalyst. Measure the concentration of residual tetracycline in the filtrate by UV spectroscopy.

[0107] Figure 18 The figure shows a comparison of the performance of nitrogen-doped graphene nanosheets in deionized water and Yangtze River water in deionized water. As can be seen from the figure, the removal rate of tetracycline decreases from 73.5% in the deionized water system to 51.5% in the Yangtze River water system. It is speculated that the sulfur atoms in the sulfur-nitrogen co-doped graphene hollow nanospheres prepared in Example 1 are the main reason for improving the removal rate of tetracycline in actual water bodies.

[0108] To further confirm the above speculation, certain metal ions were added to deionized water to simulate the Yangtze River water (iron ion concentration in the water body = 0.2 mg / L, cobalt ion concentration = 0.05 mg / L, nickel ion concentration = 0.05 mg / L, and the ion concentration was determined according to the concentration of the corresponding metal ions in the latest Yangtze River water quality report). The study found that in the Yangtze River water and the simulated Yangtze River water, the high sulfur and nitrogen co-doped graphene hollow nanospheres prepared in Example 1 had very similar removal effects on tetracycline, both exceeding 90% ( Figure 19 ).

[0109] Figure 20 This graph shows the efficiency of tetracycline removal experiments conducted solely using simulated Yangtze River water spiked with metal ions. Without the addition of carbon materials, the added metal ions only removed approximately 50.0% of tetracycline. However, with the addition of carbon materials, the tetracycline removal efficiency in the simulated Yangtze River water exceeded 90%, demonstrating that it was the carbon materials that played a role in degrading tetracycline in the Yangtze River water system, rather than the metal ions in the water that contributed to the increased removal efficiency.

[0110] To further confirm the interaction between sulfur atoms and metal ions, we characterized the sulfur 2p X-ray photoelectron spectra ( X-ray photoelectron spectroscopy) of sulfur-nitrogen co-doped graphene hollow nanospheres before and after use in deionized water and real water. Figure 21 After mixing the materials with real water and then using them for catalytic degradation of tetracycline, the peak attributable to thiophenic sulfur in these materials red-shifted by 0.24 eV compared to the sulfur-nitrogen co-doped hollow graphene nanospheres reacted in deionized water, while the peak attributable to sulfur oxide remained unchanged. This suggests a coordination interaction between thiophenic sulfur and metal ions.

[0111] Combining all the above experimental results, it can be proved that the graphitic nitrogen-pyridinic nitrogen-thiophene sulfur synergistic active sites are the sites where high sulfur and nitrogen co-doped graphene hollow nanospheres can efficiently and stably activate persulfate to degrade organic matter in actual water bodies.

[0112] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres, characterized in that: The preparation steps are as follows: 1) Thoroughly mix thiourea and zinc powder and place them in a calcination device; 2) calcining the mixture under inert gas protection; 3) adding H2O and HCl to the calcined product and heating to react; 4) The reaction product of step 3) is centrifuged, and the precipitate is washed and dried to obtain sulfur-nitrogen co-doped graphene hollow nanospheres.

2. The method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres according to claim 1, wherein: In step 1), the mass ratio of thiourea to zinc powder is 1:5 to 2:

1.

3. The method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres according to claim 1, wherein: In step 2), the calcination conditions are: a heating rate of 1 to 20° C. / min, heating to 800 to 1000° C. and calcining for 1 to 3 hours.

4. The method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres according to claim 1, wherein: In step 3), the volume of H2O added is 100-300 mL, and the volume of HCl is 10-30 mL.

5. The method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres according to claim 1, wherein: In step 3), the concentration of HCl is 8 to 12 mol / L.

6. The method for preparing sulfur-nitrogen co-doped graphene hollow nanospheres according to claim 1, wherein: In step 3), the heating reaction temperature is 60-100° C., and the heating reaction time is 24-36 hours.

7. A sulfur-nitrogen co-doped graphene hollow nanosphere, characterized in that: The hollow graphene nanospheres are prepared by the method for preparing sulfur-nitrogen co-doped graphene nanospheres according to any one of claims 1 to 5.

8. The use of sulfur-nitrogen co-doped graphene hollow nanospheres as claimed in claim 7 in removing organic pollutants in actual water bodies, characterized in that: Sulfur-nitrogen co-doped graphene hollow nanospheres have synergistic active sites of graphitic nitrogen-pyridinic nitrogen-thiophene sulfur. When removing tetracycline at a concentration of 20 mg / L in actual water bodies, the removal rate can reach 90% within 2 minutes.

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