A cathode catalyst and a method for preparing and applying the same in a heterogeneous electro-fenton-electroflocculation system for treating tetracycline hydrochloride wastewater containing oxalate

Abstract

The application discloses a kind of cathode catalyst and its preparation and application in heterogeneous electro-Fenton-electroflocculation system method for treating tetracycline hydrochloride wastewater containing oxalate, belong to electrochemical wastewater treatment technical field.The method is based on heterogeneous electro-Fenton-electroflocculation system;Through electroflocculation system, oxalate in wastewater is removed, and the formation process of floc is optimized in electro-Fenton-electroflocculation system, so that the effect of removing tetracycline hydrochloride in the system is improved;Based on the optimized electro-Fenton system coupling electroflocculation system, the formation of floc in electroflocculation process is promoted under the action of nitrogen-doped interlayer electrode, and the contribution of electroflocculation to the removal of TC is further improved.The removal rate of TC based on the method of the application is 94.7% in 60 minutes.Under the joint action of electroflocculation and electro-Fenton process, excellent TC removal rate is realized.The application provides a new direction for the process technology for treating TC wastewater, and has high practical value.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electrochemical wastewater treatment, and particularly relates to a cathode catalyst and a preparation method and application thereof in a heterogeneous electro-Fenton-electroflocculation system for treating tetracycline hydrochloride wastewater containing oxalate. BACKGROUND

[0002] Tetracycline hydrochloride (TC) is a representative broad-spectrum antibiotic and is widely used in the fields of medical treatment, livestock and aquaculture, etc. The annual consumption of TC is as high as 210,000 tons. At present, TC can be produced by fermentation, and the process mainly includes the preparation and acidification of fermentation broth, the preparation of tetracycline base product, the preparation of tetracycline hydrochloride, and the preparation of tetracycline hydrochloride, etc. In these processes, especially in the stage of fermentation broth acidification, a large amount of oxalic acid needs to be added for treatment, which results in that the final tetracycline hydrochloride wastewater contains oxalate (Ox) with a concentration of 800-4500 mg / L, which is much higher than that of TC (60-400 mg / L).

[0003] At present, the technologies for treating TC wastewater include microbial degradation, photocatalysis, adsorption, membrane separation and advanced oxidation processes (AOPs). The biological treatment method has low treatment efficiency because it inhibits the growth and metabolism of microorganisms. The adsorption method increases the operation cost because the adsorption material needs to be regenerated. As an environmentally friendly method, electro-Fenton mainly treats TC by active substances, which can decompose TC to achieve complete removal. However, when the electro-Fenton system treats TC pharmaceutical wastewater, as a small molecule organic matter, Ox will react with ·OH. Therefore, Ox will compete with TC for ·OH, thereby affecting the removal effect of the electro-Fenton system on TC; Ox has small molecular weight and is difficult to degrade, and currently the physical adsorption method is mainly used to remove Ox in wastewater.

[0004] Although the electro-Fenton system shows excellent removal effect and mineralization degree when treating simulated TC wastewater, compared with the simulated TC wastewater of a single pollutant, there are more types of pollutants in the real TC wastewater, and the situation is more complex. Therefore, only using a single electro-Fenton technology to treat real TC wastewater may not achieve the expected effect. When the electro-Fenton system treats TC wastewater containing Ox, as a small molecule organic matter, Ox will compete with TC for ·OH, thereby reducing the TC removal effect of the electro-Fenton system. It is necessary to eliminate the influence of Ox on the removal of TC by the electro-Fenton system.

[0005] Therefore, it is necessary to develop a new technology to solve the problem of Ox interfering with the treatment effect of the electro-Fenton system when treating TC wastewater. SUMMARY

[0006] This section is intended to introduce some aspects of the embodiments of the present application and to further draw the reader's attention by highlighting some preferred embodiments of the present application. It will be appreciated that the section need not be read to understand the invention, and the section can include matters, which are not prior art.

[0007] In view of the above and / or existing problems in the prior art, the present application is proposed.

[0008] Therefore, the purpose of the present application is to overcome the deficiencies in the prior art, and provide a preparation method of a cathode catalyst.

[0009] To solve the above technical problems, the present application provides the following technical scheme: including,

[0010] After the iron salt, the cobalt salt and the organic ligand substance are dissolved into the organic solvent, they are transferred to a water reaction kettle, and reacted at 110-150 DEG C for 24-48 hours and cooled to room temperature, then the supernatant is discarded after centrifugation, and the powder Fe / Co MOFs are prepared after washing and drying;

[0011] The powder Fe / Co MOFs and melamine are dispersed into ethanol, and the cathode catalyst for a heterogeneous electro-Fenton-electroflocculation system is prepared after suction filtration, drying, calcination and cooling.

[0012] As a preferred scheme of the preparation method of the cathode catalyst, the molar ratio of the iron salt to the cobalt salt is 2:1-1:2, the iron salt includes one or more of FeCl3.6H2O, Fe(NO3)3.9H2O and FeSO4.7H2O, and the cobalt salt includes one or more of Co(NO3)2.6H2O and CoCl2.6H2O.

[0013] As a preferred scheme of the preparation method of the cathode catalyst, the organic ligand includes one or more of terephthalic acid and trimesic acid, and the calcination temperature is 500-700 DEG C, and the calcination time is 60-120 min.

[0014] The purpose of the present application is to overcome the deficiencies in the prior art, and provide a cathode catalyst.

[0015] The purpose of the present application is to overcome the deficiencies in the prior art, and provide a heterogeneous electro-Fenton-electroflocculation system using the above-mentioned cathode catalyst.

[0016] To solve the above technical problems, the application provides the following technical scheme: the heterogeneous electro-Fenton-electrocoagulation system comprises a nitrogen-doped interlayer electrode, a reference electrode, a counter electrode, an electrochemical workstation and an aeration pipe; the nitrogen-doped interlayer electrode, the reference electrode and the counter electrode are connected with the electrochemical workstation respectively, and the aeration pipe is connected with the nitrogen-doped interlayer electrode from the bottom; the counter electrode is a noble metal electrode; the reference electrode comprises one or more of Ag / AgCl electrodes and mercury electrodes.

[0017] In the application, the pH of the heterogeneous electro-Fenton-electrocoagulation system is 3.5-4.5; and the cathode in the heterogeneous electro-Fenton-electrocoagulation system is a nitrogen-doped interlayer electrode prepared by combining a cathode catalyst and a hydrophobic graphite felt.

[0018] As a preferred scheme of the heterogeneous electro-Fenton-electrocoagulation system, the preparation method of the nitrogen-doped interlayer electrode comprises the following steps:

[0019] The PVDF / DMA adhesive is prepared by dissolving polyvinylidene fluoride in N,N-dimethylacetamide; the cathode catalyst NMOFsPC and the PVDF / DMA adhesive are mixed sufficiently and then uniformly coated on both sides of the hydrophobic graphite felt; after being fully immersed in water, the nitrogen-doped interlayer electrode combined by the hydrophilic catalytic layer and the hydrophobic substrate is obtained by taking out and air-drying.

[0020] In the application, the concentration of the PVDF / DMA adhesive is 5%-10%.

[0021] As a preferred scheme of the heterogeneous electro-Fenton-electrocoagulation system, the preparation method of the hydrophobic graphite felt comprises the following steps:

[0022] The graphite felt is sequentially placed in ethanol and ultrapure water, ultrasonically cleaned and dried to obtain a hydrophilic graphite felt; then the hydrophilic graphite felt is immersed in a polytetrafluoroethylene solution, taken out after sufficient soaking, calcined and cooled to obtain the hydrophobic graphite felt.

[0023] In the application, the concentration of the polytetrafluoroethylene solution is 5%-15%; the calcination temperature is 300-400 DEG C, and the calcination time is 5-15 min.

[0024] As a preferred scheme of the method for treating tetracycline hydrochloride wastewater containing oxalate by using the heterogeneous electro-Fenton-electrocoagulation system, the nitrogen-doped interlayer electrode removes tetracycline hydrochloride containing oxalate in the wastewater by electro-adsorption under the condition of aeration.

[0025] The heterogeneous electro-Fenton-electroflocculation system comprises a nitrogen-doped interlayer electrode, a reference electrode, a counter electrode, an electrochemical workstation and an aeration pipe; the nitrogen-doped interlayer electrode, the reference electrode and the counter electrode are connected with the electrochemical workstation, and the aeration pipe is connected with the nitrogen-doped interlayer electrode from the bottom.

[0026] As a preferred scheme of the method for treating tetracycline hydrochloride wastewater containing oxalate by using the heterogeneous electro-Fenton-electroflocculation system, in the electro-Fenton-electroflocculation system, the oxalate treatment concentration is 0-1000 mg / L, and the tetracycline hydrochloride treatment concentration is 0-100 mg / L.

[0027] As a preferred scheme of the method for treating tetracycline hydrochloride wastewater containing oxalate by using the heterogeneous electro-Fenton-electroflocculation system, in the electro-Fenton-electroflocculation system, the aeration gas is air or oxygen, and the aeration rate is 2-50 mL / min.

[0028] The present application has the following beneficial effects:

[0029] (1) The nitrogen-doped interlayer electrode prepared by combining the cathode catalyst with the hydrophobic felt has excellent ORR performance, and a large number of pores ensure that there is space for gaseous oxygen storage and mass transfer process in the graphite felt substrate.

[0030] (2) The present application solves the influence of Ox on the removal of TC by the electro-Fenton system by constructing an electro-Fenton-electroflocculation system, and based on the nitrogen-doped interlayer electrode, the problem of insufficient oxygen supply for the ORR reaction is solved, the utilization rate of ·OH is improved, and the electro-Fenton system is optimized.

[0031] (3) Based on the optimized electro-Fenton system coupled with the electroflocculation system to construct the electro-Fenton-electroflocculation system, the flocs generated in the electroflocculation process promote the formation of flocs in the electroflocculation process under the action of the nitrogen-doped interlayer electrode, and the contribution of the electroflocculation to the removal of TC is further improved. Based on the method of the present application, the removal rate of TC is 94.7% in 60 min.

[0032] (4) Under the joint action of electroflocculation and electro-Fenton process, excellent TC removal rate is achieved. The present application provides a new direction for the treatment technology of TC wastewater and has high practical value. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings. Among them:

[0034] Figure 1 Preparation flow chart of the cathode catalyst of the embodiment 1 of the present application.

[0035] Figure 2 Nitrogen adsorption-desorption isotherm graph (A) and pore size distribution graph (B) of three catalysts prepared at different treatment temperatures.

[0036] Figure 3 CV curve graph (A) and LSV curve graph (B) of NMOFsPC catalysts prepared at different treatment temperatures.

[0037] Figure 4 Removal effect graph of electric adsorption TC of NMOFsPC catalysts prepared at different treatment temperatures.

[0038] Figure 5 Hydrophobic treatment flow chart of the graphite felt of the embodiment 2 of the present application.

[0039] Figure 6 Preparation schematic diagram of the nitrogen-doped interlayer electrode of the embodiment 2 of the present application.

[0040] Figure 7 Performance test of the nitrogen-doped interlayer electrode prepared in the embodiment 2; wherein: (A) photo of the hydrophobized GF immersed in an aqueous solution, (B) photo of the nitrogen-doped interlayer electrode immersed in an aqueous solution, (C) contact angle.

[0041] Figure 8 SEM graph of the nitrogen-doped interlayer electrode prepared in the embodiment 2.

[0042] Figure 9 Nyquist graph of the nitrogen-doped interlayer electrode prepared in the embodiment 2.

[0043] Figure 10 Linear sweep voltammetry curve of the nitrogen-doped interlayer electrode prepared in the embodiment 2.

[0044] Figure 11 (A) TC removal effect, (B) ·OH production, (C) TOC removal effect of the nitrogen-doped interlayer electrode.

[0045] Figure 12 (A) TC removal effect, (B) Ox removal effect of the electro-Fenton-electrocoagulation system, the electro-Fenton system and the electrocoagulation system.

[0046] Figure 13 Experimental device for electrochemical system degradation platform.

[0047] Figure 14 TC removal effect of nitrogen-doped interlayer electrode electro-Fenton system (A) and Ox removal effect (B).

[0048] Figure 15 TC removal effect of electro-Fenton-electrocoagulation system without electro-Fenton effect (A), floc particle size (B), floc morphology in different stages of electro-Fenton-electrocoagulation (C) and electro-Fenton system (D).

[0049] Figure 16 TC removal effect (A), floc particle size (B) and oxygen bubble overflow (C) under different aeration rates.

[0050] Figure 17 TC removal mechanism of electro-Fenton-electrocoagulation system. DETAILED DESCRIPTION

[0051] In order to make the above objectives, features and advantages of the present application more apparent and comprehensible, the specific embodiments of the present application will be described in detail below with reference to the description and examples.

[0052] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0053] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an independent or selective embodiment that excludes other embodiments.

[0054] The raw materials used in the present application are ordinary commercially available materials without special instructions.

[0055] The wastewater used in the examples and comparative examples of the present application is simulated by the following method:

[0056] The concentration and concentration ratio of Ox and TC in the simulated wastewater are prepared according to the real tetracycline hydrochloride wastewater.

[0057] Preparation of wastewater containing TC and Ox: in a 50ml reactor, the electrolyte is 0.1M Na2SO4 solution, the initial pH is 3.5, the TC concentration is 100mg / L, and the Ox concentration is 1000mg / L.

[0058] TC wastewater preparation: In a 50ml reactor, the electrolyte is a 0.1M Na2SO4 solution, the initial pH is 3.5, and the TC concentration is 100mg / L.

[0059] Electrochemical system degradation platform construction: The experimental device is as shown in Figure 13 All experiments are carried out in a 50mL reactor via a CHI 660E electrochemical workstation.

[0060] Electro-Fenton system used in the example: platinum sheet, nitrogen-doped interlayer electrode and Ag / AgCl electrode are used as the counter electrode, working electrode and reference electrode respectively, and aeration is required.

[0061] Electrocoagulation system used in the example: aluminum sheet is used as the counter electrode and working electrode, and Ag / AgCl is used as the reference electrode.

[0062] Electro-Fenton-electrocoagulation system used in the example: aluminum sheet, nitrogen-doped interlayer electrode and Ag / AgCl electrode are used as the counter electrode, working electrode and reference electrode respectively, and aeration is required.

[0063] Electro-Fenton-electrocoagulation system without electro-Fenton effect used in the example: the electro-Fenton-electrocoagulation system is constructed, but methanol is added as a ·OH quencher in the system to eliminate the contribution of ·OH generated by the electro-Fenton process to the removal of TC, and aeration is required.

[0064] The electrochemical impedance (EIS) of the nitrogen-doped interlayer electrode prepared in Example 2 of the present application is tested by open circuit voltage: the test voltage is open circuit voltage, the frequency range is 1x10 5 to 1x10 -2 Hz, and the alternating current potential amplitude is 5Mv.

[0065] The test conditions of the linear sweep voltammetry curve of the nitrogen-doped interlayer electrode prepared in Example 2 of the present application are: pH=7, 0.1M Na2SO4, O2 flow rate is 20mL / min, test potential-0.7 to 0V vs Ag / AgCl, and scan rate is 5mV / s.

[0066] The test conditions of the TC removal effect and ·OH production of the nitrogen-doped interlayer electrode prepared in Example 2 of the present application are: potential-0.7V vs Ag / AgCl, solution pH=7, O2 rate 20mL / min, TC0=40mg / L.

[0067] The oxygen reduction (ORR) performance and electric adsorption capacity of the nitrogen-doped interlayer electrode prepared in Example 3 at different treatment temperatures were tested by an electrochemical workstation in a three-electrode system. An Ag / AgCl electrode, a platinum sheet electrode and a catalyst-coated FTO electrode were used as a reference electrode, a counter electrode and a reference electrode. The preparation conditions of the catalyst-coated FTO electrode were as follows: 7.5 mg of catalyst, 1.44 ml of isopropanol and 60 μL of perfluoro resin solution were sequentially taken into a 5 ml centrifuge tube, and the mixture was ultrasonically treated for 45 min to form a uniformly distributed ink. A certain amount of catalyst ink was added dropwise to the FTO glass (2 cm x 2 cm x 0.22 cm).

[0068] Example 1: The cathode catalyst was prepared according to the procedure shown in Figure 1

[0069] (1) Preparation of Fe / Co MOFs material

[0070] FeCl3·6H2O, Co(NO3)2·6H2O (molar ratio of 2:1, total molar number of 5 mmol) and 5 mmol of H2BDC (terephthalic acid) were dissolved in 30 mL of N,N-dimethylformamide (DMF), and the solution was transferred to a 100 mL polytetrafluoroethylene-lined water reaction kettle after stirring for 1.5 h. The solution was reacted at 110°C for 24 h and cooled to room temperature. After centrifugation at a speed of 4500 rpm for 2 min, the supernatant was discarded, and the mixture was washed with DMF three times and ethanol three times. The mixture was dried in a vacuum drying oven at 60°C for 12 h to obtain a pure powder of Fe / Co MOFs.

[0071] (2) Preparation of NMOFsPC cathode catalyst

[0072] 0.4 g of Fe / Co MOFs powder and 0.2 g of melamine were added to 20 mL of ethanol and ultrasonically dispersed for 2 h. After suction filtration, a solid mixture was obtained, which was dried in an oven for 12 h. A nitrogen-doped NMOFsPC catalyst was prepared using a tube furnace, with a treatment temperature of 600°C and a heating rate of 5°C / min, and was kept at a constant temperature for 2 h. The NMOFsPC catalyst was obtained after cooling to room temperature.

[0073] Example 2: Effect of different treatment temperatures on the performance of nitrogen-doped catalyst

[0074] In this example, NMOFsPC catalysts were prepared at treatment temperatures of 400°C, 600°C and 800°C, respectively, and were denoted as NMOFsPC-400, NMOFsPC-600 and NMOFsPC-800.

[0075] Figure 2 ​(A) shows nitrogen adsorption-desorption isotherms of three catalysts prepared at different processing temperatures. By comparing the values of the vertical coordinates of the nitrogen adsorption-desorption isotherms, it can be preliminarily obtained that the specific surface areas of the three catalysts are NMOFsPC-600 > NMOFsPC-800 > NMOFsPC-400. Increasing the temperature can increase the specific surface area of the catalyst, but too high a temperature will cause the collapse of the pore structure, thereby causing the specific surface area to decrease. Further, the specific surface areas of the three catalysts are 105, 238 and 193 m 2 / g, respectively. The pore size distribution graphs of the three catalysts are shown in Figure 2 (B) shows that the horizontal coordinates of the pore size distribution graph represent the pore size of the catalyst, where the pore size less than 2 nm is microporous, and the pore size between 2-50 nm is mesoporous. The vertical coordinates represent the pore volume size, reflecting the number of corresponding pore sizes. As can be seen from the graph, the three catalysts all exhibit a microporous-mesoporous hierarchical porous structure, among which NMOFsPC-600 has more mesopores and micropores than the other two catalysts, which can improve the ORR performance. At the same time, the mesoporous structure can effectively reduce the diffusion resistance of gas and pollutants in the porous structure, promoting the effective contact of gas and pollutants with the reaction active center.

[0076] Figure 3 (A) is the CV curve of the NMOFsPC catalyst prepared at different processing temperatures. The response current of the NMOFsPC-600 catalyst corresponding to the maximum is -34.5 mA; followed by the NMOFsPC-800 catalyst, which is 22.3 mA, and the minimum is the NMOFsPC-400, which is -14.3 mA; indicating that among the three, the NMOFsPC-600 catalyst has the strongest ability to reduce oxygen. Figure 3 (B) is the LSV curve of the three catalysts. The onset potential of the three catalysts is further analyzed to analyze the ORR performance of the catalyst. The onset potential is the potential at which the response current begins to increase significantly, i.e., the potential at which the ORR reaction on the surface of the working electrode will occur. Therefore, a more positive peak potential indicates that the electron transfer is easier to trigger and the ORR reaction is more likely to occur under the action of the catalyst. The onset potential of the NMOFsPC-600 catalyst is the most positive, which is -0.07 V vs Ag / AgCl, followed by the NMOFsPC-800, which is -0.11 V vs Ag / AgCl, and the minimum is the NMOFsPC-400, which is -0.16 V vs Ag / AgCl. Therefore, the NMOFsPC-600 catalyst has the best ORR performance.

[0077] The results of the catalyst electro-adsorption TC capacity experiment under saturated nitrogen conditions are as follows Figure 4The removal rate of TC in the FTO electrode system without catalyst loading is only 6.7%, which indicates that the electro-adsorption and direct anodic oxidation of TC on the electrode are very limited. Compared with the electrode system with non-nitrogen-doped catalyst (MOFsPC-600) loading, the three electrode systems with nitrogen-doped catalyst loading have better electro-adsorption effect on TC. This indicates that nitrogen doping can improve the adsorption performance of the catalyst on TC. Among them, the TC removal rate of NMOFsPC-600 is the highest, reaching 43.6%, followed by NMOFsPC-800, which is 40.5%, and NMOFsPC-400, which is the lowest, reaching 38.2%.

[0078] The above shows that nitrogen doping helps to improve the ability of the catalyst to adsorb TC. The pyridine-N of the NMOFsPC-600 catalyst and the TC mass transfer conditions provided by the high surface area and mesoporous structure make it have the best electro-adsorption capacity for TC.

[0079] Example 3: Preparation of nitrogen-doped sandwich electrode

[0080] (1) Hydrophobic treatment of graphite felt: The method steps are as shown in Figure 5 , first put the graphite felt (GF) into ethanol and ultrapure water successively, ultrasonic cleaning for 30 min, remove the impurities on the surface of the material, and then dry to obtain hydrophilic GF. Then cut out GF with a size of 20x20x3mm, immerse it in a 10% polytetrafluoroethylene (PTFE) solution, take it out after 2h and place it in a muffle furnace, the furnace temperature is raised from room temperature to 340℃ at a rate of 10℃ / min, annealed for 10 min, and then cooled to room temperature in the furnace to obtain hydrophobic GF.

[0081] (2) Preparation of electrode: The flow chart is as shown in Figure 6 , dissolve polyvinylidene fluoride (PVDF) in N,N-dimethylacetamide (DMA) to prepare 8% PVDF / DMA adhesive. Mix 60mg of cathode catalyst NMOFsPC-600 and 0.25mL of 8% PVDF / DMA adhesive thoroughly, then evenly coat it on both sides of the hydrophobic graphite felt, immerse it in water for 30 min, and then naturally air dry for 2h to obtain the electrode formed by the combination of the hydrophilic catalyst layer and the hydrophobic substrate.

[0082] Figure 7 The performance test results of the nitrogen-doped sandwich electrode prepared in this example are shown in Figure 7 (A), on the surface of the hydrophobic substrate, a mirror-like silver gas film can be clearly observed, which is caused by the reflection of light by the air remaining on the surface of the electrode. This indicates that the hydrophobic treated GF can prevent water from being immersed, and can provide conditions for gas mass transfer and storage. Figure 7(C) shows that the CA of the cathode catalyst layer is 45.9°, which is less than 90°. This indicates that it has good hydrophilicity, which ensures that the electrode can be in good contact with the electrolyte. The CA of the bottom of the electrode is 140.3°, which is more than 100°, indicating that it has very strong hydrophobicity, which ensures that the inside of the electrode will not be invaded by the aqueous solution. Based on the above phenomena and analysis, by combining the hydrophilic catalyst layer and the hydrophobic substrate, a space for gas mass transfer is introduced into the electrode.

[0083] Figure 8 The SEM image of the nitrogen-doped sandwich electrode prepared in this example was analyzed by a scanning electron microscope to analyze the morphological characteristics of the prepared electrode. It can be seen that the hydrophilic nitrogen-doped catalyst layer composed of NMOFsPC catalyst is distributed on the surface of the hydrophobic graphite felt, which indicates that the catalyst layer is successfully loaded on the surface of the hydrophobic graphite felt substrate, providing conditions for the effective contact of the electrode with the electrolyte solution. The hydrophilic catalyst layer is below the hydrophobic graphite felt substrate, and it can be observed that the polytetrafluoroethylene fills the pores composed of carbon fibers, making the graphite felt hydrophobic. At the same time, a large number of pores ensure that there is space in the graphite felt substrate for gaseous oxygen storage and mass transfer process.

[0084] Figure 9 The Nyquist plot of the nitrogen-doped sandwich electrode prepared in this example was analyzed by electrochemical impedance spectroscopy (EIS) to analyze the charge transfer and gas mass transfer process in the electrode. In the low frequency region of the electrode, the electrode shows a small semicircle, indicating that the R mt (charge transfer resistance) of the electrode is 1.64 Ω. This indicates that the oxygen required for the ORR reaction at the TPI (three-phase interface) in the electrode can be fully supplied through the mass transfer space in the hydrophobic substrate. By analyzing the high frequency region, the Rct(oxygen mass transfer resistance) of the electrode is 1.82 Ω. Therefore, the hydrophobic substrate structure can help oxygen mass transfer quickly, thereby improving the ORR performance of the electrode. This is due to the TPI formed by the asymmetric hydrophilic and hydrophobic properties of the sandwich electrode, which promotes charge transfer.

[0085] Figure 10 The linear sweep voltammetry curve of the nitrogen-doped sandwich electrode prepared in this example. When oxygen is continuously introduced into the system, the electrode has a very positive onset potential of -0.11 V vs Ag / AgCl, which indicates that the electrode has excellent ORR performance. When the reduction potential is higher than the onset potential, the electrode begins to undergo an oxygen reduction reaction, generating a Faraday current. The current of the electrode increases sharply, and at the potential of -0.7 V Ag / AgCl, it rises sharply to -214 mA. This indicates that the electrode uses gaseous oxygen as the oxygen source through the hydrophobic substrate, avoiding the low mass transfer rate and low solubility of dissolved oxygen, providing fast and sufficient oxygen for the ORR reaction.

[0086] The TC removal efficiency, TOC removal efficiency, and ·OH production of nitrogen-doped sandwich electrodes are as follows: Figure 11 As shown in (A)(B)(C), under a saturated nitrogen environment, the nitrogen-doped sandwich electrode removed 43.6% of total toxicity (TC) within 60 minutes via electroadsorption, demonstrating the excellent TC electroadsorption capability of the nitrogen-doped catalytic layer. Under a saturated oxygen environment, the TC removal rate reached as high as 99%, and the total organic carbon (TOC) removal rate reached 61%. This is because the nitrogen-doped sandwich electrode receives sufficient gaseous oxygen through the hydrophobic graphite felt substrate, and the electrode exhibits excellent ORR (Organic Oxygen Removal) performance. Therefore, a large amount of H₂O₂ is generated, leading to abundant ·OH⁻. Figure 11 (B) Meanwhile, based on the adsorption capacity of the nitrogen-doped catalyst for TC, TC is controlled near the electrode, thereby improving the utilization efficiency of ·OH. In summary, based on its excellent performance in generating H2O2 and utilizing ·OH, the nitrogen-doped sandwich electrode possesses excellent TC removal capabilities.

[0087] Example 4: Electro-Fenton-Electrocoagulation System

[0088] An aluminum sheet, a nitrogen-doped sandwich electrode prepared in Example 3, and an Ag / AgCl electrode were used as the counter electrode, working electrode, and reference electrode, respectively, and aeration was required. The potential was -0.7V vs Ag / AgCl, the solution pH was 3.5, the O2 rate was 20 mL / min, TC0 was 100 mg / L, and Ox0 was 1000 mg / L.

[0089] Comparative Example 1: Electro-Fenton System

[0090] A platinum sheet, a nitrogen-doped sandwich electrode, and an Ag / AgCl electrode were used as the counter electrode, working electrode, and reference electrode, respectively. The potential was -0.7V vs Ag / AgCl, the solution pH was 3.5, the O2 rate was 20mL / min, TC0 was 100mg / L, and Ox0 was 1000mg / L.

[0091] Comparative Example 2: Electrocoagulation System

[0092] Aluminum sheets were used as the counter electrode and working electrode, respectively, with Ag / AgCl as the reference electrode. The potential was -0.7V vs Ag / AgCl, the solution pH was 3.5, TC0 was 100mg / L, and Ox0 was 1000mg / L.

[0093] Figure 12 (A) shows the TC removal results of the electro-Fenton, electrocoagulation, and electro-Fenton-electrocoagulation systems. The TC removal rate of the electrocoagulation system was 42.5%, which was not significantly improved compared to the electro-Fenton system. However, the electrocoagulation system showed excellent removal efficiency for Ox. Figure 12(B), which indicates that Ox is still in the dominant position in the competition with TC for the adsorption sites of the flocs in the electrocoagulation system, and a large amount of Ox is removed by adsorption of the flocs. Further analysis of the effect of the electro-Fenton-electrocoagulation system on the removal of TC in the presence of Ox is shown in Figure 12 (A). After 60 min of reaction, the removal rate of TC is 94.7%, which is significantly higher than that of the electro-Fenton and electrocoagulation systems alone, and is significantly better than the removal of Ox by the electro-Fenton system. Therefore, most of the Ox is removed by the flocs generated by the electrocoagulation process in the electro-Fenton-electrocoagulation system, eliminating the competition of Ox for ·OH. Finally, excellent TC removal is achieved under the combined action of the electro-Fenton and electrocoagulation processes.

[0094] Example 3: Effect of Ox on the removal of TC by the electro-Fenton system

[0095] The effect of adding and not adding Ox on the removal of TC in the electrochemical system degradation platform is verified as shown in Figure 13 (A). After 60 min of reaction, the removal rate of TC is 94.7%, which is significantly higher than that of the electro-Fenton and electrocoagulation systems alone, and is significantly better than the removal of Ox by the electro-Fenton system. Therefore, most of the Ox is removed by the flocs generated by the electrocoagulation process in the electro-Fenton-electrocoagulation system, eliminating the competition of Ox for ·OH. Finally, excellent TC removal is achieved under the combined action of the electro-Fenton and electrocoagulation processes.

[0096] The results of the electro-Fenton system with and without the addition of Ox (Blank) for the removal of TC are shown in Figure 14 (A). After 60 min of electrolysis, the removal rate of TC in the system with the addition of Ox is only 32.6%, which is significantly lower than that in the system without the addition of Ox (72.4%), which is due to the competition of Ox with TC for ·OH. As shown in Figure 14 (B), 20% of Ox is removed by anodic oxidation and ·OH in the system after 60 min of reaction. This indicates that Ox will react with ·OH and form a competitive relationship with TC for ·OH during the reaction, and is in the dominant position.

[0097] Example 4: Electro-Fenton-electrocoagulation system without electro-Fenton effect

[0098] Electro-Fenton-electrocoagulation system without electro-Fenton effect: potential -0.7 V vs Ag / AgCl, solution pH = 3.5, O2 rate 20 mL·min -1 , TC0= 100 mg / L, Ox0= 1000 mg / L.

[0099] Figure 15(A) are the results of TC removal by electrocoagulation system and electro-Fenton-electrocoagulation system without electro-Fenton effect. The TC removal effect of electro-Fenton-electrocoagulation system with only electrocoagulation effect is better than that of electrocoagulation system. Although the electro-Fenton effect in electro-Fenton-electrocoagulation system is eliminated by adding methanol, i.e. the contribution of ·OH generated by electro-Fenton process to TC removal, but it does not affect the occurrence of electro-Fenton process. It is indicated that electro-Fenton process promotes the electrocoagulation process in electro-Fenton-electrocoagulation system.

[0100] The reasons for electro-Fenton process promoting electrocoagulation process are analyzed, and the results of floc morphology in different stages of two systems are shown in Figure 15 (C) and (D). By comparing the floc images in the same stage of two different systems, it can be seen that the size of floc morphology in electro-Fenton-electrocoagulation system is larger than that in electrocoagulation system at any stage. This is the same as the result of floc size analysis Figure 15 (B). The floc size in electro-Fenton-electrocoagulation system is larger than that in electrocoagulation system at any stage. However, the color of floc in electrocoagulation system is obviously deeper than that in electro-Fenton-electrocoagulation system, because multiple flocs are accumulated together to form floc, and the deeper color indicates more flocs, which indicates that the number of flocs in electrocoagulation system is more. It is indicated that the removal effect of pollutants in electrocoagulation system depends on the size of floc, rather than the number of flocs. The flocs with larger size and looser structure show stronger adsorption and aggregation effect on pollutants. Therefore, the larger size of flocs in electro-Fenton-electrocoagulation system makes its TC removal effect better than that of electrocoagulation system.

[0101] Example 5: Effect of aeration rate on TC removal by electro-Fenton-electrocoagulation system

[0102] The TC removal effect of electro-Fenton-electrocoagulation system is verified when the aeration rate is 0 mL / min, 2 mL / min, 20 mL / min and 50 mL / min, respectively.

[0103] The results of TC removal by electro-Fenton-electrocoagulation system under different aeration rates are shown in Figure 16(A) shows. Without aeration, the system still has a 46.7% removal rate for TC, which is due to the electric flocculation process in the system. When the aeration rate is 2 mL / min, the system has little effect on the removal of TC, because the consumption rate of gaseous oxygen is higher than the supply rate, the gas film disappears, the electrode internal gap is filled with electrolyte, and the mass transfer of gaseous oxygen is blocked, so the electrode can only use dissolved oxygen as the oxygen source. When the aeration rate is continuously increased to 20 mL / min, the supply rate of oxygen can maintain the consumption of oxygen in the hydrophobic substrate, and the oxygen mass transfer space and the gas film can be maintained. Then, stable and sufficient oxygen ensures the continuous generation of ·OH, so that the removal rate of TC increases to 94.7%. When the aeration rate is further increased, the results are different. The system initially shows good TC removal effect, but as the aeration rate increases, the efficiency of the system for removing TC begins to decrease, which may be due to the fact that too high aeration rate causes more oxygen to overflow from the electrode hydrophobic substrate to the electrolyte in the form of bubbles. As Figure 16 (C) shows that when the aeration rate is low, no oxygen bubbles overflow from the hydrophobic substrate. When the rate is high, it can be clearly observed that oxygen bubbles overflow. The overflow of the bubbles will affect the flocculation process and reduce the contribution of electric flocculation to the removal of TC. The analysis of the flocculation particle size in the first stable stage under different aeration rates can also explain this point, and the results are shown in Figure 16 (B).

[0104] The mechanism of the electro-Fenton-electric flocculation system for removing TC is shown in Figure 17 . Inside the nitrogen-doped interlayer electrode, the applied gaseous oxygen is transmitted from the electrode bottom to the three-phase interface (TPI) through the gas mass transfer and storage space inside the electrode, and a high-efficiency ORR reaction occurs, generating a large amount of H2O2, which is then activated by the nitrogen-doped catalyst to generate abundant ·OH. On the surface of the aluminum anode, the electric current generates Al 3+ through the sacrificial anode, and then Al 3+ and OH - generate Al(OH)3 colloid, and further generate various insoluble polynuclear aluminum through compressed double-electron layers, and finally form flocculation with high specific surface area and adsorption performance. Due to the nitrogen-doped interlayer electrode, H +For the synthesis of H2O2 instead of H2, therefore, the effect of hydrogen gas flotation on floc formation can be mitigated. In addition, the gas film formed at the bottom of the nitrogen-doped interlayer electrode can prevent the overflow of the applied oxygen in the form of bubbles, avoiding the interference of the aeration process on floc formation. Based on the above two points, the formation of floc is optimized with the participation of the electro-Fenton process, thereby producing floc with more excellent particle size and strength. In terms of pollutant removal, with the removal of Ox, TC loses the competitor for ·OH. Under the action of the "TC grab" nitrogen-doped electrode catalytic layer, TC is controlled near the electrode. Subsequently, TC is degraded / mineralized by the generated ·OH. In summary, in the electro-Fenton-electroflocculation system, the adsorption effect of the floc produced by the electroflocculation process eliminates the competition of Ox for ·OH, and the removal capacity of the floc for Ox is enhanced by optimizing the formation of the floc with the participation of the electro-Fenton process. At the same time, TC is efficiently degraded / mineralized by the abundant ·OH produced by the nitrogen-doped interlayer electrode. Finally, under the joint action of the electro-Fenton and electroflocculation processes, excellent TC removal effect is achieved.

[0105] In summary, the method of the present application is mainly used for the treatment of tetracycline hydrochloride wastewater generated in the tetracycline hydrochloride pharmaceutical industry, and is used to solve the influence of Ox on the removal of TC by the electro-Fenton system. By preparing a nitrogen-doped interlayer electrode with excellent performance, an electro-Fenton-electroflocculation system is constructed. The prepared nitrogen-doped bimetallic organic framework derived catalyst is combined with a hydrophobic graphite felt substrate to prepare an asymmetric hydrophilic-hydrophobic electrode (nitrogen-doped interlayer electrode). Based on the excellent performance of the nitrogen-doped interlayer electrode, the yield of H2O2 and the utilization rate of ·OH are improved. The influence of Ox on the removal of TC by the electro-Fenton system is solved, and the nitrogen-doped interlayer electrode exhibits excellent performance, solves the problem of insufficient oxygen supply for the ORR reaction, improves the utilization rate of ·OH, and optimizes the electro-Fenton system. The removal rate of TC by the method of the present application is 94.7% in 60 minutes. The method provides a new direction for the treatment of TC wastewater technology and has high practical value.

[0106] It should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit it. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, which should be covered by the claims of the present application.

Claims

1. A method for treating tetracycline hydrochloride wastewater containing oxalate using a heterogeneous electro-Fenton-electrocoagulation system, characterized in that: The wastewater to be treated is added to the heterogeneous electro-Fenton-electro-coagulation system. Under aeration conditions, the nitrogen-doped sandwich electrode removes tetracycline hydrochloride containing oxalate from the wastewater by electro-adsorption. A heterogeneous electro-Fenton-electrocoagulation system using a cathode catalyst is characterized in that: the heterogeneous electro-Fenton-electrocoagulation system includes a nitrogen-doped sandwich electrode, a reference electrode, a counter electrode, an electrochemical workstation, and an aeration pipe; the nitrogen-doped sandwich electrode, the reference electrode, and the counter electrode are respectively connected to the electrochemical workstation, and the aeration pipe is connected to the nitrogen-doped sandwich electrode from the bottom; the counter electrode is an active metal electrode; the reference electrode includes one or both of Ag / AgCl electrode and calomel electrode. The pH of the heterogeneous electro-Fenton-electrocoagulation system is 3.5~4.5; the cathode in the heterogeneous electro-Fenton-electrocoagulation system is a nitrogen-doped sandwich electrode made by combining a cathode catalyst with a hydrophobic graphite felt. The method for preparing the cathode catalyst includes, Iron salts, cobalt salts and organic ligands were dissolved in an organic solvent and then transferred to a water reactor. The reaction was carried out at 110℃~150℃ for 24~48h and then cooled to room temperature. After centrifugation to remove the supernatant, the Fe / CoMOFs powder was obtained after washing and drying. The cathode catalyst for heterogeneous electro-Fenton-electro-flocculation systems can be prepared by dispersing powdered Fe / CoMOFs and melamine in ethanol, filtering and drying, calcining and cooling. The molar ratio of the iron salt to the cobalt salt is 2:1 to 1:2; the iron salt includes one or more of FeCl3·6H2O, Fe(NO3)3·9H2O, and FeSO4·7H2O; the cobalt salt includes one or two of Co(NO3)2·6H2O and CoCl2·6H2O. The organic ligand includes one or two of terephthalic acid and trimesic acid; the calcination temperature is 500~700℃ and the calcination time is 60~120min; The method for preparing the nitrogen-doped sandwich electrode includes, Polyvinylidene fluoride was dissolved in N,N-dimethylacetamide to prepare PVDF / DMA binder; after the cathode catalyst NMOFsPC and PVDF / DMA binder were thoroughly mixed, they were uniformly coated on both sides of the hydrophobic graphite felt, fully immersed in water, and then taken out and dried to obtain a nitrogen-doped sandwich electrode composed of a hydrophilic catalyst layer and a hydrophobic substrate. The concentration of the PVDF / DMA adhesive is 5% to 10%.

2. The method for treating tetracycline hydrochloride wastewater containing oxalate using the heterogeneous electro-Fenton-electrocoagulation system as described in claim 1, characterized in that: The method for preparing the hydrophobic graphite felt, include, Graphite felt was placed in ethanol and ultrapure water in sequence, ultrasonically cleaned and dried to obtain hydrophilic graphite felt; then it was immersed in polytetrafluoroethylene solution, fully soaked and then taken out, calcined and cooled to obtain hydrophobic graphite felt. The concentration of the polytetrafluoroethylene solution is 5%~15%; the calcination temperature is 300℃~400℃; and the calcination time is 5~15min.

3. The method for treating tetracycline hydrochloride wastewater containing oxalate using the heterogeneous electro-Fenton-electrocoagulation system as described in claim 1, characterized in that: The oxalate treatment concentration of the electro-Fenton-electrocoagulation system was 1000 mg / L; the tetracycline hydrochloride treatment concentration was 100 mg / L.

4. The method for treating tetracycline hydrochloride wastewater containing oxalate using the heterogeneous electro-Fenton-electrocoagulation system as described in claim 1, characterized in that: The gas used for aeration is air or oxygen; the aeration rate, measured by an oxygen meter, is 2~50 mL / min.

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