Low concentration nitrate removal and conversion method based on functionalized flow electrode slurry

Abstract

The present application relates to a kind of low concentration nitrate removal and conversion method based on functionalized flow electrode slurry, comprising the following steps: in low concentration nitrate contaminated water body, using functionalized flow electrode slurry, carry out capacitive deionization processing under the first voltage applied, to realize the selective adsorption and concentration of nitrate, functionalized flow electrode slurry includes functional group modification and has electrocatalytic activity carbon material;The functionalized flow electrode slurry that nitrate has been concentrated, carry out electrochemical reduction reaction under the second voltage applied, reduce the nitrate adsorbed to ammonia, to realize the removal and resource of nitrate.The present application is designed by two-step method process, and uses functionalized flow electrode slurry, effectively solves the two big problems of low concentration nitrate deep removal and resource recycling.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for the removal and conversion of low-concentration nitrate nitrogen based on functionalized flow electrode slurry. Background Technology

[0002] Nitrate nitrogen, the most abundant nitrogen source in urban wastewater, is one of the main culprits causing eutrophication of surface water and inducing algal blooms. Existing deep nitrate nitrogen removal technologies mainly include physical transfer technologies and chemical or biological conversion technologies. Physical transfer technologies, such as reverse osmosis, electrodialysis, and capacitive deionization, can concentrate nitrate nitrogen in water into a concentrated solution. However, these methods essentially only achieve phase or spatial transfer of pollutants, failing to achieve substantial reduction of nitrate nitrogen, and the concentrated solution still requires further treatment. Chemical or biological conversion technologies convert nitrate nitrogen into other nitrogenous forms through denitrification reactions. Biological methods, however, are easily affected by mass transfer limitations and inhibited microbial activity under low concentration or high salinity conditions, resulting in low treatment efficiency. While chemical reduction methods can reduce nitrate ions to products such as ammonia under normal temperature and pressure conditions, they are also limited by insufficient reactant supply and competition from side reactions in low concentration systems, making efficient and stable operation difficult. In summary, existing denitrification processes struggle to further improve removal efficiency, becoming a significant technical bottleneck restricting the improvement of water environmental quality.

[0003] Meanwhile, ammonia (NH3), as a basic chemical raw material, plays an irreplaceable role in many fields such as fertilizer production and pharmaceutical manufacturing. Furthermore, due to its high energy density and ease of liquefaction and storage, ammonia has the potential to serve as a carbon-free fuel carrier and is considered an important component of the future green energy system. However, the traditional Haber process for ammonia synthesis relies on the high temperature and pressure conditions (350–550°C, 15–25 MPa) of fossil fuels, resulting in high energy consumption (approximately 1%–2% of global energy consumption), large carbon emissions (approximately 1.5% of global carbon emissions), and the risk of resource depletion. In contrast, electrochemical reduction of nitrate to ammonia is much milder in terms of reaction conditions, typically carried out at ambient temperature and pressure, significantly reducing energy consumption and equipment costs. It not only eliminates dependence on fossil fuels but also allows for flexible control of ammonia production, making it a promising sustainable ammonia production method in the future and a rapidly growing research hotspot in recent years. Therefore, developing a technology that can reduce low-concentration nitrate to ammonia through an electrochemical process, simultaneously achieving deep removal of nitrate and resource recovery, has significant research value and application prospects.

[0004] It should be noted that the information disclosed in the background section above is only for understanding the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] The fundamental technical problem this application aims to solve is that existing low-concentration nitrate nitrogen treatment technologies cannot simultaneously achieve deep removal and resource recovery. Specifically, existing technologies suffer from drawbacks such as poor selectivity, low efficiency, reliance on metal catalysts, limited mass transfer, and difficulties in product separation. Therefore, this application provides a method for low-concentration nitrate nitrogen removal and conversion based on a functionalized flowing electrode slurry.

[0006] The technical solution adopted in this application to solve the above-mentioned technical problems is as follows.

[0007] The first aspect of this application provides a method for the removal and conversion of low-concentration nitrate nitrogen based on functionalized flow electrode slurry, comprising the following steps:

[0008] Transfer steps: In water bodies polluted with low concentration of nitrate nitrogen, a functionalized flow electrode slurry is used to perform capacitive deionization treatment under an applied first voltage to achieve selective adsorption and concentration of nitrate nitrogen. The functionalized flow electrode slurry includes carbon materials that are functionalized and have electrocatalytic activity.

[0009] Conversion step: The functionalized flow electrode slurry, which has been concentrated by the transfer step, is subjected to an electrochemical reduction reaction under an applied second voltage to reduce the adsorbed nitrate nitrogen to ammonia, thereby achieving the removal and conversion of nitrate nitrogen.

[0010] In some embodiments, the low-concentration nitrate-nitrogen-polluted water contains competitive anions, including chloride ions, which are the most abundant in natural water bodies, and the nitrate concentration in the low-concentration nitrate-nitrogen-polluted water is less than 10 mg-N / L.

[0011] In some embodiments, the functionalized carbon material with electrocatalytic activity includes quaternized self-defective carbon nanotubes, and the preparation method of the functionalized flow electrode slurry includes the following steps:

[0012] Provide multi-walled carbon nanotubes;

[0013] Multi-walled carbon nanotubes were mixed with hydrogen peroxide solution and reacted to obtain hydroxylated self-defect carbon nanotubes.

[0014] quaternized self-defective carbon nanotubes were obtained by mixing and reacting hydroxylated self-defective carbon nanotubes with a quaternization reagent solution.

[0015] Quaternized self-defective carbon nanotubes are dispersed in a supporting electrolyte solution to form a functionalized flow electrode slurry.

[0016] In some embodiments, the reaction temperature of multi-walled carbon nanotubes with hydrogen peroxide solution is 50~70°C, and the reaction time is 12~36 hours; the reaction temperature of hydroxylated self-defective carbon nanotubes with quaternization reagent solution is 70~90°C, and the reaction time is 12~36 hours.

[0017] In some embodiments, the electrolyte solution is supported as a sodium chloride solution.

[0018] In some embodiments, the first voltage is 1.0V and the second voltage is from 1.9V to 2.5V.

[0019] In some embodiments, during the transfer step, the flow rate of the low-concentration nitrate-polluted water is 10 mL / min, and the flow rate of the functionalized flow electrode slurry is 50 mL / min.

[0020] In some embodiments, the method further includes the step of: after the conversion step, performing solid-liquid separation on the functionalized flow electrode slurry after the reaction to obtain an ammonia-containing solution and a regenerated carbon material that is functionalized and also has electrocatalytic activity.

[0021] The second aspect of this application provides a functionalized flow electrode slurry for the removal and conversion of low-concentration nitrate nitrogen, comprising a functionalized carbon material with electrocatalytic activity, dispersed in a supporting electrolyte solution; the functionalized carbon material with electrocatalytic activity is obtained by sequentially modifying the carbon material with a hydrogen peroxide solution and a quaternizing agent solution.

[0022] In some embodiments, the quaternizing agent is dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride.

[0023] In some embodiments, the electrolyte solution is supported as a sodium chloride solution.

[0024] A third aspect of this application provides a low-concentration nitrate removal and conversion system for implementing the method of the first aspect of this application, comprising: a reactor device including a first chamber for containing water to be treated and a second chamber for containing a functionalized flow electrode slurry, the first chamber and the second chamber being separated by an ion exchange membrane; a power supply device configured to apply a first voltage or a second voltage to the reactor device; and a fluid drive device configured to cause the water to be treated and the functionalized flow electrode slurry to flow in their respective circulation loops; wherein the functionalized flow electrode slurry is the functionalized flow electrode slurry of the second aspect of this application.

[0025] In some embodiments, the system further includes a solid-liquid separation device connected to the circulation loop of the functionalized flow electrode slurry for receiving the reacted functionalized flow electrode slurry after the conversion step and separating it into an ammonia-containing solution and regenerated functionalized carbon material with electrocatalytic activity; the regenerated functionalized carbon material with electrocatalytic activity is configured to be returned to the second chamber or the functionalized flow electrode slurry circulation loop for reuse.

[0026] The present invention has the following beneficial effects:

[0027] This application, through a two-step process design of "transfer first, then conversion" and the use of functionalized flowing electrode slurry, effectively solves the core challenge of simultaneously achieving deep removal of low-concentration nitrate nitrogen and resource recovery. Specifically:

[0028] This application employs a functionalized flow electrode slurry, which includes functionalized carbon materials with electrocatalytic activity, such as quaternized self-defective carbon nanotubes. By utilizing the specific high affinity between quaternary ammonium functional groups and nitrate ions, it can achieve highly selective adsorption and concentration of nitrate nitrogen in water bodies with low concentrations of nitrate nitrogen pollution containing competitive anions, such as chloride ions. This significantly improves the selectivity for target pollutants, such as nitrate nitrogen, and overcomes the poor selectivity of physical transfer technologies.

[0029] Furthermore, this application utilizes a concentrated, nitrogen-rich, functionalized flow electrode slurry to perform electrochemical reduction at a suitable second voltage. The self-defective structure (such as oxygen vacancies) inherent in the quaternized self-defective carbon nanotubes provides highly efficient catalytic active sites, enabling the efficient reduction of nitrogen to ammonia without metal catalysis. This not only avoids the leaching contamination and cost issues that metal catalysts may cause, but also, due to the concentration effect of the preceding step, provides a high concentration of reactants for the reduction reaction, significantly improving reaction efficiency and the selectivity of ammonia products.

[0030] Furthermore, this application integrates adsorption concentration and catalytic conversion functions into the same set of electrode materials (quaternized self-defective carbon nanotubes) and reaction system. The conversion reaction is carried out directly in the concentrated slurry, and the generated ammonia is enriched in the liquid phase of the slurry. Subsequently, a high-concentration ammonia-containing solution can be easily obtained for resource recovery through simple solid-liquid separation. At the same time, the separated quaternized self-defective carbon nanotubes can be recycled, realizing in-situ separation and recovery of products and simplifying the process.

[0031] In summary, the various technical features of this application work together to form a complete technical solution. Specifically, through the overall concept of "transfer and concentration first, then conversion and recovery," combined with the synergistic effect of specific adsorption material design, metal-free catalytic system construction, and in-situ product separation strategy, it overcomes multiple technical bottlenecks in traditional low-concentration nitrate nitrogen treatment technologies, such as the disconnect between "transfer" and "conversion," poor selectivity, low efficiency, dependence on metal catalysts, limited mass transfer, and difficulty in product separation. Ultimately, it achieves a highly efficient and unified deep removal and resource recovery of low-concentration nitrate nitrogen, demonstrating significant technological progress and practical application value.

[0032] Other beneficial effects of the present invention will be further described below. Attached Figure Description

[0033] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0034] Figure 1 A schematic diagram of the preparation process of a functionalized flow electrode slurry;

[0035] Figure 2 This is a schematic diagram of the transfer step apparatus in a method for deep removal and conversion of low-concentration nitrate nitrogen based on functionalized flow electrode slurry.

[0036] Figure 3 The graph shows the nitrate adsorption performance of the transfer step in Example 1 and Comparative Example 1.

[0037] Figure 4 This is a schematic diagram of the conversion step apparatus in a method for deep removal and conversion of low-concentration nitrate nitrogen based on functionalized flow electrode slurry;

[0038] Figure 5 The graphs show the effects of nitrate reduction to ammonia production in the conversion steps of Example 1 and Comparative Example 1.

[0039] Figure 6 The graph shows the nitrate adsorption performance of the transfer step in Example 2 and Comparative Example 2.

[0040] Figure 7 The diagram shows the effect of nitrate reduction to ammonia production in the conversion step of Example 2 and Comparative Example 2. Detailed Implementation

[0041] The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.

[0042] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0043] To address nitrate pollution in water bodies, existing technologies primarily focus on two directions: transfer and concentration, and conversion and reduction. Regarding transfer and concentration, existing technologies employ physical separation methods such as reverse osmosis, electrodialysis, and capacitive deionization to separate nitrate ions from the raw water. However, due to a lack of selectivity for nitrate, it is often necessary to remove other anions simultaneously, resulting in significant desalination effects and high energy consumption. A common drawback of transfer technologies is that they only achieve the enrichment and migration of nitrate nitrogen, failing to achieve substantial nitrate reduction within the system. The resulting high-concentration nitrate concentrate still requires further treatment, increasing system complexity and operating costs.

[0044] In terms of conversion and reduction, existing technologies mainly include biological denitrification and electrochemical reduction methods. Biological denitrification converts nitrate nitrogen into nitrogen gas through microbial metabolism; however, under low concentration conditions, it is limited by low mass transfer efficiency, long hydraulic retention time, and the need for external carbon sources, making efficient and deep removal difficult. Electrochemical nitrate reduction technology utilizes an external electric field to reduce nitrate ions to ammonia for resource recovery, offering advantages such as mild conditions and controllable processes, and has become an important research direction in recent years. However, in actual low-concentration water bodies, these technologies are generally affected by insufficient reactant supply and limited mass transfer, requiring long reaction times, resulting in high energy consumption and preventing deep conversion. Furthermore, the ammonia nitrogen generated by reduction is usually dispersed in large volumes of water, making subsequent separation and recovery difficult. Therefore, existing conversion technologies struggle to simultaneously achieve efficient removal and resource utilization under low-concentration nitrate nitrogen conditions.

[0045] In general, existing technological solutions often focus on optimizing a single technology for transfer or conversion, while this invention achieves efficient synergy between low-concentration nitrate removal and resource utilization by constructing a "transfer first, then conversion" technical path.

[0046] Specifically, this invention aims at the efficient and deep removal of low-concentration nitrate nitrogen and the resource utilization of ammonia. Based on a functionalized flowing electrode slurry, it constructs an integrated "transfer-conversion" reaction system. At the front end, by designing an adsorption interface that responds to the characteristic coordination structure of nitrate ions, highly selective deep transfer and concentration of low-concentration nitrate nitrogen are achieved. In the subsequent conversion stage, the functionalized flowing electrode slurry, a through-type electrode, overcomes the bottleneck of diffusion mass transfer in traditional fixed electrode systems. The electrode, in a flowing state, can also trigger special mechanisms such as intermediate product adsorption and active hydrogen supply. Combined with the high-multiple nitrate nitrogen enrichment in the transfer step, the reaction proceeds along a pathway favorable to ammonia formation, jointly enhancing the nitrate nitrogen conversion rate and specificity. This invention achieves a breakthrough in the entire process of selective capture, concentration, and efficient electroreduction of nitrate nitrogen.

[0047] In some embodiments, this application addresses the problems of poor selectivity, limited mass transfer, easy dissolution of metal catalysts, and inability to simultaneously achieve deep removal and resource recovery in existing low-concentration nitrate nitrogen treatment technologies. Based on the principle of synergistic effect of electrochemical adsorption and catalytic reduction, it proposes a method for low-concentration nitrate nitrogen removal and conversion based on a functionalized flow electrode slurry. This method involves designing functional materials with nitrate-characteristic coordination response structures into the functionalized flow electrode slurry: functionalized carbon materials with electrocatalytic activity, such as quaternized self-defective carbon nanotubes, to construct an adsorption interface responsive to nitrate-characteristic coordination structures. By designing the charge density and spatial configuration of the quaternary ammonium groups, a large adsorption free energy difference is generated between nitrate and other anions, thereby preferentially binding and adsorbing nitrate nitrogen in an environment with multiple competing anions. This results in the functionalized flow electrode slurry possessing both high selective adsorption and efficient catalytic activity. This technology adopts an integrated approach of "transfer first, then conversion": In the transfer stage, functionalized electrodes are used to achieve highly selective adsorption and concentration of low-concentration nitrate nitrogen under low voltage, effectively overcoming interference from competing anions such as chloride ions (the most concentrated anion in natural water bodies); in the conversion stage, by removing the anion exchange membrane and applying a higher voltage, the permeable mass transfer advantage of the functionalized flow electrode slurry and the oxygen vacancy defect structure on the material surface are utilized to promote the electrochemical reduction reaction of nitrate nitrogen to ammonia, while simultaneously achieving in-situ separation of the products. This technical solution solves the problems of low efficiency in deep removal of low-concentration nitrate nitrogen, difficulty in controlling the reaction path, and difficulty in recovering resource-based products. It achieves a nitrate nitrogen removal rate of over 90%, ammonia production selectivity of over 90%, and Faraday efficiency of over 80%, ensuring efficient and stable operation while also possessing the advantages of low cost, low energy consumption, no risk of metal leaching, continuous system operation, and flexible operation, providing an innovative solution for the deep treatment and resource recovery of low-concentration nitrogen-containing wastewater.

[0048] It should be noted that the competitive anion of this invention is a technical term with a recognized and clearly defined meaning in this technical field, particularly in the fields of electroadsorption, capacitive deionization, and electrochemical separation. It is defined as: the competitive anion that, in this case, reacts with the target ion (nitrate ion, NO3-) − In the treated water, there are other anions that compete with the target ions for limited adsorption sites on the electrode. The low-concentration nitrate-nitrogen polluted water in this invention refers to nitrate-nitrogen concentrations below 10 mg-N / L. Of course, this invention is still applicable to nitrate-nitrogen polluted water with nitrate-nitrogen concentrations above 10 mg-N / L.

[0049] In some embodiments, the functionalized flow electrode slurry is prepared in the following manner:

[0050] Carbon nanotubes are used as the initial raw material. Optionally, the carbon nanotubes have a purity of 99%, an inner diameter greater than 3 nm, an outer diameter less than 18 nm, and a length less than 30 μm.

[0051] Furthermore, the hydrogen peroxide solution is mixed with the above-mentioned carbon nanotubes for a reaction. Optionally, the reaction temperature is 60°C and the reaction time is more than 24 hours. After the reaction is completed, the product is centrifuged and washed multiple times with deionized water to obtain hydroxylated self-defective carbon nanotubes.

[0052] Further, a 40 wt% solution of dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride was mixed with the above-mentioned hydroxylated self-defective carbon nanotubes for a reaction. Optionally, the reaction temperature was 80 °C and the reaction time was more than 24 hours. After the reaction was completed, the product was centrifuged and washed several times with anhydrous ethanol and deionized water to obtain quaternized self-defective carbon nanotubes.

[0053] Finally, the above-mentioned quaternized self-defective carbon nanotubes were dissolved in a 2 g / L sodium chloride solution and stirred evenly to form a functionalized flow electrode slurry.

[0054] In some embodiments, the method for deep removal and resource recovery of low-concentration nitrate nitrogen based on functionalized flow electrode slurry of the present invention has the following technical solution:

[0055] The transfer process is based on flow electrode capacitive deionization technology. Using the aforementioned functionalized flow electrode slurry, selective and deep removal of nitrate nitrogen from low-concentration nitrate-polluted water is achieved in the presence of competitive anions (chloride ions). It is understood that flow electrode capacitive deionization technology typically involves a flow electrode, a reaction apparatus (including a first chamber for containing the water to be treated, a second chamber for containing the functionalized flow electrode slurry, and anion and cation exchange membranes), which will not be elaborated upon here.

[0056] Optionally, the transfer process uses an external voltage of 1.0 V, which is lower than the decomposition voltage of water (~1.23 V). This avoids water decomposition side reactions while ensuring the quaternary ammonium functional groups on the material surface effectively resist NO3. − Effective electroadsorption.

[0057] The conversion process is based on the concentration of nitrate nitrogen in the transfer process. After stopping the water supply to the reaction device and removing the anion exchange membrane, a 1 M KOH solution is flowed in the first chamber as the anode, and the electrochemical reduction to ammonia production can then proceed directly in the second chamber. The cation exchange membrane, an existing component in the flow electrode capacitive deionization technology, functions to facilitate ion migration and prevent electrode short circuits. It prevents the ammonia product from being oxidized at the anode during the nitrate nitrogen reduction to ammonia production at the cathode, thus avoiding a decrease in yield.

[0058] Optionally, the conversion process uses an external voltage of 1.9~2.5V. The second voltage range of 1.9-2.5V has been optimized and determined. Within this range, sufficient driving force can be provided to convert NO3. − It can undergo reduction reactions while effectively controlling the hydrogen evolution side reaction (HER), ensuring high ammonia selectivity and Faraday efficiency.

[0059] Preferably, the flow rate of the low-concentration nitrate-nitrogen-polluted water is 10 mL / min, and the flow rate of the functionalized flow electrode slurry is 50 mL / min. This ensures sufficient contact and reaction time between the pollutants and the electrode material, while maintaining stable operation of the flow electrode system and preventing sedimentation and clogging.

[0060] In some embodiments, this application also provides a low-concentration nitrate removal and conversion system for implementing the method of this application, comprising: a reactor device including a first chamber for containing water to be treated and a second chamber for containing functionalized flowing electrode slurry, the first chamber and the second chamber being separated by an ion exchange membrane; a power supply device configured to apply a first voltage or a second voltage to the reactor device; and a fluid drive device configured to cause the water to be treated and the functionalized flowing electrode slurry of this application to flow in their respective circulation loops.

[0061] In some embodiments, the system further includes a solid-liquid separation device connected to the circulation loop of the functionalized flow electrode slurry for receiving the reacted functionalized flow electrode slurry after the conversion step and separating it into an ammonia-containing solution and regenerated quaternized self-defective carbon nanotubes; the regenerated quaternized self-defective carbon nanotubes are configured to be returned to the second chamber or reused in the functionalized flow electrode slurry circulation loop.

[0062] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. These embodiments are merely illustrative and do not limit the present invention to the following examples.

[0063] Example 1:

[0064] like Figure 1 As shown, this embodiment uses a functionalized flow electrode slurry containing quaternized self-defective carbon nanotubes for low-concentration nitrate removal and conversion:

[0065] S1. Commercially available multi-walled carbon nanotubes are used as the initial raw material. Specifically, the carbon nanotubes have a purity greater than 99%, an inner diameter between 3 and 15 nm, an outer diameter between 12 and 18 nm, a length between 15 and 30 μm, and a specific surface area between 250 and 270 m². 2 ·g -1 The density is between 0.06 and 0.09 g·cm³. -3 The resistivity is between 1600 and 2000 μΩ·m.

[0066] S2. The above-mentioned carbon nanotubes were subjected to hydroxylation self-defect modification. Specifically, 1.8 g of carbon nanotubes were dispersed in 360 mL of hydrogen peroxide solution (30 wt%) and reacted at 60 °C for 24 hours. After the reaction, the product was centrifuged and washed three times with deionized water, and then vacuum dried at 60 °C to obtain hydroxylated self-defect carbon nanotubes. It should be noted that the self-defect is for catalyzing the electroreduction of nitrate nitrogen, while hydroxylation is for quaternization modification to facilitate the attachment of quaternary ammonium groups through CO-Si bonds.

[0067] S3. Add 1.8 g of the above-mentioned hydroxylated self-defect carbon nanotubes to 150 mL of deionized water and stir until homogeneous. Add 6.0 mL of dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution (40 wt%) and react at 80 °C for 24 hours. After the reaction is complete, centrifuge the product and wash it three times with anhydrous ethanol and deionized water, respectively. After vacuum drying at 60 °C, quaternized self-defect carbon nanotubes are obtained.

[0068] S4. Dissolve 1.8 g of the above-mentioned quaternized self-defective carbon nanotubes in a 2 g / L sodium chloride solution and stir until homogeneous to form 100 g of functionalized flow electrode slurry (carbon content 1.8 wt%).

[0069] S5. Transfer step: Using the above-mentioned functionalized flow electrode slurry, the low-concentration nitrate source water is treated by flow electrode capacitive deionization technology. The corresponding treatment device is as follows: Figure 2 The low-concentration nitrate-nitrogen polluted water body was a mixed solution (800 mL) of 10 mg / L N-NaNO3 and 100 mg / L NaCl. During operation, an external voltage of 1.0 V was used, the low-concentration nitrate-nitrogen polluted water body was circulated at a flow rate of 10 mL / min, and the functionalized flow electrode slurry flow rate was 50 mL / min.

[0070] S6. Conversion Step: After the nitrate and nitrogen are concentrated during the transfer process, the anion exchange membrane is removed, while the cation exchange membrane remains. A 1 M KOH solution is introduced into the first chamber as the anode chamber, allowing direct cathodic electrochemical reduction to ammonia production in the second chamber. The apparatus is as follows: Figure 4 As shown, the flow electrode slurry nitrification-nitrogen reduction conversion device, compared to the flow electrode slurry capacitor deionization device in the transfer step, only requires the removal of the cation exchange membrane. It will use an external voltage of 2.2 V and the functionalized flow electrode slurry flow rate will be 50 mL / min.

[0071] S7. The functionalized flow electrode slurry after the above reaction is filtered and separated to obtain a high concentration of ammonia solution and regenerated quaternized self-defective carbon nanotubes. S4 is repeated to prepare a new flow electrode slurry. After testing, the performance of the regenerated flow electrode slurry is not degraded.

[0072] Comparative Example 1:

[0073] Performance testing of unfunctionalized ordinary carbon nanotubes.

[0074] S1. Commercially available multi-walled carbon nanotubes are used as the initial raw material. Specifically, the carbon nanotubes have a purity greater than 99%, an inner diameter between 3 and 15 nm, an outer diameter between 12 and 18 nm, a length between 15 and 30 μm, and a specific surface area between 250 and 270 m². 2 ·g -1 The density is between 0.06 and 0.09 g·cm³. -3 The resistivity is between 1600 and 2000 μΩ·m.

[0075] S2. Dissolve 1.8 g of the above multi-walled carbon nanotubes in a 2 g / L sodium chloride solution and stir until homogeneous to form 100 g of flowing electrode slurry (carbon content 1.8 wt%).

[0076] S3. Transfer step: Using the above-mentioned flowing electrode slurry, the low-concentration nitrate source water is treated by flowing electrode capacitive deionization technology. The corresponding treatment device is as follows: Figure 2 The low-concentration nitrate-nitrogen polluted water body was a mixed solution (800 mL) of 10 mg / L N-NaNO3 and 100 mg / L NaCl. During operation, an external voltage of 1.0 V was used, the low-concentration nitrate-nitrogen polluted water body was circulated at a flow rate of 10 mL / min, and the functionalized flow electrode slurry flow rate was 50 mL / min.

[0077] S4. Conversion Step: After the nitrate nitrogen is concentrated during the transfer process, the cation exchange membrane is removed, while the anion exchange membrane remains. This allows for direct electrochemical reduction to produce ammonia. The apparatus is as follows: Figure 4 As shown, the flow electrode slurry nitrate-nitrogen reduction conversion device, compared to the flow electrode slurry capacitor deionization device in the transfer step, only requires the removal of the anion exchange membrane and the introduction of KOH solution. It will use an external voltage of 2.2 V and the functionalized flow electrode slurry flow rate will be 50 mL / min.

[0078] Comparing Comparative Example 1 with Example 1 shows that, during the transfer process, Comparative Example 1, in the presence of competing anions such as chloride ions, exhibits the following transfer effect: Figure 3As shown by the blank CNT lines, the chloride ion concentration in low-concentration nitrate-polluted water decreased to varying degrees under different operating times, failing to achieve selective nitrate adsorption; while in Example 1, the transfer effect was as shown in the presence of competitive anions, such as chloride ions. Figure 3 As shown in the functionalized CNTs, compared with the ordinary carbon nanotubes of Comparative Example 1 (corresponding to the blank CNT lines), the quaternized self-defective carbon nanotubes of the present invention (corresponding to the functionalized CNT lines) have excellent selective adsorption performance for nitrate nitrogen. Under different time tests, the chloride ion concentration in the low-concentration nitrate nitrogen-polluted water body did not decrease. The functionalized flow electrode slurry of the present invention can achieve near-complete removal of low-concentration nitrate nitrogen from the source water (<1 mg-N / L), while having almost no effect on chloride ions in the source water. The selectivity coefficient can reach more than 20. The nitrate nitrogen concentration in the flow electrode slurry after transfer can be concentrated to seven times the original water concentration (70 mg-N / L).

[0079] During the conversion process, the conversion effects of Example 1 and Comparative Example 1 under the mass transfer enhancement effect of the flowing electrode slurry are as follows: Figure 5 As shown, specifically, as Figure 5 The figures show a comparison of the test results of Example 2 (functionalized CNTs) and Comparative Example 1 (blank CNTs) in H-cell (H-type electrolytic cell) and flowing electrode slurry modes, respectively. (a) shows the nitrate-nitrogen conversion effect; it can be seen that the functionalized CNTs significantly improve the rate and selectivity of nitrate-nitrogen reduction to ammonia production compared to the blank CNTs in the flowing electrode slurry mode, which is attributed to the full utilization of active sites. (b) shows the energy consumption and Faradaic efficiency of nitrate-nitrogen conversion; it can be seen that the functionalized CNTs significantly reduce energy consumption and significantly improve Faradaic efficiency compared to the blank CNTs in the flowing electrode slurry mode. Furthermore, the operating performance of both materials in the flowing electrode mode is superior to that in the H-cell mode. Through the above comparison, the functionalized flowing electrode slurry based on quaternized self-defective carbon nanotubes of the present invention can achieve a nitrate-nitrogen conversion rate of over 85%, an ammonia production selectivity of over 90%, and a Faradaic efficiency of over 80%.

[0080] It should be noted that the H-cell test was conducted using conventional methods. This involved first preparing a catalyst-coated carbon paper electrode, followed by electrolysis testing within the H-cell. The preparation process of the carbon paper electrode may include the following steps: dispersing 5 mg of catalyst powder and 20 μL of a 5% Nafion solution in 1 mL of isopropanol, followed by ultrasonic treatment for 1 hour to prepare the catalyst ink. 200 μL of the ink was then dropped onto an effective working area of ​​1 × 1 cm². 2 On carbon fiber paper, a catalyst loading of 1 mg / cm³ was obtained. 2 Carbon paper electrodes.

[0081] Example 2:

[0082] This embodiment uses a functionalized flowing electrode slurry containing quaternized self-defect activated carbon for low-concentration nitrate removal and conversion:

[0083] S1. Commercial activated carbon (AC) is used as the initial raw material.

[0084] S2. The activated carbon was subjected to hydroxylation self-defect modification. Specifically, 1.8 g of activated carbon was dispersed in 360 mL of hydrogen peroxide solution (30 wt%) and reacted at 60 °C for 24 hours. After the reaction was completed, the product was centrifuged and washed three times with deionized water, and then vacuum dried to obtain hydroxylated self-defect activated carbon.

[0085] S3. Add 1.8 g of the above-mentioned hydroxylated self-defect activated carbon to 150 mL of deionized water and stir until homogeneous. Add 6.0 mL of dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride solution (40 wt%) and react at 80 °C for 24 hours. After the reaction is complete, centrifuge the product and wash it three times with anhydrous ethanol and deionized water, respectively. After vacuum drying, obtain quaternized self-defect activated carbon.

[0086] S4. Dissolve 1.8 g of the above-mentioned quaternized self-defect activated carbon in a 2 g / L sodium chloride solution and stir until homogeneous to form 100 g of functionalized flow electrode slurry (carbon content 1.8 wt%).

[0087] S5. Transfer step: Using the above-mentioned functionalized flow electrode slurry, the low-concentration nitrate source water is treated by flow electrode capacitive deionization technology. The corresponding treatment device is as follows: Figure 2 The low-concentration nitrate-nitrogen polluted water body was a mixed solution (800 mL) of 10 mg / L N-NaNO3 and 100 mg / L NaCl. During operation, an external voltage of 1.0 V was used, the low-concentration nitrate-nitrogen polluted water body was circulated at a flow rate of 10 mL / min, and the functionalized flow electrode slurry flow rate was 50 mL / min.

[0088] S6. Conversion Step: After the nitrate nitrogen is concentrated during the transfer process, the cation exchange membrane is removed, while the anion exchange membrane remains. This allows for direct electrochemical reduction to produce ammonia. The apparatus is as follows: Figure 4 As shown, the flow electrode slurry nitrification-nitrogen reduction conversion device, compared to the flow electrode slurry capacitor deionization device in the transfer step, only requires the removal of the cation exchange membrane. It will use an external voltage of 2.2 V and the functionalized flow electrode slurry flow rate will be 50 mL / min.

[0089] Comparative Example 2:

[0090] Performance testing of unfunctionalized activated carbon.

[0091] S1. Commercial activated carbon is used as the initial raw material.

[0092] S2. Dissolve 1.8 g of the above activated carbon in a 2 g / L sodium chloride solution and stir until homogeneous to form 100 g of flowing electrode slurry (carbon content 1.8 wt%).

[0093] S3. Transfer step: Using the above-mentioned flowing electrode slurry, the low-concentration nitrate source water is treated by flowing electrode capacitive deionization technology. The corresponding treatment device is as follows: Figure 2 The low-concentration nitrate-nitrogen polluted water body was a mixed solution (800 mL) of 10 mg / L N-NaNO3 and 100 mg / L NaCl. During operation, an external voltage of 1.0 V was used, the low-concentration nitrate-nitrogen polluted water body was circulated at a flow rate of 10 mL / min, and the functionalized flow electrode slurry flow rate was 50 mL / min.

[0094] S4. Conversion Step: After the nitrate nitrogen is concentrated during the transfer process, the cation exchange membrane is removed, while the anion exchange membrane remains. This allows for direct electrochemical reduction to produce ammonia. The apparatus is as follows: Figure 4 As shown, the flow electrode slurry nitrate-nitrogen reduction conversion device, compared to the flow electrode slurry capacitor deionization device in the transfer step, only requires the removal of the anion exchange membrane and the introduction of KOH solution. It will use an external voltage of 2.2 V and the functionalized flow electrode slurry flow rate will be 50 mL / min.

[0095] Comparing Comparative Example 2 with Example 2 shows that, during the transfer process, in the presence of competing anions such as chloride ions, the transfer effect of Example 2 is as follows: Figure 6 As shown in the quaternized AC line, compared with the ordinary activated carbon (blank AC) of Comparative Example 2, the quaternized self-defective activated carbon of the present invention also has excellent nitrate-nitrogen selective adsorption performance, indicating that the quaternized carbon material of the present invention has adsorption performance and is suitable for the modification of various carbon materials.

[0096] During the transformation process, refer to Figure 7 As can be seen, the functionalized AC (quaternized self-defect activated carbon) of Example 2 has significantly improved parameters in all aspects compared with the blank AC (ordinary activated carbon) of Comparative Example 2 in the flow electrode slurry mode. The functionalized flow electrode slurry based on quaternized self-defect activated carbon in Example 3 can achieve a nitrogen-nitrate conversion rate of over 80%, an ammonia production selectivity of over 80%, and a Faraday efficiency of over 70%.

[0097] In summary, the present invention has the following advantages and technical effects:

[0098] 1. Highly selective nitrate nitrogen transfer and deep removal: This invention addresses the challenge of deep removal of nitrate ions under complex anion competition environments. By designing an adsorption interface that responds to the characteristic coordination structure of nitrate ions (such as quaternary ammonium functional groups), and utilizing its adsorption energy difference regulation mechanism, highly selective deep transfer of low-concentration nitrate nitrogen (which can be reduced to <1 ppm) is achieved. Its selectivity is far higher than that of ordinary carbon nanotubes, and it does not affect other coexisting anions in the water.

[0099] 2. Highly Efficient Non-metallic Catalytic System: This invention abandons traditional metal catalysts, avoiding secondary pollution problems caused by metal leaching. Relying on functionalized carbon materials with electrocatalytic activity, such as the oxygen vacancy defect structure of quaternized self-defective carbon nanotubes, local electron enrichment centers and nitrate / nitrogen adsorption sites are constructed, achieving highly selective adsorption and stepwise electron transfer of nitrate / nitrogen, thereby converting NO3- into nitrogen. − The reduction pathway is directed to NH3 generation, ensuring the efficient and stable operation of the electrochemical nitrate reduction reaction.

[0100] 3. Integration of Adsorption-Catalysis Dual-Functional Materials: This invention innovatively integrates adsorption and catalysis functions into a single material, breaking the limitations of traditional single-function materials and simultaneously improving the capture and conversion efficiency of low-concentration nitrate nitrogen. This design also enables in-situ separation of the product ammonia from the reaction medium, avoiding complex subsequent separation steps and significantly reducing system operating costs.

[0101] 4. Enhanced Mass Transfer and Reaction Path Control with Flowing Electrode Slurry: The flow mode of the electrode in this invention breaks through the bottleneck of diffusion-based mass transfer in traditional fixed electrode systems. The flow state not only enhances mass transfer but also triggers special mechanisms such as intermediate product adsorption and active hydrogen supply, jointly promoting the reaction in a direction favorable to ammonia formation, thereby significantly enhancing the nitrate-nitrogen conversion rate and ammonia selectivity.

[0102] 5. Integrated system and continuous treatment: This invention continuously couples the selective transfer (concentration) of nitrate nitrogen with the electrochemical conversion (reduction) process in the same reactor, realizing uninterrupted operation of "transfer first and then conversion" for low-concentration nitrate nitrogen. This avoids the problems of excessively long process and increased energy consumption in traditional segmented or batch processing, and significantly improves the stability and engineering applicability of the system operation.

[0103] It should be noted that the background section of this invention may include background information about the problems or environment of this invention, and is not necessarily a description of prior art. Therefore, the content included in the background section does not constitute an admission of prior art by the applicant.

[0104] The above description provides a further detailed explanation of the present invention in conjunction with specific / preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the concept of the present invention, and all such substitutions or modifications should be considered within the scope of protection of the present invention. In the description of this specification, the reference to terms such as "an embodiment," "some embodiments," "preferred embodiment," "example," "specific example," or "some examples," etc., indicates that the specific features, structures, materials, or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made herein without departing from the scope of protection of the patent application.

Claims

1. A method for removing and converting low-concentration nitrate nitrogen based on functionalized flow electrode slurry, characterized in that, Includes the following steps: Transfer step: In water bodies polluted with low concentration of nitrate nitrogen, a functionalized flow electrode slurry is used to perform capacitive deionization treatment under an applied first voltage to achieve selective adsorption and concentration of nitrate nitrogen. The functionalized flow electrode slurry includes carbon materials that are functionalized and have electrocatalytic activity. Conversion step: The functionalized flow electrode slurry containing concentrated nitrate nitrogen is subjected to an electrochemical reduction reaction under an applied second voltage to reduce the adsorbed nitrate nitrogen to ammonia, thereby achieving the removal and conversion of nitrate nitrogen; The low-concentration nitrate nitrogen polluted water contains competitive anions, including chloride ions, which are the most abundant in natural water bodies, and the nitrate nitrogen concentration in the low-concentration nitrate nitrogen polluted water is less than 10 mg-N / L. The preparation method of the functionalized flow electrode slurry includes the following steps: providing a carbon material, including multi-walled carbon nanotubes or activated carbon; mixing and reacting the carbon material with a hydrogen peroxide solution to obtain a hydroxylated self-defective carbon material; mixing and reacting the hydroxylated self-defective carbon material with a quaternization reagent solution to obtain a quaternized self-defective carbon material; and dispersing the quaternized self-defective carbon material in a supporting electrolyte solution to form the functionalized flow electrode slurry.

2. The method according to claim 1, characterized in that, When the carbon material is a multi-walled carbon nanotube, the reaction temperature of the multi-walled carbon nanotube with hydrogen peroxide solution is 50~70℃ and the reaction time is 12~36 hours; the reaction temperature of the hydroxylated self-defective carbon nanotube with the quaternization reagent solution is 70~90℃ and the reaction time is 12~36 hours.

3. The method according to claim 1, characterized in that, The supporting electrolyte solution is a sodium chloride solution.

4. The method according to claim 1, characterized in that, The first voltage is 1.0V, and the second voltage is 1.9V to 2.5V.

5. The method according to claim 1, characterized in that, In the transfer step, the flow rate of the low-concentration nitrate-nitrogen-polluted water is 10 mL / min, and the flow rate of the functionalized flow electrode slurry is 50 mL / min.

6. The method according to claim 1, characterized in that, The method further includes the step of: after the conversion step, performing solid-liquid separation on the functionalized flow electrode slurry after the reaction to obtain an ammonia-containing solution and the regenerated carbon material modified with functional groups and possessing electrocatalytic activity.

7. A functionalized flowing electrode slurry for the removal and conversion of low-concentration nitrate nitrogen, characterized in that, The invention comprises a functionalized carbon material with electrocatalytic activity, dispersed in a supporting electrolyte solution. The functionalized carbon material with electrocatalytic activity is obtained by sequentially modifying a carbon material with a hydrogen peroxide solution and a quaternizing agent solution. The preparation method of the functionalized flow electrode slurry includes the following steps: providing a carbon material, including multi-walled carbon nanotubes or activated carbon; mixing and reacting the carbon material with a hydrogen peroxide solution to obtain a hydroxylated self-defective carbon material; mixing and reacting the hydroxylated self-defective carbon material with a quaternizing agent solution to obtain a quaternized self-defective carbon material; and dispersing the quaternized self-defective carbon material in a supporting electrolyte solution to form the functionalized flow electrode slurry.

8. The functionalized flow electrode slurry according to claim 7, characterized in that, The quaternizing agent is dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride.

9. The functionalized flow electrode slurry according to claim 7, characterized in that, The supporting electrolyte solution is a sodium chloride solution.

10. A low-concentration nitrate removal and conversion system for implementing the method according to any one of claims 1 to 6, characterized in that, Includes: a reactor device comprising a first chamber for containing water to be treated and a second chamber for containing functionalized flowing electrode slurry, the first chamber and the second chamber being separated by an ion exchange membrane; A power supply device is configured to apply a first voltage or a second voltage to the reactor device; a fluid drive device is configured to cause the water to be treated and the functionalized flow electrode slurry to flow in their respective circulation loops; wherein the functionalized flow electrode slurry is the functionalized flow electrode slurry according to any one of claims 7 to 9.

11. The system according to claim 10, characterized in that, The system further includes a solid-liquid separation device connected to the circulation loop of the functionalized flowing electrode slurry. The solid-liquid separation device is used to receive the functionalized flowing electrode slurry after the reaction following the conversion step and separate it into an ammonia-containing solution and a regenerated functionalized carbon material with electrocatalytic activity. The regenerated functionalized carbon material with electrocatalytic activity is configured to be returned to the second chamber or the circulation loop of the functionalized flowing electrode slurry for reuse.

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