A method and apparatus for defluorination based on electrochemical confinement

Abstract

The present application belongs to the technical field of defluorination, and particularly relates to a defluorination method and device based on electrochemical confinement. The present application introduces water in a polar aprotic solvent environment and generates hydroxyl ions in situ by electrolysis under the action of an electric field by constructing a specific electrochemical reaction system, thereby realizing the nucleophilic attack of the carbon-fluorine bond without the need for additional strong base and high temperature conditions. Moreover, due to the confinement effect of the cathode hydroxyl, the hydroxyl forms a local confined high concentration distribution at the electrode interface, promoting the efficient rupture of the C-F bond. At the same time, the electrochemical defluorination path proposed by the present application realizes the efficient rupture of the C-F bond in fluorine-containing organic matter under mild conditions, releases the fluorine element mainly in the form of fluoride ions and further fixes it as inorganic fluoride, avoids the generation of volatile fluorine-containing organic by-products in the traditional high-temperature treatment process, improves the safety and stability of the system operation, and reduces the risk of secondary pollution.

Description

Technical Field

[0001] This invention belongs to the field of defluorination methods, specifically relating to a defluorination method and apparatus based on electrochemical confinement. Background Technology

[0002] With the continuous advancement of industrialization and the widespread application of fluorinated chemicals, the accumulation of per- and polyfluoroalkyl substances (PFAS) in the environment has become an increasingly prominent problem. PFAS are widely found in various applications such as fire-fighting foams, textile waterproofing agents, surfactants, electroplating additives, and fluorinated polymers. They exhibit high chemical stability and environmental persistence, readily migrating and accumulating in water bodies, soil, and organisms, and have become one of the typical emerging pollutants of global concern. Therefore, achieving efficient removal and harmless transformation of PFAS has become a key technical problem urgently needing to be solved in the fields of environmental engineering and pollution control.

[0003] PFAS molecules are predominantly composed of high-energy carbon-fluorine (CF) bonds, making them difficult to degrade under conventional physical, chemical, and biological conditions. For destructive treatment of PFAS, current research suggests a class of methods that approach a complete defluorination pathway: thermochemical treatment under strong alkaline conditions (e.g., high-temperature alkaline systems). This type of method introduces excess hydroxide ions at high temperatures to promote nucleophilic substitution or elimination reactions of the CF bonds, thereby achieving stepwise defluorination. However, due to the high bond energy of the CF bonds and the large activation energy of the reaction, a high-temperature and high-concentration alkaline environment is required to drive the reaction. The need for high temperature and strong alkaline conditions further introduces a series of technical problems: First, high-temperature operation significantly increases system energy consumption and carbon emissions, which is detrimental to green and low-carbon development; second, adding large amounts of alkaline reagents (such as NaOH) increases operating costs and generates high-salt wastewater after the reaction, increasing the burden of post-treatment.

[0004] In recent years, electrochemical methods have been considered a potentially effective pathway for PFAS degradation due to their ability to generate active species in situ under relatively mild conditions. However, existing electrochemical systems are mostly based on aqueous radical oxidation pathways, which suffer from insufficient reaction selectivity, limited defluorination efficiency, and low mineralization levels, and are difficult to effectively overcome the energy barrier required for CF bond breaking. Therefore, there is an urgent need to develop a novel technology system that can achieve efficient defluorination conversion under mild conditions while avoiding the need for external strong alkalis and high temperatures. Summary of the Invention

[0005] The purpose of this invention is to provide a defluorination method and apparatus based on electrochemical confinement.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a defluorination method based on electrochemical confinement, comprising the following steps: The fluorine-containing substance, electrolyte, polar aprotic solvent, and water are mixed and electrochemically treated under normal pressure to achieve the defluorination of the fluorine-containing substance.

[0007] Preferably, the polar aprotic solvent includes at least one of N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethylformamide, acetonitrile, and cyclohexanone.

[0008] Preferably, the water volume content in the mixed system is 0.5-5%.

[0009] Preferably, the fluorinated substance includes perfluorinated and polyfluoroalkyl substances; The perfluorinated and polyfluoroalkyl substances include small molecules and / or polymers; The small molecule substance includes at least one of perfluorobutyric acid, perfluorovaleric acid, perfluorohexanoic acid, perfluorooctanoic acid, perfluorononanoic acid, and perfluorohexanesulfonic acid. The polymer includes polyvinylidene fluoride.

[0010] Preferably, in the system obtained by mixing, the concentration of the fluorine-containing substance is not less than 1 mmol / L.

[0011] Preferably, the electrolyte comprises tetrabutylammonium bromide, and the concentration of the electrolyte in the mixed system is 0.05~0.2 mol / L.

[0012] Preferably, the current density of the electrochemical treatment is 10~50 mA / cm². 2 ; The electrochemical treatment temperature is room temperature to 120°C.

[0013] Preferably, after the electrochemical treatment, the system is further separated, and a calcium source is added to the filtrate to recover the inorganic fluoride. The calcium source includes at least one of calcium oxide and calcium hydroxide.

[0014] The present invention also provides a defluorination device based on electrochemical confinement for implementing the defluorination method described above, including a cylindrical coaxial electrode structure; The cylindrical coaxial electrode structure includes a cylindrical anode and a cylindrical cathode sleeved outside the cylindrical anode and not in contact with each other.

[0015] Preferably, the ratio of the diameter of the cylindrical anode to the system to be treated is 3 cm: 2 L; the ratio of the diameter to the height of the cylindrical anode is 3: 16. The ratio of the diameter of the cylindrical anode to the outer diameter of the cylindrical cathode is 3:10; the wall thickness of the cylindrical cathode is not less than 2 mm.

[0016] This invention provides an electrochemically confined defluorination method capable of operating under low-temperature and ambient-pressure conditions. By constructing a specific electrochemical reaction system, water is introduced into a polar aprotic solvent environment and electrolyzed in situ under an electric field to generate hydroxide ions. This achieves nucleophilic attack on the carbon-fluorine bond without the need for external strong alkali or high temperatures. Furthermore, due to the hydroxide confinement effect at the cathode, a locally confined high concentration of hydroxide ions is formed at the electrode interface, promoting efficient CF bond breaking. Simultaneously, the proposed electrochemical defluorination pathway achieves efficient CF bond breaking in fluorinated organic compounds under mild conditions, releasing fluorine primarily as fluoride ions and further fixing it into inorganic fluorides. This avoids the generation of volatile fluorinated organic byproducts in traditional high-temperature treatments, improving the safety and stability of the system and reducing the risk of secondary pollution. This invention provides a relatively simplified, easily scalable, and industrially applicable technical route for treating complex water bodies or fluorinated waste, thus solving the problems of high energy consumption, high pollution risk, and limited large-scale application in existing technologies.

[0017] Specifically: (1) Water-mediated electrochemical defluorination mechanism: By introducing water into the reaction system, hydroxide ions are generated in situ under the action of an external electric field, thereby breaking the carbon-fluorine (CF) bond and providing a reaction basis for defluorination reaction without external strong alkali.

[0018] (2) OH - Confined enhancement mechanism: OH groups generated in situ during electrochemical reactions - The formation of a locally confined high-concentration distribution at the electrode interface and in the microenvironment significantly enhances its nucleophilic attack capability on CF bonds, thereby achieving efficient and selective defluorination conversion.

[0019] (3) Efficient defluorination method under mild conditions: The defluorination conversion is achieved under low temperature, normal pressure and low current density conditions, avoiding the traditional high temperature or strong alkali system, reducing energy consumption and equipment requirements, while inhibiting the generation of volatile fluorine-containing by-products and improving environmental safety.

[0020] (4) Universality of application to multiple types of PFAS: This method is applicable not only to small molecule PFAS, but also to polymeric PFAS materials (such as PVDF), demonstrating its wide applicability from small molecule systems to polymeric systems.

[0021] (5) Construction of specific electrochemical reaction systems: including the coordinated design of polar aprotic solvents, trace water systems, electrode material selection (such as DSA anode / platinum cathode) and operating parameters (current density, etc.) to achieve enhanced interfacial reaction and stable system operation.

[0022] (6) Engineering scale-up reactor structure: The electrochemical reactor adopts a coaxial electrode structure to achieve uniform electric field distribution and efficient interfacial reaction, and has good scale-up capability and continuous operation potential.

[0023] (7) Fluorine resource recovery and closed-loop process: By converting the fluoride ions generated in the reaction into inorganic fluorides to achieve resource recovery, and combined with the recycling of electrolyte, a low-emission and sustainable closed-loop treatment process is constructed, which has good industrial application prospects. Attached Figure Description

[0024] Figure 1 The results are the defluorination efficiency test results corresponding to Example 1; Figure 2 Here is a physical image of the cylindrical coaxial electrode structure reactor in Example 3 and the corresponding defluorination efficiency test results; Figure 3 This is a flowchart corresponding to Example 3; Figure 4 The results are the defluorination efficiency test results for Example 2 and Comparative Example 1. Detailed Implementation

[0025] This invention provides a defluorination method based on electrochemical confinement, comprising the following steps: The fluorine-containing substance, electrolyte, polar aprotic solvent, and water are mixed and electrochemically treated under normal pressure to achieve the defluorination of the fluorine-containing substance.

[0026] In this invention, the polar aprotic solvent preferably includes at least one selected from N,N-dimethylacetamide (DMAc), dimethyl sulfoxide, N,N-dimethylformamide, acetonitrile, and cyclohexanone. In this invention, the polar aprotic solvent can effectively dissolve or disperse fluorine-containing substances and is beneficial to OH... - Maintain a localized high concentration near the interface.

[0027] In this invention, the volume content of water in the mixed system is preferably 0.5-5%, specifically 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%.

[0028] In this invention, the fluorinated substances preferably include perfluorinated and polyfluoroalkyl substances (PFAS); the perfluorinated and polyfluoroalkyl substances preferably include small molecule substances and / or polymers; the small molecule substances preferably include at least one selected from perfluorobutyric acid, perfluorovalerate, perfluorohexanoic acid, perfluorooctanoic acid, perfluorononanoic acid, and perfluorohexanesulfonic acid; the polymers preferably include polyvinylidene fluoride. This invention does not specifically limit the source of the fluorinated substances; any source well known to those skilled in the art can be used. In this invention, when defluorination of fluorinated wastewater is required, it is also preferable to include concentration of the fluorinated wastewater; the concentration method is preferably: adsorption using resin, followed by elution using a polar aprotic solvent. This invention does not specifically limit the adsorption and elution processes; any process well known to those skilled in the art can be used.

[0029] In this invention, the concentration of the fluorine-containing substance in the mixed system is preferably not less than 1 mmol / L. In this invention, the electrolyte preferably comprises tetrabutylammonium bromide, and the concentration of the electrolyte in the mixed system is preferably 0.05~0.2 mol / L, specifically 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, or 0.2 mol / L.

[0030] In this invention, the mixing process is preferably: dissolving the fluorine-containing substance in a polar aprotic solvent, and then adding an electrolyte and water.

[0031] In this invention, the preferred current density for the electrochemical treatment is 10~50 mA / cm². 2 Specifically, it can be 10mA / cm 2 20mA / cm 2 30mA / cm 2 40mA / cm 2 50mA / cm 2 The preferred temperature for the electrochemical treatment is room temperature to 120°C, specifically room temperature, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, and 120°C. In this invention, the anode used in the electrochemical treatment preferably includes a DSA anode, a boron-doped diamond electrode, a titanium-based metal oxide electrode, a platinum electrode, or a gold electrode; the anode is capable of electrochemically activating water and maintaining the required interfacial reaction environment. In this invention, the preferred cathode includes a platinum electrode, a gold electrode, a nickel electrode, a copper electrode, a graphite electrode, or a boron-doped diamond electrode; the cathode is capable of stable operation in the system and promotes interfacial reactions.

[0032] The present invention does not impose any special limitations on the structure of the electrolytic reactor used in the electrochemical treatment. Any structure well-known to those skilled in the art can be used, as long as it can provide an effective electric field distribution, ensure sufficient contact between the reactants and the electrode interface, and form a favorable OH- ion exchange rate. - The desired defluorination effect can be achieved by creating a locally confined reaction microenvironment.

[0033] In this invention, taking PFAS as an example, under the action of an external electric field, by controlling the reaction environment at the electrode interface, the stable carbon-fluorine (CF) bonds in the PFAS molecule are induced to break in the electrode interface region, accompanied by electron transfer and proton participation, thereby realizing the conversion of fluorine elements into fluoride ions (F... - The gradual transformation and release of this form involves reactions including: Step 1, water electrolysis reaction: ; The second step is the defluorination reaction: .

[0034] In this invention, after the electrochemical treatment, the resulting system is preferably separated, and a calcium source is added to the filtrate to recover inorganic fluorides; the calcium source preferably includes at least one of calcium oxide and calcium hydroxide. In this invention, by adding a calcium source, stable calcium fluoride can be formed with the fluoride ions obtained from electrolysis and precipitated for recovery, thereby realizing the resource utilization of fluorine; at the same time, after removing the calcium fluoride precipitate, the resulting electrolyte can be returned to the electrolysis system for recycling.

[0035] The present invention does not impose any special limitation on the operation mode of the electrochemical treatment; intermittent operation, semi-continuous operation, or continuous operation may be adopted.

[0036] The present invention also provides a defluorination device based on electrochemical confinement for implementing the defluorination method described above, including a cylindrical coaxial electrode structure; The cylindrical coaxial electrode structure includes a cylindrical anode and a cylindrical cathode sleeved outside the cylindrical anode and not in contact with each other.

[0037] In this invention, the ratio of the diameter of the cylindrical anode to the system to be treated is preferably 3 cm:2 L; the ratio of the diameter to the height of the cylindrical anode is preferably 3:16; the type of cylindrical anode preferably includes a DSA anode, a boron-doped diamond electrode, a titanium-based metal oxide electrode, a platinum electrode, or a gold electrode. In this invention, the ratio of the diameter of the cylindrical anode to the outer diameter of the cylindrical cathode is preferably 3:10; the wall thickness of the cylindrical cathode is preferably not less than 2 mm; the height of the cylindrical anode is preferably the same as that of the cylindrical cathode; the type of cylindrical cathode preferably includes a platinum electrode, a gold electrode, a nickel electrode, a copper electrode, a graphite electrode, or a boron-doped diamond electrode.

[0038] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0039] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0040] Example 1 Perfluorooctanoic acid (PFOA) was dissolved in the polar aprotic solvent DMAc at a concentration of 10 mmol / L. Tetrabutylammonium bromide and water were then introduced to form a mixed reaction system, with a water volume concentration of 0.9% and a tetrabutylammonium bromide concentration of 0.2 mol / L. A cylindrical electrolytic cell with an effective reaction volume of 10 mL was used. Platinum sheets were used as the cathode and anode, respectively. Electrochemical treatment was performed under ambient pressure and an applied electric field (current density 20 mA / cm²). 2 The reaction temperature is 60℃, which achieves the decomposition of PFOA; Quantitative analysis of the fluoride ions generated during the reaction was performed, and the resulting curve is shown in the figure. Figure 1 As shown in Table 1, the specific test results are as follows; Table 1 Defluorination efficiency of Example 1

[0041] It can be seen that during the reaction, hydroxide ions are generated in situ through the electrolysis of water and form a locally confined high concentration distribution at the electrode interface, thereby promoting the gradual breaking of carbon-fluorine bonds in PFOA molecules and achieving defluorination conversion. After continuous reaction for 24 hours, the defluorination rate of PFOA in the system can reach about 99%, indicating that the method of the present invention can achieve efficient breaking of carbon-fluorine bonds and deep defluorination conversion in PFAS molecules under mild conditions.

[0042] Example 2 Polyvinylidene fluoride (PVDF) was dissolved in the polar aprotic solvent DMAc at a concentration of 0.1 mol / L. Tetrabutylammonium bromide and water were then introduced to form a mixed reaction system, with a water volume concentration of 0.9% and a tetrabutylammonium bromide concentration of 0.1 mol / L. A cylindrical electrolytic cell with an effective reaction volume of 10 mL was used. Platinum sheets were used as the cathode and anode, respectively. Electrochemical treatment was carried out under normal pressure and an applied electric field (current density of 20 mA / cm²). 2 The reaction temperature is 120℃, which achieves the decomposition of PVDF; Quantitative analysis of the generated fluoride ions during the reaction process revealed that hydroxide ions are generated in situ through water electrolysis, forming a locally confined high concentration distribution at the electrode interface and in the aprotic microenvironment. This promotes the gradual breaking of carbon-fluorine bonds in PVDF molecules, achieving defluorination conversion. Experimental results show that under the above conditions, the defluorination rate of the PVDF system can reach approximately 93% in 16 hours, indicating that the method of this invention is not only applicable to small molecule PFAS, but can also effectively achieve the breaking of carbon-fluorine bonds and defluorination conversion in polymeric PFAS materials, demonstrating good universality and application potential.

[0043] Example 3 An electrochemical reactor employing a cylindrical coaxial electrode structure is shown in the following physical structure: Figure 2 As shown, the cylindrical anode has a diameter of 3cm and a height of 16cm and is a DSA anode; the cylindrical cathode has an outer diameter of 10cm, a wall thickness of 2mm, and a height of 16cm and is a platinum-plated titanium electrode. Polyvinylidene fluoride (PVDF) was dissolved in the polar aprotic solvent DMAc, where the concentration of PVDF was 0.1 mol / L. Tetrabutylammonium bromide and water were introduced to form a mixed reaction system, where the volume concentration of water was 0.9% and the effective volume was 2 L. The concentration of tetrabutylammonium bromide in the system was 0.2 mol / L. Electrochemical treatment was carried out under normal pressure and an applied electric field (current density 10 mA / cm²). 2 The reaction temperature is room temperature, which enables the decomposition of PVDF. Quantitative analysis of the fluoride ions generated during the reaction was performed, and the resulting curve is shown in the figure. Figure 2 As shown in the figure; the specific results are shown in Table 2; Table 2 Defluorination efficiency corresponding to Example 3

[0044] The fluoride ion generation rate is the average generation rate over the time interval 0-t.

[0045] As can be seen, in this embodiment, the cathode is a platinum-plated titanium cathode, which is arranged coaxially around the anode to form a ring electrode system to ensure uniform electric field distribution and improve interface reaction efficiency; when a constant current is applied at room temperature, a defluorination rate of about 90% can be achieved.

[0046] Based on reaction volume and operating time, the PVDF treatment capacity reaches 8.5 kg·m³. -3 ·d -1 This indicates that the method of the present invention has good potential for engineering scale-up.

[0047] After the reaction is complete, solid-liquid separation is performed to obtain carbonaceous solid product and fluorine-containing filtrate. Fluoride ions in the filtrate undergo a precipitation reaction with the addition of a calcium source, converting into solid calcium fluoride, thus achieving the resource recovery of fluorine. The treated electrolyte, after appropriate purification, can be recycled back to the reaction system, realizing the recovery and reuse of solvent and electrolyte, thereby forming a low-emission, recyclable closed-loop process system. The specific process is as follows: Figure 3 As shown.

[0048] Comparative Example 1 The electrochemical treatment was carried out in accordance with Example 2, without the addition of water; The test showed that the defluorination rate was 0 within 16 hours. Specific results are as follows: Figure 4 As shown.

[0049] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A defluorination method based on electrochemical confinement, characterized in that, Includes the following steps: The fluorine-containing substance, electrolyte, polar aprotic solvent, and water are mixed and electrochemically treated under normal pressure to achieve the defluorination of the fluorine-containing substance.

2. The defluorination method according to claim 1, characterized in that, The polar aprotic solvent includes at least one of N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethylformamide, acetonitrile, and cyclohexanone.

3. The defluorination method according to claim 1, characterized in that, The water volume content in the system obtained by mixing is 0.5-5%.

4. The defluorination method according to claim 1, characterized in that, The fluorinated substances include perfluorinated and polyfluoroalkyl substances; The perfluorinated and polyfluoroalkyl substances include small molecules and / or polymers; The small molecule substance includes at least one of perfluorobutyric acid, perfluorovaleric acid, perfluorohexanoic acid, perfluorooctanoic acid, perfluorononanoic acid, and perfluorohexanesulfonic acid. The polymer includes polyvinylidene fluoride.

5. The defluorination method according to claim 1, characterized in that, In the system obtained by mixing, the concentration of the fluorine-containing substance is not less than 1 mmol / L.

6. The defluorination method according to claim 1, characterized in that, The electrolyte includes tetrabutylammonium bromide, and the concentration of the electrolyte in the mixed system is 0.05~0.2 mol / L.

7. The defluorination method according to claim 1, characterized in that, The current density of the electrochemical treatment is 10~50 mA / cm². 2 ; The electrochemical treatment temperature is room temperature to 120°C.

8. The defluorination method according to claim 1, characterized in that, The electrochemical treatment further includes separating the obtained system, adding a calcium source to the obtained filtrate, and recovering the inorganic fluoride. The calcium source includes at least one of calcium oxide and calcium hydroxide.

9. A defluorination device based on electrochemical confinement, characterized in that, The method for implementing the defluorination method according to any one of claims 1 to 8 includes a cylindrical coaxial electrode structure; The cylindrical coaxial electrode structure includes a cylindrical anode and a cylindrical cathode sleeved outside the cylindrical anode and not in contact with each other.

10. The defluorination device according to claim 9, characterized in that, The ratio of the diameter of the cylindrical anode to the system to be treated is 3 cm: 2 L; the ratio of the diameter to the height of the cylindrical anode is 3:

16. The ratio of the diameter of the cylindrical anode to the outer diameter of the cylindrical cathode is 3:10; the wall thickness of the cylindrical cathode is not less than 2 mm.

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