Method for removing perfluoroalkyl compound, and liquid-passing type capacitor

Abstract

One aspect of the present invention relates to a method for removing a perfluoroalkyl compound from a liquid to be treated, the method including causing the liquid to be treated to flow through a liquid-passing type capacitor comprising an electrode, a highly corrosion-resistant current collector, and a separator, and causing the electrode to adsorb the perfluoroalkyl compound, the electrode including a conductive substance having a BET specific surface area of 800 m2 / g or more.

Description

Method for removing perfluoroalkyl compounds and liquid-passing capacitor

[0001] The present invention relates to a method for removing perfluoroalkyl compounds from a liquid to be treated, and a flow-through capacitor.

[0002] Fluorine-containing organic compounds (PFCs) have unique properties that cannot be achieved with other substances (excellent heat and chemical resistance, usable even under harsh conditions, no light absorption, etc.), and so have been used in a variety of applications, including surfactants, emulsifiers, water repellents, fire extinguishing agents, waxes, carpet cleaning agents, coating agents, etc. Recently, they have also been increasingly used as functional materials, such as surface treatment agents for semiconductors and constituent materials for fuel cells.

[0003] However, recent reports have shown that some fluorine-containing organic compounds are accumulating in environmental waters and wildlife. Typical examples are perfluoroalkyl and polyfluoroalkyl substances (PFAS), which include perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), as well as hundreds of other similar compounds. Concerns about the human health and environmental risks of these substances are emerging, and there are concerns that they may result from widespread contamination. While some PFAS compounds with known human health risks (PFOA and PFOS) have been voluntarily phased out, legacy contamination remains. Furthermore, replacement PFAS compounds are being introduced with limited understanding of their health risks.

[0004] In response to this situation, efforts have been initiated to reduce the environmental risks of fluorinated organic compounds. However, currently, only PFOA and PFOS are regulated in the United States and other countries, and no maximum contaminant levels (MCLs) have been established to control the acceptable levels of these or other PFAS compounds in drinking water.

[0005] Meanwhile, flow-through capacitors are used to remove harmful substances and the like from liquids by utilizing electrostatic force, and it is known that they are used to demineralize liquids to be treated. It has been reported that the use of flow-through capacitors can remove various substances. For example, Patent Documents 1 and 2 describe methods for removing metals and the like using activated carbon as electrodes, as well as methods for improving the efficiency of such methods. Patent Document 3 also describes the removal of PFAS compounds using flow-through capacitors.

[0006] The desalination method using a flow-through capacitor is an energy-efficient method because it stores the electrical energy supplied during adsorption of the substance to be removed in the capacitor and recovers the electrical energy during desorption of the substance. Furthermore, the flow-through capacitor can perform desalination even at low voltage. For these reasons, the desalination method using a flow-through capacitor offers significant benefits in terms of equipment and is industrially advantageous.

[0007] However, the techniques described in Patent Documents 1 and 2 relate to the removal of inorganic substances such as metals, and there is no mention of the removal of perfluoroalkyl compounds. The technique described in Patent Document 3 relates to the removal of perfluoroalkyl compounds, but there is no mention of a specific electrode structure, and the technique does not focus on the durability of the electrode.

[0008] US Patent No. 6,709,560 JP 2016-509527 A US Patent Application Publication No. 2021 / 395113

[0009] Therefore, in view of the above-mentioned current situation, the main object of the present invention is to provide a liquid-flow capacitor that is capable of adsorbing and removing perfluoroalkyl compounds and has excellent durability, and to provide a method for removing perfluoroalkyl compounds using the liquid-flow capacitor.

[0010] As a result of intensive research to solve the above problems, the present inventors have found that the above problems can be solved by a method having the following configuration, and based on this finding, they have conducted further research and completed the present invention.

[0011] That is, one aspect of the present invention is a method for removing perfluoroalkyl compounds from a liquid to be treated, which comprises passing the liquid to be treated through a liquid-flow capacitor including electrodes, a highly corrosion-resistant current collector, and a separator, and causing the perfluoroalkyl compounds to be adsorbed onto the electrodes, and the electrodes have a BET specific surface area of ​​800 m ２ / g or more of a conductive material.

[0012] Another aspect of the present invention provides a liquid-passing capacitor comprising electrodes, a highly corrosion-resistant current collector, and a separator, wherein the electrodes have a BET specific surface area of ​​800 m ２ / g or more of a conductive material.

[0013] Fig. 1 is a schematic diagram showing an example of a liquid-flow type capacitor (1 cell) used in this embodiment. Fig. 2 is a schematic diagram showing an example of a liquid-flow type capacitor (3 cells) used in this embodiment. Fig. 3 is a schematic cross-sectional view of the liquid-flow type capacitor (3 cells) shown in Fig. 2. Fig. 4 is a top view of a container containing the liquid-flow type capacitor obtained in the example.

[0014] One aspect of the present invention is a method for removing perfluoroalkyl compounds from a liquid to be treated, which comprises passing the liquid to be treated through a liquid-flow capacitor including electrodes, a highly corrosion-resistant current collector, and a separator, and allowing the perfluoroalkyl compounds to be adsorbed onto the electrodes, the electrodes having a BET specific surface area of ​​800 m ２ / g or more of conductive material.

[0015] According to the method of this embodiment, perfluoroalkyl compounds can be removed from the liquid to be treated. Furthermore, since the flow-through capacitor used in this embodiment has excellent durability, perfluoroalkyl compounds can be stably removed over a long period of time. Furthermore, the method of this embodiment allows sufficient removal of perfluoroalkyl compounds even when the flow rate of the liquid to be treated is high.

[0016] That is, the present invention can provide a liquid-flow capacitor that can adsorb and remove perfluoroalkyl compounds and has excellent durability, and a method for removing perfluoroalkyl compounds using the same.

[0017] Hereinafter, embodiments of the present invention will be specifically described, but the present invention is not limited to these.

[0018] (Flow-through Capacitor) First, the flow-through capacitor used in the removal method of the present embodiment will be described. The present invention encompasses not only the method for removing perfluoroalkyl compounds using the following flow-through capacitor, but also the flow-through capacitor itself.

[0019] The liquid-flow capacitor of this embodiment includes electrodes, a highly corrosion-resistant current collector, and a separator, and the electrodes have a BET specific surface area of ​​800 m ２ The configuration of the capacitor is not particularly limited as long as it contains a conductive material having a conductivity of 1 / g or more and is capable of adsorbing a perfluoroalkyl compound, and one embodiment thereof will be described with reference to the drawings. The reference numerals in the drawings represent the following: 1, 5 current collector; 2, 4 electrodes; 3 separator; 6 tab portion; 7 connecting line between current collector tabs; 8 voltage; 9, 9' electrode terminals; 10 flow-through capacitor (one cell); 11 container; 20 flow-through capacitor (multiple cells).

[0020] 1 includes a separator 3 between an electrode 2 and an electrode 4. A current collector 1 and a current collector 5 are provided on the outside of the electrode 2 and the electrode 4. That is, the liquid-flow capacitor 10 has a laminated structure in which the current collector 1, the electrode 2, the separator 3, the electrode 4, and the current collector 5 are stacked.

[0021] The liquid-flow capacitor of this embodiment may have a capacitor structure 20 including a plurality of cells, with the laminate serving as one cell, as shown in Figures 2 and 3. In this case, a separator 3 may be disposed between each cell.

[0022] When stacking multiple cells, the tab portions 6 of the current collectors of the liquid-flow capacitor 10 shown in Fig. 1 are preferably arranged so that the tab portions of the nearest layers face in opposite directions, as shown in Fig. 2. Each tab portion of the current collector 1 is electrically connected to an electrode terminal (not shown).

[0023] The capacitor 20 shown in Figures 2 and 3 is housed in a container 11 (see, for example, Figure 4), which typically has a liquid supply port for supplying the liquid to be treated and a liquid drain port for draining the liquid after treatment. When a flow-through capacitor 20 such as that shown in Figures 2 and 3 is housed in a capacitor container, the tab portions of each current collector are led out of the capacitor container and alternately connected to the positive or negative electrode of the current. The tab portions of each current collector may be connected to each other by a current collector tab connection line 7, as shown in Figure 2. Tabs with the same polarity may be connected together by a metal bolt 8, as shown in Figure 3. A conductive adhesive, metal tape, or the like may be used instead of the metal bolt.

[0024] Current Collector The current collector of this embodiment is not particularly limited as long as it is a current collector that is permeable and highly corrosion-resistant, but in order to withstand the high acidity of the perfluoroalkyl compound, graphite sheet, graphite sheet, oxide film-coated aluminum foil, titanium foil, titanium mesh, stainless steel mesh, titanium punched metal, stainless steel punched metal electrode, etc., which have excellent corrosion resistance, can be used. From the viewpoints of cost, flexibility, processability, and electrical resistance, it is preferable to use a graphite sheet. Specific examples of graphite include graphite sheets formed from expanded graphite, and graphite sheets that do not use binders or binding agents that affect electrical resistance are particularly preferred.

[0025] The thickness of the current collector is not particularly limited, but if it is too thin, the current collector may deteriorate when used under acidic conditions after repeated removal of the perfluoroalkyl compound, and conductivity and strength may not be ensured. Furthermore, if it is too thick, flexibility may decrease. From the above viewpoints, the thickness is preferably in the range of 1 μm to 1 mm, more preferably in the range of 10 μm to 800 μm, and even more preferably in the range of 100 to 500 μm.

[0026] Electrodes The electrodes of the liquid-flow capacitor of this embodiment have a BET specific surface area of ​​800 m ２The electrode configuration is not particularly limited, and the electrode of this embodiment may be an electrode in which a powder or granular conductive material is formed into a sheet or coated with a binder, or a woven or nonwoven fabric made of a fibrous conductive material.

[0027] Specific surface area is 800m ２ When the specific surface area is 2400 m / g or more, the adsorption capacity is good, and further, when the polarity of the electrode is reversed to desorb the perfluoroalkyl compound adsorbed on the surface of the electrode, the perfluoroalkyl compound is easily desorbed. ２ When the BET specific surface area of ​​the conductive material is less than 800 m / g, the performance per volume is superior, and further, the amount of binder used can be reduced, and the proportion of the conductive material does not become too small, which has the advantage of superior perfluoroalkyl compound adsorption ability. ２ / g, preferably 900 to 2000 m ２ / g, more preferably 1200 to 1600 m ２ / g.

[0028] In this embodiment, the BET specific surface area means a specific surface area calculated by a nitrogen adsorption method.

[0029] As the conductive material, activated carbon is preferably used from the viewpoints of adsorption performance, conductivity, weight, corrosiveness, etc. As the activated carbon, powdered and / or granular activated carbon, or fibrous activated carbon, etc., can be used, and a mixture of these may be used as the conductive material.

[0030] Specific examples of powdered and / or granular activated carbon include plant-based activated carbons such as wood, sawdust, charcoal, fruit shells such as coconut shells and walnut shells, fruit seeds, pulp manufacturing by-products, lignin, and blackstrap molasses; mineral-based activated carbons obtained by carbonizing and activating peat, grass peat, lignite, brown coal, bituminous coal, anthracite, coke, coal tar, coal pitch, petroleum distillation residue, and / or petroleum pitch; synthetic resin-based activated carbons obtained by carbonizing and activating phenol, saran, acrylic resin, etc.; and these activated carbons may also be pulverized and used as powdered and / or granular activated carbons. Among these, plant-based activated carbons are preferred due to their excellent adsorption performance, and coconut shell activated carbon is particularly preferred.

[0031] When powdered and / or granular activated carbon is used, the particle size has a median particle diameter in the range of 1 μm to 5 mm, more preferably in the range of 3 μm to 1 mm. Here, the median particle diameter is the particle diameter (D50) at which the integrated value of the mass of all particles in the particle size distribution is 50%. When the median particle diameter of activated carbon is 1 μm or more, the amount of binder used is reduced and the proportion of activated carbon does not become too low, resulting in good perfluoroalkyl compound adsorption capacity. Furthermore, when the median particle diameter of activated carbon is 5 mm or less, the resulting activated carbon electrode has the advantage of excellent surface uniformity and excellent perfluoroalkyl compound adsorption capacity.

[0032] When powdered and / or granular activated carbon is used, the electrode of this embodiment can be obtained, for example, by forming a mixture containing the activated carbon as a conductive material and a binder into a sheet. When used for water purification, it is preferable to use a binder that is non-toxic to living organisms. The proportion of activated carbon contained in the activated carbon electrode is preferably 80 to 99% by mass, more preferably 90 to 97% by mass. When the proportion of activated carbon contained in the activated carbon electrode is within the above range, the electrode exhibits excellent perfluoroalkyl compound adsorption performance.

[0033] Examples of binders that can be used for electrodes include polytetrafluoroethylene, polyvinylidene fluoride, fluoroethylene-perfluoroalkoxyethylene copolymer, ethylene-tetrafluoroethylene copolymer, styrene-butadiene copolymer, polyethylene, polypropylene, polystyrene, ethylene-methacrylic acid copolymer, ethylene-vinyl acetate copolymer, polyethylene terephthalate, polybutylene terephthalate, polymethyl methacrylate, polyacrylonitrile, and polyamide. These may be used alone or in combination. Of these, polytetrafluoroethylene is preferred from the viewpoints of binding properties, stability, etc.

[0034] The content of the binder contained in the electrode is not particularly limited, and may be set appropriately so that the conductive material is not detached from the electrode and properties such as flexibility are maintained. For example, the proportion of the binder contained in the activated carbon electrode is in the range of 0.1% by mass to 10% by mass, preferably 0.5% by mass to 8% by mass, and more preferably 1% by mass to 6% by mass, based on the total mass of the electrode.

[0035] The electrode containing the conductive substance may further contain a conductive material. Adding a conductive material can impart superior conductivity to the electrode. Specific examples of such conductive materials include carbon-based materials such as acetylene black, ketjen black, and graphite; precious metals such as gold, platinum, and silver; and highly conductive ceramics such as titanium nitride, titanium silicon carbide, titanium carbide, titanium boride, and zirconium boride. Among these, carbon-based materials such as acetylene black, ketjen black, and graphite are preferred due to their excellent cost and processability.

[0036] The content of the conductive material contained in the electrode is not particularly limited, and when a conductive material is used, the proportion of the conductive material contained in the electrode is in the range of 0.1 mass % to 10 mass %, preferably in the range of 0.5 mass % to 8 mass %, and more preferably in the range of 1 mass % to 6 mass %, relative to the total mass of the electrode.

[0037] The thickness of the electrode containing the conductive material is not particularly limited, but the amount of adsorption is often affected by the thickness. In order to ensure a thickness that can exert an electrical effect, the thickness is usually in the range of 10 μm to 20 mm, preferably 20 μm to 18 mm, and more preferably 50 μm to 15 mm, from the viewpoint of preventing the electrical resistance from becoming too high.

[0038] The electrode of this embodiment can also use fibrous activated carbon as the conductive material. The use of fibrous activated carbon eliminates the need for inactive materials such as binders, simplifying the electrode configuration. Furthermore, because fibrous activated carbon has low electrical resistance, it can be driven at a lower voltage, resulting in energy savings. Specific examples of fibrous activated carbon include fibrous activated carbon obtained by carbonizing and activating fibers derived from acrylic resin, phenolic resin, or polyvinyl alcohol resin, recycled fibers such as rayon, or fibers such as cotton. Among these, fibrous activated carbon made from phenolic resin fibers, rayon fibers, or the like is preferred in terms of its excellent adsorption performance, adsorption capacity, and fiber strength.

[0039] The fiber diameter of the fibrous activated carbon is not particularly limited, but is preferably in the range of 1 μm to 30 μm, more preferably in the range of 5 μm to 28 μm, and even more preferably in the range of 7 μm to 25 μm.

[0040] When fibrous activated carbon is used as the conductive material, woven fabrics, nonwoven fabrics, etc., made of the fibrous activated carbon can be used as the electrode of this embodiment. In this case, a binder may be added to the woven fabric or nonwoven fabric to improve its strength. The binder used is not particularly limited as long as it is insoluble in water, but examples include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), (meth)acrylic resins such as methyl methacrylate (PMMA), polyacrylonitrile resins, polystyrene resins, styrene-butadiene copolymer resins, styrene-isoprene copolymer resins, and polyolefins. The binder content in the electrode is not particularly limited as long as it can suppress detachment of the activated carbon, but may be adjusted appropriately as long as it does not reduce electrical conductivity. Typically, the binder content is 0.1 to 10 mass%, 0.5 to 5 mass%, and more preferably 1 to 3 mass% relative to 100 mass% of the fibrous activated carbon used.

[0041] Alternatively, a mixture containing fibrous activated carbon and a binder may be formed into a sheet, and the resulting sheet may be used as an electrode. In this case, the fibrous activated carbon may be pulverized and used in powder and / or granular form. The type and amount of binder used are the same as when powder and / or granular activated carbon is used. A conductive material may also be added to enhance conductivity.

[0042] In this embodiment, the electrode described above preferably has pores in order to allow as much of the liquid to be treated as possible to flow through and enhance the adsorption of perfluoroalkyl compounds. From the viewpoint that larger pores reduce the resistance of the flowing liquid, the porosity of the electrode of this embodiment is preferably 10% or more. Although there is no particular upper limit, pores that are too large may be undesirable from the viewpoint of electronic conduction. Therefore, the porosity of the electrode of this embodiment is preferably 10% to 90%, more preferably 20 to 85%, and even more preferably 25 to 80%. Furthermore, the method for achieving the porosity of the electrode within the above range is not limited, but the porosity of the electrode can be adjusted by adjusting the binder amount or slurry concentration, or adjusting the pressure during electrode molding, etc., according to the true helium density of the conductive material.

[0043] In this embodiment, the "porosity" refers to the proportion of voids in an electrode, and can be calculated by dividing the weight per volume of the actual electrode by the weighted average of the true helium densities of the materials (conductive materials, binders, etc.) that make up the electrode, calculated based on their blending amounts, and then subtracting the result from 1.

[0044] In the liquid flow capacitor of this embodiment, the same electrode may be used as the positive electrode and the negative electrode, or different electrodes may be used as the positive electrode and the negative electrode.

[0045] Separator The separator of the present embodiment is not particularly limited as long as it is a separator that allows liquid to pass through, and specific examples include porous films, resin nets of synthetic fibers, woven fabrics, paper-like aggregates, woven fabrics or nonwoven fabrics made of accumulated synthetic fibers or recycled fibers, etc. Among these, from the viewpoints of excellent liquid permeability and economy, resin nets and nonwoven fabrics are preferred, and resin nets are more preferred.

[0046] Materials constituting the separator as described above include synthetic resins such as polyester, for example, polyethylene terephthalate, polypropylene, polyamide, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, polyether ether ketone, and mixtures thereof. Among these, polyethylene terephthalate and polypropylene are preferred, with polyethylene terephthalate being more preferred, from the viewpoints of low cost and excellent processability. In a preferred embodiment, a resin net or nonwoven fabric made of these resins is used.

[0047] The thickness of the separator is not particularly limited as long as it can maintain liquid permeability, separate the negative electrode and the positive electrode, and prevent short circuits, but is preferably 50 to 250 μm, more preferably 70 to 150 μm. If the separator thickness is 250 μm or less, the electrical resistance between the cells during current flow will not be too high, resulting in excellent perfluoroalkyl compound adsorption ability. Furthermore, if the separator thickness is 50 μm or more, there is the advantage that the liquid flow resistance can be kept relatively low.

[0048] The aperture ratio of the separator is preferably 20 to 80%, more preferably 30 to 70%. When the aperture ratio of the separator is 20% or more, the resistance to liquid passage is kept low. When the aperture ratio of the separator is 80% or less, internal short circuits at the openings are further suppressed.

[0049] In addition to the above configuration, the liquid flow capacitor of this embodiment may further include an anion exchange membrane and a cation exchange membrane.

[0050] When the liquid-flow capacitor of this embodiment includes an anion exchange membrane and a cation exchange membrane, for example, in the case of capacitor 10 shown in FIG. 1 , a laminate in which a current collector 1, an electrode 2 (first electrode), and a cation exchange membrane (not shown) are stacked in this order can be arranged to face a laminate in which a current collector 5, an electrode 4 (second electrode), and an anion exchange membrane (not shown) are stacked in this order, with a separator 3 interposed between the anion exchange membrane and the cation exchange membrane, thereby forming one cell of the capacitor.

[0051] When such an anion exchange membrane and a cation exchange membrane are provided, the following effects can be obtained.

[0052] Before current is passed through the flow-through capacitor, when the water to be treated is passed through each cell, the anions (-) and cations (+) in the water are not adsorbed by the first electrode and the second electrode, but pass directly between the two electrodes. On the other hand, when the flow-through capacitor includes a cation exchange membrane and an anion exchange membrane, when a current is passed between the two electrodes by connecting the negative side of a DC power supply to the first electrode and the positive side of a DC power supply to the second electrode, the cations are adsorbed by the first activated carbon electrode of the first electrode because they can pass through the cation exchange membrane arranged on the surface of the first electrode, and the anions are adsorbed by the second activated carbon electrode of the second electrode because they can pass through the anion exchange membrane arranged on the surface of the second electrode.

[0053] Furthermore, as described above, if the adsorption performance gradually decreases with increasing liquid flow rate through the flow-through capacitor, the polarity of the activated carbon electrode is reversed from that during the adsorption step (i), causing the cations adsorbed on the first electrode and the anions adsorbed on the second electrode to desorb and be released into the flow-through liquid. At this time, the released cations cannot pass through the anion exchange membrane disposed on the surface of the second electrode and are therefore not adsorbed by the second electrode. Similarly, the released anions cannot pass through the cation exchange membrane disposed on the surface of the first electrode and are therefore not adsorbed by the first electrode. In this way, ion re-adsorption on the first and second electrodes is suppressed, resulting in a high concentration of ions in the flow-through liquid. Therefore, when a flow-through capacitor includes an anion exchange membrane and a cation exchange membrane, the adsorption capacity of the flow-through capacitor is improved and ion re-adsorption is prevented, enabling efficient treatment of the water to be treated using the flow-through capacitor.

[0054] The anion exchange membrane that can be used in this embodiment is not particularly limited, but examples thereof include membranes containing ion exchange resins such as styrene resins, acrylic resins, or fluorine resins, which have anion exchange groups such as quaternary amino groups.Furthermore, the cation exchange membrane that can be used in this embodiment is not particularly limited, but examples thereof include membranes containing ion exchange resins such as styrene resins, acrylic resins, or fluorine resins, which have cation exchange groups such as sulfonic groups or carboxyl groups.

[0055] The method for manufacturing the liquid-flow capacitor of this embodiment is not particularly limited, and it can be obtained by laminating the above-mentioned current collectors, electrodes, separators, and, if necessary, each ion exchange membrane. The obtained laminate can be placed in a capacitor container equipped with a liquid inlet and a liquid outlet.

[0056] In the liquid-flow capacitor of this embodiment, it is preferable that the current collector and the electrode are in close contact from the viewpoint of reducing resistance. The current collector and the electrode can be brought into close contact by applying pressure, or in a preferred embodiment, the current collector and the electrode can be bonded and fixed using a conductive adhesive or pressure-sensitive adhesive. There are no particular restrictions on the conductive adhesives and pressure-sensitive adhesives that can be used, and metal-based, carbon-based, and conductive organic adhesives can be used, but in consideration of durability and conductivity, it is preferable to use a carbon-based conductive adhesive.

[0057] (Method for Removing Perfluoroalkyl Compounds) Next, a method for removing perfluoroalkyl compounds from a liquid to be treated using the flow-through capacitor of this embodiment will be described.

[0058] The method of this embodiment includes passing a liquid to be treated through a flow-through capacitor, and causing the perfluoroalkyl compound contained in the liquid to be adsorbed onto the electrodes of the flow-through capacitor.

[0059] As a liquid passing method for removing perfluoroalkyl compounds from a liquid to be treated using a liquid passing capacitor, a total filtration method in which the entire amount of the raw liquid to be treated is filtered, or a circulating filtration method may be adopted. The liquid passing conditions are not particularly limited, but are preferably 5 to 100 hours. －１It is preferable to pass the liquid parallel to the electrode surface because this allows the non-adsorbed liquid to flow without accumulating inside the electrode.

[0060] In the liquid-flow capacitor of this embodiment, liquid is passed through the capacitor while a voltage is applied to the electrodes. However, the type of DC power source that supplies power to the capacitor is not particularly limited. A 100 V household power source may be used after adjusting the voltage and converting it to DC, or a battery or storage battery may be used to supply power. Furthermore, when used outdoors, an independent power source such as a solar cell, wind power generator, fuel cell, or cogenerator may be used. Furthermore, since the liquid-flow capacitor itself has the ability to store electricity, multiple liquid-flow capacitors may be connected together, and the electricity stored in each may be used alternately as a power source.

[0061] The voltage applied to the liquid-flow capacitor is not particularly limited, and it will of course vary depending on the resistance of the electrodes. Typically, the potential of the positive electrode relative to the negative electrode is in the range of 0.01 to 10 V, preferably 0.1 to 8 V, and more preferably 1 to 5 V. If the potential difference is too low, the adsorptive force will be insufficient and the amount of capture will be small, while if the potential difference is too high, it will be difficult to suppress other side reactions, such as water electrolysis, and this is not preferable.

[0062] The perfluoroalkyl compound to be removed by the method of this embodiment is not particularly limited, and examples thereof include perfluorobutane, perfluoropentane, perfluorohexane, perfluoroheptane, perfluorooctane, perfluorononane, perfluorodecane, perfluoroundecane, perfluorododecane, perfluorotridecane, perfluorotetradecane, perfluorobutanoic acid, perfluoropentanoic acid, perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorononanoic acid, and perfluoro Decanoic acid, perfluoroundecanoic acid, perfluorododecanoic acid, perfluorotridecanoic acid, perfluorotetradecanoic acid, perfluorobutanesulfonate, perfluoropentanesulfonate, perfluorohexanesulfonate, perfluoroheptanesulfonate, perfluorooctane sulfonate, perfluorononanesulfonate, perfluorodecanesulfonate, 1H,1H,2H,2H-perfluorohexanesulfonic acid, 1H,1H,2H,2H-perfluorooctane sulfonic acid, 1H,1H,2H,2H-perfluorodecanesulfonate perfluorooctanesulfonic acid, perfluorooctanesulfonamide, N-methylperfluorooctanesulfonamide, N-ethylperfluorooctanesulfonamide, N-methylperfluorooctanesulfonamidoacetic acid, N-ethylperfluorooctanesulfonamidoacetic acid, N-methylperfluorooctanesulfonamidoethanol, N-ethylperfluorooctanesulfonamidoethanol, hexafluoropropylene oxide dimer acid, 4,8-dioxa-3H-perfluorononanoic acid, perfluoro-3-methoxypropane perfluoropropylpropanoic acid, 2H,2H,3H,3H-perfluorooctanoic acid, 3-perfluoroheptylpropanoic acid, bis(1H,1H,2H,2H-perfluorodecyl)phosphate, perfluoro-4-methoxybutanoic acid, nonafluoro-3,6-dioxaheptanoic acid, 9-chlorohexadecafluoro-3-oxanonane-1-sulfonic acid, 11-chloroeicosafluoro-3-oxaundecane-1-sulfonic acid, perfluoro(2-ethoxyethane)sulfonic acid, 3-perfluoropropylpropanoic acid, 2H,2H,3H,3H-perfluorooctanoic acid, 3-perfluoroheptylpropanoic acid, bis(1H,1H,2H,2H-perfluorodecyl)phosphate, and derivatives thereof such as salts thereof.The liquid to be treated by the method of this embodiment may contain one of these compounds alone or two or more of them.

[0063] In the method of this embodiment, the perfluoroalkyl compounds contained in the liquid to be treated are electrostatically adsorbed and captured by the positive electrode as they pass between the positive and negative electrodes of the flow-through capacitor. When a large amount of the perfluoroalkyl compounds is adsorbed to the first positive electrode (the first positive electrode in the capacitor), the supply of the liquid to be treated is stopped, the voltage applied to the flow-through capacitor is stopped, and the capacitor is discharged, allowing the concentrated perfluoroalkyl compounds to be recovered.

[0064] Furthermore, in the removal method of this embodiment, a so-called pretreatment may be performed to remove compounds other than perfluoroalkyl compounds (also referred to as "non-perfluoroalkyl compounds") from the liquid to be treated before passing the liquid through the flow-through capacitor, thereby preventing a decrease in the adsorptive power of the flow-through capacitor due to the adhesion of non-perfluoroalkyl compounds.

[0065] In the pretreatment, non-perfluoroalkyl compounds may be removed using, for example, a filter, such as a filter using an adsorbent such as activated carbon, silica, or alumina, a filter using fibers such as cellulose or cotton, or a membrane filter.

[0066] Non-perfluoroalkyl compounds removed by such pretreatment include, but are not limited to, substances that are electrically neutral or whose ionic moieties do not significantly affect the properties of the substance. Substances that cause electrode clogging and lead to a decrease in the specific surface area of ​​the electrode can also be targets for removal. More specifically, the non-perfluoroalkyl compounds in this embodiment include, for example, microorganisms in water such as bacteria and algae, solid suspended matter in water, microscopic droplets such as emulsions, and organic components such as fulvic acid, humic acid, proteins, and amino acids.

[0067] According to the method for removing perfluoroalkyl compounds using a flow-through capacitor of the present embodiment as described above, it is possible to perform a perfluoroalkyl compound removal treatment on a liquid to be treated, such as water containing perfluoroalkyl compounds. According to the method of the present embodiment, even when the flow rate of the liquid to be treated is high, the liquid to be treated can be sufficiently treated to remove perfluoroalkyl compounds, and the durability of the flow-through capacitor is also high. Therefore, according to the method of the present embodiment, even when the liquid to be treated is water or the like containing dissolved solids of perfluoroalkyl compounds at a high concentration, the perfluoroalkyl compounds can be sufficiently removed from the liquid to be treated, and long-term treatment is possible. Therefore, the method of the present embodiment is practical and useful for industrial applications.

[0068] This specification discloses various aspects of the technology as described above, but the main technologies among them are summarized below.

[0069] That is, a first aspect of the present invention is a method for removing perfluoroalkyl compounds from a liquid to be treated, which comprises passing the liquid to be treated through a liquid-flow capacitor including electrodes, a highly corrosion-resistant current collector, and a separator, and causing the perfluoroalkyl compounds to be adsorbed onto the electrodes, and the electrodes have a BET specific surface area of ​​800 m ２ / g or more of a conductive material.

[0070] A method for removing perfluoroalkyl compounds according to a second aspect of the present invention is the method for removing perfluoroalkyl compounds according to the first aspect, in which the conductive material is activated carbon.

[0071] A method for removing perfluoroalkyl compounds according to a third aspect of the present invention is the method for removing perfluoroalkyl compounds according to the second aspect, in which the activated carbon is fibrous activated carbon.

[0072] A method for removing a perfluoroalkyl compound according to a fourth aspect of the present invention is the method according to any one of the first to third aspects, wherein the porosity of the electrode is 10% or more.

[0073] A fifth aspect of the present invention relates to a method for removing a perfluoroalkyl compound, which is the method according to any one of the first to fourth aspects, wherein the flow-through capacitor further comprises an anion exchange membrane and a cation exchange membrane.

[0074] A method for removing perfluoroalkyl compounds according to a sixth aspect of the present invention is the method for removing perfluoroalkyl compounds according to any one of the first to fifth aspects, which comprises removing compounds other than perfluoroalkyl compounds from the liquid to be treated before passing the liquid through the liquid-flow capacitor.

[0075] The method for removing perfluoroalkyl compounds according to the seventh aspect of the present invention is the method for removing perfluoroalkyl compounds according to the sixth aspect, in which a filter is used to remove compounds other than perfluoroalkyl compounds.

[0076] A liquid-flow capacitor according to an eighth aspect of the present invention comprises electrodes, a highly corrosion-resistant current collector, and a separator, wherein the electrodes have a BET specific surface area of ​​800 m ２ / g or more of a conductive material.

[0077] A ninth aspect of the present invention is a flow-through capacitor according to the eighth aspect, in which the conductive material is activated carbon.

[0078] A tenth aspect of the present invention is a flow-through capacitor according to the ninth aspect, in which the activated carbon is fibrous activated carbon.

[0079] A liquid-flow capacitor according to an eleventh aspect of the present invention is the liquid-flow capacitor according to any one of the eighth to tenth aspects, wherein the porosity of the electrode is 10% or more.

[0080] A flow-through capacitor according to a twelfth aspect of the present invention is the flow-through capacitor according to any one of the eighth to eleventh aspects, further comprising an anion exchange membrane and a cation exchange membrane.

[0081] A flow-through capacitor according to a thirteenth aspect of the present invention is the flow-through capacitor according to any one of the eighth to twelfth aspects, which is used for removing perfluoroalkyl compounds.

[0082] A flow-through capacitor according to a fourteenth aspect of the present invention is the flow-through capacitor according to the thirteenth aspect, further comprising a mechanism for removing compounds other than perfluoroalkyl compounds.

[0083] A flow-through capacitor according to a fifteenth aspect of the present invention is the flow-through capacitor according to the fourteenth aspect, in which the mechanism is a filter.

[0084] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0085] First, the test methods for evaluating the properties in the present example will be described.

[0086] [Measurement of Nitrogen Adsorption Isotherm] Using a specific surface area / pore distribution measuring device (BELL Japan Co., Ltd., "BELL Sorp Mini"), the amount of nitrogen adsorbed to the conductive material at liquid nitrogen temperature (77 K) was measured as follows. The conductive material was pulverized to a particle size of approximately 3 to 10 μm and filled into a sample tube. The sample tube was cooled to -196°C, and the pressure was reduced once. Nitrogen (purity 99.999%) was then adsorbed into the conductive material at a desired relative pressure. The amount of nitrogen adsorbed by the sample when equilibrium pressure was reached at each desired relative pressure was defined as the amount of adsorbed gas v.

[0087] [Measurement of BET specific surface area] From the obtained amount of adsorbed gas v, vm was determined by the three-point method using nitrogen adsorption at liquid nitrogen temperature, using an approximate formula derived from the BET formula shown below.

[0088]

[0089] In the formula, vm is the amount of gas adsorbed (cm) required to form a monolayer on the sample surface. ３ / g), v is the measured amount of adsorbed gas (cm ３ / g), p0 is the saturated vapor pressure, p is the absolute pressure, and c is a constant (reflecting the heat of adsorption).

[0090] Next, the specific surface area (m) of the sample was calculated from the calculated vm using the following formula: ２ / g) was calculated.

[0091]

[0092] In the formula, vm is the amount of adsorbed gas (cm) required to form a monolayer on the sample surface. ３ / g), N is Avogadro's number 6.022 × 10 ２３ , a is the area occupied by the adsorbate molecule on the sample surface (molecular occupied cross-sectional area) (nm２ )

[0093] [Porosity] The electrode was cut into a 10 mm × 10 mm piece, and the thickness was measured with a micrometer. The volume (cc) and weight (g) of the electrode were measured. The porosity was calculated using the following formula, assuming that the true helium density of the activated carbon was 2.0 g / cc and that of the polytetrafluoroethylene was 2.15 g / cc.

[0094] Porosity (%)={1−(weight (g) / volume of electrode (cc)) / weighted average of true helium densities of materials constituting the electrode based on blending amount (g / cc)}×100

[0095] Example 1 [Fabrication of a Liquid-Filled Capacitor] A conductive material having a median particle diameter of 6 μm and a specific surface area of ​​1700 m ２ An activated carbon electrode A1 containing 100 parts by mass of activated carbon (activated carbon made from coconut shells, "YP-50F" manufactured by Kuraray Co., Ltd.) with a molecular weight of 1000 / g and 10 parts by mass of polytetrafluoroethylene binder was used. Electrode A1 was prepared by mixing 100 parts by mass of the activated carbon particles and 10 parts by mass of the binder in water to obtain a slurry, which was then filled into a mold and molded into a sheet by reducing the pressure. The obtained activated carbon electrode A1 had a thickness of 250 μm and an electrode porosity of 18%. Electrode A1 was cut to a size of 100 mm long x 100 mm wide before use.

[0096] The current collector was a 250 μm thick graphite sheet ("SIGRAFLEX S GRAPHITE FOIL" manufactured by SGL Carbon Japan Co., Ltd.) formed by compression molding expanded graphite. This graphite sheet had a square shape measuring 100 mm long and 100 mm wide, and had two rectangular tabs measuring 50 mm long and 30 mm wide, evenly spaced along the center of one side. Furthermore, a 6.5 mm diameter hole was provided in the center of each tab to allow passage of a fastening bolt for fastening the laminate.

[0097] The separator used was a polyester resin net ("LX60SS" manufactured by Nippon Tokushu Orimono Co., Ltd.) with a thickness of 93 μm, a wire diameter of 55 μm, a mesh size of 368 μm, and an opening ratio of 76%. The separator had a size of 108 mm long x 108 mm wide.

[0098] The activated carbon electrode, current collector, and separator were stacked to form a laminate. Specifically, as shown in Figure 1, a current collector 1, an activated carbon electrode 2, a separator 3, an activated carbon electrode 4, and a current collector 5 were stacked to form a liquid-flow capacitor (1 cell) 10 having a one-cell capacitor structure. In the liquid-flow capacitor 10, each component overlapped in an area of ​​100 mm length x 100 mm width, and the tabs 6 of each current collector were arranged so that the tabs of the closest layers sandwiching the separator 3 faced in opposite directions.

[0099] Next, as shown in Figures 2 and 3 (cross-sectional view of Figure 2), three liquid-flow capacitors (single cell) 10 were stacked to obtain a three-cell liquid-flow capacitor 20. In the liquid-flow capacitor (multiple cells) 20, each of the liquid-flow capacitors (single cell) 10 was fixed in place by fastening together multiple overlapping tabs on each side with two titanium bolts 8 and nuts.

[0100] The obtained liquid-flow capacitor (multiple cells) 20 was housed in a resin container 11. The container 11 was a rectangular parallelepiped with internal dimensions of 210 mm length, 110 mm width, and 50 mm height, and was equipped with a liquid inlet having a diameter of 15 mm and a liquid outlet having a diameter of 15 mm. Furthermore, as shown in FIG. 4 , two electrode terminals 9, 9′ were disposed on the top surface of the container 11 and electrically connected to titanium bolts 8 used to secure the liquid-flow capacitor 20 housed in the container. The liquid-flow capacitor 20 was hermetically housed in the container 11, thereby obtaining a perfluoroalkyl compound-containing liquid treatment device using the liquid-flow capacitor 20. The negative and positive sides of a DC power supply were connected to the externally exposed terminals 9, 9′, respectively.

[0101] 1 L of a simulated perfluoroalkyl compound-containing solution (PFOA-containing water 100 ng / L) was circulated through the obtained device at a flow rate of 250 mL / min, and a continuous liquid flow test was performed under constant current control with an upper limit voltage of 3.0 V and a set current of 6.0 A during adsorption, and under constant current / constant voltage control with an upper limit voltage of 1.5 V and a set current of 8.0 A during desorption.

[0102] (Example 2) The conductive material was activated carbon felt (manufactured by Kuraray Co., Ltd., thickness 1 mm, specific surface area 1150 m ２A liquid-passing capacitor was fabricated using electrodes having a molecular weight of 1.5g / g and an electrode porosity of 78%), and a continuous liquid-passing test of a simulated perfluoroalkyl compound-containing solution was carried out in the same manner as in Example 1, except that the upper limit voltage during adsorption was changed to 4.0 V.

[0103] (Example 3) As a conductive substance, a powder having a median particle diameter of 6 μm and a specific surface area of ​​1700 m ２ An activated carbon electrode A1 containing 100 parts by mass of activated carbon (activated carbon made from coconut shells, "YP-50F" manufactured by Kuraray Co., Ltd.) with a molecular weight of 1000 / g, 4 parts by mass of conductive carbon black ("Super-P (registered trademark)" manufactured by TIMCAL), and 6 parts by mass of polytetrafluoroethylene binder was used. Electrode A1 was prepared by mixing 100 parts by mass of the activated carbon particles and 10 parts by mass of the binder in water to obtain a slurry, which was then filled into a mold and molded into a sheet by reducing the pressure. The obtained activated carbon electrode A1 had a thickness of 220 μm and an electrode porosity of 8%. Electrode A1 was cut to a size of 100 mm long x 100 mm wide before use.

[0104] Comparative Example 1 A flow-through capacitor was fabricated in the same manner as in Example 1, except that the current collector was made of aluminum foil, and a continuous flow-through test of a simulated perfluoroalkyl compound-containing solution was carried out.

[0105] (Comparative Example 2) As the conductive material, 5 μm graphite particles (specific surface area 2 m) were used instead of activated carbon. ２ A continuous liquid-passing test of a simulated perfluoroalkyl compound-containing solution was carried out in the same manner as in Example 1, except that an electrode having an electrode porosity of 8% was prepared using a cellulose acylate polymer (cellulose acetate / cellulose acetate stearate).

[0106] <Evaluation Method> [PFOA Adsorption and Desorption Amounts] In the Examples and Comparative Examples, continuous liquid flow tests were carried out, and the passing liquid was sampled 10 minutes, 30 minutes, and 60 minutes after the start of liquid circulation under the above adsorption conditions, and the amount of PFOA (ng / L) in the treated liquid was measured using an HPLC MS / MS device. Thereafter, desorption was carried out under the above conditions, and after 30 minutes of passing the liquid for desorption, sampling was again carried out, and the amount of PFOA (ng / L) in the treated liquid was measured using an HPLC MS / MS device.

[0107] The results are shown in Table 1.

[0108]

[0109] (Discussion) As is clear from the results in Table 1, the method using the liquid-flow capacitor of this embodiment (Examples 1 to 3) showed excellent PFOA removal ability. In addition, desorption was also possible without any problems, and it was confirmed that the capacitor had excellent durability. In particular, ２ / g or more, and it was also confirmed that in Examples 1 and 2, in which a conductive material having a porosity of 10% or more was used, extremely excellent results were obtained.

[0110] On the other hand, in Comparative Example 1, in which a highly corrosion-resistant current collector was not used as the current collector, the electrode was corroded by PFOA, and sufficient adsorption could not be achieved. Also, in Comparative Example 2, in which a conductive material with a low specific surface area was used as the electrode, the adsorption performance of PFOA was poor.

[0111] This application is based on Japanese Patent Application No. 2024-047863 filed on March 25, 2024, the contents of which are incorporated herein by reference.

[0112] In order to express the present invention, the present invention has been properly and sufficiently described above through embodiments with reference to specific examples, etc. However, it should be recognized that those skilled in the art can easily change and / or improve the above-described embodiments. Therefore, unless changes or improvements made by those skilled in the art deviate from the scope of the claims set forth in the claims, such changes or improvements are construed as being encompassed within the scope of the claims.

[0113] The present invention has wide industrial applicability in the technical field relating to the removal of organic compounds such as perfluoroalkyl compounds.

Claims

1. A method for removing perfluoroalkyl compounds from a liquid to be treated, comprising passing the liquid to be treated through a liquid-flow capacitor having electrodes, a highly corrosion-resistant current collector, and a separator, and causing the perfluoroalkyl compounds to be adsorbed onto the electrodes, wherein the electrodes have a BET specific surface area of ​​800 m ２ / g or more of a conductive substance.

2. The removal method according to claim 1, wherein the conductive material is activated carbon.

3. The removal method according to claim 2, wherein the activated carbon is fibrous activated carbon.

4. The removal method according to claim 1, wherein the porosity of the electrode is 10% or more.

5. The removal method according to claim 1, wherein the flow-through capacitor further comprises an anion exchange membrane and a cation exchange membrane.

6. The removal method according to claim 1, further comprising removing compounds other than perfluoroalkyl compounds from the liquid to be treated before passing the liquid through the liquid-flow capacitor.

7. The removal method according to claim 6, wherein a filter is used to remove compounds other than perfluoroalkyl compounds.

8. A battery comprising an electrode, a highly corrosion-resistant current collector, and a separator, wherein the electrode has a BET specific surface area of ​​800 m ２ / g or more of a conductive material.

9. The liquid-flow capacitor according to claim 8, wherein the conductive material is activated carbon.

10. The liquid-flow capacitor according to claim 9, wherein the activated carbon is fibrous activated carbon.

11. The liquid-flow capacitor according to claim 8, wherein the porosity of the electrodes is 10% or more.

12. The flow-through capacitor according to claim 8, further comprising an anion exchange membrane and a cation exchange membrane.

13. The liquid-flow capacitor according to claim 8, which is used to remove perfluoroalkyl compounds.

14. The liquid-flow capacitor according to claim 13, further comprising a mechanism for removing compounds other than perfluoroalkyl compounds.

15. The flow-through capacitor of claim 14, wherein the mechanism is a filter.

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