Method for recovering organic fluorine compound

Abstract

The present invention recovers an organic fluorine compound safely and with high efficiency. In this method for recovering an organic fluorine compound from an aqueous phase in a two-liquid-phase system comprising the aqueous phase, which includes the organic fluorine compound, and a specific organic phase, the organic fluorine compound is brought into contact with an organic base or a pH-stable hydrophobic cation, and the organic base or the pH-stable hydrophobic cation that has leaked into the aqueous phase is removed while the organic fluorine compound is subjected to solvent extraction from the aqueous phase into the organic phase.

Description

Method for recovering organic fluorine compounds

[0001] The present invention relates to a method for recovering an organic fluorine compound contained in an aqueous phase by solvent extraction from the aqueous phase into an organic phase while concentrating the compound by bringing one or more organic bases having the properties of a Lewis base, a Brønsted base, or both, or a hydrophobic cation stable to pH change, added to an alkane-based solvent, into contact with the organic fluorine compound through a liquid-liquid interface to form an electrically neutral chemical species by a bond based on an acid-base reaction or an ion pair formation reaction, and recovering the organic base, the hydrophobic cation, or both of them that leaked into the aqueous phase during the solvent extraction by solvent extraction with an alkane-based solvent, and also relates to a method for recovering and concentrating the organic fluorine compound by back-extracting the organic fluorine compound from the organic phase into the aqueous phase while separating the organic base or the hydrophobic cation from the organic fluorine compound by bringing the organic phase containing the extracted organic fluorine compound into contact with an aqueous phase having a pH higher than that of the aqueous phase during the solvent extraction, an aqueous phase having an ionic strength higher than that of the aqueous phase during the solvent extraction, or an aqueous phase having a pH and an ionic strength higher than those of the aqueous phase during the solvent extraction, and recovering the organic base, the hydrophobic cation, or both of them that leaked into the aqueous phase during the back-extraction by solvent extraction with an alkane-based solvent.

[0002] A group of organic fluorine compounds called PFAS (perfluorinated alkyl substances) has excellent chemical properties such as heat resistance, chemical resistance, light resistance, biodegradation resistance, water / oil repellency, and insulation properties, and is used in many industrial fields such as fibers / clothing, cooking utensils, semiconductors, medical, automobiles, household appliances, construction, aviation, and food as surfactants, emulsifiers, fire extinguishing agents, coating agents, antifouling agents, food packaging agents, etching agents, photoresists, antireflection agents, refrigerants, lubricating oils, and separator materials for lithium-ion batteries.

[0003] On the other hand, PFAS are resilient substances that accumulate easily in living organisms and can travel long distances in the environment without being broken down. Some PFAS are suspected of having various health effects, including carcinogenicity, reproductive toxicity, thyroid hormone disruption, liver dysfunction, and effects on the immune system and blood cholesterol levels. Among them, PFOS (perfluorooctanesulfonic acid) and PFOA (perfluorooctanoic acid), which have eight carbon atoms and are the most widely used, are already banned or restricted internationally.

[0004] Technologies have been developed to remove PFAS (Pervasive Fatty Acids) primarily by adsorption, targeting raw water from water sources, river water, and tap water. Activated carbon is often used as the adsorbent (for example, Patent Document 1), but desorption of PFAS from activated carbon is difficult. Therefore, a method has been proposed (Patent Document 2) to remove PFAS and regenerate activated carbon using an alcohol-containing solution and a base-containing solution.

[0005] In addition to activated carbon, other methods have been proposed for using adsorbents, such as using polymers (for example, Patent Documents 3 and 4), using metal-organic structures (Patent Document 5), and using metal-inorganic compounds (Patent Document 6).

[0006] In addition to methods using adsorbents, other methods have been proposed, such as using ion exchange resins (anion exchangers) (Patent Document 7), using membranes or membrane-like filters (Patent Document 8), using microgels (Patent Document 9), and using aquatic algae (Patent Document 10).

[0007] However, conventional methods, including the adsorption described above, are suitable for removing low concentrations of PFAS, but are unsuitable for rapidly and effectively treating wastewater containing high concentrations of PFAS. When rapidly and effectively treating high concentrations of target substances in wet separation, liquid-liquid extraction (also called solvent extraction) is generally chosen. In solvent extraction, the target substance dissolved in water is extracted into a solvent that is immiscible with water (a solvent that forms a two-phase system with water).

[0008] Adsorption and absorption by solids such as adsorbents and ion exchange resins have a smaller capacity for recovering and removing target components compared to extraction into liquids (solvent extraction), and are also less rapid. The same applies to membrane separation. Furthermore, when the target substance is highly concentrated, adsorbents and membranes can be reused far less frequently than extractants used in solvent extraction, resulting in high consumable costs and the generation of large amounts of solid waste. Although adsorbents incorporating the functional groups of extractants have been developed, ion exchange resins, adsorbents, and membranes are based on macroscopic, rigid solids such as polymers and porous materials, and are therefore more susceptible to degradation compared to extractants, which are molecules themselves.

[0009] Thus, solvent extraction offers many advantages that wet separation techniques using solid agents (ion exchange, adsorption, membrane separation) do not possess. However, it also has fatal drawbacks, and for this reason, it has rarely been used in wastewater treatment until now. In other words, solvent extraction results in the contamination of wastewater with organic compounds such as solvents and extractants, leading to a significant environmental burden and making it unsuitable for wastewater treatment. In fact, solvent extraction is often cited as a prime example of an environmentally unfriendly technology. Moreover, the solvents (extraction solvents) chosen for the solvent extraction of highly polar PFAS are often harmful solvents such as ethyl acetate, toluene, and methyl butyl ether. On the other hand, saturated hydrocarbons (alkanes such as octane and hexane), which are nonpolar, chemically inert, almost insoluble in water, and have low toxicity, are considered unsuitable for the extraction of highly polar PFAS. Therefore, although solvent extraction may be used as a pretreatment for the rapid analysis of PFAS, there are no known examples of its use in purifying PFAS-containing wastewater (recovering and removing PFAS from wastewater). In other words, the use of solvent extraction for PFAS is currently limited to analytical applications.

[0010] On the other hand, a method has been proposed to remove PFAS with short carbon chains from wastewater by solvent extraction using a primary amine, secondary amine, or tertiary amine or quaternary ammonium salt as an extractant (Non-Patent Literature 1). In this document, various solvents (diluents referred to as cosolvents) for dissolving the amine or ammonium salt are also considered, and mesitylene and octanol are considered promising from the viewpoint of versatility. However, these solvents have high solubility in water compared to saturated hydrocarbons (alkane solvents), and mesitylene, in particular, which is a type of aromatic hydrocarbon, is undesirable from the viewpoint of toxicity and harmfulness. Among the solvents (diluents) considered in Non-Patent Literature 1, octane, an alkane solvent, is also included, but it is not particularly highlighted or recommended in this document. The reason for this can be inferred as follows.

[0011] Alkane solvents are nonpolar and chemically extremely inert, making them prone to forming molecular aggregates (micelles, reverse micelles, etc.) based on the self-organization of molecules and ions in a two-liquid-phase system. In other words, when alkane solvents such as octane contain surfactants, turbidity (gel formation) due to the formation of molecular aggregates is likely to occur. If a large amount of gel is formed and the turbidity is severe, the solvent extraction operation itself becomes difficult. Even slight turbidity (gel formation) means that if gel reaches the aqueous phase after treatment, the surfactants constituting the gel will leak into the wastewater. Since the amines and ammonium salts mentioned above are mostly surfactants, the main components of gels formed in alkane solvents are these extractants. In other words, since the gel is an aggregate of these extractants, the formation of the gel and its leakage into the aqueous phase is nothing more than the leakage of the extractants into the wastewater.

[0012] Compared to alkane solvents used as diluents, the aforementioned amines and ammonium salts have significantly greater impacts on the environment, ecosystems, and human health. Therefore, it is essential to thoroughly prevent the leakage of the amines or ammonium salts into wastewater due to gel formation.

[0013] It is necessary to avoid not only leakage as a gel, but also leakage in the form of dissolved molecules and ions in wastewater. For example, data shows that the solubility of trioctylamine, a representative tertiary amine, in water is 0.08659 ppm at 25°C. However, in the risk assessment of trioctylamine under the Chemical Substances Control Law, data indicates that the no-effect concentration (NOEC) for algae is less than 0.00078 ppm (0.78 ppb = 780 ppt), and the predicted no-effect concentration (PNEC) for aquatic organisms is less than 0.0000078 ppm (0.0078 ppb = 7.8 ppt). Therefore, a mechanism to sufficiently remove amines that leak into the aqueous phase due to contact with the organic phase during solvent extraction is essential. Furthermore, when selecting solvents used as diluents, it is necessary to select solvents that do not leak into the aqueous phase (wastewater) as much as possible.

[0014] Leakage of molecules and ions used to recover PFAS must be considered not only in solvent extraction but also in methods using solids such as ion exchange resins and adsorbents. This is because if a solid agent (material) incorporating functional groups with the same structure as the extractant deteriorates, the functional group components will leak into water, just as in the case of solvent extraction.

[0015] Furthermore, it must be noted that flammable solvents are typically used in solvent extraction. Especially when dealing with large volumes of wastewater requiring rapid treatment, a significant amount of organic phase is inevitably needed. Since this increases the amount of diluent used, a diluent with a lower flash point should be selected to safely recover PFAS.

[0016] While non-flammable solvents (diluents) exist, almost all of them are toxic, harmful, or have a high environmental impact, and many are highly toxic, harmful, and environmentally harmful. Therefore, the use of non-flammable solvents should be restricted. Furthermore, even ionic liquids, which are said to be highly environmentally friendly, inevitably leak their constituent components into wastewater when used in solvent extraction due to the principles of ion exchange. In addition, their high cost makes them economically unfeasible.

[0017] Japanese Patent Publication No. 2022-93398, Japanese Patent Publication No. 2022-526919, Japanese Patent Publication No. 2011-25102, Japanese Patent Publication No. 2012-101159, Japanese Patent Publication No. 2021-137805, Japanese Patent Publication No. 2022-526606, Japanese Patent Publication No. 2019-511363, Japanese Patent Publication No. 2023-521446, Japanese Patent Publication No. 2014-231056, Japanese Patent Publication No. 2009-22887

[0018] Preprint (pre-peer-review paper): Liquid-Liquid Extraction via Hydrophobic Ion-Pailing Between 2 Ultrashort-Chain Perfluoroalkyl Substances and Ammonium Salts, Social Science Research Network (SSRN), ELSEVIER, 18 Nov 2024.

[0019] Solvent extraction offers significantly larger capacity and faster processing for the recovery and removal of PFAS compared to adsorption / absorption to solids (adsorbents, ion exchange resins, etc.) or membrane separation. Furthermore, the extractants used in solvent extraction can be reused far more frequently than adsorbents, ion exchange resins, or membranes, resulting in extremely low consumable costs. Consequently, the amount of waste generated is also significantly lower. On the other hand, solvent extraction has a significant environmental impact, such as contaminating the water environment with oil in the wastewater. Therefore, currently, the use of solvent extraction for PFAS is limited to analytical applications rather than wastewater purification. Moreover, saturated hydrocarbons (alkanes), which have minimal biological impact, are considered unsuitable as extraction solvents for PFAS; therefore, in many cases, harmful solvents such as ethyl acetate, toluene, and methyl butyl ether are selected as extraction solvents.

[0020] Carboxylic acid-based PFASs, if they have a sufficiently large number of carbon atoms (for example, PFOA with 8 carbon atoms), can be extracted into pure alkane solvents by lowering the pH to make them electrically neutral (non-dissociated). However, those with a small number of carbon atoms cannot be extracted into alkane solvents alone, even in their molecular form. In such cases, since carboxylic acid-based PFASs are acidic substances, acid-base reactions with organic bases can be used for solvent extraction. Furthermore, at pH levels where organic bases (for example, amines) exist in the form of protonated cations, PFASs that exist as anions over a wide pH range, such as sulfonic acid-based PFASs, can be extracted as ion pairs in the solvent.

[0021] For example, amines such as trioctylamine are effective as extractants for solvent extraction of PFAS. Trioctylamine can be used directly as the organic phase, but its solubility in water is slightly less than 0.1 ppm (0.08659 ppm at 25°C), which is significantly higher than the no-effect concentration (NOEC) value for algae indicated in the risk assessment under the Chemical Substances Control Law (data shows 0.00078 ppm). Therefore, a method using an organic phase obtained by diluting amines with an alkane solvent can be considered. Alkane solvents have low toxicity and low solubility in water. In particular, alkane solvents with a large number of carbon atoms (for example, dodecane with 12 carbon atoms) have extremely low solubility in water and can be considered diluents with small biological and environmental impacts. As the alkane solvent, chain alkanes, cyclic alkanes, or mixtures thereof can be selected, and "chain" here includes both linear and side chains.

[0022] However, in chemically inert alkane solvents, the inertness itself leads to more pronounced interactions between solutes, and surfactants readily self-assemble to form molecular assemblies such as reverse micelles. At the same time, surfactants strongly oriented at the liquid-liquid interface between the aqueous phase and the alkane solvent phase (organic phase) are also utilized for micelle formation in the aqueous phase, which easily leads to the formation of gel-like solids and causes turbidity.

[0023] As amines effective as extractants for PFAS, long-chain amines with high hydrophobicity are usually used (for example, the trioctylamine mentioned above). Many long-chain amines have strong surface activity and readily gel in alkane solvents. The transfer of the gel to the aqueous phase means that the long-chain amine itself leaks into the wastewater.

[0024] Furthermore, even if gelation and turbidity do not occur, the concentration of amines that migrate from the alkane solvent phase (organic phase) to the aqueous phase often exceeds the indicators for environmental risk assessment (for example, NOEC, values ​​related to aquatic environmental toxicity, etc.), requiring a mechanism to thoroughly recover and remove amines that have leaked into the aqueous phase (wastewater after solvent extraction treatment).

[0025] When using organic bases such as amines as extractants for PFAS, solvent extraction (forward extraction) and back extraction are controlled by utilizing pH changes. However, hydrophobic cations that are stable to pH changes are also effective as PFAS extractants. For example, quaternary ammonium ions maintain their hydrophobic cation form regardless of pH. Therefore, they act as counterions to PFAS that exist as anions after dissociating hydrogen ions, without adjusting the pH of the wastewater, allowing PFAS to be solvent extracted into the organic phase as an ion pair. For example, this is effective for the recovery and concentration of sulfonic acid-based PFAS that exist in anionic form over a wide pH range. Furthermore, in back extraction, if the back extract solution, which has a higher ionic strength than that used during solvent extraction (forward extraction), is used as the aqueous phase, the extracted PFAS can be recovered and concentrated from the organic phase into the aqueous phase.

[0026] On the other hand, long-chain quaternary ammonium ions, used as extractants, are typical surfactants, and in a two-phase system with alkane solvents, gelation and strong turbidity are almost inevitable. Furthermore, because they are stable cations, they have high solubility in water at all pH levels, even in their elemental form (without gelation). Therefore, a mechanism is needed to thoroughly recover and remove quaternary ammonium ions that leak into the aqueous phase (wastewater).

[0027] The inventors, taking advantage of the benefits of solvent extraction, diligently studied to solve the above problems and found that by adding a Lewis base or a Brønsted base, or one or more organic bases having both properties, or a pH-stable hydrophobic cation, to an alkane solvent and bringing it into contact with an organofluorine compound contained in the aqueous phase via a liquid-liquid interface, and causing it to become an electrically neutral chemical species through bonding based on an acid-base reaction or ion-pair formation reaction, the organofluorine compound is recovered and concentrated by solvent extraction from the aqueous phase to the organic phase, while the organic base, hydrophobic cation, or both that leaked into the aqueous phase during solvent extraction are recovered by solvent extraction with an alkane solvent. We have devised a method for recovering the organic base or hydrophobic cation by back-extracting and concentrating the organic base or hydrophobic cation that leaked into the aqueous phase during back-extraction, while recovering and concentrating the organic base or hydrophobic cation that leaked into the aqueous phase during back-extraction, or both, using solvent extraction with an alkane solvent.

[0028] The organic bases mentioned above function, so to speak, as extractants for PFAS. When the organic base is a Lewis base, the combination of PFAS and the organic base can be expressed as B:PFAS, where B: represents the Lewis base. In other words, PFAS is bonded via an electron pair donated by the Lewis base. On the other hand, when the organic base is a Brønsted base, the combination of PFAS and the organic base is BH ＋ PFAS － This can be expressed as follows: BH ＋This represents a protonated Brønsted base. That is, PFAS exists as an anion with a dissociated hydrogen ion, and binds by forming an ion pair with a Brønsted base that has accepted a hydrogen ion and become a cation. Since the generation of chemical species based on such acid-base reactions can be controlled by pH, solvent extraction (forward extraction) and back extraction of PFAS can be repeatedly performed by changing the pH. Note that an organic base may have both a site that functions as a Lewis base (electron pair donor) and a site that functions as a blended base (hydrogen ion acceptor) within a single molecule.

[0029] Specific examples of Lewis bases effective as extractants for PFAS include, but are not limited to, alcohols, ketones, ethers, esters (including carboxylic acid esters, thioesters, phosphate esters, phosphite esters, phosphonic acid esters, carbonate esters, etc.), amine oxides, sulfoxides, phosphine oxides, and organophosphines. Similarly, specific examples of Brønsted bases effective as extractants for PFAS include, but are not limited to, amines. Amines that can be used include aliphatic amines (primary, secondary, or tertiary amines), aromatic amines, heterocyclic amines, and mixtures thereof. The amines listed here also include those with heteroatoms in their substituents, or those with unsaturated bonds, or both. In other words, the skeletal structure of amines other than the amine itself is arbitrary.

[0030] Another effective method involves using a mixture of Lewis bases and Brønsted bases as organic bases. Many Brønsted bases, which are highly hydrophobic (have a large number of carbon atoms), exhibit strong surface activity when protonated to cations, such as amines like trioctylamine. If dissolved alone in an alkane solvent and used for solvent extraction, gelation (turbidity) is likely to occur due to the formation of molecular aggregates. On the other hand, amines with a small number of carbon atoms tend to leak (dissolve) into the aqueous phase (wastewater) and have low extraction capacity for PFAS. Therefore, by coexisting with electrically neutral (non-ionizing) Lewis bases, the electrostatic interactions that drive the formation of molecular aggregates are mitigated, and gelation (turbidity) is eliminated. If gelation (turbidity) is eliminated, the leakage of organic bases (extractants) into wastewater is also significantly reduced.

[0031] For PFAS with a small number of carbon atoms, in addition to the method using organic bases described above, a method using pH-stable hydrophobic cations can be used. When PFAS dissociates hydrogen ions to produce anions, the pH-stable hydrophobic cation acts as a counterion, forming an electrically neutral ion pair, which allows for extraction into a nonpolar alkane solvent. For example, quaternary ammonium ions are effective as pH-stable hydrophobic cations.

[0032] However, the quaternary ammonium ions that can be used in solvent extraction are limited to those with long chains, and therefore, many of them inevitably have strong surfactant properties. Consequently, in a two-phase system of alkane solvents and PFAS-containing wastewater, gelation (turbidity) due to the formation of aggregates of quaternary ammonium ions is likely to occur. In this case as well, electrically neutral Lewis bases are effective in resolving gelation (turbidity). The reason for this is the same as in the case of organic bases described above.

[0033] Regardless of whether gel (turbidity) occurs, extractants for PFAS, such as organic bases or pH-stable hydrophobic cations, should be considered to leak into wastewater at concentrations exceeding the environmental risk assessment indicators (NOEC, etc.). This is because the molecules or ions of the extractant themselves dissolve in water. For example, data shows that the solubility of trioctylamine in water is 0.08659 ppm at 25°C, but its no-effect concentration (NOEC) for algae is indicated as 0.00078 ppm in the risk assessment under the Chemical Substances Control Law, which is less than 1 / 100th of the solubility. Furthermore, data shows that the value for aquatic environmental toxicity (acute) is 0.0022 ppm at 72 hours EC50 (50% effective concentration) for the algae (Pseudokirchnerilla subcapitata), which is about 1 / 39th of the solubility.

[0034] Furthermore, Lewis bases can leak into wastewater not only when used alone as an extractant, but also when used in combination with Brønsted bases such as amines to prevent gelation (turbidity) when alkane solvents are used as diluents. For example, 1-octanol is effective as a gelation (turbidity) inhibitor when trioctylamine is used. While data shows that the solubility of 1-octanol in water is 460 ppm at 25°C, data also shows that its 48-hour EC50 (50% effective concentration) for crustaceans (Daphnia genus) as an aquatic environmental hazard (acute) is 4.2 ppm. This value is less than 1 / 100th of the solubility of 1-octanol in water.

[0035] As a result of diligent research, the inventors discovered that organic bases (Lewis bases, Brønsted bases, or both) and pH-stable hydrophobic cations leaked into wastewater can be fully recovered by solvent extraction using the alkane solvent diluent itself. In particular, alkanes with a large number of carbon atoms within a range that does not become too viscous (a range suitable for solvent extraction) are effective because they have an extremely small environmental impact.

[0036] Specifically, alkanes with 11 or more carbon atoms that have extremely low solubility in water are effective. Although alkane solvents have low toxicity, from the perspective of reducing environmental impact, it is necessary to minimize the amount of solvent remaining in the wastewater after solvent extraction. For example, data shows that the solubility of octane (8 carbon atoms) in water is 0.7 ppm, nonane (9 carbon atoms) is 0.2 ppm, decane (10 carbon atoms) is 0.052 ppm, undecane (11 carbon atoms) is 0.004 ppm, and dodecane (12 carbon atoms) is 0.0037 ppm. In addition, data shows that the acute aquatic environmental toxicity values, measured at 48 hours EC50 (50% effective concentration) in crustaceans (Daphnia magna), are 0.029 ppm for decane and 0.011 ppm for undecane. In other words, the impact on the aquatic environment is significantly reduced when using undecane (11 carbon atoms), whose solubility is below the value for acute aquatic environmental toxicity.

[0037] When selecting a diluent for the extractant used in solvent extraction of PFAS from wastewater, a safer option should be chosen, especially from the perspective of flammability. Octane, nonane, decane, undecane, and dodecane are all classified as flammable liquids (Class 4) under the Fire Service Act. Furthermore, in the classification of petroleum hazards based on flash point, octane is classified as Class 1 petroleum (flash point less than 21°C at 1 atmosphere), nonane as Class 2 petroleum (flash point between 21°C and 70°C at 1 atmosphere), and decane, undecane, and dodecane as Class 3 petroleum (flash point between 70°C and 200°C at 1 atmosphere). In other words, under the Fire Service Act, octane is classified as Class 4, Class 1 petroleum, nonane as Class 4, Class 2 petroleum, and decane, undecane, and dodecane as Class 4, Class 3 petroleum. Therefore, from the perspective of flammable substances, alkanes (decane or higher) with 10 or more carbon atoms are preferred as diluents.

[0038] In practice, when recovering PFAS from wastewater by solvent extraction, the choice of equipment is extremely important. To perform solvent extraction efficiently, the aqueous and organic phases must be mixed until an emulsion is formed. However, mixer-settlers, which achieve this, rely on gravity to naturally separate the phases, making them prone to contaminating wastewater with oil, which is undesirable from an environmental perspective. Furthermore, centrifugal extractors, which use centrifugal force to rapidly separate the phases from the emulsion, have a complex mechanism and low robustness against solid components. Also, because they require both centrifugal force and stirring force simultaneously, they have a high power load and are not suitable for large-scale processing with large-scale equipment. Using a column-type solvent extraction system (for example, a pulse column) allows for more reliable phase separation at the expense of phase mixing efficiency, but the low step efficiency necessitates a huge system.

[0039] In recent years, the emulsion flow system developed by the Japan Atomic Energy Agency (JAEA) is an innovative solvent extraction device that, by controlling the size of droplets and their vertical linear velocity, mixes the aqueous and organic phases with a step efficiency superior to that of a mixer-settler, while simultaneously achieving phase separation of the two phases in an extremely clear state. By utilizing emulsion flow for solvent extraction of PFAS from wastewater and back extraction of PFAS from the organic phase, it becomes possible to recover PFAS from wastewater with high efficiency while avoiding secondary environmental pollution by not introducing organic phase droplets (oil droplets) into the wastewater.

[0040] As an entirely different approach to avoid secondary environmental pollution, organic base molecules can be supported on silica gel, or sites that function as Lewis bases, Brønsted bases, or both can be bonded to a resin to be used as adsorbents. By incorporating organic bases or their base sites into the base material to function as adsorbents, gelation (clouding) associated with the formation of molecular aggregates can be avoided. However, unlike extractants, which are molecules themselves, adsorbents based on macroscopic, rigid solids are much more susceptible to degradation than extractants. Furthermore, there is a possibility that the organic base sites may leak out as the adsorbent degrades.

[0041] Similarly, a hydrophobic cationic site that is stable to pH changes can be supported on silica gel or bound to a resin and used as an adsorbent. However, its susceptibility to degradation and the possibility of leakage of the hydrophobic cationic site are inevitable.

[0042] Further, both an organic base molecule and a hydrophobic cation that is stable to pH changes can be supported on silica gel or bound to a resin and used as an adsorbent. However, similarly, its susceptibility to degradation and the possibility of leakage of the organic base site and the hydrophobic cationic site are inevitable.

[0043] PFAS recovered and concentrated by an adsorbent incorporating an organic base can be desorbed from the adsorbent by contacting it with an aqueous solution having a higher pH than at the time of adsorption. Further, PFAS recovered and concentrated by an adsorbent incorporating a hydrophobic cationic site that is stable to pH changes can be desorbed from the adsorbent by contacting it with an aqueous solution having a higher ionic strength than at the time of adsorption. Furthermore, PFAS recovered and concentrated by an adsorbent incorporating both an organic base and a hydrophobic cation that is stable to pH changes can be desorbed from the adsorbent by contacting it with an aqueous solution having a higher pH and ionic strength than at the time of adsorption.

[0044] Hereinafter, the present invention will be described in more detail. As specific PFAS, carboxylic acid-based PFAS and sulfonic acid-based PFAS are selected and described as representatives, but the scope of the present invention is not limited thereto.

[0045] When the PFAS to be recovered is carboxylic acid-based, if it has a large number of carbon atoms, without using an organic base, by reducing the pH of the aqueous phase (wastewater) to molecularize the carboxylic acid-based PFAS in the form of an acid molecule type, it can be solvent-extracted into an alkane-based solvent (only a diluent). Conversely, if the pH is increased to dissociate hydrogen ions into an anionic form, it can be back-extracted from the alkane-based solvent into the aqueous phase (back extract). For example, perfluorooctanoic acid (PFOA) with 8 carbon atoms has a sufficiently hydrophobic monomer in the acid molecule type, so its solvent extraction (forward extraction) and back extraction can be controlled by pH change without using an organic base or the like.

[0046] On the other hand, even among carboxylic acid-based PFAS, those with a small number of carbon atoms cannot achieve sufficient hydrophobicity (or organic affinity) as individual molecules, even when converted to a molecular form (acid molecular form), and therefore cannot be extracted using only alkane solvents. By adding an organic base that binds to the carboxylic acid-based PFAS, a compound is formed through an acid-base reaction or ion-pair formation reaction at the liquid-liquid interface, increasing the hydrophobicity (or organic affinity) compared to the individual carboxylic acid-based PFAS molecule. This makes solvent extraction into the organic phase possible even for those with a small number of carbon atoms.

[0047] For example, perfluorohexanoic acid (PFHxA), which has six carbon atoms, and perfluorobutanoic acid (PFBA), which has four carbon atoms, are extracted with low or no extraction at all using only alkane solvents, even when the pH is sufficiently low to convert them into acid molecules.

[0048] When solvent extraction of carboxylic acid-based PFASs with a small number of carbon atoms, such as PFHxA and PFBA, a sufficiently high extraction rate can be obtained by using an organic base that binds to these PFASs based on an acid-base reaction or ion-pair formation reaction as the extractant. Organic bases can be broadly classified into two types: Lewis bases (electron pair donors) and Brønsted bases (hydrogen ion acceptors). In addition, organic bases that have both a Lewis base site and a Brønsted base site within a single molecule can also be used as extractants.

[0049] Examples of Lewis bases include alcohols with a large number of carbon atoms, such as octanol, and examples of Brønsted bases include amines, but are not limited to these. Specific examples of amines include primary, secondary, and tertiary amines, as well as monoalkylamidoamines, dialkylamidoamines, trialkylamidoamines, and tetraalkyldiglycolamides, but are not limited to these.

[0050] Many amines possess strong surfactant properties. In particular, amines with long alkyl chains often exhibit surfactant activity. Amines with surfactant activity form molecular aggregates in a two-phase system consisting of an organic phase mainly composed of an alkane solvent and an aqueous phase, and the resulting gelation causes strong turbidity.

[0051] The strong surfactant properties of amines can cause gel formation, leading to leakage of extractant components during solvent extraction. To eliminate this gelation (turbidity), the presence of a Lewis base is effective. It is believed that the uncharged Lewis base buffers electrostatic interactions, inhibiting the self-assembly of surfactant cations generated from amines, thereby eliminating the gelation (turbidity). Specifically, alcohols with a large number of carbon atoms, such as 1-octanol and 2-ethylhexanol, are effective.

[0052] On the other hand, amines that incorporate a moiety containing an oxygen atom (heteroatom), such as amides, tend not to exhibit strong surface activity. Therefore, when using this type of amine, gelation (turbidity) is less likely to occur even when using alkane solvents as diluents, and it may not be necessary to include a Lewis base.

[0053] Carboxylic acid-based PFAS extracted into the organic phase can be back-extracted by contacting it with an aqueous phase (back-extract solution) having a higher pH than that of the solvent extraction (forward extraction), regardless of the presence or absence of an organic base.

[0054] Furthermore, the method using organic bases has the advantage of allowing solvent extraction (forward extraction) at a higher pH even for carboxylic acid-based PFAS with a large number of carbon atoms (e.g., PFOA). In other words, compared to the case where only alkane solvents are used as the organic phase, the amount of acid added to the wastewater and the amount of base required to neutralize it can be significantly reduced.

[0055] Furthermore, by placing an organic phase consisting solely of an alkane solvent at the final stage of solvent extraction (forward extraction) and the final stage of back extraction, and setting the pH of the wastewater to near neutral or basic, the entire amount of organic bases (Lewis bases or Brønsted bases, or both) leaked into the aqueous phase (wastewater) after treatment by solvent extraction into the alkane solvent can be recovered. Note that selecting an alkane solvent with 11 or more carbon atoms significantly reduces the environmental impact. However, removal of organic bases from the aqueous phase (back extract) in back extraction is not necessary if PFAS is concentrated and recovered and then incinerated. In other words, removal of organic bases from the back extract should be performed as needed.

[0056] The removal of organic bases from the back extract, as described above, is a necessary step, especially when PFAS is precipitated and recovered from the back extract for reuse, considering water recycling as well. For essential-use PFAS, closed-loop recycling within the factory with zero leakage into the environment is ideal. In that case, from the perspective of water recycling, it is desirable that organic bases have been removed from the back extract.

[0057] In addition to the method using organic bases described above, another method for recovering carboxylic acid-based PFAS with a small number of carbon atoms is to use a pH-stable hydrophobic cation. When carboxylic acid-based PFAS dissociates hydrogen ions, the resulting anion is counter-ion of the pH-stable hydrophobic cation, forming an electrically neutral ion pair, which allows for extraction into a nonpolar alkane solvent. For example, quaternary ammonium ions are effective as pH-stable hydrophobic cations. This method has the advantage of eliminating the need to adjust the pH of the wastewater being treated during solvent extraction (forward extraction).

[0058] Furthermore, in the back-extraction following solvent extraction (forward extraction) of carboxylic acid-based PFAS using pH-stable hydrophobic cations, it is necessary to use an aqueous phase with high ionic strength. Specifically, an aqueous electrolyte solution containing a relatively high concentration of inorganic salt or inorganic base is used as the back-extractant. However, acidic aqueous solutions that tend to convert carboxylic acid-based PFAS into molecularized forms (acid molecular form) may not be suitable as back-extractants, even if they have high ionic strength.

[0059] The back-extraction reaction is based on ion exchange across the liquid-liquid interface between the PFAS anion and the electrolyte anion. That is, the PFAS anion in the organic phase is back-extracted into the aqueous phase as the concentration of the electrolyte anion in the aqueous phase increases. Among the electrolyte anions, those with lower hydration energy are more effective; among inorganic ions, for example, perchlorate ions are more effective.

[0060] Seawater with a high electrolyte concentration can also be used as the aqueous phase with high ionic strength for the back-extract. If a PFAS recovery system is installed in a coastal area facing the sea, the use of seawater is considered feasible.

[0061] Furthermore, by placing an organic phase consisting solely of an alkane solvent at the final stage of solvent extraction (forward extraction) and the final stage of back extraction, and by setting the wastewater to an appropriate ionic strength, it is possible to recover the entire amount of pH-stable hydrophobic cations (e.g., quaternary ammonium ions) that leaked into the aqueous phase (wastewater) after treatment by solvent extraction into the alkane solvent. Note that selecting an alkane solvent with 11 or more carbon atoms can significantly reduce the environmental impact. However, removal of pH-stable hydrophobic cations from the aqueous phase (back extract) during back extraction is not necessary if the PFAS is concentrated and recovered for incineration or other treatment. In other words, removal of pH-stable hydrophobic cations from the back extract should be performed as needed.

[0062] The removal of pH-stable hydrophobic cations from the back extract, as described above, is a necessary step, especially when PFAS is precipitated and recovered from the back extract for reuse, considering water recycling as well. For essential-use PFAS, closed-loop recycling within the factory with zero leakage into the environment is ideal. In that case, from the perspective of water recycling, it is desirable that pH-stable hydrophobic cations have been removed from the back extract.

[0063] If the extraction rate of carboxylic acid-based PFAS in solvent extraction (forward extraction) is sufficiently high, the carboxylic acid-based PFAS can be recovered while being concentrated in the organic phase. If the back extraction rate in back extraction is sufficiently high, the carboxylic acid-based PFAS can be recovered while being concentrated in the aqueous phase (back extract). In this method, PFAS, not just carboxylic acid-based PFAS, can be concentrated in two stages: solvent extraction (forward extraction) and back extraction.

[0064] By using adsorbents incorporating organic bases, pH-stable hydrophobic cations, or both, it becomes possible to recover and concentrate carboxylic acid-based PFAS while suppressing the leakage of organic components into wastewater. Furthermore, similar to forward and back extraction in solvent extraction, the adsorption and desorption of carboxylic acid-based PFAS can be controlled by changes in pH, ionic strength, or both. However, the number of adsorption and desorption cycles using adsorbents is much shorter than the number of forward and back extraction cycles using extractants.

[0065] Next, if the PFAS to be recovered is sulfonic acid-based, the sulfonic acid group maintains an anionic form with dissociated hydrogen ions over an extremely wide pH range; therefore, the presence of hydrophobic cations is essential for its solvent extraction.

[0066] When using organic bases, Lewis bases alone cannot solvent extract sulfonic acid-based PFASs; instead, ion pairs (BH) with hydrophobic cations formed by the protonation of Brønsted bases are necessary. ＋ PFAS －Solvent extraction becomes possible only after the formation of ) . In the case of sulfonic acid-based PFAS, as in the case of carboxylic acid-based PFAS, amines as Brønsted bases function as effective extractants. That is, the protonated amines act as hydrophobic cations and become counterions of the sulfonic acid-based PFAS anions, forming ion pairs (BH ＋ PFAS － This generates a compound, and the sulfonic acid-based PFAS is extracted into the organic phase.

[0067] Hydrophobic cations that are stable to pH changes are also effective as extractants for sulfonic acid-based PFAS, which exist as anions over an extremely wide pH range.

[0068] Furthermore, in solvent extraction using alkane solvents as diluents, amines and pH-stable hydrophobic cations that function as extractants tend to gel (cause turbidity), and this can be suppressed by the presence of Lewis base-type organic bases. The same applies to the recovery of sulfonic acid-based PFAS, where the entire amount of organic bases leaked into the aqueous phase (wastewater) after treatment can be recovered by solvent extraction using only alkane solvents.

[0069] Whether recovering carboxylic acid-based PFAS or sulfonic acid-based PFAS, using emulsion flow as the solvent extraction apparatus prevents the organic phase droplets (oil droplets) from being mixed into the wastewater, thus avoiding secondary environmental pollution. Furthermore, from the perspective of avoiding secondary environmental pollution, adsorbents incorporating the same functional groups as the extractant are also effective.

[0070] Next, examples of the present invention will be shown, but the present invention is not limited to these examples.

[0071] Example 1 (Solvent Extraction Test of PFOA with Triamidoamine) Solvent extraction (forward extraction) of perfluorooctanoic acid (PFOA), a carboxylic acid-based PFAS (8 carbon atoms), was performed using hexaoctylnitrilotriacetamide (HONTA), a triamidoamine. An aqueous hydrochloric acid solution adjusted to pH 2.5 containing PFOA at a concentration of 1000 ppm was prepared as the aqueous phase, and n-dodecane containing 0.02 mol / L HONTA (organic phase) was prepared as the organic phase. Both phases were placed in equal volumes in a centrifuge tube (made of polypropylene, 50 mL) and shaken for 10 minutes using a vertical shaker. Then, centrifugation was performed using a centrifuge at 3000 rpm for 5 minutes. After centrifugation, the aqueous phase was collected, the concentration of PFOA was measured, and the extraction rate (forward extraction rate) of PFOA was calculated based on the result to be 94.6%.

[0072] Example 2 (Solvent Extraction Test of PFHxS with Diamidoamines) Solvent extraction (forward extraction) of perfluorohexanesulfonic acid (PFHxS), a sulfonic acid-based PFAS (6 carbon atoms), was performed using penta-2-ethylhexyldiamidoamine (PEHDAA), a diamidoamine. An aqueous hydrochloric acid solution adjusted to pH 1.0 containing PFHxS at a concentration of 1000 ppm was prepared as the aqueous phase, and n-dodecane containing 0.02 mol / L of PEHDAA (organic phase) was prepared as the organic phase. Both phases were placed in equal volumes in a centrifuge tube (made of polypropylene, 50 mL) and shaken for 10 minutes using a vertical shaker. Then, centrifugation was performed using a centrifuge at 3000 rpm for 5 minutes. After centrifugation, the aqueous phase was collected, and the concentration of PFHxS was measured. Based on the result, the extraction rate (forward extraction rate) of PFHxS was calculated to be 92.0%.

[0073] Example 3 (Solvent Extraction Test of PFBA with Triamidoamines) Solvent extraction (forward extraction) of perfluorobutanoic acid (PFBA), a carboxylic acid-based PFAS (4 carbon atoms), was performed using hexa-2-ethylhexylnitrilotriacetamide (HEHNTA), a triamidoamine. An aqueous hydrochloric acid solution adjusted to pH 2.1 containing PFBA at a concentration of 1000 ppm was prepared as the aqueous phase, and n-dodecane containing 0.02 mol / L HEHNTA (organic phase) was prepared as the organic phase. Both phases were placed in equal volumes in a centrifuge tube (made of polypropylene, 50 mL) and shaken for 10 minutes using a vertical shaker. Then, centrifugation was performed using a centrifuge at 3000 rpm for 5 minutes. After centrifugation, the aqueous phase was collected, and the concentration of PFBA was measured. Based on the result, the extraction rate (forward extraction rate) of PFBA was calculated to be 94.8%.

[0074] Example 4 (Solvent Extraction Test of PFBS with Diamidoamines) Solvent extraction (forward extraction) was performed on perfluorobutanesulfonic acid (PFBS), a sulfonic acid-based PFAS (4 carbon atoms), using penta-2-ethylhexyldiamidoamine (PEHDAA), a diamidoamine. An aqueous hydrochloric acid solution adjusted to pH 1.0 containing PFBS at a concentration of 1000 ppm was prepared as the aqueous phase, and n-dodecane containing 0.02 mol / L of PEHDAA (organic phase) was prepared as the organic phase. Both phases were placed in equal volumes in a centrifuge tube (made of polypropylene, 50 mL) and shaken for 10 minutes using a vertical shaker. Then, centrifugation was performed using a centrifuge at 3000 rpm for 5 minutes. After centrifugation, the aqueous phase was collected, the concentration of PFBS was measured, and the extraction rate (forward extraction rate) of PFBS was calculated based on the result to be 92.0%.

[0075] Example 5 (Recovery and Removal Test of Triamidoamines by n-Dodecane) In Example 1, solvent extraction (forward extraction) of triamidoamines using HONTA was performed on the aqueous phase, and solvent extraction was performed with n-dodecane alone. The total organic carbon concentration (TOC) was measured. As a result, it was below the detection limit (0.004 ppm). Similarly, in Example 3, solvent extraction (forward extraction) of triamidoamines using HEHNTA was performed on the aqueous phase, and solvent extraction was performed with n-dodecane alone. The total organic carbon concentration (TOC) was measured, and it was below the detection limit (0.004 ppm).

[0076] Example 6 (Recovery and removal test of diamidoamines using n-dodecane) Following solvent extraction (forward extraction) of diamidoamines using PEHDAA in Examples 2 and 4, solvent extraction was performed on the aqueous phase using n-dodecane alone, and the total organic carbon concentration (TOC) was measured. As a result, in all cases the TOC was below the detection limit (0.004 ppm).

[0077] Example 7 (Effect of Lewis base on resolving gelation and turbidity) To aqueous solutions containing PFOA, PFHxS, and PFBA at 50 ppb each, an equal volume of n-dodecane solution containing 0.02 mol / L trioctylamine or n-dodecane solution containing 0.02 mol / L tetraoctylammonium chloride was added to a centrifuge tube (made of polypropylene, 50 mL). When the tubes were shaken for 10 minutes using a vertical shaker, turbidity due to gelation occurred in both cases. On the other hand, the turbidity due to gelation was resolved by adding an appropriate amount of 1-octanol or ethylhexanol to the trioctylamine or tetraoctylammonium chloride in the n-dodecane solution.

[0078] Example 8 (Solvent Extraction Experiment of PFOA with Phosphine Oxides) Solvent extraction (forward extraction) of perfluorooctanoic acid (PFOA), a carboxylic acid-based PFAS (8 carbon atoms), was tested using trioctylphosphine oxide (TOPO), a phosphine oxide. Soil leachate was prepared as the aqueous phase, with pH adjusted to 2 using hydrochloric acid containing PFOA at a concentration of 50 ppb. N-dodecane containing 0.03 mol / L of TOPO was prepared as the organic phase. Both phases were placed in equal volumes in centrifuge tubes (made of polypropylene, 50 mL) and shaken for 10 minutes using a vertical shaker. Then, centrifugation was performed using a centrifuge at 3000 rpm for 5 minutes. After centrifugation, the aqueous phase was collected and the PFOA concentration was measured. Based on the results, the extraction rate (forward extraction rate) of PFOA was calculated to be 99.9%.

[0079] Example 9 (Solvent Extraction Experiment of HFPO-TA with Phosphine Oxides) Solvent extraction (forward extraction) of hexafluoropropylene oxide trimer carboxylic acid (HFPO-TA), a carboxylic acid-based PFAS (6 carbon atoms), was tested using trioctylphosphine oxide (TOPO), a phosphine oxide. Industrial wastewater, adjusted to pH 2 with hydrochloric acid containing HFPO-TA at a concentration of 32 ppm, was prepared as the aqueous phase, and n-dodecane containing 0.01 mol / L of TOPO was prepared as the organic phase. Both phases were placed in centrifuge tubes (made of polypropylene, 50 mL) in equal volumes and shaken for 10 minutes using a vertical shaker. Then, the mixture was centrifuged at 3000 rpm for 5 minutes using a centrifuge. After centrifugation, the aqueous phase was collected and the HFPO-TA concentration was measured. Based on the results, the extraction rate (positive extraction rate) of HFPO-TA was calculated to be 99.995%.

[0080] Example 10 (Adsorption experiment of PFOA with adsorbent immobilized with diamidoamines) An adsorption test was performed on perfluorooctanoic acid (PFOA), a carboxylic acid-based PFAS (8 carbon atoms), using an adsorbent with the following structure in which penta-2-ethylhexyldiamidoamine (PEHDAA), a diamidoamine, was immobilized on a resin. In a centrifuge tube (made of polypropylene, 50 mL), PEHDAA-immobilized resin was added to a pH 2 aqueous hydrochloric acid solution containing PFOA at a concentration of 1000 ppm to a concentration of 10 wt%, and the mixture was shaken for 10 minutes using a vertical shaker. After that, the mixture was centrifuged at 3000 rpm for 5 minutes using a centrifuge. After centrifugation, the liquid phase (aqueous phase) was collected and the PFOA concentration was measured, and the adsorption rate of PFOA was calculated based on the result to be 100%.

[0081]

[0082] Example 11 (Adsorption experiment of various PFAS with adsorbent immobilized with amines) Adsorption tests were conducted on perfluorooctanoic acid (PFOA), a carboxylic acid-based PFAS (8 carbon atoms), perfluorohexanesulfonic acid (PFHxS), a sulfonic acid-based PFAS (6 carbon atoms), and perfluorobutanesulfonic acid (PFBS), a sulfonic acid-based PFAS (4 carbon atoms), using an adsorbent with the following structure in which the amine diethylhexylamine (DEHA) was immobilized on a resin. In a centrifuge tube (made of polypropylene, 50 mL), DEHA-immobilized resin was added to a pH 2 aqueous hydrochloric acid solution containing PFOA, PFHxS, or PFBS at a concentration of 1000 ppm to a total concentration of 10 wt%, and the mixture was shaken for 10 minutes using a vertical shaker. After that, the mixture was centrifuged at 3000 rpm for 5 minutes using a centrifuge. After centrifugation, the liquid phase (aqueous phase) was collected and the concentrations of PFOA, PFHxS, or PFBS were measured. Based on these results, the adsorption rates of PFOA, PFHxS, or PFBS were calculated, and all were found to be 100%.

[0083]

[0084] The present invention relates to a method for effectively and efficiently recovering organofluorine compounds from factory wastewater containing organofluorine compounds at high or low concentrations. Here, "high concentration" refers to a concentration at which conventional methods such as activated carbon adsorption, ion exchange, and membrane separation cannot ensure economic rationality in terms of efficiency and cost, considering factors such as the volume of organofluorine compounds, the speed of treatment, and the reusability of consumables.

[0085] These perfluorinated compounds are widely used in various industries, but their significant impact on health and ecosystems is a cause for concern. Perfluorinated compounds are persistent, some are hazardous, but they remain stable in the environment for long periods and easily disperse over long distances. Therefore, in recent years, there has been a rapidly growing global movement to gradually restrict or phase out their use unless they are proven to be socially essential.

[0086] On the other hand, there are many cases where substitution is difficult for industrial or consumer applications. For so-called essential uses, even if their use is not discontinued, strict restrictions are placed on discharge from factories. In other words, technology capable of handling aqueous solutions containing high concentrations of perfluorinated compounds, such as factory wastewater, is essential. Furthermore, from the perspectives of resource circulation and carbon dioxide reduction, as well as from the perspectives of health and ecosystem impact, it is necessary to consider recycling perfluorinated compounds targeted for essential uses in a completely closed system without any discharge outside the system. This invention addresses these urgent industrial needs.

Claims

1. A method for recovering an organofluorine compound contained in an aqueous phase, comprising a two-liquid-phase system consisting of an aqueous phase which is an aqueous solution containing an organofluorine compound, and an organic phase which is an alkane solvent in the form of a chain, a cyclic or mixture thereof containing at least one Lewis base or Brønsted base or an organic base having the properties of both, the method comprising the step of solvent extraction of the organofluorine compound from the aqueous phase to the organic phase by bringing the organofluorine compound in the aqueous phase and the organic base in the organic phase into contact via the interface of the two liquid phases and causing them to become electrically neutral chemical species through bonding based on an acid-base reaction or ion pair formation reaction, wherein during the solvent extraction, the organic base dissolved in the aqueous phase is recovered using solvent extraction with an alkane solvent having 11 or more carbon atoms.

2. A method for recovering an organofluorine compound, characterized in that the Lewis base is present in the same location as the Brønsted base, thereby suppressing turbidity caused by gel formation in a two-liquid-phase system due to the surface-active properties of the protonation cation of the Brønsted base, while solvent-extracting the organofluorine compound from the aqueous phase to the organic phase.

3. A method for recovering an organofluorine compound according to claim 1 or 2, characterized in that the amount or flow rate of the organic phase relative to the aqueous phase is controlled to concentrate the organofluorine compound in the organic phase while performing solvent extraction.

4. A method for recovering an organofluorine compound, characterized in that the solvent extraction of the organofluorine compound is performed using an emulsion flow mechanism, according to any one of claims 1 to 3.

5. A method for recovering an organofluorine compound, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both, according to any one of claims 1 to 4.

6. A method for recovering an organofluorine compound contained in an aqueous phase, comprising a two-liquid-phase system consisting of an aqueous phase which is an aqueous solution containing an organofluorine compound and an organic phase which is an alkane solvent containing a linear, cyclic, or mixture thereof hydrophobic cation that is stable to pH changes, the method comprising the step of solvent extraction of the organofluorine compound from the aqueous phase to the organic phase by bringing the organofluorine compound in the aqueous phase and the hydrophobic cation that is stable to pH changes in the organic phase into contact via the two-liquid-phase interface and causing it to become an electrically neutral chemical species through bonding based on an ion pair formation reaction, and recovering the hydrophobic cation dissolved in the aqueous phase during the solvent extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

7. A method for recovering an organofluorine compound according to claim 6, characterized in that the organofluorine compound is solvent-extracted while suppressing turbidity caused by gel formation in a two-liquid-phase system due to the surface-active properties of the hydrophobic cation by coexisting the Lewis base with the pH-stable hydrophobic cation.

8. A method for recovering an organofluorine compound according to claim 6 or 7, characterized in that the organofluorine compound is concentrated in the organic phase while solvent extraction is performed by controlling the amount or flow rate of the organic phase relative to the aqueous phase.

9. A method for recovering an organofluorine compound, characterized in that the solvent extraction of the organofluorine compound is performed using an emulsion flow mechanism, according to any one of claims 6 to 8.

10. A method for recovering an organofluorine compound, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both, according to any one of claims 6 to 9.

11. A method for recovering an organofluorine compound, comprising the step of solvent extraction of an organofluorine compound from an aqueous phase, which is an aqueous solution containing the organofluorine compound, using an organic phase which is a linear, cyclic, or mixture thereof, alkane solvent containing a Lewis base or a Brønsted base or an organic base having the properties of both, and a pH-stable hydrophobic cation, and recovering the organic base and the hydrophobic cation dissolved in the aqueous phase during the solvent extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

12. A method for recovering an organofluorine compound according to claim 11, characterized in that the organofluorine compound is concentrated in the organic phase while solvent extraction is performed by controlling the amount or flow rate of the organic phase relative to the aqueous phase.

13. A method for recovering an organofluorine compound according to claim 11 or 12, characterized in that the solvent extraction of the organofluorine compound is performed using an emulsion flow mechanism.

14. A method for recovering an organofluorine compound, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both, according to any one of claims 11 to 13.

15. A method for recovering organofluorine compounds, characterized by using an organic base supported on silica gel or bonded to a resin as an adsorbent, thereby adsorbing organofluorine compounds in an aqueous solution onto the silica gel or resin.

16. A method for recovering an organofluorine compound according to claim 15, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

17. A method for recovering organofluorine compounds, characterized by using a hydrophobic cation that is stable to pH changes supported on silica gel or bonded to a resin as an adsorbent, thereby adsorbing organofluorine compounds in an aqueous solution onto the silica gel or resin.

18. A method for recovering an organofluorine compound according to claim 17, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

19. A method for recovering organofluorine compounds, characterized by using an adsorbent to which both an organic base and a pH-stable hydrophobic cation are supported on silica gel or bonded to a resin, thereby adsorbing organofluorine compounds in an aqueous solution onto the silica gel or resin.

20. A method for recovering an organofluorine compound according to claim 19, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

21. A method for recovering an organofluorine compound, comprising the step of back-extracting the organofluorine compound from the organic phase to the aqueous phase by contacting an organic phase containing an organofluorine compound solvent-extracted from an aqueous phase by the method of claim 1, 6, or 11 with an aqueous phase having a pH greater than the pH of the aqueous phase at the time of solvent extraction of the organofluorine compound, thereby separating the organic base, the hydrophobic cation, or both from the organofluorine compound and reducing it to an electronegative chemical species, and recovering the organic base, the hydrophobic cation, or both dissolved in the aqueous phase during the back-extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

22. A method for recovering an organofluorine compound according to claim 21, characterized in that the organofluorine compound is concentrated in the aqueous phase while being back-extracted by controlling the amount or flow rate of the aqueous phase relative to the organic phase.

23. A method for recovering an organofluorine compound, characterized in that the back extraction of the organofluorine compound is performed using an emulsion flow mechanism, according to claim 21.

24. A method for recovering an organofluorine compound, comprising the step of back-extracting the organofluorine compound from the organic phase to the aqueous phase by contacting an organic phase containing an organofluorine compound solvent-extracted from an aqueous phase by the method of claim 1, 6, or 11 with an aqueous phase having an ionic strength greater than the ionic strength of the aqueous phase at the time of solvent extraction of the organofluorine compound, thereby separating the organic base, the hydrophobic cation, or both from the organofluorine compound and reducing it to an electronegative chemical species, and recovering the organic base, the hydrophobic cation, or both dissolved in the aqueous phase during the back-extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

25. A method for recovering an organofluorine compound according to claim 24, characterized in that the organofluorine compound is concentrated in the aqueous phase while being back-extracted by controlling the amount or flow rate of the aqueous phase relative to the organic phase.

26. A method for recovering an organofluorine compound, characterized in that the back extraction of the organofluorine compound is performed using an emulsion flow mechanism, according to claim 24.

27. A method for recovering an organofluorine compound, comprising the step of back-extracting the organofluorine compound from the organic phase to the aqueous phase by contacting an organic phase containing an organofluorine compound solvent-extracted from an aqueous phase by the method of claim 1, 6, or 11 with an aqueous phase having a pH and ionic strength greater than the pH and ionic strength of the aqueous phase at the time of solvent extraction of the organofluorine compound, thereby separating the organic base, the hydrophobic cation, or both from the organofluorine compound and reducing it to an electronegative chemical species, and recovering the organic base, the hydrophobic cation, or both dissolved in the aqueous phase during the back-extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

28. A method for recovering an organofluorine compound according to claim 27, characterized in that the organofluorine compound is concentrated in the aqueous phase while being back-extracted by controlling the amount or flow rate of the aqueous phase relative to the organic phase.

29. A method for recovering an organofluorine compound, characterized in that the back extraction of the organofluorine compound is performed using an emulsion flow mechanism, according to claim 27.

30. A method for recovering an organofluorine compound, characterized in that an adsorbent on which an organofluorine compound has been adsorbed by the method described in claim 15 is brought into contact with an aqueous solution having a pH higher than that at the time of adsorption, thereby desorbing the organofluorine compound from the adsorbent.

31. A method for recovering an organofluorine compound according to claim 30, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

32. A method for recovering an organofluorine compound, characterized in that an adsorbent on which an organofluorine compound has been adsorbed by the method described in claim 17 is brought into contact with an aqueous solution having a greater ionic strength than that at the time of adsorption, thereby desorbing the organofluorine compound from the adsorbent.

33. A method for recovering an organofluorine compound according to claim 31, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

34. A method for recovering an organofluorine compound, characterized in that the organofluorine compound is desorbed from the adsorbent by contacting the adsorbent, which has adsorbed the organofluorine compound by the method described in claim 19, with an aqueous solution having a pH and ionic strength greater than that at the time of adsorption.

35. A method for recovering an organofluorine compound according to claim 32, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

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