Method for recovering organic fluorine compound

Abstract

The present invention is capable of safely and highly efficiently recovering an organic fluorine compound. Provided is a method for recovering an organic fluorine compound in an aqueous phase in a two-liquid phase system comprising an aqueous phase containing the organic fluorine compound and a specific organic phase, wherein the organic fluorine compound is brought into contact with a metal cation, an organic base, an organic ligand, or a combination thereof, and the organic base and the organic ligand leaking into the aqueous phase are removed while the organic fluorine compound is solvent-extracted from the aqueous phase into the organic phase.

Description

Method for recovering organofluorine compounds

[0001] The present invention relates to a two-liquid-phase system comprising an aqueous phase in which a metal cation is added to an aqueous solution containing an organofluorine compound, and an organic phase which is a linear, cyclic, or mixture of alkane solvents containing at least one hydrophobic organic compound that acts as an organic base that binds to the organofluorine compound, an organic ligand that coordinates to the metal cation, or both. Based on acid-base reactions, complex formation reactions, or ion pair formation reactions, or combinations thereof, that occur between the organofluorine compound, the metal cation, and the hydrophobic organic compound, the organofluorine compound is reduced to an electrically neutral chemical species, which is then solvent-extracted from the aqueous phase into the organic phase, and the hydrophobic organic compound that leaks into the aqueous phase during solvent extraction is extracted using an alkane solvent with 11 or more carbon atoms. The present invention relates to a method for recovery using solvent extraction with an alkane-based solvent, and a method for recovering organic compounds using solvent extraction with an alkane-based solvent, which is performed after the solvent extraction, by contacting the extracted organic phase containing the organofluorine compound with an aqueous phase having a higher pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower concentration of metal cations or not containing metal cations, or an aqueous phase containing a complexing agent which is a water-soluble organic ligand, or an aqueous phase which is an arbitrary combination thereof, thereby converting the organofluorine compound into an electronegative chemical species or a molecule with low hydrophobicity, and then back-extracting it from the organic phase to the aqueous phase, and recovering the hydrophobic organic compound that leaks into the aqueous phase during back-extraction using solvent extraction with an alkane-based solvent having 11 or more carbon atoms.

[0002] The group of organofluorine compounds known as PFAS possesses excellent chemical properties such as heat resistance, chemical resistance, light resistance, biodegradability resistance, water and oil repellency, and insulation properties. They are used in many industrial fields, including textiles and clothing, cooking utensils, semiconductors, medical equipment, automobiles, home appliances, construction, aerospace, and food, as surfactants, emulsifiers, fire extinguishing agents, coatings, antifouling agents, food packaging materials, etching agents, photoresists, anti-reflective agents, refrigerants, lubricants, and lithium-ion battery separator materials.

[0003] On the other hand, PFAS are persistent 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, PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanesulfonic acid), which have eight carbon atoms and are the most widely used, are already banned or restricted in international use.

[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 also lack the speed of processing. The same applies to membrane separation. Furthermore, when the target substance is highly concentrated, adsorbents and membranes have significantly fewer reuse cycles compared to 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 unenvironmentally friendly technology. Moreover, the solvents (extraction solvents) chosen for the solvent extraction of highly polar PFAS are often harmful solvents such as acetate ether, toluene, and methyl butyl ether. On the other hand, saturated hydrocarbons (octane, hexane, and other alkanes) that 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 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 procedure itself becomes difficult. Even slight turbidity (gel formation) means that the surfactants constituting the gel are leaking into the wastewater, as the gel is mixed into the aqueous phase after treatment. Since most long-chain amines and long-chain ammonium salts used in solvent extraction are 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, 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 amines or ammonium salts into wastewater due to gel formation.

[0013] It is necessary to avoid, as much as possible, not only leakage as a gel, but also leakage into wastewater in the form of dissolved molecules and ions. 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 upon contact with the organic phase during solvent extraction is essential. Furthermore, when selecting solvents to be 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. In particular, when the wastewater to be treated is large in volume and needs to be treated rapidly, a large amount of organic phase is inevitably required. Therefore, the amount of diluent used will also increase, and in order to safely recover and concentrate PFAS, a diluent with a lower flash point should be selected.

[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, when PFAS concentrations are high (specifically, above ppb levels), the extractants used in solvent extraction can be reused far more frequently than adsorbents, ion exchange resins, or membranes, resulting in extremely low consumables. Consequently, the amount of waste generated is also extremely low. 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 PFAS extraction solvents; 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 also 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 of 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 for the alkane solvent, linear alkanes, cyclic alkanes, or mixtures thereof can be selected, and "linear" here includes both straight chains 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] The amines effective as extractants for PFAS are usually highly hydrophobic long-chain amines (for example, the trioctylamine mentioned above). Many long-chain amines have strong surface activity and readily gel (become a highly viscous solid) in alkane solvents. The transition 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 (e.g., 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] The inventors, taking advantage of the benefits of solvent extraction, diligently studied to solve the above problems and have found a two-liquid-phase system consisting of an aqueous phase in which a metal cation is added to an aqueous solution containing an organofluorine compound, and an organic phase which is a linear, cyclic, or mixture of alkane solvents containing at least one hydrophobic organic compound that acts as an organic base that binds to the organofluorine compound, an organic ligand that coordinates to the metal cation, or both. Based on acid-base reactions, complex formation reactions, or ion pair formation reactions, or combinations thereof, that occur between the organofluorine compound, the metal cation, and the hydrophobic organic compound, the organofluorine compound is reduced to an electrically neutral chemical species, which is then solvent-extracted from the aqueous phase into the organic phase, and the hydrophobic compounds that leak into the aqueous phase during solvent extraction are also removed. We have devised a method for recovering organic compounds using solvent extraction with an alkane solvent having 11 or more carbon atoms, and a method for recovering organic compounds using solvent extraction. This method involves contacting the organic phase containing the extracted organofluorine compounds with an aqueous phase having a higher pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower concentration of metal cations or containing no metal cations, or an aqueous phase containing a complexing agent which is a water-soluble organic ligand, or an aqueous phase which is a combination of these, thereby converting the organofluorine compounds into electronegative chemical species or molecules with low hydrophobicity. These are then back-extracted from the organic phase into the aqueous phase, and the hydrophobic organic compounds that leak into the aqueous phase during back-extraction are recovered using solvent extraction with an alkane solvent having 11 or more carbon atoms. It has also been found that to recover organofluorine compounds while concentrating them during forward extraction, the amount or flow rate of the organic phase relative to the aqueous phase should be controlled (reduced), and to recover organofluorine compounds while concentrating them during back-extraction, the amount or flow rate of the aqueous phase relative to the organic phase should be controlled (reduced).

[0026] The above organic bases function 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. That is, 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 and ion-pair formation 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.

[0027] 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.

[0028] Furthermore, specific examples of Brønsted bases effective as extractants for PFAS include, but are not limited to, amines. Examples of 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, those with unsaturated bonds, or both. In other words, the skeletal structure of amines other than the amine is arbitrary.

[0029] Furthermore, the solvent extraction of PFAS can also be promoted by adding a metal cation to an aqueous solution containing PFAS. In one case, the chemical species formed by the PFAS coordinating with the metal cation is extracted from the solvent, and in another case, the electrically positive chemical species formed by the organic ligand molecules added to the organic phase coordinating with the metal ion becomes a counterion, and the anionic PFAS forms an ion pair and is extracted from the solvent.

[0030] For example, an electrically neutral chemical species generated by coordination of monovalent PFAS anions to n-valent metal cations (M(PFAS) ｎ ) is solvent-extracted into the organic phase. In addition, an electrically positive chemical species generated by coordination of m organic ligand molecules (denoted as L) to an n-valent metal cation (ML ｍ ｎ＋ ) acts as a counter ion, and a monovalent PFAS anion forms an ion pair (ML ｍ ｎ＋ (PFAS － ) ｎ ｎ－ ) that is solvent-extracted into the organic phase.

[0031] When both an organic ligand and a metal cation are added, they are used in an appropriate combination. Many compounds that act as organic ligands for metal cations also act as organic bases for PFAS. Therefore, the organic ligand and the organic base may be the same compound.

[0032] As specific metal cations, all metal ions that can exist as cations are effective. However, cations such as cadmium, mercury, and lead are extremely toxic, and even if added in very small amounts, they should not be used in the method of the present invention. Metal cations that hardly adversely affect the human body or the environment even in very small amounts, such as rare earths, iron, calcium, and aluminum, are selected.

[0033] In the method of the present invention, metal cations are added at a concentration exceeding the charge equivalent to anionic PFAS (ionized PFAS) contained in wastewater. Specifically, metal cations are added to the wastewater at about 10 times the equivalent of PFAS anions. For example, when the organic ligand is electrically neutral, the chemical species generated by the complexation reaction between the metal cation and the organic ligand is a metal complex cation that maintains the valence of the metal cation. Therefore, under conditions where the organic ligand is present in large excess relative to the metal cations (conditions under which all metal cations form complexes), the required concentration of metal cations only needs to be a concentration exceeding the equivalent relative to the PFAS anions.

[0034] In other words, if the concentration of PFAS to be recovered and concentrated is low, the concentration of the added metal cation can be lower accordingly. For example, if the concentration of PFAS in the wastewater is on the order of ppt or ppb, the concentration of the metal cation added to the wastewater can also be on the order of ppt or ppb. Note that the unit of equivalent is molar concentration (mol / L), so when comparing weight concentrations such as wt%, ppm, ppb, and ppt, the required weight concentration of the metal cation can be lower for PFAS, which has a larger molecular weight.

[0035] For example, if the concentration of PFAS to be recovered is on the order of ppt or ppb, it is sufficient to simply add a metal cation at the same order of concentration as the PFAS. Therefore, by adding only a very small amount (on the order of ppt or ppb) of metal cation, PFAS in wastewater can be recovered with a high extraction rate (for example, around 95% to 98%) even from wastewater with a pH of around 7, without requiring pH adjustment of the wastewater (aqueous phase).

[0036] Furthermore, by selecting a metal cation with a higher valency as the metal cation to be added to the wastewater, the equivalent amount relative to PFAS will be smaller, thus allowing for a reduction in the amount of metal cation added. Since many PFAS dissociate into monovalent anions through acid dissociation, for example, if a trivalent metal cation is used, the equivalent amount of the trivalent metal cation will be 1 / 3 of the molar concentration of the monovalent PFAS anion. Therefore, in this respect, polyvalent metal cations are more advantageous than monovalent metal cations.

[0037] When PFAS does not ionize but coordinates to a metal cation as a molecule, a hydrophobic cation is generated in which the positive charge number of the metal cation is maintained, and this cation is extracted into the organic phase along with an anion as a counterion. If the wastewater contains a high concentration of electrolytes, the anions of the electrolytes can act as counterions. Also, if another PFAS coexists as an anion, that PFAS anion becomes a counterion and is extracted into the organic phase together with the other PFAS. This makes it possible to simultaneously solvent extract, recover, and concentrate different types of PFAS.

[0038] In the method of the present invention, an alkane solvent (in particular, one having a large number of carbon atoms) is used as a diluent, which has extremely low solubility in water and small biological and environmental impacts. However, both organic bases that function as extractants for PFAS and organic ligands that generate hydrophobic cations that bind to metal cations and act as counterions for PFAS anions have high surface activity. Therefore, gel (cloudiness) caused by the formation of molecular assemblies is prone to occur in a two-liquid phase system consisting of a chemically inert alkane solvent and an aqueous phase. Leakage of gel (cloudiness) into wastewater is nothing but the outflow of hydrophobic organic compounds such as organic bases and organic ligands, which must be strictly controlled.

[0039] For example, many protonated Brønsted bases that act as counterions for PFAS anions exhibit surface activity and form gel (cloudiness) in alkane solvents. However, the coexistence of a Lewis base can alleviate or eliminate gelation. It is believed that this effect arises from Lewis bases mitigating the electrostatic interactions that act in molecular assemblies formed by protonated cations.

[0040] Metal cations also exhibit the same effect. It is considered that this effect arises because the metal complex cations act on the molecular assemblies of protonated cations, destabilizing the molecular assemblies or inhibiting molecular association.

[0041] In addition, the simultaneous use of both a Lewis base and a metal cation can in some cases more effectively suppress the generation of gel (cloudiness).

[0042] Regardless of whether gel (cloudiness) is generated, the organic base, organic ligand, or both contained in the organic phase should be considered to leak into wastewater at concentrations exceeding the indicators for environmental risk assessment (such as NOEC). This is because these molecules or ions themselves dissolve in water. For example, there is data showing that the solubility of trioctylamine, which functions as a Brønsted base, in water is 0.08659 ppm at 25°C, while its no-observed-effect concentration (NOEC) for algae is specified as 0.00078 ppm in risk assessment under the Chemical Substance Control Law, which is a value no more than 1 / 100 of its solubility. Additionally, there is data showing that the value for hazardousness to the aquatic environment (acute) is 0.0022 ppm as the 72-hour EC50 (50% effect concentration) for algae (*Pseudokirchneriella subcapitata*), which is approximately 1 / 39 of its solubility.

[0043] Furthermore, Lewis bases can leak into wastewater not only when used alone as an extractant, but also when used in coexistence with Brønsted bases such as amines for the purpose of preventing gelation (cloudiness) when an alkane-based solvent is used as the diluent. For example, 1-octanol is effective as a gelation (cloudiness) inhibitor when trioctylamine is used. There is data showing that the solubility of 1-octanol in water is 460 ppm at 25°C, while there is also data giving the 48-hour EC50 (50% effect concentration) for crustaceans of the genus *Daphnia* in terms of hazardousness to the aquatic environment (acute) as 4.2 ppm. This value is no more than 1 / 100 of the solubility of 1-octanol in water.

[0044] As a result of intensive studies, the inventor found that the entire amount of organic base, organic ligand, or both that have leaked into wastewater can be recovered by solvent extraction using the alkane-based solvent, that is, the diluent itself. In particular, alkanes with large carbon numbers within a range that does not result in excessively high viscosity (a range suitable for solvent extraction) are effective in that their environmental impact is extremely low.

[0045] 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. Furthermore, data on aquatic environmental toxicity (acute) shows that the 48-hour EC50 (50% effective concentration) value for crustaceans (Daphnia magna) is 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.

[0046] 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.

[0047] Furthermore, the alkane solvent from which hydrophobic organic compounds (organic bases, organic ligands, or both) have been recovered can be mixed with the organic phase in the solvent extraction step of PFAS once a certain amount of the hydrophobic organic compounds has accumulated. It can also be used when preparing a new organic phase.

[0048] 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 the wastewater with oil, which is undesirable from an environmental perspective. On the other hand, centrifugal extractors, which use centrifugal force to rapidly separate the phases from the emulsion, can reduce oil contamination in the wastewater, but their mechanism is complex, and they have low robustness against contaminants and coexisting substances, especially solid components which must be thoroughly removed. Furthermore, 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 the efficiency of phase mixing between the two phases, but the low step efficiency necessitates a huge system.

[0049] 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 and concentrate PFAS from wastewater with high efficiency while avoiding secondary environmental pollution by not introducing organic phase droplets (oil droplets) into the wastewater.

[0050] As an entirely different approach to avoid secondary environmental pollution, organic bases, organic ligands, or both can be supported on silica gel, or a portion that functions as an organic base, an organic ligand, or both can be bonded to a resin and used as an adsorbent. By incorporating the organic base portion, the organic ligand portion, or both into the matrix material, 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 portion may leak out as the adsorbent degrades.

[0051] Organofluorine compounds extracted into the organic phase by solvent extraction (forward extraction) can be back-extracted from the organic phase to the aqueous phase by contacting the organic phase with an aqueous phase having a higher pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower pH than the aqueous phase used during solvent extraction, an aqueous phase having a lower concentration of metal cations than the aqueous phase used during solvent extraction or not containing metal cations at all, or an aqueous phase containing a complexing agent which is a water-soluble organic ligand, or an aqueous phase which is a combination of these, thereby reducing the organofluorine compounds to electronegative chemical species or molecules with low hydrophobicity.

[0052] Furthermore, during back extraction, organic bases, organic ligands, or both leak into the aqueous phase (back extract). Therefore, similar to solvent extraction (forward extraction), solvent extraction using an alkane solvent with 11 or more carbon atoms is used to recover the organic bases, organic ligands, or both that leaked into the aqueous phase (back extract).

[0053] PFAS extracted into the organic phase can be back-extracted by contacting the organic phase with an aqueous phase that has a lower concentration of metal cations than during solvent extraction, or an aqueous phase that does not contain metal cations. However, this is not an effective back-extraction method for organic ligands or PFAS that form strong complexes with metal cations.

[0054] When PFAS is converted from an acid-dissociated anion (dissociated form) to an acid molecule (undissociated form) by lowering the pH, its ability to coordinate with metal cations is often significantly reduced by becoming an acid molecule. In other words, complex formation with metal cations is eliminated, making it possible to back-extract PFAS into the aqueous phase (provided that the PFAS molecule has a certain degree of hydrophilicity). Furthermore, when the PFAS anion becomes molecularized and changes into an electrically neutral chemical species, the formation of ion pairs with hydrophobic cations formed by the metal cation and organic ligand, or hydrophobic cations produced by the protonation of organic bases, is also eliminated, thus promoting back-extraction. Therefore, an aqueous phase with a lower pH than that used during solvent extraction is effective for back-extraction. However, this method is not effective for back-extraction if the metal cation forms a strong complex with the PFAS anion and acidification of PFAS is difficult.

[0055] Conversely, an aqueous phase with a higher pH than that used during solvent extraction is also effective for back-extraction of PFAS. Raising the pH causes most metal cations to precipitate as hydroxides or oxide hydrates, thus destroying the complexes between the metal cations and PFAS or organic ligands, or mixed complexes between PFAS and organic ligands, or most or all of these complexes. Furthermore, increasing the pH also eliminates hydrophobic cations formed by the protonation of organic bases. Therefore, hydrophobic cations that act as counterions to PFAS anions disappear. This method is an effective back-extraction method even when metal cations form strong complexes with PFAS.

[0056] If the complex between the metal cation and PFAS, or the complex between the metal cation and the organic ligand, or both, is too rigid and back-extraction is difficult using any of the above methods, an aqueous phase containing a complexing agent, which is a water-soluble organic ligand, can be used for back-extraction. However, since the complexing agent is difficult to reuse, it is used as a consumable agent to increase OPEX.

[0057] Furthermore, by arbitrarily combining the back-extraction methods described above, it may be possible to more effectively back-extract organofluorine compounds from the organic phase to the aqueous phase.

[0058] Furthermore, similar to solvent extraction, organic bases, organic ligands, or both that leak into the wastewater (back extract) during back extraction can be fully recovered by solvent extraction using the diluent itself, which is an alkane solvent. Alkane solvents with 11 or more carbon atoms are particularly effective because they have very little environmental impact. Also, solvent extraction is not hindered as long as the viscosity does not become too high. For example, dodecane with 12 carbon atoms is extremely effective as a solvent used in this invention.

[0059] To recover organofluorine compounds while concentrating them in forward extraction, the amount or flow rate of the organic phase relative to the aqueous phase should be controlled (reduced). Similarly, to recover organofluorine compounds while concentrating them in back extraction, the amount or flow rate of the aqueous phase relative to the organic phase should be controlled (reduced).

[0060] PFAS recovered and concentrated by an adsorbent incorporating an organic base, an organic ligand, or both can also be detached from the adsorbent by contacting it with an aqueous solution having a higher pH than at the time of adsorption, an aqueous solution having a lower pH than at the time of adsorption, an aqueous solution with a lower concentration of metal cations than at the time of adsorption or without such metal cations, an aqueous solution containing a complexing agent which is a water-soluble organic ligand, or an aqueous solution which is any combination thereof, similar to back extraction after solvent extraction.

[0061] The present invention will be described in more detail below. Carboxylic acid-based PFAS and sulfonic acid-based PFAS will be selected and described as representative examples, but the scope of the present invention is not limited to these.

[0062] If the PFAS to be recovered is carboxylic acid-based, and it has a large number of carbon atoms, then by lowering the pH of the aqueous phase (wastewater) to convert the carboxylic acid-based PFAS into a molecular form (acid molecular form), it can be solvent extracted into an alkane solvent (diluent only). Conversely, by raising the pH to dissociate the hydrogen ions (anionic form), it can be back-extracted from the alkane solvent into the aqueous phase (back extract). For example, perfluorooctanoic acid (PFOA), which has 8 carbon atoms, is sufficiently hydrophobic in its acid molecular form, so even if the organic phase consists only of an alkane solvent as a pure solvent, solvent extraction (forward extraction) and back extraction can be controlled by changing the pH. On the other hand, sulfonic acid-based PFAS, which always exist in an anionic state, cannot be solvent extracted using only an alkane solvent.

[0063] Furthermore, even among carboxylic acid-based PFAS, those with a small number of carbon atoms, even when molecularized (in the acid molecular form), do not possess sufficient hydrophobicity (or organic affinity) as individual molecules, making solvent extraction impossible with alkane solvents alone. Therefore, by adding an organic base that binds to carboxylic acid-based PFAS as an extractant to the alkane solvent, a compound is formed through an acid-base reaction or ion-pair formation reaction at the liquid-liquid interface. This increases the hydrophobicity (or organic affinity) compared to the individual carboxylic acid-based PFAS molecules, enabling solvent extraction into the organic phase even for those with a small number of carbon atoms.

[0064] Organic bases can be broadly classified into two types: Lewis bases (electron pair donors) and Brønsted bases (hydrogen ion acceptors). Furthermore, organic bases that possess both Lewis and Brønsted base structures within a single molecule can also be used as extractants.

[0065] 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.

[0066] 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.

[0067] When using Lewis bases alone as organic bases as extractants for carboxylic acid-based PFAS with a small number of carbon atoms, an effective acid-base reaction will not occur unless the carboxylic acid-based PFAS is in an acid molecule state (undissociated form). Similarly, when using Brønsted bases alone as organic bases as extractants for carboxylic acid-based PFAS or sulfonic acid-based PFAS with a small number of carbon atoms, the Brønsted base will not act as a counterion for carboxylic acid-based PFAS anions (dissociated form) or sulfonic acid-based PFAS anions unless it protonates to form a hydrophobic cation. Therefore, pH adjustment is essential even when using organic bases as extractants. That is, during solvent extraction (forward extraction), it is necessary to add acid to lower the pH, and during back extraction, it is necessary to neutralize with a base to raise the pH. It should be noted that even for carboxylic acid-based PFAS with a large number of carbon atoms (e.g., PFOA), adding organic bases can significantly reduce the amount of acid and base used for pH adjustment compared to using only alkane solvents, but this does not eliminate the need for pH adjustment.

[0068] Such pH adjustments must be performed regardless of the concentration of PFAS, and consume approximately the same amount of acid and base whether the carboxylic acid-based PFAS, sulfonic acid-based PFAS, or both are present in the wastewater at high or low concentrations.

[0069] On the other hand, solvent extraction (forward extraction) and back extraction of PFAS can also be controlled by using metal cations instead of hydrogen ions. When using metal cations to control solvent extraction (forward extraction) and back extraction of PFAS, the required concentration of metal cations depends on the concentration of PFAS contained in the wastewater. In this respect, it differs from control by pH. That is, if the PFAS concentration is low, the concentration of metal cations to be added can also be low.

[0070] For example, if there is a large excess of organic ligands relative to the added metal cation, virtually all of the existing metal cations will be in the form of metal complex cations (hydrophobic cations). If the hydrophobic cations that act as counterions to the PFAS anion are present in excess, exceeding the equivalent charge of the PFAS anion, then almost the entire amount of the PFAS anion will form an ion pair, and if the hydrophobicity of that ion pair is sufficiently large, then almost the entire amount of PFAS in the aqueous solution (aqueous phase) will be solvent-extracted into the organic phase. This mechanism of ion pair extraction works similarly for anions produced when carboxylic acid-based PFAS dissociate hydrogen ions, and also for sulfonic acid-based PFAS, which always maintain anionic form.

[0071] Specifically, the amount of metal cations in molar concentration (mol / L) must be equal to or greater than the equivalent amount of PFAS. In other words, a molar concentration of about 10 times the equivalent amount of PFAS is sufficient for the required metal cations. In most cases, the molecular weight of PFAS is much larger than the atomic weight of the metal cation, and the valence of the metal cation is greater than the valence of the PFAS anion. Therefore, when compared in weight concentration (wt%, ppm, ppb, ppt, etc.), the concentrations are on roughly the same order. Thus, if the concentration of PFAS in wastewater is on the order of ppb, the required concentration of metal cations is basically also on the order of ppb.

[0072] The ligands acting for metal cations can be either the PFAS anion itself or an organic base. Many organic bases are electron pair donors and can also act as organic ligands for metal cations. In other words, in this case, the organic base not only acts as an electron pair donor or hydrogen ion acceptor for PFAS, but also coordinates to the metal cation to form a metal complex cation. Of course, it is not always necessary for one organic compound to play both roles; the organic base and the organic ligand can be added individually as separate organic compounds, but it is better to have fewer components added to the system.

[0073] As described above, the present invention, which recovers PFAS by adding a metal cation at a concentration similar to that of PFAS, eliminates the need to adjust the pH of the wastewater. If the concentration of PFAS to be recovered is low, PFAS can be quantitatively recovered simply by adding a metal cation at a similar concentration to the wastewater. In other words, it becomes possible to control solvent extraction (forward extraction) and back extraction of PFAS at a much lower cost than methods that adjust the pH using acids and bases.

[0074] For example, amines function as organic bases, which are extractants for carboxylic acid-based PFASs or sulfonic acid-based PFASs, as well as as organic ligands for metal cations during solvent extraction of these PFASs. On the other hand, many amines possess strong surfactant activity. In particular, amines with long alkyl chains often exhibit surfactant activity. Amines with surfactant activity form molecular aggregates in a two-liquid system consisting of an organic phase mainly composed of an alkane solvent and an aqueous phase, and the resulting gelation causes strong turbidity.

[0075] 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.

[0076] Furthermore, the gel (turbidity) generated by amines in alkane solvents can also be eliminated by the addition of metal cations. This is thought to be because the metal complex cations act on the surface-active cation molecular aggregates formed by the protonation of amines, destabilizing the molecular aggregates or inhibiting molecular association. In other words, metal cations not only control the solvent extraction (forward extraction) and back extraction of PFAS, but also have the effect of suppressing the generation of gel (turbidity) caused by the surface activity of organic bases and organic ligands.

[0077] 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 Lewis bases or metal cations.

[0078] 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, the entire amount of organic bases (Lewis bases, Brønsted bases, or both) and organic ligands that 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 and ligands from the aqueous phase (back extract) during back extraction is not necessary if the PFAS is concentrated and recovered in the back extract for incineration or other treatment. In other words, removal of organic bases and ligands from the back extract should be performed as needed.

[0079] When PFAS is precipitated and recovered from a back extract for reuse, removing organic bases and ligands from the back extract is a necessary step, including from the perspective of water recycling. 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 and ligands have been removed from the back extract.

[0080] If the extraction rate of PFAS in solvent extraction (forward extraction) is sufficiently high, the PFAS can be recovered while being concentrated in the organic phase. If the back extraction rate in back extraction is sufficiently high, the PFAS can be recovered while being concentrated in the aqueous phase (back extract). In this method, it is possible to concentrate PFAS in two stages: solvent extraction (forward extraction) and back extraction.

[0081] By using adsorbents incorporating organic bases, organic ligands, or both, it becomes possible to recover and concentrate carboxylic acid-based PFAS and sulfonic 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 these PFAS can be controlled by changing the pH, the concentration of metal cations, 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.

[0082] 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.

[0083] Next, examples of the present invention will be shown, but the present invention is not limited to these examples.

[0084] Example 1 (Solvent extraction test of PFOA with triamidoamines and Al) Solvent extraction (forward extraction) was performed on perfluorooctanoic acid (PFOA), a carboxylic acid-based PFAS (8 carbon atoms), in a system in which hexaoctylnitrilotriacetamide (HONTA), a triamidoamine, and an Al salt were added. An aqueous hydrochloric acid solution containing PFOA at a concentration of 1000 ppm and Al at a concentration of 1000 ppm, adjusted to pH 2.5, 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 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 concentration of PFOA was measured. Based on the results, the extraction rate (positive extraction rate) of PFOA was calculated to be 97.5%.

[0085] Example 2 (Solvent extraction test of PFHxS with diamidoamines and La) Solvent extraction (forward extraction) was performed on perfluorohexanesulfonic acid (PFHxS), a sulfonic acid-based PFAS (6 carbon atoms), in a system in which penta-2-ethylhexyldiamidoamine (PEHDAA), a diamidoamine, and La salt were added. An aqueous hydrochloric acid solution containing PFHxS at a concentration of 1000 ppm and La at a concentration of 1000 ppm, adjusted to pH 1.0, was prepared as the aqueous phase, and n-dodecane (organic phase) containing 0.02 mol / L of PEHDAA 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 concentration of PFHxS was measured. Based on this result, the extraction rate (positive extraction rate) of PFHxS was calculated to be 95.3%. Furthermore, when the aqueous phase was adjusted to pH 3.5 and the same experiment was performed, the extraction rate (positive extraction rate) of PFHxS was 95.8%, which was almost the same value as when the pH was 1.0.

[0086] Example 3 (Solvent extraction test of PFBA with triamidoamines and Al) Solvent extraction (forward extraction) was performed on perfluorobutanoic acid (PFBA), a carboxylic acid-based PFAS (4 carbon atoms), in a system in which hexa-2-ethylhexylnitrilotriacetamide (HEHNTA), a triamidoamine, and an Al salt were added. An aqueous hydrochloric acid solution containing PFBA at a concentration of 1000 ppm and Al at a concentration of 200 ppm, adjusted to pH 2.1, 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 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 concentration of PFBA was measured. Based on the results, the extraction rate (positive extraction rate) of PFBA was calculated to be 94.7%.

[0087] Example 4 (Solvent extraction test of PFBS with diamidoamines and La) Solvent extraction (forward extraction) was performed on perfluorobutanesulfonic acid (PFBS), a sulfonic acid-based PFAS (4 carbon atoms), in a system in which penta-2-ethylhexyldiamidoamine (PEHDAA), a diamidoamine, and La salt were added. An aqueous hydrochloric acid solution containing PFBS at a concentration of 1000 ppm and La at a concentration of 1000 ppm, adjusted to pH 1.0, was prepared as the aqueous phase, and n-dodecane (organic phase) containing 0.02 mol / L of PEHDAA 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 concentration of PFBS was measured. Based on this result, the extraction rate (positive extraction rate) of PFBS was calculated to be 93.8%. Furthermore, when the aqueous phase was adjusted to pH 3.5 and the same experiment was performed, the extraction rate (positive extraction rate) of PFBS was 93.9%, which was almost the same value as when the pH was 1.0.

[0088] Example 5 (Solvent extraction test of three carboxylic acid-based PFAS with triamidoamines, La, and Sm) A pH 6.8 aqueous solution containing perfluorooctanoic acid (PFOA) with 8 carbon atoms, perfluorohexanoic acid (PFHxA) with 6 carbon atoms, and perfluorobutanoic acid (PFBA) with 4 carbon atoms at concentrations of 5 ppb to 50 ppb as carboxylic acid-based PFAS, and further containing La and Sm at concentrations approximately 10 times the equivalent charge of all these carboxylic acid-based PFAS, was used as the aqueous phase. An n-dodecane solution containing 0.02 mol / L hexaoctylnitrilotriacetamide (HONTA) was used 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, the mixture was centrifuged at 3000 rpm for 5 minutes using a centrifuge. After centrifugation, the aqueous phase was collected, and the concentrations of PFOA, PFHxA, and PFBA were measured. Based on these results, the extraction rates (positive extraction rates) were calculated to be between 94.6% and 97.8%.

[0089] Example 6 (Recovery and Removal Test of Triamidoamines by n-Dodecane) Solvent extraction with n-dodecane alone was performed on the aqueous phase after solvent extraction (forward extraction) of triamidoamines using HONTA in Examples 1 and 5, and on the aqueous phase after solvent extraction (forward extraction) of triamidoamines using HEHNTA in Example 3, and the total organic carbon concentration (TOC) was measured. As a result, the TOC was below the detection limit (0.004 ppm) for all aqueous phases.

[0090] Example 7 (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, the TOC was below the detection limit (0.004 ppm) for all aqueous phases.

[0091] Example 8 (Effect of metal cations on eliminating gelation and turbidity) When an aqueous solution containing PFOA, PFHxS, and PFBA at 50 ppb each was mixed with an equal volume of n-dodecane solution containing 0.02 mol / L trioctylamine, and then shaken for 10 minutes using a vertical shaker, turbidity due to gelation occurred. On the other hand, when an appropriate amount of Al salt, Fe salt, La salt, or Sm was present in the aqueous phase, turbidity due to gelation did not occur.

[0092] This 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 capacity for handling organofluorine compounds, the speed of treatment, and the reusability of consumables. Specifically, it refers to concentrations at or above the ppb level.

[0093] These perfluorinated compounds (OCFs) are widely used in various industries, but their significant impact on health and ecosystems is a cause for concern. OCFs are persistent, some are hazardous, yet they remain stable in the environment for extended 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 their social necessity is proven. On the other hand, there are many cases where alternatives are difficult to find for industrial or consumer applications. Even if their use is not phased out for so-called essential uses, strict restrictions are placed on emissions from factories. In other words, technology capable of handling aqueous solutions containing high concentrations of OCFs, such as factory wastewater, is essential. Furthermore, not only from the perspective of health and ecosystem impacts, but also from the perspectives of resource recycling and carbon dioxide reduction, it is necessary to consider recycling OCFs 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 in which a metal cation is added to an aqueous solution containing an organofluorine compound, and an organic phase which is a linear, cyclic, or mixture of alkane solvents containing at least one hydrophobic organic compound that acts as an organic base that binds to the organofluorine compound, an organic ligand that coordinates to the metal cation, or both, the method comprising the step of causing the organofluorine compound to become an electrically neutral chemical species based on an acid-base reaction, a complex formation reaction, or an ion pair formation reaction, or a combination thereof, occurring between the organofluorine compound, the metal cation, and the hydrophobic organic compound, and solvent extraction of this species from the aqueous phase to the organic phase, wherein during the solvent extraction, the hydrophobic organic compound 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 organic base is a Lewis base, a Brønsted base, or a hydrophobic organic compound having the properties of both, according to claim 1.

3. A method for recovering an organofluorine compound, characterized in that the Lewis base, the metal cation, or both are present in the presence of 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.

4. A method for recovering an organofluorine compound, 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, according to any one of claims 1 to 3.

5. 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 4.

6. 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 5.

7. A method for recovering organofluorine compounds, characterized by using an organic base, an organic ligand, or both as an adsorbent by supporting it on silica gel or bonding it to a resin, thereby adsorbing organofluorine compounds in an aqueous solution onto the silica gel or the resin.

8. A method for recovering an organofluorine compound according to claim 7, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

9. 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 aqueous phase having a pH greater than the pH of the aqueous phase at the time of solvent extraction with an organic phase containing an organofluorine compound obtained by solvent extraction from an aqueous phase by the method of any one of claims 1 to 6, wherein the organic base, the organic ligand, or both dissolved in the aqueous phase during the back-extraction are recovered by solvent extraction with an alkane solvent having 11 or more carbon atoms.

10. A method for recovering an organofluorine compound according to claim 9, 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.

11. 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 9.

12. 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 aqueous phase having a pH lower than the pH of the aqueous phase at the time of solvent extraction with an organic phase containing an organofluorine compound obtained by solvent extraction from an aqueous phase by the method of any one of claims 1 to 6, wherein the organic base, the organic ligand, or both dissolved in the aqueous phase during the back-extraction are recovered by solvent extraction with an alkane solvent having 11 or more carbon atoms.

13. A method for recovering an organofluorine compound according to claim 12, 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.

14. 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 12.

15. 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 the organic phase containing the organofluorine compound, which has been solvent-extracted from the aqueous phase by the method of any one of claims 1 to 6, with an aqueous phase having a lower concentration of metal cations than the aqueous phase used for the solvent extraction or an aqueous phase that does not contain metal cations, and recovering the organic base, the organic ligand, or both dissolved in the aqueous phase during the back-extraction using solvent extraction with an alkane solvent having 11 or more carbon atoms.

16. A method for recovering an organofluorine compound according to claim 15, 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.

17. 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 15.

18. 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 aqueous phase containing a complexing agent which is a water-soluble organic ligand with an organic phase containing an organic phase containing an organofluorine compound obtained by solvent extraction from an aqueous phase by the method of any one of claims 1 to 6, wherein the organic base, the organic ligand, or both dissolved in the aqueous phase during the back-extraction are recovered by solvent extraction with an alkane solvent having 11 or more carbon atoms.

19. A method for recovering an organofluorine compound according to claim 18, 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.

20. 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 18.

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 aqueous phase containing an organofluorine compound, which has been solvent-extracted from the aqueous phase by the method of any one of claims 1 to 6, with an aqueous phase obtained by arbitrarily combining the methods of claims 9 to 20, and recovering the organic base, the organic ligand, or both that have 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, 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 7, with an aqueous solution having a pH higher than that at the time of adsorption.

25. A method for recovering an organofluorine compound according to claim 24, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

26. 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 7, with an aqueous solution having a pH lower than that at the time of adsorption.

27. A method for recovering an organofluorine compound according to claim 26, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

28. 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 of claim 7, with an aqueous solution in which the concentration of metal cations is lower than at the time of adsorption or which does not contain metal cations.

29. A method for recovering an organofluorine compound according to claim 28, characterized in that the organofluorine compound has a carboxyl group, a sulfonic acid group, or both.

30. A method for recovering an organofluorine compound, characterized by contacting an adsorbent that has adsorbed an organofluorine compound by the method of claim 7 with an aqueous solution containing a complexing agent which is a water-soluble organic ligand, thereby detaching 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 by desorbing the organofluorine compound from the adsorbent into an aqueous solution by arbitrarily combining the methods described in claims 24 to 31.

33. 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.