Purification method

Abstract

To provide a purification method for removing an organic fluorine compound from water to be treated with excellent purification efficiency.SOLUTION: A purification method for removing an organic fluorine compound from water to be treated, includes: making a temperature of the water to be treated appropriate; supplying bubbles to the water to be treated; and collecting bubbles from a water surface of the water to be treated.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a purification method. [Background technology]

[0002] Patent Document 1 discloses a foam separation treatment method for separating surfactants such as organic fluorine compounds contained in stored water to be treated from the water to be treated, in which gas is pumped into the stored water to be treated to generate bubbles, the surfactant is concentrated at the air-water interface of the bubbles to generate foam, and the foam is collected near the surface of the water to be treated.

[0003] Patent Document 2 discloses a water intake method in which gas is pumped into water to be extracted to generate bubbles, the bubbles are brought into contact with the water to adsorb and separate contaminants contained in the water, and the water after the bubbles have been separated is collected.

[0004] Patent Document 3 discloses a method for destroying PFAS in a PFAS-containing aqueous mixture, the method comprising: subjecting the PFAS-containing aqueous mixture to a separation procedure to separate the PFAS-containing aqueous mixture into a PFAS-enriched fraction and a PFAS-depleted fraction, heating at least a portion of the PFAS-depleted fraction to a temperature of at least 300° C. to form a heated PFAS-depleted fraction, and combining the heated PFAS-depleted fraction and the PFAS-enriched fraction to form a high-temperature PFAS-containing aqueous mixture prior to the supercritical destruction step; or separating the PFAS-containing aqueous mixture into a first salt-enriched fraction and a salt-depleted fraction, heating the salt-depleted fraction to a temperature of at least 300° C. to form a heated salt-depleted fraction, and combining the heated salt-depleted fraction with the cooler first fraction to form a high-temperature PFAS-containing aqueous mixture prior to the supercritical destruction step; and reacting the high-temperature PFAS-containing aqueous mixture with an oxidizing agent under supercritical conditions. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2023-050657 A [Patent Document 2] JP 2018-153742 A [Patent Document 3] Special Publication No. 2022-537895 Summary of the Invention [Problem to be solved by the invention]

[0006] Due to their useful properties such as surface activity, heat resistance, and chemical resistance, organic fluorine compounds have been used in a wide range of applications in manufacturing and industry. In recent years, the ecotoxicity and human toxicity of organic fluorine compounds have been reported, and restrictions on the manufacture and use of organic fluorine compounds have been tightened worldwide. On the other hand, organic fluorine compounds are difficult to decompose, and there is an urgent need to develop technology to purify organic fluorine compounds that have already accumulated in the environment.

[0007] It is against the above background that the present disclosure has been made. An object of the present disclosure is to provide a purification method for removing organic fluorine compounds from water to be treated, the purification method having excellent purification efficiency. [Means for solving the problem]

[0008] Specific means for solving the above problems include the following aspects.

[0009] <1> A purification method for removing organic fluorine compounds from water to be treated, comprising the steps of: Setting the temperature of the water to be treated to an appropriate temperature; Supplying air bubbles to the water to be treated; and recovering bubbles from the surface of the water to be treated. Purification methods. <2> The temperature of the water to be treated is adjusted to an appropriate temperature, and includes at least one of heating, lowering, and keeping the water to be treated warm. <1> The purification method according to claim 1. <3> Adjusting the temperature of the water to be treated to an appropriate temperature includes adjusting the temperature of the water to be treated to a range of 35°C to 50°C. <1> or <2> The purification method according to claim 1. <4> The water to be treated is water pumped from soil. <1> ~ <3> 13. The purification method according to claim 12, <5> The air bubbles include micro-nano bubbles. <1> ~ <4> 13. The purification method according to claim 12, <6> Further, the method includes adding at least one surfactant selected from the group consisting of anionic surfactants, cationic surfactants, and nonionic surfactants (excluding organic fluorine compounds) to the water to be treated. <1> ~ <5> 13. The purification method according to claim 12, Effect of the Invention

[0010] According to the present disclosure, there is provided a purification method for removing organic fluorine compounds from water to be treated, which has excellent purification efficiency. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing an example of a purification method according to the present disclosure. [Diagram 2] FIG. 2 is a schematic diagram showing another example of the purification method of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The cleaning method of the present disclosure will be described below. These descriptions and embodiment examples are merely illustrative, and the scope of the cleaning method of the present disclosure is not limited to the illustrative examples.

[0013] In the present disclosure, a numerical range indicated using "~" includes the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit value of the numerical range may be replaced with a value shown in the examples.

[0014] In the present disclosure, the term "step" includes not only an independent step, but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0015] <Purification method> The purification method of the present disclosure is a purification method for purifying the water to be treated by removing organic fluorine compounds from the water to be treated. The purification method of the present disclosure is based on foam separation, Adjusting the temperature of the water to be treated to an appropriate temperature (referred to as the "temperature adjustment process"); Supplying air bubbles to the water to be treated (referred to as the "air bubble supplying step"); and recovering foam from the surface of the water to be treated (referred to as the "foam recovery step").

[0016] The mechanism by which the purification method of the present disclosure has excellent purification efficiency is presumed to be as follows. The purification method disclosed herein utilizes the property of organic fluorine compounds (many of which are compounds with surface activity) gathering at the gas-liquid interface of air bubbles in water. The surface tension of water is inversely correlated with the water temperature (i.e., the higher the water temperature, the lower the surface tension of the water), so the higher the temperature of the water being treated, the more likely it is that organic fluorine compounds will gather at the gas-liquid interface of the air bubbles. On the other hand, the higher the temperature of the water being treated, the greater the mobility of molecules around the gas-liquid interface of the air bubbles, and the gas that constitutes the bubbles expands, causing the bubbles to become unstable. The purification method of the present disclosure stabilizes air bubbles and collects a large amount of organic fluorine compounds in the air bubbles by adjusting the temperature of the water to be treated to an appropriate temperature, resulting in excellent purification efficiency.

[0017] In the purification method of the present disclosure, the order of each step is not limited. In one embodiment, the temperature adjustment step is performed before the air bubble supplying step. In one embodiment, the temperature adjustment step is started before the air bubble supplying step and is continued during the air bubble supplying step. In one embodiment, the air bubble supplying step and the foam collecting step are performed in parallel. In one embodiment, the air bubble supplying step is stopped, and then the foam collecting step is continued. In one embodiment, the temperature adjustment step, the air bubble supply step, and the foam recovery step are continuously performed while circulating a part or all of the water to be treated. In one embodiment, a set of a temperature adjustment step, an air bubble supply step, and a foam recovery step is repeated multiple times.

[0018] The configuration of the purification method of the present disclosure will be described in detail below.

[0019] [Organofluorine compounds] The organofluorine compounds that the cleaning method of the present disclosure is intended to remove include all known organofluorine compounds and those that will be developed in the future. The organic fluorine compounds to be removed by the purification method of the present disclosure include PFAS (per- and polyfluoroalkyl substances). PFAS is a general term for perfluoroalkyl compounds and polyfluoroalkyl compounds. Representative compounds of PFAS include PFSAs (perfluorosulfonic acids) such as PFOS (perfluorooctane sulfonic acid) and PFHxS (perfluorohexane sulfonic acid), and PFCAs (perfluorocarboxylic acids) such as PFOA (perfluorooctanoic acid) and PFHxA (perfluorohexanoic acid).

[0020] [Water to be treated] Examples of water to be treated that can be purified by the purification method of the present disclosure include groundwater, lake water, reservoir water, river water, industrial wastewater, and effluent from waste disposal sites.

[0021] An example of the water to be treated is water pumped from soil. Examples of water pumped from soil include the following water: (1) Groundwater containing organofluorine compounds. (2) Water that contains organic fluorine compounds and that has been pumped after in-situ purification using soil flushing to remove the organic fluorine compounds. (3) Water that contains organic fluorine compounds and that has been pumped after in-situ purification using soil flushing to remove harmful substances other than organic fluorine compounds (e.g., volatile organic compounds, mineral oils, heavy metals). (4) Water that contains organic fluorine compounds and that has been pumped after in-situ purification using bioremediation to remove harmful substances other than organic fluorine compounds (e.g., volatile organic compounds, mineral oils, heavy metals).

[0022] [Temperature control process] The temperature adjustment step includes, for example, at least one of heating, decreasing the temperature, and keeping the water to be treated warm. By treating or leaving the water to be treated to aim for an appropriate temperature, heating, decreasing the temperature, or keeping the water to be treated warm is realized.

[0023] Heating the water to be treated may be achieved by artificially heating the water to be treated, or by allowing the temperature of the water to be treated to rise naturally. For example, the water to be treated may be stored until the temperature of the water to be treated reaches an appropriate range. The temperature of the water to be treated may be reduced by artificially cooling the water to be treated, or by allowing the temperature of the water to be treated to decrease naturally. For example, the water to be treated may be stored until the temperature of the water to be treated reaches an appropriate range. Keeping the water to be treated warm may involve artificially heating or cooling the water to be treated, or it may involve carrying out foam separation while maintaining the temperature of the water to be treated within an appropriate range in a water tank with heat retention capabilities.

[0024] The temperature of the water to be treated is preferably as high as possible, from the viewpoint that the surface tension of water is relatively low and organic fluorine compounds tend to collect at the gas-liquid interface of the bubbles, and is preferably 18°C ​​or higher, more preferably 20°C or higher, even more preferably 25°C or higher, even more preferably 30°C or higher, and even more preferably 35°C or higher. From the viewpoint of bubble stability, the temperature of the water to be treated is preferably 80°C or less, more preferably 70°C or less, even more preferably 60°C or less, still more preferably 55°C or less, and even more preferably 50°C or less.

[0025] The temperature of the water to be treated may be constant or may vary during the implementation of the purification method of the present disclosure.

[0026] The temperature adjustment step preferably includes measuring the temperature of the water to be treated. An example of an embodiment of the temperature adjustment step includes measuring the temperature of the water to be treated, and treating or leaving the water to be treated depending on the temperature of the water to be treated determined by the temperature measurement. One example of an embodiment of the temperature adjustment process includes measuring the temperature of the water to be treated, and includes at least one of heating, cooling, and keeping the water to be treated warm, depending on the temperature of the water to be treated determined by the temperature measurement.

[0027] [Air bubble supply process] The air bubbles may be supplied continuously or intermittently.

[0028] Examples of methods for supplying bubbles include a method of feeding gas into the water to be treated through micropores to form bubbles, a method of injecting a supersaturated gas solution into the water to be treated and forming bubbles by reducing the pressure, a method of introducing a solid that dissolves and generates bubbles into the water to be treated, a method of mixing the water to be treated and a gas in a nozzle and forming bubbles by utilizing the shear force caused by turbulence, a method of stirring the water to be treated at high speed while feeding gas into the water to be treated, etc. Water containing bubbles may be prepared using a relatively small amount of water other than the water to be treated and gas, and the water containing bubbles may be injected into the water to be treated. The source gas for the bubbles is, for example, air or carbon dioxide.

[0029] The size of the bubbles may be on the order of centimeters, millimeters, micrometers, or nanometers, or may be a mixture of these.

[0030] The bubbles are preferably micro-nano bubbles. In the present disclosure, micro-nano bubbles is a term that collectively refers to bubbles on the order of micrometers (microbubbles) and bubbles on the order of nanometers (nanobubbles). The micro-nano bubbles may be only microbubbles, only nanobubbles, or a mixture of microbubbles and nanobubbles. From the viewpoint of purification efficiency, the diameter of the micro-nano bubbles when they are formed is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 10 μm or less. The diameter of the micro-nano bubbles when formed is preferably 100 nm or more, more preferably 500 nm or more, and even more preferably 1 μm or more.

[0031] The amount of air bubbles to be supplied is not limited and may be set depending on the concentration of the organic fluorine compounds contained in the water to be treated, the volume of the water to be treated, the foam recovery speed, and the like. For example, when purifying water to be treated while continuously supplying it to a purification device, the supply amount of air bubbles (mL / min) per unit supply amount (1 L / min) of water to be treated may be set to 100 to 20,000 mL / min, 500 to 15,000 mL / min, or 1,000 to 10,000 mL / min. The amount of foam dispensed may be constant or may vary during the foam dispense step.

[0032] [Foam recovery process] Examples of the method for collecting the foam include a method of sucking the foam, a method of letting the foam flow out together with a part of the water to be treated, etc. Foam collection may be performed continuously or intermittently.

[0033] The collected foam contains concentrated organic fluorine compounds. It is desirable to detoxify the collected foam and dispose of it appropriately. The treated water, in which the concentration of organic fluorine compounds has been reduced through the foam recovery process, is, for example, discharged into a river or the sea, poured onto soil, or used as industrial water.

[0034] [Other processes] The purification method of the present disclosure preferably includes analyzing the water to be treated and quantifying the concentration of organic fluorine compounds contained in the water to be treated (organic fluorine compound quantification step). Here, the organic fluorine compounds whose concentrations are quantified preferably include at least one selected from the group consisting of PFSAs such as PFOS and PFHxS, and PFCAs such as PFOA and PFHxA, and preferably include at least PFOS and PFOA.

[0035] The organic fluorine compound quantitative determination step is preferably performed before the temperature adjustment step, the air bubble supply step, and the foam recovery step, and is preferably performed before the air bubble supply step is stopped.

[0036] The purification method of the present disclosure preferably includes adding at least one surfactant selected from the group consisting of anionic surfactants, cationic surfactants, and nonionic surfactants (excluding organic fluorine compounds) to the water to be treated (surfactant adding step). The surfactant improves the stability of air bubbles in the water and foam on the water surface.

[0037] Examples of the anionic surfactant include sodium dodecyl sulfate, fatty acid salts, polyoxyalkylene alkyl ether acetates, alkyl sulfates, polyoxyalkylene alkyl ether sulfates, polyoxyalkylene alkylamide ether sulfates, monoglyceride sulfates, olefin sulfonates, alkanesulfonates, acylated isethionates, acylated amino acids, alkyl phosphates, and polyoxyalkylene alkyl ether phosphates. Examples of the cationic surfactant include benzalkonium chloride, benzethonium chloride, cetylpyridinium chloride, hexadecyltrimethylammonium bromide, and dequalinium chloride. Examples of nonionic surfactants include polyoxyethylene alkyl ethers, glycerin fatty acid esters, propylene glycol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitan tetraoleate, polyoxyethylene polyoxypropylene glycol, polyoxyethylene polyoxypropylene alkyl ethers, polyethylene glycol fatty acid esters, polyoxyethylene castor oil, polyglycerin fatty acid esters, and alkyl glycosides. The surfactant may be used alone or in combination of two or more kinds.

[0038] In one embodiment of the cleaning method of the present disclosure, the surfactant addition step is performed before the air bubble supply step. In one embodiment of the cleaning method of the present disclosure, the surfactant addition step continues during the air foam supply step.

[0039] [Example of the purification method] The purification method of the present disclosure will be further described below with reference to the drawings. The embodiment described below is an example of the purification method, and the purification method of the present disclosure is not limited thereto.

[0040] 1 and 2 are schematic configuration diagrams each showing an example of the purification method of the present disclosure. The purification apparatus 10A shown in FIG. 1 is a foam separation apparatus including a treatment tank 12, a temperature adjustment device 14, an air bubble supply device 16A, and a foam recovery tank 18. The purification apparatus 10B shown in FIG. 2 is a foam separation apparatus including a treatment tank 12, a temperature adjustment device 14, an air bubble supply device 16B, and a foam recovery tank 18.

[0041] The treatment tank 12 is a tank for storing the water to be treated 22. The water to be treated 22 may be poured into and discharged from the treatment tank 12 by a batch method or a flow method. The treatment tank 12 may be equipped with an agitation means for agitating the water to be treated 22, a heat-retaining jacket for maintaining the temperature of the water to be treated 22, a temperature sensor for measuring the temperature of the water to be treated 22, and the like.

[0042] The temperature adjustment device 14 is provided upstream of the water injection section of the treatment tank 12 as shown in FIGS. A temperature sensor and heating means and / or cooling means are arranged inside the temperature adjustment device 14. The heating means and cooling means operate or do not operate depending on the temperature of the water to be treated 22 monitored by the temperature sensor. If the temperature of the water to be treated 22 monitored by the temperature sensor is not appropriate, the heating means or cooling means operates to heat or cool the water to be treated 22. If the temperature of the water to be treated 22 monitored by the temperature sensor is appropriate, the heating means and cooling means do not operate.

[0043] The air bubble supplying device 16A is provided at the bottom of the treatment tank 12 as shown in FIG. An air bubble generating means is disposed inside the air bubble supplying device 16A. The air bubble generating means is, for example, a means for feeding gas into water through fine pores. The air fed into the air bubble supplying device 16A becomes air bubbles 24 and is fed to the water 22 to be treated.

[0044] The air bubble supplying device 16B is provided at the bottom of the treatment tank 12 as shown in FIG. An air bubble forming means is disposed inside the air bubble supplying device 16B. The air bubble forming means is, for example, a means for mixing water and gas in a nozzle and forming air bubbles by utilizing shear force caused by turbulent flow, or a means for forming air bubbles by feeding gas into water and stirring the water at high speed. The water used by the air bubble forming means to form bubbles is a part of the water to be treated 22. Water containing air bubbles 24 is prepared inside the air bubble supplying device 16B, and the water containing air bubbles 24 is supplied to the water to be treated 22 stored inside the treatment tank 12. The water used by the bubble forming means to form bubbles may be tap water or a portion of the water drained from the treatment tank 12, but it is preferable that it be a portion of the water to be treated 22 from the viewpoint of not diluting the concentration of organic fluorine compounds in the water to be treated 22 stored inside the treatment tank 12.

[0045] The foam collection tank 18 is a tank that stores foam 26 collected from the water surface of the water to be treated 22. Inside the foam collection tank 18, a means for rendering the foam 26 harmless (for example, a high-temperature heating means) may be disposed.

[0046] The water to be treated 22 is poured into the temperature adjustment device 14, and the temperature is adjusted by the temperature adjustment device 14. The temperature-adjusted water to be treated 22 is poured into the treatment tank 12 and stored therein. Air bubbles 24 are supplied to the water to be treated 22 stored inside the treatment tank 12 from the air bubble supplying device 16A or 16B. The air bubbles 24 supplied to the water to be treated 22 rise in the water to be treated 22. While the air bubbles 24 rise, organic fluorine compounds adhere to the gas-liquid interface of the air bubbles 24. The air bubbles 24 rise to the water surface of the water to be treated 22 and form foam 26. The foam 26 contains a high concentration of organic fluorine compounds. The foam 26 is collected from the surface of the water to be treated 22. The collected foam 26 is stored in the foam collection tank 18 and, preferably, is rendered harmless and then appropriately disposed of. The water to be treated 22 with the reduced concentration of organic fluorine compounds is discharged from the treatment tank 12 .

[0047] A surfactant other than an organic fluorine compound may be added to the water to be treated 22. The surfactant may be added, for example, to the water to be treated 22 stored inside the treatment tank 12, to the water to be treated 22 upstream of the temperature adjustment device 14, or to the water to be treated 22 inside the temperature adjustment device 14.

[0048] A configuration may be adopted in which a plurality of purifiers 10A or 10B are connected together, and wastewater from a first purifier 10A or 10B is poured into a second purifier 10A or 10B. The water to be treated 22 is purified in stages as it flows from the most upstream purifier 10A or 10B to the most downstream purifier 10A or 10B. EXAMPLES

[0049] The purification method of the present disclosure will be specifically described below with reference to examples, but the purification method of the present disclosure is not limited to the following examples.

[0050] <Example 1> In order to clarify the effect of temperature on foam separation of an organic fluorine compound, foam separation was carried out according to the following procedures (1) to (9).

[0051] (1) PFOS or PFOA was added to ion-exchanged water to prepare a PFOS solution and a PFOA solution with a concentration of 1 mg / L. The solution was further diluted with ion-exchanged water to prepare a PFOS solution and a PFOA solution with a concentration of 5 μg / L, which was used as the water to be treated. (2) Sodium dodecyl sulfate (SDS) was added to ion-exchanged water to prepare an SDS solution with a concentration of 10 g / L. (3) A heating device with a thermostat was installed in a square polypropylene tank. An overflow-type protein skimmer (Kamihata Fish Farm Co., Ltd., product name: Kaido Daruma) was fixed to the edge of the tank, and the protein skimmer's intake pipe was inserted into the tank, forming a circulating foam separation device in which water circulates between the tank and the protein skimmer. (4) 16 L of water to be treated was placed in the water tank. A heating device with a thermostat was operated to raise and maintain the temperature of the water to be treated in the water tank at 20°C, 35°C, or 50°C. (5) 10 mL of an SDS solution with a concentration of 10 g / L was added to the water to be treated in the tank, and the whole was stirred slowly without creating bubbles. (6) The protein skimmer was operated to start circulating the water to be treated and bubbling by supplying air. The circulation rate of the water to be treated was 6 L / min, and the amount of air supplied was about 2 L / min. (7) The entire amount of foam from the water surface of the protein skimmer was collected from 10 minutes to 30 minutes after the protein skimmer started operating. (8) The weight of the collected foam was measured, and after mixing until homogeneous, a small amount was taken for analysis. (9) The PFOS or PFOA concentrations in the foam samples were measured using a liquid chromatography tandem mass spectrometer (LC-MS / MS). The measurement results are shown in Table 1.

[0052] The "initial concentration in the treated water" in Table 1 refers to the PFOS or PFOA concentration measured by LC-MS / MS in the treated water sampled from the tank after (5) and before (6) above.

[0053] [Table 1]

[0054] The higher the temperature in the range of 20℃ to 50℃, the higher the PFOS concentration in the foam. The higher the temperature in the range of 20℃ to 50℃, the higher the PFOA concentration in the foam. The removal efficiency of organic fluorine compounds was higher at higher temperatures in the range of 20℃ to 50℃. [Explanation of symbols]

[0055] 10 Purification Equipment 12 Treatment tank 14 Temperature control device 16A Air bubble supply device 16B Air bubble supply device 18 Foam collection tank 22 Untreated water 24 Bubbles 26 Foam

Claims

1. A purification method for removing organic fluorine compounds from water to be treated, comprising the steps of: Setting the temperature of the water to be treated to an appropriate temperature; Supplying air bubbles to the water to be treated; and recovering bubbles from the surface of the water to be treated. Purification methods.

2. 2. The purification method according to claim 1, wherein adjusting the temperature of the water to be treated to an appropriate temperature includes at least one of heating, decreasing the temperature, and keeping the water to be treated warm.

3. 2. The purification method according to claim 1, wherein adjusting the temperature of the water to be treated to an appropriate temperature includes adjusting the temperature of the water to be treated to a range of 35°C to 50°C.

4. 2. The purification method according to claim 1, wherein the water to be treated is water pumped from soil.

5. The method of claim 1 , wherein the bubbles include micro-nano bubbles.

6. The purification method according to any one of claims 1 to 5, further comprising adding at least one surfactant selected from the group consisting of anionic surfactants, cationic surfactants, and nonionic surfactants (excluding organic fluorine compounds) to the water to be treated.

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