JP6923865B2 - Selective Separation Method for Fluorine-Containing Organic Acids - Google Patents

Description

The present invention relates to a method for selectively separating fluorine-containing organic acids.

Fluorine-containing organic acids such as fluorocarboxylic acid are excellent in water-repellent action, surface-active action and the like, and have been used in water-repellent sprays, surface coating agents, digestive agents, waxes and the like. Conventionally, perfluorooctanesulfonic acid (hereinafter, also referred to as “PFOS”) and perfluorooctanecarboxylic acid (hereinafter, also referred to as “PFOA”) have been widely used as typical fluorocarboxylic acids. However, PFOS and PFOA have a large environmental load, and their production and use are restricted by regulations such as the Stockholm Convention on Persistent Organic Pollutants, and their amounts are decreasing.

For the above reasons, perfluorocaproic acid (hereinafter, also referred to as "PFHxA") and fluorocarboxylic acid having an ether bond are currently used as fluorine-containing organic acids in place of PFOS and PFOA, and their production and usage amount. Is increasing. Along with this, the amount of wastewater containing a large amount of fluorine-containing organic acids discharged as industrial wastewater is increasing. As a method for treating this waste liquid, a treatment of concentrating the waste liquid using a reverse osmosis membrane and then incinerating it is currently performed. For example, Patent Document 1 describes a method of filtering an aqueous solution containing a fluorine-containing surfactant, which is a fluoroalkanoic acid, produced in a process for producing a fluoropolymer using a reverse osmosis membrane. Further, Non-Patent Document 1 describes a method for removing 95.5 to 98.5% of PFHxA with a reverse osmosis membrane having a NaCl removal rate of 97.0%.

Japanese Patent No. 50555652

Steinle-Darling, E., Reinhard, M., 2008. Nanofiltration for trace organic contamination removal: structure, solution, and membrane fouling effects on the rejection of perfluorochemicals. Environmental science & technology 42, 5292-7

In the method for treating a waste liquid containing fluorocarboxylic acid using a reverse osmosis membrane as described above, since the pores of the reverse osmosis membrane used are very small, it is necessary to apply a large intermembrane pressure. Therefore, conventional processing methods require high energy. Further, since the treatment takes time, the treatment efficiency is low, and there is a problem that it is difficult to treat a large amount of waste liquid.

Another problem is that reverse osmosis membranes are prone to fouling (clogging) because the pore diameter is very small (0.2 nm to 1 nm) (see How the Water Cycle System Works, Natsumesha, 2010). There is a problem that the frequency of maintenance is high. This increases processing costs and reduces efficiency as processing must be stopped during maintenance.

In order to improve the processing capacity and prevent fouling, it is necessary to increase the diameter of the pores of the reverse osmosis membrane (semipermeable membrane). However, when the diameter of the pores is increased, the fluorocarboxylic acid itself used for the surfactant or the like is a relatively small molecule, so that the fluorocarboxylic acid molecule passes through the membrane and the inhibition rate of the fluorocarboxylic acid decreases. It ends up.

In addition, reverse osmosis membranes have very small pore diameters, resulting in excessive separation. That is, a substance having a small molecular weight that is not intended to be separated, such as NaCl, cannot pass through the pores, and is separated and concentrated together with the fluorocarboxylic acid. When a large amount of other substances are present in the fluorocarboxylic acid concentrate as described above, it becomes difficult to recover and reuse the fluorocarboxylic acid. As a result, this fluorocarboxylic acid concentrate is currently incinerated. Further, since hydrogen fluoride (HF) is generated during this incineration, the incinerator requires a special structure and the maintenance cost is high.

If the fluorocarboxylic acid can be selectively blocked, the above problem can be solved, but such a method is not known.

Therefore, the present invention can treat a fluorinated organic acid solution having 2 to 7 carbon atoms with low energy and a short time, and can selectively separate a fluorinated organic acid having 2 to 7 carbon atoms. The purpose is to provide a processing method.

As a result of diligent studies to solve the above problems, the present inventors have used a membrane having a large molecular weight cut-off and a negative charge on the surface as a filtration membrane (semipermeable membrane), thereby increasing the number of carbon atoms. It has been found that it becomes possible to selectively separate by repelling the negative charge of the fluorine-containing organic acids 2 to 7 and further improve the processing efficiency.

According to one gist of the present invention, it is a method for separating a fluorinated organic acid in a solution containing a fluorinated organic acid having 2 to 7 carbon atoms by filtration, and the filtration membrane contains 2 to 7 carbon atoms. Provided are methods characterized by having a fractional molecular weight greater than that of the fluoroorganic acid and having a negative surface zeta potential.

According to one gist of the present invention, it is a filtration system of a fluorine-containing organic acid solution having 2 to 7 carbon atoms including a filtration membrane, and the semipermeable membrane has a molecular weight of a fluorine-containing organic acid having 2 to 7 carbon atoms. Also provided are filtration systems characterized by having a large fractional molecular weight and having a negative surface zeta potential.

According to the present invention, by using a filtration membrane having a fractional molecular weight larger than the molecular weight of a fluoroorganic acid having 2 to 7 carbon atoms and having a negative surface zeta potential (that is, a negative charge), the membrane is used. A high molecular weight can be achieved even when the pressure applied to the substance is relatively small, and further, it can be selectively separated by repelling the negative charge of the fluorine-containing organic acid having 2 to 7 carbon atoms. .. That is, according to the present invention, it is possible to process a solution of a fluorine-containing organic acid having 2 to 7 carbon atoms with low energy in a short time.

It is a graph which shows the blocking rate of PFHxA in an Example.

In the method of the present invention, a fluorinated organic acid having 2 to 7 carbon atoms in a fluorinated organic acid solution having 2 to 7 carbon atoms is separated using a filtration membrane.

In a preferred embodiment, filtration is performed by reverse osmosis.

Fluorine-containing organic acid having 2 to 7 carbon atoms In the fluorine-containing organic acid having 2 to 7 carbon atoms contained in the solution treated by the method of the present invention, the number of carbon atoms is preferably 3 to 7, preferably 5 to 7. It is more preferable to have it, and it is particularly preferable to have 6 to 7.

Examples of the fluorine-containing organic acid having 2 to 7 carbon atoms contained in the solution treated by the method of the present invention include a fluorine-containing carboxylic acid having 2 to 7 carbon atoms and a salt thereof.

As a fluorine-containing carboxylic acid having 2 to 7 carbon atoms, the formula (i):
X ^1- Rf ^1- COOH (i)
[In the formula, X ¹ is H, F or Cl, and Rf ¹ is a linear or branched fluoroalkylene group having 1 to 6 carbon atoms and a monooxyfluoroalkylene group having 1 to 6 carbon atoms. It is a group or a group having 1 to 6 carbon atoms having a polyoxyfluoroalkylene group. ]
Examples thereof include compounds represented by.

As the linear or branched fluoroalkylene group having 1 to 6 carbon atoms in the above Rf ¹ _{, for example, CF 2} , C ₂ F ₄ , C ₃ F ₆ , C ₄ F ₈ , C ₅ F ₁₀ , C. _{6 F 12, CHF, C 2} F 3 H, C 2 F 2 H 2, C 2 FH 3, C 3 F 5 H, C 3 F 4 H 2, C 3 F 3 H 3, C 3 F 2 H 4 , C ₃ FH ₅ , C ₄ F ₇ H, C ₄ F ₆ H ₂ , C ₄ F ₅ H ₃ , C ₄ F ₄ H ₄ , C ₄ F ₃ H ₅ , C ₄ F ₂ H ₆ , C ₄ FH ₇ , C ₅ F ₉ H, C ₅ F ₈ H ₂ , C ₅ F ₇ H ₃ , C ₅ F ₆ H ₄ , C ₅ F ₅ H ₅ , C ₅ F ₄ H ₆ , C ₅ F ₃ H ₇ , C ₅ F ₂ H ₈ , C ₅ FH ₉ , C ₆ F ₁₁ H, C ₆ F ₁₀ H ₂ , C ₆ F ₉ H ₃ , C ₆ F ₈ H ₄ , C ₆ F ₇ H ₅ , C ₆ F _{6 H 6, C 6 F 5 H} 7, C 6 F 4 H 8, C 6 F 3 H 9, C 6 F 2 H 10, C 6 FH 11 and the like.

The fluorine-containing carboxylic acid having 2 to 7 carbon atoms is represented by the formula (ia):
X ^2- Rf ^2- COOH (ia)
[In the formula, X ² is H or F, and Rf ² is formula (a) :.
(CF ₂ ) _l − (CF ₂ OCF ₂ ) _m − (CF ₂ OCF (CF ₃ )) _n (a)
It is a group indicated by
In the above formula (a), l is an integer of 0 or 1 to 4, m is an integer of 0 or 1 to 3, n is 0, 1 or 2, but l + 2m + 3n does not exceed 6. , M and n are both 0, they are excluded, and the order of existence of each repeating unit enclosed in parentheses is arbitrary. ]
It is more preferably the perfluorocarboxylic acid represented by.

In the above-mentioned fluorine-containing carboxylic acid, the number of carbon atoms is preferably 3 to 7, more preferably 5 to 7, and particularly preferably 6 to 7.

As a fluorine-containing carboxylic acid having 5 to 7 carbon atoms, which is a preferred embodiment, for example,
CF ₃ OCF (CF ₃ ) CF ₂ OCF (CF ₃ ) COOH,
CF ₃ CF ₂ OCF ₂ CF ₂ OCF ₂ COOH,
CF ₃ OCF ₂ CF ₂ CF ₂ OCHFCF ₂ COOH,
CF ₃ CF ₂ OCF ₂ CF ₂ OCF ₂ COOH,
CF ₃ OCF ₂ CF ₂ CF ₂ OCHFCF ₂ COOH,
CF ₃ (CF ₂ ) ₄ COOH,
CF ₃ CF ₂ CF ₂ OCF (CF ₃ ) COOH,
F (CF ₂ ) ₄ CH ₂ CF ₂ COOH
H (CF ₂ ) ₆ COOH,
H (CF ₂ ) ₄ COOH,
CH ₂ = CFCF ₂ OCF (CF ₃ ) COOH
Etc. can be exemplified.

In the present invention, the "solution of a fluorine-containing organic acid having 2 to 7 carbon atoms" is not particularly limited as long as it is a liquid containing a fluorine-containing organic acid having 2 to 7 carbon atoms, and the fluorine-containing organic acid having 2 to 7 carbon atoms. The solution is not limited to a solution in which an organic acid is dissolved, and may be a suspension or an emulsion in which a fluorine-containing organic acid having 2 to 7 carbon atoms is dispersed. In a preferred embodiment, the fluorine-containing organic acid solution having 2 to 7 carbon atoms is a solution in which the fluorine-containing organic acid having 2 to 7 carbon atoms is dissolved.

In the solution of the fluorine-containing organic acid having 2 to 7 carbon atoms, the fluorine-containing organic acid having 2 to 7 carbon atoms may be present alone or in two or more kinds.

In a solution of a fluorinated organic acid having 2 to 7 carbon atoms, the fluorinated organic acid having 2 to 7 carbon atoms may be in an ionized form, a non-ionized form or a salt form, or two or three of these. The forms may coexist. Preferably, the fluorinated organic acid having 2 to 7 carbon atoms may be in a form in which at least a part of the fluorinated organic acid is ionized.

The solvent in the solution of the fluorine-containing organic acid having 2 to 7 carbon atoms is not particularly limited, and may be water, an organic solvent, or a mixture of two or more kinds of solvents. The organic solvent is not particularly limited and may be aqueous or non-aqueous. For example, alcohols (for example, methanol, ethanol, isopropyl alcohol, etc.), esters such as methyl acetate, ethers (diethyl ether). Etc.), aliphatic hydrocarbons (hexane, octane, etc.), ketones (acetone, etc.), acetonitrile and the like. The solvent is preferably an aqueous solvent, more preferably water.

Impurities, that is, other solutes, may be present in the solution of the fluorine-containing organic acid having 2 to 7 carbon atoms. Examples of other solutes include metal salts (NaCl, KCl, Na ₂ SO _4, etc.), organic salts, and the like.

The pH of the solution of the fluorine-containing organic acid having 2 to 7 carbon atoms is not particularly limited, but is preferably pH 4 or higher, preferably pH 7 or higher. By setting the pH to 7 or more, the inhibition rate of the fluorine-containing organic acid having 2 to 7 carbon atoms can be further increased. The pH can be adjusted without particular limitation by adding a hydrochloric acid solution or a sodium hydroxide solution.

The solution of the fluorine-containing organic acid having 2 to 7 carbon atoms can be, for example, a solution discharged as industrial wastewater or a solution discharged in a laboratory or the like.

Filtration membrane The filtration membrane (including the semipermeable membrane) used in the method of the present invention has a molecular weight cut-off larger than the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms, and has a negative surface zeta potential.

The "molecular weight cut-off" is well known to those skilled in the art as an index showing the pore size of a membrane, and membrane filtration is performed using an index substance to obtain the respective blocking rates, and the molecular weight corresponding to the blocking rate of 90% is obtained. The molecular weight is cut off (see Membrane Filtration Technology, p.54, Industrial Research Council, 2006).

The "surface zeta potential" is well known to those skilled in the art as an index showing the chargeability of the membrane, and the potential difference generated on the membrane surface when the electrolyte solution is flowed is measured and calculated by the following Helmholtz-Smoluchowski formula. ..
ζ = (ηε) × (1 / ρ) × (E / P)
ζ: Surface zeta potential η: Electrolyte viscosity (poise)
ε: Electrolyte conductivity (S / cm)
ρ: Specific resistance of solution (Ω · cm)
E: Measured potential P: Intermembrane pressure

The fractional molecular weight of the filtration membrane (semipermeable membrane) used in the present invention may be larger than the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms, but is preferably 5000 Da or more and 50,000 Da or less, and more preferably 8000 Da. It is 30,000 Da or less, and more preferably 15,000 Da or more and 25,000 Da or less. By increasing the molecular weight of the fractionation to be larger than the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms, the processing capacity of the film is improved. That is, the amount of solvent permeated per unit area of the membrane becomes large. The processing capacity improves as the molecular weight cut-off increases. Also, the pressure required for filtration (typically reverse osmosis) is reduced. Furthermore, it is possible to prevent other impurities that are not desired to be separated from being separated together with the fluorine-containing organic acid having 2 to 7 carbon atoms. Further, by setting the molecular weight cut-off to 20000 Da or less, the inhibition rate of the fluorine-containing organic acid having 2 to 7 carbon atoms can be further increased.

The surface zeta potential of the filtration membrane (semipermeable membrane) used in the present invention has a negative value, preferably -10 mV or less, more preferably -30 mV or less, still more preferably -50 mV or less, still more preferably −. It is 80 mV or less. The lower limit of the surface zeta potential is not particularly limited, but is preferably −300 mV or higher, for example, −200 mV or higher or −150 mV or higher. Since the filter membrane has a negative surface zeta potential, it contains 2 to 7 carbon atoms even when a membrane having a fractional molecular weight larger than the molecular weight of the fluoroorganic acid containing 2 to 7 carbon atoms is used. The negative charge of the fluoroorganic acid and the negative surface charge of the film repel each other, so that they can be separated with a high blocking rate. The larger the negative surface zeta potential (ie, the larger the negative charge on the film surface), the higher the fractional molecular weight of the film can be used.

The surface zeta potential of the film can be appropriately changed by selecting the material forming the film. The surface zeta potential of the membrane can also be adjusted by adjusting the pH of the solution.

In a preferred embodiment, the filtration membrane used in the present invention has a molecular weight cut-off of 5000 Da or more and 50,000 Da or less, preferably 8000 Da or more and 30000 Da or less, more preferably 15000 Da or more and 25000 Da or less, and has a molecular weight cut-off of -30 mV or less, preferably -50 mV or less. More preferably, it has a surface zeta potential of −80 mV or less.

The material constituting the filter film used in the present invention is not particularly limited as long as it can have a negative surface zeta potential, and is, for example, a total aromatic polyamide resin, a polyvinyl alcohol resin, a piperazine amide resin, and a sulfone. Examples thereof include modified polyether sulfone.

The filtration membranes used in the present invention can be obtained, for example, from Nitto Denko KK as NTR7450 and NTR7410.

The conditions (intermembrane pressure, temperature, etc.) in the method of the present invention can be appropriately set by those skilled in the art according to various factors such as the composition of the solution to be treated and the type of membrane to be used.

According to the method of the present invention, a fluorine-containing organic acid having 2 to 7 carbon atoms is satisfactorily selected even though the filter membrane has a fractionated molecular weight larger than the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms. Can be separated.

Therefore, the present invention is also a filtration system for a fluorine-containing organic acid solution having 2 to 7 carbon atoms including a filtration membrane, and the semipermeable membrane has a larger molecular weight than the fluorine-containing organic acid having 2 to 7 carbon atoms. Also provided is a filtration system characterized by having a fractionated molecular weight and having a negative surface zeta potential.

Although not bound by any theory, the present invention uses a film having a molecular weight fractionation larger than that of the fluorine-containing organic acid having 2 to 7 carbon atoms, but the fluorine-containing organic acid having 2 to 7 carbon atoms. The reason why can be separated is considered as follows. The filtration membrane used in the present invention has a negative charge on its surface. On the other hand, fluorine-containing organic acids having 2 to 7 carbon atoms also have a negative charge. As a result, the negative charges of each other repel each other, and it is considered that this electric repulsive force makes it difficult for the fluorine-containing organic acid having 2 to 7 carbon atoms to approach the film.

An aqueous solution of perfluorocaproic acid (hereinafter, also referred to as PFHxA) (initial concentration 100 ng / L) was subjected to a membrane filtration test using a cross-flow type flat membrane test apparatus. As membranes, Nitto Denko's NTR7450 (molecular weight cut-off: 10000 Da, surface zeta potential -33 mV; nominal NaCl inhibition rate: 50%) and NTR7410 (molecular weight cut-off: 20000 Da, surface zeta potential -99 mV; nominal NaCl inhibition rate: 10). %) Was used. The processing conditions are as follows. The pH was not adjusted from the initial value.
Intermembrane pressure: 0.700 MPa
Circulation flow rate: 1.0 ± 0.1 L / min Water temperature: 20 ± 2 ° C

Ｃｐ：透過液濃度（ｎｇ／Ｌ）
Ｃｓ_１：透過液採取開始時濃縮液濃度（ｎｇ／Ｌ）
Ｃｓ_２：透過液採取終了時濃縮液濃度（ｎｇ／Ｌ） After 0, 6, 12, 24, 48 and 72 hours, 500 mL of the permeate and 50 mL of the stock solution were collected at the start and end of the permeation collection, and the PFHxA concentration was analyzed. The concentration was measured by HPLC / MS / MS after concentrating the sample by the solid-phase extraction method. Based on the obtained results, the inhibition rate of PFHxA in each test was calculated by the following formula (1).

Cp: Permeate concentration (ng / L)
Cs ₁ : Concentration solution concentration (ng / L) at the start of permeate collection
Cs ₂ : Concentration solution concentration (ng / L) at the end of permeate collection

The transition of the PFHxA inhibition rate of each membrane is shown in FIG. The inhibition rates after 72 hours were 99.0% (NTR7450) and 98.3% (NTR7410), respectively, and despite the low NaCl inhibition rates (50% and 10%, respectively), the high PFHxA inhibition rates. Was confirmed to be obtained. Since these membranes are sulfonated polyether sulfone membranes and have a negative charge on the surface of the membrane, it is considered that PFHxA was blocked by repelling the negative charge of PFHxA.

Since the method of the present invention has a high processing capacity and can selectively separate a fluorine-containing organic acid having 2 to 7 carbon atoms, it is used for treating a waste liquid containing a fluorine-containing organic acid having 2 to 7 carbon atoms. be able to.

Claims (14)

This is a method of separating a fluorine-containing organic acid having 2 to 7 carbon atoms in a fluorine-containing organic acid solution having 2 to 7 carbon atoms by filtration, and the filtration film is based on the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms. A method characterized by having a large fractional molecular weight, a negative surface zeta potential, and the fluorine-containing organic acid solution having 2 to 7 carbon atoms containing Na ₂ SO _4. The method of claim 1, wherein the filtration is performed by reverse osmosis. The method according to claim 1 or 2, wherein the fractional molecular weight of the filtration membrane is 5000 Da or more and 50,000 Da or less. The method according to any one of claims 1 to 3, wherein the fractional molecular weight of the filtration membrane is 8000 Da or more and 30000 Da or less. The method according to any one of claims 1 to 4, wherein the fractional molecular weight of the filtration membrane is 15,000 Da or more and 25,000 Da or less. The method according to any one of claims 1 to 5, wherein the surface zeta potential of the filtration membrane is −30 mV or less. The method according to any one of claims 1 to 6, wherein the surface zeta potential of the filtration membrane is −50 mV or less. The method according to any one of claims 1 to 7, wherein the surface zeta potential of the filtration membrane is −80 mV or less. The method according to any one of claims 1 to 8, wherein the fractional molecular weight of the filtration membrane is 5000 Da or more and 50,000 Da or less, and the surface zeta potential is −30 mV or less. The method according to any one of claims 1 to 9, wherein the fluorine-containing organic acid having 2 to 7 carbon atoms is a fluorine-containing carboxylic acid having 2 to 7 carbon atoms or a salt thereof. The fluorine-containing carboxylic acid having 2 to 7 carbon atoms has the formula (i):
X ^1- Rf ^1- COOH (i)
[In the formula, X ¹ is H, F or Cl, and Rf ¹ is a linear or branched fluoroalkylene group having 1 to 6 carbon atoms and a monooxyfluoroalkylene group having 1 to 6 carbon atoms. It is a group or a group having 1 to 6 carbon atoms having a polyoxyfluoroalkylene group. ]
Fluorine-containing carboxylic acid represented by, or formula (ia):
X ^2- Rf ^2- COOH (ia)
[In the formula, X ² is H or F, and Rf ² is formula (a) :.
(CF ₂ ) _l − (CF ₂ OCF ₂ ) _m − (CF ₂ OCF (CF ₃ )) _n (a)
(In formula (a), l is an integer of 0 or 1-4, m is an integer of 0 or 1-3, n is 0, 1 or 2, where l + 2m + 3n does not exceed 6. , M and n are both 0, they are excluded, and the order of existence of each repeating unit in parentheses above is arbitrary.)
It is a group indicated by. ]
10. The method of claim 10, characterized in that it is a perfluorocarboxylic acid represented by. The method according to any one of claims 1 to 11, wherein the fluorine-containing organic acid having 2 to 7 carbon atoms has 5 to 7 carbon atoms. Fluorine-containing organic acids with 2 to 7 carbon atoms
CF ₃ OCF (CF ₃ ) CF ₂ OCF (CF ₃ ) COOH,
CF ₃ CF ₂ OCF ₂ CF ₂ OCF ₂ COOH,
CF ₃ OCF ₂ CF ₂ CF ₂ OCHFCF ₂ COOH,
CF ₃ CF ₂ OCF ₂ CF ₂ OCF ₂ COOH,
CF ₃ OCF ₂ CF ₂ CF ₂ OCHFCF ₂ COOH,
CF ₃ (CF ₂ ) ₄ COOH,
CF ₃ CF ₂ CF ₂ OCF (CF ₃ ) COOH,
F (CF ₂ ) ₄ CH ₂ CF ₂ COOH
H (CF ₂ ) ₆ COOH,
H (CF ₂ ) ₄ COOH, or CH ₂ = CFCF ₂ OCF (CF ₃ ) COOH
The method according to any one of claims 1 to 12, wherein the method is characterized by the above. A filtration system for a fluorine-containing organic acid solution having 2 to 7 carbon atoms having a filtration membrane, wherein the semipermeable membrane has a fractional molecular weight larger than the molecular weight of the fluorine-containing organic acid having 2 to 7 carbon atoms, and is negative. A filtration system characterized by having a surface zeta potential of 2 to 7 and containing Na ₂ SO _{4 in the fluorine-containing organic acid solution having 2 to 7 carbon atoms.}

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