Draw solute

Abstract

To provide a draw solute which can be suitably used for a draw solute having temperature sensitivity of a high-temperature dissolution type in a solution (upper critical solution temperature type: UCST type), a draw solution containing the draw solute, a water treatment device using the draw solution, and a water treatment method using the draw solute.SOLUTION: A draw solute contains a polymer which has (A) one or more structural units derived from a monomer having, as a terminal substituent, an amino group, a quaternary ammonium group, or a nitrogen heterocyclic structure, and (B) a structural unit derived from an acid group-containing monomer, and has a molar composition ratio of the structural unit (A) to the structural unit (B) of 1:10 to 2:1.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a draw solution, a water treatment method, and a water treatment device, and more particularly to a draw solution suitable for use in a forward osmosis membrane process. [Background technology]

[0002] The forward osmosis membrane method uses the phenomenon of solvent migration from the side with lower osmotic pressure to the side with higher osmotic pressure when two solutions of different concentrations are brought into contact via a semipermeable membrane, and can be used to separate the components of a solution. Compared to the reverse osmosis membrane method, which applies pressure to a solution against osmotic pressure to force the liquid through a membrane, the forward osmosis membrane method, which uses osmotic pressure to perform membrane filtration, is more energy-efficient and is expected to be applied to water treatment such as seawater desalination and power generation.

[0003] When water is treated using a forward osmosis membrane process, a solution (draw solution) with a higher osmotic pressure than the solution to be treated (the solution to be treated) is used to move a solvent (water) from the solution to be treated to the draw solution through a semipermeable membrane. Since the solvent must then be recovered from the draw solution, the draw solution must have properties that allow for easy solvent separation. Various osmotic pressure inducers (draw solutes) for preparing such draw solutions have been investigated. For example, Patent Document 1 below proposes the use of a "block copolymer having a glycerin skeleton and containing ethylene oxide groups as the hydrophilic portion and groups consisting of propylene oxide and / or butylene oxide as the hydrophobic portion" as a temperature-sensitive water absorbent (draw solute) that aggregates and separates the solvent when heated. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 156404 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to broaden the scope of application of the forward osmosis membrane method to various technologies in the future, it is desirable to increase the variety of draw solutes so that the optimum draw solution can be selected according to the process. For example, the draw solute in Patent Document 1 has the property of flocculating when heated (lower critical solution temperature: LCST type), but there is also a need to develop a draw solute that is soluble at high temperatures in solution (upper critical solution temperature: UCST type) and has temperature sensitivity.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a draw solute that can be suitably used in a UCST-type draw solution, a draw solution containing the draw solute, a water treatment device using the draw solution, and a water treatment method using the draw solute. [Means for solving the problem]

[0007] The present inventors have conducted various studies to achieve the above object and have come up with the present invention. The draw solute of the present disclosure is a draw solute containing a polymer having one or more structural units (A) selected from the group consisting of structural units represented by the following general formula (1-1), structural units represented by the general formula (1-2), structural units represented by the general formula (1-3), structural units represented by the general formula (1-4), structural units represented by the general formula (1-5), and structural units represented by the general formula (1-6), and structural units (B) derived from an acid group-containing monomer, wherein the molar ratio of the structural units (A) to the structural units (B) is 1:10 to 2:1.

[0008] [ka]

[0009] [ka]

[0010] [ka] In the general formulae (1-1) to (1-6), R0 represents a hydrogen atom or a methyl group, R2, R3, and R4 each independently represent an organic group having 1 to 20 carbon atoms, and R2 and R3 may be bonded to form a ring structure, provided that in the general formula (1-3), R2 and R3 may be a hydrogen atom. In the general formula (1-1) and the general formula (1-2), X is an oxygen atom or a nitrogen atom, and R1 is an alkylene group having 1 to 4 carbon atoms which may have a substituent. In the general formula (1-1) and the general formula (1-2), n1 is the number of hydrogen atoms bonded to X, and is 0 when X is an oxygen atom, and is 0 or 1 when X is a nitrogen atom. In general formula (1-1) and general formula (1-2), n2 is the number of amino groups represented by the structure in parentheses that bond to X, and is 1 when X is an oxygen atom, and is 1 or 2 when X is a nitrogen atom. In the general formulas (1-1) and (1-2), when X is a nitrogen atom, the sum of n1 and n2 is 2. In general formulas (1-3) to (1-4), R5 represents a direct bond, CH2, or any of CH2CH2, OCH2CH(—OH)CH2, CH2OCH2CH(—OH)CH2, CH2CH2OCH2CH(—OH)CH2, or an optionally substituted cyclic hydrocarbon group having 5 to 20 carbon atoms. In the general formula (1-1) and the general formula (1-3), the amino group may be neutralized or may be an amine oxide. In the general formula (1-6), R6 represents a group containing a ring structure containing at least one nitrogen atom. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a draw solute for a forward osmosis membrane process that can be suitably used in a UCST-type draw solution, a draw solution containing the draw solute, and a water treatment device using the draw solution. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present disclosure will be described in detail below. Note that a combination of two or more of the individual preferred embodiments of the present disclosure described below is also a preferred embodiment of the present disclosure.

[0013] [Terminology] In the present disclosure, a structural unit derived from an acid group-containing monomer generally refers to a structural unit having a structure formed by polymerization of an acid group-containing monomer. The structural unit derived from an acid group-containing monomer is not limited to a structural unit actually obtained by polymerization of an acid group-containing monomer; even a structural unit formed by a different method is included in the structural unit derived from an acid group-containing monomer as long as it has the same structure as the structure formed by polymerization of the acid group-containing monomer. For example, when the acid group-containing monomer is acrylic acid (CH═CH(COOH)), the structural unit derived from the acid group-containing monomer can be represented by —CH—CH(COOH)—.

[0014] In the present disclosure, a structural unit derived from a nonionic group-containing monomer generally refers to a structural unit having a structure formed by polymerization of a nonionic group-containing monomer. The structural unit derived from a nonionic group-containing monomer is not limited to a structural unit actually obtained by polymerization of a nonionic group-containing monomer; even a structural unit formed by a different method is included in the structural unit derived from the nonionic group-containing monomer as long as it has the same structure as the structure formed by polymerization of the nonionic group-containing monomer. For example, when the nonionic group-containing monomer is methyl acrylate (CH═CH(COOCH),), the structural unit derived from the acid group-containing monomer can be represented by -CH—CH(COOCH)—.

[0015] [Draw solute] <Polymer of the Present Disclosure> The draw solute of the present disclosure preferably contains a polymer (hereinafter also referred to as the polymer of the present disclosure) having one or more structural units (A) selected from the group consisting of structural units represented by the following general formula (1-1), structural units represented by the general formula (1-2), structural units represented by the general formula (1-3), structural units represented by the general formula (1-4), structural units represented by the general formula (1-5), and structural units represented by the general formula (1-6), and a structural unit (B) derived from an acid group-containing monomer, wherein the molar ratio of the structural units (A) to the structural units (B) is 1:10 to 2:1.

[0016] (Structural unit (A)) The draw solute of the present disclosure includes the polymer of the present disclosure described above.

[0017] The polymer of the present disclosure may contain only one type of structural unit selected from the group consisting of structural units represented by the above general formulas (1-1) to (1-6), or may contain two or more types. When two or more types of structural units selected from the group consisting of structural units represented by the above general formulas (1-1) to (1-6) are contained, the combination thereof is not particularly limited. Furthermore, the polymer may contain two or more types of any one of the structural units represented by the above general formulas (1-1) to (1-6), for example, the structural unit represented by the general formula (1-1).

[0018] In the above general formulas (1-1) to (1-4) and (1-6), R0 represents a methyl group or a hydrogen atom. In general formulas (1-1) and (1-2), R0 is preferably a methyl group from the viewpoint of stability at high temperatures, and in general formulas (1-3) and (1-4), R0 is preferably a methyl group from the viewpoint of production (polymerizability of raw material monomers).

[0019] In the general formulas (1-1) and (1-2), X represents an oxygen atom or a nitrogen atom.

[0020] In the above general formulas (1-1) to (1-2), R1 represents an alkylene group having 1 to 4 carbon atoms which may have a substituent. In the above general formulas (1-1) to (1-2), R1 is more preferably CH2CH2, CH2CH2CH2, or CH2CH(-OH)CH2. The substituent is not particularly limited, but examples include the groups described below.

[0021] In the above general formulas (1-3) to (1-4), R5 represents a direct bond, CH2, or any of CH2CH2, OCH2CH(—OH)CH2, CH2OCH2CH(—OH)CH2, CH2CH2OCH2CH(—OH)CH2, or a cyclic hydrocarbon group having 5 to 20 carbon atoms which may have a substituent. The cyclic hydrocarbon group having 5 to 20 carbon atoms, which may have a substituent, may be either monocyclic or polycyclic, and may have one or more substituents. Examples of the cyclic hydrocarbon group having 5 to 20 carbon atoms include a phenyl group, a naphthyl group, a cyclohexyl group, and a benzyl group. In the cyclic hydrocarbon group having 5 to 20 carbon atoms, which may have a substituent, only the cyclic hydrocarbon group may be bonded to a residue other than R5, or one or more substituents may be bonded to a residue other than R5. The substituent is not particularly limited, and examples thereof include the groups described below.

[0022] In the general formulae (1-1) to (1-5), R2, R3, and R4 each independently represent an organic group having 1 to 20 carbon atoms, provided that in the general formula (1-3), R2 and R3 may be a hydrogen atom. The organic group having 1 to 20 carbon atoms is not particularly limited, but examples thereof include an alkyl group, an aryl group, and an alkylene group, which may have one or more substituents. The substituent is not particularly limited, but examples thereof include a hydroxyl group, an oxyalkylene group, an oxyaryl group, an amino group, a carbonyl group, an ester group, an amide group, a halogen atom; an acid group such as a sulfonic acid group, a sulfate ester group, a phosphoric acid group, a phosphate ester group, and a carboxy group, and salts thereof. When the organic group having 1 to 20 carbon atoms has a substituent, the number of carbon atoms including the substituent may be 1 to 20.

[0023] In general formula (1-1) and general formula (1-3), the amino group may be neutralized or may be an amine oxide. The amino group in general formula (1-1) and general formula (1-3) may be neutralized with an inorganic acid or base. In this case, the counter anion is not particularly limited, and examples thereof include inorganic anions such as chloride ion, bromide ion, borate ion, carbonate ion, bicarbonate ion, and phosphate ion; and organic acid anions such as acetate ion, succinate ion, citrate ion, malate ion, lactate ion, and fumarate ion.

[0024] The general formulae (1-2), (1-3), and (1-5) usually have a counter anion. There are no limitations on the counter anion, but examples include the counter anions exemplified in the description of the general formulae (1-1) and (1-3).

[0025] In general formula (1-6), R6 represents a group containing a ring structure containing at least one nitrogen atom. The ring structure may be monocyclic or polycyclic. The number of atoms constituting the ring structure is preferably 4 to 20, more preferably 5 to 10. The atoms constituting the ring structure may include one or more nitrogen atoms, and it is preferable that the number of nitrogen atoms is one or more and five or less. The ring structure may have one or more substituents, and preferred examples of the substituents include the substituents exemplified above as the substituents for the organic group having 1 to 20 carbon atoms. The ring structure is not particularly limited, but examples thereof include a pyridine ring, an imidazole ring, a pyrrole ring, a morpholine ring, and an oxazoline ring.

[0026] The polymer of the present disclosure more preferably contains a structural unit containing a quaternary amine salt, since this tends to reduce pH dependency.

[0027] The method for forming the structural unit (A) is not particularly limited, but it may be formed by polymerizing a monomer component essentially containing a monomer selected from the following (2-1) to (2-6).

[0028] [ka]

[0029] [ka]

[0030] [ka]

[0031] The embodiments and preferred embodiments of R0, X, R1, R2, R3, R4, and R5 in the structural formulae (2-1) to (2-6) are the same as those in the structural formulae (1-1) to (1-6) above.

[0032] The monomer represented by general formula (2-1) is not particularly limited, and examples thereof include dialkylaminoalkyl(meth)acrylates such as dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, and dimethylaminopropyl acrylate; alkylaminoalkyl(meth)acrylamides such as dimethylaminoethyl acrylamide, dimethylaminoethyl methacrylamide, and dimethylaminopropyl acrylamide; alkylamine adducts such as dimethylamine and diethylamine to glycidyl acrylate or glycidyl methacrylate; 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propane-1-sulfonic acid; and 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1-sulfonic acid.

[0033] The monomer represented by general formula (2-2) is not particularly limited, but examples thereof include monomers in which the amino group of the monomer represented by general formula (2-1) is quaternized, and examples of monomers in the form of a counter anion include methacroylcholine chloride, dimethylaminoethyl acrylate benzyl chloride quaternary salt, and dimethylaminopropylacrylamide methyl chloride quaternary salt.

[0034] The monomer represented by the general formula (2-3) is not particularly limited, but examples thereof include vinylamine, allylamine, and alkylamine adducts such as dimethylamine and diethylamine to allyl glycidyl ether.

[0035] The monomer represented by the general formula (2-4) is not particularly limited, but examples thereof include a monomer in which the amino group of the monomer represented by the general formula (2-3) is quaternized.

[0036] The monomer represented by the general formula (2-5) is not particularly limited, but examples thereof include diallyldimethylammonium chloride.

[0037] The monomer represented by the general formula (2-6) is not particularly limited, but examples thereof include vinylpyridine, vinylimidazole, acryloylmorpholine, methacryloylmorpholine, and 2-isopropenyl-2-oxazoline.

[0038] The method for forming the structural unit (A) is not particularly limited, and it may be formed by a method in which a monomer component essentially containing a monomer modified with a monomer selected from the above (2-1) to (2-6) is polymerized and then post-treated. For example, instead of using vinylamine as the monomer represented by general formula (2-3), the structural unit may be formed by hydrolyzing vinylformamide or vinylacetamide after polymerization.

[0039] (Structural unit (B)) The polymer of the present disclosure contains a structural unit (B) derived from an acid group-containing monomer. The polymer of the present disclosure contains one or more types of structural units (B) derived from an acid group-containing monomer. The acid group contained in the structural unit (B) is not particularly limited as long as it is a structural unit having an acid group, and examples thereof include a carboxy group, a sulfonic acid group, a carboxy group, a sulfonic acid group, a sulfate ester group, a phosphoric acid group, and salts or acid anhydrides thereof.

[0040] The structural unit (B) derived from an acid group-containing monomer is not particularly limited, but examples thereof include the structural unit of the following general formula (3-1).

[0041] [ka]

[0042] In the general formula (3-1), R7 represents a hydrogen atom or an alkyl or alkenyl group having 1 to 5 carbon atoms which may have a substituent. In the general formula (3-1), R8 represents a direct bond, CH2, or CH2CH2, or a monocyclic or polycyclic hydrocarbon group which may have a substituent. In the general formula (3-1), Y represents a direct bond, an oxygen atom, -C(=O)-O-, or -C(=O)-NH-. In general formula (3-1), R9 represents a direct bond, an alkyl or alkenyl group having 1 to 5 carbon atoms which may have a substituent, or a monocyclic or polycyclic hydrocarbon group which may have a substituent. In the general formula (3-1), A represents an acid group, which may be neutralized. In the general formula (3-1), B represents a hydrogen atom or an acid group. When B is an acid group, it may be neutralized or may form an acid anhydride with A. In the general formula (3-1), when R7 to R9 have a substituent, preferred examples of the substituent include the substituents exemplified above as the substituents of the organic group having 1 to 20 carbon atoms. In general formula (3-1), when A and / or B are neutralized, they form, for example, metal salts, ammonium salts, or organic amine salts, and examples thereof include, but are not limited to, alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts and calcium salts; salts of transition metals such as aluminum and iron; alkanolamine salts such as monoethanolamine salts, diethanolamine salts, and triethanolamine salts; alkylamine salts such as monoethylamine salts, diethylamine salts, and triethylamine salts; and polyamine salts such as ethylenediamine salts and triethylenediamine salts. A and / or B is preferably an acid form (unneutralized), an alkali metal salt, or an ammonium salt, more preferably an acid form (unneutralized), a sodium salt, or a potassium salt.

[0043] Examples of the acid group-containing monomer include unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, α-hydroxymethylacrylic acid, and salts thereof; unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, methylene glutaric acid, methylene malonic acid, and salts or acid anhydrides thereof; 3-(meth)allyloxy-2-hydroxypropanesulfonic acid, 2-(meth)allyloxyethylenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-styrenesulfonic acid, and the like. acid, α-methyl-p-styrenesulfonic acid, vinylsulfonic acid, vinylsulfamic acid, (meth)allylsulfonic acid, isoprenesulfonic acid, 4-(allyloxy)benzenesulfonic acid, 1-methyl-2-propene-1-sulfonic acid, 1,1-dimethyl-2-propene-1-sulfonic acid, 3-butene-1-sulfonic acid, 1-butene-3-sulfonic acid, 2-acrylamido-1-methylpropanesulfonic acid, 2-acrylamidopropanesulfonic acid, 2-acrylamido-n-butanesulfone Examples of the alkyl acrylate include unsaturated sulfonic acids such as 2-acrylamido-2-phenylpropanesulfonic acid and 2-((meth)acryloyloxy)ethanesulfonic acid, and salts thereof; 2-(meth)acryloyloxyethyl acid phosphate, 2-(meth)acryloyloxypropyl acid phosphate, polyoxypropylene allyl ether phosphate ester, vinylphosphonic acid, allylphosphonic acid, 2-acrylamido-2-methylpropanephosphonic acid, α-phosphonostyrene, 2-methylacrylamido-2-methylpropanephosphonic acid, 2-phosphoethyl(meth)acrylate, 2-phosphopropyl(meth)acrylate, 3-phosphopropyl(meth)acrylate, and salts thereof; sulfates of polyoxyethylene alkyl allyl ethers, sulfates of polyoxyethylene polyoxypropylene alkyl allyl ethers, sulfates of polyoxyalkylene mono(meth)acrylates, and sulfates of polyoxyalkylene bisphenol A mono(meth)acrylates.

[0044] (Structural unit (C)) The polymer of the present disclosure may have a structural unit (C) derived from a nonionic group-containing monomer. The structural unit (C) is not particularly limited, but is introduced for the purpose of improving the separability of the draw solute of the present disclosure from water or adjusting the interaction between the polymers of the present disclosure. The structural unit (C) is not particularly limited as long as it contains a nonionic group, however, in the present disclosure, structural units corresponding to the structural unit (A) or the structural unit (B) are not included in the structural unit (C).

[0045] The nonionic group-containing monomer is not particularly limited, but examples thereof include (meth)acrylic acid alkyl ester monomers such as butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and dodecyl (meth)acrylate; vinyl aryl monomers such as styrene, vinyl toluene, and indene; olefin monomers such as isobutylene; vinyl acetate and acrylonitrile; amide monomers such as (meth)acrylamide, N,N-dimethylacrylamide, N-isopropylacrylamide, N-vinylpyrrolidone, N-vinylcaprolactam, acryloylmorpholine, N-vinylformamide, and N-vinylacetamide; unsaturated alcohols such as (meth)allyl alcohol and isoprenol; monomers having a structure in which alkylene oxide is added to the above-mentioned unsaturated alcohols; and alkylene glycol chain monomers such as polyalkylene glycol (meth)acrylate. The polymer of the present disclosure may optionally contain one or more types of structural unit (C).

[0046] (Polymer composition of the present disclosure, etc.) In the polymer of the present disclosure, the molar composition ratio of the structural unit (A) to the structural unit (B) is preferably 1:10 to 2:1, more preferably 1:7 to 2:1, and even more preferably 1:2 to 2:1. When the molar ratio is within this range, the polymer tends to be more suitable for use in UCST-type draw solutions.

[0047] When calculating the above composition, if a structural unit corresponds to both the structural unit (A) and the structural unit (B), the composition is calculated by dividing equally. For example, if a structural unit that corresponds to both the structural unit (A) and the structural unit (B) is contained at 50 mol%, the composition is calculated assuming that the structural unit (A) and the structural unit (B) are contained at 25 mol% each.

[0048] The polymer of the present disclosure may contain any structural unit other than the structural unit (A) and the structural unit (B). The total composition of the structural unit (A) and the structural unit (B) in the polymer of the present disclosure is not particularly limited, and may be, for example, 30% by mass or more, 50% by mass or more, or 70% by mass or more, and 100% by mass or less, 90% by mass or less, or 80% by mass or less, relative to 100% by mass of the polymer of the present disclosure.

[0049] The content of the structural unit (C) in the polymer of the present disclosure is not particularly limited, and is, for example, 0% by mass or more, 10% by mass or more, or 20% by mass or more, and 70% by mass or less, 50% by mass or less, or 30% by mass or less, relative to 100% by mass of the polymer of the present disclosure.

[0050] In the polymer of the present disclosure, the structural unit (A) and the structural unit (B) may be present randomly, regularly, or in a block form.

[0051] The weight-average molecular weight of the polymer of the present disclosure is not particularly limited, but is, for example, 2,000 or more, 3,000 or more, 5,000 or more, or 7,000 or more, and is, for example, 200,000 or less, 100,000 or less, or 50,000 or less. The weight-average molecular weight can be measured by GPC (gel permeation chromatography). When the weight-average molecular weight is in the above range, it tends to be more suitable for use in UCST-type draw solutions.

[0052] The polymer of the present disclosure may be produced by, but is not particularly limited to, radical polymerization of a monomer component. The polymer of the present disclosure is preferably produced by solution polymerization, preferably aqueous solution polymerization. The polymer of the present disclosure is preferably produced by polymerizing the monomer component in the presence of a polymerization initiator, and a chain transfer agent may be used. The polymer of the present disclosure may be produced by optionally including a post-treatment step, a purification step, a pH adjustment step, etc. after the polymerization step.

[0053] <Draw Solute Forms of the Present Disclosure> The draw solute of the present disclosure preferably contains one or more polymers of the present disclosure. The draw solute of the present disclosure is not particularly limited, but may contain, for example, 0.1% by mass or more and 100% by mass or less of the polymer of the present disclosure. The draw solute of the present disclosure may contain optional components other than the polymer of the present disclosure. Examples of such optional components include, but are not limited to, antioxidants, preservatives, UV absorbers, antibacterial agents, surfactants, etc.

[0054] The draw solute of the present disclosure may contain, in place of part or all of the polymer of the present disclosure, a polymer (a) containing the structural unit (A) and a polymer (b) containing the structural unit (B). In this case, the molar composition ratio of the structural unit (A) to the structural unit (B) in the total amount of the polymer (a) and the polymer (b) contained in the draw solute of the present disclosure is preferably 1:10 to 2:1, more preferably 1:7 to 2:1, and even more preferably 1:2 to 2:1.

[0055] [Drawing solution] The draw solution of the present disclosure contains the draw solute. The content of the draw solute is preferably 5 to 100% by mass, more preferably 10 to 70% by mass, and even more preferably 15 to 60% by mass, based on the total amount of the draw solution. The draw solution may contain a solvent. The solvent may be appropriately selected depending on the conditions of the forward osmosis membrane method using the draw solution, and one or more solvents selected from water, methanol, ethanol, etc. may be used. It is more preferable that the draw solution contains the same solvent as the solution to be treated (sometimes referred to as the feed solution). The content of the solvent may be, for example, 80 to 0 mass % of the total amount of the draw solution.

[0056] The draw solution may contain draw solutes other than the draw solutes (other draw solutes), but the content of the other draw solutes is preferably 20 mass % or less relative to the total amount of the draw solutes. The draw solution is preferably composed of the draw solutes, any solvent, and any other draw solutes, and more preferably composed of the draw solutes and solvent.

[0057] The draw solution preferably has a cloud point (upper critical solution temperature). The cloud point is the temperature at which a transparent or translucent liquid undergoes phase separation and becomes opaque. By lowering the temperature below the cloud point, phase separation between the draw solute and the solvent can occur.

[0058] For example, when the forward osmosis membrane process is applied to a site where wastewater treatment at high temperatures is required, such as produced water from oil drilling, it is preferable that the draw solution does not undergo phase separation at the high temperatures used in the forward osmosis membrane process, but rather undergoes phase separation at temperatures around room temperature. The cloud point of the draw solution used in such applications is preferably, for example, 0°C to 100°C, more preferably 30°C to 80°C, and even more preferably 40°C to 60°C.

[0059] [Water treatment method] In the forward osmosis membrane process, a feed solution and a draw solution are brought into contact with each other through a semipermeable membrane, and a solvent migrates from the feed solution, which has a lower osmotic pressure, to the draw solution, which has a higher osmotic pressure. As the solvent migrates, the concentration of the draw solution gradually decreases. Therefore, in order to continue the forward osmosis membrane process, it is necessary to separate the draw solute and the solvent contained in the draw solution. When the draw solution has a cloud point, the draw solute and the solvent can be phase-separated by heating.

[0060] In the forward osmosis membrane method using a draw solution having such a cloud point, the forward osmosis membrane method can be continuously carried out by repeating the following treatment, for example. (1) A feed solution is placed on one side of a semipermeable membrane and a draw solution on the other side so that they are in contact with the semipermeable membrane, and the solvent is transferred from the feed solution side to the draw solution side through the semipermeable membrane. (2) The draw solution with reduced concentration is removed and its temperature is adjusted to cause phase separation between the draw solute and the solvent. (3) The phase-separated draw solute is again circulated to the other side. (4) The phase-separated solvent is further purified, for example, using a nanofiltration membrane (NF membrane), to obtain the desired processed product (purified water, etc.).

[0061] As another method, a method can be applied in which a draw solution in which the compatibility with the solvent has been increased by making the draw solute absorb an acidic gas, the solvent is made to permeate through a membrane from the supply liquid side to the draw solution side, and then the acidic gas is removed from the draw solute to cause phase separation between the draw solute and the solvent.

[0062] Examples of the acidic gas include carbon oxides such as carbon monoxide and carbon dioxide, sulfur oxides such as sulfur monoxide, sulfur dioxide and sulfur trioxide, and nitrogen oxides such as nitrogen monoxide, nitrogen dioxide, nitrous oxide, dinitrogen trioxide, dinitrogen tetroxide and dinitrogen pentoxide. Of these, carbon dioxide is preferred as the acidic gas.

[0063] As the semipermeable membrane used in the forward osmosis membrane method, conventionally known membranes can be used, but in order to maintain the strength of the membrane, it is preferable to use a combination of a dense active layer, which determines the selective permeability of the membrane, and a porous support layer. Since the support layer is more likely to adsorb dirt than the active layer, from the viewpoint of reducing membrane fouling, it is generally preferable to provide the active layer of the semipermeable membrane on the feed liquid side.

[0064] [Water treatment equipment] In water treatment, a device equipped with a semipermeable membrane is usually a cylindrical or box-shaped container in which a semipermeable membrane is placed, and membrane-filtered water is passed through one chamber separated by the semipermeable membrane, while a draw solution is passed through the other chamber. Any known semipermeable membrane device can be used, and commercially available products can be used.

[0065] The form of the semipermeable membrane is not particularly limited, and may be any of a flat membrane, a tubular membrane, a hollow fiber membrane, and the like.

[0066] The device equipped with the semipermeable membrane may be supplied with filtrate obtained by filtering water to be treated, or may include a filtering device as a water treatment device.

[0067] The semipermeable membrane may be any membrane that is selectively permeable to water, and may be a reverse osmosis membrane (RO membrane), but is preferably a forward osmosis membrane (FO membrane).

[0068] The forward osmosis membrane may be an organic membrane such as cellulose acetate, polyvinylidene fluoride, or polyolefin, or may be an inorganic membrane such as a metal membrane (stainless steel membrane, etc.), glass membrane, filter cloth, ceramic membrane (zeolite membrane, zirconia membrane, alumina membrane, silicon carbide membrane, silica membrane, etc.), or carbon membrane (graphene, etc.).

[0069] The draw solution, which has undergone solvent transfer to the draw solution side through the semipermeable membrane, can be phase-separated by adjusting the temperature. Temperature adjustment can be performed using a heat exchanger or the like. The phase-separated liquid can be obtained, for example, using an oil-water separator.

[0070] [Use of draw solute] The draw solute of the present disclosure is a solution with water as a solvent and can be applied when treating seawater, sewage, industrial wastewater, accompanying water during oil excavation, etc. Here, as a method for separating the accompanying water, for example, oil-water separation is performed by sedimentation, etc. The draw solute of the present disclosure has improved separability, and for example, contaminants such as evaporation residues and organic substances contained in purified water tend to be easily reduced.

Example

[0071] The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to only these examples and is not restricted by these examples.

[0072] <Evaluation of UCST property> The draw solution was placed in a dryer at 80 °C, and after visually confirming whether it became a uniform aqueous solution, the uniform aqueous solution was taken out of the dryer and visually confirmed whether it became turbid when cooled in a water bath.

[0073] <Production Example 1> Into a 500 mL glass separable flask equipped with a reflux condenser and a stirrer, 74.2 g of pure water, 51.9 g of an 80% methacryloylcholine chloride aqueous solution, 114.4 g of a 40% 2-acrylamido-2-methylpropanesulfonic acid aqueous solution (hereinafter referred to as 40% AMPS), and 16.7 g of a 48% sodium hydroxide aqueous solution (hereinafter referred to as 48% NaOH) were charged, and the temperature was raised to 85 °C with stirring to form a polymerization reaction system. Next, 95.9 g of a 1% sodium persulfate aqueous solution was continuously dropped into the polymerization reaction system maintained at 85 °C over 180 minutes while stirring. After the dropping was completed, the reaction solution was further maintained at 85 °C for 30 minutes (aged) to complete the polymerization. In this way, an aqueous polymer solution (1) with a solid content concentration of 25% containing a polymer having a structural unit derived from methacryloylcholine as a cationic site and sodium 2-acrylamido-2-methylpropanesulfonate as an anionic site was obtained.

[0074] <Production Example 2> A 500 mL glass separable flask equipped with a reflux condenser and a stirrer was charged with 92.4 g of pure water and heated to 85°C with stirring to form a polymerization reaction system. Next, 135.1 g of 80% aqueous acrylic acid (hereinafter referred to as 80% AA), 67.6 g of 40% AMPS, 9.8 g of 48% NaOH, 25.9 g of 15% aqueous sodium persulfate (hereinafter referred to as 15% NaPS), and 10.8 g of 30% aqueous sodium hypophosphite (hereinafter referred to as 30% SHP) were added dropwise from separate nozzles to the polymerization reaction system maintained at 85°C. The addition times for each solution were 180 minutes for 80% AA, 40% AMPS, 48% NaOH, and 30% SHP, and 210 minutes for 15% NaPS. The addition of all the solutions began simultaneously. After the dropwise addition of 15% NaPS was completed, the reaction solution was maintained (aged) at 85°C for an additional 30 minutes to complete the polymerization. In this way, an aqueous polymer solution (2) was obtained with a solids concentration of 43%, which contained a polymer having structural units derived from acrylic acid and sodium 2-acrylamido-2-methylpropanesulfonate as anionic moieties.

[0075] Example 1 A draw solution (1) with a solid content of 15% was prepared by mixing 2.00 g of the polymer aqueous solution (1) obtained in Production Example 1 and 1.33 g of ion-exchanged water. The UCST properties were evaluated and the results are shown in Table 1.

[0076] <Example 2> A draw solution (2) with a solids content of 40% was prepared by mixing 1.19 g of a water-soluble anionic polymer (DL-40S manufactured by Nippon Shokubai Co., Ltd.), 1.65 g of a water-soluble cationic polymer (PAS-H-1L manufactured by Nittobo Medical Co., Ltd.) concentrated to a solids content of 61.32%, and 1.16 g of ion-exchanged water. The UCST properties were evaluated and the results are shown in Table 1. Example 3 A draw solution (Example 3) with a solids content of 30% was prepared by mixing 2.26 g of the polymer aqueous solution (2) obtained in Production Example 2, 0.73 g of a water-soluble cationic polymer (PAS-H-1L manufactured by Nittobo Medical Co., Ltd.), and 1.0128 g of ion-exchanged water. The UCST properties were evaluated and the results are shown in Table 1.

[0077] [Table 1]

[0078] From Table 1, it is clear that the draw solution containing the draw solute of the present disclosure has UCST properties and can therefore be suitably used as a UCST-type draw solution.

Claims

1. One or more structural units (A) selected from the group consisting of structural units represented by the following general formula (1-1), structural units represented by the general formula (1-2), structural units represented by the general formula (1-3), structural units represented by the general formula (1-4), structural units represented by the general formula (1-5), and structural units represented by the general formula (1-6), and a structural unit (B) derived from an acid group-containing monomer, A draw solute containing a polymer in which the molar composition ratio of structural unit (A) to structural unit (B) is 1:10 to 2:

1. 【Chemical 1】 【Chemistry 2】 【Chemistry 3】 In the general formulas (1-1) to (1-6), R ０ represents a hydrogen atom or a methyl group, R ２ , R ３ , and R ４ each independently represents an organic group having 1 to 20 carbon atoms; R 2 and R ３ may be bonded to form a ring structure. ２ and R ３ may be a hydrogen atom. In the general formula (1-1) and the general formula (1-2), X is an oxygen atom or a nitrogen atom, and R １ represents an alkylene group having 1 to 4 carbon atoms which may have a substituent. In the general formulas (1-1) and (1-2), n1 is the number of hydrogen atoms bonded to X, and is 0 when X is an oxygen atom, and is 0 or 1 when X is a nitrogen atom. In general formula (1-1) and general formula (1-2), n2 is the number of amino groups represented by the structure in parentheses that bond to X, and is 1 when X is an oxygen atom, and is 1 or 2 when X is a nitrogen atom. In the general formulas (1-1) and (1-2), when X is a nitrogen atom, the sum of n1 and n2 is 2. In the general formulas (1-3) and (1-4), R ５ is a direct bond, CH ２ , and C.H. ２ CH ２ , OCH ２ CH(-OH)CH ２ , C.H. ２ OCH ２ CH(-OH)CH2,CH ２ CH ２ OCH ２ CH(-OH)CH ２ or a cyclic hydrocarbon group having 5 to 20 carbon atoms which may have a substituent. In the general formula (1-1) and the general formula (1-3), the amino group may be neutralized or may be an amine oxide. In general formula (1-6), R ６ represents a group containing a ring structure containing at least one nitrogen atom.

2. The draw solute according to claim 1, wherein the polymer further comprises a structural unit (C) derived from a nonionic group-containing monomer.

3. The draw solute according to claim 2, wherein the content of the structural unit (C) derived from the nonionic group-containing monomer is 0 to 50 mol % relative to 100 mol % of the total of the structural units (A), (B), and (C).

4. 3. The draw solute according to claim 1, wherein the acid group is a carboxy group, a sulfonic acid group, a sulfate ester group, a phosphoric acid group, or a salt or anhydride thereof.

5. One or more structural units (A) selected from the group consisting of structural units represented by the following general formula (1-1), structural units represented by the general formula (1-2), structural units represented by the general formula (1-3), structural units represented by the general formula (1-4), structural units represented by the general formula (1-5), and structural units represented by the general formula (1-6), and a structural unit (B) derived from an acid group-containing monomer, a draw solution (I) containing a polymer having a molar ratio of structural unit (A) to structural unit (B) of 1:10 to 2:1; The water to be treated (II), A method for treating water to be treated, comprising a step of contacting water through a semipermeable membrane (III). 【Chemistry 4】 【Chemistry 5】 【Chemistry 6】 In the general formulas (1-1) to (1-6), R ０ represents a hydrogen atom or a methyl group, R ２ , R ３ , and R ４ each independently represents an organic group having 1 to 20 carbon atoms; R 2 and R ３ may be bonded to form a ring structure. ２ and R ３ may be a hydrogen atom. In the general formula (1-1) and the general formula (1-2), X is an oxygen atom or a nitrogen atom, and R １ represents an alkylene group having 1 to 4 carbon atoms which may have a substituent. In the general formulas (1-1) and (1-2), n1 is the number of hydrogen atoms bonded to X, and is 0 when X is an oxygen atom, and is 0 or 1 when X is a nitrogen atom. In general formula (1-1) and general formula (1-2), n2 is the number of amino groups represented by the structure in parentheses that bond to X, and is 1 when X is an oxygen atom, and is 1 or 2 when X is a nitrogen atom. In the general formulas (1-1) and (1-2), when X is a nitrogen atom, the sum of n1 and n2 is 2. In the general formulas (1-3) and (1-4), R ５ is a direct bond, CH ２ , and C.H. ２ CH ２ , OCH ２ CH(-OH)CH ２ , C.H. ２ OCH ２ CH(-OH)CH2,CH ２ CH ２ OCH ２ CH(-OH)CH ２ or a cyclic hydrocarbon group having 5 to 20 carbon atoms which may have a substituent. In the general formula (1-1) and the general formula (1-3), the amino group may be neutralized or may be an amine oxide. In general formula (1-6), R ６ represents a group containing a ring structure containing at least one nitrogen atom.

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