JP2006008732A - Metal ion scavenger and method for scavenging metal ion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a metal ion scavenger for rapidly scavenging a number of metal ions, to provide a metal ion scavenging method for rapidly scavenging metal ions and especially to obtain a metal ion scavenger that rapidly scavenges a number of metal ions and is formed into various shapes. <P>SOLUTION: The metal ion scavenger is obtained by coating the crystal of a polymer having a straight-chain polyethyleneimine skeleton with silica and mutually associating fibrous compound materials having 15-100 nm thickness. The method for scavenging metal ions comprises bringing the metal ion scavenger into contact with a solution containing metal ions and coordinating the metal ions contained in the solution on the straight-chain polyethylene imine skeleton in the metal ion scavenger. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a capture agent that captures metal ions, and a method for capturing metal ions using the capture agent.

  Separation, removal, and recovery of metal ions are increasingly emphasized as a social issue that goes beyond industrial issues from the viewpoint of environmental purification and environmental protection. Therefore, in technical development for efficient metal ion scavengers or trapping methods, there are many advantages over conventional organic polymers such as ion exchange resin systems, such as fast ion exchange rate, high ion selectivity, high There is a tendency to switch to a new material system having ion exchange rate, simple ion recovery, heat resistance, environmental suitability, and the like.

  In recent years, silica (silicon) -based scavengers are attracting attention as such metal ion scavengers. Silicon is the second most abundant existence after oxygen on the earth's crust, and natural quartz, quartz, opal, mica, asbestos, etc. are well-known oxides of silicon. Silica-based materials having a regular spatial structure or pattern structure are extremely promising as various functional materials, for example, materials for separation, purification, or catalyst support, and this function or effect is more than that of conventional polymer resin systems. Due to its excellence, technological development in this regard has made great progress over the past few years.

  As the silica-based metal ion scavenger, mesoporous silica is mainly used. As a method for expressing a mesoporous cavity structure in silica, a molecular organization, for example, a low molecular weight or high molecular weight aggregate is used as a template, and a sol-gel reaction of alkoxysilane, which is a silica source, around the aggregate. Doing gives silica gel. The silica gel is sintered at a high temperature to remove the organic template inside, thereby leaving a cavity in the silica. As the template, a surfactant (see Non-Patent Document 1), a block polymer (see Non-Patent Document 2), a virus, a bacterium (see Non-Patent Document 3), and the like are used. Depending on the type of template, the cavities in the resulting silica vary in the range of about 2-20 nm, and the cavities are regularly arranged as parallel channels. For this reason, mesoporous silica materials have attracted a lot of interest in the fields of catalysts, electronic materials, nanofilters, chromatography, separation, biotechnology, and the like.

  However, in order to use mesoporous silica as a metal ion scavenger, a functional group having a coordination ability to metal ions such as amine and thiol must be introduced into the silica cavity surface (see Non-Patent Documents 4 to 6). . Mesoporous silicas can be made into spherical shapes, but in many cases these silicas are uncontrolled shaped powders or lumps. Therefore, in terms of the process of capturing metal ions, metal ions enter into the cavities in mesoporous silica, and in order for the metal ions to be stably coordinated with functional groups such as amines and thiols in the cavities, It took a long time in the whole process such as ion diffusion and coordination. In addition, there is a limit to the number of functional groups that can be immobilized in mesoporous silica, and it is necessary to increase the amount of silica when capturing a large amount of metal ions.

C.T.Kresge et al., Nature, (1992), 359, p.710 A. Monnier et al., Science, (1993), 261, p1299 S.A.Davis et al., Nature, (1997), 385, p420 T. Kang et al., J. Mater. Chem. (2004) 14, p1043 B. Lee et al., Langmuir, (2003) 19, p4246 K. Zakir et al., Adv. Mater. (2002) 14, p1053)

  The problem to be solved by the present invention is to provide a metal ion scavenger that can quickly capture metal ions and a metal ion capture method that can quickly capture metal ions, and in particular, quickly capture many metal ions. An object of the present invention is to provide a metal ion scavenger that can be formed into various shapes.

  In the present invention, a silica-coated polymer crystal having a linear polyethyleneimine skeleton having a high coordination ability to metal ions is associated with a fibrous composite having a thickness on the order of nanometers. Since the formed trapping agent can secure a small space and a large surface area inside a sponge like a sponge, a large amount of metal ions can be quickly trapped. Furthermore, since it consists of an aggregate of fibrous composites with a thickness of the order of nanometers, it can be molded into any shape.

  That is, the present invention provides a metal ion scavenger formed by associating fibrous composites having a thickness in the range of 15 to 100 nm, in which a polymer crystal having a linear polyethyleneimine skeleton is coated with silica. It is in.

  Furthermore, the present invention comprises a solution containing metal ions and a fibrous composite having a thickness in the range of 15 to 100 nm, in which a silica crystal is coated with a polymer crystal having a linear polyethyleneimine skeleton. Metal capture method comprising contacting metal ion scavenger and coordinating metal ions contained in the solution to linear polyethyleneimine skeleton in the transition metal scavenger, and metal ion scavenger It is an object of the present invention to provide a method for separating metal ions trapped in a metal scavenger by an acid treatment.

  The metal ion scavenger of the present invention has a three-dimensional network structure in which fibrous composites of nanometer order are associated, that is, a silica sponge structure. Therefore, the inside of the sponge is composed of a fibrous composite and a void, and the surface area of the internal fibrous composite is very large. For this reason, when the metal ion scavenger of the present invention is brought into contact with a metal ion aqueous solution, the metal ions can come into contact with the entire surface of silica, and many metal ions can be quickly captured. Furthermore, since the metal ion scavenger of the present invention is easy to mold, it can be used as a nanofilter type metal ion scavenger by processing into a column or plate.

  Further, the polymer having a linear polyethyleneimine skeleton in the metal ion scavenger of the present invention can be changed to a protonated cationic state or a deprotonated amine state by changing the pH of the solution. For this reason, the metal ion scavenger of the present invention can selectively capture metal ions.

  Furthermore, in the metal ion scavenger of the present invention, the polymer having excellent acid resistance, oxidation resistance, and organic solvent resistance possessed by silica and having a linear polyethyleneimine skeleton in the fibrous composite is the fibrous composite. It is not eluted from. Therefore, it is possible to maintain a state in which the active part coordinated with the metal is stably immobilized in the fibrous composite.

  Further, in the formation of the scavenger comprising the polymer crystal and silica of the present invention, it is possible to contain a functional compound such as a fluorescent substance in the crystal filament, and the functional substance contained The derived function can also be expressed.

  In addition, according to the trapping method using the metal ion scavenger of the present invention, many metal ions can be quickly trapped, and further, the metal ions trapped in the silica fiber can be washed with an acidic aqueous solution to thereby remove the metal. It can be easily recovered.

  The scavenger of the present invention is obtained by associating fibrous composites having a thickness in the order of nanometers, in which a polymer crystal having a linear polyethyleneimine skeleton is coated with silica.

[Polymer having linear polyethyleneimine skeleton]
The linear polyethyleneimine skeleton as used in the present invention refers to a polymer skeleton having an ethyleneimine unit of a secondary amine as a main structural unit. In the skeleton, structural units other than the ethyleneimine unit may exist, but in order to form a polymer crystal, it is preferable that a constant chain length of the polymer chain is a continuous ethyleneimine unit. The length of the linear polyethyleneimine skeleton is not particularly limited as long as the polymer having the skeleton can form a crystal, but in order to form a polymer crystal suitably, the ethyleneimine unit of the skeleton portion is not limited. The number of repeating units is preferably 10 or more, and particularly preferably in the range of 20 to 10,000.

  The polymer used in the present invention may be any polymer as long as it has a linear polyethyleneimine skeleton in its structure. Even if the shape is linear, star-shaped or comb-shaped, it is water or water and a water-soluble organic solvent. Crystals can be provided in a mixed aqueous medium.

  Further, these linear, star-shaped or comb-shaped polymers may be composed of only a linear polyethyleneimine skeleton or a block composed of a linear polyethyleneimine skeleton (hereinafter abbreviated as a polyethyleneimine block). And a block copolymer of the other polymer block. Other polymer blocks include, for example, water-soluble polymer blocks such as polyethylene glycol, polypropionylethyleneimine, polyacrylamide, or polystyrene, polyoxazoline polyphenyloxazoline, polyoctyloxazoline, polydodecyloxazoline, polyacrylates Hydrophobic polymer blocks such as polymethyl methacrylate and polybutyl methacrylate can be used. By using a block copolymer with these other polymer blocks, the shape and characteristics of the polymer crystal can be adjusted.

  When the polymer having a linear polyethyleneimine skeleton is a block copolymer, the proportion of the linear polyethyleneimine skeleton in the polymer is not particularly limited as long as the polymer crystal can be formed. In order to form the polymer, the proportion of the linear polyethyleneimine skeleton in the polymer is preferably 40 mol% or more, and more preferably 50 mol% or more.

  The polymer having a linear polyethyleneimine skeleton is a polymer having a linear skeleton composed of polyoxazolines serving as a precursor thereof (hereinafter abbreviated as a precursor polymer) under acidic conditions or alkaline conditions. It can be easily obtained by hydrolysis. Therefore, the shape of a polymer having a linear polyethyleneimine skeleton such as a linear shape, a star shape, or a comb shape can be easily designed by controlling the shape of the precursor polymer. Further, the degree of polymerization and the terminal structure can be easily adjusted by controlling the degree of polymerization and the terminal functional group of the precursor polymer. Furthermore, when forming a block copolymer having a linear polyethyleneimine skeleton, the precursor polymer is used as a block copolymer, and the linear skeleton composed of polyoxazolines in the precursor is selectively hydrolyzed. Can be obtained at

  The precursor polymer can be synthesized by a cationic polymerization method or a synthesis method such as a macromonomer method using an oxazoline monomer. By selecting an appropriate synthesis method and initiator, a linear polymer can be obtained. It is possible to synthesize precursor polymers having various shapes such as a star shape or a comb shape.

  Oxazoline monomers such as methyl oxazoline, ethyl oxazoline, methyl vinyl oxazoline, and phenyl oxazoline can be used as the monomer that forms a linear skeleton composed of polyoxazolines.

  As the polymerization initiator, a compound having a functional group such as an alkyl chloride group, an alkyl bromide group, an alkyl iodide group, a toluenesulfonyloxy group, or a trifluoromethylsulfonyloxy group in the molecule can be used. These polymerization initiators can be obtained by converting the hydroxyl groups of many alcohol compounds into other functional groups. Of these, brominated, iodinated, toluene sulfonated and trifluoromethyl sulfonated functional groups are preferred because of high polymerization initiation efficiency, and alkyl bromide and toluene sulfonate alkyl are particularly preferred.

  Moreover, what converted the terminal hydroxyl group of poly (ethylene glycol) into bromine or iodine, or converted into a toluenesulfonyl group can also be used as a polymerization initiator. In that case, the degree of polymerization of poly (ethylene glycol) is preferably in the range of 5 to 100, particularly preferably in the range of 10 to 50.

  Also, dyes having a functional group having the ability to initiate cationic ring-opening living polymerization and having a skeleton of any of a porphyrin skeleton, a phthalocyanine skeleton, or a pyrene skeleton having a light emission function, an energy transfer function, and an electron transfer function by light Can impart a special function to the resulting polymer.

  The linear precursor polymer is obtained by polymerizing the oxazoline monomer with a polymerization initiator having a monovalent or divalent functional group. Examples of such a polymerization initiator include methyl benzene, methyl benzene bromide, methyl benzene iodide, methyl benzene toluene sulfonate, methyl benzene trifluoromethyl sulfonate, methane bromide, methane iodide, toluene sulfonic acid. Monovalent ones such as methane or toluenesulfonic anhydride, trifluoromethylsulfonic anhydride, 5- (4-bromomethylphenyl) -10,15,20-tri (phenyl) porphyrin, or bromomethylpyrene, dibromo Divalent ones such as methylbenzene, diiodomethylbenzene, dibromomethylbiphenylene, or dibromomethylazobenzene can be mentioned. In addition, linear polyoxazolines that are industrially used such as poly (methyloxazoline), poly (ethyloxazoline), or poly (methylvinyloxazoline) can be used as a precursor polymer as they are.

  The star-shaped precursor polymer is obtained by polymerizing the oxazoline monomer as described above with a polymerization initiator having a trivalent or higher functional group. Examples of the trivalent or higher polymerization initiator include trivalent compounds such as tribromomethylbenzene, tetravalent compounds such as tetrabromomethylbenzene, tetra (4-chloromethylphenyl) porphyrin, and tetrabromoethoxyphthalocyanine. A pentavalent or higher valent compound such as hexabromomethylbenzene and tetra (3,5-ditosilylethyloxyphenyl) porphyrin can be used.

  In order to obtain a comb-like precursor polymer, an oxazoline monomer can be polymerized from the polymerization initiating group using a linear polymer having a polyvalent polymerization initiating group. It can also be obtained by halogenating a hydroxyl group of a polymer having a hydroxyl group in a side chain such as polyvinyl alcohol with bromine or iodine, or converting it to a toluenesulfonyl group and then using the converted moiety as a polymerization initiating group. .

  As a method for obtaining a comb-like precursor polymer, a polyamine type polymerization terminator can also be used. For example, a monovalent polymerization initiator is used to polymerize oxazoline, and the end of the polyoxazoline is bonded to the amino group of a polyamine such as polyethyleneimine, polyvinylamine, or polypropylamine to obtain a comb-shaped polyoxazoline. Can do.

  Hydrolysis of the linear skeleton composed of the polyoxazolines of the precursor polymer obtained as described above may be carried out under either acidic conditions or alkaline conditions. Hydrolysis under acidic conditions can obtain polyethyleneimine hydrochloride, for example, by stirring polyoxazoline under heating in an aqueous hydrochloric acid solution. By treating the obtained hydrochloride with excess ammonium water, a basic polyethyleneimine crystal powder can be obtained. The hydrochloric acid aqueous solution used may be concentrated hydrochloric acid or an aqueous solution of about 1 mol / L, but it is desirable to use a 5 mol / L aqueous hydrochloric acid solution for efficient hydrolysis. The reaction temperature is preferably around 80 ° C.

  For hydrolysis under alkaline conditions, polyoxazoline can be converted into polyethyleneimine by using, for example, an aqueous sodium hydroxide solution. After reacting under alkaline conditions, the reaction solution is washed with a dialysis membrane to remove excess sodium hydroxide and to obtain polyethyleneimine crystal powder. The concentration of sodium hydroxide to be used may be in the range of 1 to 10 mol / L, and is preferably in the range of 3 to 5 mol / L in order to perform a more efficient reaction. The reaction temperature is preferably around 80 ° C.

  In the hydrolysis under acidic conditions or alkaline conditions, the amount of acid or alkali used may be 1 to 10 equivalents relative to the oxazoline unit in the polymer. In order to improve reaction efficiency and simplify post-treatment. About 3 equivalents are preferred.

  By the hydrolysis, a linear skeleton composed of polyoxazolines in the precursor polymer becomes a linear polyethyleneimine skeleton, and a polymer having the polyethyleneimine skeleton is obtained.

  In the case of forming a block copolymer of a linear polyethyleneimine block and another polymer block, the precursor polymer is a block copolymer composed of a linear polymer block composed of polyoxazolines and another polymer block. And a linear block composed of polyoxazolines in the precursor polymer can be selectively hydrolyzed.

  When the other polymer block is a water-soluble polymer block such as poly (N-propionylethyleneimine), poly (N-propionylethyleneimine) is converted to poly (N-formylethyleneimine) or poly (N-acetyl). A block copolymer can be formed by taking advantage of its higher solubility in organic solvents than ethyleneimine). That is, 2-oxazoline or 2-methyl-2-oxazoline is subjected to cationic ring-opening living polymerization in the presence of the above-described polymerization initiating compound, and then 2-ethyl-2-oxazoline is further polymerized to the obtained living polymer. Thus, a precursor polymer composed of a poly (N-formylethyleneimine) block or a poly (N-acetylethyleneimine) block and a poly (N-propionylethyleneimine) block is obtained. The precursor polymer is dissolved in water, and an emulsion is formed by mixing and stirring an organic solvent incompatible with water in which the poly (N-propionylethyleneimine) block is dissolved in the aqueous solution. A linear polyethyleneimine block is obtained by preferentially hydrolyzing a poly (N-formylethyleneimine) block or a poly (N-acetylethyleneimine) block by adding an acid or an alkali to the aqueous phase of the emulsion. And a block copolymer having a poly (N-propionylethyleneimine) block.

  When the valence of the polymerization initiating compound used here is 1 or 2, a linear block copolymer is obtained. When the valence is higher than that, a star-shaped block copolymer is obtained. Further, by making the precursor polymer a multistage block copolymer, the resulting polymer can also have a multistage block structure.

[Polymer crystals]
In the polymer having a polyethyleneimine skeleton, the linear polyethyleneimine skeleton in the primary structure exhibits crystallinity in water or a mixed solution of water and a hydrophilic solvent to form a polymer crystal. The polymer crystals can also form a hydrogel having a three-dimensional network structure by physical bonding between the polymer crystals in the presence of water, and further chemically crosslinked by crosslinking the polymer crystals with a crosslinking agent. Crosslinked hydrogels with bonds can also be formed.

  Polyethyleneimine that has been widely used heretofore is a branched polymer obtained by ring-opening polymerization of cyclic ethyleneimine, and has primary amine, secondary amine, and tertiary amine in its primary structure. Therefore, branched polyethyleneimine is water-soluble, but has no crystallinity. Therefore, in order to make a hydrogel using branched polyethyleneimine, a network structure must be provided by covalent bonding with a crosslinking agent. However, the linear polyethyleneimine that the polymer used in the present invention has as a skeleton is composed only of a secondary amine, and the secondary amine type linear polyethyleneimine is water-soluble and has excellent crystallinity. Have

  Such linear polyethyleneimine crystals are known to vary greatly in polymer crystal structure depending on the number of water of crystallization contained in the ethyleneimine unit of the polymer (Y. Chatani et al., Macromolecules, 1981). Year, Vol. 14, pp. 315-321). Anhydrous polyethylenimine prefers a crystal structure characterized by a double helix structure, but if the monomer unit contains two molecules of water, the polymer is known to grow into a crystal characterized by a zigzag structure. Yes. Actually, the crystal of linear polyethyleneimine obtained from water is a crystal containing bimolecular water in one monomer unit, and the crystal is insoluble in water at room temperature.

  The polymer crystal having a linear polyethyleneimine skeleton in the present invention is formed by crystal expression of the linear polyethyleneimine skeleton in the same manner as described above, and the polymer shape is linear, star-shaped, or comb-like. Even if it is a shape such as a shape, a polymer crystal can be obtained if it is a polymer having a linear polyethyleneimine skeleton in the primary structure.

  The presence of the polymer crystal in the present invention can be confirmed by X-ray scattering, and the linear polyethyleneimine skeleton in the crystalline hydrogel in the vicinity of 20 °, 27 °, and 28 ° in terms of 2θ angle values in a wide angle X-ray diffractometer (WAXS). Confirmed by the derived peak value.

  Moreover, although the melting point in the differential scanning calorimeter (DSC) of the polymer crystal in the present invention depends on the primary structure of the polymer having a polyethyleneimine skeleton, the melting point generally appears at 45 to 90 ° C.

  The polymer crystal in the present invention takes various shapes due to the influence of the geometric shape of the polymer structure constituting the crystal, the molecular weight, the non-ethyleneimine moiety that can be introduced into the primary structure, and the formation conditions of the polymer crystal. For example, it has a fiber shape, a brush shape, a star shape, or the like.

  The polymer crystal is based on a nanometer-order fibrous polymer crystal (hereinafter, the crystal is abbreviated as a fibrous nanocrystal) of about 5 to 30 nm, and is present on the surface of the fibrous nanocrystal. By the free ethyleneimine chain, the fibrous nanocrystals are connected by physical bonds by hydrogen bonds, arranged in the space, and grow in the three-dimensional shape as described above. These polymer crystals are further physically bonded to form a three-dimensional network structure of the polymer crystals. Since these occur in the presence of water, the result is a hydrogel that includes water in the three-dimensional network structure.

  The three-dimensional network structure here is different from ordinary polymer hydrogels. Micro- or nano-scale crystals are physically cross-linked by hydrogen bonds of free ethyleneimine chains existing on the crystal surface. Say a net structure. Therefore, at a temperature higher than the melting point of the crystal, the crystal is dissolved in water, and the three-dimensional network structure is also dismantled. However, when it returns to room temperature, polymer crystals grow, and physical crosslinks due to hydrogen bonds are formed between the crystals, so that a three-dimensional network structure appears again.

  The polymer crystal of the present invention can be obtained by dissolution-crystal conversion of a polymer having a linear polyethyleneimine skeleton in water utilizing the property that a polymer having a linear polyethyleneimine skeleton is insoluble in water at room temperature. Can do.

  In the hydrogel, a polymer having a linear polyethyleneimine skeleton is first dispersed in water, and the dispersion is heated to obtain a transparent aqueous solution of the polymer having a polyethyleneimine skeleton. Subsequently, it is obtained by cooling the heated polymer aqueous solution to room temperature. The hydrogel is deformed by an external force such as a shearing force, but has a state like an ice cream capable of maintaining a general shape, and can be deformed into various shapes.

  The heating temperature of the polymer dispersion is preferably 100 ° C. or less, and more preferably in the range of 90 to 95 ° C. The polymer content in the polymer dispersion is not particularly limited as long as the hydrogel can be obtained, but is preferably in the range of 0.01 to 20% by mass, and a hydrogel composed of a stable-shaped crystal is obtained. Therefore, the range of 0.1 to 10% by mass is more preferable. Thus, in the present invention, when a polymer having a linear polyethyleneimine skeleton is used, a hydrogel can be formed even with a very small polymer concentration.

  The crystal shape in the obtained hydrogel can be adjusted by the process of lowering the temperature of the aqueous polymer solution to room temperature. For example, the polymer aqueous solution is kept at 80 ° C. for 1 hour, then brought to 60 ° C. over 1 hour, and kept at this temperature for another 1 hour. Thereafter, the temperature is lowered to 40 ° C. over 1 hour, and then naturally lowered to room temperature, whereby a hydrogel crystal in which the water of the aqueous solution has lost its fluidity can be obtained. In addition, after the above polymer aqueous solution is cooled at once with freezing water at a freezing point, or with a refrigerant liquid of methanol / dry ice or acetone / dry ice below freezing point, the state of the polymer is held in a water bath at room temperature, thereby hydrogel Crystals can be obtained. Furthermore, a hydrogel crystal can be obtained by lowering the temperature of the aqueous polymer solution to room temperature in a water bath or room temperature air environment.

  Since the step of lowering the temperature of the aqueous polymer solution strongly affects the shape of the polymer crystals in the resulting hydrogel, the crystalline form of the polymers in the hydrogel obtained by the different steps is not the same.

  When the temperature of the polymer aqueous solution is lowered in a multistage manner with a constant concentration, the polymer crystal form in the hydrogel can be made into a fiber-like polymer crystal form. When this is rapidly cooled and then returned to room temperature, it can be made into a petal-like polymer crystal form. Moreover, when this is rapidly cooled again with acetone on dry ice and returned to room temperature, it can be made into a wavy polymer crystal form. Thus, the crystal form of the polymer in the hydrogel of the present invention can be set to various shapes.

  The hydrogel obtained as described above is an opaque gel, in which a polymer crystal having a polyethyleneimine skeleton is formed, and is physically crosslinked by hydrogen bonding between the crystals, and thus a three-dimensional physical gel is formed. A network structure is formed. Once formed, the polymer crystals in the hydrogel remain insoluble at room temperature, but when heated, the polymer crystals dissociate and the hydrogel changes to a sol state. Therefore, the physical hydrogel of the present invention can be reversibly changed from sol to gel and from gel to sol by heat treatment.

  The hydrogel referred to in the present invention contains at least water in the three-dimensional network structure, and a hydrogel containing an organic solvent can be obtained by adding a water-soluble organic solvent during preparation of the hydrogel. Examples of the hydrophilic organic solvent include water-soluble organic solvents such as methanol, ethanol, tetrahydrofuran, acetone, dimethylacetamide, dimethylsulfone oxide, dioxirane, and pyrrolidone.

  The content of the organic solvent is preferably in the range of 0.1 to 5 times the volume of water, more preferably in the range of 1 to 3 times.

  By containing the hydrophilic organic solvent, the form of the polymer crystal can be changed, and a crystal having a form different from that of a simple aqueous system can be provided. For example, even in a branched crystal form having a fiber-like spread in water, when a certain amount of ethanol is included in the preparation, a spherical crystal form in which the fiber contracts can be obtained.

  The hydrogel containing a water-soluble polymer can be obtained by adding another water-soluble polymer during the preparation of the hydrogel in the present invention. Examples of the water-soluble polymer include polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, poly (N-isopropylacrylamide), polyhydroxyethyl acrylate, polymethyloxazoline, polyethyloxazoline, and the like.

  The content of the water-soluble polymer is preferably in the range of 0.1 to 5 times, more preferably in the range of 0.5 to 2 times the mass of the polymer having a linear polyethyleneimine skeleton.

  By containing the water-soluble polymer, the form of the polymer crystal can be changed, and a crystal having a form different from that of a simple aqueous system can be provided. It is also effective in increasing the viscosity of the hydrogel and improving the stability of the hydrogel.

  By treating the hydrogel obtained by the above method with a compound containing at least two functional groups that react with the amino group of polyethyleneimine, a crosslinked hydrogel in which polymer crystal surfaces in the hydrogel are linked by chemical bonds can be obtained. it can.

  As the compound containing at least two functional groups capable of reacting with the amino group at room temperature, aldehyde crosslinking agents, epoxy crosslinking agents, acid chlorides, acid anhydrides, and ester crosslinking agents can be used. Examples of the aldehyde crosslinking agent include malonyl aldehyde, succinyl aldehyde, glutaryl aldehyde, adifoyl aldehyde, phthaloyl aldehyde, isophthaloyl aldehyde, terephthaloyl aldehyde, and the like. Examples of the epoxy crosslinking agent include polyethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, glycidyl chloride, and glycidyl bromide. Examples of the acid chlorides include malonyl chloride, succinyl chloride, glutaryl chloride, adifoyl chloride, phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride, and the like. Examples of the acid anhydride include phthalic anhydride, succinic anhydride, and glutaric anhydride. Examples of the ester crosslinking agent include malonic acid methyl ester, succinic acid methyl ester, glutaric acid methyl ester, phthaloyl acid methyl ester, and polyethylene glycol carboxylic acid methyl ester.

  The cross-linking reaction can be carried out by immersing the obtained hydrogel in a solution of the cross-linking agent or by adding the cross-linking agent solution into the hydrogel. At this time, the cross-linking agent penetrates into the hydrogel as the osmotic pressure changes in the system, and connects the crystals with hydrogen bonds to cause a chemical reaction with the nitrogen atom of ethyleneimine.

  The crosslinking reaction proceeds by reaction with free ethyleneimine on the polyethyleneimine crystal surface, but in order to prevent the reaction from occurring inside the crystal, the reaction is performed at a temperature below the melting point of the crystal forming the hydrogel. It is desirable to carry out the crosslinking, and it is most desirable to carry out the crosslinking reaction at room temperature.

  When the crosslinking reaction proceeds at room temperature, the crosslinked hydrogel can be obtained by leaving the hydrogel mixed with the crosslinking agent solution. The time for the crosslinking reaction may be from a few minutes to a few days, and the crosslinking proceeds suitably by leaving it to stand overnight.

  The amount of the crosslinking agent may be 0.05 to 20% with respect to the number of moles of ethyleneimine units in the polymer having a polyethyleneimine skeleton used for hydrogel formation, and more preferably 1 to 10%.

  In the hydrogel, since the gelling agent is a crystalline polymer, it can express gel structures having various morphologies. Further, even a small amount of polymer crystal has a high water retention property because it suitably forms a three-dimensional network structure in water. Furthermore, the polymer having a linear polyethyleneimine skeleton to be used is easy to design and synthesize, and easy to adjust the hydrogel. Moreover, the shape of a hydrogel is fixable by bridge | crosslinking between the polymer crystals in this hydrogel with a crosslinking agent.

[Fibrous composite, association of fibrous composite]
The metal ion scavenger of the present invention is subjected to a sol-gel reaction at room temperature by adding a solution in which a silica source is dissolved in a solvent that can be used in a normal sol-gel reaction to the above-described dispersion of polymer crystals in water or hydrogel of polymer crystals. Can be easily obtained.

  The polymer crystal surface inevitably has many free polyethyleneimine chains that are not related to the crystal, and these free chains exist like a brush on the crystal surface. These brush-like chains are scaffolds that attract the silica source and at the same time act as a catalyst to polymerize the silica source. As a result, a sol-gel reaction proceeds on the surface of the fiber nanocrystal of the polymer having a linear polyethyleneimine skeleton, and the surface of the fiber nanocrystal is coated with silica, and as a result, the fiber-like nanocrystal is coated with silica. A metal ion scavenger in a state in which the formed fibrous composites are associated with each other is obtained.

  Examples of the compound used as the silica source include tetraalkoxysilanes and alkyltrialkoxysilanes.

  Examples of tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, and tetra-t-butoxysilane.

  Examples of alkyltrialkoxysilanes include methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, iso-propyltrimethoxysilane, iso -Propyltriethoxysilane, 3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-glycitoxypropyltrimethoxysilane, 3-glycitoxypropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-mercaptopropyltomethoxysilane, 3-mercaptotriethoxysilane, 3,3,3-trifluoropropylto Methoxysilane, 3,3,3-trifluoropropyltriethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxylane, p-chloromethylphenyltrimethoxy Orchid, p-chloromethylphenyltriethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diethyldimethoxysilane, diethyldiethoxycyan and the like.

  The sol-gel reaction that gives the metal ion scavenger of the present invention proceeds in the presence of polymer crystals in an aqueous liquid such as water or a mixed solution of water and a hydrophilic organic solvent, but the reaction does not occur in the aqueous liquid phase. Instead, it proceeds on the surface of the crystal. Accordingly, the reaction conditions are arbitrary as long as the polymer crystals are not dissolved under the complexing reaction conditions.

  In order to make the polymer crystals insoluble, the presence of water is preferably 20% or more in the aqueous liquid containing the hydrophilic organic solvent in the sol-gel reaction, and more preferably 40% or more.

  In the sol-gel reaction, a scavenger can be suitably formed if the amount of alkoxysilane as a silica source is excessive with respect to ethyleneimine as a monomer unit of polyethyleneimine. The degree of excess is preferably in the range of 4 to 1000 times equivalent to ethyleneimine.

  Moreover, it is preferable that the polymer concentration in the water and the aqueous liquid when forming the polymer crystal is 0.1 to 30% based on the amount of polyethyleneimine contained in the polymer.

  The sol-gel reaction time varies from 1 minute to several days. In the case of methoxysilanes having a high alkoxysilane reaction activity, the reaction time may be from 1 minute to 24 hours. It is more preferable to set it to 30 minutes to 5 hours. In the case of ethoxysilanes and butoxysilanes having low reaction activity, the sol-gel reaction time is preferably 24 hours or longer, and it is also desirable to set the time to about one week.

  The metal ion scavenger of the present invention is a fibrous composite having a thickness of 15 to 100 nm, which is obtained by coating a polymer crystal having a linear polyethyleneimine skeleton with silica (hereinafter simply referred to as a fibrous composite). It has a three-dimensional mesh shape formed from the association and spatial arrangement. The metal ion scavenger has a three-dimensional network structure in which a fibrous composite having a thickness of nanometer order is used as a basic unit, and the basic units associate with each other to form a three-dimensional network structure. Inside the metal ion scavenger, when the fibrous composite serving as a basic unit has a fine association shape by transferring a secondary shape of the micrometer order formed by fibrous nanocrystals In many cases, the finely-aggregated aggregates further associate to form a metal ion scavenger having a three-dimensional network structure as a whole.

  The secondary shape formed by the fibrous composite on the order of micrometers is the geometric shape of the polymer structure as described above, the molecular weight, the non-ethyleneimine moiety that can be introduced into the primary structure, and the polymer crystals. It can be adjusted according to the formation conditions, and can be adjusted to various shapes such as lettuce, fiber, sponge, aster, cactus, and dandelion. These shapes are highly dependent on the molecular structure of the polymer used, the degree of polymerization, the composition, and the method of temperature reduction during polymer crystal preparation.

  Moreover, since the shape of the hydrogel whose shape can be arbitrarily controlled can be fixed with silica, the metal ion scavenger of the present invention can be formed into any shape. In this case, it is preferable to use a cross-linked hydrogel in which polymer crystals are cross-linked by a chemical bond because molding becomes easy. The shape and size of the metal ion scavenger can be the same as the size and shape of the container used in the preparation of the hydrogel, for example, any shape such as a disk shape, a column shape, a plate shape, a spherical shape, etc. Can be prepared. Further, the crosslinked hydrogel can be cut or scraped to be formed into a desired shape. By bringing the hydrogel thus formed into contact with a silica source solution by dipping or the like, a molded body composed of polymer crystals and silica can be easily obtained. The time for immersing in the silica source solution varies from 1 hour to 1 week depending on the type of silane source to be used, so it is necessary to prepare it appropriately. However, in the methoxysilane solution, it may be about 1 to 48 hours. In the solution of ethoxysilanes, about 1 to 7 days is preferable.

  Usually, the scavenger of the present invention is composed of polymer crystals and silica, and the content of silica in the scavenger varies within a certain range depending on reaction conditions and the like, but is in the range of 30 to 90% by mass of the entire scavenger. You can get things. Among them, those having a silica content of 40 to 80% by mass of the total scavenger can be preferably used.

  The silica content increases as the amount of polyethyleneimine polymer used in the sol-gel reaction, ie the polymer concentration in the hydrogel, increases. Moreover, it increases by lengthening the sol-gel reaction time.

  The metal ion scavenger of the present invention can incorporate a fluorescent substance into the polymer having a linear polyethyleneimine skeleton in the fibrous composite. For example, after porphyrin having an acidic group and polyethyleneimine are mixed, a sol-gel reaction is performed, whereby the porphyrin residue is incorporated into the metal ion scavenger. Further, for example, by using a polymer crystal in which a side chain of a polymer having a linear polyethyleneimine skeleton is reacted with a small amount of pyrene, for example, pyrenealdehyde (preferably, 10 mol% or less based on imine), pyrene is used. Residues can be incorporated into the metal ion scavenger. Furthermore, a fluorescent dye such as porphyrins, phthalocyanines, pyrenes having an acidic group, for example, a carboxylic acid group or a sulfonic acid group is added to the base of the polymer having a linear polyethyleneimine skeleton (preferably in the number of moles of imine). These fluorescent substances can be incorporated into a metal ion scavenger that is mixed in a small amount and uses the crystal as a template.

  The nitrogen atom content of the metal ion scavenger of the present invention can be changed by changing the polymer concentration having a linear polyethyleneimine skeleton in the hydrogel when preparing the metal ion scavenger. As the concentration increases, the content of nitrogen atoms in the obtained silica scavenger increases. Usually, the content of nitrogen atoms can be in the range of 1 to 15% by mass. The nitrogen atom in the metal ion scavenger of the present invention is a part having a high coordination ability to the metal ion, and the amount of metal ion that can be captured by the metal ion scavenger of the present invention is determined according to the content of the nitrogen atom. The greater the nitrogen atom content that can be prepared, the greater the amount of metal ions that can be captured. The maximum amount of metal ions that can be captured by the metal scavenger of the present invention is that 1 mol of metal ions can be captured per 4 mol of nitrogen atoms in the metal ion scavenger.

  Since the metal ion scavenger of the present invention has a three-dimensional network structure in which nanometer-order fibrous composites are associated, there are many voids in the interior and the surface area of the fibrous composite is very large. For this reason, when the metal ion scavenger of the present invention is brought into contact with a metal ion aqueous solution, the metal ions can come into contact with the entire surface of silica, and many metal ions can be quickly captured. Furthermore, since the metal ion scavenger of the present invention is easy to mold, it can be used as a nanofilter type metal ion scavenger by processing into a column or plate.

  Further, the polymer having a linear polyethyleneimine skeleton in the metal ion scavenger of the present invention can be changed to a protonated cationic state or a deprotonated amine state by changing the pH of the solution. For this reason, the metal ion scavenger of the present invention can selectively capture metal ions.

  Furthermore, in the metal ion scavenger of the present invention, the polymer having excellent acid resistance, oxidation resistance, and organic solvent resistance possessed by silica and having a linear polyethyleneimine skeleton in the fibrous composite is the fibrous composite. Cannot be eluted. Therefore, it is possible to maintain a state in which the active part coordinated with the metal is stably immobilized in the fibrous composite.

  The metal ion scavenger of the present invention can be used in a process similar to the use conditions of a conventional ion exchange resin in which a ligand is bound to an organic polymer. In addition, the organic polymer ion exchange resin is weak in oxidation resistance and acid resistance and may not be used under such conditions. However, the metal ion scavenger of the present invention has high oxidation resistance and acid resistance of silica. Can be used in such an environment. That is, it can be used sufficiently stably even in an environment of pH <1.

  Since the scavenger in the present invention functions as a support, silica can be used not only in water but also in water containing an organic solvent or in an organic solvent.

[Metal ion separation method]
In the metal ion scavenger of the present invention, the metal ions are taken in by the strong coordination ability of the polymer having a polyethyleneimine skeleton contained in the fibrous composite to the metal ions. At this time, silica functions as a support for supporting the ligand.

  Examples of metal ions that can be captured by the metal ion scavenger of the present invention include alkali metal ions, alkaline earth metal ions, transition metal ions, lanthanum metal ions, polyoxometalate ions, and the like.

The mechanism by which the scavenger in the present invention captures metal ions is that the polyethyleneimine in silica is coordinated to the metal ions so that the metal ions are taken into the silica from the aqueous liquid phase and immobilized in the silica as it is. Is done. Unlike processes such as ionic bonding, this is trapped by the coordination of polyethyleneimine, whether the metal is a cation or an anion. Therefore, the metal ion scavenger of the present invention can be particularly suitably used for trapping transition metal ions. As a transition metal ion, even if it is a metal cation (M ^{n +} ), an acid radical anion (MO _x ⁿ⁻ ) consisting of a bond with oxygen, or an anion (ML _x ⁿ⁻ ) consisting of a halogen bond Even so, the present scavengers can be suitably used for capturing, fixing, removing and recovering them.

Transition metal ions include the following transition metal cations (M ^{n +} ), such as Cu, Zn, Co, Ni, Cr, Mn, Mo, Fe, Ru, W, Rh, Cd, Os, Hg, Cd, and Ir. , Ag, Pt, Sn, Pb, Ga monovalent, divalent, trivalent or tetravalent cations. In capturing these metal cations, the counter anion of the metal cation may be Cl, NO ₃ , SO ₄ , a polyoxometalate anion, or an organic anion of a carboxylic acid.

In addition, the following transition metal anions (MO _x ⁿ⁻ ), for example, anions of MnO ₄ , MoO ₄ , ReO ₄ , WO ₃ , RuO ₄ , CoO ₄ , CrO ₄ , VO ₃ , NiO ₄ can be mentioned.

  The scavenger in the present invention can be suitably used for metal compounds of polyoxometalates, but specifically, molybdate, tungstate and vanadate in combination with a transition metal cation. is there.

Further, an anion (ML _x ⁿ⁻ ) containing the following metal, for example, AuCl ₄ , PtCl ₆ , RhCl ₄ , RoF ₆ , NiF ₆ , CuF ₆ , RuCl ₆ , In ₂ Cl ₆ , the metal is arranged on the halogen. It can also be suitably used for trapping, concentrating, recovering or removing anions after being positioned.

Examples of the alkali metal include lithium, sodium, potassium, cesium and the like. In this case, as a use condition of the medium, the pH is preferably in the range of 5 to 14, and more preferably in the range of 7 to 9. In capturing alkali metal ions, when the alkali metal ion is a cation, the counter anion of the alkali metal cation is Cl, NO ₃ , SO ₄ , PO ₄ , ClO ₄ , PF ₆ , BF ₄ or the like. It can be preferably used for some systems.

  Examples of the alkaline earth metal ion include barium, magnesium, calcium and the like.

  Examples of the lanthanum metal ion include trivalent cations such as La, Eu, Gd, Yb, and Eu.

  The metal ion scavenger of the present invention can be used in various shapes such as powders, particles, polyhedrons and cylinders. For this reason, when capturing metal ions using the metal ion scavenger of the present invention, these silica scavengers can be used in a batch method or a flow method.

  When capturing by the batch method, the metal ions can be preferably captured by a method of adding silica powder or particles to a metal ion aqueous solution, a method of adding a metal ion aqueous solution to the silica powder, or the like.

  When capturing metal ions by the batch method, the mixing system is shaken from 10 minutes to 24 hours. Although the shaking time varies greatly depending on the type of metal, a cation metal ion can be suitably captured within a few hours.

  In the case of the flow method, silica ions, particles, and the like are filled in a glass column in a wet manner, and metal ions can be suitably captured by flowing a metal ion aqueous solution through the column. In particular, the flow method is suitable for cationic metal ions.

  Since the content of nitrogen derived from the polymer having a linear polyethyleneimine skeleton in the metal ion scavenger of the present invention is an active point that coordinates to the metal ion, if all the nitrogen atoms are used for coordination, the metal The ion scavenger becomes saturated. At this time, the metal ion scavenger is taken out and washed with an aqueous hydrochloric acid solution or an aqueous nitric acid solution, and the metal ions can be easily decoordinated. That is, the scavenger is regenerated. The concentration of the hydrochloric acid or nitric acid aqueous solution used for this regeneration may be 1 to 7M, preferably 3 to 5M.

  After desorbing the metal ions captured by the acid from the metal ion scavenger, the metal ion scavenger is washed with distilled water, 0.001 M aqueous alkali solution or distilled water, and then used again to capture the metal ions. Can do.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto. Unless otherwise specified, “%” represents “mass%”.

[Analysis of scavenger by X-ray diffraction method]
Place the isolated and dried sample on the holder for the measurement sample, set it on the wide-angle X-ray diffractometer “Rint-Ultma” manufactured by Rigaku Corporation, Cu / Kα ray, 40 kV / 30 mA, scan speed 1.0 ° / min. The measurement was performed under conditions of a scanning range of 10 to 40 °.

[Shape analysis of capture agent by scanning electron microscope]
The isolated and dried sample was placed on a glass slide and observed with a surface observation device VE-7800 manufactured by Keyence.

[Observation of capture agent by transmission electron microscope]
The isolated and dried sample was placed on a carbon-deposited copper grid and observed with a JEM-200CX transmission electron microscope manufactured by JEOL.

(Production Example 1)
<Synthesis of linear polyethyleneimine (L-PEI)>
5 g of polyethyloxazoline (number average molecular weight 500000, average polymerization degree 5000, manufactured by Aldrich) was dissolved in 20 mL of 5M aqueous hydrochloric acid. The solution was heated to 90 ° C. in an oil bath and stirred at that temperature for 10 hours. Acetone 50 mL was added to the reaction solution to completely precipitate the polymer, which was filtered and washed with methanol three times to obtain white polyethyleneimine powder. When the obtained powder was identified by ¹ H-NMR (heavy water), peaks 1.2 ppm (CH ₃ ) and 2.3 ppm (CH ₂ ) derived from the side chain ethyl group of polyethyloxazoline completely disappeared. It was confirmed that That is, it was shown that polyethyloxazoline was completely hydrolyzed and converted to polyethyleneimine.

  The powder was dissolved in 5 mL of distilled water, and 50 mL of 15% aqueous ammonia was added dropwise to the solution while stirring. The mixture was allowed to stand overnight, the precipitated polymer crystal powder was filtered, and the crystal powder was washed with cold water three times. The crystal powder after washing was dried at room temperature in a desiccator to obtain linear polyethyleneimine (L-PEI). The yield was 4.2 g (including crystal water). In polyethyleneimine obtained by hydrolysis of polyoxazoline, only the side chain reacts and the main chain does not change. Therefore, the polymerization degree of L-PEI is the same as 5000 before hydrolysis.

<Production of scavenger from linear polyethyleneimine system>
The L-PEI powder obtained in Synthesis Example 1 was dispersed in distilled water, and the dispersion was heated to 90 ° C. in an oil bath to obtain a completely transparent aqueous solution having a concentration of 2%. The aqueous solution was left at room temperature and naturally cooled to room temperature to obtain an opaque L-PEI hydrogel. The obtained hydrogel was deformed when a shearing force was applied, but was an ice cream-like hydrogel capable of maintaining a general shape.

  As a result of X-ray diffraction measurement of the obtained hydrogel, it was confirmed that peaks of scattering intensity appeared at 20.7 °, 27.6 °, and 28.4 °. Moreover, the endothermic peak was confirmed at 64.5 ° C. from the measurement result of the endothermic state change by the calorimeter. From these measurement results, the presence of L-PEI crystals in the hydrogel was confirmed.

  20 mL of a mixture of tetramethoxysilane (TMSO) and ethanol (volume ratio) of 1/1 (volume ratio) is added to 20 mL of the hydrogel thus obtained, and the ice cream is lightly stirred for 1 minute and then left for 40 minutes. did. Then, it wash | cleaned with excess acetone and performed it 3 times with the concentric separator. The solid was collected and dried at room temperature to obtain a white metal ion scavenger. From the X-ray diffraction measurement of the scavenger, L-PEI crystal-derived peaks appeared at 20.5 °, 27.2 °, and 28.2 °.

  When the obtained metal ion scavenger was observed with a scanning microscope, it had a three-dimensional network structure of a fibrous composite as shown in FIG.

  Elemental analysis of the scavenger confirmed that the nitrogen content was 8.5%. From this, in 1 gram of the scavenger, the number of millimolar nitrogen of the ligand was converted to 6.07. This was extremely large compared to the fact that the ligand introduction in the mesoporous silica was usually 2 mmol or less.

Example 1
<Capture of copper ions>
50 mg of the metal ion scavenger obtained above was weighed, and 10 mL of 1 mM copper nitrate aqueous solution was added thereto. The white metal ion scavenger in the mixture turned blue in an instant. Instead, the liquid part was almost colorless. As a result, it was confirmed that copper ions were rapidly taken into the metal ion scavenger. The solution was allowed to stand for 2 hours, and then the liquid part was taken out and the absorption spectrum was measured. It was determined that the remaining copper ions were not in a recognized concentration. That is, copper ions were almost completely trapped by the metal ion scavenger.
The metal ion scavenger dyed blue was collected by filtration and washed with a 2M aqueous nitric acid solution, and it was confirmed that the aqueous solution turned blue. From this, it was shown that the copper ion trapped by the metal ion scavenger is easily recovered.

(Example 2)
<Palladium ion capture and reduction>
20 mg of the above metal ion scavenger was weighed and added to 4 mL of 0.5 M palladium nitric acid aqueous solution and allowed to stand for 2 hours. After the mixture was filtered and the metal ion scavenger was washed with distilled water, it was added into 3 mL of NaBH ₄ aqueous solution (weight concentration 2%) and shaken for 30 minutes. The mixture was filtered and it was washed with distilled water and then acetone. The white metal ion scavenger turned black. Thereby, it was shown that the palladium ion trapped by the metal ion scavenger was reduced to palladium metal by the reducing agent.
As a result of observing the metal ion scavenger by XRD, peaks derived from metal palladium crystals appeared remarkably at 36, 43 and 66 °. From TEM observation (FIG. 2), it was confirmed that a large amount of palladium nanoparticles were fixed in the silica fiber. From this, it was shown that the metal ions captured by the capturing agent can be immobilized in a metallic state.

It is a scanning electron micrograph of the metal ion scavenger in Production Example 1 of the present invention. It is a transmission electron micrograph of the metal ion scavenger to which the palladium metal crystal was fixed in Example 3 of the present invention.

Claims (8)

A metal ion scavenger formed by assembling together fibrous composites having a thickness in the range of 15 to 100 nm, in which a polymer crystal having a linear polyethyleneimine skeleton is coated with silica. The metal ion scavenger according to claim 1, wherein the association between the fibrous composites is a three-dimensional network association. The metal ion scavenger according to any one of claims 1 to 3, wherein the polymer having a linear polyethyleneimine skeleton is composed of a block copolymer of a linear polyethyleneimine block and another polymer block. The metal ion scavenger according to any one of claims 1 to 4, wherein a ratio of the polyethyleneimine skeleton in the polymer having the linear polyethyleneimine skeleton is 40 mol% or more. The metal ion scavenger according to any one of claims 1 to 5, wherein the silica content is in the range of 40 to 80% by mass. A solution containing metal ions and the metal ion scavenger according to any one of claims 1 to 6 are brought into contact with each other to convert the metal ions contained in the solution into linear polyethylene in the metal ion scavenger. A method for capturing a metal ion, characterized by coordinating to an imine skeleton. The metal ion according to claim 7, wherein the metal ion is at least one metal ion selected from alkali metal ions, alkaline earth metal ions, transition metal ions, polyoxometalates, and lanthanum metal ions. Capture method. The metal ion separation method which isolate | separates the metal ion trapped by the metal ion scavenger by the method of Claim 7 from a metal scavenger by an acidic process.