JP2013212495A - Purification technology of metal ion contaminated water using water-soluble polymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple purification technology of metal at low cost.SOLUTION: A purification technology of metal ion contaminated water includes: a mixed solution production step of producing a mixed solution by mixing a water-soluble polymer in an aqueous solution containing a metal ion; a pH adjustment mixed solution production step of adjusting pH of the mixed solution to the pH in a range in which metal forms the water-soluble polymer and an inclusion complex; a precipitate formation step of forming a precipitate by adding an inorganic salt to a pH adjustment mixed solution; and a step of removing the metal from the aqueous solution by removing the precipitate. Further, a step of blending an organic ligand in the mixed solution production step.

Description

  The present invention relates to an inexpensive and simple purification technology for metal ion contaminated water. More specifically, the present invention relates to a metal ion removal method for removing metal ions from an aqueous solution by utilizing the property that metal ions form an inclusion complex with a water-soluble polymer.

  In recent years, various products have been developed with the development of science and technology. The industrial process for producing or destroying the developed product necessitates the use of various chemicals. As a result of the use of this chemical, many environmental pollutions have been taken up as problems. Among these environmental pollution, it has become clear that heavy metals were the cause of past environmental pollution problems. Even today, environmental pollution problems caused by heavy metals continue to spread worldwide as a cause of environmental pollution in developing countries.

Examples of environmental pollution problems caused by heavy metals in Japan include Minamata disease and Itai-itai disease, which are one of the four major pollutions. Minamata disease was caused by organic mercury in factory effluent, and itai-itai disease was also caused by cadmium leaching from the mine. In response to these environmental pollution problems, the “Water Pollution Prevention Law” that regulates wastewater was enacted in 1970. This will regulate the amount of wastewater containing hazardous substances such as copper, mercury, and cadmium, and the content of harmful substances for wastewater discharged from public facilities such as factories and business establishments. became.
In this way, heavy metals can be a cause of harm to the environment and the human body, but it is also a fact that they are indispensable for modern industry. For this reason, it is necessary to convert heavy metals into alternative substances and develop purification technologies after contamination with heavy metals.

Another example of metal contamination is the Fukushima Daiichi nuclear power plant accident.
In the accident at the Fukushima Daiichi Nuclear Power Station, the treatment of radioactively contaminated water has become a problem, and the problem of radioactive cesium contained in radioactively contaminated water has become apparent. Radioactive cesium is a toxic substance that is highly reactive and can cause internal exposure, and its half-life is relatively long at 30.1 years. An example of a method for removing radioactive cesium is a method using a metal scavenger such as zeolite. However, there are many problems such as difficulty in recovering radioactive cesium after trapping adsorption. For this reason, there is a need for a method for purifying radioactive cesium that is easy to remove and recover.

  Heavy metal purification techniques include sulfide precipitation, water purification, hydroxide precipitation, mining removal, and the use of metal scavengers (Non-Patent Documents 1 and 2). Among these technologies, the heavy metal purification technologies currently used are the sulfide precipitation method and the hydroxide precipitation method.

  The sulfide precipitation method is a technique that enables heavy metals to be separated from contaminated water by precipitating heavy metals as sulfides by blowing hydrogen sulfide into the contaminated water and removing the precipitates. The sulfide precipitation method has the advantage that the metal ion concentration can be easily reduced because the solubility product of sulfide is low. However, since the precipitates of sulfides are fine, precipitation separation is low, hydrogen sulfide is generated in the process of generating sulfides using a sulfiding agent, and bad odors are generated. The law has many challenges.

  In the hydroxide precipitation method, a heavy metal is precipitated as a hydroxide by adding an alkaline agent to the contaminated water to raise the pH, and the heavy metal can be separated from the contaminated water by removing this precipitate. It is. However, in the hydroxide precipitation method, there are problems that the precipitate produced is small and there are metals that cannot be separated, or the pH of the wastewater after treatment must be adjusted before discharging.

A purification technology that has the potential to eliminate the disadvantages of these conventional technologies is a purification technology using a polymer. Some polymers interact with heavy metals to cause precipitation. Among these, the disadvantages of the purification technology mentioned above can be compensated by using a polymer that does not burden the environment.
In recent years, a purification technique using polyglutamic acid (hereinafter referred to as PGA) has attracted attention as a purification technique using such a polymer (Patent Document 1, Non-Patent Document 3, Non-Patent Document 4).

JP2007-152188A

Sato Yuya (2001) Soil pollution and countermeasures, Environmental research, 122, 96-103. Naito, Katsuhiko (2002) Environmental research, 127, 28-39. K. Hikichi, H. Tanaka, A. Konno, Polym. J. 22,103 (1990) D. Gonzales, K. Fan and M, Sevoian, J Polym Sci Part A: Polym Chem, 34, 2019 (1996) H Yokoi, S Kawata, MIwaizumi, J. Am. Chem. Soc., 108, 3358-3361 (1986)

Patent Document 1 and the like disclose a purification technique using PGA (hereinafter, PGA purification technique).
In PGA purification technology, crosslinked PGA obtained by crosslinking PGA by irradiating PGA with γ rays is used as a purification agent (hereinafter referred to as PGA purification agent). By mixing this PGA purifier with contaminated water containing heavy metals, the PGA purifier coordinates with the heavy metals to form a precipitate. It is a purification technology that enables heavy metals to be separated from contaminated water by removing the precipitate.
PGA purification technology has the advantage that PGA itself is biodegradable and has a low environmental impact. However, because the PGA purifier itself is relatively expensive, it is not suitable for wastewater treatment in large quantities, and is currently used only for drinking water purification in developing countries. In particular, toxic heavy metals such as Cu ²⁺ cause serious environmental pollution, and there is a need for a relatively inexpensive and simple heavy metal purification technology that can be used in developing countries.

  In view of the above circumstances, an object of the present invention is to develop an inexpensive and simple metal ion purification technique.

The inventors focused on a report on Cu ²⁺ using polyvinyl alcohol (hereinafter referred to as PVA).
In other words, Cu ²⁺ aqueous solution becomes Cu (OH) ₂ at pH ≧ 6 and precipitates. However, when PVA is added to Cu ²⁺ aqueous solution, it becomes a green transparent solution regardless of pH ≧ 6. (Non-Patent Document 5). Non-patent document 5 reports that PVA has only an alcoholic hydroxyl group with low coordination ability, and PVA cannot be coordinated to Cu ²⁺ , so PVA surrounds Cu (OH) ₂ . It has been reported that it forms a kind of inclusion complex and interacts differently from coordination.

The inventors developed this knowledge and completed the present invention. That is, metal ions such as Cu ²⁺ have a property of forming an inclusion complex with a water-soluble polymer such as PVA in a specific pH range (hereinafter referred to as inclusion complex forming pH). By utilizing this property of clathrate complex formation and adding an inorganic salt to an aqueous solution in which metal ions form an clathrate complex with a water-soluble polymer, the water-soluble polymer includes metal ions. It was found that a precipitate was formed in the form. The invention relating to the metal ion removal method of removing metal ions from metal ion contaminated water by separating and removing the generated precipitate was completed.

In addition, the inventors have also completed an invention in which the metal ion removal method has been developed. In other words, metal ions such as Cu ²⁺ have the property of forming an inclusion complex with a water-soluble polymer such as PVA at the inclusion complex forming pH. It was found that metal ions and water-soluble polymers form inclusion complexes in the pH range. Based on this finding, the inventors have completed an invention relating to a method for removing metal ions in which an organic ligand is further added.

  The present invention comprises the following.

The first configuration of the present invention is a mixed solution preparation step in which a water-soluble polymer is mixed in an aqueous solution containing metal ions to prepare a mixed solution, and a pH adjusted mixed solution preparation in which the mixed solution is adjusted to an inclusion complex forming pH A step of forming a precipitate by adding an inorganic salt to a pH-adjusted mixed solution, and a method of removing metal ions from an aqueous solution by removing the precipitate, the inclusion complex forming pH However, in the method for removing heavy metal ions, the metal ion has a pH in a range in which an inclusion complex forms with the water-soluble polymer.
A second configuration of the present invention is the metal ion removal method according to the first configuration, wherein the water-soluble polymer is selected from any of polyvinyl alcohol, polyacrylic acid, and dextran.
According to a third configuration of the present invention, the metal is Cu ²⁺ , Fe ³⁺ , Cr ³⁺ , Ni ²⁺ , Zn ²⁺ , Pb ²⁺ , Mn ²⁺ , Cd ²⁺ , Cr ³⁺ , Cs. The metal ion removal method according to the first or second configuration, characterized in that it is ⁺ .

A fourth configuration of the present invention is the metal ion removal method according to the first to third configurations, wherein an organic ligand is further mixed in the mixed solution preparation step.
A fifth configuration of the present invention is the metal ion removing method according to the fourth configuration, wherein the organic ligand is a nitrogen-containing ligand.
A sixth configuration of the present invention is the metal ion removal method according to the fifth configuration, wherein the nitrogen-containing ligand is a nitrogen-containing amino acid.
A seventh configuration of the present invention is the metal ion removal method according to the fifth configuration, wherein the nitrogen-containing ligand is a nitrogen-containing aromatic ring compound.
The eighth configuration of the present invention is the metal ion removal method according to claim 4, wherein the organic ligand is an oxygen-containing cyclic compound.
According to a ninth configuration of the present invention, the organic ligand is 2,2′-bipyridyl, tryptophan, histidine, pyridine, imidazole, dibenzo-21-crown-7-ether, dibenzo-24-crown-8-ether. The metal ion removal method according to the fourth configuration, wherein the metal ion removal method is selected from any one or more of the above.

According to the present invention, it is possible to provide a metal ion purification technique that is relatively inexpensive and simple. That is, according to the metal ion removal method of the present invention, it is possible to remove metal ions from contaminated water and the like relatively inexpensively and easily by using a relatively inexpensive water-soluble polymer such as PVA. In addition, by using an organic ligand, metal ions can be removed from contaminated water in a wider pH range, and convenience can be improved. In addition, radioactive cesium contained in radioactively contaminated water can be expected to be relatively inexpensive and easy to remove.

The figure explaining the principle of the metal ion removal method concerning this invention Figure showing how Cu ²⁺ and PVA form an inclusion complex Example of metal ion removal method according to the present invention using Cu ²⁺ and PVA Example of metal ion removal method according to the present invention using Cu ²⁺ and PVA Diagram showing precipitation at pH 2 Diagram showing precipitation at pH 10 Example of metal ion removal method according to the present invention using Ni ²⁺ and PVA Example of metal ion removal method according to the present invention using Fe ³⁺ and PVA Example of metal ion removal method according to the present invention using Cu ²⁺ , PVA, and bpy Example of metal ion removal method according to the present invention using Zn ²⁺ and PVA Example of metal ion removal method according to the present invention using Pb ²⁺ and PVA Example of metal ion removal method according to the present invention using Mn ²⁺ and PVA Example of metal ion removal method according to the present invention using Cd ²⁺ and PVA Example of metal ion removal method according to the present invention using Cr ²⁺ and PVA Example of metal ion removal method according to the present invention using Cu ²⁺ and PVA in the presence of tryptophan Figure showing the pale blue color of the precipitate when tryptophan is used Diagram showing the structure of the crown ether used in the study

  Hereinafter, the metal ion removal method of the present invention will be described in detail.

First, the metal ion removal method of the present invention will be described based on the principle with reference to FIG.
Outside the range of inclusion complex formation pH, metal ions do not form inclusion complexes with water-soluble polymers and are dissolved in aqueous solution (Fig. 1, a). Even if inorganic salt is added and salted out in this state, only the water-soluble polymer precipitates, and the metal ions remain dissolved in the aqueous solution.
On the other hand, in the range of inclusion complex formation pH, metal ions form inclusion complexes with water-soluble polymers (Fig. 1, b). When an inorganic salt is added and salting out in the state where this inclusion complex is formed, precipitation occurs in a form in which the water-soluble polymer contains metal ions (Fig. 1, c). By removing this precipitate from the aqueous solution, metal ions can be removed.
In the case of the metal ion-organic ligand complex, the hydrophobic interaction with the water-soluble polymer can be enhanced by the organic ligand compared to the case of the metal ion alone. Inclusion complex formation in a wide pH range is possible.

  Next, the metal ion removal method of the present invention will be described based on each step (mixed solution preparation step, pH adjustment mixed solution preparation step, precipitate formation step).

  In the mixed solution preparation step, a water-soluble polymer is mixed with an aqueous solution containing metal ions to prepare a mixed solution.

The metal ion is not particularly limited, and any metal ion capable of forming an inclusion complex directly or indirectly with the water-soluble polymer can be used. For example, Cu ²⁺ is known to form an inclusion complex with polyvinyl alcohol, which is a water-soluble polymer, as a dihydroxide as shown in FIG. As another example, Fe ³⁺ and Cr ³⁺ are also known to form inclusion complexes with water-soluble polymers as hydroxides. Further, as will be described later, metal ions that indirectly form inclusion complexes with water-soluble polymers by coordination with organic ligands can also be used.

  The water-soluble polymer is not particularly limited as long as it forms an inclusion complex with a metal ion or the like and can cause precipitation by salting out. As such a water-soluble polymer, for example, polyvinyl alcohol, polyacrylic acid, or dextran can be used. Among these water-soluble polymers, since polyvinyl alcohol is relatively inexpensive, it is most preferable to use polyvinyl alcohol, which has the effect of reducing the cost of the metal ion removal method of the present invention.

  In the mixed solution preparation step, an organic ligand can be further mixed. As a result, the metal ion is coordinated with the organic ligand, thereby making it easier to form an inclusion complex by hydrophobic interaction with the water-soluble polymer. As a result, since the water-soluble polymer and the metal ion easily form an inclusion complex in a wider pH range, it may not be necessary to substantially adjust the pH. The convenience of the metal ion removal method of the present invention Has the effect of dramatically improving the. In particular, it is useful in that metal ions can be removed even in a low pH region.

  The organic ligand is not particularly limited as long as it is an organic compound having an unshared electron pair and a coordination ability with a metal ion. However, from the viewpoint of interaction with a water-soluble polymer, organic coordination is not necessary. The ligand needs to have a certain degree of hydrophobicity. Moreover, since the organic ligand coordinated to the metal ion needs to be included in the water-soluble polymer by hydrophobic interaction or the like, an organic compound having an excessively large molecular size as the organic ligand is not preferable. In addition, since the coordination ability of the organic ligand varies depending on the type of metal ion, it is also important to select an organic ligand that matches the metal to be removed. As such an organic ligand, for example, an organic compound having a nitrogen atom, an oxygen atom, a sulfur atom, a phosphorus atom or the like having an unshared electron pair can be mentioned.

As the organic ligand, it is preferable to use a nitrogen-containing ligand or an oxygen-containing ligand from the viewpoint of coordination ability with a metal ion.
An amino acid can be preferably used as the nitrogen-containing ligand. Since amino acids can be obtained relatively easily and inexpensively, the metal ion removal method according to the present invention has the effect of being able to be performed more inexpensively and easily. For example, tryptophan can be used as the amino acid.

  In another embodiment, the nitrogen-containing ligand can be a nitrogen-containing aromatic ring compound. Since the nitrogen-containing aromatic ring compound is excellent in coordination ability with metal ions, the effect of enhancing the efficiency of the method for removing metal ions according to the present invention can be expected. Examples of the nitrogen-containing aromatic ring compound include 2,2'-bipyridyl, tryptophan, histidine, pyridine, imidazole and the like.

  Preferably, an oxygen-containing cyclic compound can be used as the oxygen-containing ligand. Since the oxygen-containing cyclic compound is excellent in coordination ability with metal ions, the effect of improving the efficiency of the metal ion removal method according to the present invention can be expected. As the oxygen-containing cyclic compound, for example, a crown ether derivative can be used.

  In the pH adjustment mixed solution preparation step, the pH of the mixed solution prepared in the mixed solution preparation step is adjusted to the inclusion complex formation pH.

Inclusion complex formation pH is defined as the pH within which metal ions form an inclusion complex with a water-soluble polymer. This inclusion complex formation pH can be measured by the technical common knowledge of those skilled in the art.
That is, the spectrum of a mixed solution of metal ions and water-soluble polymers adjusted to a plurality of pH is measured using nuclear magnetic resonance analysis (NMR) or electron spin resonance (ESR). By comparing the changes in the spectrum, it is possible to measure the inclusion complex formation pH.

  The adjustment method for adjusting the inclusion complex formation pH is not particularly limited, and may be adjusted by adding an acid such as HCl or an alkali such as NaOH to the mixed solution.

  The pH adjustment mixed solution preparation step can be performed simultaneously with the mixed solution preparation step. That is, when the inclusion complex formation pH is reached at the same time as the preparation of the mixed solution (for example, when the nitrogen-containing ligand described above is mixed), the pH adjustment mixed solution step is substantially omitted. In this case, the pH adjustment mixed solution step is performed simultaneously with the mixed solution preparation step.

In the precipitation formation step, an inorganic salt is added to the mixed solution after pH adjustment to form a precipitate.
As the inorganic salt, an inorganic salt that can be precipitated with a water-soluble polymer and precipitate can be used. As the inorganic salt, a neutral salt can be preferably used. As a result, it is possible to reduce the influence of pH change of the pH-adjusted mixed solution, and the effect of improving the precipitation efficiency can be expected. Examples of such a neutral salt include NaCl, Na ₂ SO ₄ , KNO _{3 and the} like.

  The precipitate formed by the precipitation forming step can be removed by a commonly used method. For example, a filtration method or a centrifugal separation method.

The metal ion removal method according to the present invention has been described above. In the present invention, it is possible not only to remove metal ions but also to remove specific metal ions step by step by selecting the inclusion complex forming pH. For example, the Fe ³⁺ by 4 from less than 6 clathrate formation pH, the Cu ²⁺ by from 6 8, can be removed Ni ²⁺ by 9 or more.

  Below, based on an Example, the metal ion removal method concerning this invention is demonstrated in detail, However, The content of this invention is not limited to the following naturally.

<< Examples 1 to 6, Examination of Precipitation Formation by Interaction between PVA and Cu 2 ⁺ >>
Experiments such as Example 1 were conducted for the purpose of whether Cu ²⁺ can be separated by salting out in a mixed solution of Cu ²⁺ and polyvinyl alcohol (hereinafter referred to as PVA).

1. experimental method
(1) Each Example and Comparative Example and their respective solution compositions are listed in Table 1. The PVA concentration is the concentration as a monomer unit.
(2) For each example and comparative example, experimental solutions were prepared as follows. First, PVA was heated and dissolved in distilled water, and then CuCl ₂ · 2H ₂ O and NaClO ₄ were added to the PVA solution for the purpose of adjusting the ionic strength. Thereafter, HCl or NaOH was added to adjust the pH, and an experimental solution was prepared so as to finally have a concentration shown in Table 1.
(3) To each prepared experimental solution, NaCl was added until saturation, and stirred to precipitate PVA.
(4) After precipitation, suction filtration was performed using coarse filter paper to obtain a filtrate. Further, the obtained filtrate was subjected to syringe filtration (filter size, 0.2 μm) to remove fine precipitates.
(5) The absorbance of Cu ^{2+ in} the filtrate was measured by atomic absorption spectrometry for the filtrate obtained by filtration. For this atomic absorption analysis, an atomic absorption photometer, AA-6200, manufactured by Shimadzu Corporation was used.
The absorbance (6) measured Cu ^2+, apart from the Examples and the like, using a calibration curve prepared using a standard sample, was calculated for Cu ²⁺ concentrations. In each example, PVA is added to the experimental solution, so the experimental solution is viscous. In order to prevent errors in Cu ²⁺ absorbance measurement due to viscosity, the standard sample is a standard sample with the addition of a predetermined amount of PVA corresponding to each example, and the solution after the salting-out / filtration operation with Cu ²⁺ added. Atomic absorption analysis was performed as a sample, and a calibration curve was prepared.

2. Results and discussion
(1) The results are shown in FIG. 3 and FIG. In the figure, the vertical axis represents the Cu ²⁺ concentration in the filtrate, and the horizontal axis represents the pH. The higher the Cu ²⁺ concentration in the filtrate, the more Cu ²⁺ is not removed as a precipitate. Also, the less Cu ²⁺ concentration in the filtrate means that Cu ²⁺ often are removed as a precipitate.
(2) Looking at Fig. 3 and Fig. 4, the Cu ²⁺ concentration in the filtrate is high when the pH is 5 or less, slightly varied when the pH is 6, but extremely low when the pH is 7 or more. I found out that
(3) In addition, as a qualitative analysis, the coloration of precipitates carried out at pH 2 and pH 10 was compared. The precipitate at pH 2 was white (Fig. 5), whereas the precipitate at pH 10 showed green color. (FIG. 6). This green coloring itself indicates that it is precipitated in a form including Cu ²⁺ .
(4) From these results, it was found that the Cu ²⁺ concentration in the filtrate changed depending on the pH regardless of the molecular weight and saponification degree of PVA. In other words, it was found that Cu ²⁺ was removed as a precipitate by salting out at a pH of 6 where Cu ²⁺ forms an inclusion complex with PVA.

<< Example 7, Examination of precipitate formation by interaction between PVA and Ni ²⁺ >>
Similar to Cu ²⁺ , experiments such as Example 7 were conducted with the aim of separating Ni ²⁺ by salting out in a mixed solution of Ni ²⁺ and PVA.

1. Experimental Method Experiments were performed with the solution compositions shown in Table 2 in accordance with the experimental method of Example 1 and the like.

2. result
(1) Looking at FIG. 7, it was found that the Ni ²⁺ concentration in the filtrate was high at pH 8 or lower, decreased at pH 9 and extremely low at pH 10 or higher.
(2) From these results, it was found that Ni ²⁺ was removed as a precipitate by salting out at a pH of 9 where Ni ²⁺ forms an inclusion complex with PVA.

<< Example 8, Examination of precipitate formation by interaction of PVA and Fe ³⁺ >>
Similar to Cu ²⁺ , an experiment such as Example 8 was conducted for the purpose of whether Fe ³⁺ can be separated by salting out in a mixed solution of Fe ³⁺ and PVA.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 3 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 8, it was found that the Fe ³⁺ concentration in the filtrate was high when the pH was 3 or less and extremely low when the pH was 4 or more.
(2) From these results, it was found that Fe ³⁺ was removed as a precipitate by salting out at a pH of 4 where Fe ³⁺ forms an inclusion complex with PVA.

<< Examples 9 to 11, examination of precipitate formation by interaction of PVA and Cu ^{2+ in the} presence of bpy >>
The purpose of investigating how Cu ²⁺ separation is affected by adding 2,2'-bipyridyl (hereinafter referred to as bpy), an organic ligand, to a mixed solution of Cu ²⁺ and PVA Experiments such as Example 9 were conducted.

1. experimental method
The experiment was conducted with the solution composition shown in Table 4 in the same manner as in Example 1 except that NaClO ₄ was not contained and bpy was contained.

2. result
(1) FIG. 9 shows the results of Examples 9 to 11. Note that the gray curve in FIG. 9 shows a Cu ²⁺ concentration curve that was tested under the same conditions as in Examples 9 to 11, except that bpy was not added.
(2) In any of the studies in Examples 9 to 11, it was found that the Cu ²⁺ concentration in the filtrate was extremely low at all pHs.
(3) From these results, it was found that Cu ²⁺ precipitates were formed in a very wide pH range compared to the case where bpy was not added.

<< Example 12, Examination of precipitate formation by interaction between PVA and Zn ²⁺ >>
Similar to Cu ²⁺ , experiments such as Example 12 were conducted for the purpose of being able to separate Zn ²⁺ by salting out in a mixed solution of Zn ²⁺ and PVA.

1. Experimental Method Experiments were performed with the solution compositions shown in Table 5 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 10, it was found that the Zn ²⁺ concentration in the filtrate was high when the pH was 8 or less and extremely low when the pH was 9 or more.
(2) From these results, it was found that Zn ²⁺ was removed as a precipitate by salting out at a pH of 9 where Zn ²⁺ forms an inclusion complex with PVA.

<< Example 13, Examination of precipitate formation by interaction between PVA and Pb2 ⁺ >>
Similar to Cu ²⁺ , experiments such as Example 13 were conducted with the aim of separating Pb ²⁺ by salting out in a mixed solution of Pb ²⁺ and PVA.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 6 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 11, it was found that the Pb ²⁺ concentration in the filtrate was high when the pH was 6 or less, slightly decreased when the pH was 7, and extremely low when the pH was 8 or more.
(2) From these results, it was found that Pb ²⁺ was removed as a precipitate by salting out at a pH of 7 where Pb ²⁺ forms an inclusion complex with PVA.

<< Example 14, Examination of precipitate formation by interaction between PVA and Mn2 ⁺ >>
As in the case of Cu ²⁺ , an experiment such as Example 14 was conducted for the purpose of whether Mn ²⁺ can be separated by salting out in a mixed solution of Mn ²⁺ and PVA.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 7 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 12, it was found that the Mn ²⁺ concentration in the filtrate was high when the pH was 9 or less and extremely low when the pH was 10 or more.
(2) From these results, it was found that Mn ²⁺ was removed as a precipitate by salting out at a pH of 10 which forms an inclusion complex with PVA.
This precipitation was confirmed to be that Mn ²⁺ was oxidized to Mn ³⁺ . In other words, the hydroxide of Mn ²⁺ is light pink and the hydroxide of Mn ³⁺ is brown. When the PVA-Mn ²⁺ mixed aqueous solution was adjusted to pH ≧ 10, it became brown. , It was confirmed that the resulting precipitate was due to Mn ³⁺ . This is thought to be because Mn ²⁺ became Mn ³⁺ in the air at pH ≧ 10 and became Mn (OH) ₃ , a brown precipitate.

<< Example 15, Examination of precipitate formation by interaction of PVA and Cd2 ⁺ >>
Similar to Cu ²⁺ , experiments such as Example 15 were conducted with the aim of separating Mn ²⁺ by salting out in a mixed solution of Cd ²⁺ and PVA.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 8 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 13, it was found that the Cd ²⁺ concentration in the filtrate was high when the pH was 9 or less, and extremely low when the pH was 10 or more.
(2) From these results, it was found that Cd ²⁺ was removed as a precipitate by salting out at a boundary of 10 where Cd ²⁺ forms an inclusion complex with PVA.

<< Example 16, Examination of Precipitation Formation by Interaction between PVA and Cr ^{3 +} >>
As in the case of Cu ²⁺ , an experiment such as Example 16 was conducted for the purpose of separating Cr ³⁺ by salting out in a mixed solution of Cr ³⁺ and PVA.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 9 in accordance with Example 1 and the like.

2. result
(1) Looking at FIG. 13, it was found that the Cr ³⁺ concentration in the filtrate was high when the pH was 5 or less, slightly decreased when the pH was 6, and extremely low when the pH was 7 or more.
(2) From these results, it was found that Cr ³⁺ was removed as a precipitate by salting out at a pH of 10 where Cr ³⁺ forms an inclusion complex with PVA.

<< Example 17, Precipitation formation by the interaction of PVA and Cu ^{2+ in the} presence of tryptophan and examination of presence or absence of coloration >>
Experiments such as Example 17 were conducted for the purpose of separating Cu ²⁺ by adding tryptophan, which is an organic ligand, to a mixed solution of Cu ²⁺ and PVA and performing salting out.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 10 in accordance with Example 9 and the like.

2. result
(1) The results are shown in FIG. 15 and FIG.
(2) Looking at FIG. 15, it was found that the Cu ²⁺ concentration in the filtrate was high when the pH was 3 or less and extremely low when the pH was 4 or more. As a result, it was confirmed that Cu ²⁺ was precipitated at a wider pH compared to Example 1 and the like.
(3) As shown in the left part of FIG. 16, the precipitate showed a pale white coloration. As a result, it was confirmed that Cu ²⁺ was contained in the precipitate by adding a tryptophan and performing a salting-out reaction.

<< Examples 18 and 19, Examination of precipitate formation by interaction of PVA and Cs ^{+ in} presence of crown ether >>
Experiments such as Example 18 were conducted for the purpose of separating Cs ⁺ by adding crown ether (FIG. 17), which is an organic ligand, to a mixed solution of Cs ⁺ and PVA and performing salting out. It was. Example 18 was performed using DB21C7, and Example 19 was performed using DB24C8.

1. Experimental Method Experiments were conducted with the solution compositions shown in Table 10 in accordance with Example 9 and the like.

2. result
(1) While the precipitation removal rate in the comparative example was 17.0%, the precipitation removal rate in Example 18 was 34.0% and in Example 19 was 32.7%, which was approximately twice as high.
(2) From this, it was confirmed that the precipitation removal rate was improved by crown ether.

Claims (9)

A mixed solution preparation process in which a water-soluble polymer is mixed with an aqueous solution containing metal ions to prepare a mixed solution;
A pH adjustment mixed solution preparation step of adjusting the mixed solution to an inclusion complex forming pH;
a precipitation forming step of forming a precipitate by adding an inorganic salt to the pH adjusted mixed solution;
A metal ion removal method for removing metal ions from an aqueous solution by removing the precipitate,
The method for removing metal ions, wherein the inclusion complex forming pH is within a range in which the metal ions form an inclusion complex with a water-soluble polymer. The method for removing metal ions according to claim 1, wherein the water-soluble polymer is selected from polyvinyl alcohol, dextran, and polyacrylic acid. The metal is Cu ²⁺ , Fe ³⁺ , Cr ³⁺ , Ni ²⁺ , Zn ²⁺ , Pb ²⁺ , Mn ²⁺ , Cd ²⁺ , Cr ³⁺ , Cs ⁺ Item 3. The method for removing metal ions according to Item 1 or 2 4. The method for removing metal ions according to claim 1, wherein an organic ligand is further mixed in the mixed solution preparation step. 5. The method for removing metal ions according to claim 4, wherein the organic ligand is a nitrogen-containing ligand. 6. The method for removing metal ions according to claim 5, wherein the nitrogen-containing ligand is an amino acid. 6. The method for removing metal ions according to claim 5, wherein the nitrogen-containing ligand is a nitrogen-containing aromatic ring compound. 5. The method for removing metal ions according to claim 4, wherein the organic ligand is an oxygen-containing cyclic compound. The organic ligand is selected from one or more of 2,2'-bipyridyl, tryptophan, histidine, pyridine, imidazole, dibenzo-21-crown-7-ether, dibenzo-24-crown-8-ether 5. The method for removing metal ions according to claim 4