JPH0899022A - Method for electrodialysis - Google Patents

Abstract

PURPOSE: To perform removal and concn. of bromide ion and nitrate ion by decreasing permeability of bromide ion and improving permeability of nitrate ion by means of electrodialysis using an ion-exchange membtrane. CONSTITUTION: In electrodialysis of a water soln. contg. bromide ion and/or nitrate ion, a composite membrane wherein polypyrrole exists in a range of e.g. 0.01-2mg/cm<2> -dry membrane in an anion exchange membrane base.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selectively permeating bromine ions and selectively permeating nitrate ions by electrodialysis to remove or concentrate these ions.

[0002]

2. Description of the Related Art Today, various methods are used to remove or concentrate harmful ionic species from the viewpoint of environmental protection. Of these, the removal and concentration of inorganic ions with an ion exchange membrane is a very important problem, and a technique for freely controlling the selective permeability for various inorganic ions is required. For example, in seawater-concentrated salt production by electrodialysis, membranes that can block the permeation of bromine ions and sulfate ions and membranes that selectively permeate harmful nitrate ions, which have increased abnormally in groundwater in recent years, are widely used in various fields. It has been demanded. In addition to the above, from the viewpoint of environmental protection, there is a need for an anion exchange membrane that selectively permeates a specific anion in a chemical synthesis process or chemical reaction process, or a membrane that does not permeate.

[0003]

Generally, bromine ions having a small radius of hydrated ions are more likely to permeate the anion exchange membrane than chlorine ions. However, no anion exchange membrane that is difficult to permeate bromine ions has been reported to date.

Further, nitrate ions are usually easier to permeate through an anion exchange membrane than chlorine ions. Further, anion exchange membranes having excellent performance have already been announced, but in any case, the electric resistance of the membranes increases. There is a need for an anion exchange membrane that selectively permeates nitrate ions without increasing electrical resistance.

[0005]

From such a point of view, the present inventors have proposed an electrodialysis method in which bromine ions are less likely to permeate and nitrate ions are selectively permeated as compared with conventional anion exchange membranes. As a result of intensive studies, when a membrane in which polypyrrole is present in the anion exchange membrane substrate is used, unexpectedly no decrease in current efficiency is caused, and no increase in electric resistance of the membrane is caused. The inventors have found that the permeability of bromine ions is suppressed and at the same time the permeability of nitrate ions is improved, and have completed the present invention.

That is, the present invention is an electrodialysis method characterized by using a composite membrane in which polypyrrole is present in an anion exchange membrane substrate when electrodialyzing an aqueous solution containing bromine ions and / or nitrate ions. .

The aqueous solution to be treated in the present invention contains one or both of bromine ion and nitrate ion, and other anions such as sulfate ion, chloride ion, fluoride ion and carbonate. Inorganic ions such as ions and bicarbonate ions, and in some cases, organic ions may be contained. In particular, it selectively permeates ionic species other than bromine ions from an aqueous solution containing trace amounts of bromine ions, such as seawater, or an aqueous solution containing trace amounts of bromine ions and other anions, and nitrate ions. It is preferably used for removing nitrate ions from a salt solution, particularly for removing nitrate ions from groundwater contaminated with nitrate ions. There is no particular limitation on the salt concentration of the aqueous solution containing bromine ions and / or nitrate ions, and the total salt concentration is several 1
It may be in the range of 00 ppm to several stipulations and is selected according to the purpose. The salt concentration of the aqueous solution does not significantly change the permeability of bromine ions and / or nitrate ions.

The anion exchange membrane used for forming the composite membrane in which the polypyrrole used in the present invention is present in the anion exchange membrane substrate is not particularly limited, and known anion exchange membranes can be used. When carrying out electrodialysis of a neutral salt solution, a strongly basic anion exchange group such as an onium base, that is, a quaternary ammonium base, a tertiary sulfonium base, or a quaternary phosphonium base must be bound. Is preferred,
When carrying out electrodialysis of an acidic aqueous solution, an anion exchange membrane having a weakly basic anion exchange group such as primary, secondary or tertiary amino group bonded thereto may be used. The types of anion exchange membranes include homogeneous membranes, heterogeneous membranes, membranes composed of polymerized polymers, membranes composed of condensed polymers, hydrocarbon anion exchanged membranes,
Any of fluorocarbon-based anion exchange membranes may be used without any particular limitation.

In the case of forming a complex with polypyrrole, it is preferable that polypyrrole be dispersed and present in the anion-exchange membrane substrate as uniformly as possible. In order to make polypyrrole uniformly exist in the anion exchange membrane substrate, a method by chemical oxidative polymerization is preferable. As the oxidizing agent in the chemical oxidative polymerization, known ones such as persulfate ion, hydrogen peroxide, Fe ³⁺ and Cu ²⁺ can be used, but Fe ³⁺ is particularly preferable.

The specific method for allowing polypyrrole to exist in the substrate of the anion exchange membrane is not particularly limited, and for example, pyrrole is dissolved in a solvent that does not deteriorate or abnormally swell the anion exchange membrane, such as water or alcohol. Then, an anion exchange membrane may be dipped in the membrane, and the pyrrole may be sufficiently impregnated or adsorbed in the membrane, and then immersed in an oxidant solution for polymerization. In addition, a method in which an oxidant is adsorbed on or ion-exchanged with an anion exchange membrane in advance and the resultant is immersed in a pyrrole solution can also be mentioned. These methods can be appropriately selected according to the purpose, but when the latter is desirable in order to make the polypyrrole uniformly exist in the anion exchange membrane and hardly increase the electric resistance of the anion exchange membrane. There are many.

The amount of polypyrrole present in the anion-exchange membrane substrate is not particularly limited, but it is selective impermeability of bromide ion to other ions and selective permeation of nitrate ion to other ions. Considering sex,
It is preferably in the range of 0.01 to ² mg / cm ² -dry film.

The following is a general method for producing a composite membrane containing polypyrrole used in the present invention. When an anion exchange membrane having a quaternary ammonium salt group is used, a group capable of reacting with a tertiary amine to form a quaternary ammonium salt group, for example, a polymer film having a chloromethyl group, Specifically, a chloromethylstyrene-divinylbenzene copolymer film material or the like is immersed in an acetone-water mixed solution of a tertiary amine such as trimethylamine to introduce a quaternary ammonium base. The thus obtained anion exchange membrane was immersed in a concentrated iron oxide aqueous solution to adsorb Fe ³⁺ , and this membrane was immersed in a 5% pyrrole aqueous solution without washing with water and the Fe ³⁺ adsorbed in the membrane. The pyrrole is polymerized in the film. The membrane is thoroughly washed with water or acid to remove iron ions in the membrane and then subjected to electrodialysis.

When an anion exchange membrane having a pyridinium base is used, a polymer membrane having a pyridyl group, specifically, 4-vinylpyridine-divinylbenzene copolymer membrane is used. The pyridine ring is alkylated by immersing it in a hexane solution of an alkylating agent such as methyl iodide. Then, the obtained anion exchange membrane is immersed in a concentrated aqueous solution of copper sulfate to adsorb Cu ²⁺ into the membrane,
It is also possible to immerse this in an aqueous solution of pyrrole and polymerize the pyrrole in the film to uniformly form a matrix of polypyrrole.

By using such a composite membrane,
The permeation amount of bromine ions with respect to chloride ions can be reduced, and the permeability of nitrate ions with respect to chloride ions can be increased.

In the present invention, a conventionally known cation exchange membrane is used as the cation exchange membrane for electrodialyzing an aqueous solution containing bromine ions and / or nitrate ions. For example, various cation exchange membranes are appropriately used regardless of the type of polymerization system, condensation system, homogeneous system, heterogeneous system, etc., the manufacturing method, and the presence or absence of a reinforcing material. Further, an ordinary sulfonic acid type cation exchange membrane can be used, or a monovalent cation selective permeable membrane having a thin layer of positive charge on the membrane surface can be used depending on the purpose.

Then, a large number of such cation exchange membranes and the above-mentioned composite membranes are alternately arranged, and electrodialysis is carried out by a conventional method using a conventionally known electrodialysis tank. For example, either a tightening type or a water tank type electrodialysis tank can be used, and an electrodialysis tank with a limiting current density as high as possible is suitable. As a method for operating the electrodialysis tank, a known method can be adopted.

Further, only an anion exchange membrane is arranged between the anode and the cathode, which can be regarded as a kind of electrodialysis, and an aqueous solution containing bromine ions and / or nitrate ions is supplied to the cathode chamber so that the bromine ion is supplied to the cathode chamber. The method of the present invention can also be employed for leaving or for collecting and concentrating nitrate ions in the anode chamber. As the electrodialysis tank (electrolysis tank) in this case, a conventionally known one can be used without any limitation.

[0018]

[Function] The details of the change in anion permeability due to the composite of polypyrrole and anion exchange membrane are unknown.
Polypyrrole itself is a kind of conductive polymer and at the same time is an anion exchanger having a weakly basic anion exchange group.
Further, since it has a conjugated double bond, it is extremely rigid as a polymer. Coexistence of weakly basic anion-exchange group, formation of rigid polymer matrix, and existence of polymer with electronic conductivity, etc. interacts subtly with halogen ions or interacts with nitrate ions. It seems that the selective permeability between the anions changes.

[0019]

According to the method of the present invention, the permeability of bromine ions can be reduced and the permeability of nitrate ions can be improved as compared with the conventional anion exchange membrane. Therefore, the present invention provides a wide field of application of ion-exchange membrane electrodialysis, and its industrial significance is great.

[0020]

Example The permeability of bromine ion or nitrate ion was measured by a four-chamber electrodialysis cell having silver-silver chloride electrodes at both ends as shown in FIG. The anode chamber 1 and the cathode chamber 4 were filled with a sodium chloride solution so as to have the same ion concentration as the measurement solution contained in the desalting chamber 2 and the concentrating chamber 3.
That is, 0.02 normal, 0.07
When it is filled with a 5: 1, 0.25N sodium chloride solution and a 1: 1 mixed aqueous solution of 0.02N, 0.075N, 0.25N sodium bromide, 0.04 each.
Normal, 0.15, and 0.5 normal sodium chloride solutions were filled. Similarly, when measuring the membrane permeability of nitrate ion and chlorine ion, 0.005N, 0.02N, 0.
075 normal sodium nitrate aqueous solution and 0.005 normal,
Measurement was performed using a 1: 1 mixed aqueous solution of 0.02N and 0.075N sodium chloride aqueous solution, and the anode chamber 1 and the cathode 4 have the same salt concentration as the desalting chamber 2 and the concentrating chamber 3. Sodium chloride aqueous solution was filled.

C is a cation exchange membrane (4 cm × 5 cm) used as a diaphragm, and A is an anion exchange membrane (2 cm × 5 c).
m). The current density was changed in proportion to the concentration. After electrodialysis, the concentrations of bromine ion, chloride ion and nitrate ion were analyzed by ion chromatography to determine the permeation equivalent ratio of bromine ion or nitrate ion to chlorine ion permeating the anion exchange membrane A.

Example 1 Commercially available anion exchange membrane NEOSEPTA AM-1
(Manufactured by Tokuyama Corp., trade name) is immersed in an aqueous solution of 4.0 N ferrous chloride at room temperature and allowed to stand for 24 hours, and then the surface is lightly wiped with filter paper without washing with water. ,
It was immersed in a 5% aqueous solution of pyrrole and left for 16 hours. The membrane changed color from slightly yellow to black, but the flexibility of the membrane did not change at all. This film was immersed in 1N hydrochloric acid with stirring, the hydrochloric acid was replaced repeatedly, and the operation was repeated until the hydrochloric acid no longer colored. The amount of polypyrrole dispersed in the anion exchange membrane substrate was 0.08 g / cm ² -dry membrane. Next, this membrane was immersed in a solution used for measurement and equilibrated, and then used for measurement.
The permeability of bromine ion to chloride ion (P ^Br _Cl ) was measured by changing the salt concentration, and the results are shown in Table 1.

The (P ^Br _Cl ) shown here is defined by the following equation.

[0024]

[Chemical 1]

T _Br , t _Cl : _Equivalent number of bromine ion and chlorine ion transmitted through the membrane C _Br , C _Cl : average concentration of bromine ion and chlorine ion in the desalting chamber 2 during electrodialysis.

The electric resistance of the film was measured in a 0.5N saline solution at 1000 cycle AC. As a result, NE
OSEPTA AM-1 was 1.2 Ω-cm ² , but the composite film in which polypyrrole was dispersed was 0.9 to 1.0 Ω.
Was −cm ² .

[0027]

[Table 1]

Example 2 The results of electrodialyzing a mixed solution of sodium nitrate and sodium chloride in the same manner as in Example 1 using the composite membrane synthesized in Example 1 are shown in Table 2.

[0029]

[Table 2]

The P ^NO3 _Cl shown here is the equivalent ratio of nitrate ions to chlorine ions that have permeated the membrane, as in Example 1.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of an electrodialysis tank used in an embodiment of the present invention.

[Explanation of symbols] C: Cation exchange membrane A: Anion exchange membrane 1: Anode chamber 2: Desalination chamber 3: Concentration chamber 4: Cathode chamber

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C02F 1/58 ZAB LP

Claims (1)

[Claims] 1. A method of electrodialysis, which comprises using a composite membrane in which polypyrrole is present in an anion exchange membrane substrate when electrodialyzing an aqueous solution containing bromine ions and / or nitrate ions.