JPS61269095A - Method of treating radioactive substance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Objective of the Invention 1 The object of the present invention is to provide equipment and equipment used in places such as nuclear power plants that handle radioactive materials or where such materials are generated.
We provide peelable coatings for treating radioactive substances that are useful for preventing equipment or building walls, floors, and ceilings from being contaminated by radioactive substances, and for removing radioactive substances from the surface of contaminated base materials. The objective is to enable the work to be performed more efficiently.

[Technical Background 1] With the recent spread of nuclear power plants, work within such power plants has become increasingly diverse, and at the same time, issues of the working environment are becoming more important. For example, it is unavoidable that radioactive substances adhere to appliances, equipment, walls, floors, etc. in a power plant, but if these substances become suspended matter from such contaminated base materials and dissipate into the air, they can be released at extremely low concentrations. Even though it is radiation, it accumulates over time, and therefore the quality of radiation prevention technology has a huge impact on the issue of radiation exposure for workers.

[Prior art 1] Various proposals have been made for the treatment of radioactive substances adhering to the surface of such substrates, and one of them is a method of using strippable paint. be. (For example, Tokuko Sho 38-2580
Publication No. 7, JP-A No. 57-56799, JP-A-58-7
98 Publication, 58-799 Publication, etc.) As a treatment method using this paint, a peelable paint is applied on a base material to which radioactive substances have adhered, and after drying, it is peeled off to remove contaminants (hereinafter referred to as , this is called decontamination), or by applying a peelable paint in advance to a base material that may be contaminated with radioactive materials, and then peeling it off after it has been contaminated to protect the base material from contamination (
There are two known methods (hereinafter referred to as resist dyeing).

[Problem to be solved by the invention 1] However, in the special environment of a nuclear power plant, and in order to target a special substance such as a radioactive substance, it is sufficient to simply repurpose a general-purpose peelable paint. It's not that. In other words, paints used for such special purposes and locations are required to have better performance in various respects than those used for general dust, rust, etc. Among these required performances, the quality of the peeling performance when peeling off the paint film from the base material is directly related to the length of the processing time (this is extremely important as it has a large effect on the radiation exposure of the person performing the work) In other words, if the coating cannot be removed quickly and completely, the radioactive materials will remain in place indefinitely, so the coating must be completely removed through regular operations. First, the time required for such work is a major problem.

However, it is essential that paints used for such applications have good adhesion to the target substrate and that no sagging or flaking of the dried paint film occurs during application, so removability is essential. Although adhesion and adhesion are contradictory performances, it is important in this technical field to maintain the minimum necessary adhesion without losing the balance between the two, while improving releasability and increasing anticontamination and decontamination effects. This has become a big problem.

Means for Solving Problem E 1 However, in view of the above problems, the inventors of the present invention took into account the special circumstances in places where radioactive materials are handled, such as nuclear power plants, and developed a method to remove dust with radioactive materials attached. As a result of extensive research into the most suitable strippable paint for treatment, we have found that either a strippable paint is applied to a substrate contaminated with radioactive substances, dried, and then the coating film is peeled off or removed. In a radioactive substance treatment method in which a peelable paint is applied to a base material that may be contaminated with radioactive substances in advance with a needle, and the coating film is peeled off after contamination, the peelable paint is of an emulsion type. , and the chlorine content (atomic weight equivalent) in the paint is 3p
It has been found that the above-mentioned problems can be successfully solved when a peelable paint having a phosphorus content of 5 ppm or less is used, and the present invention has been completed.

The present invention is characterized in that an emulsion-type peelable paint is used, and the contents of chlorine and phosphorus in the paint are below specified values. That is, the inventors have found that the content of chlorine and phosphorus in the peelable paint used in conventional applications is considerably higher than that of the paint of the present application. It is a novel finding discovered for the first time by the present inventors that reducing the content to an extremely low level has a significant effect on the peeling performance.

In other words, the paint of the present invention has significantly improved peelability compared to peelable paints with relatively high chlorine and phosphorus contents conventionally used for radioactive substance treatment, and as a result, once the paint film is The radioactive substances attached to the paint can be removed more efficiently by being peeled off together with the paint.

The peelable paint used in the present invention will be explained in more detail below.

Emulsion-type Komagu, which is the main ingredient (vehicle) for peelable paints, includes acrylic resins, styrene resins, vinyl acetate resins, ethylene resins, ethylene-vinyl acetate copolymer resins, butadiene resins, and acetylene resins. Although any resin can be used, it is preferable to use an acrylic resin because even better results can be expected when using the resin.

The acrylic resin is a resin whose main component is an acrylic monomer, such as an acrylic acid alkyl ester, a methacrylic acid alkyl ester, and other derivatives of acrylic acid or methacrylic acid. Representative examples of such monomers include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, pentyl acrylate, hebutyl acrylate, octyl acrylate, decyl acrylate, hexadecyl acrylate, octadecyl acrylate,
Methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, pentyl methacrylate, heptyl methacrylate, octyl methacrylate, decyl methacrylate,
Acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, etc., such as hexadecyl methacrylate, are used alone or in combination.

Among these, at least one selected from (meth)acrylic acid esters or acrylonitrile in which the alkyl group has a carbon number of 4 to 3, and at least one selected from (meth)acrylic esters in which the alkyl group has 4 to 10 carbon atoms. More preferably, a copolymer formed by combining at least one monomer selected from methyl methacrylate and acrylonitrile and at least one monomer selected from butyl acrylate and 2-ethylhexyl acrylate, especially its composition. Weight ratio of former/latter = 50/SO~80
A copolymer having an SO of 150 to 70/30, more preferably 150 to 70/30, has the best overall performance.

In the acrylic resin, there is no problem even if a third component other than the above-mentioned acrylic monomer that can be copolymerized is introduced in an amount of about 0 to 20 parts by weight per 100 parts by weight of the acrylic monomer. Such third components include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid,
Fumaric acid, itaconic acid, and their mono- or dialkyl esters, anhydrides, etc. other than the above-mentioned acrylic monomers are preferably used.

Unsaturated monomers used in addition to acrylic resins include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl persate, vinyl palmitate, vinyl stearate, and styrene. , ethylene, propylene, etc. Of course, it is also possible to use these in combination with acrylic monomers as appropriate.

Thus, the form of the peelable paint used in the present invention is an emulsion type paint, more preferably an acrylic paint, in order to maximize the anticontamination/decontamination effect and workability and safety in special environments such as nuclear power plants. Resin-based emulsion
Must be paint. The emulsion type tends to further improve the radioactive substance absorption effect, and its anticontamination and decontamination effects are superior to the solvent type.

To produce such an emulsicon type resin, emulsion polymerization,
Alternatively, any known method can be employed, such as a method of post-emulsifying a resin produced by suspension polymerization, bulk polymerization, solution polymerization, etc., but emulsion polymerization is most preferred.

When carrying out emulsion polymerization, water, emulsifier and/or
・In the presence of a polymeric protective colloid and/or a nonionic surfactant and a polymerization catalyst, an unsaturated monomer is temporarily or continuously added, heated, and stirred. It can be implemented. The amount of emulsifier used varies somewhat depending on the type of emulsifier, the resin content of the emulsion required, etc., but is usually selected from a range of about 1 to 8% by weight based on the entire emulsion polymerization reaction system.

In addition, when producing an emulsion by the post-emulsification method, the emulsifier is dissolved in water, and a solution or molten resin is added dropwise to this and stirred. do it. There is no particular need for heating etc. for emulsification, but if necessary, 45 to 8
It may be heated to about 5°C.

The peelable paint used in the present invention has a final chlorine content (in terms of atomic weight, all elemental content standards are the same below) of 3p.
p + m or less, the phosphorus content is 5 ppm or less, and as described later, preferably the fluorine content is a pl)-
Hereinafter, it is necessary that the sulfur content be spp- or less, so when carrying out the emulsion polymerization, sufficient consideration must be given to satisfying this condition.

In other words, as a general rule, it is necessary to use chemicals that do not contain elements such as chlorine, fluorine, sulfur, and phosphorus, but also to take great care such as performing purification as necessary to avoid containing trace components as impurities. is necessary. From this point of view, examples of preferred agents for producing the emulsion resin used in the present invention include hydrogen peroxide and the like as inorganic peroxide catalysts as polymerization catalysts, diethyl peroxide-cyisopropyl peroxide, and diethyl as organic peroxides. peroxide, distearyl peroxide, g-t-
Butyl peroxide, di-amyl peroxide, dicumyl peroxide, (-butyl high-V rober oxide, t-amyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, acetyl peroxide, propionyl peroxide) , Lauroilver Oxai V,
Steer 0 Il peroxide, Malonyl peroxide V
, succiloyl peroxide, phthaloyl peroxide, benzoyl peroxide, methyl ethyl ketone peroxide, cyclohexanone peroxide, acetyl benzoyl peroxide, propionyl benzoyl peroxide, 2,2゛-azobis(imbutyronitrile), 2,2' Examples include -7zobis(2-methylbutyronitrile), 1,1゛-azobis(1-cyclohexanecarbonitrile), and combinations of hydrogen peroxide and ascorbic acid, and hydrogen peroxide and tartaric acid as redox catalysts. is used. In particular, redox catalysts are advantageous in increasing the polymerization rate and lowering the polymerization process temperature.

The amount of these catalysts used is 0.00 parts by weight per 100 parts by weight of resin.
It is 1 to 1.5 parts by weight.

As the polymer protective colloid, one or more selected from polyvinyl alcohol, cellulose derivatives, poly N-methylol acrylamide, polyacryl 7mide, polyethylene oxide, etc. can be used. As the cellulose derivative, water-soluble cellulose derivatives such as alkyl cellulose, carboxyalkyl cellulose, and hydroxyalkyl cellulose are used. Polydi-N-7 tyroacrylamide and polyacrylamide can be produced by polymerizing monomers such as N-methylol acrylamide and acrylamide, or by copolymerizing them with other vinyl monomers in an amount that does not impair water solubility. obtained by. Polyethylene oxide is obtained by polymerizing ethylene oxide or ethylene glycol. Although many low molecular weight polyethylene oxides are used, it is preferable that the polyethylene oxide used in the present invention has a high molecular weight.

The amount of the protective fluid used is 0 to 30 parts by weight, more preferably 5 to 15 parts by weight, based on 100 parts by weight of the resin.

Various other emulsifiers such as nonionic surfactants can be used in place of the protective colloids mentioned above. Of course, nonionic surfactants can also be used in combination with protective colloids.

Such nonionic surfactants include polyethylene alkyl ether, polyoxyethylene furkylphenol,
Examples include polyoxyethylene alkyl ester, sorbitan alkyl ester, polyoxyethylene sorbitan alkyl ester, copolymers of polyoxyethylene and polyoxypropylene, and particularly useful ones include polyoxyethylene octylphenol needles and polyoxyethylene nonyl. Polyoxyethylene alkylphenol ethers such as phenyl ether are used.

The amount of the surfactant used is 0 to 100 parts by weight of the resin.
The amount is 20 parts by weight, preferably 0.5 to 10 parts by weight.

In addition to the above, aliphatic alcohols having 1 to 4 carbon atoms, ie, methanol, ethanol, furopanol, isopropanol, butanol, isobutanol, and t-butanol can also be used in combination. The amount of alcohol used is 100% of the resin.
1 to 30 parts by weight, more preferably 5 to 2 parts by weight
It is 0 parts by weight.

A peelable paint is produced using the resin thus obtained as a main ingredient. The paint can exhibit sufficient effects with the resin component alone, but if necessary, at least one selected from organic acid compounds, polymeric acid compounds, and chelating agents may be added to further improve the physical properties of the paint. It is preferable to do so. Organic acid compounds include formic acid, acetic acid, butyric acid, L-ascorbic acid, etc., and polymeric acid compounds include polyacrylic acid and (
Copolymers of ethylenically unsaturated carboxylic acids such as meth)acrylic acid, maleic acid, fumaric acid, and itaconic acid, etc.; as a chelating agent, ethylenediaminetetraacetate (E
DTA) or its neutralized product, oxalic acid, citric acid, pyrophosphoric acid, etc., and the blending amount thereof is 0 to 15 parts by weight, more preferably 0.5 to 15 parts by weight, based on 100 parts by weight of the solid content of the emulsion resin. If the amount is 10 parts by weight or more than 15 parts by weight, it tends to impair the stability of the coating material, which is not preferable.

Furthermore, if necessary, auxiliary agents such as pigments, dispersants, antifoaming agents, oil agents, etc. may be appropriately mixed.

Of course, with regard to these organic acid compounds, polymeric acid compounds, chelating agents, and other auxiliary agents, care must be taken regarding trace components, as in the case of the emulsion polymerization described above.

However, as mentioned above, the emulsion type peelable paint obtained as described above has a chlorine content of 3p in the entire paint system.
pm or less, more preferably 2 ppm or less, and the phosphorus content is 5 ppm or less, more preferably 3 ppm or less.
It must satisfy the condition that it is less than or equal to m. If these conditions are not met, the peeling performance will be greatly reduced, and the radioactive substance processing performance will also be reduced accordingly.

If the paint used in the method of the present invention has a chlorine content within the above range, the purpose can be fully achieved, but in order to further improve the performance and stability of the paint, In addition to the above-mentioned chlorine component as a trace component, the fluorine content is further reduced to 3 ppm or less;
Preferably, the sulfur content is 5 ppm or less.

In addition to these, the content of so-called low melting point metals such as sodium, bismuth, arsenic, and thimonium is also kept as low as possible.
It is desirable to keep each content to 5 ppm or less at maximum.

[Function] When carrying out the method of the present invention, the peelable paint described above is applied onto the surface of an already contaminated base material, and the radioactive substance is adsorbed and occluded in the paint, and then the paint film is removed. In some cases, it is used for the purpose of decontamination, in which the surface of the base material is not contaminated, and in other cases, it is used for the purpose of anti-staining, in which paint is applied in advance on the surface of a substrate that is not yet contaminated, and after the surface of the substrate is contaminated, the paint film is removed. Both are possible.

The coating amount is in the range of 50 to 500 g/m2 in terms of resin solid content, more preferably 100 to 300 H/+o2,
The application method is not limited to this, and methods such as brush application, roll application, and spraying can be used.

The areas to be coated include everything that is placed in an environment that is susceptible to radioactive contamination, such as walls, floors, and ceilings in nuclear power plants, as well as tools and equipment used there.
It can be an effective means of decontamination and anti-contamination for jigs, materials, etc.

[Effect 1] As mentioned above, the method of the present invention is characterized in that it uses a paint that has excellent peeling performance that cannot be obtained with conventionally known peelable paints, especially for the purpose of treating radioactive substances. It also makes it easier to use in special environments such as nuclear power plants and minimizes the amount of radiation exposure from working tools.
In addition to being highly functional, it also has superior properties, ie, coatability, drying properties, etc., compared to conventional peelable paints.

[Example 1] Hereinafter, the present invention will be explained in more detail by giving examples. still
: In the examples, "parts" and "%" are based on weight unless otherwise specified.

The abbreviations used below have the following meanings.

MMA: Methyl methacrylate B A Butyl diacrylate 2-EHA 2-ethylhexyl diacrylate 8 N: 7
Acrylic acid EVA: Ethylene-vinyl acetate copolymer emulsion Example 1 In a glass polymerization vessel with an internal volume of 500+++ equipped with a reflux condenser, a dropping funnel, a thermometer, and a stirrer. 95 parts of deionized water, 30 parts of ethanol, 9 parts of Gohnanol GM-14 (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., polyvinyl alcohol), nonionic surfactant (polyoxyethylene nonylphenyl ether type, 1-ILB170) Prepare 3 parts, 200
The mixture was dissolved by stirring at rp+*, and the internal temperature was raised to 70°C. Next, a mixed solution of 65 parts of AN and 35 parts of 2-EHA was added, thoroughly emulsified, and 7.5 parts of a 10% aqueous hydrogen peroxide solution was added.
and 10 parts of a 10% solution of L-ascorbic acid.
Polymerization was carried out for 3 hours.

After the reaction was completed, the mixture was aged for 2 hours while maintaining the internal temperature at 80°C.
Acrylic resin emulsion was obtained.

The thus obtained emulsion was adjusted to a solid content concentration of Wo% and used as a peelable paint. The content of chlorine, phosphorus, fluorine, and sulfur (hereinafter referred to as C1, P
, F, S) was measured, Cl: 0.5
ppm (units are abbreviated below), P: 0.6, F: 0
．． 5, S: 0.5.

Each performance test was conducted in the following manner.

(A) Peel strength test/laboratory evaluation Two types of base materials (sus-304, !9P
eO-8p (trade name manufactured by Nippon Test Panel Co., Ltd.) was coated with the above paint at a coating amount of 100 g/l112 (solid content IK), and after drying, the tensile strength was 10 using a Shimadzu autograph.
00mm/win, 180° peeling, 20℃, 65%RH
The peel strength was measured under the following conditions.

・Contaminated metal wall (sus-304) inside a nuclear power plant
The above paint was applied at a coating amount of 100g/m2 (solid content equivalent) on the surface of the surface of the paint, and after drying, it was peeled off by hand, and the ease of peeling at this time was evaluated using ○, Δ, and ×. The average time required to peel off the coating was measured.

(B) Decontamination test - During the above (A) Peel strength test inside the nuclear power plant evaluation, the radioactivity intensity of the wall surface before paint was applied and the radioactivity of the wall surface when the paint film was peeled off after it was applied and dried. The strength was measured.

At the same time, the coatability (occurrence of sagging) during coating of the paint was visually measured and evaluated.

(C) Resistance test: Apply 100g of the above peelable paint to unused equipment in advance.
/a12 (solid content equivalent), and after drying, it was brought into a nuclear power plant and used for 50 days, and the coating film was removed.

The degree of resistance against radioactive substances at this time was evaluated by measuring the radioactivity intensity before and after the removal of the coating film.

The above results are summarized in Tables 1 and 2.

Examples 2 to 8 In place of the peelable paint used in Example 1, one having the following composition was used. However, unless otherwise specified, the emulsifiers and other chemicals used in resin production were the same as in Example 1, and the reaction conditions were also the same. As shown in Table 1, the amounts of C1, P, F, and S in the resulting paint were all below the amounts specified in the present application.

- Paint resin composition of Example 2 AN/MMA/2-EHA=20/3015
0 (wt%, same hereinafter) Paint resin composition of Example 3 AN/BA=60/40 Nonionic surfactant polyoxyethylene lauryl
1 Ether (HL B 16.O) 3 parts Coating resin composition of Example 4 MMA/2-EHA=70/30 blending agent
Polyacrylic acid 0.7 parts/Paint resin composition of Example 5 MMA/2-EHA/M
A n =69.8/3010.2 Compounding agent 0.5 part of EDTA (ethylenedi7minetetra7cetate) Coating resin composition of Example 6 MMA/AN/BA/AA=30/20/4
9,5/, 0.5 Compounding agent Oxalic acid 0.5 part / Paint resin composition of Example 7 EVA (ethylene content 20%) Compounding agent
Citric acid 0.5 parts / Paint resin composition of Example 8 MMA/BA = 60/40 pH adjuster Sodium hydrogen phosphate hydrate (Na, HP
The above-mentioned performance tests (A) to (C) were conducted using 0.01 part or more of the peelable paint obtained in the above manner, and the performance was evaluated.

The approximate compositions of the strippable paints of Examples 1 to 8, the added compounding agents, and the content of trace components in each paint are shown in Table 1, and the results of the performance tests (A) to (C) are shown in Table 2. They are summarized in .

Control Example 1 Tap water (CI) was used instead of deionized water in Example 1.
A coating material was produced according to the same procedure except that emulsion polymerization was carried out using about 20 ppm), and its performance was evaluated.

Control Example 2 A paint was produced in the same manner as in Example 1, except that water with a different CI content (about 10 ppm CI) was used for emulsion polymerization instead of deionized water, and its performance was evaluated6. Control Example 3 Groundwater (CI) was used instead of deionized water in Example 1.
A coating material was produced in accordance with the same procedure except that emulsion polymerization was carried out using about 40 ppm of F and about 20 ppm of F, and its performance was evaluated.

Control Example 4 In Example 1, ground water (approximately 40 ppm CI, approximately 20 ppm F) was used instead of deionized water, and 10
% hydrogen peroxide solution 7.5 parts and 10% Ronlit aqueous solution 3 parts
．． A peelable paint was obtained in the same manner as in the same example except that 5 parts of the mixed solution and 0.35 parts of sodium hydrogen phosphate hydrate (Na2HPO4.12H20) were used as the pH adjuster, and the performance was evaluated.

Comparative Example 5 A peelable paint was obtained in the same manner as in Example 1 except that 0.02 part of sodium hydrogen phosphate hydrate was further used as a pH adjuster, and its performance was evaluated.

The approximate compositions of the strippable paints of Control Examples 1 to 5 and the content of trace components in each paint are summarized in Table 1, and the results of the performance tests (A) to (C) are summarized in Table 2.

As can be understood from the above results, the content of chlorine and phosphorus in the paint has a very large effect on the peeling performance, and the content of trace components other than chlorine and phosphorus, such as fluorine and sulfur, should be kept as low as possible. It is understood that good results are brought about in the peelability and other properties of the peelable paint.

In other words, in the control examples in Table 2, the peeling performance of the paint film was not good, so in all cases, the paint film could not be completely removed in one peeling operation, and therefore all the paint films could not be completely removed. This shows that the time required for removing the slag is much longer than in the example. Further, the peeling removal rate of the coating film that can be removed by one peeling operation under the same conditions as in the example is about 45% (unit area ratio). Note that the radioactivity intensity was measured under these conditions (the coating film that was not removed by one peeling was left as it was).

This method of measuring trace component content CI, P, S: 0.2 g of paint is placed on an aluminum plate with a diameter of 3 mm.
It was coated so as to have a diameter of 0φ, was accurately weighed, dried with an infrared lamp, and quantified using fluorescent X-ray analysis.

F: The paint was decomposed using an oxygen pump method, and the absorbed liquid was quantified using an ion-selective electrode method.

Table 1 (Paint composition and @quantity component content) (
Margin below)

Claims (1)

[Scope of Claims] 1. After applying a peelable paint to a base material contaminated with radioactive substances and drying, the coating film is peeled off, or the base material that may be contaminated with radioactive substances is In a radioactive substance treatment method in which a strippable paint is applied to a material in advance and the paint film is peeled off after contamination, the peelable paint is of an emulsion type and the chlorine content (atomic weight (converted, the same applies hereafter) is 3 ppm or less, phosphorus content is 5 ppm
A method for treating radioactive substances, characterized by using the following paint. 2. The chlorine content in the peelable paint is 3 ppm or less, the phosphorus content is 5 ppm or less, and the fluorine content is 3 ppm or less.
The processing method according to claim 1, wherein the treatment method is less than or equal to m. 3. The chlorine content in the peelable paint is 3 ppm or less, the phosphorus content is 5 ppm or less, and the fluorine content is 3 ppm or less
The treatment method according to claim 1, wherein the sulfur content is 5 ppm or less. 4. The treatment method according to claim 1, wherein the peelable paint is a paint whose main ingredient is an acrylic resin emulsion. 5. The acrylic resin emulsion contains at least one selected from (meth)acrylic esters having an alkyl group having 1 to 3 carbon atoms or acrylonitrile and (meth)acrylic esters having an alkyl group having 4 to 10 carbon atoms. The treatment method according to claim 4, which is a copolymer of. 6. Claim 4, wherein the acrylic resin emulsion is a copolymer of at least one selected from methyl methacrylate and acrylonitrile and at least one selected from butyl acrylate and 2-ethylhexyl acrylate.
or the treatment method described in Section 5. 7. The treatment method according to claims 1 to 4, wherein the peelable paint contains at least one of an organic acid compound, a polymeric acid compound, and a chelating agent. 8. The treatment method according to claim 7, wherein the organic acid compound is at least one selected from formic acid, acetic acid, butyric acid, and L-ascorbic acid. 9. The treatment method according to claim 7, wherein the polymeric acid is at least one polymer or copolymer selected from (meth)acrylic acid, maleic acid, fumaric acid, and itaconic acid. 10. The treatment method according to claim 7, wherein the chelating agent is at least one selected from ethylenediaminetetraacetate or its neutralized product, oxalic acid, citric acid, and pyrophosphoric acid.