JP2014006111A - Decontamination method for improving classification and cleaning effect in soil contaminated with radioactive cesium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for improving an effect in classification and cleaning to be performed in order to reduce the volume of waste soil to be generated when soil contaminated with radioactive cesium is decontaminated by removing surface soil.SOLUTION: When polyions exist in performing classification and cleaning with respect to soil contaminated with radioactive cesium, fine particles attached to rough particles are easily detached by an aggregation action with respect to fine particles held by polyions and also the reattachment of fine particles is suppressed. Thus, a classification and cleaning effect is improved.

Description

  The present invention relates to a decontamination method for improving the classification and cleaning effect of radioactive cesium-contaminated soil. In particular, in decontamination of radioactive cesium-contaminated soil, the present invention relates to a method for improving the classification / cleaning effect by utilizing the aggregating action of polyion on fine particles in soil.

  The soil was extensively contaminated by radioactive material scattered by the accident at the TEPCO Fukushima Daiichi NPS on March 11, 2011. As of May 2012, when more than one year has passed since the accident occurred, there is almost no radioactive iodine with a short half-life, and a long half-life of radioactive cesium (Cs-137: half-life 30 years, Cs-134: half) 2 years) is the main pollutant.

  Radioactive cesium tends to stay in the surface soil (surface soil) while adsorbed and immobilized on fine particles such as clay. Therefore, for the decontamination of contaminated soil, it is most effective to peel only the topsoil while many radioactive cesiums are present in the topsoil. The detached topsoil must be properly shielded and stored for a long period of time, approximately 10 times the half-life of radioactive cesium, but the amount of waste soil generated is enormous. In Japan, where the land area is small, there is a limit to securing a disposal site for shielding and storing waste soil, and volume reduction of waste soil is essential. As methods for reducing the volume of waste soil, methods such as an acid extraction method (see Non-Patent Document 1) and a soil combustion method (see Non-Patent Document 2) are also known. There is a problem in terms of size and safety, and the soil combustion method has a large problem in terms of cost.

  On the other hand, as a purification method for contaminated soil with non-radioactive substances, harmful components (such as heavy metal ions) and fine particles such as clay that concentrate them are separated and classified from the entire soil by classification (sieving) and washing, A soil purification technique called soil classification / cleaning method for reducing the volume is known (see Non-Patent Document 3 and Non-Patent Document 4). However, in the normal classification / cleaning method, there is a problem that the classification / cleaning does not proceed efficiently because fine particles adhere to the surface of large particles.

  In order to effectively and efficiently peel off the contaminated soil surface at a necessary minimum depth, a method of solidifying the soil surface layer by spraying a molecular polymer or self-hardening cement is known. Although the molecular polymer can solidify the soil surface layer in a short time, it takes a long time to classify and wash the solidified state with water. Since self-hardening cement cannot be softened with water once it has solidified, it cannot be classified and washed with water.

  In the field of water treatment, polymer flocculants that aggregate and flocculate and separate suspended components in wastewater are widely used, and polycation or polyanionic polymer flocculants are known ( Patent Document 1 and Patent Document 2). On the other hand, in the classification / cleaning technology, although the contrivance to improve the cleaning effect by using a surfactant or a chemical that binds to a contaminant (substance to be removed) has been empirically performed, the classification / cleaning effect is improved. For the purpose of improving, a drug having an action of aggregating fine particles has never been used.

  In the present invention, the flocculant works to improve the classification / cleaning effect by preventing the reattachment of fine particles attached to the surface of the coarse particles. The effect of the agent has never been noticed. In order to improve the classification / cleaning effect, devices and machines have been devised exclusively based on physical methods such as surface polishing. For example, there is a method of improving the polishing ability by mechanically generating friction between particles (see Non-Patent Document 4 and Non-Patent Document 5).

JP-A-9-164389 Japanese Patent Laid-Open No. 7-224104

“Succeeded in extracting cesium in soil with low-concentration acid,” AIST press release, August 31, 2011 “A new decontamination test in Iitate, Fukushima: Incineration of high-concentration soil,” Kyodo News, October 26, 2011 “Regional Measures Improvement Guidebook”, Ministry of the Environment, Soil Environment Division, Water and Air Environment Bureau, 43-46 (2011). Http://www.env.go.jp/water/dojo/gb_me/ Tatsuya Futami, “An example of practical technology for remediation of heavy metal-contaminated soil by classification, specific gravity selection, surface polishing”, Geographical Journal, 116 (6), 932-940 (2007) Outline of FY2011 “Decontamination Technology Demonstration Test Project”, Japan Atomic Energy Agency, Fukushima Technology Division, 7th Atomic Energy Commission Document 1-2, p10, 12, February 21, 2012

  It is an object of the present invention to provide a method for reducing the volume of radioactive waste soil produced by decontamination treatment of soil contaminated with radioactive cesium.

  As a result of extensive research, the inventors of the present application have found that polyion, which is expected to be used as a soil treatment agent, has an effective action in classification and washing of waste soil, and greatly improves its efficiency. As a result, the present invention has been completed.

  For the decontamination of radioactive cesium-contaminated soil, stripping off the surface soil (surface soil) where most of the radioactive cesium exists is the most effective. Since most of the radioactive cesium is strongly adsorbed and immobilized on the clay-like fine particle components, only the fine particles are classified and collected while washing the coarse particles, and only the fine particle components containing the radioactive cesium are collected in the radioactive waste soil. Process as. Since the washed coarse particle component does not contain radioactive cesium, it can be returned to its original location or reused as cement aggregate.

  According to the present invention, after spraying the polyion aqueous solution on the soil, peeling off the topsoil, and then classifying and washing the topsoil removed using water, the fine particles adhering to the surface of the large particles are washed out, The agglomeration effect on the fine particles of the polyion prevents the washed out fine particles from aggregating and reattaching to the large particles. In this way, fine particles that are attached to large particles and difficult to separate by the conventional method can be excluded from the large particles, and the classification and cleaning effect can be improved.

  Specifically, a polyion aqueous solution is sprayed on the soil contaminated with radioactive cesium, and then the surface layer of the soil is peeled and removed, and then the generated radioactive waste soil is classified and washed, and the classification size is less than 0.075 mm. Provided is a method for decontamination of radioactive cesium-contaminated soil using the particle fraction as radioactive waste soil. As described in detail in Examples, according to the method of the present invention, for example, about 85% of radioactive cesium can be concentrated to a fine particle fraction of less than 0.075 mm by using a 0.075 mm mesh sieve. The particle fraction of 0.075 mm or more, which does not contain much radioactive cesium, does not constitute radioactive waste soil and can achieve a significant volume reduction. Fraction that is not treated as radioactive waste soil can be backfilled or reused as aggregate. In addition, as a sieve used for classification, a sieve having a standard of 0.075 mm mesh is preferable, but a sieve having a finer mesh (for example, 0.05 mm mesh) can also be used.

  There are two types of polyions: polycations and polyanions. In the above method, polycations include cationized cellulose, cationized starch, polymers having amino groups, or polymers of quaternary ammonium salts. Available. As the polyanion, carboxymethyl cellulose, carboxymethyl amylose, lignin sulfonic acid and its salt, polyacrylic acid and its salt, polysulfonic acid and its salt and the like can be used. Moreover, a polycation and a polyanion can be mixed and used. In this case, it is preferable to add 2 to 6 wt% of a salt (sodium chloride, potassium chloride, potassium sulfate, ammonium sulfate, etc.) so as not to form a gel-like polyion complex when sprayed on soil.

  According to the present invention, after the surface layer of soil contaminated with radioactive cesium is peeled off, the effect can be improved in classification and washing performed to reduce the volume.

Preferred embodiment

  In the decontamination method of the present invention, a polyion aqueous solution is sprayed on soil contaminated with radioactive cesium, the surface layer of the soil is peeled and removed, the resulting radioactive waste soil is classified, and the fine particle fraction with a classification size of less than 0.075 mm is obtained. The painting is radioactive waste soil. Classification fractions with a classification size exceeding 0.075 mm are washed, backfilled or reused.

  As an aqueous polyion solution, an aqueous solution containing a polycation selected from cationized cellulose, cationized starch, a polymer having an amino group or a polymer of a quaternary ammonium salt; carboxymethylcellulose, carboxymethylamylose, lignin sulfonic acid and salts thereof, An aqueous solution containing a polyanion selected from polyacrylic acid and a salt thereof, polysulfonic acid and a salt thereof, or an aqueous solution containing a polycation and a polyanion can be preferably used. In particular, it is preferable to use it as a polyion aqueous solution without forming a gel-like polyion complex, since it can be easily applied to contaminated soil and diffused into fine particles in the soil. Formation of a polyion complex can be prevented by containing 1 to 6 wt% sodium chloride, potassium sulfate, or ammonium sulfate as a buffering agent that prevents the polycation and polyanion from binding to the polyion aqueous solution. it can.

FIG. 1 is a graph for comparing the weight ratio of each fraction in Examples and Comparative Examples. FIG. 2 is a graph comparing the distribution ratios of the radioactivity concentrations of the fractions of the examples and comparative examples.

Hereinafter, the present invention will be specifically described by taking the influence of polyion in classification and washing of soil treated with polyion as an example, but the present invention is not limited thereto.
[Example 1]
When classifying and cleaning for the purpose of reducing the volume of detached topsoil, it is necessary to understand the effects of soil treatment agents on classification and cleaning. Therefore, the soil treated with polyion and untreated soil were classified and washed by the same method, and the effects were compared.

Specifically, in the area of 1m square in Iitate, Iitate Village, Fukushima Prefecture, we collected polyion-treated soil (soil that was sprayed with a polyion aqueous solution and left for 2 days) and untreated soil in close proximity to it. Used for washing test. A polyion aqueous solution was prepared using cationized cellulose as the polycation and carboxymethylcellulose as the polyanion. The polyion aqueous solution is an aqueous solution containing 3 wt% of the polycation and the polyanion, so that the polyion complex is not generated (so that it can be dispersed as a normal aqueous solution in which the gel of the polyion complex is not generated). % Sodium chloride. In addition, the polyion aqueous solution was sprayed on the soil in an amount of 5 L / m ² .

  The soil sample subjected to the above polyion treatment was classified and washed using water (tap water). For classification, stainless steel sieves with mesh sizes of 0.075 mm, 0.125 mm, 0.25 mm, 0.5 mm, 2.0 mm, and 4.75 mm are used, and water is carefully used while utilizing mechanical force such as vibration. Washed. Also, untreated soil was classified and washed by the same method, and compared with the soil treated with polyion. Classification size <0.075mm, 0.075-0.125mm, 0.125-0.25mm, 0.25-0.5mm, 0.5-2.0mm, 2.0-4.75mm,> 4 It was fractionated into 7 regions of .75 mm, and dry weight (g) and radioactivity (Bq) were measured for each classification size. From these measurement results, the weight ratio (%) and the radioactivity ratio (%) in each classification size were determined. Radioactivity was measured with a NaI scintillation detector. Table 1 shows the results of classification / washing of untreated soil, and Table 2 shows the results of classification / washing of polyion-treated soil.

  Comparing Table 1 and Table 2, it can be seen that in the polyion-treated soil, the radioactivity ratio in the classification size of less than 0.075 mm is larger than in the untreated soil. This means that the soil subjected to polyion treatment has higher classification and washing efficiency. Since the polycation and polyanion have an aggregating action on fine particles, it is considered that the efficiency of peeling and removing fine particles adhering to the surface of large particles is enhanced. That is, it is considered that only the fine particles can be effectively agglomerated while preventing the fine particles from re-adhering to the large particles, so that the efficiency of classification and cleaning is improved.

  When the decontamination method of the present invention is applied to the field soil of Iitate village Iitate where fine particle components less than 0.075 mm account for about 50%, nearly 90% of the total radioactivity may be collected in a classification size of less than 0.075 mm. all right. If it is judged that the cesium removal rate of nearly 90% is sufficient, the volume of waste soil can be reduced to about half. In addition, if classification to a size smaller than 0.075 mm is possible, there is a possibility that the amount of discarded soil can be further reduced.

  The volume reduction rate of waste soil depends greatly on the proportion of clay in the soil. This is because many fine particles of less than 0.075 mm are clayey. In Iitate village Iitate, soil has a lot of fine particles of clay, so the volume reduction of waste soil should not be high, but it is thought that a large volume reduction can be obtained in soil with little clay.

[Comparative Example 1]
Molecular polymers such as polyvinyl acetate have the advantage of drying and solidifying soil in a shorter time than polyion, and are effective in decontaminating contaminated soil (removing topsoil) in a short period of time. Therefore, the suspension of polyvinyl acetate is also sprayed on 1m square soil adjacent to the polyion-treated soil, dried and peeled off, and then classified and treated in the same manner as shown in Example 1. Washing was performed. However, since the soil treated with polyvinyl acetate takes some time to soften the solidified polyvinyl acetate with water, the treated soil was soaked in water for 6 hours, and then classified and washed. Table 3 shows the results of classification and washing of the field soil of Iitate village treated with polyvinyl acetate.

  Comparing Table 1 and Table 3, the radioactivity ratio in the classification size of less than 0.075 mm is not significantly different between the soil treated with the molecular polymer (polyvinyl acetate) and the untreated soil.

  As examples and comparative examples, classification / washing data in untreated soil, polyion-treated soil, molecular polymer-treated soil was shown, but even if the numerical values shown in Tables 1 to 3 above were directly compared, untreated It is not a fair comparison of the differences between the polyion treatment and the molecular polymer treatment. This is because even if samples collected from the same field are used, the soil is not homogeneous, and the distribution of classification sizes is different for each sample. On the other hand, if the soil is in the vicinity of the same field, the particle composition in each fraction (classification size) can be regarded as almost the same. However, in consideration of the state in which fine particles adhere to coarse particles, practically perfect fractionation is impossible. In fact, some radioactivity has also been measured in the fraction of coarse particles (particles greater than 0.075 mm) after washing. This is because slight fine particles enriched with radioactive cesium remain. Moreover, the amount of fine particles remaining in each fraction varies depending on the treatment method. Therefore, the weight ratio of each fraction was compared between the untreated, polyion treated, and molecular polymer treated. The result is shown in FIG. As described above, the polyion-treated soil has a large radioactivity ratio in the fraction less than 0.075 mm (conversely the radioactivity ratio in the fraction of coarse particles is small), and the amount of residual fine particles compared to other soils. There should be few. If the weight of the fine particles remaining in each fraction is not negligible with respect to the total weight of the fraction, in the case of polyion-treated soil with a small amount of fine particles remaining, coarse particles (0.075 mm or more The pile of particles) should be lower than in other soils. However, as shown in FIG. 1, there is no significant difference between the three in the height of the peak of the coarse particle fraction. Therefore, the weight ratio of the remaining fine particles in the coarse particle fraction was estimated to be sufficiently small relative to the total weight of the fraction.

  From the above, it is possible to apply the concept of radioactivity concentration distribution rate as an index for judging the effect of classification and cleaning. The method for obtaining the distribution ratio of the radioactivity concentration in each fraction and the numerical values of the obtained radioactivity concentration and distribution ratio are shown below.

  First, the radioactivity concentration in each fraction was determined for the untreated, polyion-treated, and molecular polymer treated, and the radioactivity concentration (Bq / g) calculated for each fraction was summarized in Table 4. . The radioactivity concentration is a value defined below.

  The following formula is used to obtain the distribution rate (%) from the radioactivity concentration.

In the formula, D _i is the distribution rate (%) in fraction i, C _i is the radioactivity concentration in fraction i (Bq / g), ΣC _i is the sum of the radioactivity concentrations in all fractions (total radioactivity concentration) ). Further, as described above, the sum of the distribution ratios in all the fractions is 100.

  Table 5 shows the results of comparing the distribution ratio (%) of the radioactivity concentration in each fraction of untreated soil, polyion-treated soil, and molecular polymer-treated soil.

  The data of Table 5 is graphed and shown in FIG. In untreated soil, the distribution rate of radioactivity concentration increases almost uniformly as the classification size increases. On the other hand, in polyion-treated soil, the distribution rate of radioactivity concentration suddenly increases at a classification size of less than 0.075 mm (minimum size fraction), and a low distribution ratio is maintained at a classification size of 0.075 mm or more. . From this, it can be seen that polyion-treated soil has a much higher classification / cleaning effect than untreated soil. Moreover, it turns out that the effect of classification and washing | cleaning is higher in the polyion treatment soil compared with the molecular polymer treatment soil. Although the molecular polymer-treated soil is located between the untreated soil and the polyion-treated soil, the tendency is similar to the polyion-treated soil. Therefore, it is suggested that the molecular polymer (polyvinyl acetate) may have a slight aggregating action on the fine particles.

  In classification and washing, the volume of waste soil is reduced by collecting only the fine particle fraction (minimum size fraction), so when compared in terms of the distribution ratio of radioactivity concentration to fractions less than 0.075 mm, It is 28.0% for untreated soil and 36.3% for molecular polymer treated soil, whereas it is 51.7% for polyion treated soil, showing the largest value. Compared to untreated soil, the distribution ratio of the radioactivity concentration to the fine particle fraction (minimum size fraction) in polyion-treated soil is about 1.9 times that of untreated soil.

  The amount of waste soil generated by decontamination of soil contaminated with radioactive cesium (removal of surface soil) is enormous, and there is concern about the shortage of disposal sites. Therefore, if the volume of waste soil can be reduced, the problem of a shortage of disposal sites can be reduced. The water classification and cleaning method is the most realistic method for reducing the volume of contaminated soil in terms of cost, low environmental impact, and safety. In addition, since most of the radioactive cesium is strongly adsorbed and fixed to fine particles such as clay, there is an advantage that it can be disposed of in a stable state (low risk of environmental spillage).

  In order to perform classification and cleaning effectively, the problem is how to eliminate fine particles adhering to the surface of coarse particles, and the present invention using the aggregating action of polyion on fine particles is the subject of this problem. It is very effective against. Polyion is an inexpensive material that can be procured in large quantities, and is a material that is used in daily life such as in the food and cosmetic fields, so there are no safety issues. In other words, in the classification and washing with water, by adding polyion (or by the presence of polyion added as a soil treatment agent), the merit of greatly improving the efficiency is great. It is extremely useful as a decontamination technique considering volume reduction.

Claims (6)

After spraying polyion aqueous solution to soil contaminated with radioactive cesium,
Peeling off the surface layer of the soil,
The generated radioactive waste soil is classified and washed in the presence of water,
A decontamination method for radioactive cesium-contaminated soil using a fine particle fraction having a classification size of less than 0.075 mm as radioactive waste soil.   After spraying a polyion aqueous solution to the soil contaminated with the radioactive cesium to gel the polyion in the soil, the surface of the soil is peeled off to wash out the fine particles adhering to the surface of the large soil particles, and the radioactive cesium The method for decontamination of radioactive cesium-contaminated soil according to claim 1, wherein fine particles adsorbed and fixed are agglomerated and classified and washed while preventing reattachment of large particles to the surface. The polyion aqueous solution is:
An aqueous solution containing a polycation selected from cationized cellulose, cationized starch, polymers having amino groups or polymers of quaternary ammonium salts;
In an aqueous solution containing a polyanion selected from carboxymethylcellulose, carboxymethylamylose, lignin sulfonic acid and salts thereof, polyacrylic acid and salts thereof, polysulfonic acid and salts thereof, or an aqueous solution containing poly cations and poly anions The decontamination method according to claim 1 or 2.   The decontamination method according to claim 3, wherein the polyion aqueous solution is an aqueous solution containing 1 to 6 wt% of a polycation and / or a polyanion.   The decontamination method according to claim 3 or 4, wherein the polyion aqueous solution does not produce a gel-like polyion complex.   When the polyion aqueous solution contains both a polycation and a polyanion, the polyion aqueous solution further includes a salt selected from sodium chloride, potassium chloride, ammonium chloride, magnesium chloride, sodium sulfate, potassium sulfate, ammonium sulfate, and magnesium sulfate. The decontamination method according to claim 5.