JP7678449B2 - Cesium adsorbent and method for treating cesium-containing contaminated water - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (Part 1) Posting date: March 8, 2021, Posting address: https://confit.atlas.jp/guide/event-img/mmij2021a/3K0408-14-06/public/pdf?type=in (Part 2) Held on March 10, 2021, 2021 Spring Conference of the Resource and Materials Society of Japan (Held online) (Part 3) Held on September 3, 2020, 9th (online) Research Presentation of the Society for Environmental Radioactivity Decontamination, Held by the Society for Environmental Radioactivity, Its Decontamination, Intermediate Storage and Environmental Restoration (Part 4) Posting date: June 4, 2021, Posting address: https://store-confit.atlas.jp/guide/event-img/mmij2021a/3K0408-14-06/public/pdf?type=in jp/kenkyu/kenkyu26/static/20210615095129541_ja.pdf (Part 5) Date: June 24, 2021, 26th Research Meeting on Groundwater and Soil Contamination and Its Prevention Measures, Saitama Conference (Held online) held by the Soil Environment Center, a general incorporated association.

The present invention relates to a method for removing cesium from contaminated liquid discharged from nuclear facilities, etc., and to an adsorbent material used therefor.

In contaminated liquids such as low to high level radioactive waste liquid generated at nuclear power plants and nuclear fuel reprocessing facilities, there is a strong demand for the removal of radioactive cesium Cs137, as it has a long half-life and is the main component of radioactive materials. There is also a demand for the removal of radioactive cesium from contaminated liquids such as seawater and freshwater that are contaminated with radioactive materials.

Removing cesium from radioactive liquid waste makes it easier to treat the remaining liquid waste. If most of the radioactive material could be separated as a small amount of solids from a large volume of solution and the remaining liquid waste could be treated as low-level waste, significant cost savings could be expected. If the concentration of residual radioactive material in the low-level liquid waste were low enough, it would be possible to discharge it directly. Even if this is not possible, low-level waste is much less dangerous and its treatment is much easier than that of high-level liquid waste.

If the cesium separated from the radioactively contaminated liquid is in solid form, it has the advantage that concentration procedures are simplified or unnecessary, and it can be easily solidified and stabilized, or stored in a container, etc.

Special Publication No. 2000-512759 JP 2001-133594 A Japanese Patent Application Publication No. 5-254828 Patent No. 6572320

Methods for removing cesium from contaminated liquids include precipitation and ion exchange. Another method is to use iron cyano complexes, such as ferrocyanide, to remove cesium using insoluble ferrocyanide as an adsorbent.

Patent document 1 discloses a method for removing cesium from contaminated water using ferricyanide.

Patent Document 2 discloses that radionuclides are adsorbed and removed by an adsorbent capable of selectively adsorbing cations such as cesium contained in the cooling water upstream of a desalination ion exchange resin tower installed in the purification line of the reactor cooling water, and discloses that titanate, titanium oxide hydroxide, or ferrocyanide is used as the adsorbent.

Patent Document 3 discloses a method for recovering cesium in which an adsorbent made of insoluble ferrocyanide is brought into contact with a cesium-containing aqueous solution in an adsorption step to adsorb cesium, and in a desorption step, the insoluble ferrocyanide that has adsorbed cesium is brought into contact with an oxidizing desorbent solution to convert it to insoluble ferricyanide, thereby desorbing and recovering cesium, and the resulting insoluble ferricyanide is brought into contact with a regenerant solution to convert it to insoluble ferrocyanide.

Patent Document 4 discloses a cesium recovery material that contains ferrocyanide or ferricyanide supported by a binder in a nonwoven fabric packaging material, and highly water-absorbent polymer powder. Here, it is expected that the ferrocyanide or ferricyanide will adsorb cesium, and the highly water-absorbent polymer powder will absorb water and swell.

The present invention provides a radioactive cesium removal material that can easily adsorb cesium in contaminated liquid with high selectivity using a readily available adsorbent, and a removal method using the same.

After extensive research into materials capable of adsorbing cesium in aqueous solutions, the inventors discovered that a certain type of water-absorbent polymer selectively adsorbs cesium in aqueous solutions, and thus completed the present invention.

That is, the present invention is a cesium adsorbent for cesium-containing contaminated water, characterized by being made of a polyacrylate-based water-absorbing polymer.

As the water-absorbent polymer, an alkali metal salt of polyacrylic acid is suitable, and a sodium or potassium salt-based polymer is particularly suitable.

The present invention also provides a method for treating cesium-containing contaminated water, which comprises reducing the electrolyte concentration in the cesium-containing contaminated water to 10 mmol/L or less, adding the above-mentioned water-absorbing polymer to the cesium-containing contaminated water and allowing it to absorb water to form a water-absorbent polymer, evaporating or distilling the water from the water-absorbent polymer to dry it or concentrate the cesium, burning or compressing the dried or concentrated water-absorbent polymer to further reduce its volume, and storing the reduced-volume combustion ash or compressed material in a container.

According to the present invention, water-absorbent polymers can be used as cesium adsorbents, making them easy to obtain. In addition, water-absorbent polymers have the advantage that they can absorb large amounts of contaminated water, and therefore continue to adsorb cesium even after absorbing the water. In addition, the removal material used to remove cesium needs to be safely transported or stored after being separated from the aqueous solution, but the removal material of the present invention can be easily dried, burned, or compressed, making it easy to reduce its weight and suitable for long-term storage in a container.

This is a Langmuir plot.

The present invention will now be described.
The cesium-containing aqueous solution to be treated by the present invention may be wastewater discharged from nuclear facilities such as nuclear reactors, or may be seawater contaminated with radioactive cesium, and is not limited as long as it contains radioactive cesium. It may also be contaminated liquid generated by washing soil, rubble, structures, etc. contaminated with radioactivity, or leachate from soil. The cesium concentration may be low to high, but the present invention is suitable for contaminated liquids with a cesium concentration of about 0.0001 to 100 mg/L. The contaminated liquid may contain components other than cesium, and the present invention is particularly effective for contaminated liquids containing the same amount of NaCl as seawater as an adsorption inhibitor that may compete during adsorption.

The cesium adsorbent of the present invention is made of a polyacrylate-based water-absorbent polymer.
Suitable water-absorbent polymers include sodium polyacrylates and potassium polyacrylates. In addition, the water-absorbent polymers preferably have a high maximum water absorption rate (amount of water absorbed in g/amount of water-absorbent polymer in g), and are preferably about 20 to 1000.

Polyacrylate-based water-absorbing polymers are made of a network of alkali metal salts of polyacrylic acid, which ionize into -COO (-) and alkali metal positive ions (+) when they absorb water, and can capture and hold a large amount of water molecules in the gaps created by the repulsion of the negative ions. In this invention, the cesium ions in the contaminated water are not physically captured along with the absorbed water molecules, but are adsorbed by ion exchange due to the difference in ionization tendency (sodium < potassium < cesium) between the sodium ions or potassium ions present in the alkali metal positive ions (+) that make up the structure of the polyacrylate-based water-absorbing polymer and the cesium ions contained in the cesium-containing water. Therefore, it is possible to adsorb cesium even when the water-absorbing performance of the water-absorbing resin is impaired by contaminated water containing a large amount of sodium ions.

There is no restriction on the method for making the cesium adsorbent absorb cesium-containing contaminated water, but the impregnation method is suitable. The amount of water content varies depending on the water absorption rate of the water-absorbing polymer, but it is good to make it 100% or less of the maximum water absorption rate, and it is preferable to be in the range of 50 to 99%. From another point of view, it is desirable to make the water absorbent polymer absorb 10 to 1000 times by weight, preferably 20 to 500 times by weight of water.
The cesium adsorbent may contain components other than the water-absorbent polymer if necessary, but does not contain cesium removers such as ferrocyanides or ferricyanides.

When adding the water-absorbing polymer, it is advisable to adjust the electrolyte concentration in the cesium-containing contaminated water to 10 mmol/L or less, preferably 5 mmol/L or less, before adding the water-absorbing polymer. If the electrolyte concentration is high, the distribution rate decreases. Here, if the electrolyte is a salt such as NaCl, 1 mol of salt is calculated as 1 mol of electrolyte.

After absorbing water to form a water-absorbent polymer, the water is evaporated or distilled off to dry or concentrate the cesium.

The water-absorbent polymer that has absorbed the cesium-containing contaminated water is then dried by evaporation or drying. If the drying process is carried out at 80°C or less, preferably 60°C or less, the polymer can be added to the cesium-containing contaminated water again and absorb water. By repeating drying and water absorption in this way, the amount of cesium adsorbed by the water-absorbent polymer per unit weight increases.
Drying may also be done by sun drying or air drying, and such drying methods make it possible to minimize heating. Note that water-absorbent polymers have the property of absorbing moisture from the surroundings when the temperature drops and as the drying progresses, so it is advisable to keep the temperature at 30°C or higher in the latter half of the drying process, although this depends on the type of water-absorbent polymer and the humidity of the atmosphere.

The drying process can involve either evaporating the water in the water-containing absorbent polymer or distilling it off. If evaporation is used, the evaporated water should be discharged as exhaust gas, and if distillation is used, it should be discharged as wastewater.

When drying by evaporation, one method is to evaporate slowly while exposing it to the atmosphere at low temperature, but when treating a large amount, it is better to evaporate while flowing a gas such as air. When evaporating while flowing gas in this way, the water in the exhaust gas is sufficiently diluted, and even if there is a trace amount of radioactive gas, it is easy to satisfy the regulatory value. When performing drying while flowing gas, it is better to put the water-absorbent polymer in a perforated container or bag.
As the drying progresses, the cesium concentration of the water in the water-containing water-absorbing polymer increases, but this does not migrate into the gas, so it can be increased sufficiently. If the drying temperature is made higher than the ambient temperature, it is possible to effectively use the relatively low-temperature waste heat generated during the operation of nuclear power plants or accident treatment. Reducing the humidity of the circulating gas is also effective in promoting drying, and in this case too, the waste heat or exhaust gas from nuclear power plants can be used. The drying process time should be at least one day, depending on the surface area of the water-containing water-absorbing polymer, and is preferably around 5 to 15 days.
Furthermore, the natural drying method using sunlight has the advantage that the heating and cooling means can be greatly simplified. In this case, drying can be achieved simply by placing the water-containing water-absorbent polymer in an open-topped container or bag indoors or outdoors where there is a window that lets in sunlight, and storing it in sunlight or indoors where the temperature is raised by the greenhouse effect. This drying method requires a long processing time and certain humidity and exhaust gas control, but it can significantly reduce the drying processing costs compared to forced drying. If necessary, heated air can be used for indoor ventilation.

When the drying treatment is carried out by distillation, a device such as a distillation device or a device such as that used for the evaporation may be used. In either case, at least a part of the generated or discharged gas is cooled and distilled to become wastewater.
Drying should be performed until 80% or more, preferably 90% or more, and more preferably 95% or more of the water contained in the initial water-absorbent polymer that absorbed the cesium-containing contaminated water is removed. On the other hand, if the water removal rate is low, the volume of the remaining water-absorbent polymer increases, and the amount of storage increases. In addition, if it is burned, the combustion efficiency decreases.

After drying, concentrated water-absorbent polymers containing concentrated cesium in them will far exceed the regulated value, so it is best to store them in containers until the volume falls below the regulated value. Advantageously, the dried or concentrated water-absorbent polymer can be burned or compressed to further reduce its volume, and the reduced volume combustion ash or compressed material can be stored in containers. Since the volume has been significantly reduced by drying or burning, and the polymer is solid overall, it can be easily stored.

As the water-absorbing polymer, a cross-linked acrylic acid polymer with partial sodium salt is typically used. Its characteristics include the ability to hold water up to 100 times its weight, or up to 1000 times its weight depending on the material, and the fact that its water-absorbing performance decreases when electrolytes are present, but remains almost unchanged when non-electrolytes are present. Because of these characteristics, by using a highly water-absorbing polymer, treated water can be stored in a tank in a solid form, allowing safe storage with a lower risk of leakage compared to liquid form. It also requires no energy and allows natural evaporation while controlling the cesium concentration, and once cesium is held in the polymer, it remains relatively stable even if new water is added, as long as the amount does not exceed the water-absorbing capacity.

When radioactive cesium with a long half-life coexists in contaminated water containing volatile radionuclides such as tritium, it is expected that the dangers of water leakage and ocean release can be prevented if the cesium can be effectively removed.

Example 1
As the radioactive cesium, ^137Cs was used, which is easy to quantify by gamma ray measurement. In order to quantitatively evaluate the water absorption rate and ^137Cs adsorption performance of the water-absorbent polymer, it is necessary to separate the water-containing polymer from the liquid, so we followed the Japanese Industrial Standard JIS K 7223-1996 "Test method for water absorption of superabsorbent resins".
A commercially available water-absorbent polymer (BC-283FHA) of partially cross-linked acrylic acid polymer (manufactured by TAISAP) was used as the water-absorbent polymer. 0.2 g of the polymer was placed in a non-woven fabric tea bag measuring 6.5 cm x 9.5 cm and 3 cm in maximum width at the bottom, and then placed in a polyethylene bottle with an internal volume of 250 mL to facilitate easy separation.
Into this polyethylene bottle, 200 mL of ¹³⁷ Cs radioactive solution (Eckert & Ziegler, Normal Solution, 100 kBq/mL, cesium chloride concentration 0.05 mg/mL) containing non-radioactive cesium iodide (High Purity Chemical Laboratory, purity 99% or more) was added as an electrolyte, making the cesium concentration 2.5 mmol/L (specific gravity 1.0) per bottle, the ¹³⁷ Cs radioactivity concentration 40 Bq/mL, and the specific radioactivity 16 MBq/moL.

The bag containing the water-absorbing polymer was immersed in the bottle. To allow ^{the 137} Cs to reach equilibrium in the bottle, the bottle was closed and left to stand at room temperature for 7 days. After that, the entire liquid outside the nonwoven bag was removed with a pipette, and the mass was measured to determine the water absorption rate (%) of the water-absorbing polymer that had absorbed water.
The radioactivity concentration (Bq/kg) of ¹³⁷ Cs in the liquid was quantified by measuring 10 mL of the removed liquid with a germanium semiconductor detector for 20 minutes, and the amount of ¹³⁷ Cs remaining in the aqueous solution was calculated. The ¹³⁷ Cs concentration A (Bq/kg) in the water-absorbent polymer after water absorption was calculated using the following formula.
A = B/W
B: Radioactivity of ¹³⁷ Cs added to the bottle (Bq) - Concentration of ¹³⁷ Cs in the aqueous solution (Bq/kg) x Mass of the liquid (kg)
W: Mass of water added to the bottle (kg) - Mass of liquid (kg)
The adsorption rate of ¹³⁷ Cs adsorbed to the water-absorbent polymer was calculated from the above data. Furthermore, the distribution ratio D was calculated by dividing the ¹³⁷ Cs concentration (Bq/kg) in the water-absorbent polymer after water absorption by the ¹³⁷ Cs concentration (Bq/kg) in the remaining aqueous solution.

Example 2
The same procedure as in Example 1 was carried out, except that the cesium concentration of ^{the 137} Cs radioactive solution placed in the bottle was 5.0 mmol/L (specific gravity 1.0), the radioactivity concentration of ¹³⁷ Cs was 40 Bq/mL, and the specific activity was 8.0 MBq/moL.

Reference Example The same procedures as in Example 1 were carried out using the same chemicals, containers, and equipment as in Example 1, except that the cesium concentration was 10 mmol/L (specific gravity 1.00), the ¹³⁷ Cs radioactivity concentration was 40 Bq/mL, and the specific radioactivity was 4.0 MBq/moL.

The results are summarized in Table 1. Here, the water absorption rate is the water absorption rate of the water-absorbing polymer, the residual ¹³⁷ Cs amount is the amount of ¹³⁷ Cs remaining in the aqueous solution, the amount of ¹³⁷ Cs in the polymer is the amount of ¹³⁷ Cs in the water-absorbing polymer, and the ¹³⁷ Cs adsorption rate is the amount of ¹³⁷ Cs adsorption in the water-absorbing polymer.

In Examples 1 and 2, the distribution coefficient is 1 or more, which indicates that ^137Cs in the solution is adsorbed from the solution to the water-absorbent polymer. In addition, it is understood from the reference example that the adsorption performance is improved by setting the electrolyte concentration to less than 10 mmol/L.

In addition, when it is assumed that the adsorbate molecules are adsorbed to the adsorption site in a monolayer, the saturated adsorption capacity from the solution to the substance can be calculated from the Langmuir adsorption isotherm by changing the concentration of the solution to obtain the change in the amount of adsorption. Here, C is the concentration of the solution (the combined concentration of non-radioactive cesium and radioactive cesium- ^137Cs contained in the electrolyte), and q is the amount of adsorption per unit. The results of Examples 1 and 2 and the Reference Example are shown in Table 2.

Here, C is the solution concentration, q is the amount of adsorption per unit, K is the adsorption equilibrium constant, and _qm is the saturated adsorption capacity. The amount of adsorption per unit, q, is calculated by dividing (saturated adsorption capacity, _qm, x adsorption equilibrium constant, K, x solution concentration, C) by (1 + adsorption equilibrium constant, K, x solution concentration, C). Rearranging this equation, C/q becomes C/ _qm + 1/( _qm x K).
In other words, if a Langmuir plot is taken with C/q on the vertical axis and C on the horizontal axis and approximated as a straight line, the reciprocal of the slope will be the saturated adsorption capacity.
The Langmuir plot is shown in FIG.

From FIG. 1, the approximate straight line is y = 0.3163x + 0.4985 ( ^r2 = 1), and from the reciprocal of the slope, the cesium saturated adsorption capacity of 1 g of the water absorbent resin of this example is 3.16 (mg/g), which is equivalent to 10.1 GBq/g when converted to ¹³⁷ Cs.

Claims (4)

A cesium adsorbent for use in cesium-containing contaminated water where the cesium concentration is less than 10 mmol/L, characterized in that the cesium adsorbent is made of an absorbent polymer based on an alkali metal (excluding lithium) salt of polyacrylic acid. 2. The cesium adsorbent according to claim 1 , wherein the water-absorbent polymer is a sodium salt-based polymer of polyacrylic acid. 2. The cesium adsorbent according to claim 1 , wherein the water-absorbent polymer is a polymer based on a potassium salt of polyacrylic acid. A method for treating cesium-containing contaminated water, comprising the steps of: reducing the cesium concentration in the cesium-containing contaminated water to less than 10 mmol/L; adding the water-absorbing polymer according to claim 1 to absorb water to form a water-containing water-absorbing polymer; evaporating or distilling the water from the water-containing water-absorbing polymer to dry it or concentrate the cesium ; burning or compressing the dried water-absorbing polymer or the water-containing water-absorbing polymer with concentrated cesium to further reduce its volume; and storing the reduced-volume combustion ash or compressed material in a container.

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