Cesium adsorbent and method for treating cesium-containing contaminated water

Abstract

To provide a removal material for radioactive cesium capable of simply adsorbing cesium in polluted water with high selectivity using a readily available adsorbent, and a removal method using the removal material.SOLUTION: A cesium adsorbent in cesium-containing polluted water comprises a polyacrylate-based water-absorbing polymer. As the water-absorbing polymer, a sodium salt or potassium salt-based polymer of polyacrylic acid is suitable. A method for treating cesium-containing polluted water comprises: setting an electrolyte concentration in cesium-containing polluted water to 10 mmol / L or less; adding the water-absorbing polymer to the polluted water to absorb water; drying the polymer by evaporating or distilling this moisture; and burning or compressing the polymer to reduce its volume and then store it in a container.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for removing cesium from contaminated liquid discharged from nuclear power facilities and the like, and to an adsorbent material used therein. [Background technology]

[0002] 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, which has a long half-life and is the main component of radioactive materials. In addition, there is a demand for the removal of radioactive cesium from contaminated liquids such as seawater and freshwater contaminated with radioactive materials.

[0003] Removing the cesium from radioactive liquid waste makes it easier to treat the remaining liquid. Significant cost savings can be expected if most of the radioactive material can be separated as a small amount of solids from a large volume of solution, and the remaining liquid can be treated as low-level waste. If the concentration of residual radioactive material in the low-level waste is low enough, it may 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 waste.

[0004] If the cesium separated from the radioactively contaminated liquid is in solid form, it not only has the advantage of simplifying or eliminating the need for concentration procedures, but also makes it easier to solidify and stabilize it, or to store it in a container, among other disposal methods. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2000-512759 [Patent Document 2] JP 2001-133594 A [Patent Document 3] Japanese Patent Application Publication No. 5-254828 [Patent Document 4] Patent No. 6572320

[0006] Methods for removing cesium from contaminated liquids include precipitation and ion exchange, as well as the use of iron cyano complexes such as ferrocyanide salts, which use insoluble ferrocyanides as adsorbents to remove cesium.

[0007] US Patent No. 5,399,633 discloses a method for removing cesium from contaminated water using ferricyanide.

[0008] Patent Document 2 discloses that radionuclides are adsorbed and removed by an adsorbent capable of selectively adsorbing cations such as cesium contained in reactor cooling water upstream of a desalination ion exchange resin tower installed in a purification line for reactor cooling water, and discloses that titanate, hydrous titanium oxide, or ferrocyanide is used as the adsorbent.

[0009] 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 insoluble ferricyanide thus produced is brought into contact with a regenerant solution to convert it into insoluble ferrocyanide.

[0010] Patent Document 4 discloses a cesium recovery material containing ferrocyanide or ferricyanide supported by a binder and highly water-absorbent polymer powder in a nonwoven fabric packaging material, where the ferrocyanide or ferricyanide is expected to adsorb cesium, and the highly water-absorbent polymer powder is expected to absorb water and swell. Summary of the Invention [Problem to be solved by the invention]

[0011] 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. [Means for solving the problem]

[0012] The present inventors have conducted extensive research into materials capable of adsorbing cesium in an aqueous solution, and as a result have discovered that a certain type of water-absorbent polymer selectively adsorbs cesium in an aqueous solution, thereby completing the present invention.

[0013] That is, the present invention is a cesium adsorbent for cesium-containing contaminated water, which is characterized by comprising a polyacrylate-based water-absorbent polymer.

[0014] As the water-absorbent polymer, an alkali metal salt of polyacrylic acid is suitable, and a polymer based on a sodium salt or potassium salt is particularly suitable.

[0015] The present invention also relates to 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-absorbent polymer to the cesium-containing contaminated water and allowing it to absorb water to form a water-containing water-absorbent polymer, evaporating or distilling the water from the water-containing 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. Effect of the Invention

[0016] According to the present invention, the water-absorbing polymer can be used as a cesium adsorbent, and therefore it is easy to obtain. In addition, since the water-absorbing polymer absorbs a large amount of contaminated water, there is also the advantage that the adsorption of cesium continues even after absorbing water. In addition, the removal material used for removing 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, so that it is easy to reduce the weight and is suitable for long-term storage in a container. [Brief description of the drawings]

[0017] [Figure 1] This is a Langmuir plot. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] 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.

[0019] 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 polymer preferably has a high maximum water absorption rate (water absorption amount g / water-absorbent polymer g), and is preferably about 20 to 1000.

[0020] A polyacrylate-based water-absorbing polymer is a network of alkali metal salts of polyacrylic acid, which ionize into -COO(-) and alkali metal positive ions (+) when absorbing water, and can take in and hold a large amount of water molecules in the gaps created by the repulsion of the negative ions. In the present invention, the cesium ions are not physically taken in as cesium ions in the contaminated water along with the water molecules absorbed at this time, 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 form 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 absorption performance of the water-absorbing resin is inhibited by contaminated water containing a large amount of sodium ions.

[0021] 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 preferable 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.

[0022] When adding the water-absorbing polymer, it is recommended 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.

[0023] After absorbing water to form a water-absorbent polymer, the water is evaporated or distilled off to dry or concentrate the cesium.

[0024] 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.

[0025] The drying process may involve evaporating or distilling the water in the water-containing water-absorbing polymer. When evaporating, the evaporated water is preferably discharged as exhaust gas, and when distilling, it is preferably discharged as wastewater.

[0026] 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 proceeds, 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 sufficiently increased. When the drying temperature is made higher than the atmospheric temperature, it is possible to effectively utilize the relatively low-temperature waste heat generated during the operation of nuclear power plants or accident treatment. In addition, lowering the humidity of the circulating gas is effective in promoting drying, and in this case, the waste heat or exhaust gas from nuclear power plants can be used. The drying treatment time is preferably one day or more, and preferably about 5 to 15 days, depending on the surface area of ​​the water-containing water-absorbing polymer. 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.

[0027] 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.

[0028] The concentrated water-absorbing polymer that has been dried and contains cesium concentrated in the water-absorbing polymer will greatly exceed the regulated value, so it is recommended that it be stored in a container until it falls below the regulated value. Advantageously, the dried or concentrated water-containing water-absorbing polymer is burned or compressed to further reduce its volume, and the reduced volume combustion ash or compressed material is stored in a container. Since the volume is greatly reduced by the drying or burning process, and the polymer is solid as a whole, it is easy to store.

[0029] As the water-absorbing polymer, a partially sodium salt-crosslinked acrylic acid polymer is typically used. Its characteristics include the ability to hold water 100 times or, depending on the material, 1000 times its weight, 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, the use of highly water-absorbing polymers allows safe storage of treated water in a tank by turning it into a solid state, reducing the risk of leakage compared to liquid water, and allowing natural evaporation without the need for energy while controlling the cesium concentration. 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.

[0030] When long-half-life radioactive cesium 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. EXAMPLES

[0031] Example 1 As radioactive cesium, it is easy to quantify using gamma ray measurement. 137 Cs was used. The water absorption rate and 137 In order to quantitatively evaluate the Cs adsorption performance, 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." As the absorbent polymer, a commercially available acrylic acid polymer partially cross-linked with sodium salt (BC-283FHA) manufactured by TAISAP was used. 0.2 g of the polymer was placed in a nonwoven 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. This polyethylene bottle contains non-radioactive cesium iodide (High Purity Chemical Laboratory, purity 99% or more) as an electrolyte. 137Add 200 mL of Cs radioactive solution (Eckert & Ziegler, Normal Solution, 100 kBq / mL, cesium chloride concentration 0.05 mg / mL) to make the cesium concentration 2.5 mmol / L (specific gravity 1.0) per bottle. 137 The radioactivity concentration of Cs was 40 Bq / mL, and the specific activity was 16 MBq / moL.

[0032] A bag containing a water-absorbent polymer was immersed in this bottle. 137 In order to allow the Cs to reach equilibrium in the bottle, the bottle was closed and left to stand at room temperature for 7 days. After that, all the liquid on the outside of the nonwoven bag was removed with a pipette, and the water absorption rate (%) of the absorbent polymer was calculated by measuring the mass. 10 mL of the removed liquid was measured for each sample for 20 minutes using a germanium semiconductor detector to determine the liquid 137 Quantify the radioactivity concentration of Cs (Bq / kg) remaining in the aqueous solution. 137 The amount of Cs in the water-absorbent polymer after water absorption was measured. 137 The Cs concentration A (Bq / kg) was calculated using the following formula. A=B / W B: Added to the bottle 137 Radioactivity of Cs (Bq) in aqueous solution 137 Cs concentration (Bq / kg) × liquid mass (kg) W: Mass of water added to the bottle (kg) - Mass of liquid (kg) In addition, the calculation of the amount of water adsorbed by the water-absorbent polymer 137 The adsorption rate of Cs was calculated. 137 Cs concentration (Bq / kg) in the remaining aqueous solution 137 The distribution ratio D was calculated by dividing by the Cs concentration (Bq / kg).

[0033] Example 2 Bottled 137 The cesium concentration of the Cs radioactive solution is 5.0 mmol / L (specific gravity 1.0), 137 The same procedure as in Example 1 was carried out except that the Cs radioactivity concentration was 40 Bq / mL and the specific radioactivity was 8.0 MBq / mol.

[0034] Reference example Using the same chemicals, containers, and equipment as in Example 1, the cesium concentration was set to 10 mmol / L (specific gravity 1.00), 137 The same procedure as in Example 1 was carried out, except that the Cs radioactivity concentration was 40 Bq / mL and the specific radioactivity was 4.0 MBq / mol.

[0035] The results are summarized in Table 1. The water absorption rate is the water absorption rate of the water-absorbing polymer, and the remaining 137 Cs amount remains in the aqueous solution 137 Cs content in the polymer 137 The amount of Cs in the water-absorbing polymer 137 is the amount of Cs, 137 The Cs adsorption rate in the water-absorbing polymer 137 Cs adsorption rate.

[0036] [Table 1]

[0037] In Examples 1 and 2, the distribution coefficient is 1 or more, and 137 It is shown that Cs is adsorbed from the solution to the water-absorbent polymer. Also, from the reference example, it is found that the adsorption performance is improved by setting the electrolyte concentration to less than 10 mmol / L.

[0038] In addition, when it is assumed that the adsorbate molecules are adsorbed in a monolayer on the adsorption site, the saturated adsorption capacity of a solution to a substance can be calculated from the Langmuir adsorption isotherm by changing the concentration of the solution and determining the change in the amount of adsorption. Here, C is the concentration of the solution (non-radioactive cesium and radioactive cesium contained in the electrolyte). 137 Regarding q: combined concentration of Cs) and q: amount of adsorption per unit, the results of Examples 1 and 2 and the Reference Example are shown in Table 2.

[0039] [Table 2]

[0040] where C is the solution concentration, q is the amount of adsorption per unit, K is the adsorption equilibrium constant, and q m If the adsorption capacity is saturated, the adsorption amount per unit q is (saturated adsorption capacity q m × adsorption equilibrium constant K × solution concentration C) divided by (1 + adsorption equilibrium constant K × solution concentration C). By rearranging this equation, C / q becomes C / q m +1 / (q m ×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 Figure 1.

[0041] From Figure 1, the approximate line is y=0.3163x+0.4985(r 2 = 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), 137 Converted to Cs, this is equivalent to 10.1 GBq / g.

Claims

1. 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. 2. The cesium adsorbent according to claim 1, wherein the water-absorbent polymer is a sodium salt-based polymer of polyacrylic acid.

3. 2. The cesium adsorbent according to claim 1, wherein the water-absorbent polymer is a potassium salt of polyacrylic acid.

4. 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-containing water-absorbing polymer or the water-containing water-absorbing polymer in which the cesium has been concentrated to further reduce its volume; and storing the reduced-volume combustion ash or compressed product in a container.

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