How to make the hydrogel decontaminant

Abstract

To provide a hydrogel decontamination reagent and manufacturing method of the hydrogel decontamination reagent that are excellent in ability for recovering radioactive cesium, and easy to handle after capturing the radioactive cesium even though relatively large amount of alkali metals are resolved.SOLUTION: A manufacturing method of a hydrogel decontamination reagent for decontaminating waste water including radioactive cesium, or a slurry is configured to: dissolve a sodium alginate in water to prepare a sodium alginate aqueous solution; mix a ferrocyanide aqueous potassium with a metal sulfate salt aqueous solution to prepare a ferrocyanide metal salt slurry; prepare a mixed solution mixing the sodium alginate aqueous solution with the ferrocyanide metal salt slurry; inject the mixed solution into an aqueous solution containing a calcium ion to generate a hydrogel molded article of calcium alginate; and recover the hydrogel molded article to thereby obtain a hydrogel decontamination reagent.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This invention is a hydrogel decontamination method that is effective for decontaminating radioactive cesium. Agent This is related to the manufacturing method, and in particular to the decontamination technology for molten fly ash containing radioactive cesium. [Background technology]

[0002] The national government (Ministry of the Environment) has a policy to complete the final disposal of decontamination waste, etc., currently in intermediate storage within Fukushima Prefecture, outside of Fukushima Prefecture by 2045, and under this policy, various projects and technological developments have been carried out to contribute to the reduction of the volume of final disposal. Among these, a facility that thermally melts and reduces the volume of incineration residues (bottom ash and fly ash) generated from temporary incinerators within the countermeasures area and combustible materials (such as vegetation contained in flexi-containers and removed soil) generated from soil sorting facilities has been in operation since March 2020.

[0003] Various options for reducing the volume of highly radioactive molten fly ash have also been considered. Unlike soil, molten fly ash has a high ratio of water-soluble cesium, so it is possible to reduce the volume of the molten fly ash by washing it. In this case, the washing process of the molten fly ash generates a large amount of wastewater containing radioactive cesium, and this wastewater must be treated.

[0004] The present applicant has previously filed a patent application for a decontamination device for fly ash containing radioactive cesium, the decontamination device using an adsorbent containing magnetic iron nanoparticles carrying a ferrocyanide compound (Patent Document 1). As a related prior art, Patent Document 2 discloses a granular radioactive substance removal agent in which a porous granular body containing an alkaline earth metal salt of alginic acid and a radioactive substance removal functional substance is used as a base particle, and an alginic acid metal salt of a metal having a smaller ionization tendency than sodium is present at least in the surface layer of the base particle. Patent Document 3 discloses a trapping hydrogel having a three-dimensional mesh structure, which contains magnetic powder, an adsorbent that traps a target substance in a liquid, and water within the three-dimensional mesh structure, the magnetic powder being iron powder, which shows alkaline properties, and which can be recovered by magnetic force. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-117981 A [Patent Document 2] JP 2014-32107 A [Patent Document 3] Patent No. 6667880 Summary of the Invention [Problem to be solved by the invention]

[0006] The invention described in Patent Document 1 is a method that does not use hydrogel such as alginate, and there is room for improvement in handling after capture of radioactive cesium, etc. In the invention described in Patent Document 2, the granular radioactive material remover is produced by drying the gel to form a porous granular body, and there are problems with energy consumption and economics during production. In addition, although it is possible to treat even when the sodium content is high, since an alkaline earth metal salt of alginic acid and a metal salt of alginic acid with iron, aluminum, zinc, nickel, cobalt, copper, etc. are mixed, the hydrogel tends to harden, and the capture ability of radioactive cesium, etc. is not necessarily sufficient. The glucomannan disclosed in the examples of Patent Document 3 does not necessarily have a sufficiently large swelling property in water. In addition, glucomannan requires heating in a saturated steam environment when hardening the gel, and there are problems with the equipment and economy when supplying a large amount of material.

[0007] The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide a hydrogel decontamination method that has an excellent ability to recover radioactive cesium even when a relatively large amount of alkali metals is dissolved therein, and is easy to handle after capturing the radioactive cesium. Agent A manufacturing method is provided. [Means for solving the problem]

[0008] The inventors have studied the mixing of an aqueous solution of potassium ferrocyanide and an aqueous solution of a metal sulfate to prepare a slurry of metal ferrocyanide, and dispersing the slurry in a hydrogel made of calcium alginate. As a result, they have found that it is possible to produce a hydrogel decontamination agent that has a relatively high water content, swells uniformly in water to the inside, and has an excellent ability to recover radioactive cesium. The present invention is based on this finding.

[0009] (1) A method for producing a hydrogel decontamination agent for decontaminating wastewater or slurry containing radioactive cesium, comprising the steps of: dissolving sodium alginate in water to prepare a sodium alginate aqueous solution; mixing an aqueous solution of potassium ferrocyanide with an aqueous solution of a metal sulfate to prepare a metal ferrocyanide slurry; preparing a mixed solution by mixing the aqueous solution of sodium alginate with the metal ferrocyanide slurry; and injecting the mixed solution into an aqueous solution containing calcium ions to produce a hydrogel molded article of calcium alginate. The hydrogel molded article is then recovered, and the hydrogel decontamination agent is then produced. The mixed solution is injected into a calcium ion-containing aqueous solution to produce a hydrogel molded article of calcium alginate, and the hydrogel molded article is then immersed in the aqueous solution for 20 minutes or more before being collected. Method for producing hydrogel decontaminant.

[0010] (2) A method for producing the hydrogel decontamination agent described in (1) above, comprising the step of adding magnetic metal powder to the mixed solution.

[0012] ( 3 2.) The method according to claim 1, wherein the mixed solution is injected into a calcium ion-containing aqueous solution, and the calcium ion-containing aqueous solution is repeatedly used to produce a hydrogel molded article of calcium alginate. Or the above (2) A method for producing the hydrogel decontamination agent described in claim 1.

[0013] ( 4 ) The calcium ion-containing aqueous solution according to any one of (1) to (5), characterized in that the calcium ion-containing aqueous solution is an aqueous solution of a calcium salt of an organic acid. 3 ) A method for producing the hydrogel decontamination agent according to any one of the above. Effect of the Invention

[0019] The hydrogel decontamination agent of the present invention has an excellent ability to recover radioactive cesium even when a relatively large amount of alkali metals is dissolved therein, and is easy to handle after capturing the radioactive cesium. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a process diagram of a method for producing a hydrogel decontamination agent according to the present embodiment. [Diagram 2] FIG. 2 is a process diagram of a method for using the hydrogel decontamination agent of the present embodiment. [Diagram 3] This is the reaction formula for producing calcium alginate from sodium alginate. [Figure 4] 1 is a graph showing the change in cesium concentration over time in an example. [Diagram 5] Graph (a) showing the change in Ca concentration and Na concentration in a calcium lactate solution versus the cumulative amount of sodium alginate added in an example, and graph (b) showing the change in Ca concentration and Na concentration in a dried recovered gel versus the number of times sodium alginate gel was added in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The following is a detailed description of an embodiment of the present invention. However, the embodiment described below is merely an example of the present invention, and the present invention is not limited to the contents thereof, and various modifications can be made within the scope of the present invention.

[0022] The present invention treats wastewater or slurry containing radioactive cesium. Specifically, the treatment target is a filtrate obtained by solid-liquid separation after washing fly ash containing radioactive cesium, or a slurry of fly ash containing radioactive cesium.

[0023] (Type of adsorbent) In the past, inorganic adsorbents such as (i) zeolites, (ii) metal ferrocyanides, and (iii) silicic titanates have been widely used as adsorbents (capture compounds) for adsorbing and recovering radioactive cesium in water.

[0024] Among the above inorganic adsorbents, (i) zeolite adsorbs cesium (Cs + ), and potassium (K + ) and sodium (Na +) may not function as an adsorbent for cesium when it is present in high concentrations. On the other hand, (ii) metal ferrocyanide salts and (iii) silicic titanates have high selectivity for cesium, and therefore can exhibit a certain level of adsorption performance even in the presence of high concentrations of alkali metals.

[0025] The wastewater obtained after washing the molten fly ash, which is the subject of treatment in the present invention, contains a large amount of dissolved alkali metals in addition to radioactive cesium, and therefore possible adsorbents that can be used include metal ferrocyanides and silicititanates. It is known that, among the two, the metal ferrocyanide salt is far superior to the silica titanate salt in terms of the maximum adsorption amount. Also, the metal ferrocyanide salt is cheaper in terms of production cost. Therefore, the present invention adopts the metal ferrocyanide salt as the adsorbent for radioactive cesium.

[0026] (Form of adsorbent: In case of powder) When powdered metal ferrocyanide salt is used, the surface area becomes large and it is expected that the adsorption capacity for radioactive cesium can be fully utilized. However, when recovering the adsorbent after adsorbing the radioactive cesium, sludge containing high concentrations of radioactivity must be handled, which raises issues regarding the cost of separation and recovery equipment and the safety of handling.

[0027] (Form of adsorbent: Granular) When the adsorbent is granulated, it can be packed into a cylindrical adsorption tower and continuously passed through it to treat wastewater. On the other hand, granulation using various binders etc. leads to a decrease in the adsorption performance of the metal ferrocyanide salt. In addition, the manufacturing cost of the material is high.

[0028] In light of the above circumstances, the present inventors investigated the existence form of the adsorbent metal ferrocyanide salt, not in the form of a powder or granules, but by dispersing it in a calcium alginate hydrogel having a three-dimensional network structure. That is, we devised a manufacturing method that enables the wet preparation of a metal ferrocyanide slurry by mixing an aqueous solution of potassium ferrocyanide with an aqueous solution of a metal sulfate, and then uniformly dispersing the slurry in a hydrogel made of calcium alginate. As a result, we succeeded in producing a hydrogel decontamination agent that has a relatively high water content, swells uniformly in water to the inside, and has excellent ability to recover radioactive cesium.

[0029] In other words, the hydrogel decontamination agent of this embodiment is a hydrogel decontamination agent that is composed of a hydrogel molded product of calcium alginate having a three-dimensional mesh structure, and is characterized in that it contains a metal ferrocyanide salt, which is a radioactive cesium capturing compound, in the hydrogel molded product of calcium alginate.

[0030] Here, a gel is generally composed of a three-dimensional network with nanoscale intervals and a dispersion medium that infiltrates the network. The contact area is very large, so there is strong interaction, and the polymer network and the dispersion medium do not easily separate. A hydrogel is a gel in which the dispersion medium is water. Compared with other crosslinked types, polymer gels are stable against a wider range of changes in heat, solution type, ionic strength, and pH, and the polymer network is less likely to collapse and can maintain its shape. The hydrogel made of calcium alginate in this embodiment corresponds to this polymer gel.

[0031] Figure 3 shows the reaction formula for producing calcium alginate from sodium alginate. Calcium alginate is produced by injecting (dropping) sodium alginate into an aqueous solution containing calcium ions such as calcium lactate. Since the calcium ions of calcium alginate can form a network with other ions, it is possible to form a hydrogel molded article of calcium alginate with a three-dimensional mesh structure.

[0032] In the case of the hydrogel decontamination agent of this embodiment, the primary particles of the metal ferrocyanide salt generated by the solution reaction (potassium ferrocyanide aqueous solution + metal sulfate aqueous solution) are fine particles of about 10 nm to 100 nm. It is considered that these particles undergo secondary aggregation to become particles of the order of micrometers, and are dispersed throughout the three-dimensional mesh structure.

[0033] The hydrogel decontamination agent of this embodiment can compensate for the disadvantages of powders and granular materials described above, can be easily separated and recovered without requiring special equipment, and can be handled safely in terms of radioactivity concentration.

[0034] The advantages of the decontamination agent having the form of a hydrogel according to this embodiment can be summarized as follows. (a) The hydrogel after radioactive cesium adsorption can be easily collected using a wire mesh or the like. If magnetic metal powder is added to the hydrogel, it can be easily collected using magnetic force. As the magnetic metal powder, micro-sized iron powder with an average particle size of about 80 μm is preferable. Unlike the method of directly supporting a capturing compound on nano-sized iron powder described in Patent Document 1, when using micro-sized iron powder, it is sufficient to disperse the micro-sized iron powder in a three-dimensional mesh structure, so that magnetic separation properties can be easily imparted. In addition, since micro-sized iron powder is inexpensive, the cost of the magnetic material can be significantly reduced. (b) The moisture content can be set to 80 to 90% by mass, and the radioactivity concentration can be reduced. As a result, the surface dose rate of the adsorbent is significantly reduced, making it possible to handle it safely. (c) After radioactive cesium is adsorbed, it is possible to reduce or concentrate the volume at any time. For example, by removing the moisture and drying, the volume can be reduced to about one-tenth of its original size. In addition, metal ferrocyanide salts can be thermally decomposed at a relatively low temperature of about 300°C while suppressing the volatilization of cesium. In addition, by combining with an appropriate stabilization technology (e.g., ceramicization, vitrification, etc.), it can be converted into a stable substance containing radioactivity. (d) The metal ferrocyanide salt may be at least one selected from nickel ferrocyanide, copper ferrocyanide, iron ferrocyanide, and cobalt ferrocyanide. Any metal ferrocyanide salt may be selected and produced on-site. In addition, since the reaction occurs under room temperature conditions, the metal ferrocyanide salt may be produced on-site.

[0035] (Method of manufacturing hydrogel decontamination agent) 1 is a process diagram of a method for producing a hydrogel decontamination agent of this embodiment. The hydrogel decontamination agent of this embodiment is produced through the following 10 steps from step S1 to step S10.

[0036] Step S1 is a step of dissolving sodium alginate in water to prepare a sodium alginate aqueous solution. For example, when sodium alginate powder (industrial product; medium viscosity) is dissolved in tap water at 3% by mass and allowed to stand for a while, air bubbles naturally escape from the viscous sodium alginate solution, and the solution becomes smooth and starch syrup-like. Step S2 is a step of mixing an aqueous solution of potassium ferrocyanide and an aqueous solution of a metal sulfate (for example, an aqueous solution of nickel sulfate). There is no limitation on the order of steps S1 and S2. Step S3 is a step of precipitating a metal ferrocyanide salt (eg, nickel ferrocyanide) to prepare a metal ferrocyanide salt slurry. Step S4 is a step of preparing a mixed solution by mixing an aqueous sodium alginate solution with a metal ferrocyanide salt slurry (eg, nickel ferrocyanide slurry). Step S5 is a step of adding magnetic metal powder. Step S5 is performed as necessary. The magnetic metal powder is, for example, micro-sized iron powder (manufactured by Kobe Steel, Ltd.: average particle size 80 μm). Step S6 is a step of stirring the mixed solution containing the magnetic metal powder, and is carried out when step S5 is carried out. In step S6, the mixture is stirred uniformly for 2 to 3 minutes using a stirrer to prepare a mixed solution in which the sodium alginate gel aqueous solution, the metal ferrocyanide salt slurry, and the magnetic metal powder are mixed. Step S7 is a step of dissolving calcium lactate in water to prepare an aqueous calcium lactate solution. The water may be tap water. Calcium lactate is dissolved in tap water in a range of, for example, 2 to 5% by mass. The aqueous calcium ion-containing solution is preferably an aqueous solution of a calcium salt of an organic acid, and examples of the organic acid include lactic acid, acetic acid, citric acid, succinic acid, and phthalic acid. Step S8 is a step in which the gel of the mixed solution is dispensed into a syringe or the like and quickly injected (ejected, dropped) into an aqueous calcium lactate solution to produce a calcium alginate hydrogel molded article. Step S9 is a step of leaving the hydrogel molded article obtained in step S8 for a while. After the hydrogel molded article of calcium alginate is produced, it is immersed in the aqueous solution for 5 minutes or more, preferably 20 minutes or more, until it is collected. By immersing in the aqueous solution for 20 minutes or more, a stable hydrogel can be obtained. Step S10 is a step of recovering the produced gel (hydrogel molded article). The hydrogel molded article is in the form of a thread or string (diameter: about 2 to 3 mm), and is cut to a predetermined length (for example, about several mm to 10 cm) depending on the application to obtain a hydrogel decontamination agent.

[0037] (How to use hydrogel decontamination agent) 2 is a process diagram of a method for using the hydrogel decontamination agent of this embodiment. The hydrogel decontamination agent of this embodiment can adsorb and recover radioactive cesium in fly ash through the following six steps from step S11 to step S16. In step S11, fly ash containing radioactive material (radioactive cesium) is added to water to prepare fly ash slurry. By stirring and mixing for 30 to 60 minutes, it is possible to dissolve the water-soluble cesium in the water. Step S12 is a step of adding an appropriate amount of a hydrogel decontamination agent. Step S13 is a step of stirring the solution obtained in step S12 for a period of time to cause a reaction. That is, step S13 is a step of adsorbing the radioactive cesium in the solution by the hydrogel decontamination agent. Step S14 is a step of recovering the hydrogel decontamination agent that has adsorbed radioactive cesium using a magnetic separation tool or device. Specifically, a general-purpose device using a permanent magnet (neodymium magnet), such as a magnet bar or a magnetic separator, can be used. In step S15, the solution after step S14 is filtered using an appropriate solid-liquid separator to separate the filtration residue (solids, washed fly ash) from the filtrate. Note that in cases where solid-liquid separation is performed in advance using a high-pressure filter press or the like and only the filtrate in which radioactive cesium has been dissolved is to be decontaminated, this step S15 may be performed immediately after S11. Step S16 is a step of measuring the radioactivity concentration. The amount of recovered radioactive cesium is quantified by measuring the radioactivity concentration of the filtered residue (washed fly ash) and the filtrate.

[0038] The hydrogel decontamination agent of this embodiment is manufactured by the manufacturing method and the method of use described above, and the following effects can be expected by using the hydrogel decontamination agent and the method of use described above. (a) By adjusting the amount of adsorbent (metal ferrocyanide salt) contained in the hydrogel molded product, it is possible to provide a decontamination agent that does not have an excessively high radioactive concentration and can be handled safely. (b) Separation and recovery is possible by utilizing the shape of the hydrogel molded product. For example, it can be physically recovered using a mesh material. Also, if the hydrogel molded product contains magnetic metal powder, it can be separated and recovered by magnetism. No special solid-liquid separation equipment (high-pressure filter press, etc.) is required. (c) It is possible to synthesize the adsorbent at room temperature using general-purpose equipment. As a result, it is possible to provide the required amount of adsorbent on the spot. It is also possible to manage the quality by a certain method. EXAMPLES

[0039] The present invention will be described in more detail below using examples, but the technical scope of the present invention is not limited to these examples.

[0040] [Experimental Example 1: Indoor production test of hydrogel decontamination agent using a stirrer and evaluation of the product] (1) Purpose of the study We will produce a hydrogel decontamination agent and determine its cesium removal performance both in the case of small-scale synthesis at the syringe level and in the case of large-scale synthesis of the order of 1 kg using an indoor mixing device as a test scale simulating actual production.

[0041] (2) Test method In Experimental Example 1, seven cases (No. 1 to 7) were examined. Table 1 shows the manufacturing conditions of the hydrogel decontamination agents used in No. 1 to 7. In the case of small-scale synthesis, the viscosity of sodium alginate, the concentration of calcium lactate, and the amount of nickel ferrocyanide added were changed. The sodium alginate used was a medium viscosity type (Newtex FM, manufactured by Fuji Chemical Industry Co., Ltd.), with No. 6 being the only one that used a high viscosity type (Newtex F-SH, manufactured by Fuji Chemical Industry Co., Ltd.). The hydrogel decontamination agent was produced and used in accordance with the steps shown in FIGS. The basic procedure for testing cesium adsorption using a hydrogel decontamination agent is as follows. (i) Cesium chloride was dissolved in distilled water to prepare a simulated solution with a cesium concentration of 10 mg / L. (ii) 5% by mass of hydrogel decontamination agent was added to the above cesium simulant solution. (iii) The mixture was stirred at 10 rpm using a rotary shaker. (iv) After 10 minutes, 30 minutes, and 24 hours, the hydrogel decontamination agent was separated using a magnet, and the cesium and nickel concentrations in the liquid phase were quantitatively analyzed by ICP / MS. The supernatant after standing was used for analysis (unfiltered). Radioactivity concentration was also measured.

[0042] The specific amounts of materials used are as follows: In No. 1, a nickel ferrocyanide slurry was prepared from 0.6 ml of a 0.6 M solution of sodium ferrocyanide and 0.6 ml of a 0.6 M solution of nickel sulfate, and added to 2.4 g of a 3% by mass aqueous solution of sodium alginate. Then, 0.2 g of iron powder was added, and the mixture was injected into 0.5 L of a 2% by mass aqueous solution of calcium lactate. No. 2 was the same as No. 1 except that the amounts were doubled (1.2 ml of 0.6 M sodium ferrocyanide solution and 1.2 ml of 0.6 M nickel sulfate solution). For Nos. 3 to 6, the amounts added were changed according to the description in Table 1. In the case of small-scale synthesis, the mixture was mixed and synthesized in a syringe, and the mass of the obtained hydrogel decontamination agent was about 50 g. In the case of large-scale synthesis, a practical device was used, and the mass of the obtained hydrogel decontamination agent was about 5 kg.

[0043] [Table 1]

[0044] (3) Test results The test results are shown in Table 2. Figure 4 shows the change in cesium (Cs) concentration over time. As shown in Table 2, in all of No. 1 to No. 7, the cesium removal rate was 98% or more 30 minutes after adding the hydrogel decontamination agent prototype, and it was 99% or more except for No. 2 and No. 5. In addition, in the case of large-scale synthesis, the cesium concentration removal rate after 30 minutes was greater than in the case of small-scale synthesis. From this, it is estimated that economies of scale can be expected during manufacturing.

[0045] Moreover, even if the amount of nickel ferrocyanide added was increased, the removal rate did not improve significantly. From this, it was thought that the cesium removal rate depends solely on the passage speed within the gel, and does not depend on the amount of nickel ferrocyanide retained within the gel. However, although this was not evaluated this time, the maximum adsorption amount increases when a large amount of nickel ferrocyanide is retained, so it is presumed to be advantageous in repeated use.

[0046] In addition, from the measured values ​​of nickel, which is a component of the capture compound contained in the liquid phase, it was confirmed that there was almost no occurrence of nickel ferrocyanide, which is a capture compound, being released from inside the gel and eluted outside the system. As can be seen from Figure 4, the cesium concentration after 24 hours was slightly higher than after 30 minutes. This is thought to be due to the effect of a small amount of the capturing compound being released from the gel when immersed for a long period of time, causing the cesium concentration to rise slightly.

[0047] [Table 2]

[0048] [Experimental Example 2: Effect of immersion time during hydrogel production] (1) Purpose of the study The effect of immersion time in calcium lactate aqueous solution on the sodium and calcium content in the hydrogel decontamination agent is evaluated.

[0049] (2) Test method (i) A 3% by mass aqueous solution of sodium alginate was prepared under the following conditions on the day before the test. Dissolve 15g of sodium alginate powder in 500ml of distilled water and stir. (ii) A 5% by weight aqueous solution of calcium lactate was prepared on the day of the test under the following conditions: 100 g of calcium lactate powder was dissolved in 2000 ml of distilled water and stirred with a stirrer. (iii) A nickel ferrocyanide slurry was prepared according to the following conditions and was ready on the day of testing: 50 ml of a 0.6 mol / l aqueous solution of potassium ferrocyanide was dispensed into each of five disposable dishes, and 50 ml of a 0.6 ml / l aqueous solution of nickel sulfate was added and reacted for about an hour. (iv) A hydrogel decontamination agent was prepared by the following procedure. Dispense 20g of sodium alginate gel into a 50ml syringe. · Dispense 10ml of nickel ferrocyanide slurry into a 20ml syringe. Mix thoroughly using a three-way stopcock. Add 1.7g of iron powder and mix thoroughly. Transfer the entire mixed gel into a 50ml syringe. Inject into calcium lactate solution. After soaking for the specified time, collect the material promptly. The immersion times were 30 sec, 2 min, 5 min, 20 min, and 60 min. The obtained hydrogel was cut into pieces with scissors to prepare gels for measurement tests. (v) Decontamination Tests A cesium chloride solution with a concentration of 50 mg / l was prepared. -2g (5wt%) of hydrogel was added to 40ml of the above cesium solution. The cesium concentration was measured 10 minutes and 30 minutes after the start of stirring. Separately, the hydrogel was subjected to quantitative analysis of water content and elements by fluorescent X-ray analysis for the solid part. Water content was measured by placing the hydrogel in a heating oven heated to 110℃ and measuring the mass change after drying.

[0050] (3) Test results The test results are shown in Table 3. Figure 5(a) shows a graph indicating the change in Ca concentration and Na concentration in the calcium lactate solution versus the cumulative amount of sodium alginate added. Figure 5(b) shows a graph indicating the change in Ca concentration (content) and Na concentration (content) in the dried recovered gel versus the number of times sodium alginate gel was added. The residual rate of sodium (Na) in the obtained hydrogel decreased with increasing immersion time, and was below the lower limit of quantification (ND) when immersed for 20 minutes or more. It was confirmed that the content of calcium (Ca), which replaces sodium and forms cross-links, increases with immersion time. Regarding the cesium decontamination effect, the hydrogels with immersion times of 20 and 60 minutes showed relatively good results. From the above, it was determined that a good product can be obtained if the immersion time during hydrogel production is 20 minutes or more.

[0051] [Table 3]

[0052] [Experimental Example 3: Examination of the possibility of repeated use of calcium lactate aqueous solution] (1) Purpose of the study The mixed solution is injected into a calcium ion-containing aqueous solution to confirm whether the calcium ion-containing aqueous solution can be repeatedly used when producing a calcium alginate hydrogel molded article.

[0053] (2) Test method 500 g of an aqueous solution containing 2% calcium lactate by mass was repeatedly used as the calcium lactate aqueous solution. The calcium content was 1.74 g. The sodium alginate aqueous solution used was one in which sodium alginate was dissolved at a concentration of 3% by mass. The above sodium alginate solution was added to the above calcium lactate solution a total of seven times, and the formation of hydrogel was repeated. A total of 607 g of sodium alginate aqueous solution was added as hydrogel (Table 4). This corresponds to 18.2 g of powder and 2.12 g of Na. The hardened calcium alginate gel was collected each time, and the calcium lactate solution was also sampled each time. Although sodium alginate gel was constantly added and then collected after hardening, the amount of solution in the beaker never decreased.

[0054] (3) Test results The results are shown in Table 4. It was confirmed that the calcium lactate aqueous solution can be repeatedly used up to the point where 7.1 g of sodium alginate (in powder equivalent) is added per 10 g of calcium lactate (in powder equivalent). The calcium lactate aqueous solution can be used at any concentration within the range of, for example, 2% to 5% by mass, as long as it does not exceed the saturated solubility. Sodium alginate is a viscous liquid and can be selected within a range that is easy to handle, for example, a concentration of 3% by mass is suitable.

[0055] [Table 4]

[0056] [Experimental Example 4: Decontamination test of fly ash slurry containing radioactive cesium] (1) Purpose of the study The feasibility of decontamination treatment will be evaluated for actual samples of municipal waste fly ash containing radioactive cesium.

[0057] (2) Test method Using the same method as in Experiments 1 and 2, a hydrogel decontamination agent was created containing nickel ferrocyanide and iron powder in calcium alginate gel. · Fly ash slurry was prepared by adding 5 times the amount of distilled water to the municipal waste fly ash. 5% by mass of hydrogel decontamination agent was added to the fly ash and reacted for 30 minutes while stirring at a rotation speed of 200 rpm. The decontamination agent was collected using a magnetic bar and turned into treated slurry. The above slurry was filtered through a 0.45 μm mesh filter, and the filtration residue (treated fly ash) and filtrate (treated water) were separated, and the radioactivity concentration of each was measured.

[0058] (3) Test results The test results are shown in Table 5. - The initial radiation concentration of the fly ash was approximately 20,000 Bq / kg, but the radioactivity concentration of the treated slurry had been decontaminated to approximately 1,700 Bq / kg, well below the target of 8,000 Bq / kg for recycling. The radioactivity concentration of the filtrate had also fallen to a level that was sufficiently sufficient to meet the discharge standards.

[0059] [Table 5]

[0060] (4) Supplementary analysis results The analysis results of the components contained in the fly ash and the slurry to which water was added used in the test are shown in Table 6. In particular, the fly ash sample contained 3.1% by mass and 5.0% by mass of sodium and potassium, which are alkali metals that compete with cesium, and the concentrations in the liquid when slurried were high at 5,900 mg / L and 8,000 mg / L, respectively. From the above, it was confirmed that the decontamination test was conducted in an environment containing high concentrations of alkali metals.

[0061] [Table 6]

Claims

1. A method for producing a hydrogel decontamination agent for decontaminating wastewater or slurry containing radioactive cesium, comprising: A step of dissolving sodium alginate in water to prepare an aqueous solution of sodium alginate; mixing an aqueous potassium ferrocyanide solution with an aqueous metal sulfate solution to form a metal ferrocyanide slurry; preparing a mixed solution by mixing the sodium alginate aqueous solution and the metal ferrocyanide salt slurry; and injecting the mixed solution into an aqueous solution containing calcium ions to produce a hydrogel molded article of calcium alginate. The hydrogel molded article is collected and used as a hydrogel decontamination agent. A method for producing a hydrogel decontamination agent, comprising injecting the mixed solution into a calcium ion-containing aqueous solution to produce a hydrogel molded article of calcium alginate, and then immersing the hydrogel molded article in the aqueous solution for 20 minutes or more before collecting the hydrogel molded article.

2. The method for producing a hydrogel decontamination agent according to claim 1, further comprising the step of adding a magnetic metal powder to the mixed solution.

3. 3. The method for producing a hydrogel decontamination agent according to claim 1, wherein the mixed solution is injected into a calcium ion-containing aqueous solution, and the calcium ion-containing aqueous solution is repeatedly used to produce a hydrogel molded article of calcium alginate.

4. 4. The method for producing a hydrogel decontamination agent according to claim 1, wherein the calcium ion-containing aqueous solution is an aqueous solution of a calcium salt of an organic acid.

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