Radioactive liquid waste treatment method

Abstract

To provide a radioactive liquid waste treatment method comprising a concentration step using an RO device and a step of applying adsorption treatment to RO-concentrated liquid using an adsorbent, which prevents scale adhesion in the RO device and enables sufficient adsorption treatment.SOLUTION: A radioactive liquid waste treatment method is provided, involving adding a non-phosphorus scale dispersant to radioactive liquid waste and passing the liquid waste through a reverse osmosis membrane, adjusting pH of the resultant concentrated liquid to a range of 5 to 6, and then passing the concentrated liquid through an adsorption tower filled with an inorganic oxide adsorbent for Sr adsorption, where the inorganic oxide adsorbent is crystalline silicotitanate or the like.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for treating radioactive liquid waste, and more particularly to a method for treating radioactive liquid waste comprising a concentration step using a reverse osmosis membrane device (hereinafter sometimes referred to as an RO device) and an adsorption treatment step of RO concentrated water. [Background technology]

[0002] Conventionally, radioactive liquid waste has been treated by pre-treating it using methods such as coagulation and filtration, and then passing the water through an adsorption tower filled with an adsorbent to adsorb and remove the radioactive substances (for example, Patent Document 1).

[0003] However, with this method, the concentration of radioactive materials in radioactively contaminated water is generally extremely low, so the adsorption efficiency to the adsorbent is poor, and it is necessary to use a large amount of adsorbent and treat it at a low flow rate. This requires a large-capacity adsorption tower, and the disposal of the large amount of waste adsorbent that is discharged is also a major problem. Therefore, methods have been proposed to remove radioactive materials from radioactively contaminated water using reverse osmosis membranes (RO membranes).

[0004] Crystalline silicotitanate has been proposed as a suitable adsorbent for adsorbing radioactive strontium and cesium (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-145687 [Non-patent literature]

[0006] [Non-Patent Document 1] Olga Oleksiienko, Christian Wolkersdorfer, Mika Sillanpaa “Titanosilicates in cation adsorption and cation exchange-A review”Chemical Engineering Journal 317(2017)570-585) Summary of the Invention [Problem to be solved by the invention]

[0007] When radioactive liquid waste is concentrated using an RO device and the concentrated water is treated by adsorption with an adsorbent, a scale dispersant is added to the water to be treated to suppress scale buildup on the RO membrane.

[0008] When the water to be treated contains a large amount of Ca and Mg derived from seawater, phosphorus-based scale dispersants such as phosphonic acid and PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid) are used as scale dispersants.

[0009] When radioactive wastewater containing alkaline earth metals such as Ca and Mg is concentrated using RO membranes, adding phosphorus-containing scale dispersants such as phosphonic acid or PBTC results in concentrated alkaline earth metals chelated by the scale dispersant in the RO concentrate. When RO concentrate containing alkaline earth metals is removed by coagulation and sedimentation or adsorption, the chelated alkaline earth metals, especially Sr, are not adequately adsorbed. This results in a short breakthrough rate for the adsorbent, and a large amount of used adsorbent waste is generated.

[0010] The present invention aims to prevent scale in the RO device and perform sufficient adsorption treatment in a method for treating radioactive liquid waste, which includes a concentration step using an RO device and a step of adsorbing the RO concentrated water using an adsorbent. [Means for solving the problem]

[0011] The present invention has the following gist.

[0012] [1] A method for treating radioactive wastewater by adding a non-phosphorus scale dispersant to the radioactive wastewater, passing the water through a reverse osmosis membrane device, adjusting the pH of the concentrated water to 5-6, and then passing the water through an adsorption tower filled with inorganic oxide adsorbent for Sr adsorption.

[0013] [2] The method for treating radioactive liquid waste according to [1], wherein the inorganic oxide adsorbent is a crystalline silicotitanate.

[0014] [3] The method for treating radioactive liquid waste according to [1], wherein the concentration ratio of the reverse osmosis membrane device is 2 to 5 times.

[0015] [4] The method for treating radioactive liquid waste according to any one of [1] to [3], wherein the non-phosphorus-based scale dispersant contains an acrylic acid-based polymer or a maleic acid-based polymer. [Effects of the Invention]

[0016] In the present invention, a non-phosphorus-based scale dispersant is used as a scale dispersant in a radioactive liquid waste treatment method that includes a concentration step using an RO device and a step of adsorbing the RO concentrated water with an adsorbent. By using such a non-phosphorus-based scale dispersant, Sr is prevented from being chelated, and the breakthrough amount of Sr in the adsorption step is extended.

[0017] In one embodiment of the present invention, a crystalline silicotitanate is used as the adsorbent, which allows operation without reducing the water flow rate even in the presence of a scale dispersant, and achieves a high breakthrough water flow rate.

[0018] The non-phosphorus-based scale dispersant used in one embodiment of the present invention is acidic, so that the pH of the RO concentrate to which it is added becomes less than 6, enabling the expulsion of carbon dioxide. This suppresses the decrease in the adsorbent surface area due to the formation of calcium carbonate, and increases the breakthrough water flow rate of Sr.

[0019] Because the Sr adsorption water flow rate of conventional adsorbents is suitable under alkaline conditions, conventional technology requires complex pH adjustment, such as adjusting the pH to make the solution alkaline after driving out the carbon dioxide. In contrast, the non-phosphorus-based scale dispersant used in the present invention is acidic, so adding the scale dispersant reduces the pH to less than 6. By using an adsorbent that has high Sr adsorption performance even under neutral conditions, a high Sr breakthrough water flow rate can be achieved even under pH levels below 6. This eliminates the need for complex pH adjustment. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a flow diagram illustrating an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment will be described with reference to FIG.

[0022] After the radioactive liquid waste is separated into solid and liquid in solid-liquid separation means 1, a non-phosphorus scale dispersant is added, and a pH adjuster is further added to adjust the pH to 5 to 6, before being supplied to RO device 2, where it is separated into permeate and concentrated water. The concentration ratio of RO device 2 is preferably about 2 to 5 times. The concentrated water is sent to adsorption tower 3, where it is subjected to adsorption treatment, and the water effluent from the adsorption tower is taken out as treated water. Adsorption tower 3 is filled with crystalline silicotitanate.

[0023] An example of radioactive wastewater is that discharged from nuclear power plants. This radioactive wastewater may contain SS components in addition to ions such as Ca and Mg, so the SS in the wastewater is separated into solid and liquid using solid-liquid separation means 1, such as a UF membrane separator. A scale dispersant is added to the solid-liquid separated treated water from which the SS has been removed. The addition of the dispersant suppresses scale formation in the RO membrane, allowing the concentration ratio of the RO device 2 to be increased.

[0024] As the scale dispersant, a non-phosphorus-based scale dispersant that does not contain phosphorus, specifically one that contains an acrylic acid-based polymer or a maleic acid-based polymer, is used. The concentration of the non-phosphorus-based scale dispersant added is preferably about 10 to 100 mg / L.

[0025] After adding the non-phosphorus-based scale dispersant, an acid such as hydrochloric acid or sulfuric acid is added as needed to drive off carbon dioxide, adjusting the pH to 6 or less (preferably 5.5 or less). Depending on the water to be treated, the pH may drop to around 3 after adding the non-phosphorus-based scale dispersant. A pH of around 3 is good for driving off carbon dioxide, but may reduce the treatment performance of the adsorbent, so it is preferable to adjust the pH to 5 to 6, for example, around 5.5.

[0026] RO concentrated water containing high concentrations of coexisting ions and radioactive materials is passed through the adsorption tower in the presence of a dispersant, without removing Ca, Mg, etc. Sr and Cs are adsorbed and removed by the crystalline silicotitanate-based adsorbent packed into the adsorption tower. This process does not require coagulation and sedimentation, eliminating the sludge generated by coagulation and sedimentation and reducing radioactive waste.

[0027] Crystalline silicotitanates have the chemical formula M2Ti2O3SiO4·2H2O, where Ti may be partially replaced with Nb, and M is an alkali metal such as potassium, sodium, or lithium.

[0028] Conventionally used scale dispersants are phosphorus-based scale dispersants such as phosphonic acid and PBTC, which have a strong bonding force with alkaline earth metals. If the scale dispersant is coexisting with the scale dispersant and flows out to a downstream stage, there is a high possibility that this will cause poor coagulation of the alkaline earth metals or inhibit adsorption in the adsorption tower. In the present invention, a non-phosphorus-based scale dispersant is used, so alkaline earth metals are efficiently adsorbed onto the crystalline silicotitanate. This is because the adsorption force between crystalline silicotitanate and Sr is stronger than the bonding force between non-phosphorus-based scale dispersants and Sr, and Sr is sufficiently adsorbed onto the adsorbent even in the presence of a non-phosphorus-based scale dispersant.

[0029] Because non-phosphorus-based scale dispersants are acidic, adding them reduces the pH of the solid-liquid separation treated water to less than 6. This converts the bicarbonate ions in the solid-liquid separation treated water into carbonic acid, which is removed from the treated water. As a result, the decrease in the adsorbent surface area due to the formation of calcium carbonate is suppressed, thereby increasing the breakthrough water flow rate of the adsorbent. [Example]

[0030] Comparative Examples, Examples, and Reference Examples will be described below.

[0031] In the following Comparative Examples, Examples and Reference Examples, the following crystalline silicotitanate was used as the adsorbent.

[0032] <Crystalline silicotitanate> This crystalline silicotitanate was produced by the method described in Non-Patent Document 1. Specifically, 3.43 g of titanium isopropoxide, 3.33 g of tetraethyl orthosilicate, and 50.54 g of NbO were each dispersed in 50 mL of an aqueous solution of NaOH (0.66 g), mixed for 30 minutes, and then allowed to stand in a pressure vessel at 200°C for 3 days to cause precipitation, followed by granulation to a particle size of approximately 0.5 mm.

[0033] 5 mL of this crystalline silicotitanate was packed into a column with an inner diameter of 14.8 mm to form an adsorption column.

[0034] [Comparative Example 1] Test water was prepared with the composition of the water to be treated shown in Table 1, and 0.001 wt% (10 mg / L) of Kurita Water Industries Ltd.'s dispersant for RO treatment, "Kuriverter (registered trademark) N-500," was added as a phosphorus-based scale dispersant. The water was then passed through the adsorption column at a linear velocity of 0.68 cm / min.

[0035] [Example 1] The experiment was conducted under the same conditions as in Comparative Example 1, except that a non-phosphorus scale dispersant (Kurita Water Industries Ltd.'s dispersant for RO treatment, "Kuriverter (registered trademark) N-195") was added at 0.005 wt% (50 mg / L) as the scale dispersant.

[0036] Comparative Example 2 The experiment was carried out under the same conditions as in Comparative Example 1, except that no scale dispersant was added.

[0037] [Results, discussion] Table 1 shows the measurement results of the 10% breakthrough water flow rate (Bv) of Sr in Comparative Examples 1 and 2 and Example 1.

[0038] [Table 1]

[0039] As shown in Table 1, despite the higher Sr and Cs concentrations in Example 1 than in the comparative examples, a 10% Sr breakthrough water flow rate of 4000 BV or more was achieved, more than twice that of Comparative Example 1. Furthermore, Example 1, to which a non-phosphorus-based dispersant was added, showed a 14% increase in water flow rate compared to Comparative Example 2, to which no dispersant was added. These results demonstrate that the Sr removal efficiency can be improved by adsorbing Sr after removing carbonate ions from untreated water. Furthermore, it was demonstrated that this adsorbent can achieve a high water flow rate even without adjusting the pH to alkaline after decarbonation.

[0040] [Reference examples 1~4] To a solution containing 2 g / L of NaCl and 10 mmol / L of HEPES buffer, strontium chloride solution was added at 0.97 mg / L (Reference Example 1), 0.95 mg / L (Reference Example 2), 51 mg / L (Reference Example 3), or 48 mg / L (Reference Example 4) in terms of Sr, and the resulting solution was used as test water (raw water).

[0041] To 50 mL of test water, the non-phosphorus scale dispersant was added in the amount shown in Table 2, and 0.05 g of the crystalline silicotitanate granules was also added. After shaking this test water for 4 days, the supernatant water was separated and the Sr concentration was measured. The measurement results of the Sr concentrations in the test water (raw water) and treated water (supernatant water) are shown in Table 2.

[0042] [Table 2]

[0043] As shown in Table 2, it was confirmed that the coexistence of a non-phosphorus scale dispersant did not affect the amount of Sr adsorption or distribution coefficient. [Explanation of symbols]

[0044] 1 Solid-liquid separation means 2 RO device 3 Adsorption tower

Claims

1. A method for treating radioactive waste liquid by adding a non-phosphorus scale dispersant to the radioactive waste liquid, passing the water through a reverse osmosis membrane device, adjusting the pH of the resulting concentrated water to 5-6, and then passing the water through an adsorption tower filled with an inorganic oxide adsorbent for Sr adsorption.

2. 2. The method for treating radioactive liquid waste according to claim 1, wherein said inorganic oxide adsorbent is a crystalline silicotitanate.

3. 2. The method for treating radioactive liquid waste according to claim 1, wherein the concentration ratio of said reverse osmosis membrane device is 2 to 5 times.

4. 4. The method for treating radioactive liquid waste according to claim 1, wherein the non-phosphorus-based scale dispersant contains an acrylic acid-based polymer or a maleic acid-based polymer.

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