JP2014098558A - Method for purifying contaminated water containing radioactive cesium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying contaminated water containing radioactive cesium that generates radioactive waste whose quantity is small after its treatment and that requires no time-consuming work during the treatment.SOLUTION: A method for purifying contaminated water containing radioactive cesium comprises: an adsorption step of mixing ferrocyanide with contaminated water to adsorb radioactive cesium on the ferrocyanide; a precipitation step of precipitating the ferrocyanide having adsorbed radioactive cesium by the use of a flocculation precipitant in the contaminated water; and a separation step of separating flocculated precipitate containing radioactive cesium from a supernatant solution in the contaminated water after the precipitation step. The flocculation precipitant contains silicon dioxide and alumina. Further, the flocculation precipitant preferably has an absolute value of zeta potential of 15 mV or more.

Description

  The present invention relates to a method for purifying contaminated water containing radioactive substances, and more particularly to a technique for removing radioactive cesium from contaminated water containing radioactive cesium.

  In the special decontamination area, radioactive contaminants containing radioactive cesium (mainly Cs134, Cs135, Cs137) adhering to the wall of the house or the surface of the tree are being removed by washing with water. Substance-containing contaminated water (hereinafter referred to as “radiated contaminated water”) is generated in large quantities.

  Radioactive water is allowed to be released into the environment by removing all radionuclides stipulated by law until each is below the emission threshold. For example, Patent Document 1 discloses a method for obtaining clean treated water by oxidizing a radioactive liquid waste containing organic matter, adding an adsorbent and then filtering.

  The cesium compound has a very high solubility in water and exists in an ionic state in water. In this state, it is adsorbed and held by an adsorbent such as zeolite or bitumen. Here, bitumen is a blue pigment mainly composed of ferric ferrocyanide and can be produced by adding iron sulfate and an oxidizing agent to a solution of potassium ferrocyanide.

  On the other hand, Patent Document 2 discloses a wastewater treatment method for treating wastewater containing oil such as heavy crude oil and sludge, COD components, and nitrogen components.

JP-A-7-260997 Japanese Patent Application Laid-Open No. 2004-275484

  When removing radioactive cesium with zeolite, an adsorption tower packed with zeolite is required. In addition, since zeolite has a low amount of adsorption of cesium per unit volume, a large amount of zeolite is required, and a large amount of radioactive waste is generated after treatment.

  On the other hand, when removing radioactive cesium by fine powder bitumen, the amount of radioactive waste is small because the amount of cesium adsorbed per unit volume of fine powder bitumen is high, but it is fine powder. Therefore, since it is dispersed in the liquid phase, it takes time for aggregation and precipitation. Therefore, there is a problem that it takes a lot of time to process a large amount of radioactively contaminated water.

  When fine bitumen adsorbed with radioactive cesium is separated and removed using a filter, since the bitumen has a very small particle size, it takes time for separation and a pressure operation is required. There is a problem that becomes complicated.

  In addition, when removing radioactive cesium using a material obtained by processing bitumen into a granular form or a material in which a bitumen is supported on a porous single body, these materials have a low cesium adsorption amount per unit volume. Further, there is a problem that the purification of radioactive contaminated water increases in cost and the amount of radioactive waste generated increases.

  The present invention is a method for purifying contaminated water containing radioactive cesium, an adsorption step in which ferrocyanide is mixed with the contaminated water, and the radioactive cesium is adsorbed on the ferrocyanide; A precipitation step for precipitating the ferrocyanide adsorbed with radioactive cesium using an agent; and a separation step for separating the aggregated precipitate containing radioactive cesium and the supernatant in the contaminated water after the precipitation step; And the coagulating precipitant comprises silicon dioxide and alumina.

  Moreover, it is preferable that the absolute value of a zeta potential is 15 mV or more as for the said aggregation precipitation agent.

  Further, in the precipitation step, it is preferable to promote the coagulation precipitation by mixing the coagulating precipitant at a mass ratio of 1 / 1,000,000 to 1/10 with respect to the contaminated water.

  Further, in the precipitation step, the aggregated precipitate and the supernatant liquid are mixed without stirring and dewatering treatment such as filtration, pressing, and centrifugation by mixing the aggregated precipitant with the contaminated water and stirring. It is preferable to make it separable.

  In the precipitation step, it is preferable to further mix an antifoaming agent to suppress the generation of bubbles and promote coagulation precipitation.

  Moreover, it is preferable that the aggregating precipitation agent has an absolute value of zeta potential of 30 mV or more.

  The method for purifying contaminated water containing radioactive cesium of the present invention adsorbs radioactive cesium using a ferrocyanide typified by bitumen, so that the amount of radioactive waste generated after treatment can be reduced as compared with the prior art, and By using a coagulating precipitation agent containing silicon dioxide and alumina, the ferrocyanide adsorbed with radioactive cesium can be rapidly coagulated and precipitated.

It is the schematic of the purification system of radioactive polluted water which concerns on this invention.

  In order to precipitate ferrocyanide adsorbed with radioactive cesium in a short time, the present inventors have conducted a number of tests using various aggregating precipitants. However, while conducting various tests with various devices added to the coagulating precipitant, using a specific coagulating precipitant, it was determined that the precipitation of ferrocyanide proceeded unexpectedly quickly, and the present invention was completed. .

  The purification system for radioactive contaminated water shown in FIG. 1 includes an adsorption step 10, a precipitation step 20, and a separation step 30, and can remove radioactive cesium from the radioactive contaminated water.

In the adsorption step 10, ferrocyanide is mixed with radioactively contaminated water, and radioactive cesium is adsorbed on the ferrocyanide. Here, the ferrocyanide can be produced by a reaction between potassium ferrocyanide and a metal salt. Here, various metal salts such as Ni (nickel), Co (cobalt), Fe (iron), and Zn (zinc) can be used as the metal salt. A typical ferrocyanide composition formula is Fe (III) ₄ [Fe (II) (CN) ₆ ] ₃ (bitumen) and the like. Examples of ferrocyanides that are relatively easily available at present include ferric ferrocyanide, copper ferrocyanide, and cobalt ferrocyanide, and these are mainly used as pigments.

  In the precipitation step 20, after the adsorption step 10, ferrocyanide having adsorbed radioactive cesium is precipitated in the radioactively contaminated water using an aggregating precipitant. Here, the coagulating precipitant contains silicon dioxide and alumina, and the absolute value of the zeta potential is 15 mV or more. If the zeta potential of the coagulating precipitant is 15 mV or more, the particles of the coagulating precipitant will not aggregate due to the repulsive force between the particles of the coagulating precipitant in the contaminated water, and it becomes easier to react with the ferrocyanide. It is possible to rapidly agglomerate and precipitate Russian compounds. Here, the value of the zeta potential can be adjusted by adjusting the contents of silicon dioxide and alumina.

  It should be noted that the zeta potential of the coagulating precipitant is basically preferably as large as possible. Specifically, if the zeta potential of the coagulating precipitant is 15 mV or more, a remarkable effect can be obtained, but it is preferably 20 mV or more, more preferably 25 mV or more, and further preferably 30 mV or more. On the other hand, if the zeta potential of the coagulating precipitant is excessively large, the effect is reduced. Therefore, the upper limit of the zeta potential of the coagulating precipitant is preferably 35 mV.

  The particle size of the coagulating precipitant is preferably an average particle size of 50 μm to 350 μm, more preferably 75 μm to 150 μm. If the average particle size is less than 50 μm, particles having a relatively large particle size must be pulverized to a fine particle size, resulting in higher production costs. If the average particle size exceeds 350 μm, the amount of aggregation per volume is increased. This is not preferable because it becomes low and a large amount of the coagulating precipitant needs to be mixed. Further, in order to increase the sedimentation rate of the aggregate and shorten the treatment time, it is preferable that the specific gravity of the aggregated precipitant is 1.5 or more.

  Here, the zeta potential is a measurable value defined for use as an index of dispersion stability of dispersed particles, and can be measured using an electrophoretic light scattering measurement method.

  Here, the electrophoretic light scattering measurement method refers to applying an electric field to a system in which charged particles are dispersed, irradiating particles moving in the electric field with laser light, and using the Doppler effect of the light, the frequency of the scattered light. The speed of the particles is obtained by measuring the shift amount. From the velocity of the particles obtained by the electrophoretic light scattering measurement method and the strength of the applied electric field, the zeta potential can be calculated by the following calculation.

When a sample dispersed in a solvent having a refractive index n is irradiated with laser light having a wavelength λ and the scattered light is detected at a scattering angle θ, the relationship between the migration velocity V and the Doppler shift amount Δv is expressed by (Equation 1). It is represented by
Δv = {2Vnsin (θ / 2)} / λ (Formula 1)
Here, n is the refractive index of the solvent, and θ is the detection angle (scattering angle).
From the migration velocity V and the electric field E obtained by Equation 1, the electric mobility U is obtained by Equation (2).
U = V / E (Formula 2)
The zeta potential ζ is obtained from the electric mobility U by (Equation 3).
ζ = 4πηU / ε (Formula 3)
Here, η is the viscosity of the solvent, and ε is the dielectric constant of the solvent.

  In the separation step 30, after the precipitation step 20, the aggregated precipitate containing radioactive cesium and the supernatant liquid purified by removing the radioactive cesium are separated from the radioactive contaminated water using a sedimentation concentration device such as thickener. . For example, radioactive contaminated water is supplied to the thickener, aggregated sediment containing radioactive cesium is taken out as a slurry from the outlet at the center of the bottom of the thickener, and the supernatant liquid overflowing from the outer periphery of the upper portion of the thickener is discharged.

  In addition, in the precipitation step 20, it is preferable to promote the aggregation precipitation by mixing the aggregation precipitation agent in a mass ratio of 1 / 1,000,000 to 1/10 with respect to the radioactive polluted water.

  Further, in the separation step 30, a dehydration process such as filtration, pressing, and centrifugation may be performed.

  In the precipitation step 20, the aggregated precipitant is mixed and stirred in the radioactively contaminated water, and in the separation step 30, it is allowed to stand for a certain period of time without performing dehydration such as filtration, pressing, and centrifugation, and decantation. It is also possible to separate the aggregated precipitate and the supernatant liquid simply by doing so.

  Furthermore, in the precipitation step 20, an antifoaming agent can be mixed to suppress the generation of bubbles and promote aggregation precipitation.

  The present invention is also effective for decontamination of radioactive Sr (strontium) and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
In the adsorption step 10, ferrocyanide (ferric ferrocyanide, which is a kind of bitumen: Fe (III) ₄ [Fe (II) (CN) ₆ ] ₃ ) is mixed at a rate of 250 mg to 1 L of radioactive polluted water. Then, radioactive cesium was adsorbed on the ferrocyanide to obtain dark blue radioactively contaminated suspended water.

  In the precipitation step 20, 80 mL (100 Bq / kg) of radioactively contaminated suspended water after the adsorption step is taken, and a powder of the aggregated precipitant having an absolute value of zeta potential of about 32 mV mainly composed of silicon dioxide and alumina is added to the radioactivity. It was added at a rate of 2 g with respect to 1 L of contaminated suspended water and stirred vigorously for about 30 seconds. Immediately, aggregation and precipitation occurred strongly, and after about 1 minute, the supernatant liquid became colorless and transparent.

  In the separation step 30, the supernatant was collected by decantation. When the radiation dose of the supernatant was measured, it was below the detection limit (6 Bq / kg or less).

  The supernatant is ICP-AES (ICP emission spectroscopy, the sample solution is atomized and introduced into Ar plasma, and the light emitted when the excited element returns to the ground state is spectroscopically analyzed. As a result, the Fe from bitumen was below the detection limit (7 ppb or less), and Cu, Pb, Cd, and Zn were also below the detection limit.

(Comparative Example 1)
On the other hand, in the same manner as in Example 1 above, instead of the powder of coagulating precipitant having an absolute value of zeta potential of about 32 mV mainly composed of silicon dioxide and alumina used in Example 1, general-purpose coagulant is generally used. PAC-250A (polyaluminum chloride liquid products, the general formula _{[Al 2 (OH) n Cl} 6-n] m) used in manner was used.

  PAC-250A was used as a general flocculant at a rate of 2 g with respect to 1 L of radioactively contaminated suspended water after the adsorption step.

  Since 30 minutes after the adsorption step and the precipitation step, the supernatant liquid remained dark blue, no ferrocyanide precipitation was observed, and the radioactively contaminated suspended water remained suspended. In the separation step 30, the supernatant liquid could not be collected.

(Example 2)
In the adsorption step 10, 80 g of suspended water (500 Bq / kg) containing radioactively contaminated soil with a mass ratio of 5% is taken as a sample, and the ferrocyanide used in Example 1 is in a ratio of 250 mg to 1 L of the suspended water. Added and shaken.

  In the precipitation step 20, a powder of a coagulating precipitant having an absolute value of zeta potential of about 32 mV mainly composed of silicon dioxide and alumina is added at a rate of 100 mg to 1 L of suspended water after the adsorption step, and stirred vigorously for about 30 seconds. did. Aggregation precipitation immediately occurred, and after about 2 minutes, the supernatant became colorless and transparent.

  In the separation step 30, the supernatant was collected by decantation. When the radiation dose of the supernatant was measured, it was below the detection limit. When the supernatant was measured by ICP-AES, Fe derived from bitumen was below the detection limit, and Cu, Pb, Cd, and Zn were also below the detection limit.

  As can be seen from Examples 1 and 2, according to the method for purifying contaminated water containing radioactive cesium of the present invention, since radioactive cesium is adsorbed using ferrocyanide, the amount of radioactive waste generated after treatment is reduced. In addition, the coagulating and precipitating agent mainly composed of silicon dioxide and alumina can be used to rapidly coagulate and precipitate the compound adsorbed with radioactive cesium.

1 Purification system of radioactive contaminated water 10 Adsorption step 20 Precipitation step 30 Separation step

Claims (6)

A method for purifying contaminated water containing radioactive cesium,
An adsorption step of mixing ferrocyanide with the contaminated water and adsorbing radioactive cesium to the ferrocyanide;
In the contaminated water, a precipitation step of precipitating the ferrocyanide adsorbed with radioactive cesium using a coagulating precipitant;
A separation step of separating the aggregated precipitate containing radioactive cesium and the supernatant in the contaminated water after the precipitation step;
The said coagulating precipitation agent contains a silicon dioxide and an alumina, The purification method characterized by the above-mentioned.   The purification method according to claim 1, wherein the coagulating precipitant has an absolute value of zeta potential of 15 mV or more. In the precipitation step,
The purification method according to claim 1 or 2, wherein the aggregated precipitant is mixed with the contaminated water at a mass ratio of 1 / 1,000,000 to 1/10. In the precipitation step,
The purification method according to any one of claims 1 to 3, wherein the aggregated precipitant is mixed and stirred with respect to the contaminated water. In the precipitation step,
The purification method according to any one of claims 1 to 4, wherein an antifoaming agent is mixed.   The purification method according to any one of claims 1 to 5, wherein the coagulating precipitant has an absolute value of zeta potential of 30 mV or more.

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