JP6028545B2 - How to recover cesium - Google Patents

Description

  The present invention relates to a method for recovering cesium using Prussian blue nanoparticles, and more particularly to a method suitable for recovering radioactive cesium dissolved in water.

  As a method of recovering radioactive cesium dissolved in water, a method of recovering with an adsorbent can be mentioned. At present, various materials including zeolite are studied as adsorbents, and some are used in actual decontamination sites. It is desirable that the adsorbent has a high adsorbing ability capable of adsorbing a large amount of cesium so that the amount of waste after decontamination is small. In addition, in the environment, a much larger amount of other metal ions than radioactive cesium is present, so the adsorbent is required to have a highly selective adsorption ability for adsorbing only cesium.

  It has been reported that cesium tends to elute into water from municipal waste incineration fly ash. The ability to adsorb cesium from incinerated ash washing water of bentonite, which is well known to adsorb radioactive cesium ions well in pure water, is significantly lower than that of pure water, about 1/100 (Ministry of the Environment) (Fifth Disaster Waste Safety Evaluation Review Material). One reason for this is that bentonite adsorbs not only cesium but also other ions in the incinerated ash washing water.

  Prussian blue is a candidate for a highly selective cesium adsorbent capable of highly efficient recovery of cesium. Prussian blue has been given to livestock during the Chernobyl nuclear power plant accident to reduce cesium in milk.

  The reason why Prussian blue has cesium selective adsorption properties is considered to be because the hydration radius of cesium ions matches the size of the internal pores of Prussian blue.

  Patent Document 1 describes a radioactive cesium adsorbent composed of Prussian blue nanoparticles and a method for separating radioactive cesium using the same.

In addition, the present applicant has made a presentation on radioactive cesium decontamination using Prussian blue nanoparticles (August 24, 2011, August 31, 2011, AIST press release). It was 5-10 nanometers (nm) when the primary particle diameter of this Prussian blue nanoparticle for this cesium adsorbent was estimated from the result of the powder X-ray diffraction. Moreover, it was recognized from an electron microscope image that the primary particle size is about 10 nm. In general, it is believed that the adsorption capacity increases as the specific surface area increases. The specific surface area determined by the BET method of the nanoparticle adsorbent granulated to about 60 μm is 390 m ² / g, which is much larger than the value (about 100 m ² / g) reported as a conventional Prussian blue.

JP 2011-200856 A

  As described above, Prussian blue nanoparticles have a large amount of cesium adsorption and are excellent in selective adsorption of cesium. When these Prussian blue nanoparticles are supplied into cesium-containing water, cesium ions are selectively absorbed. And it is rapidly adsorbed on Prussian blue nanoparticles. Therefore, if the Prussian blue nanoparticles having adsorbed radioactive cesium ions are separated from water, the water contaminated with radioactive cesium can be decontaminated. However, since the Prussian blue nanoparticles are nano-sized, solid-liquid separation processing is not easy.

  There is also a need for a technique for reducing the volume of Prussian blue nanoparticles that have adsorbed radioactive cesium ions.

  An object of the present invention is to provide a method for recovering cesium that easily and efficiently decontaminates radioactive cesium, and further reduces the amount of waste.

  The method for recovering cesium of the present invention includes an adsorption step in which Prussian blue nanoparticles are added to a liquid to be treated containing cesium ions, and cesium ions are adsorbed on the Prussian blue nanoparticles, and then the Prussian blue nanoparticles are contained. Filtering the liquid by a filtering means having a deposited layer of Prussian blue nanoparticles on the filter medium.

  In the present invention, it is preferable to circulate the Prussian blue nanoparticle-containing liquid through the filter medium to form the attached deposit layer on the filter medium.

  After the adsorption step, the Prussian blue nanoparticle-containing liquid is centrifuged to separate a sediment and a supernatant, and the supernatant is circulated through the filter medium to deposit the deposited deposit on the filter medium. A layer may be formed. In this case, it is preferable to return the sediment to the adsorption step.

  The primary particle size of the Prussian blue nanoparticles is preferably 50 nm or less.

The filter medium is preferably a synthetic resin fiber woven fabric. In this case, the air permeability of the woven fabric is preferably 0.1 to ⁵ cm ³ / cm ² · sec.

  When Prussian blue nanoparticles are added to the liquid to be treated containing cesium ions, the cesium ions are quickly adsorbed on the Prussian blue nanoparticles. The liquid containing Prussian blue nanoparticles having adsorbed cesium ions is filtered with a filter medium having an attached deposit layer of Prussian blue nanoparticles. Prussian blue nanoparticles have a very small particle size and are hardly separated by the filter medium alone, but are sufficiently trapped in the deposited layer of Prussian blue nanoparticles.

  By circulating the Prussian blue nanoparticle-containing liquid through a filtration means having an attached deposit layer, filtered water having a significantly low Prussian blue concentration can be obtained. Since Prussian blue nanoparticles are remarkably excellent in the selective adsorption property of cesium, the concentration of radioactive cesium in the filtered water is remarkably low.

  The adhesion deposit layer is formed by passing and depositing the Prussian blue nanoparticle-containing liquid through the filter medium, so that secondary particles having a relatively large particle diameter adhere and deposit on the filter medium. That is, the Prussian blue nanoparticles have a very small particle size and pass through the filter medium almost through, but some of the secondary particles having a large particle size are trapped by the filter medium. When the liquid flow is continued, particles with a smaller particle diameter gradually become trapped, and the amount of Prussian blue nanoparticles trapped gradually increases. Finally, Prussian blue nanoparticles adhere to the filter medium. A deposit layer is formed. According to this deposit layer, Prussian blue nanoparticles having a small particle diameter are efficiently captured.

  In the present invention, the Prussian blue nanoparticle-containing liquid is first centrifuged, and only the supernatant is passed through a filtration means having an attached deposit layer, whereby filtration can be performed efficiently. About the sediment of centrifugation, when the cesium adsorption capacity remains in Prussian blue, it can return to an adsorption process and can be used for adsorption of radioactive cesium. In this way, Prussian blue nanoparticles containing a high concentration of radioactive cesium are recovered. However, even if the cesium adsorption capacity remains, it may be taken out of the system and stored without being reused.

  In the present invention, it is preferable that the supernatant is passed through a filter medium having no or almost no deposit deposit layer to form the deposit deposit layer on the filter medium.

  If the deposited layer of Prussian blue nanoparticles deposits thickly, the filter medium is backwashed. A large amount of radioactive cesium can be adsorbed on Prussian blue nanoparticles by returning the backwash waste water to the adsorption step. However, the backwash waste water may be centrifuged to form a dehydrated cake containing radioactive cesium.

  Thus, according to the present invention, a large amount of cesium can be adsorbed to Prussian blue nanoparticles. Since this Prussian blue nanoparticle has a large amount of cesium adsorption, the amount (volume) is remarkably small compared to the stock solution (radioactive waste liquid). Therefore, storage of radioactive cesium adsorption Prussian blue nanoparticles is easy.

  In addition, Prussian blue nanoparticles are excellent in selective adsorption of cesium and have a large amount of saturated adsorption of cesium, so that cesium can be selectively concentrated and recovered at a high level, and the concentration of cesium in treated water is extremely low. It will be a thing.

It is sectional drawing explaining the filtration method. It is a flowchart explaining the method of this invention. It is a crystal structure diagram of Prussian blue.

  Hereinafter, the present invention will be described in detail. In the following description, Prussian blue nanoparticles may be simply referred to as nanoparticles.

  The liquid to be treated by the method of the present invention contains cesium, particularly radioactive cesium. The liquid to be treated includes nuclear power plant facilities contaminated with radioactive materials, lake water in contaminated areas, river water, ground water, pooled water such as pools, decontamination wastewater, and radioactive material contaminated soil. Examples include acid extraction water, waste incineration ash washing waste water, and the like. These liquids to be treated contain various metal ions and solids in addition to cesium. The cesium concentration of the liquid to be treated is not particularly limited, and it can be processed from a low concentration contaminated water of about 100 Bq / L to a high concentration contaminated water of about 100,000 Bq / L.

  Prior to the cesium adsorption treatment by adding Prussian blue nanoparticles, it is desirable to remove solids from the liquid to be treated by filtration, centrifugation, or the like. The radioactive cesium is ionized and dissolved, and the amount of attached radioactive cesium of the removed solid matter is very small.

As described above, Prussian blue nanoparticles are added to the liquid to be treated after the solid removal treatment as necessary, and cesium is adsorbed. Prussian blue nanoparticles having a primary particle diameter (average particle diameter) of 50 nm or less and a secondary particle diameter (aggregation diameter) (average particle diameter) of about 5 nm to 1 mm are cesium adsorption performance, and an attached sediment layer. Although preferable from the viewpoint of forming ability, pigments having a large primary particle size and a secondary particle size of about 10 to 100 μm, so-called “bitumen” or the like can also be used. (Measurement method (primary particle size): measured with an X-ray diffractometer, and a value obtained by calculating a crystal lattice diameter from a diffraction peak. Measurement method (secondary particle size): measured with a laser diffraction / scattering particle size distribution analyzer Value.)
The specific surface area measured by the BET method using nano-particle N ₂ gas is preferably about 150 to 2500 m ² / g in terms of adsorption capacity and handling. A suitable composition of the Prussian blue nanoparticles will be described later. The amount of Prussian blue nanoparticles added is preferably about 0.2 to 10 kg / m ^3, particularly about 1 to 5 kg / m ³ . As the cesium concentration in the liquid to be treated is higher, it is preferable to increase the amount of Prussian blue nanoparticles added within the above range.

  Since the Prussian blue nanoparticles have a very high cesium adsorption rate, the adsorption is completed within about 0.1 to 1 hr after the Prussian blue nanoparticles are added to the liquid to be treated. Therefore, the nanoparticle-added water is preferably centrifuged by a decanter or the like, and separated into a sediment and a supernatant. When a sufficient adsorption capacity remains in the Prussian blue nanoparticles in the sediment, it is preferable to return to the adsorption step and reuse (add to the liquid to be treated). If Prussian blue nanoparticles are close to saturated adsorption, the sediment is taken out of the system and stored.

  The supernatant from this centrifugation is subjected to a filtration step. In the filtration step, the supernatant is passed through the filter medium. Preferably, the supernatant is circulated and filtered. Centrifugation is not essential, and the additive solution of Prussian blue nanoparticles may be directly used in the filtration step.

  An example of this filtration system is shown in FIG. 1, and an example of the processing flow is shown in FIG. The liquid in the tank 1 (the liquid added with Prussian blue or the supernatant obtained by centrifuging it) is fed by the pipe 2 and the pump 3, and enters the shell (outer shell) 6 of the filter 5 from the inlet 4. Inflow. In the shell 6, a cylindrical support body 8 having a large number of holes 8 a and a filter medium 7 surrounding the outer periphery of the cylindrical support body 8 are installed. The cylindrical support 8 may be a punching plate or a lattice having rods arranged in the longitudinal and circumferential directions, but is not limited thereto.

The filter medium 7 is preferably made of a porous cloth, sheet or film, among which a woven cloth of synthetic resin fibers having a thickness of about 0.5 to 1.2 mm, particularly about 0.9 to 1 mm is preferable. It is. As the synthetic resin, polypropylene, polyphenylene sulfide (PPS), polyester, polytetrafluoroethylene, or the like can be used, but is not limited thereto. Examples of the woven fabric include plain weave, satin weave and twill weave, but are not limited thereto. The air permeability of the woven fabric is preferably about 0.1 to ⁵ cm ³ / cm ² · sec, particularly about 0.3 to 1.3 cm ³ / cm ² · sec.

  The liquid flows into the cylindrical support 8 through the filter medium 7 and the hole 8a, then flows into the header chamber 10 through the communication port 9, and is returned to the tank 1 through the outlet 11 and the pipe 12. .

  At the beginning of the filtration operation, the deposited sediment layer M is not formed, and most of the Prussian blue nanoparticles in the liquid pass through the filter medium 7 in a straight line. The secondary particles are trapped by the filter medium 7 and the trapping amount gradually increases. As a result, particles having a relatively small particle diameter are also trapped, and finally, the deposited deposit layer M is formed. When the deposited deposit layer M is formed, Prussian blue nanoparticles having a small particle size are also captured by the deposited deposit layer M, and the thickness of the deposited deposit layer M increases. In addition, when filtering after filtering the liquid from a filtration process, it is preferable to use a thing with a small opening as the filter medium 7 compared with the case where it filters directly without centrifugation.

  When the thickness of the deposit layer M attached to the filter medium 7 exceeds a predetermined value, the introduction of the liquid into the filter 5 is stopped, and the filter medium 7 is back-washed with a gas such as water or air. When water or air or the like is supplied to the header 9 by a backwash line (not shown), the adhered deposit layer adhering to the filter medium 7 is peeled off and falls in the shell 6. When the bottom discharge valve 15 is opened, the slurry containing the deposited deposit layer flows out from the discharge port 14. When the residual adsorption capacity of Prussian blue nanoparticles in the slurry is large, the slurry is returned to the cesium adsorption step. When the cesium adsorption amount of Prussian blue nanoparticles is close to saturation adsorption, or when storing Prussian blue with a small amount of cesium adsorption, the slurry of the adsorption process is centrifuged to form a dehydrated cake and stored in a storage container etc. And store.

  For storage of Prussian blue nanoparticles adsorbed with cesium, it is preferable to use a sealed container made of a corrosion-resistant material such as stainless steel, and automatic filling into the sealed container is also possible.

  After the backwashing of the filter medium 7 is completed, the liquid is circulated through the filter 5. Also in this case, when the liquid passage is started, first, secondary particles having a large particle size are captured by the filter medium 7 to form an adhering deposit layer M, and then Prussian blue nanoparticles having a small particle size are also captured and adhering deposit. The layer thickness of the layer M increases.

  In addition, since the actual volume of the tank 1 is larger than the volume of the filter 5, a plurality of, for example, two filters 5 are connected in parallel to the tank 1, and filtration is performed by some of the filters 5. Sometimes it is preferred to backwash with another filter.

  When the Prussian blue nanoparticle concentration in the liquid circulating from the tank 1 through the pipe 2, the filter 5, and the pipe 12 becomes a predetermined concentration or less, filtered water is taken out from the system through the outlet 16.

  In the present invention, a filter aid may be used.

  FIG. 3 shows the crystal structure of Prussian blue. Prussian blue has a cubic lattice shape and a lattice constant of about 0.5 nm. Since the hydrated cesium ion has a size that almost completely enters this lattice, Prussian blue selectively adsorbs cesium.

  By using Prussian blue as nanoparticles, the adsorption rate of cesium is increased. In addition, since the distance that cesium ions entering the lattice gap of Prussian blue nanoparticles diffuse and move to the core of the particles is short, almost all of the Prussian blue nanoparticles are used for adsorption of cesium, and quickly become almost saturated. Cesium can be adsorbed until

The chemical formula of Prussian blue is represented by Fe (III) ₄ [Fe (II) (CN) ₆ ] ₃ , but Prussian blue used in the present invention contains water of crystallization, or some iron ions may be One or two metals selected from the group consisting of metals such as vanadium, chromium, manganese, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium It may be a Prussian blue-type metal complex represented by the following formula, which is substituted with a metal of more than one species.
_{A x M A [M B (} CN) 6] y · zH 2 O
A is an atom derived from a cation. x is 0 to 2, y is 1 to 0.3, and z is 0 to 20. M _{A, M B} is a metal atom.

  In order to produce Prussian blue nanoparticles, for example, an aqueous solution of iron nitrate is added in an equimolar amount to an aqueous solution of sodium ferrocyanide, stirred and reacted, and the produced Prussian blue is precipitated by centrifugation, and then water is added as necessary. Wash with methanol or the like. However, the manufacturing method of Prussian blue is not limited to this. The primary particles of the precipitate are nanoparticles having a particle size of 50 nm or less, for example, an average particle size of 10 to 20 nm. The particle size of the nanoparticles is a value measured from an image taken with a transmission electron microscope.

  When added to the liquid to be treated in the adsorption step, it is preferably added in the form of a dispersion, a slurry, a water-containing solid, etc., in order to prevent Prussian blue from scattering, but it is added as a dried product of Prussian blue nanoparticles. Also good. Even if Prussian blue is added to the liquid to be treated in any form, Prussian blue immediately disperses in the liquid and adsorbs cesium.

[Example 1]
<Preparation of Prussian blue nanoparticles>
An aqueous solution in which 14.5 g of sodium ferrocyanide decahydrate was dissolved in 60 mL of water was mixed with 30 mL of an aqueous solution in which 16.2 g of iron nitrate nonahydrate was dissolved in water and stirred for 5 minutes. The precipitated blue Prussian blue precipitate was centrifuged, washed 3 times with water and then once with methanol and dried under reduced pressure. The yield of this time was 11.0 g, the yield was _{Fe 4 [Fe (CN) 6} ] 3 · 15H 97.4% as ₂ O.

As a result of analyzing the produced Prussian blue with a powder X-ray diffractometer, it was consistent with Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) retrieved from the standard sample database. Also in the FT-IR measurement, a peak due to Fe-CN stretching vibration appeared in the vicinity of 2080 cm ^-1 , indicating that this solid was Prussian blue. This Prussian blue had a BET specific surface area of 300 m ² / g, an average primary particle size of 16 nm, and an average secondary particle size of 55 μm.

<Preparation of simulated treatment liquid>
Cesium carbonate was dissolved in pure water and hydrochloric acid was added to prepare a cesium solution having a pH of 6 and a cesium concentration of 100 ppb.

<Filter media>
A cylindrical bag (diameter 80 mm, length 240 mm) of polypropylene woven fabric (plain weave, air permeability 0.4 cm ³ / cm ² · sec, thickness 1 mm) was used as the filter medium.

<Adsorption treatment>
40 g of the Prussian blue was added to 50 L of the liquid to be treated and stirred for 10 minutes to adsorb cesium.

<Filtration treatment>
This Prussian blue additive solution was passed through the woven fabric with a pump at a pressure of 0.3 MPa gauge at 0.52 L / min for one pass. At the beginning of water flow, Prussian blue was transparent, but the formation of an adhering sediment layer was recognized from about 5 minutes, and the color of the filtrate began to fade with this, and after 50 minutes, the filtrate became colorless and transparent. . The water flow rate after 50 minutes decreased to 0.295 L / min, but after 100 minutes, the decrease in water flow rate decreased to 0.255 L / min.

When the Prussian blue content in the filtrate was measured,
After 0 min: Prussian blue content 800ppm
After 15 min: Prussian blue content 50ppm
After 30 min: Prussian blue content 10 ppm
After 50 min: Prussian blue content 0 ppm
After 70 min: Prussian blue content 0 ppm
After 100 min: Prussian blue content 0 ppm
Met.

From this experiment, it was confirmed that cesium in the cesium aqueous solution can be almost completely removed. A total of 30 L of water was passed through this filter medium, and 13 L of colorless and transparent water was obtained during 50 min from the time of 50 min to the time of 100 min. The volume of the deposited deposit formed by treating 30L of the liquid to be treated was 19 cm ³ . This is 1/1573 of the original aqueous solution 30L, meaning that cesium is concentrated 1573 times.

  In addition, the 5000 ppm slurry of Prussian blue nanoparticles was dehydrated with a centrifugal separator to obtain a 50 ppm slurry supernatant and a dehydrated cake having a water content of 90% by weight or less.

1 tank 5 filter 7 filter medium 8 cylindrical support

Claims (11)

An adsorption step of adding Prussian blue nanoparticles to the liquid to be treated containing cesium ions, and adsorbing the cesium ions to the Prussian blue nanoparticles;
Thereafter, the Prussian blue nanoparticle-containing liquid is filtered by a filtering means having a deposited deposit layer of Prussian blue nanoparticles on the filter medium;
A method for recovering cesium having   2. The method for recovering cesium according to claim 1, wherein the Prussian blue nanoparticle-containing liquid is passed through the filter medium to form the attached deposit layer on the filter medium.   3. The method for recovering cesium according to claim 1, wherein the Prussian blue nanoparticle-containing liquid is circulated through the filter medium.   In Claim 1, the said Prussian blue nanoparticle containing liquid is centrifuged and isolate | separated into a sediment and a supernatant liquid, This supernatant liquid is passed through the said filter medium, and the said adhesion deposit layer is formed on this filter medium. A method for recovering cesium, characterized by comprising:   5. The method for recovering cesium according to claim 4, wherein the supernatant is circulated through the filter medium.   6. The method for recovering cesium according to claim 4 or 5, wherein the sediment is returned to the adsorption step.   The method for recovering cesium according to any one of claims 1 to 6, wherein the filtering means is backwashed, and the backwash waste water is returned to the adsorption step.   The method for recovering cesium according to any one of claims 1 to 6, wherein the filtration means is back-washed, and the backwash waste water is centrifuged to obtain a dehydrated cake of cesium-adsorbed Prussian blue nanoparticles.   The method for recovering cesium according to any one of claims 1 to 8, wherein the primary particle size of the Prussian blue nanoparticles is 50 nm or less.   The method for recovering cesium according to any one of claims 1 to 9, wherein the filter medium is made of a woven fabric of synthetic resin fibers. The method for recovering cesium according to claim 10, wherein the air permeability of the woven fabric is 0.1 to ⁵ cm ³ / cm ² · sec.

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