JP7641760B2 - Manufacturing method of carbon-based nonwoven fabric for cesium adsorption and cesium adsorbent - Google Patents

Description

The present invention relates to a method for producing a carbon-based nonwoven fabric for cesium adsorption and a cesium adsorbent. More specifically, the present invention relates to a method for producing a carbon-based nonwoven fabric for cesium adsorption having cesium adsorption performance and a cesium adsorbent.

The accident at Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant in 2011 released a large amount of radioactive material into the environment. Among the radioactive materials released, cesium-134 (Cs-134) and cesium-137 (Cs-137), which are the products of nuclear reactions of uranium, were released in large quantities and have long half-lives of 2.1 years and 30 years, respectively. Furthermore, these cesium species have a high tendency to bind to soil, so they remain in the environment for a long time.

Since this cesium behaves in a similar way to potassium, which is essential for living organisms, and is expected to have an impact on ecosystems, it is necessary to remove it from the environment.

A method of using zeolite as an inorganic ion exchanger has been known for removing cesium. Specifically, there is a method of mixing zeolite with water containing cesium (the target for removing cesium; cesium ions) and then obtaining a precipitate containing cesium, or a method of passing water through a cylinder filled with zeolite. The contaminated water treatment device "SARRY" (manufactured by Toshiba Energy Systems & Solutions Corporation) used at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant employs a method of passing water through a cylinder filled with zeolite.

Methods other than those using zeolite are known, such as those described in Patent Documents 1 and 2. Specifically, these include a method using a PVA membrane carrying a cation exchange resin or adsorbent, and a method using a cation exchange membrane with an exchange group introduced. Furthermore, a method is being explored in which Prussian blue, a dye that specifically adsorbs to cesium ions, is used directly or after being adsorbed onto fibers, etc.

JP 2014-108379 A International Publication No. 2014/010417

However, in the removal method using zeolite, it takes time and effort to precipitate the cesium-containing zeolite (to obtain a precipitate), and then to dehydrate and remove the precipitate. Furthermore, used zeolite tends to be large in volume, which can lead to issues such as the need to secure space for waste management and the associated costs, leaving room for improvement.

In addition, methods that use polyvinyl alcohol (PVA) membranes or cation exchange membranes tend to release harmful substances when they are incinerated for disposal, and there is still room for improvement in terms of ease of disposal. Methods that use Prussian blue have issues, such as the difficulty of separating the substance (cesium-Prussian blue complex) from the free form (cesium ions and Prussian blue) after adsorption.

The present invention was made in consideration of such conventional techniques, and its objective is to provide a method for producing a carbon-based nonwoven fabric for cesium adsorption that can recover radioactive cesium from the environment and is easy to dispose of, and a cesium adsorbent.

The present invention provides a method for producing a carbon-based nonwoven fabric for cesium adsorption, and a cesium adsorbent, as shown below.

[1] A method for producing a carbon-based nonwoven fabric for cesium adsorption, which is obtained by immersing a carbon fiber nonwoven fabric in a surfactant solution containing a surfactant and then drying it.

[2] The method for producing a carbon-based nonwoven fabric for cesium adsorption described in [1] above, in which the surfactant contained in the surfactant solution is polyoxyalkylene lauryl ether.

[3] The method for producing a carbon-based nonwoven fabric for cesium adsorption described in [1] or [2] above, in which the concentration of the surfactant contained in the surfactant solution is 0.05 to 5 mass %.

[4] A method for producing a carbon-based nonwoven fabric for cesium adsorption according to any one of [1] to [3], in which the nonwoven fabric is made of carbon fibers that have been treated to be water-repellent with a first water-repellent resin.

[5] The method for producing a carbon-based nonwoven fabric for cesium adsorption described in [4] above, in which the nonwoven fabric is made by coating the carbon fiber with a coating agent containing a second water-repellent resin and a conductive material after the carbon fiber has been treated to be water-repellent with the first water-repellent resin.

[6] The first water-repellent resin and the second water-repellent resin are each a fluororesin,
The method for producing a carbon-based nonwoven fabric for cesium adsorption according to [5] above, wherein the conductive material is carbon black.

[7] A method for producing a carbon-based nonwoven fabric for cesium adsorption, comprising: preparing a carbon fiber nonwoven fabric; and subjecting the carbon fibers of the nonwoven fabric to a water-repellent treatment using a first water-repellent resin to obtain a carbon-based nonwoven fabric for cesium adsorption.

[8] The method for producing the carbon-based nonwoven fabric for cesium adsorption described in [7] above, in which the nonwoven fabric is made by subjecting the carbon fibers to the water-repellent treatment and then coating the carbon fibers with a coating agent containing a second water-repellent resin and a conductive material.

[9] A nonwoven fabric made of carbon fiber;
A cesium adsorbent comprising: a surfactant attached to the surface of the nonwoven fabric.

[10] The cesium adsorbent according to [9], wherein the surfactant is polyoxyalkylene lauryl ether.

The method for producing a carbon-based nonwoven fabric for cesium adsorption of the present invention has the effect of producing a carbon-based nonwoven fabric for cesium adsorption (cesium adsorbent) that can recover radioactive cesium from a nuclear reactor or the environment and is easy to dispose of when disposed of.

The cesium adsorbent of the present invention has the advantage that it can recover radioactive cesium from a nuclear reactor or the environment, and can be easily disposed of.

FIG. 1 is an explanatory diagram showing a schematic diagram of an apparatus used for electrochemical measurement in which a carbon-based nonwoven fabric manufactured by the method for manufacturing a carbon-based nonwoven fabric for cesium adsorption of the present invention is arranged. FIG. 2 is a shading diagram illustrating the carbon-based nonwoven fabric in the device shown in FIG. 1 and solution phases of different concentrations separated by the carbon-based nonwoven fabric. 1 is a graph showing the measurement results of the effect of the coexistence of inorganic ions on the critical micelle concentration of a surfactant (polyoxyalkylene lauryl ether) used in the manufacturing method of the carbon-based nonwoven fabric for cesium adsorption of the present invention. 1 is a chart showing the results of mass spectrometry (Time-Of-Flight mass spectrometry; TOF-MS) of each carbon-based nonwoven fabric.

The following describes the mode for carrying out the present invention, but the present invention is not limited to the following embodiment. In other words, it should be understood that modifications and improvements to the following embodiment, based on the ordinary knowledge of a person skilled in the art, as long as they do not deviate from the spirit of the present invention, also fall within the scope of the present invention.

(1) Manufacturing method of carbon-based nonwoven fabric for cesium adsorption (first embodiment):
One embodiment of the method for producing a carbon-based nonwoven fabric for cesium adsorption of the present invention is a production method in which a nonwoven fabric made of carbon fibers is immersed in a surfactant solution containing a surfactant, and then dried to obtain a carbon-based nonwoven fabric for cesium adsorption (cesium adsorbent).

The method for producing a carbon-based nonwoven fabric for cesium adsorption of the present invention makes it possible to recover radioactive cesium from the environment, and to produce a carbon-based nonwoven fabric for cesium adsorption that is easy to dispose of (specifically, does not require burial, and does not emit harmful substances such as dioxins when incinerated, making disposal easy).

(1-1) Carbon fiber nonwoven fabric:
The carbon fiber nonwoven fabric used in the present invention is a web or sheet composed of only carbon fibers, or carbon fibers that have been subjected to a water-repellent treatment or a coating treatment after the water-repellent treatment. In such a nonwoven fabric, the carbon fibers (i.e., filaments, which are a plurality of single fibers) are gathered together to form a strand, which is a bundle. Therefore, the nonwoven fabric has a three-dimensional net shape, and a plurality of pores are formed. Because of this structure, the nonwoven fabric can be used as a filter for removing desired fine particles, etc.

The length of the carbon fibers is not particularly limited, but can be, for example, about 30 to 100 mm. The length of the carbon fibers is the average length of 100 carbon fibers randomly selected from a carbon fiber nonwoven fabric.

There are no particular limitations on the thickness of the carbon fibers, but it can be, for example, about 5 to 30 μm. The thickness of the carbon fibers is the average value of 100 carbon fibers randomly selected from a carbon fiber nonwoven fabric, and is the average value of the thickness of a single fiber (filament) (if the thickness of a single fiber is not constant, it is the thickness of the thinnest part).

The thickness of the nonwoven fabric is not particularly limited and can be set according to the application. For example, it is preferably about 100 to 500 μm. When two or more sheets of carbon fiber nonwoven fabric are laminated, the thickness refers to the thickness of the laminate in which two or more sheets of carbon fiber nonwoven fabric are laminated. In other words, it is preferable that the overall thickness of the nonwoven fabric that is the filter falls within the above range.

The basis weight of the nonwoven fabric is not particularly limited and can be, for example, about 40 to 400 g/ ^m2 .

The average pore diameter of the nonwoven fabric is preferably about 0.01 to 500 μm, more preferably about 0.02 to 300 μm, and even more preferably about 0.03 to 100 μm. If the average pore diameter is within the above range, a nonwoven fabric with better cesium adsorption capacity can be obtained. The average pore diameter of the nonwoven fabric is a value measured by mercury intrusion porosimetry.

The nonwoven fabric may be carbon fiber that has been treated to be water-repellent with a first water-repellent resin. By using a nonwoven fabric made of carbon fiber that has been treated to be water-repellent in this way, it is possible to produce a carbon-based nonwoven fabric with improved cesium adsorption capacity.

The first water-repellent resin can be a fluororesin. The use of a fluororesin provides chemical stability to the entire membrane and can improve the cesium adsorption capacity.

As a nonwoven fabric, carbon fibers may be used that have been treated to be water repellent with a first water repellent resin, and then coated with a coating agent containing a second water repellent resin and a conductive material (MPL (microporous layer) coating). By adopting such a coating process, a microporous layer is formed on the surface of the nonwoven fabric by the coating process, making it possible to produce a carbon-based nonwoven fabric with even better cesium adsorption ability.

The second water-repellent resin may be a fluororesin. By using a fluororesin, a carbon-based nonwoven fabric with better cesium adsorption properties can be manufactured. The second water-repellent resin may be the same as the first water-repellent resin.

Examples of fluororesins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA). Of these, polytetrafluoroethylene (PTFE) is preferred.

Furthermore, examples of conductive materials include carbon black such as furnace black, acetylene black, lamp black, and thermal black, as well as flake graphite, scaly graphite, earthy graphite, artificial graphite, expanded graphite, and flake graphite. Of these, it is preferable to use carbon black because it has pores.

The carbon fiber nonwoven fabric used in the present invention may be a single sheet or a laminate of two or more sheets.

(1-2a) Preparation of carbon fiber nonwoven fabric:
The carbon fiber nonwoven fabric can be produced by a conventionally known method, for example, as follows: That is, the carbon fiber nonwoven fabric can be produced by a method having a step A of producing a carbon fiber precursor fiber nonwoven fabric using carbon fiber precursor fibers, and a step B of carbonizing the carbon fiber precursor fiber nonwoven fabric produced in the step A.

Here, carbon fiber precursor fibers are fibers that are converted into carbon fibers by carbonization treatment. Such fibers are not particularly limited, but examples include polyacrylonitrile (PAN)-based fibers, pitch-based fibers, lignin-based fibers, polyacetylene-based fibers, polyethylene-based fibers, polyvinyl alcohol-based fibers, cellulose-based fibers, and polybenzoxazole-based fibers. Note that the carbon fiber precursor fibers can be used alone or in combination of two or more types.

In the above-mentioned step A, a web is formed using the carbon fiber precursor fibers, and the web is bonded into a cloth-like shape by entangling, heat fusion, binder adhesion, etc., to produce a carbon fiber precursor fiber nonwoven fabric.

In the above-mentioned step B, the carbon fiber precursor fiber nonwoven fabric produced in step A is carbonized. The carbonization conditions are not particularly limited, and methods known in the field of carbon fiber materials can be appropriately adopted. For example, the carbonization process is a process in which the carbon fiber precursor fiber is baked in an inert gas atmosphere. More specifically, the carbon fiber precursor fiber can be heated to, for example, 800°C or higher in an inert gas atmosphere while supplying an inert gas such as nitrogen or argon.

In addition, when performing water-repellent treatment, after the above-mentioned step B, the carbonized carbon fiber precursor fiber nonwoven fabric can be coated with a first water-repellent resin (i.e., a water-repellent agent) and then heat-treated.

Furthermore, when forming a microporous layer on the surface of carbon fibers by coating treatment, a coating agent containing a second water-repellent resin and a conductive material can be applied to a carbon fiber precursor fiber nonwoven fabric that has been subjected to a water-repellent treatment to form a coating film, and then the coating film can be dried at a temperature of, for example, 80 to 150°C, and then heat-treated. The coating agent can be applied by a known method such as screen printing, slot die coating, doctor blade coating, etc.

(1-2) Surfactant solution:
The surfactant solution used in the present invention (first embodiment) contains a surfactant. By using such a surfactant solution, a nonwoven fabric (filter) with good cesium adsorption capacity can be manufactured. That is, a carbon-based nonwoven fabric for cesium adsorption can be manufactured. In the present invention (first embodiment), a surfactant is used in the manufacturing process and the surfactant is attached to the surface of the carbon fiber nonwoven fabric, thereby making it possible to obtain a nonwoven fabric with improved cesium adsorption capacity.

Examples of surfactants contained in the surfactant solution include polyoxyalkylene lauryl ether, sorbitan tristearate, and n-octanoyl-N-methyl-D-glucamine (MEGA-8). Among these, polyoxyalkylene lauryl ether is preferable. If polyoxyalkylene lauryl ether is used, a nonwoven fabric with improved cesium adsorption capacity can be obtained more reliably.

The surfactant solution may contain one type of surfactant, or two or more types of surfactants.

There are no particular limitations on the concentration of the surfactant contained in the surfactant solution, but it is preferably 0.05 to 10% by mass, and particularly preferably 0.1 to 2% by mass. If the surfactant concentration is within the above range, a nonwoven fabric with improved cesium adsorption capacity can be obtained. If the concentration is below the above lower limit, there is a tendency that sufficient improvement in cesium adsorption capacity cannot be obtained. If the concentration is above the above upper limit, there is a limit to the amount of surfactant adsorbed by the nonwoven fabric (filter), and the amount of adsorption tends not to increase sufficiently.

(1-3) Immersion:
The time for which the nonwoven fabric is immersed in the surfactant solution is not particularly limited, but may be, for example, 1 to 10 minutes.

(1-4) Drying:
The drying conditions for the nonwoven fabric after immersion in the surfactant solution are not particularly limited, but may be, for example, at 50 to 80° C. for 0.5 to 2 hours.

(2) Manufacturing method of carbon-based nonwoven fabric for cesium adsorption (second embodiment):
A second embodiment of the method for manufacturing a carbon-based nonwoven fabric for cesium adsorption is a manufacturing method in which a nonwoven fabric made of carbon fiber is prepared, and the carbon fibers of this nonwoven fabric are treated to make them water-repellent with a first water-repellent resin, thereby obtaining a carbon-based nonwoven fabric for cesium adsorption.

The carbon fiber nonwoven fabric may be the same as the carbon fiber nonwoven fabric shown in the first embodiment described above.

As the first water-repellent resin, the same resin as the first water-repellent resin and the second water-repellent resin in the first embodiment described above can be used, and the nonwoven fabric can be further treated with both water repellency and coating, as in the first embodiment described above.

(3) Cesium adsorbent (first embodiment):
The cesium adsorbent of the present invention comprises a nonwoven fabric made of carbon fibers and a surfactant attached to the surface of the nonwoven fabric.

Such cesium adsorbents can recover radioactive cesium from the environment. They are also easy to dispose of when disposed of. That is, the carbon-based nonwoven fabric of the present invention can prevent the generation of SOx, NOx, dioxins, etc. during incineration, so it can be easily disposed of after use.

(3-1) Carbon fiber nonwoven fabric:
The carbon fiber nonwoven fabric may be appropriately made of the same carbon fiber nonwoven fabric as that explained in the above-mentioned method for producing a carbon-based nonwoven fabric for cesium adsorption of the present invention.

Here, the carbon fiber nonwoven fabric may be composed of only carbon fibers, as described above, or may be composed of carbon fibers that have been subjected to a water-repellent treatment using a water-repellent resin, or may be composed of carbon fibers that have been further coated after the water-repellent treatment. That is, the carbon fiber nonwoven fabric may be (a) a nonwoven fabric made of carbon fibers, or (b) a nonwoven fabric made of carbon fibers whose surfaces have been treated to be water-repellent (i.e., a nonwoven fabric made of carbon fibers (water-repellent carbon fibers) whose surfaces have been treated to be water-repellent), or (c) a nonwoven fabric made of carbon fibers whose surfaces have been treated to be water-repellent and further having a microporous layer on the water-repellent surface (i.e., a nonwoven fabric made of water-repellent carbon fibers having a microporous layer on the surface).

(3-2) Surfactant:
The surfactant may be the same as that explained in the above-mentioned method for producing a carbon-based nonwoven fabric for cesium adsorption of the present invention.

As mentioned above, examples of surfactants include polyoxyalkylene lauryl ether, sorbitan tristearate, and n-octanoyl-N-methyl-D-glucamine (MEGA-8). Among these, polyoxyalkylene lauryl ether is preferable. If polyoxyalkylene lauryl ether is used, a nonwoven fabric with improved cesium adsorption capacity can be obtained more reliably.

(3-3) Physical properties:
The cesium adsorbent of the present invention preferably has an air permeability of 50 to 800 L/( ^m2 ·sec), more preferably 100 to 500 L/( ^m2 ·sec). Alternatively, the air permeability is preferably 1 to 100 sec, more preferably 5 to 80 sec in Gurley seconds (ISO 5636-5). When the air permeability is within the above range, a good balance between filtration time and filtration efficiency is achieved. The air permeability is a value measured according to DIE EN ISO 9237.

(4) Cesium adsorbent (second embodiment):
The cesium adsorbent of the present invention (second embodiment) may have a carbon fiber nonwoven fabric on the surface of which a water-repellent resin is disposed. That is, it may be manufactured by the second embodiment of the manufacturing method of the carbon-based nonwoven fabric for cesium adsorption described above. This cesium adsorbent is an embodiment that does not have a surfactant.

The water-repellent resin may be the same as the water-repellent resin described above.

(5) Method for using the cesium adsorbent of the present invention:
The cesium adsorbent of the present invention can be used as a filter for removing cesium. More specifically, it can be used as a decontamination filter for removing radioactive cesium from contaminated water generated at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant, and from soil, water areas, agricultural products, and other contaminated materials due to environmental release.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

(Examples 1 to 7, Comparative Examples 1 and 2)
First, a "carbon fiber nonwoven fabric" manufactured by Freudenberg Performance Materials was prepared as shown in Table 1. Then, the carbon fiber nonwoven fabric was divided into two groups: one that was impregnated with a surfactant solution containing a surfactant (Examples 1, 3, 5, and 7), and one that was not impregnated with the solution (Comparative Example 1, Examples 2, 4, and 6).

The surfactant solution was prepared by dissolving a surfactant in pure water. Polyoxyethylene lauryl ether (CAS No. 9002-29-0, manufactured by Nacalai Tesque, Inc.) was used as the surfactant. The concentration was 0.2% by mass. The immersion time was 1 minute. The liquid temperature of the surfactant solution was 40°C.

Then, the samples that had been soaked in the surfactant solution (Examples 1, 3, 5, and 7) were dried at 60°C for 1 hour.

In Examples 2, 4, and 6, the carbon fibers of the nonwoven fabric were treated with a water-repellent resin to make them water-repellent. In other words, a carbon fiber nonwoven fabric was prepared, and the carbon fibers of this nonwoven fabric were treated with a water-repellent resin to make them water-repellent, resulting in a carbon-based nonwoven fabric for cesium adsorption (cesium adsorbent).

(Comparative Example 2)
A commercially available cellulose membrane (manufactured by Advantec Toyo Co., Ltd.), which is a filter paper made of cellulose, was used. Using this cellulose membrane, the same tests as in Example 1 were carried out. Note that the cellulose membrane has no cation adsorption ability and diffusion within the membrane is free, so the membrane potential is approximately 0.

In Table 1, "Freudenberg, H23" is a nonwoven fabric made of carbon fiber only (i.e., no water-repellent treatment, coating treatment, etc.). "Freudenberg, H23I2" is a nonwoven fabric made of carbon fiber treated with polytetrafluoroethylene (PTFE) to make it water-repellent. "Freudenberg, H24C3" and "Freudenberg, H23C9" are nonwoven fabrics made of carbon fiber treated with PTFE to make it water-repellent, and then MPL-coated with a coating liquid containing carbon black and PTFE. In particular, the difference between H24C3 and H23C9 is mainly the type of carbon used in the MPL coating treatment, with H24C3 using both carbon black and graphite (type 1) and H23C9 using carbon black (type 2).

(1) Measurement of membrane potential:
Regarding the cesium ion adsorption of the nonwoven fabric and cellulose membrane, the membrane potential was measured electrochemically using a concentration system in which the same electrolyte was placed in both aqueous solution phases at different concentrations through the membrane (see Figure 2). The membrane potential was measured for three types of salts, CsCl, NaCl, and _MgCl2 , using a measuring device with a measuring cell (see Figure 1) (see Tables 2 to 4). Note that measurements of NaCl and _MgCl2 were performed for comparison.

The measurement device shown in Figure 1 has two spaces separated by a nonwoven fabric or cellulose membrane as a filter, and each space is filled with the same electrolyte solution of a different concentration. During the measurement of the membrane potential, each solution phase is stirred at 90 rpm using a stirrer. The membrane potential was measured using a reference electrode connected to a high input impedance voltmeter. Figure 1 shows a perspective view of the inside of the measurement device.

More specifically, the membrane potential was measured by creating two aqueous solution phases (first phase 21, second phase 22) via the test subject nonwoven fabric or cellulose membrane (filter) 10 as shown in Figure 2, and varying the salt concentration of the first phase 21 in the range from 0.001 M to 0.1 M, while fixing the salt concentration of the second phase 22 at 0.1 M. At this time, the potential difference of the first phase 21 relative to the second phase 22 was taken as the membrane potential.

The membrane potential was measured while each aqueous solution phase (first phase 21, second phase 22) was stirred. After the entire system reached a steady state, salt bridges were placed in both aqueous solution phases and connected to a reference electrode. The membrane potential was then measured using a DUAL DISPLAY MULTIMETER (DL-2051; KENWOOD).

All membrane potential measurements were performed in a thermostatic reflux device at 25°C. Reagents were from Wako Pure Chemical Industries, Ltd. and used without further purification, and pure water with a resistance of 18 MΩ or higher was used.

Here, if the membrane (filter) has a high cation adsorption capacity, it will have a fixed charge on the membrane surface, and as a result, the membrane potential will be large, and this property can be used to measure the cation adsorption capacity. Cellulose membranes have no cation adsorption capacity and ions diffuse freely within the membrane, so the membrane potential is nearly zero. Therefore, the auto-zero function of the measuring device was used to correct the membrane potential of the cellulose membrane to zero before measurement.

Next, the results of the adsorption tests for cesium ions are shown in Table 2. The results are explained below.

Comparing Comparative Example 1 and Comparative Example 2, it can be seen that the nonwoven fabric made of carbon fibers has a membrane potential of 1.19 and has a higher adsorption capacity for Cs than the cellulose membrane (membrane potential is 0).

Comparing Example 1 and Comparative Example 1, it can be seen that the membrane potential of the nonwoven fabric made only of carbon fibers (Comparative Example 1) increases and the Cs adsorption capacity improves when treated with a surfactant (Example 1).

Comparing Examples 2 and 3 with Comparative Example 1, it can be seen that the Cs adsorption capacity is further improved by subjecting the nonwoven fabric made of carbon fiber (Comparative Example 1) to a water-repellent treatment using PTFE (Examples 2 and 3).

From Examples 4 to 7, it can be seen that if a carbon fiber nonwoven fabric is treated with a water repellent agent using PTFE and then coated with an MPL coating made of carbon black and PTFE, an even higher Cs adsorption capacity can be obtained.

In particular, Examples 5 and 7 show that further surfactant treatment results in better cation adsorption capacity.

Based on the magnitude of the membrane potential, it is believed that the carbon fiber nonwoven fabrics produced in Examples 1 to 7 have the strongest cation adsorption capacity in the order of Cs > Na > Mg (see Tables 2 to 4).

In Table 2, the numbers indicate the membrane potential. The more positive (larger) the membrane potential value, the better the performance.

In Table 2, "water-repellent treatment" indicates that the carbon fibers of the nonwoven fabric made of carbon fibers are treated with polytetrafluoroethylene (PTFE) to make them water-repellent. "Water-repellent treatment + MPL coating (type 1)" and "water-repellent treatment + MPL coating (type 2)" indicate that the carbon fibers of the nonwoven fabric made of carbon fibers are treated with PTFE to make them water-repellent, and then MPL-coated with a coating liquid containing carbon black and PTFE. "Surfactant treatment" indicates that, as shown in Example 1, a surfactant solution using polyoxyethylene lauryl ether (CAS No. 9002-29-0, manufactured by Nacalai Tesque) is used as the surfactant, and the nonwoven fabric is immersed in the surfactant solution and dried (surfactant treatment).

In Tables 2 to 4, if the membrane potential is 2.0 or more and less than 4.3, it is marked as "+". If the membrane potential is 4.3 or more and less than 6.6, it is marked as "++". If the membrane potential is 6.6 or more and less than 9.0, it is marked as "+++".

(2) Ion specificity of surfactant:
The critical micelle concentration (CMC) was measured in a surfactant (polyoxyethylene lauryl ether) aqueous solution in which Na ⁺ , Cs ⁺ , Ca ²⁺ , and Mg ²⁺ cations were not present (control) or were present. In the measurement, the critical micelle concentration of each surfactant (polyoxyethylene lauryl ether) was measured in the presence of 0.1M NaCl, CsCl, CaCl _{2, and} MgCl _2. The Wilhelmy method was used for the surface tension experiment. This is a method that is widely used as a method for measuring surface tension and has been used for a long time.

Specifically, a platinum plate is immersed in each aqueous solution, and the force generated by each aqueous solution in the direction of pulling the platinum plate is measured with a balance to calculate the surface tension. The actual measurements were performed using a surface tensiometer "One Attension (manufactured by Biolin Scientific)" and an automatic titrator "TITRONIC (manufactured by Biolin Scientific)."

When ions are present in an aqueous solution, the cations are absorbed into the surfactant to hydrate, and the surfactant's hydrophilicity decreases, decreasing the critical micelle concentration (CMC). The effect of this action is greater as the valence of the ion increases, since the electrical interaction with the water molecules is greater.

As a result of the measurements, as shown in Figure 3, Cs ⁺ , despite being monovalent, showed a low critical micelle concentration comparable to that of divalent Mg2 ⁺ , and polyoxyethylene lauryl ether showed a specifically high affinity for Cs ⁺ ions.

(3) Adsorption to nonwoven fabric:
Mass spectrometry was performed to confirm the adsorption of the surfactant to the nonwoven fabric, and the results are shown in Figures 4(a) to 4(d).

Compared to the second from the top in the mass spectrometry (TOF-MS) results shown in Figure 4, "(b) Untreated carbon-based nonwoven fabric," spectra with a consistent m/z interval were detected in the nonwoven fabrics treated with surfactants ((a), (c), (d)). The interval between each spectral peak was approximately 44 Da.

This interval corresponds to the ethylene oxide (EO, -CH ₂ CH ₂ O-) of the treated surfactant, and this result indicates that the surfactant is water-soluble but is adsorbed to the nonwoven fabric even when placed in an aqueous phase without falling off.

In FIG. 4, "(a) Surfactant (polyoxyethylene lauryl ether)" is the result of mass spectrometry of only the surfactant polyoxyethylene lauryl ether. In FIG. 4, "(b) Untreated carbon-based nonwoven fabric" is the result of mass spectrometry of only the carbon fiber nonwoven fabric (no surfactant treatment, water-repellent treatment, etc.). In FIG. 4, "(c) Carbon-based nonwoven fabric treated with surfactant concentration of 2.0 wt %" is the result of mass spectrometry of the carbon fiber nonwoven fabric ((b) untreated carbon-based nonwoven fabric) after treatment with a surfactant solution containing 2.0 mass % surfactant (polyoxyethylene lauryl ether) (but without further treatment such as water-repellent treatment). In FIG. 4, "(d) Carbon-based nonwoven fabric treated with a surfactant concentration of 0.20 wt %" is the result of mass spectrometry of a carbon fiber nonwoven fabric ((b) untreated carbon-based nonwoven fabric) after treatment with a surfactant solution containing 0.20 mass % of a surfactant (polyoxyethylene lauryl ether) (but without further treatment such as water repellency treatment).

Although not shown, scanning electron microscope (SEM) images confirmed that the nonwoven fabric treated with the surfactant did not change the physical structure of the carbon fibers.

As can be seen from the above, the carbon fiber nonwoven fabrics of Examples 1 to 7 can be used as filters that adsorb cesium, compared to the nonwoven fabric of Comparative Example 1 and the cellulose membrane of Comparative Example 2. In other words, it can be seen that the use of the carbon fiber nonwoven fabrics of Examples 1 to 7 makes it possible to recover radioactive cesium from the environment. In addition, since the nonwoven fabrics of Examples 1 to 7 are made of carbon fiber, they are easy to dispose of when disposed of.

According to the manufacturing method of the carbon-based nonwoven fabric for cesium adsorption of the present invention, it is possible to manufacture a carbon-based nonwoven fabric that can be used as a filter for adsorbing cesium. In addition, the cesium adsorbent of the present invention can be used as a filter for adsorbing cesium.

10: Nonwoven fabric or cellulose membrane (filter)
21: 1st phase 22: 2nd phase

Claims (5)

A method for producing a carbon-based nonwoven fabric for cesium adsorption, comprising the steps of: immersing a carbon fiber nonwoven fabric in a surfactant solution containing a surfactant; and then drying the fabric to obtain a carbon-based nonwoven fabric for cesium adsorption,
The method for producing a carbon-based nonwoven fabric for cesium adsorption uses, as the nonwoven fabric, the carbon fiber that has been subjected to a water-repellent treatment with the first water-repellent resin and then coated with a coating agent containing a second water-repellent resin and a conductive material . The method for producing a carbon-based nonwoven fabric for cesium adsorption according to claim 1, wherein the surfactant contained in the surfactant solution is polyoxyalkylene lauryl ether. The method for producing a carbon-based nonwoven fabric for cesium adsorption according to claim 1 or 2, wherein the concentration of the surfactant contained in the surfactant solution is 0.05 to 10 mass %. the first water-repellent resin and the second water-repellent resin are each a fluororesin,
The method for producing a carbon-based nonwoven fabric for cesium adsorption according to claim 1 , wherein the conductive material is carbon black. A method for producing a carbon-based nonwoven fabric for cesium adsorption, comprising the steps of: preparing a carbon fiber nonwoven fabric; and subjecting the carbon fibers of the nonwoven fabric to a water-repellent treatment using a first water-repellent resin to obtain a carbon-based nonwoven fabric for cesium adsorption,
The method for producing a carbon-based nonwoven fabric for cesium adsorption includes using the nonwoven fabric which has been subjected to the water-repellent treatment and then coated with a coating agent containing a second water-repellent resin and a conductive material .

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