Cesium sorption device, cesium sorption system, and cesium sorption method

Abstract

A cesium sorption device 1 comprises: a container 11 for accommodating a solution containing Cs+; a Prussian blue thin film 12 provided at one end of the container 11; a counter electrode 13 provided at the other end of the container 11; an electrode substrate 14 provided on the side opposite the Prussian blue thin film 12 of the container 11; and a voltage control unit 15 connected to the counter electrode 13 and the electrode substrate 14. The voltage control unit 15 controls a positive voltage and a negative voltage such that positive voltage is applied to the electrode substrate 14 for a time t1 to bring the Prussian blue thin film 12 into an oxidized state, and the negative voltage is then applied to the electrode substrate 14 for a time t2 to bring the Prussian blue thin film 12 into a reduced state.

Description

Cesium sorption device, cesium sorption system and cesium sorption method

[0001] This application claims priority from Japanese Patent Application No. 2024-151798, filed on September 3, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to cesium sorption devices, cesium sorption systems and cesium sorption methods.

[0003] A method has been proposed for removing cesium ions from an aqueous solution containing cesium by passing the aqueous solution through a column packed with insoluble Prussian blue nanoparticles (see, for example, Patent Document 1).

[0004] JP 2013-50313 JP 2013-160666

[0005] R. Chen et al., “Thermodynamics and Mechanism Studies on Electrochemical Removal of Cesium Ions from Aqueous Solution Using a Nanoparticle Film of Copper Hexacyanoferrate” ACS Appl. Mater. Interfaces 5, 12984-12990 (2013)

[0006] In the event of a nuclear power plant accident, large amounts of radioactive material may be dispersed into the environment. Radioactive cesium-134 and cesium-137 in particular are known to disperse over long distances. After falling to the ground, the dispersed radioactive cesium contaminates forest soil and accumulates in riverbeds and ocean bottoms via river and seawater, so it must be removed.

[0007] Prussian blue (ferric ferrocyanide) is a dye that inhibits the absorption of Cs in solution. ＋ It is known that Prussian blue nanoparticles penetrate and sorb (monovalent cesium ions) and are used as cesium sorption materials. For example, Patent Document 2 discloses a method for removing radioactive cesium from cesium-dissolved water in which radioactive cesium is dissolved in water, using Prussian blue nanoparticles.

[0008] However, powder-type sorbents such as nanoparticles have the disadvantage of being difficult to handle. To address this issue, a method has been proposed in which Prussian blue is made into a thin film and positive and negative potentials are applied to the thin film to allow cesium ions in a solution to enter and exit the film (e.g., Non-Patent Document 1).

[0009] However, the Cs present in the contaminated liquid recovered from the soil ＋ The concentration of K ＋ (Potassium monovalent ion) and Na ＋ (sodium monovalent ion) 10 －４ In the case of solutions containing such dilute cesium, even if attempts are made to remove cesium using conventional techniques, it takes a long time (for example, about one month) to reach sorption equilibrium, making it impossible to concentrate and recover cesium efficiently. This poses a major practical problem.

[0010] Furthermore, with conventional technology, it is not possible to extract and isolate cesium from the recovered Prussian blue, so the Prussian blue is subjected to hydrothermal treatment as is. As a result, in addition to cesium, Prussian blue also becomes iron oxide and is included as a residue. This residue must be disposed of by, for example, turning it into cement. However, this requires a large amount of processing space and costs. Furthermore, in this case, the Prussian blue thin film (or powder) used for cesium adsorption cannot be reused. As such, conventional technology also has the problem of requiring a large amount of processing space and costs.

[0011] The technology disclosed herein has been made in light of these circumstances, and its first objective is to recover cesium with high efficiency using a Prussian blue thin film.

[0012] Furthermore, a second object of the present disclosure is to reduce the residue after cesium recovery and make the Prussian blue thin film reusable by eluting and isolating the recovered cesium.

[0013] In order to solve the above problems, the cesium sorption device of one embodiment of the present invention is ＋the Prussian blue thin film is disposed on one end of the container, a counter electrode is disposed on the other end of the container, an electrode substrate is disposed on the opposite side of the container from the Prussian blue thin film, and a voltage control unit connected to the counter electrode and the electrode substrate. The voltage control unit controls the applied voltage so that a positive voltage higher than the potential at which the Prussian blue thin film is neutral is applied to the electrode substrate for a time t1 to oxidize the Prussian blue thin film, and then a negative voltage lower than the potential at which the Prussian blue thin film is neutral is applied to the electrode substrate for a time t2 to reduce the Prussian blue thin film.

[0014] In one embodiment of the cesium sorption device, when the Prussian blue thin film is in a neutral state at a potential of 0.3V to 0.8V, the positive voltage may be 0.9V or more and the negative voltage may be 0.1V or less.

[0015] In one embodiment, the voltage control unit of the cesium sorption device may control the applied voltage so as to repeat one or more cycles including applying a negative voltage to the electrode substrate for time t2 that is lower than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, applying a positive voltage to the electrode substrate for time t3 that is higher than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, and applying a negative voltage to the electrode substrate for time t4 that is lower than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, where t3<t1.

[0016] After repeating the cycle, the voltage control unit of an embodiment of the cesium sorption device may control the applied voltage so that a positive voltage higher than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate for a time t5, thereby bringing the Prussian blue thin film into an oxidized state.

[0017] In some embodiments of the cesium sorption device, t3<t5≦t1 or t3<t1≦t5.

[0018] Another aspect of the present invention is a cesium sorption system comprising a plurality of the cesium sorption devices described above.

[0019] Yet another aspect of the present invention is a cesium sorption method. ＋ the Prussian blue thin film to be brought into an oxidized state by applying a positive voltage to the Prussian blue thin film for a time t1, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state; and the Prussian blue thin film to be brought into a reduced state by applying a negative voltage to the Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state.

[0020] A cesium sorption method in one embodiment may include a step of repeating one or more cycles including the steps of applying a negative voltage to the Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, applying a positive voltage to the Prussian blue thin film for a time t3, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, and applying a negative voltage to the Prussian blue thin film for a time t4, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, where t3<t1.

[0021] In an embodiment, the cesium sorption method may include, after the step of repeating the cycle, a step of applying a positive voltage to the Prussian blue thin film for a time t5, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, to oxidize the Prussian blue thin film.

[0022] In an embodiment of the cesium sorption method, t3<t5≦t1 or t3<t1≦t5.

[0023] Yet another aspect of the present invention is also a cesium sorption device. ＋the Prussian blue thin film is provided with a container for containing a solution containing the Prussian blue thin film, a Prussian blue thin film disposed at one end of the container, a counter electrode disposed at the other end of the container, an electrode substrate disposed on the opposite side of the container from the Prussian blue thin film, and a voltage control unit connected to the counter electrode and the electrode substrate. The voltage control unit controls the applied voltage by applying a positive voltage to the electrode substrate for a time t1, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film yellow (Prussian yellow), and then applying a negative voltage to the electrode substrate for a time t2, the negative voltage being lower than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film colorless (Prussian white).

[0024] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure.

[0025] According to an embodiment of the present disclosure, cesium can be recovered with high efficiency using a Prussian blue thin film.

[0026] Furthermore, according to another aspect of the present disclosure, by isolating and isolating the recovered cesium, it is possible to reduce the residue after cesium recovery and to reuse the Prussian blue thin film.

[0027] Schematic diagram showing the crystal structure of Prussian blue in the neutral state, oxidized state, and reduced state. The chemical formula of Prussian blue crystal is KFe(III)[Fe(II)CN ６ In the unit cell, there are trivalent iron ions (Fe ３＋ ) and divalent iron ions (Fe ２＋ ) and four cyanide ions (CN － ) are included. Although not shown in the figure, the unit lattice of the crystal contains 24 potassium ions (K ＋ ) is contained in the container. It is a cyclic voltammogram (CV) showing the current density flowing through Prussian blue when the voltage applied to the Prussian blue is changed. It is a schematic diagram of the cesium sorption device according to the first embodiment. Cesium (Cs ＋) after being filled with a solution containing cesium (Cs ＋ ) is sometimes abbreviated as "cesium solution." This is a schematic diagram showing the state when a positive voltage is applied to the electrode substrate, causing the Prussian blue thin film to be in an oxidized state. This is a schematic diagram showing the state when a negative voltage is applied to the electrode substrate, causing the Prussian blue thin film to be in a reduced state. This is a schematic diagram showing the state when a positive voltage is applied to the electrode substrate for a short period, causing the Prussian blue thin film to be in an oxidized state for a short period. This is a schematic diagram showing the state when a Prussian blue thin film is in a neutral state after repeated oxidation and reduction. This is a schematic diagram showing the state when a positive voltage is applied to the electrode substrate, causing the Prussian blue thin film to be in an oxidized state. This is a schematic diagram of a cesium sorption system according to a fourth embodiment. This is a flowchart showing the processing steps of a cesium sorption method according to a fifth embodiment. This is a flowchart showing the processing steps of a cesium sorption method according to a seventh embodiment. This is a flowchart showing the processing steps of a cesium sorption method according to an eighth embodiment. This is a diagram showing the time control of externally applied voltage and current density under two conditions performed in Experiment 1. This is a diagram showing the results of Experiment 1. PB is an abbreviation for Prussian Blue. This is a diagram showing one of the voltage controls performed in Experiment 2. 1 shows the results of Experiment 2. PB is an abbreviation for Prussian Blue.

[0028] Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and redundant explanations will be omitted where appropriate. Furthermore, the embodiments are illustrative and do not limit the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0029] In addition, the dimensions (thickness, length, width, etc.) of each member shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple members do not necessarily represent the relative size of each other, and even if a member A is depicted as being thicker than another member B in the drawings, member A may actually be thinner than member B.

[0030] Before describing specific embodiments, the basic knowledge will be explained. Prussian blue is generally represented by the chemical formula K ＋ ４ Fe ３＋ [Fe ２＋ (CN － ) ６ ] is a deep blue complex. Figure 1 is a schematic diagram showing the crystal structure of Prussian blue in a neutral state. Prussian blue crystals are cubic crystals, and in the neutral state, Fe is located at the center of the face-centered cubic structure. ２＋ is located at the center of each side of the cube, and Fe ３＋ is located. ２＋ and Fe ３＋ What is CN － Fe ２＋ -CN － -Fe ３＋ It is cross-linked in the form of Fe ２＋ and Fe ３＋ The center-to-center distance between the cations and the cations is about 0.5 nm, and there are relatively large jungle-gym-like void sites inside. Various cations and molecules can be inserted and removed into these void sites. In particular, Cs in solution can be inserted into these void sites. ＋ By deintercalating Prussian blue, it can be used as a cesium sorbent.

[0031] As mentioned above, Prussian blue in its neutral state contains two types of valence iron (i.e., Fe ２＋ and Fe ３＋ By applying a voltage to this, it can be changed into an oxidized or reduced state. Specifically, when a voltage higher than the neutral potential is applied to the Prussian blue thin film, Fe ２＋ is Fe ３＋ Instead of Fe ３＋ -CN － -Fe ３＋ On the other hand, when a voltage lower than the neutral potential is applied to the Prussian blue thin film, Fe ３＋ is Fe ２＋ Instead of Fe ２＋ -CN － -Fe ２＋Prussian blue is in a reduced (negative) state. That is, by controlling the voltage applied to Prussian blue, the charge state can be controlled between the oxidized, neutral, and reduced states. Figure 2 is a schematic diagram showing Prussian blue in the neutral, oxidized, and reduced states.

[0032] In this specification, the potential when the Prussian blue thin film is in a neutral state is used as a reference, and any potential higher than this is referred to as a "positive potential," and the potential difference at this point is referred to as a "positive voltage." Similarly, the potential when the Prussian blue thin film is in a neutral state is used as a reference, and any potential lower than this is referred to as a "negative potential," and the potential difference at this point is referred to as a "negative voltage."

[0033] Figure 3 shows cyclic voltagrams (CV) that show the current density flowing through Prussian blue when the voltage applied to the Prussian blue is changed over time. To control the potential of the working electrode within the range of -0.3 V to 1.3 V (V vs. Ag / AgCl@3M-KCl), a voltage in the range of -1.5 V to 1.3 V was applied between the working electrode and the counter electrode. Point A is the initial state, where the potential of Prussian blue is 0.5 V and Prussian blue is in a neutral state. From this point, the potential is increased at a time change rate of 0.01 V / s. Then, initially, a positive current flows through Prussian blue near point B (potential is approximately 0.9 V). This is due to the oxidation reaction of Prussian blue (Fe ２＋ →Fe ３＋ ), from which Prussian blue is ３＋ -CN － -Fe ３＋ The potential is further increased, and when it reaches 1.3 V, the potential is decreased. Then, near point C (potential is about 0.8 V), a negative current flows through the Prussian blue. This is due to the reduction reaction of Prussian blue (Fe ３＋ →Fe ２＋ ), from which Prussian blue is ３＋ -CN － -Fe ２＋ When the potential is further increased, a negative current flows through the Prussian blue again near point D (potential is about 0.1 V). This is due to the reduction reaction of Prussian blue (Fe ３＋ →Fe２＋ ), from which Prussian blue is ２＋ -CN － -Fe ２＋ The potential is further increased, and when it reaches -0.3 V, the potential is increased again. Then, near point E (potential is about 0.3 V), a positive current flows through the Prussian blue. This is due to the oxidation reaction of Prussian blue (Fe ２＋ →Fe ３＋ ), from which Prussian blue is ３＋ -CN － -Fe ２＋ The potential is further increased until it returns to point A, completing one cycle. Figure 3 shows the results when this cycle is repeated 10 times. The solid line indicates the first cycle, and the dotted line indicates the tenth cycle.

[0034] Based on the above findings, the present inventors have developed a Prussian blue thin film using Cs ＋ They discovered that cesium can be highly efficiently concentrated and recovered by contacting the Prussian blue thin film with a solution containing cesium and controlling the voltage applied to the thin film.

[0035] 4 is a schematic diagram of a cesium sorption device 1 according to a first embodiment. The cesium sorption device 1 includes a container 11, a Prussian blue thin film 12, a counter electrode 13, an electrode substrate 14, and a voltage control unit 15.

[0036] The container 11 is Cs ＋ Such a cesium solution contains a solution containing K ＋ and Na ＋ etc. Cs ＋ It may be contained at a higher concentration. In this case, for example, Cs ＋ The concentration of K ＋ and Na ＋ Such a concentration of 10 －４ ~10 －２ That is, the cesium solution may be a dilute solution of cesium. Container 11 may optionally include an inlet 16 for introducing the cesium solution into container 11 and an outlet 17 for discharging the treated liquid out of container 11.

[0037] The Prussian blue thin film 12 is placed at one end of the container 11. The Prussian blue thin film 12 may be produced, for example, by spin-coating a colloidal solution of Prussian blue nanoparticles onto the electrode substrate 14.

[0038] The counter electrode 13 is placed at the other end of the container 11 (on the opposite side of the container 11 from the Prussian blue thin film 12). The counter electrode 13 is preferably made of, but not limited to, carbon (such as graphite) or platinum. The counter electrode 13 is preferably a parallel plate type electrode having the same area as the electrode substrate 14 and arranged in parallel, but not limited to, this. The potential control range is preferably −0.3 V to 1.2 V vs. Ag / AgCl@3M-KCl. This is because, within this range, the counter electrode 13 does not dissolve into the electrolyte.

[0039] The electrode substrate 14 is placed on the opposite side of the Prussian blue thin film 12 from the container 11 (the opposite side of the container 11 across the Prussian blue thin film 12). The material of the electrode substrate 14 is preferably, but not limited to, carbon (such as graphite), platinum, tantalum, or indium tin oxide (ITO). The electrode substrate 14 is preferably, but not limited to, a parallel plate type electrode having the same area as the counter electrode 13 and arranged parallel to it. Furthermore, the electrode substrate 14 is considered to exhibit good performance when made of a material with a large specific surface area. The potential control range is preferably −0.3 V to 1.2 V vs. Ag / AgCl@3M-KCl. This is because within this range, the electrode substrate 14 does not dissolve into the electrolyte.

[0040] The voltage control unit 15 is connected to the counter electrode 13 and the electrode substrate 14 and controls the voltage applied to the Prussian blue thin film 12. As a non-limiting example, the voltage control unit 15 may be configured using a potentiostat. In this case, the voltage control unit 15 includes the electrode substrate 14 as a working electrode, the counter electrode 13 as a counter electrode, and a reference electrode (not shown).

[0041] The operation of the cesium sorption device 1 according to this embodiment will be described below. Figures 5 to 8 are schematic diagrams of the portion from the container 11 to the electrode substrate 14 of the cesium sorption device 1 in Figure 1. Figure 5 shows the state after the container 11 is filled with a cesium solution. In the following example, the cesium solution contains Cs ＋ and K. ＋ It consists of Cs ＋ The concentration of ＋ At this stage, no voltage is applied to the electrode substrate 14. The Prussian blue thin film 12 is in a neutral state. ＋ To take in K, the vacant site ＋ exists.

[0042] Next, using the voltage control unit 15, a voltage (hereinafter referred to as a positive voltage) higher than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate 14 for a sufficiently long time. This causes the Prussian blue thin film 12 to enter an oxidized (positive) state. Figure 6 shows the state when a positive voltage is applied to the electrode substrate 14 for time t1, causing the Prussian blue thin film 12 to enter an oxidized state.

[0043] The voltage value at this time is preferably 1.2 V or more (a voltage range in which an oxidation reaction is sufficiently induced in the Prussian blue thin film). The time depends on the voltage value, but is preferably 1 to 10 minutes (until the Prussian blue thin film is sufficiently oxidized and the K ＋ It is desirable that the conditions are such that sufficient ions are released.

[0044] As the Prussian blue thin film 12 becomes oxidized, excess K sorbed in the void sites is released. ＋ is released into the cesium solution. As a result, the void sites in the Prussian blue thin film 12 become "empty" and Cs ＋ You'll be ready to import.

[0045] Next, using the voltage control unit 15, a voltage (hereinafter referred to as a negative voltage) lower than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate 14 for a sufficiently long time. This causes the Prussian blue thin film 12 to enter a reduced (negative) state. Figure 7 shows the state when a negative voltage is applied to the electrode substrate 14 for time t2, causing the Prussian blue thin film 12 to enter a reduced state.

[0046] The voltage value at this time is preferably -1.4 V or less (a voltage range in which a reduction reaction is sufficiently induced in the Prussian blue thin film). The time depends on the voltage value, but is preferably 100 to 120 minutes or more (conditions under which the Prussian blue thin film is sufficiently reduced and Cs ions are sufficiently incorporated). The amount of Cs ions incorporated increases with the voltage application time and saturates after about 100 to 120 minutes.

[0047] The Prussian blue thin film 12 is reduced, and the cations in the cesium solution (i.e., Cs ＋ and K. ＋ ) diffuses through the nanospace and is sorbed into the void sites.

[0048] As described above, the Prussian blue thin film 12 is once oxidized to remove K from the vacant sites. ＋ After releasing the Prussian blue thin film 12, the Cs in the cesium solution is reduced. ＋ By controlling the voltage applied to the Prussian blue thin film, the charged state of the Prussian blue thin film can be changed between neutral, oxidized, and reduced states, allowing for highly efficient recovery of cesium. This method is a novel idea devised by the inventors.

[0049] As an example, as shown in FIG. 3, when the Prussian blue thin film is in a neutral state at a potential of 0.3V to 0.8V, the positive voltage may be 0.9V or more and the negative voltage may be 0.1V or less.

[0050] [Second embodiment] In the second embodiment, cesium is recovered with higher efficiency using a cesium sorption device 1 having the same configuration as the first embodiment. In the state shown in Figure 7, cesium can be recovered with higher efficiency than in the prior art, but the K sorbed in the void sites is ＋ also exists.

[0051] After the state shown in FIG. 7 is reached, a positive voltage is applied to the electrode substrate 14 for a short time using the voltage control unit 15. This causes the Prussian blue thin film 12 to enter an oxidized state for a short time. FIG. 8 shows the state when a positive voltage is applied to the electrode substrate 14 for a short time (for a time t3) to cause the Prussian blue thin film 12 to enter an oxidized state for a short time. If the time for applying this positive voltage is sufficiently short, the Cs sorbed in the vacant sites will be oxidized. ＋ and K. ＋ Among them, K ＋ This allows selective release of only Cs into the cesium solution. ＋ The mass of K ＋ Since the mass of Cs is more than three times larger than that of Cs, if it is oxidized for a short time, ＋ is not released from the void sites.

[0052] The voltage value at this time is preferably, for example, 1 to 1.5 V. The time is preferably, for example, 10 seconds.

[0053] Next, a negative voltage is applied to the electrode substrate 14 using the voltage control unit 15. This causes the Prussian blue thin film 12 to enter a reduced (negative) state. Figure 9 shows the state when a negative voltage is applied to the electrode substrate 14 for time t4, causing the Prussian blue thin film 12 to enter a reduced state. As in Figure 7, the Prussian blue thin film 12 enters a reduced state, causing cations in the cesium solution to be sorbed into the vacant sites. ＋ It can be seen that the proportion of sorption is large.

[0054] The voltage value at this time is preferably -1.4 V or less (a voltage range in which a reduction reaction is sufficiently induced in the Prussian blue thin film). The time depends on the voltage value, but is preferably 6 to 10 minutes or more (conditions under which the Prussian blue thin film is sufficiently reduced and Cs ions are sufficiently taken up). The amount of Cs ions taken up (sorbed) increases with the voltage application time and saturates after about 100 to 120 minutes. The amount of Cs ions taken up after 6 minutes is about 70% of the saturated value, but considering that a cycle such as that shown in Figure 13 is performed, a Cs take-up time (reduction time) of about 6 minutes is considered appropriate.

[0055] Such short-term oxidation (K ＋ By repeating the cycle of selective release of Cs (selective release of Cs) and subsequent reduction (sorption of cations in the cesium solution) one or more times, ＋ Only the Prussian blue can be concentrated and recovered in the Prussian blue thin film 12.

[0056] As described above, it is desirable that the time t1 be sufficiently long and the time t3 be sufficiently short, i.e., it is desirable that t3<t1.

[0057] [Third embodiment] In the third embodiment, the cesium sorption device 1 having the same configuration as that of the first embodiment is used to concentrate and sorb Cs. ＋ The method involves extracting the cesium in a recovery solution. As mentioned above, in the past, it was not possible to separate the cesium recovered using Prussian blue by elution, so in addition to the cesium, iron oxide generated by the hydrothermal treatment of Prussian blue also remained as a residue. This residue had to be disposed of by, for example, turning it into cement, which posed problems in terms of processing space and costs. Another problem with this method was that the Prussian blue thin film once used for cesium adsorption could not be reused.

[0058] After the state shown in Figure 9 is reached, a positive voltage is applied to the electrode substrate 14 for a sufficiently long time using the voltage control unit 15. This causes the Prussian blue thin film 12 to enter an oxidized (positive) state. Figure 10 shows the state when a positive voltage is applied to the electrode substrate 14 for time t5, causing the Prussian blue thin film 12 to enter an oxidized state. If the time for applying this positive voltage is long enough, the Cs sorbed in the vacant sites will be oxidized. ＋ is released into the recovery solution.

[0059] The voltage value at this time is preferably 1.2 V or more (a voltage range in which an oxidation reaction is sufficiently induced in the Prussian blue thin film), and the time, although depending on the voltage value, is preferably 10 minutes or more (a condition under which the Prussian blue thin film is sufficiently oxidized and Cs ions are sufficiently released).

[0060] According to this embodiment, the cesium recovered by the Prussian blue thin film 12 can be eluted, isolated, and released into the recovery solution. This eliminates the need for waste disposal such as cementation, significantly reduces storage space, and allows the Prussian blue thin film 12 to be reused multiple times.

[0061] In order to recover cesium more efficiently, it is desirable that t3<t5≦t1 or t3<t1≦t5.

[0062] [Fourth embodiment] Figure 11 is a schematic diagram of a cesium sorption system 1000 according to a fourth embodiment. The cesium sorption system 1000 is configured by connecting the three cesium sorption devices 100, 200, and 300 described above in series. A cesium solution is introduced into the inlet 116 of the cesium sorption device 100. The outlet 117 of the cesium sorption device 100 is connected to the inlet 216 of the cesium sorption device 200. The outlet 217 of the cesium sorption device 200 is connected to the inlet 316 of the cesium sorption device 300. The treated liquid is discharged from the outlet of the cesium sorption device 300.

[0063] In this way, by configuring a cesium sorption system equipped with a plurality of the above-mentioned cesium sorption devices, it is possible to concentrate and recover cesium with even higher efficiency.

[0064] 11, in which three cesium sorption devices are connected in series, is merely one non-limiting example. The embodiment is not limited to this. For example, the number of cesium collection devices included in the cesium collection system may be any number other than three, or some or all of the multiple cesium collection devices may be connected in parallel.

[0065] 12 is a flowchart showing the procedure of a cesium sorption method according to a fifth embodiment of the present invention. This method includes steps S1, S2, and S3.

[0066] In step S1, Cs ＋ The solution containing the compound is brought into contact with the Prussian blue thin film.

[0067] After step S1, in step S2, a positive voltage is applied to the Prussian blue thin film to bring the Prussian blue thin film into an oxidized (positive) state. This causes excess K sorbed in the void sites of the Prussian blue thin film to be removed. ＋ is released into the cesium solution. As a result, the void sites in the Prussian blue thin film become "empty" and Cs ＋ You'll be ready to import.

[0068] After step S2, in step S3, a negative voltage is applied to the Prussian blue thin film to bring the Prussian blue thin film into a reduced (negative) state. This reduces the cations (i.e., Cs ＋ and K. ＋ ) is sorbed while diffusing into the void sites.

[0069] According to this embodiment, cesium can be recovered with high efficiency using a Prussian blue thin film.

[0070] [Sixth embodiment] The cesium recovery device according to the sixth embodiment has the same configuration as the cesium recovery device 1 shown in Fig. 4. That is, the cesium recovery device according to the sixth embodiment recovers Cs ＋the Prussian blue thin film is provided with a container for containing a solution containing the Prussian blue thin film, a Prussian blue thin film disposed at one end of the container, a counter electrode disposed at the other end of the container, an electrode substrate disposed on the opposite side of the container from the Prussian blue thin film, and a voltage control unit connected to the counter electrode and the electrode substrate. The voltage control unit controls the applied voltage by applying a positive voltage to the electrode substrate for a time t1, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film yellow, and then applying a negative voltage to the electrode substrate for a time t2, the negative voltage being lower than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film colorless.

[0071] When a voltage higher than the neutral potential is applied to the Prussian blue thin film, Fe ２＋ is Fe ３＋ Instead of Fe ３＋ -CN － -Fe ３＋ The Prussian blue becomes oxidized. At this time, the color of the Prussian blue thin film changes to yellow. Conversely, when a voltage lower than the potential at which the Prussian blue thin film is in a neutral state is applied, the Fe ３＋ is Fe ２＋ Instead of Fe ２＋ -CN － -Fe ２＋ The Prussian blue is reduced. At this time, the color of the Prussian blue thin film changes to colorless. This embodiment applies this principle.

[0072] According to this embodiment, the applied voltage can be controlled based on the color change of the Prussian blue thin film depending on whether it is in an oxidized or reduced state.

[0073] Seventh Embodiment Figure 13 is a flowchart showing the procedure of a cesium sorption method according to a seventh embodiment. This method includes steps S1, S2, S3, and S4. Steps S1 to S3 are the same as those in the fifth embodiment, so their explanation will be omitted.

[0074] After step S3, in step S4, a cycle is carried out in which a positive voltage is applied to the Prussian blue thin film for a short time to bring the Prussian blue thin film into an oxidized state, and then a negative-positive voltage is applied to the Prussian blue thin film to bring the Prussian blue thin film into a reduced state. This allows the Cs sorbed in the void sites of the Prussian blue thin film to be removed. ＋ and K. ＋ Among them, K ＋ Only Cs in the cesium solution is preferentially released. ＋ is sorbed into the void sites.

[0075] The cycle of step S4 is repeated until it is executed a predetermined number of times. ＋ Only Prussian blue is concentrated and sorbed in the thin film.

[0076] According to this embodiment, cesium can be concentrated and recovered with even higher efficiency using a Prussian blue thin film.

[0077] Eighth Embodiment Figure 14 is a flowchart showing the procedure of a cesium sorption method according to an eighth embodiment. This method includes steps S1, S2, S3, S4, and S5. Steps S1 to S4 are the same as those in the fourth embodiment, so their explanation will be omitted.

[0078] After step S4, in step S5, a positive voltage is applied to the Prussian blue thin film to bring the Prussian blue thin film into an oxidized (positive) state. This causes the Cs sorbed in the void sites of the Prussian blue thin film to be desorbed. ＋ is released into the recovery solution.

[0079] According to this embodiment, the cesium recovered by the Prussian blue thin film can be eluted and isolated and released into the recovery solution. This eliminates the need for waste disposal such as cementation, significantly reduces storage space, and allows the Prussian blue thin film to be reused multiple times.

[0080] [Evaluation Experiment] The present inventors conducted an evaluation experiment to confirm the effectiveness of the present disclosure. The experiment contents are as follows.

[0081] (Experiment 1) Experiment 1 compares the effect of cesium sorption in the cesium sorption device 1 of Figure 4 when the voltage control of the embodiment is not performed, when the voltage control of the first embodiment is performed, and when the voltage control of the second embodiment is performed.

[0082] FIG. 15 shows the time control of voltage and current density under two conditions in Experiment 1. Condition 1 corresponds to the voltage control in the first embodiment. Here, a positive voltage was applied to the electrode substrate for 600 seconds to oxidize the Prussian blue thin film, and then a negative voltage was applied to the electrode substrate for 7,200 seconds to neutralize and then reduce the Prussian blue thin film. Condition 2 corresponds to the voltage control in the second embodiment. Here, a positive voltage was applied to the electrode substrate for 600 seconds to oxidize the Prussian blue thin film, and then a negative voltage was applied to the electrode substrate for 360 seconds to neutralize and then reduce the Prussian blue thin film. This cycle, which includes applying a positive voltage to the electrode substrate for a short period (10 seconds) to neutralize and then oxidize the Prussian blue thin film, and then applying a negative voltage to the electrode substrate for 360 seconds to neutralize and then reduce the Prussian blue thin film, was repeated seven times. An experiment was also conducted under Condition 0. In condition 0, a positive voltage was applied to the electrode substrate for 600 seconds to oxidize the Prussian blue thin film, and then a negative voltage (-0.2 V) was applied to the electrode substrate for 360 seconds to neutralize the Prussian blue thin film. This cycle, which included applying a positive voltage (1.1 V) to the electrode substrate for a short period (10 seconds) to oxidize the Prussian blue thin film and then applying a negative voltage to the electrode substrate for 360 seconds to neutralize the Prussian blue thin film, was repeated seven times.

[0083] Fig. 16 shows the results of Experiment 1 and the results of an experiment in which voltage control was performed under Condition 0. Fig. 16 shows the results of comparing four cases: before the cesium-potassium mixed solution (K:Cs = 100:1) was placed in the container (before immersion), when no voltage was applied after the cesium-potassium mixed solution was placed in the container (no voltage application), when voltage control under Condition 0 was performed after the cesium-potassium mixed solution was placed in the container, when voltage control under Condition 1 was performed after the cesium-potassium mixed solution was placed in the container, and when voltage control under Condition 2 was performed after the cesium-potassium mixed solution was placed in the container.

[0084] The upper part of Fig. 16 shows the ion number density of K per unit cell of the Prussian blue thin film. The middle part of Fig. 16 shows the ion number density of Cs per unit cell of the Prussian blue thin film. The lower part of Fig. 16 shows the enrichment of Cs in the Prussian blue thin film after sorption. The enrichment of Cs is defined as the Cs composition ratio in the Prussian blue thin film (Cs / K) divided by the Cs composition ratio in the solution (Cs / K).

[0085] Before immersion, only K ions were present in the Prussian blue thin film. When voltage control was performed under condition 0, the number densities of K ions and Cs ions were nearly identical to those without voltage control. When voltage control was performed under condition 1, the number density of Cs ions was approximately 1.7 (= 1.36 / 0.79) times higher than without voltage control, demonstrating that a large amount of cesium was sorbed. However, in this case, the number density of K ions also increased approximately twice as much as without voltage control. When voltage control was performed under condition 2, the number density of Cs ions was approximately 2.7 (= 2.11 / 0.79) times higher than without voltage control, demonstrating that even more cesium was sorbed. Meanwhile, the number density of K ions decreased to approximately 0.34 times lower than without voltage control. Furthermore, when voltage control was performed under condition 2, Cs was highly concentrated and recovered in the Prussian blue thin film to more than 1,000 times its concentration in the solution. Thus, it was verified that voltage control under condition 2 enables selective enrichment and sorption of only cesium with extremely high efficiency.

[0086] (Experiment 2) Experiment 2 compares the effectiveness of cesium elution and isolation after sorption when voltage control according to the second embodiment is performed and when voltage control according to the third embodiment is performed in the cesium sorption device 1 of FIG.

[0087] FIG. 17 shows one of the voltage controls (condition 3) performed in experiment 2. Condition 3 corresponds to the voltage control of the third embodiment. Here, the voltage control of condition 2 is performed to reduce Cs ＋ After the Prussian blue thin film was taken in, the solution was exchanged and a positive voltage was applied to the electrode substrate for 120 minutes to bring the Prussian blue thin film into an oxidized (positive) state.

[0088] The results of Experiment 2 are shown in Figure 18. Figure 18 shows the results of comparing three cases: before the cesium-potassium mixed solution (K:Cs = 100:1) was placed in the container (before immersion), when voltage control under condition 2 was performed after the cesium-potassium mixed solution was placed in the container, and when voltage control under condition 3 was performed after the cesium-potassium mixed solution was placed in the container.

[0089] The upper part of Fig. 18 shows the ion number density of K per unit cell of the Prussian blue thin film, and the lower part of Fig. 18 shows the ion number density of Cs per unit cell of the Prussian blue thin film.

[0090] When voltage control was performed under condition 1, large amounts of K and Cs ions were sorbed onto the Prussian blue thin film. The Cs enrichment was 111.5. In contrast, when voltage control was performed under condition 3, both K and Cs ions were reduced to levels roughly equivalent to those before immersion. In other words, it was verified that by performing voltage control under condition 3, almost all of the cations after recovery were eluted and isolated, reducing the residue after cesium recovery and enabling the Prussian blue thin film to be reused.

[0091] [Each Aspect of the Present Disclosure] Each aspect of the present disclosure will be summarized below. ＋the Prussian blue thin film is disposed on one end of the container, a counter electrode is disposed on the other end of the container, an electrode substrate is disposed on the opposite side of the container from the Prussian blue thin film, and a voltage control unit connected to the counter electrode and the electrode substrate. The voltage control unit controls the applied voltage so that a positive voltage higher than the potential at which the Prussian blue thin film is neutral is applied to the electrode substrate for a time t1 to oxidize the Prussian blue thin film, and then a negative voltage lower than the potential at which the Prussian blue thin film is neutral is applied to the electrode substrate for a time t2 to reduce the Prussian blue thin film.

[0092] According to this embodiment, cesium can be recovered with high efficiency using the Prussian blue thin film.

[0093] In one embodiment, when the Prussian blue thin film is in a neutral state at a potential of 0.3 V to 0.8 V, the positive voltage is 0.9 V or more and the negative voltage is 0.1 V or less.

[0094] According to this embodiment, the voltage conditions under which cesium can be recovered with higher efficiency can be quantified.

[0095] In one embodiment, the voltage control unit of the cesium sorption device controls the applied voltage so as to repeat one or more cycles including applying a negative voltage to the electrode substrate for time t2 that is lower than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, applying a positive voltage to the electrode substrate for time t3 that is higher than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, and applying a negative voltage to the electrode substrate for time t4 that is lower than the potential at which the Prussian blue thin film is in a neutral state to reduce the Prussian blue thin film, where t3<t1.

[0096] According to this aspect, cesium can be concentrated and recovered with even higher efficiency using a Prussian blue thin film.

[0097] In one embodiment, after repeating the cycle, the voltage control unit of the cesium sorption device controls the applied voltage so that a positive voltage higher than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate for a time t5, thereby bringing the Prussian blue thin film into an oxidized (positive) state.

[0098] According to this embodiment, by eluting and isolating the recovered cesium, it is possible to reduce the residue after the recovery of cesium and to reuse the Prussian blue thin film.

[0099] In one embodiment of the cesium sorption device, t3<t5≦t1 or t3<t1≦t5.

[0100] According to this embodiment, the time condition for more efficiently recovering cesium can be quantified.

[0101] Another aspect of the present disclosure is a cesium sorption system of the present invention, which includes a plurality of the cesium sorption devices described above.

[0102] According to this embodiment, cesium can be highly efficiently concentrated and recovered using the Prussian blue thin film.

[0103] Yet another aspect of the present disclosure is a cesium sorption method. ＋ the Prussian blue thin film to be brought into an oxidized state by applying a positive voltage to the Prussian blue thin film for a time t1, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state; and the Prussian blue thin film to be brought into a reduced state by applying a negative voltage to the Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state.

[0104] According to this embodiment, cesium can be recovered with high efficiency using the Prussian blue thin film.

[0105] A cesium sorption method according to one embodiment includes a step of repeating one or more cycles including the steps of applying a negative voltage to a Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, applying a positive voltage to the Prussian blue thin film for a time t3, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, and applying a negative voltage to the Prussian blue thin film for a time t4, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, where t3<t1.

[0106] According to this aspect, cesium can be concentrated and recovered with even higher efficiency using a Prussian blue thin film.

[0107] In one embodiment of the cesium sorption method, after the step of repeating the cycle, a step of applying a positive voltage to the Prussian blue thin film for a time t5, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state, to oxidize the Prussian blue thin film.

[0108] According to this embodiment, by eluting and isolating the recovered cesium, it is possible to reduce the residue after the recovery of cesium and to reuse the Prussian blue thin film.

[0109] In one embodiment of the cesium sorption method, t3<t5≦t1 or t3<t1≦t5.

[0110] According to this embodiment, the time condition for more efficiently recovering cesium can be quantified.

[0111] In another aspect of the present disclosure, the cesium sorption device ＋the Prussian blue thin film is provided with a container for containing a solution containing the Prussian blue thin film, a Prussian blue thin film disposed at one end of the container, a counter electrode disposed at the other end of the container, an electrode substrate disposed on the opposite side of the container from the Prussian blue thin film, and a voltage control unit connected to the counter electrode and the electrode substrate. The voltage control unit controls the applied voltage by applying a positive voltage to the electrode substrate for a time t1, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film yellow, and then applying a negative voltage to the electrode substrate for a time t2, the negative voltage being lower than the potential at which the Prussian blue thin film is in a neutral state, to turn the Prussian blue thin film colorless.

[0112] According to this embodiment, cesium can be recovered with high efficiency using the Prussian blue thin film.

[0113] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and treatment processes, and that such modifications are also within the scope of the present invention.

[0114] Any combination of the above-described embodiments and modifications is also useful as an embodiment of the present disclosure. A new embodiment resulting from the combination has the combined effects of the combined embodiments and modifications.

[0115] When understanding the abstract technical ideas of the embodiments, the technical ideas should not be interpreted as being limited to the contents of the embodiments. The above-described embodiments and variations are merely illustrative examples, and many design modifications, such as changes, additions, and deletions of components, are possible. In the embodiments, the contents in which such design modifications are possible are emphasized by adding the notation "embodiment." However, design modifications are also permitted even in contents without such notation.

[0116] The cesium sorption device, cesium sorption system, and cesium sorption method disclosed herein can be widely used to remove radioactive cesium contained in soil in forests, farmland, pastures, urban areas, etc., and radioactive cesium contained in water such as rivers, lakes, dams, and seawater.

[0117] DESCRIPTION OF SYMBOLS 1: Cesium sorption device, 11: Container, 12: Prussian blue thin film, 13: Counter electrode, 14: Electrode substrate, 15: Voltage control unit, 16: Inlet, 17: Outlet, 100: Cesium sorption device, 116: Inlet, 117: Outlet, 200: Cesium sorption device, 216: Inlet, 217: Outlet, 300: Cesium sorption device, 316: Inlet, 317: Outlet, 1000: Cesium sorption system, S1: Cs ＋ S2: A step of applying a positive voltage to the Prussian blue thin film to bring the Prussian blue thin film into an oxidized state; S3: A step of applying a negative voltage to the Prussian blue thin film to bring the Prussian blue thin film into a reduced state; S4: A step of applying a positive voltage to the Prussian blue thin film for a short period to bring the Prussian blue thin film into an oxidized state, and then applying a negative-positive voltage to the Prussian blue thin film to bring the Prussian blue thin film into a reduced state; S5: A step of applying a positive voltage to the Prussian blue thin film to bring the Prussian blue thin film into an oxidized state.

Claims

1. Cs ＋ a Prussian blue thin film disposed at one end of the container; a counter electrode disposed at the other end of the container; an electrode substrate disposed on the opposite side of the container from the Prussian blue thin film; and a voltage control unit connected to the counter electrode and the electrode substrate, wherein the voltage control unit controls the applied voltage so that a positive voltage of a potential higher than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate for a time t1 to bring the Prussian blue thin film into an oxidized state, and then a negative voltage of a potential lower than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate for a time t2 to bring the Prussian blue thin film into a reduced state.

2. The cesium sorption device according to the amended claim 1, characterized in that when the Prussian blue thin film is in a neutral state at a potential of 0.3V to 0.8V, the positive voltage is 0.9V or more and the negative voltage is 0.1V or less.

3. The cesium sorption device described in claim 1, characterized in that the voltage control unit controls the applied voltage to repeat one or more cycles including applying a negative voltage to the electrode substrate for time t2, the potential of which is lower than the potential at which the Prussian blue thin film is in a neutral state, to reduce the Prussian blue thin film, then applying a positive voltage to the electrode substrate for time t3, the potential of which is higher than the potential at which the Prussian blue thin film is in a neutral state, to reduce the Prussian blue thin film, and applying a negative voltage to the electrode substrate for time t4, the potential of which is lower than the potential at which the Prussian blue thin film is in a neutral state, to reduce the Prussian blue thin film, wherein t3<t1.

4. The cesium sorption device of claim 3, wherein the voltage control unit controls the applied voltage so that after repeating the cycle, a positive voltage having a potential higher than the potential at which the Prussian blue thin film is in a neutral state is applied to the electrode substrate for a time t5, thereby bringing the Prussian blue thin film into an oxidized state.

5. A cesium sorption device according to claim 4, characterized in that t3<t5≦t1 or t3<t1≦t5.

6. A cesium sorption system comprising a plurality of cesium sorption devices according to any one of claims 1 to 5.

7. Cs ＋ a step of bringing a Prussian blue thin film into contact with a solution containing the compound; a step of applying a positive voltage to the Prussian blue thin film for a time t1, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state, to bring the Prussian blue thin film into an oxidized state; and a step of applying a negative voltage to the Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to bring the Prussian blue thin film into a reduced state.

8. The method according to claim 7, comprising a step of repeating one or more cycles of applying a negative voltage to the Prussian blue thin film for a time t2, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, followed by applying a positive voltage to the Prussian blue thin film for a time t3, the potential of which is higher than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, and applying a negative voltage to the Prussian blue thin film for a time t4, the potential of which is lower than the potential of the Prussian blue thin film in its neutral state, to reduce the Prussian blue thin film, wherein t3<t1.

9. The method according to claim 8, further comprising, after the step of repeating the cycle, applying a positive voltage to the Prussian blue thin film for a time t5, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, to bring the Prussian blue thin film into an oxidized state.

10. The method according to claim 9, wherein t3<t5≦t1 or t3<t1≦t5.

11. Cs ＋ a Prussian blue thin film disposed at one end of the container; a counter electrode disposed at the other end of the container; an electrode substrate disposed on the opposite side of the container from the Prussian blue thin film; and a voltage control unit connected to the counter electrode and the electrode substrate, wherein the voltage control unit controls the applied voltage by applying a positive voltage to the electrode substrate for a time t1, the positive voltage being higher than the potential at which the Prussian blue thin film is in a neutral state, causing the Prussian blue thin film to turn yellow, and then applying a negative voltage to the electrode substrate for a time t2, the negative voltage being lower than the potential at which the Prussian blue thin film is in a neutral state, causing the Prussian blue thin film to turn colorless.

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