Method for separating palladium ions

Abstract

To separate palladium ions from a solution containing plural kinds of metal ions at a high speed with high selectivity.SOLUTION: The method for separating Pd ions is a method for selectively separating Pd ions from a solution containing a plurality of kinds of metal ions including at least Pd ions. The method for separating Pd ions includes a sorption step of electrochemically depositing and sorbing Pd on the surface of a thin film 12 by applying a reduction potential to a base material 11 in a state where a sorption body 10 having the base material 11 formed of a conductive material and the thin film 12 formed of Prussian blue (PB) and covering the surface of the base material 11 is immersed in a solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for separating palladium ions. [Background technology]

[0002] High-level radioactive liquid waste (HLLW) generated from one ton of spent nuclear fuel from a light-water reactor (LWR) (burnup: 30,000 MWd / t, cooling period: 150 days) contains large amounts of platinum group metals (PGMs): ruthenium (Ru), rhodium (Rh), and palladium (Pd), at 2.09 kg, 0.36 kg, and 1.20 kg, respectively. These PGMs (Ru, Rh, and Pd) are vitrified along with long-lived nuclides such as americium and are disposed of in deep geological formations. Because PGMs are important rare metals for industrial applications, such as in pharmaceutical synthesis and automotive exhaust purification catalysts, securing their supply chain is a crucial policy issue for Japan, a resource-poor country. Furthermore, recovering PGMs from HLLW allows for more HLLW to be packed into glass, thereby reducing the number of vitrified bodies and reducing disposal space and costs in geological repositories.

[0003] In order to solve the above-mentioned problems, it is necessary to separate the platinum group elements from the high-level radioactive liquid waste. In addition, among the platinum group elements contained in high-level radioactive liquid waste, palladium-107 (107Pd) is a radioactive nuclide with a long half-life of 6.5 million years, so it is necessary to selectively separate palladium ions.

[0004] Conventional methods for removing platinum group elements from high-level radioactive liquid waste include chemical precipitation and solvent extraction. Furthermore, there is a conventional method in which a platinum group element is electrochemically deposited on a cathode from a solution containing the platinum group element (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2-38536 Summary of the Invention [Problem to be solved by the invention]

[0006] Chemical precipitation and solvent extraction methods have problems such as the large number of steps, which makes them complicated and time-consuming, and also the large amount of organic solvent waste liquid that generates, which places a heavy burden on the environment. In the case of the method of electrochemically depositing a platinum group element, although the platinum group element can be deposited on the cathode, palladium ions cannot be selectively electrodeposited and separated.

[0007] These problems are not limited to those that selectively separate palladium ions from high-level radioactive liquid waste, but also apply to those that selectively separate palladium ions from other industrial (mainly electronic equipment) waste (urban mines) and natural mines. [Means for solving the problem]

[0008] Various embodiments of a method for separating palladium ions to solve the above problems will be described below. [Aspect 1] A method for selectively separating palladium ions from a solution containing a plurality of types of metal ions including at least palladium ions, comprising: a sorption step of electrochemically depositing and sorbing palladium on the surface of the thin film by applying a reduction potential to a sorption body, the sorption body having a substrate made of a conductive material and a thin film formed of at least one of Prussian blue and a Prussian blue analogue and covering the surface of the substrate, while the sorption body is immersed in the solution. A method for separating palladium ions.

[0009] According to this method, palladium ions are separated at a higher speed and with a higher selectivity than when a reduction potential is not applied to the substrate of the sorption body, and therefore, palladium ions can be separated at a higher speed and with a higher selectivity from a solution containing multiple types of metal ions.

[0010] [Aspect 2] The substrate is in a sheet form. A method for separating palladium ions according to embodiment 1.

[0011] According to this method, the substrate, i.e., the sorption body, is in a sheet form, making it easy to handle the sorption body. [Aspect 3] a recovery step of applying an oxidation potential to the substrate while the sorbent body is immersed in an electrolyte solution after the sorption step, thereby dissolving palladium deposited on the surface of the thin film as ions into the electrolyte solution and recovering the palladium. The method for separating palladium ions according to embodiment 1 or embodiment 2.

[0012] According to this method, the sorbent after the sorption step, i.e., the sorbent with palladium deposited on the surface of the thin film, is immersed in an electrolytic solution, and when an oxidation potential is applied to the substrate, palladium ions are eluted into the solution. This allows palladium ions to be separated from the thin film of the sorbent. Therefore, palladium ions can be recovered quickly and with high selectivity. Furthermore, the sorbent can be reused after palladium ions have been recovered.

[0013] [Aspect 4] The solution contains rhodium ions, which are metal ions, When the sorption step is a first sorption step and the recovery step is a first recovery step, a second sorption step of electrochemically depositing and sorbing rhodium on the surface of the thin film by applying a reduction potential to the base material while the sorption body after the first recovery step is immersed in the solution in which the palladium ions have been recovered. A method for separating palladium ions according to embodiment 3.

[0014] According to this method, rhodium ions are sorbed at a higher speed and with a higher selectivity than when a reduction potential is not applied to the substrate of the sorption body. Therefore, rhodium ions can be separated from a solution containing multiple types of metal ions at a higher speed and with a higher selectivity. Therefore, rhodium ions can be separated from a solution containing multiple types of metal ions at a higher speed and with a higher selectivity.

[0015] [Aspect 5] a second recovery step of immersing the sorbent after the second sorption step in an electrolyte different from the electrolyte, and applying an oxidation potential to the substrate to elute rhodium deposited on the surface of the thin film as ions into the electrolyte, thereby recovering the rhodium. A method for separating palladium ions according to embodiment 4.

[0016] According to this method, when the sorbent, which has rhodium deposited on the surface of the thin film after the second sorption step, is immersed in an electrolyte and an oxidation potential is applied to the substrate, rhodium ions are dissolved into the solution. This allows rhodium ions to be separated from the thin film of the sorbent. Therefore, rhodium ions can be recovered quickly and with high selectivity. Furthermore, the sorbent can be reused after rhodium ions have been recovered. [Effects of the Invention]

[0017] According to the present invention, palladium ions can be separated from a solution containing a plurality of types of metal ions at high speed and with high selectivity. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view of a sorbent body used in a method for separating palladium (Pd) ions according to one embodiment. [Figure 2]FIG. 2(a) is a two-dimensional schematic diagram showing the crystal structure of Prussian blue (PB) forming the thin film of the sorption body of Examples 1 and 2, and FIG. 2(b) is a three-dimensional schematic diagram showing 1 / 8 of the unit lattice of the PB crystal. [Figure 3] Figure 3(a) is a schematic diagram of the crystalline structure of PB in a reduced state, Figure 3(b) is a schematic diagram of the crystalline structure of PB in a neutral state, and Figure 3(c) is a schematic diagram of the crystalline structure of PB in an oxidized state. [Figure 4] 4(a) to 4(d) are graphs showing the number density of ions (Fe, Ru, Rh, Pd) contained in the unit cell of the PB crystal before and after immersion in the solution and after application of a reduction potential in Example 1, respectively. [Figure 5] FIG. 5 is a graph showing the relationship between the potential applied to the substrate of the sorption body, the number density of ions (Ru, Rh, Pd) contained in the PB unit cell, and the selectivity for Pd ions. [Figure 6] FIG. 6 is an SEM image of the thin film surface showing the deposition of Pd. [Figure 7] Figure 7(a) is a schematic diagram showing PB crystals and metal ions in solution when no potential is applied to the substrate of the sorption body, Figure 7(b) is a schematic diagram showing PB crystals and metal ions in solution when a reduction potential is applied to the substrate, and Figure 7(c) is a schematic diagram showing a state where a reduction potential is applied to the substrate and Pd is precipitated near the surface of the thin film. [Figure 8] FIG. 8 is a graph showing the relationship between the potential applied to the substrate of the sorption body and the number density of ions (Ru, Rh, Pd, Mo) contained in the PB unit cell and the selectivity for Pd ions. [Figure 9] FIG. 9 is a graph showing the relationship between the immersion time and the total amount of sorption, which is the total amount of ions (Ru, Rh, Pd, Mo) sorbed per unit area of ​​the sorption body. [Figure 10] FIG. 10 is a graph showing the relationship between the immersion time and the selectivity of ions (Ru, Rh, Pd, Mo). [Figure 11]Figure 11(a) is an SEM image of the thin film surface of a sorbent immersed for 15 minutes with a reduction potential applied, and Figure 11(b) is an SEM image of the thin film surface of a sorbent immersed for 60 minutes with a reduction potential applied. [Figure 12] FIG. 12(a) is a graph showing the sample mass before sorption, after sorption, and after recovery, and FIG. 12(b) is a graph showing the proportion of Pd before sorption, after sorption, and after recovery. [Figure 13] FIG. 13 is a three-dimensional perspective view of one-eighth of a unit cell of copper ferrocyanide (PBA) forming a thin film of an alternative sorbent body. [Figure 14] Figure 14(a) is a graph showing the number density of ions (Pd, Ru, Rh) contained in the unit cell of the PB crystal after immersion, and Figure 14(b) is a graph showing the number density of ions (Pd, Ru, Rh) contained in the unit cell of the PBA crystal after immersion. [Figure 15] FIG. 15 is a graph showing the relationship between the potential applied to the substrate of the sorption body and the number density of Pd ions contained in the PBA unit cell and the selectivity of Pd ions. DETAILED DESCRIPTION OF THE INVENTION

[0019] One embodiment of a method for separating palladium ions will be described below with reference to Figures 1 to 12. The method for separating palladium ions of this embodiment is a method for selectively separating Pd ions from a solution containing multiple types of metal ions including at least palladium (Pd) ions.

[0020] As shown in FIG. 1, the method for separating Pd ions includes a sorption step in which a sorption body 10 having a substrate 11 made of a conductive material and a thin film 12 formed of at least one of Prussian blue (PB, also known as iron ferrocyanide) and a PB analogue and covering the surface of the substrate 11 is immersed in the above-mentioned solution, and a reduction potential is applied to the substrate 11 to electrochemically deposit and sorb Pd on the surface of the thin film 12.

[0021] The reduction potential in the sorption step is preferably −0.1 V or less, more preferably −0.2 V or less. The method for separating palladium ions also includes a recovery step in which, while the sorbent 10 after the sorption step is immersed in an electrolyte, an oxidation potential is applied to the substrate 11, thereby dissolving Pd deposited on the surface of the thin film 12 as ions into the electrolyte and recovering the ions. The electrolyte in this embodiment is a potassium chloride solution. However, the electrolyte is not limited to a potassium chloride solution and may be another electrolyte, such as a sodium chloride solution.

[0022] The oxidation potential in the sorption step is preferably 1.0 V or higher. (sorbent 10) 1, the substrate 11 constituting the sorption body 10 is preferably in the form of a sheet. The substrate 11 is, for example, a square with one side measuring 100 mm. The substrate 11 is made of, for example, tantalum (Ta).

[0023] The thin film 12 is preferably in the form of a sheet made of PB. The thickness of the thin film 12 is, for example, several tens to several hundreds of nm. Next, the crystal structure of PB will be described with reference to FIG.

[0024] As shown in Figures 2(a) and 2(b), PB absorbs trivalent iron ions (Fe 3+ ) and divalent iron ions (Fe 2+ ) and cyanide ion (CN - ) and all CN - But two Fe 3+ ,Fe 2+ By combining with each other, the PB as a whole has a lattice structure like a jungle gym.

[0025] In the PB crystal lattice, adjacent trivalent iron ions (Fe 3+ ) and divalent iron ions (Fe 2+ The center-to-center distance between the PB crystal lattice is 0.5 nm. There are tiny spaces inside the PB crystal lattice.

[0026] Next, the relationship between the potential applied to the substrate 11 of the sorption body 10 and the PB crystals will be described with reference to FIG. As shown in FIG. 3(b), when no potential is applied to the substrate 11 of the sorption body 10, the PB crystals are ionized by trivalent iron ions (Fe 3+ ) and divalent iron ions (Fe 2+ ) and is in a neutral state. The chemical formula of PB crystals immersed in potassium chloride solution is KFe(III)[Fe(II)CN6]. The unit cell of the PB crystal contains 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 cell of the PB crystal contains 24 potassium ions (K + ) is included.

[0027] As shown in FIG. 3(a), when a reduction potential is applied to the substrate 11 of the sorption body 10, the trivalent iron ions (Fe 3+ ) gives up electrons to form divalent iron ions (Fe 2+ ) (negative ionization).

[0028] As shown in FIG. 3(c), when an oxidation potential is applied to the substrate 11 of the sorption body 10, the divalent iron ions (Fe 2+ ) loses electrons to become trivalent iron ions (Fe 3+ ) (positive ionization).

[0029] Example 1 In Example 1, the sorption body 10 was immersed in a solution containing Pd ions, ruthenium (Ru) ions, and rhodium (Rh) ions, all of which are polyvalent metal ions, and a reduction potential was applied to the substrate 11, while the following various measurements were performed.

[0030] First, referring to Figure 4, we will explain the measurement results of the number density of ions (Fe, Ru, Rh, Pd) contained in the unit cell of the PB crystal (PB unit cell) before immersion in the solution, after immersion, and after application of a reduction potential (-0.2 V).

[0031] As shown in FIGS. 4(a) to 4(d), before immersion, the number density of Fe ions was 8, and the number densities of Ru ions, Rh ions, and Pd ions were all 0. After immersion (without application of a reduction potential), the number density of Fe ions was 7.7, the number density of Ru ions was 0.07, the number density of Rh ions was 0.01, and the number density of Pd ions was 0.20.

[0032] The increase in the number density of Ru ions, Rh ions, and Pd ions was approximately equal to the decrease in the number density of Fe ions before and after immersion, which indicates that Fe ions were replaced by Ru ions, Rh ions, and Pd ions through immersion. In particular, the increase in the number density of Pd ions through immersion was greater than that of Ru ions and Rh ions.

[0033] After immersion (application of a reducing potential), the number density of Fe ions was 7.2, that of Ru ions was 0.02, that of Rh ions was 0.09, and that of Pd ions was 0.63. The number densities of Rh ions and Pd ions increased by applying a reducing potential.

[0034] The increase in the number density of Ru ions, Rh ions, and Pd ions was approximately equal to the decrease in the number density of Fe ions before and after immersion. From these results, it can be said that the application of a reduction potential promoted the replacement of Fe ions with Rh ions and Pd ions. In particular, the increase in the number density of Pd ions due to the application of a reduction potential was larger than that of Ru ions and Rh ions.

[0035] Next, with reference to Fig. 5, the relationship between the potential n (V) applied to the substrate 11 of the sorption body 10, the number density of ions (Ru, Rh, Pd) contained in the PB unit cell, and the selectivity (%) for Pd ions will be described. The selectivity for Pd ions is the ratio of the number density of Pd ions to the sum of the number densities of Ru ions, Rh ions, and Pd ions contained in the PB unit cell. The immersion time was 2 hours (120 minutes) in both cases.

[0036] As shown by "◯" in FIG. 5, within the range of potential n from -0.4 V to 0.5 V, the number density of Pd ions increased as the potential n decreased. When the potential n was -0.1 V, the number density of Pd ions was 0.45 / unit cell. When the potential n was -0.2 V, the number density of Pd ions was 0.63 / unit cell. When the potential n was -0.3 V, the number density of Pd ions was 6.4 / unit cell. When the potential n was -0.4 V, the number density of Pd ions was 8.3 / unit cell, which was 42 times higher than when no reduction potential, a negative potential, was applied.

[0037] As shown by "x" and "Δ" in FIG. 5, within the range of potential n from -0.4 V to 0.5 V, the number densities of Ru ions and Rh ions hardly increased even when the potential n decreased.

[0038] As shown by "□" in Figure 5, the selectivity for Pd ions increased as the potential n decreased within the range of -0.4 V to 0.5 V. When the potential n was -0.1 V or less, the selectivity for Pd ions was 80% or more. When the potential n was -0.3 V and -0.4 V, the selectivity for Pd ions was approximately 100%.

[0039] From these findings, it is believed that when the potential n is greater than −0.1 V, substitutional sorption occurs, as will be described in detail later, and when the potential n is −0.1 V or less, surface electrodeposition occurs.

[0040] FIG. 6 shows an SEM image of the surface of the thin film 12 of the sorption body 10 when the potential n is −0.3V. As shown in Figure 6, numerous crystals (white) were observed on the surface of the thin film 12. Elemental analysis (EDS) of the surface of the thin film 12 confirmed that the numerous crystals were metallic Pd. This indicates that Pd is deposited on the surface of the thin film 12, i.e., surface electrodeposition has occurred.

[0041] Next, the mechanism of substitutional sorption and surface electrodeposition of Pd ions will be explained with reference to FIG. As shown in FIG. 7(a), in the PB crystals in the solution when no potential is applied to the substrate 11 of the sorption body 10, as described above, trivalent iron ions (Fe 3+ ) and divalent iron ions (Fe 2+ ) maintains an electrically neutral state.

[0042] As shown in FIG. 7(b), when a reduction potential is applied to the substrate 11, trivalent iron ions (Fe 3+ ) is reduced to divalent iron ions (Fe 2+ ), the PB crystal becomes negatively charged as a whole. This causes the positive ions (Rh 3+ , Pd 2+ ) are electrically attracted by the PB crystal. Note that Ru ions are not shown in the figure.

[0043] Pd ions exist as planar tetracoordinate complexes in solution, and because Pd ions are large enough to easily enter tiny spaces (vacant sites) in the PB crystal lattice, they are thought to be able to easily enter these sites.

[0044] Furthermore, first-principles calculations show that when no potential is applied, there is a potential energy pocket inside the PB crystal lattice into which Pd ions fall. On the other hand, when a reduction potential is applied and all of the iron ions constituting the PB crystal lattice become divalent, the potential energy inside the PB crystal lattice becomes flat, and this pocket inside the PB crystal lattice is thought to disappear. This is thought to make it easier for Pd ions to penetrate deep into the thin film 12 without falling into this pocket.

[0045] Furthermore, by applying a reduction potential, the divalent iron ions (Fe 2+ ) is converted to divalent Pd ions (Pd 2+ ) and the divalent iron ions (Fe 2+ ) is released into solution, resulting in displacement sorption.

[0046] 7(c), as the degree of supersaturation of Pd ions increases near the surface of the thin film 12 due to the progression of substitutional sorption of Pd ions, the crystal growth of Pd is promoted with the Pd ions as nuclei. The Pd crystals grown near the surface of the thin film 12 act as a barrier, making it difficult for Rh ions and Ru ions in the solution to penetrate into the thin film 12. As a result, surface electrodeposition occurs, in which a large amount of Pd is electrochemically deposited on the surface of the thin film 12.

[0047] <Example 2> In Example 2, the sorption body 10 was immersed in a solution containing molybdenum (Mo) ions in addition to Pd ions, Ru ions, and Rh ions, and a reduction potential was applied to the substrate 11, and the following various measurements were performed.

[0048] First, referring to Fig. 8, the relationship between the potential n (V) applied to the substrate 11 of the sorption body 10, the number density of ions (Ru, Rh, Pd, Mo) contained in the PB unit cell, and the selectivity (%) for Pd ions will be described. The selectivity for Pd ions is the ratio of the number density of Pd ions to the sum of the number densities of Ru ions, Rh ions, Pd ions, and Mo ions contained in the PB unit cell. The immersion time was 2 hours (120 minutes) in both cases.

[0049] As shown by "□" in FIG. 8, within the range of potential n from -0.4 V to 0.5 V, the number density of Pd ions increased as the potential n decreased. When the potential n was -0.1 V, the number density of Pd ions was 2.3 / unit cell. When the potential n was -0.2 V, the number density of Pd ions was 3.2 / unit cell. When the potential n was -0.3 V, the number density of Pd ions was 7.4 / unit cell. When the potential n was -0.4 V, the number density of Pd ions was 21 / unit cell.

[0050] As shown by "x", "△", and "◇" in Figure 8, within the range of potential n from -0.4 V to 0.5 V, the number densities of Ru ions, Rh ions, and Mo ions hardly increased even when the potential n decreased.

[0051] As shown by "◯" in Fig. 8, the selectivity of Pd ions increased as the potential n decreased within the range of potential n from -0.4 V to 0.5 V. Furthermore, when the potential n was -0.1 V or less, the selectivity of Pd ions was 95% or more.

[0052] From these findings, it can be said that the relationship between the potential n and the number density of Pd ions, and the relationship between the potential n and the selectivity for Pd, do not change even in a solution that contains Mo ions in addition to Pd, Ru, and Rh ions.

[0053] Next, referring to FIG. 9, the relationship between the immersion time (minutes) in the solution when the potential n is −0.3 V and the total sorption amount (mg / cm ), which is the total weight of ions sorbed per unit area of ​​the sorbent body 10, is shown. 2 The masses of Ru, Rh, and Pd contained in the solution before immersion were 6.47 mg, 6.81 mg, and 6.59 mg, respectively.

[0054] As shown in FIG. 9, within the range of immersion times from 0 to 120 minutes, the total amount of sorption increased with increasing immersion time. Next, with reference to FIG. 10, the relationship between the immersion time (minutes) in the solution when the potential n is −0.3 V and the selectivity of ions (Ru, Rh, Pd, Mo) will be described.

[0055] As shown in FIG. 10, when the immersion time was within the range of 15 to 40 minutes, the selectivity for Pd ions was 90% or higher. When the immersion time was 60 minutes, the selectivity for Pd ions was 85.6±9.1%, the selectivity for Rh ions was 6.0±7.1%, the selectivity for Ru ions was 3.1±2.8%, and the selectivity for Mo ions was 0.84±0.9%.

[0056] When the immersion time was 120 minutes, the selectivity for Pd ions was 60.2±0.8%, the selectivity for Rh ions was 27.4±0.7%, the selectivity for Ru ions was 11.5±0.3%, and the selectivity for Mo ions was 1.1±0.46%.

[0057] FIG. 11(a) shows an SEM image of the surface of the thin film 12 of the sorption body 10 after an immersion time of 15 minutes. FIG. 11(b) shows an SEM image of the surface of the thin film 12 of the sorption body 10 after immersion for 60 minutes.

[0058] The potential n is −0.3 V in all cases. It was confirmed that when the immersion time was 60 minutes, the crystals (white) on the surface of the thin film 12 became larger than when the immersion time was 15 minutes, that is, the surface electrodeposition progressed.

[0059] In Example 2, the sorption body 10 after the sorption step was immersed in a potassium chloride solution, and an oxidation potential was applied to the substrate 11, and the following measurements were carried out. The relationship between the sample mass (mg) and the proportion (%) of Pd ions before the sorption step, after the sorption step, and after the recovery step will be described with reference to Fig. 12. In the sorption step, the reduction potential applied to the substrate 11 was -0.3 V, and the immersion time was 240 minutes. In the recovery step, the oxidation potential applied to the substrate 11 was 2.0 V, and the oxidation potential was applied for 12 hours.

[0060] As shown in Figure 12(a), the sample mass after the sorption step increased by 0.770 mg compared to before the sorption step, and the sample mass after the recovery step decreased by 0.785 mg compared to after the sorption step (before the recovery step).

[0061] As shown in Figure 12(b), the proportion of Pd ions was approximately 60% after the sorption process, and 54.2±14% after the recovery process. Since the sample mass decreased after the recovery process and the proportion of Pd ions also decreased, it can be said that the Pd ions sorbed in the PB crystals were dissolved into the potassium chloride solution.

[0062] Next, the effects of this embodiment will be described. (1) The method for separating Pd ions includes a sorption step in which a sorption body 10 having a substrate 11 made of a conductive material and a thin film 12 formed of at least one of Prussian blue (PB) and a Prussian blue analogue and covering the surface of the substrate 11 is immersed in a solution, and a reduction potential is applied to the substrate 11 to electrochemically deposit and sorb Pd on the surface of the thin film 12.

[0063] According to this method, Pd ions are sorbed at a higher speed and with a higher selectivity than when no reduction potential is applied to the substrate 11 of the sorption body 10. Therefore, Pd ions can be separated from a solution containing multiple types of metal ions at a higher speed and with a higher selectivity.

[0064] (2) Since the substrate 11, that is, the sorption body 10, is in a sheet form, the sorption body 10 can be easily handled. (3) The method for separating Pd ions includes a recovery step in which the sorption body 10 after the sorption step is immersed in a potassium chloride solution, and an oxidation potential is applied to the substrate 11 to dissolve and recover Pd deposited on the surface of the thin film 12 as ions into the potassium chloride solution.

[0065] According to this method, when the sorbent 10 after the sorption step, i.e., the sorbent 10 with Pd deposited on the surface of the thin film 12, is immersed in a potassium chloride solution and an oxidation potential is applied to the substrate 11, the deposited Pd is eluted as ions. This allows Pd ions to be separated from the thin film 12 of the sorbent 10. Therefore, Pd ions can be recovered quickly and with high selectivity. Furthermore, the sorbent 10 can be reused after the Pd ions have been recovered.

[0066] <Example of change> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0067] The sorption step in Example 1 above is referred to as the first sorption step, and the recovery step is referred to as the first recovery step. A second sorption step may be provided in which the sorbent 10 after the first recovery step is immersed in the solution from which Pd ions have been recovered, and a reduction potential is applied to the substrate 11 to electrochemically deposit and sorb Rh on the surface of the thin film 12. This method allows Rh ions to be sorbed at a higher speed and with a higher selectivity than when a reduction potential is not applied to the substrate 11 of the sorbent 10. Therefore, Rh ions can be separated from a solution containing multiple types of metal ions at a higher speed and with a higher selectivity.

[0068] Furthermore, a second recovery step may be provided in which the sorption body 10 after the second sorption step is immersed in a potassium chloride solution different from the potassium chloride solution of Example 1, and an oxidation potential is applied to the substrate 11 to elute Rh deposited on the surface of the thin film 12 as ions into the potassium chloride solution and recover the Rh.

[0069] According to this method, when the sorbent 10, in which Rh is deposited on the surface of the thin film 12 after the second sorption step, is immersed in a potassium chloride solution and an oxidation potential is applied to the substrate 11, Rh ions are eluted into the solution. This allows Rh ions to be separated from the thin film 12 of the sorbent 10. Therefore, Rh ions can be recovered at high speed and with high selectivity. In addition, the sorbent 10 can be reused after the Rh ions have been recovered.

[0070] In the above-described embodiment and modified examples, the sorbent body 10 has a thin film 12 formed from PB (Prussian Blue), i.e., iron ferrocyanide, but is not limited to this. The thin film 12 may also be formed from a Prussian Blue Analogue (PBA). Also, the thin film 12 may be formed from both PB and PBA.

[0071] In the modified examples shown in FIGS. 13 to 15, the thin film 12 of the sorption body 10 is formed of copper ferrocyanide, which is one of the PBAs. The crystal structure of copper ferrocyanide will be described with reference to FIG.

[0072] As shown in Figure 13, copper ferrocyanide reacts with divalent copper ions (Cu 2+ ) and divalent iron ions (Fe 2+ ) and cyanide ion (CN - ) and all CN - But two Cu 2+ ,Fe 2+By bonding with each other, copper ferrocyanide has a lattice structure like a jungle gym as a whole.

[0073] With reference to FIG. 14, the measurement results of the number density of ions (Pd, Ru, Rh) contained in the unit cell of the PB crystal (PB unit cell) after immersion in the solution (without application of a reduction potential) will be described.

[0074] As shown in Figures 14(a) and 14(b), the number density of Pd ions in PB was 0.2, whereas the number density of Pd ions in copper ferrocyanide was 0.48.

[0075] The number density of Ru ions in PB was 0.07, whereas the number density of Ru ions in copper ferrocyanide was 0.01. Furthermore, the number density of Rh ions in PB was 0.01, whereas the number density of Rh ions in copper ferrocyanide was 0.01.

[0076] In a state where no reduction potential is applied to the thin film 12 of the sorption body 10, copper ferrocyanide can be said to have a higher sorption ability for Pd ions than PB. Next, the relationship between the potential n (V) applied to the substrate 11 of the sorption body 10, the number density of Pd ions contained in the copper ferrocyanide unit lattice, and the selectivity (%) for Pd ions will be described with reference to Fig. 15. The selectivity for Pd ions is the ratio of the number density of Pd ions to the sum of the number densities of Ru ions, Rh ions, and Pd ions contained in the copper ferrocyanide unit lattice. The immersion time was 2 hours (120 minutes) in both cases.

[0077] As shown by "□" in FIG. 15, within the range of potential n from -0.2 V to 0.4 V, the number density of Pd ions increased as the potential n decreased. When the potential n was -0.1 V, the number density of Pd ions was 1.1 per unit cell.

[0078] As shown by "▽" in FIG. 15, when the potential n was in the range of -0.2V to 0.4V, the selectivity of Pd ions was 80% or more. The above-mentioned Example 1 illustrates a method for selectively separating Pd ions from a solution containing Pd ions, Ru ions, and Rh ions, all of which are polyvalent metal ions. The above-mentioned Example 2 illustrates a method for selectively separating Pd ions from a solution containing Pd ions, Ru ions, Rh ions, and Mo ions, all of which are polyvalent metal ions. However, the present invention is not limited to this. The solution may contain at least Pd ions, and may also contain other polyvalent metal ions or monovalent metal ions.

[0079] While the above-described embodiment and modified examples illustrate a method for selectively separating Pd ions from high-level radioactive liquid waste, the present invention is not limited thereto. For example, the present invention can also be embodied as a method for selectively separating Pd ions from other industrial wastes, urban mines, etc.

[0080] The substrate 11 of the sorption body 10 may be made of any conductive material, and may be changed to a carbon material instead of Ta. The substrate 11 is not limited to a sheet shape, but can be changed to a fibrous shape such as carbon fiber. [Explanation of symbols]

[0081] 10...Sorption body 11...Base material 12...Thin film

Claims

1. A method for selectively separating palladium ions from a solution containing a plurality of types of metal ions including at least palladium ions, comprising: a sorption step of electrochemically depositing and sorbing palladium on the surface of the thin film by applying a reduction potential to a sorption body, the sorption body having a substrate made of a conductive material and a thin film formed of at least one of Prussian blue and a Prussian blue analogue and covering the surface of the substrate, while the sorption body is immersed in the solution. A method for separating palladium ions.

2. The substrate is in a sheet form. The method for separating palladium ions according to claim 1.

3. a recovery step of applying an oxidation potential to the substrate while the sorbent body is immersed in an electrolyte solution after the sorption step, thereby dissolving palladium deposited on the surface of the thin film as ions into the electrolyte solution and recovering the palladium. The method for separating palladium ions according to claim 1 or 2.

4. The solution contains rhodium ions, which are metal ions, When the sorption step is a first sorption step and the recovery step is a first recovery step, a second sorption step of electrochemically depositing and sorbing rhodium on the surface of the thin film by applying a reduction potential to the base material while the sorption body after the first recovery step is immersed in the solution in which the palladium ions have been recovered. The method for separating palladium ions according to claim 3.

5. a second recovery step of immersing the sorbent after the second sorption step in an electrolyte different from the electrolyte, and applying an oxidation potential to the substrate to elute rhodium deposited on the surface of the thin film as ions into the electrolyte, thereby recovering the rhodium. The method for separating palladium ions according to claim 4.

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