Method for recovering cations, and method for producing cation-containing liquid having reduced cation concentration

Abstract

The purpose of the present invention is to provide a method for electrochemically recovering target ions, which is a technique for selectively recovering the target ions in a sufficient amount relative to the amount of non-target ions with a voltage at which the electrolysis of water does not occur. The present invention is a method for recovering target cations, the method comprising: an adsorption step in which a first electrode and a second electrode are immersed in an aqueous solution of interest which contains one or more types target cations and one or more types of non-target cations that are different from the target cations, and then a negative electrode of a direct-current power source is connected to the first electrode and a positive electrode of the direct-current power source is connected to the second electrode so as to distribute an electric power at a constant current, thereby causing the adsorption of the target cations onto the first electrode, wherein the first electrode contains one or more types of Prussian blue analogs and the content of the target cations in the first electrode is equal to or less than a predetermined amount; a specific desorption step; and the like.

Description

Method for recovering cations and method for producing cation-containing liquid with reduced cation concentration

[0001] The present invention relates to a method for recovering cations to be recovered and a method for producing a cation-containing liquid in which the concentration of the cations to be recovered is reduced.

[0002] Cations contained in water may need to be recovered or removed depending on their harmfulness or usefulness. For example, ammonium ions (NH ４ ＋ ) is a useful nutrient for plants. On the other hand, ammonium ions (NH ４ ＋ High concentrations of ammonium ions (NH ４ ＋ Aqueous solutions with high concentrations of potassium ions (K) cannot be released into the environment. ＋ ) is also one of the three major nutrients for plants. ＋ ) is used as a raw material for lithium-ion batteries.

[0003] In this way, cations in water can be recovered and used as industrial or agricultural materials, and by recovering them from wastewater, the wastewater can be made harmless. Furthermore, if the cations to be recovered can be concentrated during recovery, the cost of post-treatment aimed at using them as resources can be reduced.

[0004] Methods for removing or recovering ammonium ions using adsorbents have been reported. For example, Patent Document 1 discloses an ammonium ion adsorbent of a metal cyano complex (also known as a Prussian blue analogue). Patent Documents 2 and 3 disclose the ammonium ion adsorption capacity of zeolite. Furthermore, Patent Document 3 discloses the ammonium ion adsorption capacity of amorphous aluminosilicate.

[0005] Methods of utilizing adsorbents for ammonium ions or potassium ions as electrochemical electrodes have also been investigated. For example, Non-Patent Document 1 discloses a method of using a copper-substituted Prussian blue analogue as an electrode. Non-Patent Document 2 discloses a method of recovering potassium ions using a cobalt-substituted Prussian blue analogue. Furthermore, Patent Documents 4, 5, and 6 disclose methods of adsorbing target ions using a Prussian blue analogue.

[0006] The electrochemical ion adsorption selectivity of Prussian blue analogues has been investigated in detail in Non-Patent Document 3. In particular, alkali metal ions and NH ４ ＋ The hydration energy of ions varies depending on the ion, and when these ions are adsorbed onto Prussian blue analogues, the water molecules separate. Therefore, ions with larger hydration energy lose more energy during adsorption, resulting in different potentials at which ion adsorption and desorption occur due to oxidation-reduction. In particular, in the case of copper-substituted Prussian blue analogues, the difference is clear, with the ions that are easiest to adsorb being Cs ＋ ion, Rb ＋ Ion, NH ４ ＋ Ion, K ＋ Ion, Na ＋ ions. Furthermore, Ca ２＋ Ions and Mg ２＋ It is known that other ions, such as ions, have a large ionic radius and are therefore even less likely to be adsorbed onto Prussian blue analogues. ４ ＋ Ions and Na ＋ When ions coexist, NH ４ ＋ ions, K ＋ Ions and Na ＋ When ions coexist, K ＋ Therefore, when a Prussian blue analogue is used as an electrochemical electrode, it is possible, in principle, to adsorb and recover specific ions as target ions according to the above-mentioned hierarchy.

[0007] Japanese Patent No. 6789523 JP 2022-141197 JP 2023-162591 JP 2021-63810 JP 5669183 JP 2008-46001

[0008] Env. Sci. Technol. Lett. , 2018, 5, 9, 578-583 Materials, 2021, 14, 3592. RSC Adv. ,2018,8,37356-37364

[0009] The methods using the adsorbents described in Patent Documents 1, 2, and 3 are capable of adsorbing ammonium ions, but do not disclose a method for recovering and separating the adsorbed ammonium ions. If they do, they disclose a method of causing ion exchange by contacting the adsorbent with another cation, or a method using heating. The ion exchange method requires the preparation of another cation, and the heating method requires a large amount of energy because the entire adsorbent must be heated.

[0010] As mentioned above, by utilizing electrochemical adsorption / desorption as disclosed in Non-Patent Document 1 and elsewhere, it is possible, in principle, to selectively recover target ions using an electric signal, which allows for significant cost reduction compared to ion exchange and heating. However, in practice, there are challenges to achieving this. For example, the method disclosed in Non-Patent Document 1 can remove ammonium ions by applying a constant voltage, but it also has the problem of low selectivity for ammonium ions relative to sodium ions, i.e., the ratio of the amount of ammonium ions recovered to the amount of sodium ions recovered is insufficient.

[0011] Non-Patent Document 2, Patent Document 5, and Patent Document 6 show the adsorption of ions by constant voltage or voltage sweep, but all of them use thin film electrodes of 1 μm or less, which results in a problem that the amount of adsorption is small and the ratio of the amount of recovered ammonium ions to the amount of recovered sodium ions cannot be sufficiently increased. In addition, concentration during desorption is also difficult because the amount of recovered ions is small.

[0012] As described above, the prior art has not been able to sufficiently increase the ratio of ammonium ion recovery to sodium ion recovery. Furthermore, no method capable of concentrating ammonium ions has been clearly demonstrated. Generally, increasing the membrane thickness is expected to increase the adsorption amount, but the internal resistance of the electrode increases. Therefore, when performing constant voltage or voltage sweep, an additional voltage is required depending on the internal resistance. In practice, when recovering ions from water, the voltage cannot be increased above the electrolysis voltage of water, and this method cannot be expected to recover ions efficiently. Patent Document 4 also has the same problem, as cesium is adsorbed at potentials below 0.45 V and dissociated at potentials above 0.45 V, and cesium is absorbed at potentials below 0.45 V, dissociated at potentials above 0.45 V, and the adsorption and desorption of ions are controlled by controlling the voltage.

[0013] Even if electrodes with sufficient thickness are prepared to ensure a sufficient amount of adsorption, a technology is needed to selectively recover specific target ions at a voltage that does not cause water electrolysis.

[0014] Therefore, the present invention aims to provide a method for electrochemically recovering target ions, which is a technology that selectively recovers target ions in sufficient amounts relative to non-target ions at a voltage that does not cause water electrolysis.

[0015] The present invention, which has achieved the above object, is as follows. [1] An adsorption step of immersing a first electrode containing one or more Prussian blue analogs and having a content of the cations to be recovered of a predetermined amount or less, and a second electrode, in an aqueous solution to be treated containing one or more cations to be recovered and one or more cations other than the cations to be recovered, connecting a cathode of a DC power supply to the first electrode, connecting an anode of a DC power supply to the second electrode, and applying a constant current to adsorb the cations to be recovered onto the first electrode; a desorption step of immersing the first electrode, to which the cations to be recovered have been adsorbed, together with a second electrode, in a recovery liquid instead of the treated liquid after the adsorption step, connecting an anode of a DC power supply to the first electrode, connecting a cathode of a DC power supply to the second electrode, and applying a constant current to desorb the cations to be recovered adsorbed onto the first electrode into the recovery liquid; and a step of obtaining a recovery liquid containing the cations to be recovered obtained by performing the desorption step. [2] The method according to [1], wherein the aqueous solution to be treated in the adsorption step is the treated liquid after the adsorption step. [3] The method according to [1] or [2], wherein the total concentration of the cations not to be recovered in the aqueous solution to be treated in the adsorption step (mass of cations not to be recovered / volume of aqueous solution) is at least five times the concentration of the cations to be recovered (mass of cations to be recovered / volume of aqueous solution). [4] The method according to any one of [1] to [3], wherein the aqueous solution to be treated in the adsorption step further contains halogeno ions, wherein the halogeno ions are adsorbed onto a second electrode in the adsorption step, and the halogeno ions adsorbed onto the second electrode are desorbed into the aqueous liquid in the desorption step. [5] The method according to any one of [1] to [4], wherein the second electrode includes a current collector and a carbon-based material. [6] The method according to [5], wherein the mass of the carbon-based material contained in the second electrode is at least one time the mass of the Prussian blue analogue contained in the first electrode. [7] The method according to any one of [1] to [6], wherein the cations to be recovered are ammonium ions and / or potassium ions.[8] A method for producing a liquid containing target cations in which the concentration of the target cations has been reduced, the method comprising: an adsorption step of immersing a first electrode containing one or more Prussian blue analogues and having a content of the target cations equal to or less than a predetermined amount, and a second electrode in an aqueous solution to be treated, the first electrode connecting a cathode of a DC power source to the first electrode and an anode of the DC power source to the second electrode, and applying a constant current to the first electrode; and a solution recovery step of separating the treated liquid after the adsorption step from the first and second electrodes.

[0016] According to the first aspect of the present invention, when a Prussian blue analogue is used as an electrode, applying a voltage to obtain a constant current allows selective adsorption of specific target cations at a sufficiently low voltage that water electrolysis does not occur, even when a thick electrode is used. The adsorbed target ions are then desorbed into another recovery liquid, thereby obtaining a solution containing only the target ions. Applying a sufficiently low constant voltage can avoid water electrolysis, but the amount of target cations that can be adsorbed is limited. Applying multiple constant voltages can avoid water electrolysis and increase the adsorption amount, but this requires consideration of the timing of voltage switching. Using a constant current eliminates this consideration of timing. Furthermore, according to the second aspect of the present invention, target cations can be selectively adsorbed and removed, efficiently removing harmful ions from the aqueous solution being treated.

[0017] Figure 1 is a graph showing the powder X-ray diffraction pattern of the Prussian blue analog obtained in Preparation Example. Figure 2 is a graph showing the cyclic voltammetry curve of the Fe—Fe analog. Figure 3 is a graph showing the cyclic voltammetry curve of the Cu—Fe analog. Figure 4 is a graph showing the NH ４ ＋ 5 is a graph showing the constant current adsorption characteristics of ions. ４＋ Graph showing constant current desorption characteristics of ions. FIG. 6 is a graph showing constant current selective adsorption characteristics of Cu—Fe analogs. FIG. 7 is a graph showing constant current selective adsorption characteristics of Ni—Fe analogs. FIG. 8 is a graph showing cycle characteristics of Cu—Fe analogs. FIG. 9 is a conceptual diagram of multistage adsorption treatment. FIG. 10 is a graph showing NH in a multistage cell using electrodes containing Ni—Fe analogs. ４ ＋ 11 is a graph showing the evolution of ion concentration in a multi-stage cell using electrodes containing Cu—Fe analogs. ４ ＋ 1 is a graph showing the transition of ion concentration. FIG. 12 is a graph showing the ion adsorption characteristics when activated carbon is used in an amount 10 times the weight of the Prussian blue analogue. FIG. 13 is a graph showing the ion adsorption characteristics when activated carbon is used in an amount 1.5 times the weight of the Prussian blue analogue. FIG. 14 is a graph showing the ion adsorption characteristics when activated carbon is used in an amount 1 time the weight of the Prussian blue analogue. FIG. 15 is a graph showing the ion adsorption characteristics when activated carbon is used in an amount 0.5 times the weight of the Prussian blue analogue. FIG. 16 is a graph showing the pH of the aqueous solution to be treated when activated carbon is used in an amount 10 times the weight of the Prussian blue analogue. FIG. 17 is a graph showing the pH of the aqueous solution to be treated when activated carbon is used in an amount 0.5 times the weight of the Prussian blue analogue. FIG. 18 is a graph showing the amount of ammonium ions in an electrode when an electrode with ammonium ions adsorbed thereon is immersed in a high-concentration aqueous ammonium chloride solution to desorb the ammonium ions from the electrode. FIG. 19 is a graph showing the amount of Na ＋ , K. ＋ , and N.H. ４ ＋ 20 is a graph showing the constant current selective adsorption characteristics of a thick film electrode containing a Ni—Fe analogue when the current value is changed.

[0018] <<First Aspect: Method for Recovering Target Cations>> The method for recovering target cations according to the first aspect of the present invention is characterized by carrying out an electrochemical reaction at a constant current using a first electrode containing a Prussian blue analogue and a second electrode. The constant current allows a sufficient amount of target cations to be adsorbed to the Prussian blue analogue on the first electrode with high selectivity over the non-target cations from a target aqueous solution containing target cations and non-target cations. The target cations can then be recovered into a recovery liquid by immersing the first electrode with the target cations adsorbed thereon together with the second electrode in a recovery liquid, replacing the treated solution after the adsorption step. The target cations adsorbed to the first electrode are then desorbed into the recovery liquid by an electrochemical reaction in a desorption step, and the recovery liquid containing the target cations after the desorption step is recovered. The ions to be recovered here are cations that have a higher adsorption rank to the Prussian blue analog than other cations contained in the aqueous solution to be treated. ＋ , Rb ＋ , N.H. ４ ＋ , K. ＋ , Na ＋ , Li ＋ The order of adsorption for Cs is as follows: ＋ >Rb ＋ >NH ４ ＋ >K ＋ >Na ＋ >Li ＋ For example, if the aqueous solution to be treated contains NH ４ ＋ and Na ＋ If it contains NH ４ ＋ is the ion to be collected, and Na ＋ is a cation that is not the target of recovery. ＋ and Na ＋ If it contains K ＋ is the target ion to be recovered, Na ＋ Ca, which does not appear in this order, is a cation that is not the target of recovery. ２＋ , Mg ２＋ alkaline earth metals such as Zn ２＋, Fe ２＋ , Fe ３＋ , Co ２＋ , Co ３＋ , Ni ２＋ transition metal ions such as Pt ２＋ , Ag ＋ Cations such as precious metal ions such as NH are poorly adsorbed by Prussian blue analogues and are not the target ions for recovery. ４ ＋ and Zn ２＋ If it contains NH ４ ＋ In addition, when three or more types of ions are included, multiple ions may be collected depending on their order. For example, NH ４ ＋ , K. ＋ , Na ＋ When the ions to be collected are included, depending on the current setting, the ions to be collected may be NH ４ ＋ and K. ＋ and Na ＋ However, there are cases where the cations are not the target of recovery.

[0019] In other words, the cations contained in the aqueous solution to be treated are Cs ＋ , Rb ＋ , N.H. ４ ＋ , K. ＋ , Na ＋ and Li ＋ In the case where the cations contained in the aqueous solution to be treated are one selected from group X consisting of Cs and one or more cations selected from group Y consisting of alkaline earth metals, transition metals, and noble metals, the one selected from group X is the object to be recovered, and all cations selected from group Y are not the object to be recovered. ＋ , Rb ＋ , N.H. ４ ＋ , K. ＋ , Na ＋ and Li ＋In the case where n (2≦n≦6) cations are selected from the group X consisting of Cs, one or more (n−1) cations in the group X in descending order of their adsorption to the Prussian blue analogue can be recovered. ＋ , Rb ＋ , N.H. ４ ＋ , K. ＋ , Na ＋ and Li ＋ and one or more cations selected from group Y consisting of alkaline earth metals, transition metals, and noble metals, one to n cations in group X in descending order of their adsorption to the Prussian blue analogue may be the cations to be recovered, and all cations in group X other than the cations to be recovered, as well as the cations of alkaline earth metals, transition metals, and noble metals, are not the cations to be recovered.

[0020] The cation to be recovered is NH ４ ＋ and K. ＋ At least one of the above is preferred.

[0021] <Adsorption Step> In the adsorption step, a first electrode containing a Prussian blue analogue and having a content of the target cations to be recovered of a predetermined amount or less, and a second electrode are immersed in an aqueous solution to be treated that contains one or more target cations to be recovered and one or more target cations not to be recovered, the cathode of a DC power supply is connected to the first electrode, the anode of the DC power supply is connected to the second electrode, and a constant current is applied to the first electrode to adsorb the target cations to be recovered.

[0022] By connecting the cathode of a DC power supply to the first electrode and the anode of the DC power supply to the second electrode and applying current, a reduction reaction occurs in the Prussian blue analog at the first electrode, allowing the target cations to be recovered to be inserted between the lattices of the crystal lattice of the Prussian blue analog. In the present invention, it is important to apply current at a constant current. When the output from the DC power supply that supplies power is set to a constant current, the voltage increases and then stabilizes at a voltage that allows the target cations to be selectively inserted. Therefore, the target cations can be selectively adsorbed onto the Prussian blue analog at this stable voltage.

[0023] After the voltage stabilizes at a level that allows selective insertion of the target cations, this voltage continues for a certain period of time, after which the voltage increases again. The adsorption process may be carried out for a predetermined period of time from the start of application of the voltage that allows selective insertion of the target cations, and may continue from the time the voltage begins to stabilize until it ends (the time the voltage begins to increase further). The voltage that adsorbs the target cations varies depending on the composition of the Prussian blue analog and the type of target cation, but is, for example, in the range of 0 to 1000 mV (particularly, greater than 0 mV and less than 1000 mV). The important point here is that the voltage must not cause water electrolysis. The voltage at which water electrolysis occurs varies depending on the type of voltage, the type and concentration of ions dissolved in water, and other substances coexisting in the water (such as organic substances such as alcohol), but is generally around 1500 mV to 2000 mV.

[0024] Furthermore, by obtaining a cyclic voltammetry (CV) curve in advance using the first and second electrodes in an aqueous solution containing the target cations, it is possible to determine the voltage at which the reduction reaction of the Prussian blue analogue occurs and the target cations can be adsorbed, and whether the target cations can actually be adsorbed and desorbed. For example, in the case of the Prussian blue analogue in the examples described below, NH ４ ＋ The voltage required for selectively adsorbing ions is approximately 580 mV for Fe-Fe analogs, approximately 350 mV for Cu-Fe analogs, and approximately 470 mV for Ni-Fe analogs. The current value is not critical as long as the voltage is appropriately controlled and the target cations can be selectively recovered. Increasing the current can increase the amount recovered per unit time, but this requires injecting or releasing a correspondingly large number of electrons and ions into or from the electrode, which increases the applied voltage and reduces selectivity due to the depletion of target cations near the electrode. The specific current range is affected by the concentration of the solution, the thickness of the electrode, and other factors, so if it is necessary to consider each case individually, a current of, for example, 100 mA / cm may be used. ２ (positive electrode area ratio) or less, preferably 50 mA / cm ２(positive electrode area ratio) or less. The lower limit of the current amount is not limited from the viewpoint of the applied voltage, selectivity, etc. However, since the recovery amount per unit time is proportional to the current amount, it is preferable that the current amount is 0.01 mA / cm. ２ The above is practical. That is, the specific current range is 100 to 0.01 mA / cm ２ (positive electrode area ratio) is preferably 50 to 0.01 mA / cm ２ In this case, the amount of ions collected per unit time can be obtained by calculating the number of electrons from the current and multiplying the result by the area of ​​the positive electrode.

[0025] As described above, in the adsorption step of the present invention, the target cations to be recovered can be adsorbed to the Prussian blue analogue of the first electrode with high selectivity over non-target cations, and the selectivity coefficient X represented by the following formula can be set to less than 1, preferably 0.9 or less, more preferably 0.8 or less, even more preferably 0.7 or less, and even more preferably 0.55 or less, with the lower limit being 0.2 or more (i.e., the selectivity coefficient X can be set to less than 1 to 0.2, preferably 0.9 to 0.2, more preferably 0.8 to 0.2, even more preferably 0.7 to 0.2, and even more preferably 0.55 to 0.2). Selectivity coefficient X=(C Ｎ１ / C Ｎ０ ) / (C Ａ１ / C Ａ０ ) Above C Ａ０ represents the total concentration (mg / L) of non-recovery target cations in the aqueous solution to be treated before the adsorption step, and C Ａ１ represents the total concentration (mg / L) of non-recovery target cations in the treated liquid after the adsorption step, and C Ｎ０ represents the concentration (mg / L) of the cation to be recovered in the aqueous solution to be treated before the adsorption step, and Ｎ１ represents the concentration (mg / L) of the cation to be recovered in the treated liquid after the adsorption step.

[0026] The selectivity of the adsorption of the target cations can also be expressed as the ratio of the removal rate of the target cations to the removal rate of the non-target cations (removal rate of target cations / removal rate of non-target cations), and this ratio is preferably 10 or more, more preferably 13 or more, and even more preferably 15 or more. The removal rate is the amount of removed cations divided by the amount of cations before the adsorption step. Note that if the amount of non-target cations remains almost unchanged before and after the adsorption step or increases for some reason, including measurement error, the removal rate can be considered to be as close to 0 as possible, and there is no particular upper limit to the removal rate of target cations / removal rate of non-target cations.

[0027] Furthermore, when the ratio of the concentration of cations to be recovered to those not to be recovered is extremely large, some of the cations not to be recovered may be adsorbed. In this case, the positive electrode after adsorption can be immersed again in the aqueous solution to be treated, and the adsorbed cations not to be recovered can be replaced by the cations to be recovered through ion exchange between the adsorbed cations not to be recovered and the cations in the aqueous solution. Furthermore, once adsorption has been completed, desorption of cations from the electrode can produce an aqueous solution in which the concentration of cations to be recovered / non-recovered is higher than that of the aqueous solution to be treated. Therefore, by performing adsorption and desorption again from this aqueous solution, it is possible to separate and recover only the cations to be treated.

[0028] The pH of the aqueous solution to be treated at the start of the adsorption step is, for example, 4 to 9. If the pH is 5 to 7, the pH of the treated solution at the end of the adsorption step may be 6 or less, 4 or less, or 3 or less, and is preferably 1 or more (i.e., the pH of the treated solution at the end of the adsorption step may be 6 to 1, 4 to 1, or 3 to 1).

[0029] <First Electrode> The first electrode contains a Prussian blue analogue and has a content of a target cation to be recovered of not more than a predetermined amount. The first electrode typically includes a current collector and a positive electrode active material-containing layer laminated on the current collector, and the Prussian blue analogue is preferably contained in the positive electrode active material-containing layer.

[0030] It is important that the current collector has low electrical resistance so that the voltage to be applied remains within a predetermined range, and that corrosion is suppressed to such an extent that the device is not destroyed even when electrochemical reactions are repeated. For example, at least one selected from the group consisting of simple elements such as C, Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In, or alloys such as SUS304, SUS316, SUS316L, DSALOY400, and DSA760 can be used, with C, Ti, or SUS316L being preferred, and Ti being particularly preferred. The thickness of the current collector is, for example, 0.01 to 10 mm, preferably 0.1 to 5 mm. There are no particular restrictions on the area of ​​the main surface of the current collector as long as an increase in electrical resistance is suppressed, but it is generally 1 to 10,000 cm. ２ The thickness of the positive electrode active material layer is, for example, 1 μm to 10 mm, preferably 0.3 to 5 mm, more preferably 0.5 to 4 mm, and even more preferably 1 to 3 mm, and the area of ​​the main surface of the positive electrode active material layer is, for example, 1 to 10,000 cm ２ is.

[0031] The key to the fabrication method of the first electrode is that the electrical resistance of the current collector layer and the active material-containing layer is low, as well as the interfacial resistance between the two layers. As mentioned above, if the electrical resistance of these layers is high, the resulting voltage drop results in only a portion of the applied voltage being used for the electrochemical reaction of the Prussian blue analog, resulting in a higher voltage being required for oxidation-reduction, which in turn induces reactions unrelated to the adsorption and desorption of the target cations, such as water electrolysis. As long as the electrical resistance can be reduced, any method for fabricating the electrode is acceptable. For example, a first electrode including a current collector and a positive electrode active material-containing layer laminated on the current collector can be fabricated by applying a positive electrode mixture containing a Prussian blue analog and, optionally, at least one of a binder, an organic solvent, and a conductive agent to the current collector and drying the mixture. Alternatively, a first electrode including a current collector and a positive electrode active material-containing layer laminated on the current collector can be fabricated by molding the positive electrode mixture using a press molding machine or the like and adhering it to the current collector with a conductive adhesive such as carbon paste.

[0032] The introduction of a binder is expected to improve the strength of the positive electrode when it is necessary to maintain the strength. Examples of binders include polyacrylonitrile, poly(meth)acrylic acid, poly(meth)acrylic acid methyl ester, poly(meth)acrylic acid ethyl ester, acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), and styrene butadiene rubber (SBR). The amount of binder is 150 parts by mass or less, preferably 100 parts by mass or less, and more preferably 50 parts by mass or less, per 100 parts by mass of the Prussian blue analog. If the strength can be maintained without the binder, the binder need not be included. That is, the amount of binder is preferably 150 to 0 parts by mass, more preferably 100 to 0 parts by mass, and even more preferably 50 to 0 parts by mass, per 100 parts by mass of the Prussian blue analog.

[0033] The conductive agent is introduced if necessary to improve the electronic conductivity of the positive electrode. Examples of the conductive agent include carbon black such as acetylene black, furnace black, and ketjen black, and carbon nanotubes. The amount of the conductive agent is, for example, 1 to 20 parts by mass per 100 parts by mass of the Prussian blue analog.

[0034] The first electrode generally has a flat plate structure in which an active material-containing layer is laminated on a current collector layer, but the form is not critical as long as the active material-containing layer is in contact with both the current collector layer and the aqueous solution. For example, the flat plate may be rolled into a roll, or the active material layer may be coated on the surface of a current collector structure having many internal defects. Furthermore, if the active material-containing layer alone can sufficiently reduce electrical resistance and maintain structural strength, the current collector layer may be omitted. When the first electrode is flat, the mass of the Prussian blue analog per unit area of ​​the positive electrode active material layer is 50 to 2000 mg / cm. ２ is preferably 100 to 800 mg / cm ２ It is more preferable that:

[0035] The amount of cations to be recovered contained in the first electrode used in the adsorption step may be appropriately set within a range that is less than the maximum amount of cations to be recovered that can be adsorbed by the Prussian blue analog of the first electrode. The amount of cations to be recovered in the first electrode used in the adsorption step may be, for example, 0.8 mmol or less, 0.6 mmol or less, 0.3 mmol or less, or 0 mmol per 1 g of Prussian blue analog (i.e., 0.8 to 0 mmol, 0.6 to 0 mmol, or 0.3 to 0 mmol). For example, when the cations to be recovered are NH ４ ＋ In the case of cations, the amount of cations to be recovered on the first electrode used in the adsorption step may be, for example, 14 mg or less, 10 mg or less, 5 mg or less, or 0 mg per gram of Prussian blue analog (i.e., 14 to 0 mg, 10 to 0 mg, or 5 to 0 mg). Such a first electrode may be an electrode immediately after preparation by the above method, or an electrode after a predetermined amount of cations to be recovered adsorbed on the first electrode used in the adsorption step have been desorbed from the first electrode in a desorption step. As shown in the examples described below, the first electrode used in the present invention retains the ability to adsorb and desorb cations to be recovered even after undergoing multiple adsorption and desorption steps.

[0036] <Prussian Blue Analogues> Prussian blue is generally ３＋ ４ [Fe ２＋ (CN) ６ ] ３ It is a compound having a cubic crystal structure represented by the formula: ２＋ and Fe ３＋ is crosslinked by cyano groups (-CN-), and has a nanopore structure of about 0.5 nm that can incorporate cations. ３＋Prussian blue analogues in which Fe is substituted with other metals also have the same crystal structure as Prussian blue. In the present invention, the term "Prussian blue analogues" refers to both Prussian blue and Prussian blue analogues, and the Fe in the composition of Prussian blue is ３＋ Analogs in which the metal occupying position M' are designated M'-Fe analogs.

[0037] Prussian blue analogues can be produced by known production methods, such as by reacting a salt containing hexacyanoferric ions with an inorganic salt of a metal in an aqueous solution. However, any production method may be used as long as the resulting product conforms to the formula described below.

[0038] The Prussian blue analogue can be represented by the following formula (1): １ ｘ M ２ [Fe(CN) ６ ] ｙ ・zH ２ O... (1) [In the formula, M １ is the M of the crystal structure of the Prussian blue analogue １ There is no limitation as long as the ion can be placed in the position of ４ , Li, Na, K, Rb, Cs, Ca, Mg, and Ba, and M ２ is at least one element selected from the group consisting of V, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Cu, Ag, Zn, La, Eu, Gd, Lu, Ba, Sr, and Ca, and x, y, and z are each 0≦x≦3, 0.1≦y≦1.5, and 0≦z≦6.

[0039] The above M ２ Prussian blue analogs containing ２ Considering the ionic radius and number of valence electrons, it is believed to have a void structure of approximately 0.5 nm, which allows the insertion and desorption of the target cations. In addition, Prussian blue analogs can be obtained by, for example, the following oxidation-reduction reaction formula: １ NH on the site ４ ＋ The target cations, such as ions, can be inserted and removed. １ When some metals are selected as１ In some cases, adsorption and desorption of ions occurs due to oxidation and reduction of the metal.

[0040]

[0041] In the above formula, M １ are Na, K and NH ４ It is preferable that at least one or two or more of the following be used. ２ is preferably at least one selected from the group consisting of Fe, Cu, Ni, Zn, Mn and V, and more preferably at least one selected from the group consisting of Fe, Cu and Ni.

[0042] Although it depends on the composition of the Prussian blue analog, the target cations (especially NH ４ ＋ The maximum amount of the ions is, for example, 0.5 mmol to 5.0 mmol.

[0043] <Aqueous solution to be treated> The aqueous solution to be treated contains one or more target cations to be recovered and one or more non-target cations. According to the present invention, by connecting a first electrode containing a Prussian blue analog to the cathode of a DC power supply and applying a current, the Prussian blue analog is reduced, and the target cations can be selectively adsorbed onto the Prussian blue analog in preference to the non-target cations from the aqueous solution to be treated. In particular, since the current is applied at a constant current in the present invention, the target cations can be selectively adsorbed even when the concentration of the non-target cations is higher than that of the target cations.

[0044] There are no particular limitations on the total concentration of cations not to be recovered (mass of cations not to be recovered / volume of aqueous solution) in the aqueous solution to be treated in the adsorption step. However, the effect of the present invention is enhanced when the concentration is at least three times the concentration of cations to be recovered (mass of cations to be recovered / volume of aqueous solution), and even more so when it is at least five times the concentration. When there are multiple types of cations to be recovered, the concentration of cations to be recovered may be the total concentration. The upper limit is not particularly limited, but it may be about 15 times. That is, the total concentration of cations not to be recovered (mass of cations not to be recovered / volume of aqueous solution) in the aqueous solution to be treated in the adsorption step is preferably 3 to 15 times, and more preferably 5 to 15 times, the concentration of cations to be recovered (mass of cations to be recovered / volume of aqueous solution). According to the present invention, cations to be recovered can be adsorbed from the aqueous solution to be treated even when there is a concentration difference within this range.

[0045] There are no limitations on the concentration of the cations to be recovered and the concentration of the cations not to be recovered in the aqueous solution to be treated, but from the viewpoint of separating and concentrating the cations to be recovered, a lower concentration of the cations to be recovered is preferable. The concentration of the cations to be recovered in the aqueous solution to be treated is, for example, 450 mmol / L or less, preferably 350 mmol / L or less, more preferably 280 mmol / L or less, and the lower limit may be 60 mmol / L (i.e., for example, 450 to 60 mmol / L, preferably 350 to 60 mmol / L, and more preferably 280 to 60 mmol / L). When the cations to be recovered are NH ４ ＋ In this case, the NH ４ ＋ The concentration is, for example, 8000 mg / L or less, preferably 6000 mg / L or less, more preferably 5000 mg / L or less, and the lower limit may be 1000 mg / L (i.e., for example, 8000 to 1000 mg / L, preferably 6000 to 1000 mg / L, and more preferably 5000 to 1000 mg / L). The higher the total concentration of non-target cations in the aqueous solution to be treated, the greater the effect of the present invention. The concentration of non-target cations in the aqueous solution to be treated is, for example, 400 mmol / L or more, preferably 850 mmol / L or more, and more preferably 1250 mmol / L or more. When the non-target cations are Na＋ In the case of Na in the aqueous solution to be treated ＋ There is no limit to the concentration, but separation from the target cations is possible even at 30,000 mg / L or more. There is no particular upper limit to the total concentration of non-target cations, but a concentration at which the non-target cations do not precipitate is practical.

[0046] The aqueous solution to be treated contains anions as counter ions to the cations to be recovered and the cations not to be recovered. There are no limitations on the counter ions, but halogen ions, organic ions, hydroxide ions, etc. can be used. In particular, it is preferable that the aqueous solution contains halogen ions, and the halogen ions are F － , Cl － ,Br － and I － Preferably, it is at least one selected from the group consisting of Cl － It is more preferable that:

[0047] The halogeno ions preferably form salts with the cations to be recovered and also with the cations not to be recovered.

[0048] When the aqueous solution to be treated contains halogen ions, it is preferable that the halogen ions can be electrostatically adsorbed onto the positively charged second electrode in the adsorption step, and the halogen ions adsorbed onto the second electrode can be desorbed into the aqueous liquid in the desorption step. In this way, the target cations can be contained in the recovery liquid as halide salts in the desorption step.

[0049] The aqueous solution to be treated may be the treated solution after the adsorption step has been carried out once or twice or more times. That is, the aqueous solution to be treated may be the treated solution after the adsorption step has been carried out once or twice or more times. By carrying out the adsorption step twice or more times on the aqueous solution to be treated, it is possible to further adsorb the target cations to be recovered even from an aqueous solution having a higher concentration of the target cations to be recovered.

[0050] The amount of the aqueous solution to be treated can be adjusted depending on the purpose. For example, when the amount of the Prussian blue analog that can adsorb the target cations to be recovered is 30 mg / g, and the mass of the Prussian blue analog per unit area (unit area of ​​the main surface of the positive electrode active material layer) is 1 g / cm ２ , the area of ​​the main surface of the positive electrode active material layer is 100 cm ２ In this case, the mass of the Prussian blue analog is 1 x 100 = 100 g, and the amount of adsorption in the positive electrode active material layer is 30 x 1 x 100 = 3000 mg. If the concentration of the cations to be recovered in the aqueous solution to be treated is 1000 mg / L, and the volume of the solution to be treated is 3 L, the amount of ions to be recovered in the solution to be treated will be 3000 mg. In other words, in principle, all of the ions to be recovered can be adsorbed and their concentration in the aqueous solution to be treated reduced to zero. Furthermore, if the volume of the aqueous solution to be treated is 6 L, the total amount of ions to be recovered is 6000 mg, so the concentration in the aqueous solution to be treated can be reduced to zero by two adsorption treatments. When the concentration of ions to be recovered in the aqueous solution to be treated is to be reduced below a certain value, the mass per unit area of ​​the Prussian blue analog and the area of ​​the positive electrode active material layer can be determined based on this approach. For example, if the NH ４ ＋ This is the case when the objective is to reduce the ion concentration so that the aqueous solution to be treated can be discharged.

[0051] On the other hand, when the objective is to recover useful ions from a solution to be treated, it is not necessarily necessary to recover all of the ions to be recovered. For example, when recovering Na ions and / or K ions from seawater, it is not necessary to reduce the concentration of these ions in the seawater below a certain value. Furthermore, when practicing the present invention on a beach or the like, there is essentially no limit to the amount of seawater, and therefore there is also essentially no limit to the amount of aqueous solution to be treated, so the amount of aqueous solution to be treated with 1 g of a Prussian blue analogue may be infinite.

[0052] <Second Electrode> The second electrode typically includes a current collector and a layer containing a negative electrode active material laminated on the current collector. The negative electrode active material is preferably a carbon-based material, a Prussian blue analogue similar to that described above, or a conductive polymer material, with a carbon-based material being more preferred. Examples of the carbon-based material include graphite, activated carbon, graphene, nanocarbons such as carbon nanotubes, amorphous carbon, and cokes, with activated carbon being preferred.

[0053] As with the first electrode, it is important that the current collector has low electrical resistance so that the voltage to be applied remains within a predetermined range, and that corrosion is suppressed to a degree that the device will not be destroyed even if the electrochemical reaction is repeated. Generally, any material similar to those included in the exemplified group is sufficient, with C, Ti, or SUS316L being preferred, and Ti being particularly preferred. However, since the voltages applied during the reaction differ between the positive electrode and the negative electrode, and therefore the required corrosion strength also differs, the current collectors for the first electrode and the second electrode may be different, or it may be desirable to use different current collectors. The thickness of the current collector is, for example, 0.01 to 10 mm, preferably 0.1 to 5 mm. The thickness of the negative electrode active material layer is, for example, 1 μm to 40 mm, preferably 1 to 40 mm, more preferably 4 to 40 mm, and even more preferably 8 to 20 mm.

[0054] Important points in the fabrication method of the second electrode are that the electrical resistance of the current collector layer and the active material-containing layer are low, and that the interfacial resistance between the two layers is low. As long as the low resistance requirement is met, any method for fabricating the electrode is acceptable. For example, a second electrode including a current collector and a negative electrode active material-containing layer laminated on the current collector can be fabricated by applying a negative electrode mixture containing a negative electrode active material and, optionally, at least one of a binder, an organic solvent, and a conductive agent to the current collector and drying the mixture. Alternatively, a second electrode including a current collector and a negative electrode active material-containing layer laminated on the current collector can be fabricated by molding the negative electrode mixture using a press molding machine or the like and adhering it to the current collector with a conductive adhesive such as carbon paste. The binder can be made from the same materials as those exemplified for the first electrode. The binder amount is, for example, 0 to 25 parts by mass per 100 parts by mass of the negative electrode active material. The conductive agent can also be made from the same materials as those exemplified for the first electrode. The amount of the conductive agent is, for example, 0 to 20 parts by mass with respect to 100 parts by mass of the negative electrode active material.

[0055] The mass of the negative electrode active material contained in the second electrode is, for example, 0.1 times or more, preferably 1 time or more, and more preferably 1.5 times or more, the mass of the Prussian blue analogue contained in the first electrode. By setting the mass within this range, halogen ions in the aqueous solution to be treated in a preferred embodiment can be sufficiently adsorbed onto the second electrode, and the upper limit is, for example, 15 times or less (i.e., for example, 0.1 to 15 times, preferably 1 to 15 times, and more preferably 1.5 to 15 times). In particular, it is preferable that this range is satisfied when the negative electrode active material is a carbon-based material. When the ratio of the mass of the negative electrode active material contained in the second electrode to the mass of the Prussian blue analogue is a predetermined value or less, the second electrode can sufficiently adsorb halogen ions in the aqueous solution to be treated. －It is believed that the adsorption process adsorbs ions, and the pH of the treated solution after the adsorption process tends to decrease. For example, when the negative electrode active material contained in the second electrode is activated carbon and the ratio of its mass to the mass of the Prussian blue analogue is 7 times or less, 5 times or less, or 3 times or less, such a decrease in pH is likely to occur. For this reason, it is effective to set the amount of negative electrode active material according to the target value of the post-treatment pH of the aqueous solution to be treated. For example, when the objective is to discharge the aqueous solution to be treated after treatment, a neutral pH is desirable, so it is desirable to provide a sufficient amount of negative electrode active material. On the other hand, when the objective is to lower the pH of the aqueous solution to be treated, this objective can be achieved by setting the amount of negative electrode active material to a small amount. In addition, in this case, the negative electrode active material contains OH, not the counter anion, － The ions are adsorbed, and in the subsequent desorption step, an aqueous hydroxide solution of the target ions can be produced. Therefore, even when the target is to produce hydroxides of the target cations rather than salts, it is desirable to set the amount of negative electrode active material in this way.

[0056] The mass of the negative electrode active material per unit area of ​​the negative electrode active material layer is 1 to 5000 mg / cm ２ is preferably 100 to 200 mg / cm ２ It is more preferable that:

[0057] In a preferred embodiment, when the aqueous solution to be treated contains halogeno ions and the negative electrode active material is activated carbon, the maximum amount of halogeno ions that can be adsorbed per gram of activated carbon is, for example, 0.05 mmol to 0.8 mmol.

[0058] <Desorption Step> In the desorption step, instead of the treated liquid after the adsorption step, the first electrode to which the target cations to be recovered are adsorbed is immersed in the recovery liquid together with the second electrode, and the anode of a DC power supply is connected to the first electrode, the cathode of the DC power supply is connected to the second electrode, and current is applied to desorb the target cations adsorbed on the first electrode into the recovery liquid. By connecting the anode of the DC power supply to the first electrode and the cathode of the DC power supply to the second electrode and applying current, an oxidation reaction occurs in the Prussian blue analog on the first electrode, and the target cations inserted between the crystal lattices of the Prussian blue analog can be desorbed. In the desorption step, current may be applied at either a constant current or a constant voltage. The voltage value in the desorption step is, for example, 0 to 1000 mV, and the current value is, for example, 5 to 100 mA / cm. ２ is.

[0059] As described above, in a preferred embodiment, when the aqueous solution to be treated contains halogen ions and a sufficient negative electrode active material layer is provided, the halogen ions adsorbed on the second electrode in the adsorption step can be desorbed from the second electrode in the desorption step, and the target cations can be contained in the aqueous liquid as halides. Also, when the amount of negative electrode active material is small, the target cations can be contained in the aqueous liquid as hydroxides.

[0060] The recovery liquid is preferably an aqueous liquid, which may contain, in addition to water, solvents other than water, such as methanol, ethanol, or acetonitrile. The proportion of water in 100% by mass of the aqueous liquid is preferably 70% by mass or more, more preferably 80% by mass or more, or may be 100% by mass (i.e., 70 to 100% by mass is preferred, and 80 to 100% by mass is more preferred).

[0061] Furthermore, by reducing the amount of recovery liquid compared to the amount of the aqueous solution to be treated, it becomes possible to concentrate the cations to be recovered. For example, if the amount of aqueous solution to be treated / the amount of recovery liquid = α, the concentration of the cations to be recovered in the recovery liquid will be α times the concentration in the aqueous solution to be treated. Furthermore, if the concentration of the cation salt to be recovered in the concentrated recovery liquid is equal to or greater than the saturation solubility, it will be possible to obtain it as a solid by precipitation. For example, if the cation to be recovered is NH ４ ＋ The halogen ions contained in the aqueous solution to be treated are Cl － In this case, the concentration of ammonium chloride in the recovery liquid increases during the desorption process. For example, the solubility of ammonium chloride at 30°C is 41.4 g / 100 g of water, so if the concentration exceeds this value during the desorption process, precipitation will occur. Even if the solubility is not exceeded, efficient solid recovery is possible by combining a cooling process to lower the temperature after the desorption process, a salting-out process to add another salt to lower the solubility, or a precipitation process to reduce the amount of water by evaporation.

[0062] <Step of Obtaining a Recovery Liquid Containing the Cations to be Recovered> By recovering the recovery liquid containing the cations to be recovered obtained by carrying out the desorption step, the cations to be recovered contained in the aqueous solution to be treated can be separated from the metal ions. The recovery liquid can be reused as an ammonium ion-containing liquid in a series of steps, in which nitrogen compounds contained in domestic or industrial wastewater are converted into ammonium ions, and the ammonium ions are recovered as ammonium water through membrane treatment or the like.

[0063] <<Second Aspect: Method for Producing a Liquid Containing a Target Cation Having a Reduced Concentration of the Target Cation>> The second aspect of the present invention relates to a method for producing a liquid containing a target cation having a reduced concentration, which includes an adsorption step similar to that of the first aspect, and a step of separating and recovering the treated liquid after the adsorption step has been performed from the first and second electrodes.

[0064] The descriptions of the adsorption step, first electrode, Prussian blue analog, aqueous solution to be treated, and second electrode described in the first embodiment, including preferred embodiments, can all be referred to in the second embodiment. According to the second embodiment, even if the aqueous solution to be treated has a high concentration of cations to be recovered, the concentration of cations to be recovered in the recovered aqueous solution can be made equal to or less than the wastewater standard. In particular, when the aqueous solution to be treated in the adsorption step is a treated liquid after the adsorption step has been performed once or twice or more times, i.e., when the aqueous solution to be treated is subjected to the adsorption step twice or more times, the concentration of cations to be recovered in the recovered aqueous solution can be appropriately adjusted.

[0065] The first aspect of the present invention is useful for recovering and utilizing cations that can be used as industrial or agricultural materials from an aqueous solution to be treated, and the second aspect of the present invention is useful for removing harmful cations from an aqueous solution to be treated to render the aqueous solution harmless.

[0066] This application claims the benefit of priority based on Japanese Patent Application No. 2024-004865, filed on January 16, 2024. The entire contents of the specification of Japanese Patent Application No. 2024-004865, filed on January 16, 2024, are incorporated herein by reference.

[0067] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can of course be practiced with appropriate modifications within the scope of the above and below-described aims, all of which are included in the technical scope of the present invention.

[0068] Preparation Example 1-1: Fe-Fe Analog: 20 mL of a 0.4 mol / L aqueous solution of iron nitrate and 20 mL of a 0.3 mol / L aqueous solution of sodium hexacyanoferrate(II) were mixed and shaken for 1 hour to obtain a precipitate. The liquid containing this precipitate was subjected to solid-liquid separation at a centrifugal force of 16,000 G for 10 minutes, and the supernatant was removed. Ultrapure water was added to the precipitate, and the container was then capped and shaken up and down to stir the precipitate in the ultrapure water. This series of steps—centrifugation, removal of the supernatant, addition of ultrapure water, and stirring—was repeated six more times. The supernatant was then removed, and the precipitate was vacuum-dried at room temperature for 4 hours to obtain a powder of the Fe-Fe analog. The powder X-ray diffraction pattern of the obtained Fe-Fe analog is shown in Figure 1. The diffraction pattern of the Fe-Fe analogue shown in Figure 1 was similar to the diffraction pattern described in Reference A (X. Wu, wt. al., Cryst. Growth Des. 2006, 6, 26-28). ｘ Fe[Fe(CN) ６ ] ｙ ・zH ２ O (wherein x, y, and z are 0≦x≦3, 0.1≦y≦1.5, and 0≦z≦6, respectively).

[0069] Preparation Example 1-2 Cu—Fe Analog A Cu—Fe analog was obtained in the same manner as in Preparation Example 1-1, except that the aqueous iron nitrate solution was changed to an aqueous copper sulfate solution, the copper sulfate concentration was 0.6 mol / L, and sodium hexacyanoferrate (II) was changed to potassium hexacyanoferrate (III), and the concentration was 0.4 mol / L. The powder X-ray diffraction pattern of the obtained Cu—Fe analog is shown in FIG. 1. Since the powder X-ray diffraction pattern of the Cu—Fe analog shown in FIG. 1 was similar to the diffraction pattern described in the above-mentioned literature A, this Cu—Fe analog was found to be K ｘ Cu[Fe(CN) ６ ] ｙ ・zH ２ O (wherein x, y, and z are 0≦x≦3, 0.1≦y≦1.5, and 0≦z≦6, respectively).

[0070] Preparation Example 1-3 Ni—Fe Analog A Ni—Fe analog was obtained in the same manner as in Preparation Example 1-1, except that the aqueous iron nitrate solution was changed to an aqueous nickel sulfate solution, the nickel sulfate concentration was 0.6 mol / L, and sodium hexacyanoferrate (II) was changed to potassium hexacyanoferrate (III), and the concentration was 0.4 mol / L. The powder X-ray diffraction pattern of the obtained Ni—Fe analog is shown in FIG. 1. The diffraction pattern of the Ni—Fe analog shown in FIG. 1 was similar to the diffraction pattern described in the above-mentioned literature A, and therefore, this Ni—Fe analog was K ｘ Ni[Fe(CN) ６ ] ｙ ・zH ２ O (wherein x, y, and z are 0≦x≦3, 0.1≦y≦1.5, and 0≦z≦6, respectively).

[0071] Preparation Example 2-1: Thin-Film Electrode Film Formation 30 mL of water was added to the Fe-Fe analog powder prepared in Preparation Example 1-1, and a solution of 2 mmol of sodium hexacyanoferrate (II) dissolved in 20 mL of water was added and stirred for three days. The resulting liquid was evaporated (Tokyo Rika Kikai Co., Ltd., N-1000) to obtain a solid, which was then dissolved in water to a solid concentration of 0.1 g / L. The resulting slurry was spin-coated (Active Corporation, Model ACT-300A) to form a film (thickness approximately 1-5 μm) with an absorbance of 1 at a visible light wavelength of 700 nm.

[0072] Preparation Example 2-2: Thick Film Electrode Formation 1 A wet powder was prepared by stirring a slurry containing 2.23 g of the Cu—Fe analog prepared in Preparation Example 1-2, polyvinylidene fluoride (PVDF, binder), and 1-methyl-2-pyrrolidone (NMP, solvent), and carbon black. The wet powder was then molded using a benchtop press (Tester Sangyo Co., Ltd., SA-303) to obtain a film measuring 2 cm in length, 8 cm in width, and 2 mm in thickness. A Prussian blue electrode was fabricated by bonding the molded film to a titanium substrate (Yokoyama Techno Co., Ltd., TP340) serving as a current collector using carbon paste. In addition, similar to the preparation of the Prussian blue electrode, ground activated carbon (Fujifilm Wako Pure Chemical Industries, Ltd., model number 034-02125) was mixed with a slurry containing polyvinylidene fluoride (binder) and 1-methyl-2-pyrrolidone (NMP, solvent), and carbon black, and pressed to obtain a film of the same size. This was then adhered to a titanium substrate with carbon paste to prepare an activated carbon electrode. Furthermore, a thick film electrode of Ni—Fe analog was also prepared using the Ni—Fe analog prepared in Preparation Example 1-3 in the same manner as the thick film electrode of Cu—Fe analog described above.

[0073] Preparation Example 2-3: Thick Film Electrode Formation 2 18.56 g of the Ni—Fe analog prepared in Preparation Example 1-3, a slurry containing polyvinylidene fluoride (PVDF, binder) and 1-methyl-2-pyrrolidone (NMP, solvent), and carbon black were stirred to prepare a wet powder, which was then molded using a benchtop press (Tester Sangyo Co., Ltd., SA-303) to obtain a film measuring 70 mm in length, 190 mm in width, and 2 mm in thickness. A Prussian blue electrode was fabricated by bonding the molded film to a titanium substrate (Yokoyama Techno Co., Ltd., TP340) serving as a current collector using carbon paste. In addition, 42.75 g of pulverized activated carbon (Fujifilm Wako Pure Chemical Industries, Ltd., model number 034-02125) was mixed with a slurry containing polyvinylidene fluoride (binder) and 1-methyl-2-pyrrolidone (NMP, solvent) and carbon black, and pressed to obtain a film of the same size, which was then attached to a titanium substrate with carbon paste to produce an activated carbon electrode.

[0074] Preparation Example 2-4: Formation of Thick Film Electrode 3 69.76 g of the Ni—Fe analog prepared in Preparation Example 1-3, a slurry containing polyvinylidene fluoride (PVDF, binder) and 1-methyl-2-pyrrolidone (NMP, solvent), and carbon black were stirred to prepare a wet powder, which was then molded using a benchtop press (Tester Sangyo Co., Ltd., SA-303) to obtain a film measuring 200 mm in length, 250 mm in width, and 2 mm in thickness. A Prussian blue electrode was prepared by bonding the molded film to a titanium substrate (Yokoyama Techno Co., Ltd., TP340) serving as a current collector using carbon paste. In addition, 222 g of pulverized activated carbon (Fujifilm Wako Pure Chemical Industries, Ltd., model number 034-02125) was mixed with a slurry containing polyvinylidene fluoride (binder) and 1-methyl-2-pyrrolidone (NMP, solvent) and carbon black, and pressed to obtain a film of the same size, which was then attached to a titanium substrate with carbon paste to produce an activated carbon electrode.

[0075] Reference Example 1 Cyclic voltammetry (CV) measurement with Fe—Fe analog electrode Two thin film electrodes of the Fe—Fe analog of Preparation Example 2-1 were used as both electrodes, and 0.1 mol / L NH ４ The Prussian blue analogue was immersed in 7 ml of Cl electrolyte and CV measurement was performed at 5 mV / s. ４ It is oxidized and reduced in Cl electrolyte, and NH ４ The CV measurement results are shown in Figure 2. The reaction of the following formula (1) occurred at each potential, and NH ４ This indicates that an oxidation-reduction reaction occurs even in a Cl electrolyte, and that ammonium ions can be adsorbed.

[0076]

[0077] Reference Example 2: Cyclic Voltammetry (CV) Measurement of Cu—Fe Analogue CV measurements were performed on the Cu—Fe analogue electrode prepared in Preparation Example 2-2 using an activated carbon electrode as the counter electrode. The electrode was immersed in a 0.1 mol / L ammonium chloride electrolyte, and CV measurements were performed in the range of 0 to 1.4 V. The results are shown in Figure 3. The reduction and oxidation peaks resulting from the reaction of the following formula (2) were observed at 0.35 V and 1.15 V, respectively.

[0078]

[0079] Reference example 3 NH ４ ＋ Adsorption and desorption of ions NH ４ Ion adsorption and desorption were evaluated. An electrode consisting of 2.59 g of Prussian blue analog (Cu—Fe analog or Ni—Fe analog) and 7.37 g of activated carbon (approximately 2.8 times the weight of the Prussian blue analog) was placed in a 1800 mg-NH ４ ＋ / L's NH ４ The sample was immersed in 27.5 ml of Cl electrolyte and measured at ±40 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ Cl was adsorbed and desorbed. NH of Cu-Fe analogue and Ni-Fe analogue ４ The ion adsorption results are shown in Figure 4. In Figure 4, NH ４ ＋ The ion concentration (mg / L) is shown by the solid line, and the NH ４ ＋ The amount of ions adsorbed (mmol / g) is shown by the dotted line. In Figure 4, the Cu-Fe analogue and Ni-Fe analogue are ４ ＋ It adsorbs ions and removes NH ４ ＋ Specifically, the Cu-Fe analogue had a NH ion concentration of 0.67 mmol / g after 13,000 seconds, and the Ni-Fe analogue had a NH ion concentration of 0.79 mmol / g after 13,000 seconds. ４ ＋ The pH of the solution before adsorption was 5.5, and after adsorption, the pH dropped to 1.56.

[0080] Also, the NH of Cu-Fe analogues and Ni-Fe analogues ４ ＋ The ion desorption results are shown in Figure 5. In Figure 5, NH ４ ＋ The ion concentration (mg / L) is shown by the solid line, and the NH ４ ＋The amount of ions desorbed (mmol / g) is shown by the dotted line. In Fig. 5, the amount of NH adsorbed by Cu-Fe analogues and Ni-Fe analogues is shown by the dotted line. ４ ＋ This indicates that ions can be desorbed.

[0081] Example 1 NH ４ ＋ Selectivity of ion adsorption 1 Using an electrode prepared in the same manner as in Preparation Example 2-2, NH ４ ＋ The adsorption selectivity of ions was evaluated. An electrode consisting of 2.59 g of Prussian blue analogue and 7.37 g of activated carbon was placed on a 3600 mg-NH ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ The electrode was immersed in 27.5 ml of a mixed electrolyte of Cl and NaCl, and subjected to an electrochemical evaluation test at 40 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ The electrodes containing Cu-Fe analogs selectively adsorbed NH ４ ＋ Ion, Na ＋ NH in the electrolyte when ions are adsorbed ４ ＋ Ion concentration and Na ＋ The ion concentrations are shown in Figure 6. Also, NH ４ ＋ Ion, Na ＋ NH in the electrolyte when ions are adsorbed ４ ＋ Ion concentration and Na ＋ The ion concentrations are shown in Figure 7. In Figures 6 and 7, the Cu-Fe analogue and the Ni-Fe analogue are NH ４ ＋ It selectively adsorbs ions and NH ４ ＋ Specifically, in the Cu-Fe analogue, Na ＋ ions at 34,200 mg / L, NH ４ ＋ ions was 1972 mg / L, and in the Ni-Fe analogue, Na ＋ ions at 31,920 mg / L, NH４ ＋ The ion was 1512 mg / L.

[0082] Reference Example 4 Cycle Test Using the electrode prepared in Preparation Example 2-2, NH ４ ＋ Ion adsorption and desorption were evaluated. An electrode containing 2.59 g of Prussian blue analogue and 7.37 g of activated carbon was placed on a 1800 mg-NH ４ ＋ / L's NH ４ The sample was immersed in 27.5 ml of Cl electrolyte and measured at ±40 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ After the second time, a new electrolyte was prepared and the same electrode was used 30 times. ４ ＋ The results of ion adsorption and desorption are shown in Figure 8. ４ The amount of ion adsorption was 1.53 mmol / g, while the 30th NH ４ ＋ The amount of ions adsorbed was 0.39 mmol / g, and the amount of ions desorbed was 0.26 mmol / g.

[0083] Example 2 Multi-stage adsorption Using an electrode prepared in the same manner as in Preparation Example 2-2, NH ４ ＋ The ion concentration is 321 mg NH, which is the general wastewater standard. ４ ＋ The cells were treated in multiple steps until the concentration was below 1 / L. An electrode containing 2.59 g of Prussian blue analogue and 7.37 g of activated carbon was added to 3600 mg of NH ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ The electrode was immersed in 27.5 ml of a mixed electrolyte of Cl and NaCl, and subjected to an electrochemical evaluation test at 40 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ At this time, NH ４ As shown in the conceptual diagram in Figure 9, from the second test onwards, a new electrode was prepared, and the electrolyte was the same as that at the end of the previous adsorption test.４ Adsorption tests were performed using electrolytes adjusted to the NH ion and Na ion concentrations. ４ The test was continued until the ion concentration was below the general wastewater standard. ４ The results of ion adsorption and desorption are shown in Figure 10 (Ni-Fe analogue) and Figure 11 (Cu-Fe analogue). ４ The ion concentration is 3600 mg-NH ４ ＋ / L, it was confirmed that the discharge capacity was below the standard level. Specifically, when a Ni-Fe analog was used for the electrode, 3600 mg-NH ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ 1512 mg of NH from Cl and NaCl mixed electrolyte ４ ＋ In the second adsorption test, the concentration was reduced to 456 mg-NH ４ ＋ The concentration could be reduced to 154 mg / L in the third test. ４ ＋ / L. When a Cu-Fe analog was used for the electrode, 3600 mg-NH ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ 1726 mg of NH from Cl and NaCl mixed electrolyte ４ ＋ In the second adsorption test, the concentration was reduced to 572 mg-NH ４ ＋ / L, and the third time it was 196 mg-NH ４ ＋ The concentration could be reduced to 1 / L.

[0084] Example 3: Investigation of the ratio of the amount of Prussian blue analogue to activated carbon, the amount of ion adsorption, and pH. The required thickness of the activated carbon electrode was investigated using an electrode prepared in the same manner as in Preparation Example 2-2. Electrodes with activated carbon of 10 times, 1.5 times, 1 time, and 0.5 times the weight of 2.59 g of Cu-Fe Prussian blue analogue were used. ４ ＋ / L, 2500mg-Na ＋ / L's NH ４ The electrode was immersed in 27.5 ml of a mixed electrolyte of Cl and NaCl, and subjected to an electrochemical evaluation test at 40 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ At the end of the reaction, NH in the electrolyte was adsorbed. ４ ＋ ions and Cl － The ion concentration and pH were investigated, and the results are shown in Figure 12 (10 times weight), Figure 13 (1.5 times weight), Figure 14 (1 time weight), Figure 15 (0.5 times weight), Figure 16 (10 times weight), and Figure 17 (0.5 times weight).

[0085] The amount of ions adsorbed by both electrodes at the end of the test was NH ４ ＋ :0.279mmol, Cl － : 0.277 mmol, and when an electrode made of activated carbon with 1.5 times the weight was used, NH ４ ＋ :0.247mmol, Cl － : 0.236 mmol, and when an electrode made of activated carbon with the same weight was used, NH ４ ＋ :0.238mmol, Cl － : 0.116 mmol, and when an electrode made of activated carbon with 0.5 times the weight was used, NH ４ ＋ :0.245mmol, Cl － The amount of ions adsorbed was 0.094 mmol. ４ ＋ Regarding Prussian blue analog, Cl －The adsorption amount per 1 g of activated carbon is shown. If the weight of the activated carbon electrode is not sufficient for the Prussian blue analog electrode, the Cl － The amount of ions adsorbed decreases, while NH ４ ＋ It was found that the amount of ions adsorbed remained unchanged.

[0086] As shown in Figures 16 and 17, when the weight of activated carbon is 10 times the weight of the Prussian blue analogue, the pH is almost constant, whereas when it is 0.5 times, the pH of the electrolyte drops from 5 to 1.9. － Does not adsorb ions and OH － It is thought that ions are adsorbed.

[0087] Example 4 Adsorption of NH in a highly concentrated aqueous ammonium chloride solution ４ ＋ Using the electrode prepared in Preparation Example 2-1, it was confirmed whether further adsorbed ammonium ions could be desorbed in a high-concentration ammonium chloride aqueous solution. A silver-silver chloride electrode was prepared as the reference electrode, and platinum as the counter electrode, forming a three-electrode system. A solution with an ammonium chloride concentration of 80% of the saturated concentration was prepared using 29.76 g of ammonium chloride and 100 g of water. Desorption was attempted from this aqueous solution for 250 seconds at a constant current of 35 μA / sec, and infrared spectroscopy of the film was performed before and after the test. Ammonium ions were measured at 1410 cm －１ Since there is an absorption peak near the ４ ＋ The amount of ions adsorbed can be seen. The results are shown in Figure 18. It can be seen that the peak after the test is reduced compared to before the test. This demonstrates that this electrode is capable of desorbing ammonium ions even in solutions with high concentrations of dissolved ammonium ions. In other words, if desorption occurs in multiple stages or if a sufficient amount of ions is released from the positive electrode, it is possible to increase the concentration to a solution close to the saturated concentration. This test demonstrates that it is possible to concentrate the target ions by recovering them from a low-concentration solution and increasing the concentration.

[0088] Example 5: Ions to be collected are K＋ and / or NH ４ ＋ , the non-recovery target ion is Na ＋ Testing: Using the thick film electrode of Ni-Fe analogue prepared in Preparation Example 2-2, the adsorption behavior when three kinds of cations were contained was evaluated. ＋ , K. ＋ , N.H. ４ ＋ A solution containing 0.14 mol / L of each of these was prepared. An electrode containing activated carbon was prepared as the negative electrode, with a weight ratio of positive electrode to negative electrode of 1:4.5. The positive electrode and negative electrode were immersed in the aqueous solution to be treated, and a current of 2.5 mA / cm was applied to the positive electrode. ２ The voltage was applied so that the constant current was 0.01%. The change in concentration of the aqueous solution to be treated in this case is shown in Figure 19. ＋ There is almost no change in concentration of NH ４ ＋ , K. ＋ It can be seen that the concentration of NH ４ ＋ , K. ＋ , Na ＋ When treating an aqueous solution containing NH ４ ＋ and K. ＋ The non-recovery target cation is Na ＋ Similarly, NH ４ ＋ does not include K ＋ and Na ＋ A similar test was carried out using an aqueous solution containing only K. ＋ The concentration decreases, and Na ＋ There was almost no change in the concentration. ＋ is the target cation, Na ＋ It can be seen that cations that are not the target of recovery.

[0089] Example 6 NH ４ ＋ Selectivity of ion adsorption 2 Using the electrode prepared in Preparation Example 2-3, NH ４ ＋The adsorption selectivity of ions was evaluated. The Prussian blue analog electrode prepared in Preparation Example 2-3 and the activated carbon electrode were ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ The electrode was immersed in 228 ml of a mixed electrolyte of Cl and NaCl, and measured at 2.5 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ The electrodes containing Ni-Fe analogs selectively adsorbed NH ４ ＋ Ion, Na ＋ NH in the electrolyte when ions are adsorbed ４ ＋ Ion concentration (circles in the figure) and Na ＋ The ion concentrations (square marks in the figure) are shown in Figure 20. In Example 6, the Ni-Fe analogue was NH ４ ＋ It selectively adsorbs ions and NH ４ ＋ This shows that the ion concentration decreases. ＋ ions at 31,124 mg / L, NH ４ ＋ The ion was 1526 mg / L.

[0090] Example 7 NH ４ ＋ Selectivity of ion adsorption 3 Using the electrode prepared in Preparation Example 2-4, NH ４ ＋ The adsorption selectivity of ions was evaluated. The Prussian blue analog electrode prepared in Preparation Example 2-4 and the activated carbon electrode were ４ ＋ / L, 33000mg-Na ＋ / L's NH ４ The electrode was immersed in 1700 ml of a mixed electrolyte of Cl and NaCl, and measured at 2.5 mA / cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ Apply a current of NH ４ The electrodes containing Ni-Fe analogs selectively adsorbed NH ４ ＋ Ion, Na ＋NH in the electrolyte when ions are adsorbed ４ ＋ Ion concentration (black circle in the figure) and Na ＋ The ion concentrations (black squares in the figure) are shown in Figure 20. In Example 7, the Ni-Fe analogue was NH ４ ＋ It selectively adsorbs ions and NH ４ ＋ This shows that the ion concentration decreases. ＋ ions at 31,460 mg / L, NH ４ ＋ The ion was 1536 mg / L.

[0091] From the results of Examples 1, 6 and 7, it can be seen that if the solid-liquid ratio of the electrode and the electrolyte is similar, the Ni-Fe analogue can be used in the NH ４ ＋ This indicates that ions were selectively adsorbed.

[0092] Example 8 NH ４ ＋ Current value dependence of ion adsorption / desorption. NH ４ The current dependence of ion adsorption and desorption was evaluated. An electrode consisting of 2.59 g of Ni—Fe analogue and 7.37 g of activated carbon (approximately 2.8 times the weight of the Prussian blue analogue) was placed in a 2000 mg-NH ４ ＋ / L and 110,000 mg-Na ＋ / L's NH ４ The electrode was immersed in 27.5 ml of a mixed electrolyte of Cl and NaCl, and subjected to electrochemical evaluation at 20 mA / 16 cm using an electrochemical evaluation device (HZ-pro, Hokuto Denko Corporation). ２ , 10mA / 16cm ２ , 2.50mA / 16cm ２ , 1.25mA / 16cm ２ Apply a current of NH ４ Cl was adsorbed and desorbed. NH ４ ＋ Ion concentration and Na ＋ The ion concentration is shown in Figure 21. In Figure 21, by reducing the current value per electrode area, the NH ４ ＋This indicates that the ion concentration decreases.

Claims

1. A first electrode containing one or more Prussian blue analogs and having a content rate of the target cation to be recovered below a predetermined amount, and a second electrode are immersed in an aqueous solution to be treated containing one or more target cations to be recovered and one or more non-target cations other than the target cations to be recovered. The cathode of a DC power supply is connected to the first electrode, the anode of the DC power supply is connected to the second electrode, and electricity is passed to achieve a constant current, so as to perform an adsorption step of adsorbing the target cation to be recovered on the first electrode. After the adsorption step, instead of the treated solution after the adsorption step, the first electrode adsorbed with the target cation to be recovered is immersed in a recovery liquid together with the second electrode. The anode of the DC power supply is connected to the first electrode, the cathode of the DC power supply is connected to the second electrode, and electricity is passed to desorb the target cation to be recovered adsorbed on the first electrode into the recovery liquid, and a desorption step is performed. A step of obtaining a recovery liquid containing the target cation to be recovered obtained by performing the desorption step is included, and a method for recovering the target cation to be recovered is provided.

2. The method according to claim 1, wherein the aqueous solution to be treated in the adsorption step is the treated solution after the adsorption step is performed.

3. The method according to claim 1, wherein the total concentration of the non-target cations in the aqueous solution to be treated in the adsorption step (non-target cation mass / aqueous solution volume) is 5 times or more the target cation concentration (target cation mass / aqueous solution volume).

4. The aqueous solution to be treated in the adsorption step further contains a halogeno ion. In the adsorption step, the halogeno ion is adsorbed on the second electrode, and in the desorption step, the halogeno ion adsorbed on the second electrode is desorbed into the aqueous liquid. The method according to claim 1.

5. The method according to claim 1, wherein the second electrode includes a current collector and a carbon-based material.

6. The method according to claim 5, wherein the mass of the carbon-based material contained in the second electrode is 1 time or more the mass of the Prussian blue analog contained in the first electrode.

7. The method according to any one of claims 1 to 6, wherein the target cation to be recovered is an ammonium ion and / or a potassium ion.

8. A method for producing a recovered cation-containing solution with a reduced concentration of cations to be recovered, comprising: an adsorption step of immersing a first electrode containing one or more Prussian blue analogs and a second electrode in an aqueous solution to be treated containing one or more cations to be recovered and one or more non-recovered cations other than the cations to be recovered, with the content rate of the cations to be recovered being equal to or less than a predetermined amount, connecting the cathode of a DC power supply to the first electrode, connecting the anode of the DC power supply to the second electrode, and energizing the electrodes so as to obtain a constant current, thereby adsorbing the cations to be recovered onto the first electrode; and a solution recovery step of separating the treated solution after the adsorption step from the first and second electrodes.

Images