Layered double hydroxide, method for removing nitrate ions, and method for recovering nitrate ions

Abstract

To provide a layered double hydroxide that exhibits high nitrate ion selectivity and allows efficient recovery of nitrate ions after adsorption.SOLUTION: A layered double hydroxide represented by the following formula [1] and having an adsorption distribution coefficient (Kd) for nitrate ions of 70 or more. [Mg3(Al1-xZrx)(OH)8] [An- mH2O] [1], where, in formula [1], x is a number satisfying 0.2≤x≤0.4, m is a number of 0 or more and 4 or less, n is a natural number of 1 or more and 4 or less, and An- represents at least one n-valent ion-exchangeable anion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a layered double hydroxide, a method for removing nitrate ions, and a method for recovering nitrate ions. [Background technology]

[0002] When nitrogen compounds such as nitrate nitrogen contained in wastewater are released into the environment, they have a serious impact on the natural environment, such as eutrophication of lakes, rivers, and oceans, and contamination of groundwater. Therefore, nitrogen components contained in domestic wastewater are removed at sewage treatment plants using a water treatment microbial community called activated sludge, which contains a mixture of more than several thousand species of microorganisms.

[0003] In the manufacturing industry, the use of nitric acid to wash products during the manufacturing process generates wastewater containing high concentrations of nitrate nitrogen, which is poor in organic matter. This wastewater treatment is essential. However, the conventional activated sludge process can only remove low concentrations of nitrate nitrogen (usually less than 100 mg / L). This makes it difficult to directly apply this method to industrial wastewater. The activated sludge process requires diluting high-concentration wastewater to a treatable concentration and adding organic matter as nutrients. This difficulty in wastewater treatment has hindered the smooth production of products in manufacturing plants. Furthermore, the only available methods for removing nitrate nitrogen other than microbial treatment are physical and chemical methods such as electrodialysis, which require high energy costs. Therefore, a more efficient wastewater treatment method was needed.

[0004] Layered double hydroxides have been investigated as other materials for removing anions such as nitrate ions. For example, Patent Document 1 discloses a Ni-Fe-based adsorbent as an adsorbent capable of adsorbing nitrate ions from an aqueous solution. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4000370 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 describes methods for desorbing adsorbed nitrate ions, including a method using an alkaline solution, an alkaline carbonate solution, a method for removing NO3 by heating, and a method using a 1 to 2 M alkaline solution. However, such a desorption method is not simple.

[0007] An object of the present invention is to provide a layered double hydroxide that has high nitrate ion selectivity and enables the adsorbed nitrate ions to be recovered easily and efficiently. It is also an object of the present invention to provide a method for removing nitrate ions using the layered double hydroxide. It is also an object of the present invention to provide a method for recovering nitrate ions using the layered double hydroxide. [Means for solving the problem]

[0008] The present inventors have conducted extensive research into the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by employing the following configuration, thereby completing the present invention.

[0009] The present invention provides the following: <1> It is expressed by the following formula [1]: A layered double hydroxide characterized by having an adsorption distribution coefficient Kd of nitrate ions of 70 or more as measured by the following measurement method. [Mg3(Al 1-x Zr x )(OH)8]·[A n- mH2O] [1] In formula [1], x is a number that satisfies 0.2≦x≦0.4, m is a number that is 0 or more and 4 or less, n is a natural number that is 1 or more and 4 or less, and A n- is at least one n-valent ion-exchangeable anion. <Measurement method> 1. 1 g of layered double hydroxide is added to 30 ml of a mixed aqueous solution containing 390 ppm of nitrate ions, 2300 ppm of chloride ions, and 5800 ppm of sulfate ions, and the mixture is stirred at 27°C for 1 hour. 2. Next, the nitrate ion concentration in the supernatant is analyzed using ion chromatography. 3. Next, calculate the adsorption distribution coefficient Kd of nitrate ions using the following formula. (Adsorption distribution coefficient of nitrate ions, Kd (mL / g)) = [amount of nitrate ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of nitrate ions in 1 mL of solution after adsorption (mol / mL)].

[0010] The layered double hydroxide contains Mg as a divalent metal ion. 2+ and Al as the trivalent metal ion. 3+ The layered double hydroxide has the formula [1], where some of the Al sites of the layered double hydroxide are substituted with Zr. Specifically, in formula [1], Al is substituted with Zr within the range of x satisfying 0.2≦x≦0.4. Because Al is substituted with Zr within the range of x satisfying 0.2≦x≦0.4, the interlayer distance of the layered double hydroxide is a distance suitable for selectively adsorbing nitrate ions (specifically, 8.1 Å or more and 8.2 Å or less). That is, the layered double hydroxide having x satisfying 0.2≦x≦0.4 in formula [1] can selectively adsorb nitrate ions. Furthermore, by substituting Zr, an amphoteric element, for Al and Mg, which are highly basic, basicity can be adjusted, and adsorbed nitrate ions can be easily desorbed. That is, adsorbed nitrate ions can be recovered with high efficiency. As described above, according to the above-described configuration, it is possible to selectively adsorb nitrate ions, and also to recover the adsorbed nitrate ions with high efficiency.

[0011] Conventionally, layered double hydroxides are the only inorganic compounds known to be capable of adsorbing anions. Although layered double hydroxides can adsorb anions, it has been difficult to desorb the adsorbed anions. The only methods available for desorption, such as those described in Patent Document 1 above, are those using alkaline solutions or those involving heating, which are not particularly convenient. The present invention is particularly advantageous in that it can desorb adsorbed nitrate ions easily and efficiently, allowing for highly efficient recovery.

[0012] Furthermore, the present invention provides the following: <2> The x is a number satisfying 0.2≦x≦0.3. <1> The layered double hydroxide according to claim 1.

[0013] When the x is a number that satisfies the condition 0.2≦x≦0.3, nitrate ions can be more selectively adsorbed, and the adsorbed nitrate ions can be recovered with higher efficiency.

[0014] Furthermore, the present invention provides the following: <3> The aforementioned <1> or the above <2> 1. A method for removing nitrate ions, comprising the step of contacting the layered double hydroxide according to claim 1 with an aqueous anion solution containing nitrate ions.

[0015] According to the above-mentioned configuration, the layered double hydroxide is used, so that nitrate ions can be selectively adsorbed.

[0016] Furthermore, the present invention provides the following: <4> The anion aqueous solution containing nitrate ions is <1> or the above <2> 2. A method for removing nitrate ions, comprising the step of passing water through a column packed with the layered double hydroxide according to claim 1.

[0017] According to the above-mentioned configuration, the layered double hydroxide is used, so that nitrate ions can be selectively adsorbed.

[0018] Furthermore, the present invention provides the following: <5> The nitrate ions are adsorbed <1> or the above <2> 2. A method for recovering nitrate ions, comprising the step of contacting a leachate with the layered double hydroxide according to claim 1 to recover nitrate ions.

[0019] According to the above-mentioned configuration, since the layered double hydroxide is used, the adsorbed nitrate ions can be recovered with high efficiency. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a layered double hydroxide that has high nitrate ion selectivity and is capable of recovering adsorbed nitrate ions with high efficiency. It is also possible to provide a method for removing nitrate ions using the layered double hydroxide. It is also possible to provide a method for recovering nitrate ions using the layered double hydroxide. [Brief explanation of the drawings]

[0021] [Figure 1] 1 shows X-ray diffraction spectra of layered double hydroxides of Examples and Comparative Examples. [Figure 2] 1 shows X-ray diffraction spectra of layered double hydroxides of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to these embodiments. In this specification, zirconium is a general term and contains impurities of 10 mass% or less in terms of oxides including hafnium. In addition, in this specification, the expressions "contain" and "comprise" include the concepts of "contain," "comprise," "substantially consist," and "consist only of."

[0023] The maximum and minimum values ​​of the content of each component shown below are independently the preferred minimum and maximum values ​​of the present invention, regardless of the content of other components. Furthermore, the maximum and minimum values ​​of the various parameters (measured values, etc.) shown below are independently the preferred minimum and maximum values ​​of the present invention, regardless of the content (composition) of each component.

[0024] [Layered double hydroxide] The layered double hydroxide according to this embodiment is It is expressed by the following formula [1]: The adsorption distribution coefficient Kd of nitrate ions measured by the following measurement method is 70 or more. [Mg3(Al 1-x Zr x )(OH)8]·[A n- mH2O] [1] In formula [1], x is a number that satisfies 0.2≦x≦0.4, m is a number that is 0 or more and 4 or less, n is a natural number that is 1 or more and 4 or less, and A n- is at least one n-valent ion-exchangeable anion. <Measurement method> 1. 1 g of layered double hydroxide is added to 30 ml of a mixed aqueous solution containing 390 ppm of nitrate ions, 2300 ppm of chloride ions, and 5800 ppm of sulfate ions, and the mixture is stirred at 27°C for 1 hour. 2. Next, the nitrate ion concentration in the supernatant is analyzed using ion chromatography. 3. Next, calculate the adsorption distribution coefficient Kd of nitrate ions using the following formula. (Adsorption distribution coefficient of nitrate ions, Kd (mL / g)) = [amount of nitrate ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of nitrate ions in 1 mL of solution after adsorption (mol / mL)]. Here, the denominator on the right side of the above equation, [amount of nitrate ions (mol / mL) in 1 mL of post-absorption solution], is obtained by converting the unit of the nitrate ion concentration (μg / mL) of the supernatant (post-absorption solution) measured in step 2 of the above measurement method into mol / mL. Regarding the numerator on the right side of the above equation, the amount of nitrate ions adsorbed per gram of layered double hydroxide (μg / g) can be calculated using the following formula. [Amount of nitrate ions adsorbed per 1 g of layered double hydroxide (μg / g)] = [[390 (μg / mL)] - [nitrate ion concentration of the solution after adsorption (μg / mL)]] × 30 (mL)] / 1 (g) In the above formula, "390 (μg / mL)" is the initial nitrate ion concentration of 390 ppm, "30 (mL)" is the amount of the mixed aqueous solution, and "1 (g)" is the amount of the layered double hydroxide. After determining the amount of nitrate ions adsorbed per gram of layered double hydroxide (μg / g), the unit can be converted to moles to obtain the numerator on the right side of the above equation (the amount of nitrate ions adsorbed per gram of layered double hydroxide (mol / g)).

[0025] As described above, the layered double hydroxide according to this embodiment is represented by the following formula [1]. [Mg3(Al 1-x Zr x )(OH)8]·[A n- mH2O] [1] In formula [1], x is a number that satisfies 0.2≦x≦0.4, m is a number that is 0 or more and 4 or less, n is a natural number that is 1 or more and 4 or less, and A n- is at least one n-valent ion-exchangeable anion.

[0026] The layered double hydroxide contains Mg as a divalent metal ion. 2+ and Al as the trivalent metal ion. 3+The layered double hydroxide has the formula [1], where some of the Al sites of the layered double hydroxide are substituted with Zr. Specifically, in formula [1], Al is substituted with Zr within the range of x satisfying 0.2≦x≦0.4. Because Al is substituted with Zr within the range of x satisfying 0.2≦x≦0.4, the interlayer distance of the layered double hydroxide is a distance suitable for selectively adsorbing nitrate ions (specifically, 8.1 Å or more and 8.2 Å or less). That is, the layered double hydroxide having x satisfying 0.2≦x≦0.4 in formula [1] can selectively adsorb nitrate ions. Furthermore, by substituting Zr, an amphoteric element, for Al and Mg, which are highly basic, the basicity can be adjusted, and the adsorbed nitrate ions can be easily desorbed. That is, the adsorbed nitrate ions can be recovered with high efficiency. As described above, the layered double hydroxide can selectively adsorb nitrate ions, and can recover the adsorbed nitrate ions with high efficiency.

[0027] The x is preferably a number that satisfies 0.2≦x≦0.3. When the x is 0.2≦x≦0.3, the interlayer distance becomes a distance more suitable for adsorbing nitrate ions, and the basicity can be adjusted to be suitable for more easily desorbing the adsorbed nitrate ions, which is more preferable.

[0028] The above A n- The anion is not particularly limited as long as it is an n-valent ion-exchangeable anion, but examples thereof include OH - , Cl - , F - , Br - , HCO3 ― , NO3 - , NO2 - , CO3 2- , SO4 2- , PO4 3- , AsO3 3- , Fe(CN)6 4- , tartrate ion.

[0029] The value of m is not particularly limited as long as it is a number between 0 and 4, since the amount of water of hydration of the layered double hydroxide varies depending on the relative humidity. For example, the m is not particularly limited as long as it is a number of 0 or more and 4 or less when analyzed using TG-DTA (simultaneous differential thermal-thermogravimetric analysis) under conditions of leaving the sample at rest for 240 hours at a temperature of 20°C and a humidity of 65% for no more than 2 hours.

[0030] As described above, the layered double hydroxide according to this embodiment has an adsorption distribution coefficient Kd of nitrate ions of 70 or more, as measured by the following measurement method. Because the adsorption distribution coefficient Kd of nitrate ions is 70 or more, it can be said that the layered double hydroxide can selectively adsorb nitrate ions. <Measurement method> 1. 1 g of layered double hydroxide is added to 30 ml of a mixed aqueous solution containing 390 ppm of nitrate ions, 2300 ppm of chloride ions, and 5800 ppm of sulfate ions, and the mixture is stirred at 27°C for 1 hour. 2. Next, the nitrate ion concentration in the supernatant is analyzed using ion chromatography. 3. Next, calculate the adsorption distribution coefficient Kd of nitrate ions using the following formula. (Adsorption distribution coefficient of nitrate ions, Kd (mL / g)) = [amount of nitrate ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of nitrate ions in 1 mL of solution after adsorption (mol / mL)].

[0031] The adsorption distribution coefficient Kd of the nitrate ions is preferably 70 or more, more preferably 75 or more.

[0032] The interlayer distance of the layered double hydroxide is preferably 8.1 Å to 8.2 Å, more preferably 8.1 Å to 8.18 Å. When the interlayer distance is 8.1 Å to 8.2 Å, nitrate ions can be more suitably adsorbed.

[0033] The particle size of the layered double hydroxide is not particularly limited. From the viewpoint of being able to adsorb nitrate ions effectively, a relatively small particle size is preferable. However, if particles of 0.1 mm or less are packed into a column, pressurization may be required to pass water through, as in HPLC (high performance liquid chromatography). Therefore, it is preferable to adjust the particle size to be in the range of 0.5 mm to 2 mm. However, the particle size is not limited to this range as long as it allows water to pass through without pressurization.

[0034] The layered double hydroxide can be used alone as a nitrate ion adsorbent that adsorbs nitrate ions. The layered double hydroxide may be used in combination with other particles.

[0035] [Method for producing layered double hydroxide] An example of a method for producing a layered double hydroxide will be described below, but the method for producing a layered double hydroxide of the present invention is not limited to the following example.

[0036] The method for producing the layered double hydroxide according to this embodiment includes the steps of: A step A of preparing a mixed aqueous solution of magnesium chloride, aluminum chloride, and zirconium oxychloride octahydrate while maintaining a pH of 9 or more and 10 or less; and step B of aging the mixed aqueous solution.

[0037] In the step A, the order of mixing is not particularly limited as long as a mixed aqueous solution of magnesium chloride, aluminum chloride, and zirconium oxychloride octahydrate is obtained. For example, an aqueous solution of zirconium oxychloride is added to an aqueous solution of magnesium chloride and aluminum chloride, and the resulting solution is dropped into pure water while being kept alkaline (pH 9 or more and 10 or less) with an aqueous solution of sodium hydroxide or the like to obtain an aqueous solution of the mixture. Another method is to drop a mixed aqueous solution of magnesium chloride and aluminum chloride into pure water while maintaining the solution in an alkaline state (pH 9 to 10) using an aqueous sodium hydroxide solution or the like, and then dropwise add an aqueous solution of zirconium oxychloride while maintaining the solution in an alkaline state (pH 9 to 10), thereby obtaining a mixed aqueous solution. During the above mixing, nitrogen gas or the like may be bubbled to minimize the incorporation of carbonate ions between the layers of the layered double hydroxide.

[0038] After step A, the resulting mixed aqueous solution is aged (step B). The aging temperature is not particularly limited, but may be within the range of 20° C. to 100° C., for example, room temperature (25° C.). The aging time is not particularly limited, but is preferably 30 minutes to 240 minutes.

[0039] After the step B, if necessary, filtration washing with distilled water, drying, etc. may be carried out. The drying method is not particularly limited, and examples thereof include drying with a dryer, spray drying, and vacuum drying.

[0040] The method for producing a layered double hydroxide according to this embodiment has been described above.

[0041] [Method for removing nitrate ions] An example of a method for removing nitrate ions will be described below, but the method for removing nitrate ions of the present invention is not limited to the following example.

[0042] The method for removing nitrate ions according to the first embodiment includes the following steps: The method includes a step of contacting the layered double hydroxide with an aqueous anion solution containing nitrate ions.

[0043] As a method for contacting the layered double hydroxide with an aqueous anion solution containing nitrate ions, a method in which the layered double hydroxide is introduced into the aqueous anion solution can be mentioned.

[0044] The method for removing nitrate ions according to the second embodiment is as follows: The method includes a step of passing an aqueous anion solution containing nitrate ions through a column packed with the layered double hydroxide.

[0045] In the method for producing a layered double hydroxide according to the first embodiment and the method for producing a layered double hydroxide according to the second embodiment, the aqueous anion solution containing nitrate ions is not particularly limited as long as it contains nitrate ions. The aqueous anion solution containing nitrate ions may contain only nitrate ions, or may contain nitrate ions and other anions. Examples of the aqueous anion solution containing nitrate ions include general industrial wastewater, which may contain sulfate ions, chloride ions, and the like in addition to nitrate ions. The concentration of nitrate ions in the aqueous anion solution containing nitrate ions is not particularly limited.

[0046] According to the method for producing a layered double hydroxide according to the first embodiment and the method for producing a layered double hydroxide according to the second embodiment, the layered double hydroxide is used, so that nitrate ions can be selectively adsorbed and the nitrate ions can be suitably removed from the aqueous anion solution.

[0047] In the method for producing a layered double hydroxide according to the first embodiment and the method for producing a layered double hydroxide according to the second embodiment, the pH of the aqueous anion solution containing nitrate ions is not particularly limited. In the method for producing a layered double hydroxide according to the first embodiment and the method for producing a layered double hydroxide according to the second embodiment, the layered double hydroxide is used, and therefore nitrate ions can be selectively adsorbed regardless of the pH value (at least in the pH range of 2 to 9). This is also clear from the examples.

[0048] [Method for recovering nitrate ions] An example of a method for recovering nitrate ions will be described below, but the method for recovering nitrate ions of the present invention is not limited to the following example.

[0049] The method for recovering nitrate ions according to this embodiment includes the steps of: The method includes a step of contacting the layered double hydroxide having nitrate ions adsorbed thereon with a release liquid to recover the nitrate ions.

[0050] The desorption liquid is not particularly limited as long as it can desorb nitrate ions from the layered double hydroxide to which nitrate ions have been adsorbed, and examples thereof include hydrochloric acid, sulfuric acid, etc. The desorption liquid may be mixed with a phosphate, an acetate, a buffer solution such as Tris-HCl, or a neutral salt such as NaCl, KCl, or Na2SO4.

[0051] One method for recovering nitrate ions by bringing the eluate into contact with the layered double hydroxide to which nitrate ions have been adsorbed is to pass the eluate through a column packed with the layered double hydroxide to which nitrate ions have been adsorbed. This method allows nitrate ions to be desorbed from the layered double hydroxide, making it possible to reuse the layered double hydroxide as a nitrate ion adsorbent. Furthermore, the recovered eluate contains the nitrate ions desorbed from the layered double hydroxide, allowing for highly efficient recovery of nitrate ions. In particular, the basicity of the layered double hydroxide is adjusted by substituting the amphoteric element Zr for Al, which has high basicity, and the adsorbed nitrate ions can be easily desorbed using the desorbent. [Example]

[0052] The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. The layered double hydroxides in the examples and comparative examples contain hafnium as an unavoidable impurity in an amount of 1.3 to 2.5 mass % in terms of oxide relative to zirconium (calculated using the following formula (X)). <Formula (X)> ([Mass of hafnium oxide] / ([Mass of zirconium oxide]+[Mass of hafnium oxide]))×100(%)

[0053] [Preparation of layered double hydroxides] Example 1 First, a mixed aqueous solution of magnesium chloride (MgCl2) and aluminum chloride (AlCl3) (molar ratio Mg:Al=3:0.6) (MgCl2 concentration: 1.5 mol / L, AlCl3 concentration: 0.3 mol / L) was prepared. Next, 500 mL of the prepared mixed solution was dropped into a beaker containing pure water (200 mL). At this time, a 3 mol / L NaOH aqueous solution was added dropwise so that the pH of the solution in the beaker became 10, and a layered double hydroxide precursor was obtained. After that, the mixture was stirred for 30 minutes. Next, a zirconium oxychloride solution (concentration: 0.2 mol / L) was added dropwise to the mixed solution containing the layered double hydroxide precursor, so that the molar ratio of Mg:Al:Zr was 3:0.6:0.4, yielding a layered double hydroxide. During the addition, a 3 mol / L NaOH aqueous solution was added dropwise to maintain the pH at 10. During the above mixing, nitrogen gas was bubbled through the mixture to minimize the incorporation of carbonate ions between the layers of the layered double hydroxide. Thereafter, this mixed aqueous solution was aged at room temperature (25° C.) for 30 minutes, thoroughly filtered and washed with distilled water, and then dried using a freeze dryer to obtain the layered double hydroxide according to Example 1.

[0054] (Examples 2-3, Comparative Examples 1-5) The layered double hydroxides of Examples 2-3 and Comparative Examples 1-5 were obtained in the same manner as in Example 1, except that the mixing ratio of magnesium chloride and aluminum chloride was changed so that the ratio of Mg, Al, and Zr was as shown in Table 1, and the amount of the aqueous zirconium oxychloride solution added was changed.

[0055] [X-ray diffraction spectrum] X-ray diffraction spectra were obtained for the layered double hydroxides of the Examples and Comparative Examples using an X-ray diffractometer (RINT2500, manufactured by Rigaku). The measurement conditions were as follows. The results are shown in Figures 1 and 2. In Figure 1, (a) is the X-ray diffraction spectrum of Comparative Example 1, (b) is the X-ray diffraction spectrum of Comparative Example 2, (c) is the X-ray diffraction spectrum of Example 1, and (d) is the X-ray diffraction spectrum of Example 2. In Figure 2, (e) is the X-ray diffraction spectrum of Example 3, (f) is the X-ray diffraction spectrum of Comparative Example 3, (g) is the X-ray diffraction spectrum of Comparative Example 4, and (h) is the X-ray diffraction spectrum of Comparative Example 5. <Measurement conditions> Measurement equipment: X-ray diffraction equipment (Rigaku, RINT2500) Source: CuKα source Tube voltage: 40kV Tube current: 30mA Scanning speed: 2θ=5~60°: 2° / min

[0056] With the exception of Comparative Example 5(h), the layered double hydroxide (LDH) peak (●) is observed in all patterns in Figures 1 and 2, indicating that layered double hydroxide was produced.

[0057] Furthermore, the interlayer distance was determined from the (003) peak in the X-ray diffraction spectrum, and was 8.02 Å for (a) Comparative Example 1, 8.05 Å for (b) Comparative Example 2, 8.10 Å for (c) Example 1, 8.12 Å for (d) Example 2, 8.18 Å for (e) Example 3, 8.27 Å for (f) Comparative Example 3, and 8.21 Å for (g) Comparative Example 4. Note that no (003) peak was present in (h) Comparative Example 5.

[0058] [Analysis of Layered Double Hydroxide Composition] The compositions of the layered double hydroxides prepared in the examples and comparative examples were analyzed using ICP-AES (ULTIMA-2, HORIBA) and energy dispersive X-ray spectroscopy (EDX-7200, Shimadzu). Because the amount of HO varies depending on the storage conditions (temperature and relative humidity) of the layered double hydroxide, the samples were left to stand at a temperature of 25°C and a humidity of 40% for 240 hours, after which they were analyzed using TG-DTA (Thermo plus TG8120, Rigaku Corporation) within 2 hours.

[0059] From the results of X-ray diffraction spectrum and composition analysis, it was found that the layered double hydroxides produced in the examples and comparative examples had the following compositions. Example 1: [Mg(Al 0.8 Zr 0.2 )(OH)8]·[Cl - 2.8H2O] Example 2: [Mg(Al 0.7 Zr 0.3 )(OH)8]·[Cl -2.6H2O] Example 3: [Mg(Al 0.6 Zr 0.4 )(OH)8]·[Cl - 3.0 H2O] Comparative example 1: [Mg3Al(OH)8]·[Cl - 2.5H2O] Comparative example 2: [Mg3(Al 0.9 Zr 0.1 )(OH)8]·[Cl - 2.4H2O] Comparative example 3: [Mg3(Al 0.5 Zr 0.5 )(OH)8]·[Cl - 2.9H2O] Comparative example 4: [Mg3(Al 0.3 Zr 0.7 )(OH)8]·[Cl - 2.6H2O] Comparative example 5: [Mg3Zr(OH)8]·[Cl - 1.7H2O]

[0060] [Measurement of partition coefficient] 1 g of each layered double hydroxide from each Example and Comparative Example was added to 30 ml of a mixed aqueous solution containing 390 ppm nitrate ions, 2300 ppm chloride ions, and 5800 ppm sulfate ions, and the mixture was stirred at 27°C for 1 hour. The nitrate ion concentration in the supernatant was then analyzed using an ion chromatograph ("HIC-SP," manufactured by Shimadzu Corporation). The distribution coefficient Kd was then calculated. The distribution coefficient Kd was calculated as follows: (Adsorption distribution coefficient of nitrate ions, Kd (mL / g)) = [amount of nitrate ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of nitrate ions in 1 mL of solution after adsorption (mol / mL)]. (Adsorption distribution coefficient of sulfate ions, Kd (mL / g)) = [amount of sulfate ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of sulfate ions in 1 mL of solution after adsorption (mol / mL)]. (Adsorption distribution coefficient of chloride ions, Kd (mL / g)) = [amount of chloride ions adsorbed in 1 g of layered double hydroxide (mol / g)] / [amount of chloride ions in 1 mL of solution after adsorption (mol / mL)]. <Ion chromatograph equipment and conditions> Manufacturer: Shimadzu Corporation Equipment name: HIC-SP SPD-40V and SPD-20A (absorbance detectors) are used to detect nitrate ions. CTO-40C Column Oven CDD-10 Electrical conductivity is used for other than nitrate ions Column: IC-SA2, 250L x 4.0 (anion analysis column) Eluent (NaHCO312mmol / L, Na2CO30.6mmol / L) Flow rate: 1mL / min

[0061] [Table 1]

[0062] [Batch-type nitrate ion adsorption rate when using a nitrate ion aqueous solution] 1 g of each layered double hydroxide of the Examples and Comparative Examples was added to 30 mL of an aqueous solution containing 500 ppm nitrate ions, and the mixture was stirred at 27°C for 1 hour. The aqueous solution was prepared using sodium nitrate. The solid-liquid separation was then carried out by suction filtration, and the nitrate ion concentration in the resulting solution (filtrate) was analyzed by ion chromatography. The nitrate ion adsorption rate was then calculated using the following formula. The results are shown in Table 2. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in filtrate) / (nitrate ion concentration in aqueous solution (500 ppm))]] x 100

[0063] [Table 2]

[0064] [Adsorption rate of various anions in a batch system using simulated wastewater] One gram of each layered double hydroxide from each Example and Comparative Example was added to 30 ml of a mixed aqueous solution (simulated wastewater) containing 500 ppm nitrate ions, 2500 ppm chloride ions, and 6000 ppm sulfate ions, and the mixture was stirred at 27°C for one hour. The aqueous solution was prepared by weighing out 0.6855 g of sodium nitrate, 4.116 g of sodium chloride, and 8.878 g of sodium sulfate, dissolving them in deionized water, and adjusting the total volume to 1 L using a measuring flask. Next, solid-liquid separation was performed by suction filtration, and the concentration of each anion in the resulting solution (filtrate) was analyzed by ion chromatography. The adsorption rate of each anion was then calculated using the following formula. The results are shown in Table 3. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in filtrate) / (nitrate ion concentration in simulated wastewater (500 ppm))]] × 100 (Chloride ion adsorption rate (%)) = [1 - [(chloride ion concentration in filtrate) / (chloride ion concentration in simulated wastewater (2500 ppm))]] x 100 (Sulfate ion adsorption rate (%)) = [1 - [(sulfate ion concentration in filtrate) / (sulfate ion concentration in simulated wastewater (6000 ppm))]] x 100

[0065] [Table 3]

[0066] [Nitrate ion adsorption rate in a column adsorption test (continuous type) using a nitrate ion aqueous solution] Ten grams of the layered double hydroxides of the Examples and Comparative Examples, classified into particle sizes of 0.5 to 2.0 mm, were packed into a column (length: 80 mm, inner diameter: 18 mm), and 1.0 L of an aqueous solution adjusted to a nitrate ion concentration of 1000 ppm (specifically, 1.371 g of sodium nitrate was weighed out, dissolved in deionized water, and the total volume was adjusted to 1 L using a measuring flask) was passed through the column at a flow rate of 7.0 mL / min. After passing through the column, the entire 1.0 L of aqueous solution was collected, and the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion adsorption rate was then calculated using the following formula. The results are shown in Table 4. Note that a column test could not be performed on Comparative Example 1 due to powdering. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in the filtrate) / (nitrate ion concentration in the aqueous solution)]] × 100.

[0067] [Table 4]

[0068] [Nitrate ion desorption rate in a column adsorption test (continuous type) using a nitrate ion aqueous solution] A 200 mL mixed aqueous solution of 0.005 mol / L HCl and 13% by mass NaCl was passed through the column used in the above "Nitrate ion adsorption rate in a column adsorption test (continuous) using an aqueous nitrate ion solution" (the column after passing 1.0 L of aqueous solution and allowing nitrate ions to be adsorbed onto the layered double hydroxide) at a flow rate of 1.5 mL / min. After passing through the column, the entire 200 mL of aqueous solution was collected and filtered with a syringe filter, and the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion desorption rate was then calculated using the following formula. The results are shown in Table 5. Note that the column test could not be performed for Comparative Example 1 due to powdering. (Nitrate ion desorption rate (%)) = ([amount of nitrate ion desorbed (mol)] / [amount of nitrate ion adsorbed (mol)]) × 100.

[0069] [Table 5]

[0070] [Adsorption rate of various anions in a column adsorption test (continuous type) using simulated wastewater] Ten grams of layered double hydroxides of Examples and Comparative Examples, classified into particle sizes of 0.5 to 2.0 mm, were packed into a column (length: 80 mm, inner diameter: 18 mm), and 1.0 L of simulated wastewater (a mixed aqueous solution containing 500 ppm nitrate ions, 2500 ppm chloride ions, and 6000 ppm sulfate ions) was passed through at a flow rate of 7.0 mL / min. After passing through the column, the entire 1.0 L of aqueous solution was collected and filtered through a syringe filter, after which the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion adsorption rate was then calculated using the following formula. The results are shown in Table 6. Note that a column test could not be performed on Comparative Example 1 due to powdering. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in filtrate) / (nitrate ion concentration in aqueous solution)]] x 100

[0071] [Table 6]

[0072] [Nitrate ion desorption rate in column adsorption test (continuous type) using simulated wastewater] A 200 mL mixed aqueous solution of 0.005 mol / L HCl and 13% by mass NaCl was passed through the column used in the above "Nitrate ion adsorption rate in a column adsorption test (continuous) using simulated wastewater" (the column after passing 1.0 L of simulated wastewater and allowing various anions to be adsorbed onto the layered double hydroxide) as the eluent at a flow rate of 1.5 mL / min. After passing through the column, the entire 200 mL of the aqueous solution was collected and filtered with a syringe filter, and the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion desorption rate was then calculated using the following formula. The results are shown in Table 6. Note that the column test could not be performed for Comparative Example 1 due to powdering. (Nitrate ion desorption rate (%)) = ([amount of nitrate ion desorbed (mol)] / [amount of nitrate ion adsorbed (mol)]) × 100.

[0073] [Table 7]

[0074] [Confirmation of pH dependency] 30 mL of aqueous solutions were prepared, each of which had a nitrate ion concentration of 500 ppm and a pH adjusted to 2 to 10. The pH was adjusted using 1 mol / L HCl or 1 mol / L NaOH. 0.1 g of the layered double hydroxide of Example 1 was added to each of the above aqueous solutions and stirred at 27°C for 1 hour. Next, solid-liquid separation was performed by suction filtration, and the nitrate ion concentration in the resulting solution (filtrate) was analyzed by ion chromatography. Thereafter, the nitrate ion adsorption rate was calculated using the following formula. The results are shown in Table 8. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in filtrate) / (nitrate ion concentration in aqueous solution (500 ppm))]] x 100

[0075] [Table 8]

[0076] From the above test results, it was confirmed that the layered double hydroxide of Example 1 exhibited a high value of nitrate ion adsorption rate without significant fluctuation even when the pH varied within the range of 2 to 10.

[0077] [Nitrate ion adsorption rate in a column adsorption test (continuous type) using river water] Ten grams of the layered double hydroxide of Example 3, classified into particle sizes of 0.5 to 2.0 mm, was packed into a column (length: 80 mm, inner diameter: 18 mm). 1.0 L of water samples collected from two locations in the Tama River basin (Tamagawa Ryokuchi Park (2-43-43-1 Someji, Chofu City, Tokyo) and the remains of Yaguchi Ferry (near 3-17-3 Yaguchi, Ota Ward, Tokyo)) were passed through the column at a flow rate of 7.0 mL / min. The entire volume was collected and filtered using a syringe filter. The nitrate ion concentration was then analyzed by ion chromatography. Thereafter, the adsorption rate of nitrate ions was calculated using the following formula: The results are shown in Table 9. (Nitrate ion adsorption rate (%)) = [1-[(nitrate ion concentration in filtrate) / (nitrate ion concentration in aqueous solution)]] x 100 At the time of sampling, the nitrate ion concentration at Tamagawa Green Park was 12.8 ppm, and the nitrate ion concentration at the site of Yaguchi Ferry was 12.5 ppm.

[0078] [Table 9]

[0079] [Nitrate ion adsorption rate and nitrate ion desorption rate in a column adsorption test (continuous type) using seawater] 10 g of the layered double hydroxide of Example 3, classified into particle sizes of 0.5 to 2.0 mm, was packed into a column (length: 80 mm, inner diameter: 18 mm), and 1.0 L of seawater collected from Yokohama Port was passed through at 7.0 mL / min. After passing through the column, the entire 1.0 L of aqueous solution was collected and filtered through a syringe filter. The nitrate ion concentration was then analyzed by ion chromatography. The nitrate ion adsorption rate was then calculated using the following formula: (Nitrate ion adsorption rate (%)) = [1-[(nitrate ion concentration in filtrate) / (nitrate ion concentration in aqueous solution)]] x 100 Next, 200 mL of a mixed aqueous solution of 0.005 mol / L HCl and 13% by mass NaCl was passed through the column at a flow rate of 1.5 mL / min. After passing through the column, the entire 200 mL of aqueous solution was collected and filtered through a syringe filter. The nitrate ion concentration was then analyzed by ion chromatography. The nitrate ion desorption rate was then calculated using the following formula. The results are shown in Table 10. (Nitrate ion desorption rate (%)) = ([amount of nitrate ion desorbed (mol)] / [amount of nitrate ion adsorbed (mol)]) × 100.

[0080] [Table 10]

Claims

1. It is represented by the following formula [1]: A layered double hydroxide characterized by having an adsorption distribution coefficient Kd of nitrate ions of 70 or more as measured by the following measurement method. [Mg ３ (Al １－ｘ Zr ｘ ) (OH) ８ ]・[A ｎ－ ・mH ２ O] [1] In the formula [1], x is a number that satisfies 0.2≦x≦0.4, m is a number that is 0 or more and 4 or less, n is a natural number that is 1 or more and 4 or less, and A ｎ－ is at least one n-valent ion-exchangeable anion. <Measurement method> 1. 1 g of layered double hydroxide is added to 30 ml of a mixed aqueous solution containing 390 ppm of nitrate ions, 2300 ppm of chloride ions, and 5800 ppm of sulfate ions, and the mixture is stirred at 27° C. for 1 hour.

2. Next, the nitrate ion concentration in the supernatant is analyzed using ion chromatography.

3. Next, calculate the adsorption distribution coefficient Kd of nitrate ions using the following formula: (Adsorption distribution coefficient Kd (mL / g) of nitrate ion) = [adsorption amount of nitrate ion (mol / g) in 1 g of layered double hydroxide] / [amount of nitrate ion (mol / mL) in 1 mL of solution after adsorption].

2. 2. The layered double hydroxide according to claim 1, wherein x is a number that satisfies the following condition: 0.2≦x≦0.

3.

3. 3. A method for removing nitrate ions, comprising the step of contacting the layered double hydroxide according to claim 1 or 2 with an aqueous anion solution containing nitrate ions.

4. 3. A method for removing nitrate ions, comprising the step of passing an aqueous anionic solution containing nitrate ions through a column packed with the layered double hydroxide according to claim 1 or 2.

5. 3. A method for recovering nitrate ions, comprising the step of contacting a leachate with the layered double hydroxide according to claim 1 or 2 to which nitrate ions have been adsorbed, thereby recovering the nitrate ions.

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