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

Abstract

The present invention provides a layered double hydroxide which has high selectivity for nitrate ions and is capable of recovering adsorbed nitrate ions with high efficiency. [Solution] A layered double hydroxide represented by the following formula [1], having an adsorption distribution coefficient Kd of nitrate ions of 70 or more. [Mg3(Al 1-x Zr x )(OH)8]·[A n- mH2O] [1] In the formula [1], x is a number satisfying 0.2≦x≦0.4, m is a number between 0 and 4, n is a natural number between 1 and 4, and A n- is at least one kind of n-valent ion-exchangeable anion.

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 discharged into the environment, they have serious effects on the natural environment, such as eutrophication of lakes, rivers, and oceans, and contamination of groundwater. For this reason, the 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, wastewater containing high concentrations of nitrate nitrogen, which is poor in organic matter, is generated during the manufacturing process using nitric acid to wash products, making it essential to treat the wastewater. However, the nitrate nitrogen that can be removed by the general activated sludge method is limited to low concentrations (usually less than 100 mg / L). It has been difficult to directly apply this method to industrial wastewater. The activated sludge method requires diluting high-concentration wastewater to a treatable concentration and adding organic matter as nutrients. This difficulty in wastewater treatment has been a factor that has hindered the smooth production of products in manufacturing plants. In addition, the only option for removing nitrate nitrogen other than microbial treatment is physical and chemical methods such as electrodialysis, which have high energy costs, so a more efficient wastewater treatment method has been required.

[0004] Layered double hydroxides have been considered 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 a method for desorbing adsorbed nitrate ions, using an alkaline solution, an alkaline carbonate solution, and heating to remove NO 3 and a method using an alkaline solution of 1 to 2 M. 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 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 in that the adsorption distribution coefficient Kd of nitrate ions is 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 satisfying 0.2≦x≦0.4, m is a number between 0 and 4, n is a natural number between 1 and 4, and A n- is at least one kind of 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 ion Kd (mL / g)) = [Amount of nitrate ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of nitrate ion 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+ In this layered double hydroxide, a part of the Al site of the layered double hydroxide is replaced by Zr. Specifically, in formula [1], Al is replaced by Zr in a range where x satisfies 0.2≦x≦0.4. Since Al is replaced by Zr in a range where x satisfies 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, in formula [1], the layered double hydroxide in which x satisfies 0.2≦x≦0.4 can selectively adsorb nitrate ions. In addition, by replacing Al with Zr, an amphoteric element, for Al and Mg, which have high basicity, 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, according to the above-mentioned configuration, it is possible to selectively adsorb nitrate ions, and also to recover the adsorbed nitrate ions with high efficiency.

[0011] In the past, layered double hydroxides were the only inorganic compounds known to be capable of adsorbing anions. Although layered double hydroxides can adsorb anions, it was difficult to desorb the adsorbed anions. That is, as in the above-mentioned Patent Document 1, only methods of desorption using an alkaline solution or desorption by heating were available, which were not simple. The present invention is particularly excellent in that it can easily and suitably desorb the adsorbed nitrate ions and can be recovered with high efficiency.

[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 0.2≦x≦0.3, nitrate ions can be more selectively adsorbed, and the adsorbed nitrate ions can be more efficiently recovered.

[0014] Furthermore, the present invention provides the following: <3> The above <1> or the above <2> 2. 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, since the layered double hydroxide is used, 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, since the layered double hydroxide is used, nitrate ions can be selectively adsorbed.

[0018] Furthermore, the present invention provides the following: <5> The above where nitrate ions are adsorbed <1> or the above <2> 2. A method for recovering nitrate ions, comprising the step of contacting a desorbent 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. Effect 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 description of the drawings]

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

[0022] Hereinafter, the 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 oxide including hafnium. In addition, in this specification, the expressions "contain" and "include" include the concepts of "contain", "include", "substantially consists of" 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 contents of other components. Furthermore, the maximum and minimum values ​​of various parameters (measured values, etc.) shown below are independently 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 ion measured by the following measurement method is 70 or more. [Mg 3 (Al 1-x Zr x )(OH) 8 ]·[A n- mH 2 O] [1] In the formula [1], x is a number satisfying 0.2≦x≦0.4, m is a number between 0 and 4, n is a natural number between 1 and 4, and A n- is at least one kind of 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 ion Kd (mL / g)) = [Amount of nitrate ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of nitrate ion 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 formula, first, the amount of nitrate ions adsorbed per gram of layered double hydroxide (μg / g) can be calculated using the following formula. [Amount of nitrate ion adsorbed in 1 g of layered double hydroxide (μg / g)] = [[390 (μg / mL)] - [nitrate ion concentration of solution after adsorption (μg / mL)]] x 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 1 g of layered double hydroxide (μg / g)], the numerator on the right hand side of the above equation [amount of nitrate ions adsorbed per 1 g of layered double hydroxide (mol / g)] can be obtained by converting the unit to (mol).

[0025] As described above, the layered double hydroxide according to this embodiment is represented by the following formula [1]. [Mg 3 (Al 1-x Zr x )(OH) 8 ]·[A n- mH 2 O] [1] In the formula [1], x is a number satisfying 0.2≦x≦0.4, m is a number between 0 and 4, n is a natural number between 1 and 4, and A n- is at least one kind of 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+In this layered double hydroxide, a part of the Al site of the layered double hydroxide is replaced by Zr. Specifically, in formula [1], Al is replaced by Zr in a range where x satisfies 0.2≦x≦0.4. Since Al is replaced by Zr in a range where x satisfies 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, in formula [1], the layered double hydroxide in which x satisfies 0.2≦x≦0.4 can selectively adsorb nitrate ions. In addition, by replacing Al with Zr, an amphoteric element, for Al and Mg, which have high basicity, 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 above 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- As the anion, there is no particular limitation so long as it is an n-valent ion-exchangeable anion. For example, OH - , Cl - , F - , Br - , HCO 3 ― , NO 3 - , NO 2 - , CO 3 2- , S.O. 4 2- , P.O. 4 3- , AsO 3 3- , Fe(CN) 6 4-, and tartrate ion.

[0029] The above-mentioned 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, m is not particularly limited as long as it is a number between 0 and 4 when analyzed using TG-DTA (simultaneous differential thermal-thermogravimetric analysis) under conditions of leaving the mixture 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. Since 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 ion Kd (mL / g)) = [Amount of nitrate ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of nitrate ion in 1 mL of solution after adsorption (mol / mL)].

[0031] The adsorption distribution coefficient Kd of the nitrate ion is preferably 70 or more, and 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 suitable adsorption of nitrate ions, it is preferable that the particle size is small to some extent, but if particles of 0.1 mm or less are packed in a column, pressurization may be required to pass water, for example, as in HPLC (high performance liquid chromatography). Therefore, it is preferable to prepare the particle size within 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 without pressurization.

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

[0035] [Method of producing layered double hydroxide] An example of a method for producing a layered double hydroxide of the present invention will be described below, however, 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 to 10) 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 keeping it alkaline (pH 9 to 10) with an aqueous sodium hydroxide solution or the like, and then drop an aqueous solution of zirconium oxychloride into the pure water while keeping the solution alkaline (pH 9 to 10), thereby obtaining a mixed aqueous solution. During the above mixing, bubbling with nitrogen gas or the like may be carried out in order to minimize the incorporation of carbonate ions between the layers of the layered double hydroxide.

[0038] After the step A, the mixed aqueous solution obtained is aged (step B). The aging temperature is not particularly limited, but may be within the range of 20° C. to 100° C., and may be, 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, filtration washing with distilled water, drying, etc. may be carried out as necessary. 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, although 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 is as follows: 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 the aqueous anion solution containing nitrate ions, there can be mentioned a method in which the layered double hydroxide is introduced into the aqueous anion solution.

[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 anion aqueous solution containing nitrate ions is not particularly limited as long as it contains nitrate ions. The anion aqueous solution containing nitrate ions may contain only nitrate ions, or may contain nitrate ions and other anions. Examples of the anion aqueous 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 anion aqueous solution containing nitrate ions is not particularly limited.

[0046] According to the method for producing a layered double hydroxide of the first embodiment and the method for producing a layered double hydroxide of the second embodiment, the layered double hydroxide is used, so that nitrate ions can be selectively adsorbed and 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 anion aqueous 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 range of pH 2 to 9). This is also clear from the examples.

[0048] [Method of recovering nitrate ions] An example of the method for recovering nitrate ions will be described below, although 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 are adsorbed, and examples of the desorption liquid include hydrochloric acid, sulfuric acid, etc. Examples of the desorption liquid include phosphate, acetate, buffer solutions such as Tris-HCl, NaCl, KCl, Na 2 SO 4 Neutral salts such as these may also be mixed.

[0051] As a method for recovering nitrate ions by contacting the layered double hydroxide with nitrate ions adsorbed thereon with a eluent, there may be mentioned a method for passing the eluent through a column packed with the layered double hydroxide with nitrate ions adsorbed thereon. As a result, nitrate ions are desorbed from the layered double hydroxide, and the layered double hydroxide can be reused as a nitrate ion adsorbent. In addition, the recovered eluent contains the nitrate ions desorbed from the layered double hydroxide, and therefore, the nitrate ions can be recovered with high efficiency. In particular, the layered double hydroxide has its basicity adjusted by substituting the amphoteric element Zr for Al, which has a high basicity, and therefore the adsorbed nitrate ions can be easily desorbed by the desorption liquid. EXAMPLES

[0052] The present invention will be described in detail below with reference to 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 1.3 to 2.5 mass % of hafnium as an inevitable impurity in terms of oxide relative to zirconium (calculated according to the following formula (X)). <Expression(X)> ([Mass of hafnium oxide] / ([Mass of zirconium oxide]+[Mass of hafnium oxide]))×100(%)

[0053] [Preparation of layered double hydroxide] Example 1 First, magnesium chloride (MgCl 2 ) and aluminum chloride (AlCl 3 ) and aqueous solution (molar ratio: Mg:Al = 3:0.6) (MgCl 2 Concentration: 1.5mol / L, AlCl 3 A 0.3 mol / L solution was prepared. Next, 500 mL of the prepared mixed solution was dropped into a beaker containing pure water (200 mL). At this time, 3 mol / L NaOH aqueous solution was dropped so that the pH of the solution in the beaker became 10, and a layered double hydroxide precursor was obtained. After that, it was stirred for 30 minutes. Next, a zirconium oxychloride solution (concentration: 0.2 mol / L) was added dropwise to the mixed solution in which the layered double hydroxide precursor was dispersed, so that the molar ratio was Mg:Al:Zr=3:0.6:0.4, to obtain a layered double hydroxide. During the addition, a 3 mol / L NaOH solution was appropriately added dropwise to maintain the pH at 10. During the above mixing, nitrogen gas was bubbled in order 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) Layered double hydroxides according to 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 Figs. 1 and 2. In Fig. 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 Fig. 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] Except for Comparative Example 5(h), a layered double hydroxide (LDH) peak (●) was observed in both the patterns of FIG. 1 and FIG. 2, indicating that a layered double hydroxide was produced.

[0057] Furthermore, the interlayer distances were determined from the (003) peak of the X-ray diffraction spectrum, and were 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" manufactured by HORIBA) and energy dispersive X-ray spectroscopy (EDX-7200 manufactured by Shimadzu Corporation). 2 Since the amount of O varies depending on the storage conditions (temperature and relative humidity) of the layered double hydroxide, it was analyzed using TG-DTA ("Thermo plus TG8120", Rigaku Corporation) under these conditions within 2 hours after leaving it at rest for 240 hours at a temperature of 25°C and a humidity of 40%.

[0059] From the results of the 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 3 (Al 0.8 Zr 0.2 )(OH) 8 ]·[Cl - 2.8H 2 O] Example 2: [Mg 3 (Al 0.7 Zr 0.3 )(OH) 8 ]·[Cl - 2.6H 2 O] Example 3: [Mg 3 (Al 0.6 Zr 0.4 )(OH) 8 ]·[Cl - 3.0H 2 O] Comparative example 1: [Mg 3 Al(OH) 8 ]·[Cl - 2.5H 2 O] Comparative example 2: [Mg 3 (Al 0.9 Zr 0.1 )(OH) 8 ]·[Cl - 2.4H 2 O] Comparative example 3: [Mg 3 (Al 0.5 Zr 0.5 )(OH) 8 ]·[Cl - 2.9H 2 O] Comparative example 4: [Mg 3 (Al 0.3 Zr 0.7 )(OH) 8 ]·[Cl - 2.6H 2 O] Comparative example 5: [Mg 3 Zr(OH) 8 ]·[Cl - 1.7H 2 O]

[0060] [Measurement of distribution coefficient] 1 g of the layered double hydroxide of each of the examples and comparative examples was added to 30 ml of a mixed aqueous solution containing 390 ppm of nitrate ion, 2300 ppm of chloride ion, and 5800 ppm of sulfate ion, and stirred at 27°C for 1 hour. Next, the nitrate ion concentration in the supernatant was analyzed using an ion chromatograph ("HIC-SP" manufactured by Shimadzu Corporation). Thereafter, the distribution coefficient Kd was calculated. The distribution coefficient Kd was calculated as follows. (Adsorption distribution coefficient of nitrate ion Kd (mL / g)) = [Amount of nitrate ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of nitrate ion in 1 mL of solution after adsorption (mol / mL)]. (Adsorption distribution coefficient Kd (mL / g) of sulfate ion) = [Amount of sulfate ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of sulfate ion in 1 mL of solution after adsorption (mol / mL)]. (Adsorption distribution coefficient of chloride ion Kd (mL / g)) = [Amount of chloride ion adsorbed in 1 g of layered double hydroxide (mol / g)] / [Amount of chloride ion in 1 mL of solution after adsorption (mol / mL)]. <Ion chromatograph equipment and conditions> Manufacturer: Shimadzu Corporation Equipment name: HIC-SP SPD-40V SPD-20A (Absorbance detector) is used to detect nitrate ions CTO-40C Column Oven CDD-10 Electrical conductivity is used for other than nitrate ion Column: IC-SA2, 250L x 4.0 (column for anion analysis) Eluent (NaHCO 3 12mmol / L, Na 2 CO 3 0.6mmol / L) Flow rate: 1mL / min

[0061] [Table 1]

[0062] [Nitrate ion adsorption rate in a batch system using a nitrate ion aqueous solution] 1 g of the layered double hydroxide of each of the examples and comparative examples was added to 30 mL of an aqueous solution with a nitrate ion concentration of 500 ppm, and stirred at 27°C for 1 hour. The aqueous solution was prepared using sodium nitrate. Next, solid-liquid separation was performed by suction filtration, and the nitrate ion concentration in the obtained solution (filtrate) was analyzed by ion chromatography. Thereafter, the nitrate ion adsorption rate was calculated by 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] 1 g of the layered double hydroxide of the Example and Comparative Example was added to 30 ml of a mixed aqueous solution (simulated wastewater) containing 500 ppm of nitrate ion, 2500 ppm of chloride ion, and 6000 ppm of sulfate ion, and stirred at 27°C for 1 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 obtained solution (in the filtrate) was analyzed by ion chromatography. Then, the adsorption rate of each anion was calculated by 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))]] x 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] 10 g of the layered double hydroxides of the Examples and Comparative Examples, 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 an aqueous solution prepared 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. The entire 1.0 L of the aqueous solution after passing through the column 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 the column test could not be carried out for Comparative Example 1, since it was pulverized. (Nitrate ion adsorption rate (%)) = [1 - [(nitrate ion concentration in the filtrate) / (nitrate ion concentration in the aqueous solution)]] x 100.

[0067] [Table 4]

[0068] [Nitrate ion desorption rate in a column adsorption test (continuous type) using a nitrate ion aqueous solution] A mixed aqueous solution of 0.005 mol / L HCl and 13 mass% NaCl was passed through the column (column after passing 1.0 L of aqueous solution through the column used in the above "Nitrate ion adsorption rate in column adsorption test (continuous type) when using nitrate ion aqueous solution)" at a flow rate of 1.5 mL / min as a desorption liquid. The entire 200 mL of aqueous solution after passing through the column was collected and filtered with a syringe filter, and then the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion desorption rate was then calculated by the following formula. The results are shown in Table 5. In addition, the column test could not be performed for Comparative Example 1 because it was powdered. (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] 10 g of the layered double hydroxides of the Examples and Comparative Examples, 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 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 the aqueous solution was collected and filtered with a syringe filter, and the nitrate ion concentration was analyzed by ion chromatography. The adsorption rate of nitrate ions was then calculated using the following formula. The results are shown in Table 6. Note that the column test could not be carried out for Comparative Example 1, since it was pulverized. (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 mass% NaCl was passed through the column (column after passing 1.0 L of simulated wastewater and adsorbing various anions to the layered double hydroxide) used in the above "Nitrate ion adsorption rate in column adsorption test (continuous type) when using simulated wastewater" at a flow rate of 1.5 mL / min as a desorption liquid. The entire 200 mL of the aqueous solution after passing through the column was collected and filtered with a syringe filter, and then the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion desorption rate was then calculated by the following formula. The results are shown in Table 6. In addition, the column test could not be performed for Comparative Example 1 because it was powdered. (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 an aqueous solution was prepared, in which the nitrate ion concentration was 500 ppm and the pH was 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 obtained solution (filtrate) was analyzed by ion chromatography. Thereafter, the nitrate ion adsorption rate was calculated by 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 did not vary significantly in nitrate ion adsorption rate even when the pH varied within the range of 2 to 10, and showed a high value.

[0077] [Nitrate ion adsorption rate in a column adsorption test (continuous type) using river water] 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 water samples collected from two water sampling points in the Tama River basin (Tama River Green Park (43-43-1 Someji 2-chome, Chofu City, Tokyo) and the remains of Yaguchi Ferry (near 17-3 Yaguchi 3-chome, Ota Ward, Tokyo)) were passed through the column at a flow rate of 7.0 mL / min. The entire amount was collected and filtered with a syringe filter, after which the nitrate ion concentration was 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 in Tamagawa Green Park was 12.8 ppm, and the nitrate ion concentration at the remains 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. The entire 1.0 L of aqueous solution after passing through the column was collected and filtered with a syringe filter, and then the nitrate ion concentration was analyzed by ion chromatography. The adsorption rate of nitrate ions 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 Then, 200 mL of a mixed aqueous solution of 0.005 mol / L HCl and 13 mass% NaCl was passed through the column at a flow rate of 1.5 mL / min. The entire 200 mL of aqueous solution after passing through the column was collected and filtered with a syringe filter, and the nitrate ion concentration was analyzed by ion chromatography. The nitrate ion release 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. Represented by the following formula [1]: A layered double hydroxide having an adsorption distribution coefficient Kd of nitrate ions of 70 or more and 80.93 or less, 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 satisfying 0.2≦x≦0.4, m is a number from 0 to 4, n is a natural number from 1 to 4, and A ｎ－ is at least one kind of 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 by 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 in 1 g of layered double hydroxide (mol / g)] / [amount of nitrate ion in 1 mL of solution after adsorption (mol / mL)].

2. 2. The layered double hydroxide according to claim 1, wherein the x is a number that satisfies 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 anion 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 release liquid with the layered double hydroxide according to claim 1 or 2 having nitrate ions adsorbed thereon, thereby recovering the nitrate ions.

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