JP7541445B2 - Water purification method and water purification device - Google Patents

Description

This invention relates to a water purification method and apparatus for removing contaminants from polluted water.

Layered double hydroxides have been used to treat contaminated water containing contaminants such as boron, arsenic, and phosphorus. Here, granular layered double hydroxides have been used because of the ease with which contaminated water can pass through them (for example, Patent Document 1).

WO 2008/59618

However, producing granular layered double hydroxide requires drying after synthesis, which is costly and time-consuming.

The present invention aims to provide a water purification method and apparatus that uses a slurry or gel adsorbent made by synthesizing layered double hydroxides.

The water purification method described in claim 1 is a water purification method in which contaminants in contaminated water are removed by adsorbing them onto a layered double hydroxide, comprising the steps of: placing a slurry or gel adsorbent containing the layered double hydroxide (excluding one that has been dried after synthesis) in a container; passing the contaminated water through the container; separating the layered double hydroxide from the water using a filter provided at the end of a discharge pipe on the container side and sand provided around the filter ; preventing the layered double hydroxide from being discharged outside the container; and discharging the water outside the container through the discharge pipe.

In this case, the adsorbent may contain paper fibers.

The filter has a mesh size smaller than the particle size of the layered double hydroxide.

The adsorbent may also be one from which salts produced during the synthesis of the layered double hydroxide have been removed.

The water purification device described in claim 6 is a water purification device that removes contaminants from contaminated water by adsorbing them onto a layered double hydroxide, and comprises a container to which the contaminated water is supplied, a slurry or gel-like adsorbent stored in the container and containing the layered double hydroxide (excluding layered double hydroxide that has been dried after synthesis), a filter provided at the container side end of a discharge pipe that separates the layered double hydroxide from the water, prevents the layered double hydroxide from being discharged outside the container, and discharges the water outside the container, and sand provided around the filter .

According to the present invention, a slurry or gel adsorbent made by synthesizing layered double hydroxides can be used, so pollutants can be removed from polluted water without incurring costs or time.

1 is a cross-sectional view showing a water purification device of the present embodiment. FIG. 4 is a cross-sectional view showing another water purification device of the present embodiment.

The water purification method of the present invention will be described. This embodiment is a water purification method in which contaminants in contaminated water are removed by adsorbing them onto layered double hydroxides. A slurry or gel adsorbent containing layered double hydroxide (excluding those dried after synthesis) is placed upstream of a flow path having a separation section capable of separating the layered double hydroxide from water, and contaminated water is passed through the flow path. The contaminants refer to substances that can be adsorbed by layered double hydroxides, such as boron, arsenic, and phosphorus.

1 shows a schematic cross-sectional view of the water purification device 100 of this embodiment. As shown in FIG. 1, the water purification device 100 includes a container 10 as a flow path forming member, a water supply pipe 12 provided in the container 10, and a drain pipe 14. A first filter 4 made of a filter cloth or the like is provided at the end of the drain pipe 14 located inside the container 10. A second filter 31 is provided around the filter cloth. A slurry or gel-like adsorbent 2 containing layered double hydroxide is provided around the second filter 31. Here, the mesh (opening) of the second filter 31 is smaller than the particle size of the layered double hydroxide contained in the adsorbent 2 and larger than the mesh (opening) of the first filter 4. The first filter 4 and the second filter 31 constitute a separation section 16 that separates the layered double hydroxide from water. The presence of this separation section 16 makes it possible to prevent the adsorbent 2 from being discharged to the outside of the container 10.

The contaminated water 1 is supplied into the container 10 from the water supply pipe 12, and after the contaminants are removed by the adsorbent 2, the contaminated water 1 is separated from the adsorbent 2 by the separation section 16 (the second filter 31 and the first filter 4) and discharged to the outside of the container 10 through the drain pipe 14. That is, it can be said that a flow path is formed inside the container 10 in which the adsorbent 2 and the separation section 16 (the second filter 31 and the first filter 4) are arranged in this order from the upstream side. If the first filter 4 is a filter cloth, it may be wrapped around the drain pipe 14 or may be provided inside the drain pipe 14. The filter cloth may also be made into a bag shape and inserted into the drain pipe 14.
The second filter 31 may be made of a resin such as urethane, paper, or nonwoven fabric.

Here, the layered double hydroxide is a non-stoichiometric compound represented by the general formula M2 ⁺ _1- ^xM3 + _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} (where M2 ⁺ is a divalent metal, M3 ⁺ is a trivalent metal, ^An- is an n-valent anion, 0<x<1, m>0), and is sometimes called a hydrotalcite-like compound. Examples of the divalent metal ion (M2 ⁺ ) include Mg2 ⁺ , Fe2 ⁺ , Zn2 ⁺ , Ca2 ⁺ , Li2 ^{+, Ni2+ ,} Co2 ⁺ , and Cu2 ⁺ . Examples of the trivalent metal ion (M3 ⁺ ) include Al3 ⁺ , Fe3 ⁺ , Cr3 ⁺ , and Mn3 ⁺ . Examples of anions (A ^n- ) include ClO ₄ ^- , CO ₃ ^2- , HCO ₃ ^- , PO ₄ ^3- , SO ₄ ^2- , SiO ₄ ^4- , OH ^- , Cl ^- , NO _2- ^, NO _3- ^, etc. The divalent metal ion (M ²⁺ ) and trivalent metal ion (M ³⁺ ) contained in the general formula do not have to be of one type, and may contain multiple types.

The layered double hydroxide according to this embodiment may use any divalent metal ion (M ²⁺ ), any trivalent metal ion (M ³⁺ ), or any anion (A ⁿ⁻ ). For example, the metal ion may be ^{Mg2 +} _1- ^xAl3+ _x (OH) ₂ (A ^n- ) _{x/ n.mH2O} (Mg ^- Al type) where the divalent metal ion (M2 ⁺ ) is Mg2 ⁺ and the trivalent metal ion (M3 ⁺ ) is ^Al3+ , Mg2 ⁺ _1- xFe3 ⁺ _x (OH)2(A n-) _{x/n.mH2O (Mg-Fe type) where the divalent metal ion (M2+) is Mg2+ and the trivalent metal ion (M3+) is Fe3+, or Fe2+1-xFe3+x(OH)2(A n-)x/} ^{n.mH2O ( Fe} _- ^Fe _{type )} ^where _{the divalent} metal ion ( ^{M2 +} ) is _Fe2 ⁺ and the trivalent metal ion (M3 ⁺ ) is _Fe3 ⁺ . The Mg-Fe type is superior to the Mg-Al type in that it is more effective at adsorbing arsenic, has a higher specific gravity and is therefore easier to separate by settling, and can reduce raw material costs.

The layered double hydroxide according to this embodiment preferably has a crystallite size of 20 nm or less, more preferably 10 nm or less. It is also preferable that the average crystallite size is 10 nm or less.

The specific surface area of the layered double hydroxide according to this embodiment is not particularly limited, but a larger specific surface area is preferable in terms of improving the adsorption performance. The layered double hydroxide may have a specific surface area of 20 m ² /g or more, preferably 30 m ² /g or more, more preferably 50 m ² /g or more, and even more preferably 70 m ² /g or more, as measured by the BET method. The upper limit of the specific surface area is not particularly limited. The specific surface area as measured by the BET method can be determined, for example, by measuring the nitrogen adsorption/desorption isotherm using a specific surface area/pore distribution measuring device and creating a BET-plot from the measurement results. For example, if the crystallite size of the layered double hydroxide is 20 nm or less, the specific surface area can be 20 m ² /g or more.

The layered double hydroxide is synthesized by mixing an acidic solution containing divalent metal ions and trivalent metal ions with an alkaline solution. The smaller the crystallite size of the layered double hydroxide synthesized here, the larger its specific surface area can be. Therefore, the shorter the maturation time after synthesis, the better, and it is better to neutralize the solution within at least 120 minutes, preferably within 60 minutes, after mixing the acidic solution and the alkaline solution, and more preferably, immediately after mixing. In order to ensure that maturation does not occur, it is also good to wash the layered double hydroxide promptly after mixing the acidic solution and the alkaline solution is completed.

The adsorbent containing layered double hydroxide may contain salts such as NaCl that are generated during the synthesis process, but when used for purifying drinking water, the salts may be removed by washing with water.

Next, as an example, a method for synthesizing a layered double hydroxide represented by the structural formula Mg ²⁺ _1-x Al ³⁺ _x (OH) ₂ (A ^n- ) _x/n ·mH ₂ O will be described.

First, prepare an acidic solution containing aluminum ions and magnesium ions.

The aluminum source for the aluminum ions is not limited to any particular substance, as long as it generates aluminum ions in water. For example, alumina, sodium aluminate, aluminum hydroxide, aluminum chloride, aluminum nitrate, bauxite, alumina production residue from bauxite, aluminum sludge, etc. can be used. Furthermore, any of these aluminum sources may be used alone, or two or more types may be used in combination.

The magnesium source of magnesium ions is not limited to any specific substance, as long as it generates magnesium ions in water. For example, brucite, magnesium hydroxide, magnesite, baked magnesite, etc. can be used. These magnesium sources can be used alone or in combination of two or more types.

The aluminum compound as the aluminum source and the magnesium compound as the magnesium source do not need to be completely dissolved as long as aluminum ions and magnesium ions are present in the acidic solution.

It is also known that a highly crystalline layered double hydroxide represented by Mg2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/n.mH2O has} a molar ratio of aluminum ions to magnesium ions of 1:3 (x=0.25). Therefore, the molar ratio of aluminum ions to magnesium ions in the acidic solution is preferably in the range of 1:5 to 1:2. By setting the ratio in this range, the layered double hydroxide can be produced advantageously in terms of material balance without wasting the aluminum source and the magnesium source.

The acid contained in the acidic solution is not particularly limited as long as it makes the aqueous solution acidic, but for example, nitric acid or hydrochloric acid can be used.

An alkaline solution is also prepared. The alkali contained in the alkaline solution is not particularly limited as long as it makes the aqueous solution alkaline, but for example, sodium hydroxide, calcium hydroxide, etc. can be used. Sodium carbonate, potassium carbonate, ammonium carbonate, aqueous ammonia, sodium borate, potassium borate, etc. can also be used. Any of these may be used alone, or two or more types may be used in combination. The alkaline solution may be adjusted to a pH of 8 to 14, and preferably to a pH of 8 to 11.

Next, an acidic solution containing aluminum ions and magnesium ions is mixed with an alkaline solution containing an alkali in a predetermined ratio. This produces a layered double hydroxide. The mixing can be done by adding the acidic solution to the alkaline solution all at once or by dropping the acidic solution into the alkaline solution, but other methods are also possible.

The shorter the maturation time after the completion of mixing the acidic and alkaline solutions, the more the crystal growth can be suppressed, and layered double hydroxides with small crystallite sizes and large specific surface areas can be produced.

As a method for stopping the aging, after the mixing of the acidic solution and the alkaline solution is completed, the pH of the mixed solution may be lowered to a value at which the crystal growth of the layered double hydroxide stops. For example, in the case of a layered double hydroxide represented by the general formula Mg ²⁺ _1-x Al ³⁺ _x (OH) ₂ (A ^n- ) _x/n ·mH ₂ O, the pH may be adjusted to 9 or less. Specifically, the aging can be stopped by diluting with water within 120 minutes, preferably within 60 minutes, and more preferably simultaneously with the mixing after the mixing of the acidic solution and the alkaline solution is completed. In order to ensure that the aging does not occur, it is also good to wash the layered double hydroxide promptly after the mixing of the acidic solution and the alkaline solution is completed.

The adsorbent containing layered double hydroxide may contain salts such as NaCl that are generated during the synthesis process, but when used for purifying drinking water, the salts may be removed by washing with water.

In the above explanation, the case where aluminum ions and magnesium ions are contained in the acidic solution side has been described, but this is not limited to this. It is also possible to contain aluminum ions in the acidic solution side and magnesium ions in the alkaline solution side, to contain magnesium ions in the acidic solution side and aluminum ions in the alkaline solution side, or to contain both aluminum ions and magnesium ions in the alkaline solution side.

As another example, a method for synthesizing a layered double hydroxide having a structural formula of Mg ²⁺ _1-x Fe ³⁺ _x (OH) ₂ (A ^n- ) _x/n ·mH ₂ O will be described.

First, prepare an acidic solution containing iron ions and magnesium ions.

The iron source of iron ions is not limited to a specific substance, as long as it generates iron ions in water. For example, iron chloride can be used. These iron sources can be used alone or in combination of two or more.

The magnesium source of magnesium ions is not limited to any specific substance, as long as it generates magnesium ions in water. For example, brucite, magnesium hydroxide, magnesite, baked magnesite, etc. can be used. These magnesium sources can be used alone or in combination of two or more types.

The iron compound as the iron source and the magnesium compound as the magnesium source do not need to be completely dissolved as long as iron ions and magnesium ions are present in the acidic solution.

It is also known that a highly crystalline layered double hydroxide represented by Mg2 ⁺ _1- ^xFe3+ _x (OH) ₂ ( ^An- ) _{x/n.mH2O has} a molar ratio of iron ions to magnesium ions of 1:3 (x=0.25). Therefore, the molar ratio of iron ions to magnesium ions in the acidic solution is preferably in the range of 1:5 to 1:2. By setting the molar ratio in this range, the layered double hydroxide can be produced in an advantageous manner in terms of material balance without wasting the iron source and magnesium source.

The acid contained in the acidic solution is not particularly limited as long as it makes the aqueous solution acidic, but for example, nitric acid or hydrochloric acid can be used.

An alkaline solution is also prepared. The alkali contained in the alkaline solution is not particularly limited as long as it makes the aqueous solution alkaline, but for example, sodium hydroxide, calcium hydroxide, etc. can be used. Sodium carbonate, potassium carbonate, ammonium carbonate, aqueous ammonia, sodium borate, potassium borate, etc. can also be used. Any of these may be used alone, or two or more types may be used in combination. The alkaline solution may be adjusted to a pH of 8 to 14, and preferably to a pH of 8 to 11.

Next, the acidic solution containing iron ions and magnesium ions is mixed with an alkaline solution containing an alkali in a predetermined ratio. This produces a layered double hydroxide. The mixing can be performed by adding the acidic solution to the alkaline solution all at once or by dropping the acidic solution into the alkaline solution, but other methods are also acceptable.

The shorter the maturation time after the completion of mixing the acidic and alkaline solutions, the more the crystal growth can be suppressed, and layered double hydroxides with small crystallite sizes and large specific surface areas can be produced.

As a method for stopping the aging, after the mixing of the acidic solution and the alkaline solution is completed, the pH of the mixed solution may be lowered to a value at which the crystal growth of the layered double hydroxide stops. For example, in the case of a layered double hydroxide represented by the general formula Mg ²⁺ _1-x Fe ³⁺ _x (OH) ₂ (A ^n- ) _x/n ·mH ₂ O, the pH may be adjusted to 9 or less. Specifically, the aging can be stopped by diluting with water within 120 minutes, preferably within 60 minutes, and more preferably simultaneously with the mixing after the mixing of the acidic solution and the alkaline solution is completed. In order to reliably prevent aging, it is also good to wash the layered double hydroxide promptly after the mixing of the acidic solution and the alkaline solution is completed.

The adsorbent containing layered double hydroxide may contain salts such as NaCl that are generated during the synthesis process, but when used for purifying drinking water, the salts may be removed by washing with water.

In the above explanation, the case where the acidic solution contains iron ions and magnesium ions has been described, but this is not limited to the above. It is also possible to have the acidic solution contain iron ions and the alkaline solution contain magnesium ions, or to have the acidic solution contain magnesium ions and the alkaline solution contain iron ions, or to have the alkaline solution contain both iron ions and magnesium ions.

It has been found that the layered double hydroxides synthesized in this way have a higher ability to adsorb pollutants from polluted water when used in a slurry or gel form without drying, rather than when used after drying. Therefore, it is preferable that the adsorbent 2 containing the layered double hydroxide is in a slurry or gel form.

The adsorbent 2 may further contain paper fibers. The higher the content of paper fibers, the more the water permeability of the adsorbent can be improved. Note that the adsorbent 2 containing paper fibers may be prepared by synthesizing a layered double hydroxide and then mixing it with paper fibers, or may be prepared by synthesizing a layered double hydroxide in the presence of paper fibers.

Pressure may also be applied to the contaminated water so that the water can pass through the separation section. In this case, any pressure may be applied to the contaminated water as long as the water can pass through the separation section, but a pressure that ensures the water flow time required for the adsorbent to fully adsorb the contaminants is desirable. For example, the pressure may be adjusted to a range of 0.1 MPa to 1 MPa. The pressure can also be reduced by adding paper fibers to the adsorbent 2.

As described above, in this embodiment, the contaminated water can be purified by placing the adsorbent 2 in a slurry or gel state containing layered double hydroxide in the container 10 and passing the contaminated water 1 through the adsorbent 2. In addition, the action of the separation section 16 allows the purified contaminated water to be discharged outside the container 10 without discharging the adsorbent 2 outside the container 10. This makes it possible to easily obtain purified water, unlike a method in which, for example, a slurry or gel adsorbent and contaminated water are placed in a container and stirred, and the adsorbent and contaminated water in the container are filtered with a filter after stirring to obtain purified water.

The separation section 16 is for separating the layered double hydroxide from the water, and may be any material that can hold the adsorbent 2 (layered double hydroxide) on the upstream side of the flow path and allow water to pass through. For example, the separation section 16 may be made of a sponge, nonwoven fabric, paper, or the like. Also, as in the water purification device 200 shown in FIG. 2, sand 32 may be used instead of the second filter 31 in FIG. 1. As the sand 32, any material that can separate the layered double hydroxide from the water may be used, and for example, sand having a grain size of JIS standard sand No. 2, No. 3, or No. 4 may be used. Also, the grain size of the sand 32 does not have to be uniform throughout. For example, the grain size may be finer closer to the first filter 4, and the grain size may be coarser farther from the first filter 4. When using sand 32, for example, if the volume of the container 10 is 10 and the volume of the layer of the adsorbent 2 is about 3 to 5, it is preferable that the volume of the layer of the sand 32 is about 1 to 5. It should be noted that granular filtering materials such as anthracite may be used as the granular material other than the sand 32. The density of the saturated sand 32 is about 2 ton/ ^m3 , and the density of water applied to the contaminated water is 1 ton/ ^m3 . Since the density of the slurry or gel adsorbent 2 containing layered double hydroxide is about 1.07 to 1.10 ton/ ^m3 , the adsorbent 2 can be provided between the contaminated water 1 and the sand 32.

1 and 2, a case is described in which a flow path for contaminated water is formed inside the container 10, but this is not limited to the above. A flow path means a route through which water flows, so it may have any shape as long as there is a water flow. For example, the flow path may be formed inside a cylindrical member.

Next, the boron adsorption performance was confirmed using various adsorbents.

[Example 1]
First, magnesium chloride hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) and aluminum chloride hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) were dissolved in distilled water to prepare an acidic solution. Sodium hydroxide (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in distilled water to prepare an alkaline solution. Next, the acidic solution and the alkaline solution were mixed, and a sufficient amount of distilled water was quickly mixed into the mixed solution without any time delay (within 5 minutes) to prepare a slurry-like adsorbent 1 containing layered double hydroxide. The density of the adsorbent 1 was 1.06 mg/ml, and the Cl concentration was 175.562 ppm.
Next, 23.7 ml of a solution with a boron concentration of 97.789 mg/l was added to 3 g of Adsorbent 1 containing 0.237 g of layered double hydroxide. The results are shown in Table 1.

[Example 2]
3 g of the adsorbent 1 was dried to prepare adsorbent 2 consisting of 0.237 g of granular layered double hydroxide.

Next, 23.7 ml of a solution with a boron concentration of 97.789 mg/l was added to 0.237 g of adsorbent 2. The results are shown in Table 1.

[Example 3]
Adsorbent 1 and paper fiber were mixed to prepare 50 g of a slurry-like adsorbent 3 containing 1 g of layered double hydroxide. Next, 100 ml of a solution with a boron concentration of 100.428 mg/l was added to 50 g of adsorbent 3. The results are shown in Table 1.

[Example 4]
Adsorbent 1 was dehydrated to prepare 4.321 g of gel-like adsorbent 4 containing 1 g of layered double hydroxide. The Cl content of adsorbent 4 was 1.585 mg/g.
Next, 100 ml of a solution with a boron concentration of 103.837 mg/l was added to 4.321 g of the adsorbent 4. The results are shown in Table 1.

[Example 5]
4.321 g of adsorbent 4 and 1 g of paper fiber were mixed to prepare 5.321 g of adsorbent 5.
Next, 100 ml of a solution with a boron concentration of 103.837 mg/l was added to 5.321 g of the adsorbent 5. The results are shown in Table 1.

[Example 6]
4.321 g of the adsorbent 4 was dried to prepare 1 g of granular adsorbent 6.
Next, 100 ml of a solution with a boron concentration of 103.837 mg/l was added to 1 g of the adsorbent 6. The results are shown in Table 1.

[Example 7]
Adsorbent 1 and paper fiber were mixed to prepare 11.369 g of adsorbent 7 in a slurry form containing 2 g of paper fiber and 1 g of layered double hydroxide.
Next, 100 ml of a solution with a boron concentration of 99.214 mg/l was added to 11.369 g of the adsorbent 7. The results are shown in Table 1.

Comparing Example 1 with Example 2, and Example 4 with Example 6, it is clear that the amount of boron adsorbed is greater when an adsorbent made of layered double hydroxide that is not dried and remains in a slurry or gel state is used than when an adsorbent made of dried layered double hydroxide is used.

Reference Signs List 1 contaminated water 2 adsorbent 4 first filter 10 container 12 water supply pipe 14 drain pipe 16 separation section 31 second filter 32 sand

Claims (7)

A water purification method for removing contaminants from contaminated water by adsorbing them onto a layered double hydroxide, comprising the steps of:
A slurry or gel adsorbent containing the layered double hydroxide (excluding one that has been dried after synthesis) is placed in a container;
A water purification method characterized by passing the contaminated water through the container, separating the layered double hydroxide from the water using a filter provided at the container side end of a discharge pipe and sand provided around the filter , preventing the layered double hydroxide from being discharged outside the container, and discharging the water outside the container through the discharge pipe. The water purification method according to claim 1, wherein the adsorbent contains paper fibers. 3. The water purification method according to claim 1, wherein the filter has mesh sizes smaller than the particle size of the layered double hydroxide. The method for purifying water according to any one of claims 1 to 3 , further comprising applying pressure to the contaminated water so as to allow the water to pass through the filter . 5. The water purification method according to claim 1 , wherein the adsorbent is obtained by removing salt generated during synthesis of the layered double hydroxide. A water purification device that removes pollutants from polluted water by adsorbing them onto a layered double hydroxide,
a container into which the contaminated water is supplied;
a slurry or gel adsorbent that is contained in the container and contains the layered double hydroxide (excluding one that has been dried after synthesis);
a filter provided at an end of a discharge pipe on the container side, which separates the layered double hydroxide from water to prevent the layered double hydroxide from being discharged outside the container and discharges the water outside the container, and sand provided around the filter . The water purification device according to claim 6 , wherein the filter has mesh sizes smaller than the particle size of the layered double hydroxide.

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