JP6644805B2 - Anion adsorption method - Google Patents

Description

The present invention relates to a method for performing adsorption using an anion adsorbent containing iron oxyhydroxide as a main component.
This application claims priority to Japanese Patent Application No. 2015-200830 filed on October 9, 2015, the content of which is incorporated herein by reference.

In order to remove and purify substances that are harmful to the environment and the human body from various types of wastewater, or to recover useful substances such as rare metals, adsorbents, adsorption methods using them, and desorption / The collection method is being actively studied.
For example, phosphorus is an indispensable component in fertilizers and in the chemical industry, but in Japan almost 100% relies on imports. On the other hand, if a large amount of phosphorus is contained in the wastewater, it causes eutrophication, and it is not preferable for the environment to discharge such wastewater. In order to solve these problems at once, attention has been paid to the removal and recovery of phosphorus compounds such as phosphoric acid contained in wastewater.
As an adsorbent capable of efficiently adsorbing and recovering such a phosphorus compound, an adsorbent composed of iron oxyhydroxide has been developed and described in Patent Documents 1, 2, and 3 and the like. These are preferably made of iron oxyhydroxide having a β-type crystal structure. β-iron oxyhydroxide has a structure of FeO (OH) _x Cl _1-x (0 <x <1) in which a part of a hydroxyl group is substituted by chlorine, and there are many unclear points of an adsorption mechanism thereby. However, the following is known.
Patent Document 2 discloses that iron nitrate ions and the like are adsorbed by exchanging with chloride ions contained in the adsorbent by comparing the iron oxyhydroxide adsorbent with and without the chloride ion. It is suggested that Phosphate ion adsorption is possible without chloride ion, but it is also suggested that there is exchange with chloride ion as well. Further, it is described that the pH of the water to be treated is preferably set to about 3 to 9 for the adsorption of phosphate ions.
Patent Document 3 discloses that in the step of adsorbing hypophosphite or phosphite, pH 3 or more (when targeting hypophosphite) or pH 3 to 5 (when targeting phosphorous). It is described that, when hydrochloric acid is used for pH adjustment, the amount of adsorbed hypophosphite or phosphite is increased by chloride ions. However, it is also described that the mechanism of adsorption of these ions is different from that of phosphate (orthophosphate) ions.

JP 2006-124239 A WO2006 / 088083 pamphlet JP 2011-235222A

  The anion adsorbent as described above also has a high phosphate ion adsorption efficiency. However, a method for further increasing the phosphate ion adsorption efficiency has been required.

The present inventor has made intensive studies to further increase the adsorption efficiency of phosphate ions in an adsorbent made of iron oxyhydroxide.
As a result, they found that phosphate ion adsorption was promoted under the condition that the pH of the water subjected to the adsorption treatment was higher than the pH of the water before the treatment. The present invention has been completed based on this finding.

That is, the present invention relates to the following inventions.
(1) A method of adsorbing an anion by passing water containing a target anion and having a certain pH through an adsorption device filled with an anion adsorbent containing iron oxyhydroxide as a main component. The method according to claim 1, wherein the pH of the treated water at the breakthrough point is higher than the pH of the water before the treatment.
(2) The adsorption method according to (1), wherein the treatment is performed under the condition that the pH of the treated water at the breakthrough point is higher than the pH of the water before the treatment by 0.2 or more.
(3) The adsorption method according to (1) or (2), wherein the pH of the water before the treatment is adjusted to 2.5 to 5.0 with an acid.
(4) The adsorption method according to any one of (1) to (3), wherein the iron oxyhydroxide is β-iron oxyhydroxide.
(5) The adsorption method according to (4), wherein the average crystallite diameter of the iron oxyhydroxide is 10 nm or less.
(6) The adsorption method according to (4) or (5), wherein the crystal shape of the β-iron oxyhydroxide is granular.
(7) The adsorption method according to any one of (1) to (6), wherein the iron oxyhydroxide has a specific surface area of 250 m ² / g or more.
(8) The adsorption method according to any one of (1) to (7), wherein the concentration of the target anion contained in the water before the treatment is 100 mg / L or less.
(9) The adsorption method according to any one of (1) to (8), wherein the water before the treatment contains 100 mg / L or more of chloride ions.
(10) The adsorption method according to any one of (1) to (9), wherein the target anion is a phosphate ion.

  In the method of adsorbing anions using an adsorbent made of iron oxyhydroxide, adsorption of anions is promoted by setting the pH of the adsorbed water to be higher than that of the water before the treatment. You.

It is a figure which shows the TEM image of a crystal of iron oxyhydroxide. It is a figure which shows the result of the phosphoric acid adsorption test of Example 1-1 [powder B, test liquid I (pH 2.9), water flow rate SV10]. It is a figure which shows the result of the phosphoric acid adsorption test of Example 1-2 [powder B, test liquid I (pH 2.9), water flow rate SV20]. It is a figure which shows the result of the phosphoric acid adsorption test of Comparative Example 1-1 [powder B, test liquid J (pH 7.0), water flow rate SV10]. It is a figure which shows the result of the phosphoric acid adsorption test of Comparative Example 1-2 [powder B, test liquid J (pH 7.0), water flow rate SV20]. It is a figure which shows the result of the phosphoric acid adsorption test of Example 2 [powder C-3, test liquid I (pH 2.9), water flow rate SV30].

The adsorption method of the present invention is a method for adsorbing a target anion by passing water through an adsorption device filled with an anion adsorbent containing iron oxyhydroxide as a main component. The process is performed under the condition that the pH of (outflow water from the adsorption device) is higher than the pH of water (inflow into the adsorption device) before treatment at the same point.
Breakthrough is the point at which the concentration of the desired anion in the treated water begins to rise. Specifically, it may be determined according to the type of the target anion, the concentration of the ion in the water before the treatment, and the like. When the target is a phosphate ion, for example, 10 mg-P / L (phosphate Concentration).
By adopting this condition, the amount of adsorption is 1.5 to 2.0 times as large as the condition in which the pH of the treated water at the breakthrough point is equal to or lower than the pH of the water before the treatment. To improve.
The pH of the treated water is preferably 0.2 or more higher than the pH of the water before the treatment.
This adsorption treatment may be carried out under the same conditions as those actually performed, as long as the water is continued at least up to the breakthrough point so that the above pH condition is achieved. The flow may be continued until or beyond the break point, or the water flow may be stopped before the break point is reached. If the flow is stopped before reaching the breakthrough point, the pH of the treated water should be higher than the pH of the water before the treatment, preferably at or before the time of stopping the flow. .2 or higher, it can be determined that the above pH conditions have been achieved.
The adsorbed target anions can be desorbed from the anion adsorbent by performing an alkali treatment or the like after stopping the flow of water. Thereafter, the anion adsorbent can be regenerated by treating with hydrochloric acid or the like.

In the method of the present invention, the pH of the water before the treatment is preferably in an acidic range. The adsorption efficiency is low from near neutral to basic. Also, in this pH range, the pH of the treated water tends to be rather low. On the other hand, if the acidity is too strong, iron may be eluted. Specifically, the pH is preferably 2.5 to 5.0, and more preferably 2.5 to 4.0.
The water before the treatment can be used as it is as long as it is originally in the above-mentioned pH range, but otherwise the pH is preferably adjusted. It is necessary to adjust the pH of the treated water, which was originally neutral or basic, using an acid. In this case, hydrochloric acid is preferably used from the viewpoint of not adversely affecting the adsorption of the target anion.

As described above, the fact that the pH of the treated water is higher than the pH of the water before the treatment indicates that hydroxyl ions have flowed out of the adsorbent by the adsorption treatment.
As described above, exchange of chloride ions is important for adsorption of anions by iron oxyhydroxide, but exchange of hydroxyl ions was not known.
This is because water having a pH of about 5 or less was mainly used as water before being treated before, and water having a pH of 5 or less was not used. In addition, the present inventors used a material having a small particle size as an anion adsorbent containing iron oxyhydroxide as a main component, so that the property of adsorption by exchange with hydroxyl ions was particularly prominent. You might also say that.

  The concentration of the target anion contained in the water before the treatment is not necessarily limited, and depends on the amount of the adsorbent. However, from the viewpoint of preventing leakage and increasing the adsorption efficiency, the concentration is not more than 100 mg / L. Preferably, there is.

  It is preferable that the water before the treatment contains chlorine ions in addition to the target anions, since the adsorption of the target anions is promoted. The concentration of the chloride ion is not particularly limited, but is preferably 100 mg / L or more, and particularly preferably 500 mg / L or more. In order to adjust the chloride ion concentration, for example, sodium chloride may be added as a salt that contains chloride ions and does not adversely affect the adsorbent, or hydrochloric acid may be used for pH adjustment described below. .

The anion to be adsorbed by the anion adsorbent is an anion other than a chloride ion, and is a halogen ion such as a fluorine ion, a bromine ion, and an iodine ion; a carbonate ion, a silicate ion, a phosphate ion, a pyrophosphate ion; Nitrate, nitrite, sulfide, persulfate, sulfate, sulfite, hyposulfite, thiosulfate, selenate, selenite, borate, tetraborate, arsenate , Arsenite, perchlorate, chlorate, chlorite, hypochlorite, bromate, iodate, aluminate, manganate, permanganate, chromate Oxoacid ions such as chromium, dichromate, tungstate, vanadate, bismuthate and titanate Carboxylate ion, sulfonate ion, organic acid ion such as phosphonate; cyanide, cyanate ion, isocyanate ion, thiocyanate ion, azide ion, and the like.
Of these, phosphate ions and fluorine ions are preferred, and phosphate ions are particularly preferred.

Iron oxyhydroxide includes α-type, β-type, γ-type, and amorphous type depending on the crystal structure.
Among these, β-iron oxyhydroxide has vacancies that readily adsorb chloride ions, and a part of the hydroxyl groups present therein is easily replaced by chloride ions. Such chloride ions are further substituted with fluorine ions, nitrate ions, and the like, and the vacancies are considered to play a major role in the adsorption of these anions.
On the other hand, large anions such as phosphate ions do not reach the small holes that adsorb chloride ions, and it is thought that different types of holes function in the adsorption.
The iron oxyhydroxide used as the anion adsorbent in the present invention is preferably β-iron oxyhydroxide having the above properties.

Iron oxyhydroxide, which is a main component of the adsorbent used in the present invention, preferably has an average crystallite diameter of 10 nm or less, more preferably 5 nm or less.
The present inventors have clarified that the smaller the average crystallite diameter, the higher the phosphoric acid adsorption rate when used as a phosphoric acid adsorbent in water.
The average crystallite diameter D is calculated from the diffraction line near 2θ = 35 ° characteristic of β-iron oxyhydroxide by X-ray diffraction using the following Scherrer equation.
D = Kλ / βcosθ
Here, β is the full width at half maximum of the true diffraction peak corrected for the mechanical width caused by the apparatus, K is the Scherrer constant, and λ is the wavelength of the X-ray.

The anion adsorbent used in the present invention preferably has a content of iron oxyhydroxide as a main component of 99% by mass or more. Most preferably, the content of iron oxyhydroxide is substantially 100% by mass.
The iron oxyhydroxide preferably has a BET specific surface area of 250 m ² / g or more, and a pore volume area distribution (dV / dR) calculated by the BJH method of 100 to 300 mm ³ / g / nm. Is preferred.

  As described above, the β-iron oxyhydroxide used as the anion adsorbent used in the present invention is preferably one in which a part of the hydroxyl groups is replaced by chloride ions. Phosphate ions are adsorbed even in such β-iron oxyhydroxide from which chloride ions have been completely removed, but those which are partially replaced by chloride ions have a lower adsorption efficiency of phosphate ions. Are better.

  The shape of the crystal of β-iron oxyhydroxide used in the present invention is preferably granular. Here, “granular” means that the crystal is not needle-like or plate-like, and more specifically, the ratio of the major axis / minor axis of the crystal is 3 or less.

As the β-iron oxyhydroxide, for example, a dry gel obtained by a method including a step of reacting an iron compound-containing solution with a base to generate a precipitate at pH 9 or less can be used.
As the iron compound, an iron salt, particularly, a trivalent iron salt is preferable. Specific examples include ferric chloride, ferric sulfate, and ferric nitrate. Of these, ferric chloride is particularly preferred.
The base is used to neutralize the aqueous solution of the acidic iron compound and generate a precipitate containing iron oxyhydroxide. Specific examples include sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, sodium carbonate, potassium carbonate, calcium carbonate, and the like. Of these, sodium hydroxide is particularly preferred.
It is more preferable that the pH at the time of generation of the precipitate is adjusted to a range of pH 3.3 to 6. If necessary to adjust the pH, a pH adjuster may be used. Specific examples of the pH adjuster include the above bases and strong inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid.
The precipitate containing β-iron oxyhydroxide as a main component obtained by the above method can be collected by filtration, and dried to form a dried gel.

Further, after the above steps, it is preferable to carry out a step of drying the precipitate, and a step of bringing the dried substance into contact with water and then drying.
The two drying steps are preferably performed at 140 ° C. or lower, more preferably at 100 to 140 ° C. The drying temperature takes a long time at a low temperature, and is not suitable for efficient production. At high temperatures, the number of anion adsorption sites tends to decrease, and further at high temperatures, it changes to iron oxide, which is not preferable. Drying can be performed in air, in a vacuum, or in an inert gas.
In the step of bringing the dried product into contact with water, it is considered that impurities such as sodium chloride are eluted and pores are left behind, and the specific surface area increases and the number of anion adsorption sites increases.
After the dried product is brought into contact with water, the water is removed and dried again. This drying step is also preferably performed under the same conditions as described above.

  Further, although not limited, β-iron oxyhydroxide having a small particle size is preferably used. Specifically, the average particle diameter d50 is 70 μm or less. This can be obtained, for example, by classifying β-iron oxyhydroxide by a method such as sieving. If necessary, dry pulverization may be performed, and then classification may be performed by a method such as sieving.

  Next, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Measurement method (powder X-ray diffraction)
The X-ray diffraction (XRD) pattern was measured using an X-ray diffractometer Ultima IV (manufactured by Rigaku Corporation). A CuKα bulb was used for the measurement. The average crystallite diameter was calculated from XRD according to Scherrer's formula.
(Specific surface area)
The specific surface area was measured by a gas adsorption method using a specific surface area measuring device MacsorbHM 1210 (manufactured by Mountech).
(TEM observation and FFT analysis)
TEM (transmission electron microscope) observation of the sample was performed using a transmission electron microscope JEM 2010F (manufactured by JEOL, accelerating voltage 200 kV). The FFT (Fast Fourier Transform) analysis was performed using a Digital Micrograph manufactured by Gatan.
(Content of chlorine ion in iron oxyhydroxide)
After dissolving the iron oxyhydroxide sample in 3M sulfuric acid, dilute it with an alkaline solution to precipitate the iron content, collect the filtrate by filtration with a filter, and quantify it by ion chromatography (DX-500, manufactured by Nippon Dionex). did.

Production Example 1 (production of iron oxyhydroxide)
An aqueous solution of sodium hydroxide (NaOH) was added dropwise to an aqueous solution of ferric chloride (FeCl ₃ ) while adjusting the pH to 6 or less at room temperature, and the reaction was performed with the final addition amount of NaOH being NaOH / FeCl ₃ (molar ratio) = 2.75. Then, a particle suspension of iron oxyhydroxide was obtained. The average particle diameter d50 of the particles in the obtained suspension was 17 μm.
After the suspension was separated by filtration, the suspension was dried at 120 ° C. in air, washed with ion-exchanged water, and further dried at 120 ° C. in air to obtain iron oxyhydroxide powder.
The particle diameter of the oxyhydroxide powder (hereinafter referred to as “powder A”) obtained as described above was 0.25 mm to 5 mm. X-ray diffraction confirmed that the crystal structure was β-iron oxyhydroxide and the average crystallite size was 5 nm.
FIG. 1 shows a state observed by a transmission electron microscope (TEM). The crystal shape was granular. The crystallite diameter by TEM observation was 5 to 10 nm, and the individual crystals were granular, and these were coagulated to form particles.
The specific surface area was 280 m ² / g, and the chloride ion content was 5.8 wt%.

Production Example 2 (Production of iron oxyhydroxide adsorbent powder and classified products)
Powder A of iron oxyhydroxide was dry-pulverized with a pin mill to obtain powder B. The particle size range of the powder B was 0.6 to 300 µm, and the average particle size was 26.5 µm.
Powder B was classified with a sieve to obtain the following powders.
Powder C-1: Particle size 10 to 32 μm
Powder C-2: Particle size 32-45 μm
Powder C-3: Particle size of 45 to 75 μm

Reference measurement example 1 (Batch type phosphoric acid adsorption test of adsorbent particles)
Dissolve potassium dihydrogen phosphate in ion-exchanged water, adjust the pH to 3.5 with hydrochloric acid, or adjust the pH to 7.0 with sodium hydroxide, and test for a concentration of 400 mg-P / L (concentration as phosphorus). Solutions G (pH 3.5) and H (pH 7.0) were prepared.
After adding 1 g of each of the powders C-1 to C-3 to 150 mL of the test liquids G and H, the mixture was stirred and an adsorption test was performed. After a predetermined time, the liquid was collected, separated from the solid content with a filter syringe, and the phosphorus concentration in the solution was analyzed by ICP (inductively coupled plasma) to calculate the amount of adsorption. At the same time, the pH was measured. The results are shown in Table 1.

  When powders C-1 to C-3 are used as the adsorbent and those having a pH of 3.5 are used as the test liquid, it can be seen that the pH increases with the adsorption of phosphate ions. On the other hand, when a test solution having a pH of 7.0 is used, the pH drops significantly at the initial stage, and thereafter, the pH only slightly increases and does not exceed pH 7.0. In addition, when the pH of the test solution is 3.5, the amount of adsorption is smaller than when the pH is 7.0. This suggests that at pH 3.5 phosphate ions are adsorbed by exchange with hydroxide ions.

Measurement example 1 (Water type phosphoric acid adsorption test of adsorbent particles)
Dissolve potassium dihydrogen phosphate in ion-exchanged water, adjust the pH to 2.9 with hydrochloric acid, or adjust the pH to 7.0 with sodium hydroxide, and test for a concentration of 100 mg-P / L (concentration as phosphorus). Solutions I (pH 2.9) and J (pH 7.0) were prepared.
The column was filled with 20 g of powder B and 20 g of powder C-3, respectively, and test liquids I (Example) and J (Comparative Example) were passed through the column from the top of the column at a flow rate (SV) of 10 (2.6 mL / min), 20 ( Water was passed through the column at 5.3 mL / min) or 30 (7.8 mL / min), the liquid coming out from the lower part of the column was collected, separated from the solid content with a filter syringe, and the phosphorus concentration in the solution was analyzed by ICP. And the amount of adsorption was calculated. The breakthrough point was defined as the point at which the concentration of phosphorus in the liquid discharged from the lower part of the column became 10 mg-P / L. At the same time, the pH was measured. The test conditions are as follows.
Example 1-1: Powder B, Test Solution I, SV10, twice Example 1-2: Powder B, Test Solution I, SV20, Twice Comparative Example 1-1: Powder B, Test Solution J, SV10 Comparative Example 1-2: powder B, test liquid J, SV20, twice Example 2: powder C-3, test liquid I, SV30

The results are shown in FIGS. The pH of the treated water at the breakthrough was as follows:
Example 1-1: pH 3.8 (first time), pH 3.8 (second time)
Example 1-2: pH 3.3 (first time), 3.8 (second time)
Comparative Example 1-1: pH 3.2 (first time), 3.2 (second time)
Comparative Example 1-2: pH 3.0 (first time), 3.0 (second time)
Example 2: pH 3.4

  The water passage ratio (flow rate [ml] / adsorbent amount [ml]) at the breakthrough point is about 280 to 330 in Examples 1-1, 1-2 and 2, and in Comparative Examples 1-1 and 1-2. It is about 140, and it can be seen that the adsorption amount is clearly larger in the example. The pH of the treated water at the breakthrough point is 0.2 or more higher than the pH 2.9 of the water before the treatment in the examples, and it can be seen that the adsorption amount is greatly increased by this condition.

Claims (9)

In a method of adsorbing the anion by passing water containing a target anion and having a constant pH to an adsorption device filled with an anion adsorbent containing β -iron oxyhydroxide as a main component. The β-iron oxyhydroxide has an average crystallite diameter of 10 nm or less and a specific surface area of 250 m ² / g or more, and the pH of the treated water at the breakthrough point is the water before the treatment. An adsorption method, wherein the adsorption is carried out under a condition of at least 0.2 higher than the pH of the solution. The adsorption method according to claim 1, wherein the content of β-iron oxyhydroxide in the anion adsorbent is 99% by mass or more. The adsorption method according to claim 1 or 2, wherein the average particle diameter d50 of the anion adsorbent is 70 µm or less. The adsorption method according to any one of claims 1 to 3, wherein the constant pH is 2.5 to 5.0. The adsorption method according to any one of claims 1 to 4 , wherein the pH of the water before the treatment is adjusted to 2.5 to 5.0 with an acid. The adsorption method according to any one of claims 1 to 5, wherein the β-iron oxyhydroxide crystals are granular. The adsorption method according to any one of claims 1 to 6 , wherein the concentration of the target anion contained in the water before the treatment is 100 mg / L or less. The adsorption method according to any one of claims 1 to 7 , wherein the water before the treatment contains 100 mg / L or more of chloride ions. The purpose of the anions phosphate ion, fluorine ion, bromine ion, iodine ion, selenium ion, selenite ion, bromate ion, claim is one or more anions selected from iodate 1-8 The adsorption method according to any one of the above.