Adsorbent, method for producing adsorbent, and method for immobilizing phosphorus or heavy metal using adsorbent

Abstract

To provide a technique for facilitating the treatment of an adsorbent after the adsorption of phosphorus or heavy metals while developing excellent phosphorus adsorbing capacity.SOLUTION: Provided are an adsorbent containing a polyurethane and a powder pH-adjusted iron precipitate having a powder pH of 3 or more, in which the powder pH-adjusted iron precipitate is contained in the adsorbent at a mass ratio of 65% or more, and related techniques thereof.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an adsorbent, a method for producing the adsorbent, and a method for immobilizing phosphorus or heavy metals using the adsorbent. [Background technology]

[0002] Patent Document 1 discloses a method for immobilizing heavy metals in soil and the like, capable of sufficiently immobilizing and insolubilizing heavy metals, and a method for inhibiting the transfer of heavy metals to plants, using FeO(OH). 8-2x (SO4) x It is described that heavy metals are immobilized using neutralized schwertmannite obtained by reacting calcium carbonate with schwertmannite having the structure (where x is 1≦x≦1.75). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-47397 Summary of the Invention [Problem to be solved by the invention]

[0004] In addition to heavy metals, environmental pollution by phosphates is also a problem that cannot be ignored. In this specification, the term "phosphorus or heavy metals" is used to include heavy metals (especially arsenic) and phosphorus.

[0005] The present inventors have investigated the possibility of adsorbing phosphate ions using the neutralized schwertmannite described in Patent Document 1. However, the neutralized schwertmannite described in Patent Document 1 has fine particles, which makes it difficult to filter. As a result, they have found that it is not practical to mix the neutralized schwertmannite with wastewater and perform adsorption filtration.

[0006] The present invention provides a technique for facilitating treatment of an adsorbent after adsorption of phosphorus or heavy metals while exhibiting excellent phosphorus or heavy metal adsorption capacity. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems.

[0008] As a result, we came up with a method of solidifying a precipitate (hereinafter referred to as "powder pH-adjusted iron precipitate") with polyurethane, which is similar to the neutralized schwertmannite described in Patent Document 1. The precipitate is obtained by adding alkali to a precipitate containing iron hydroxide, iron oxide, or the like as its main components (hereinafter referred to as "iron precipitate"), which is obtained by oxidizing divalent Fe ions contained in an acidic aqueous solution using iron-oxidizing bacteria, and then adjusting the powder pH (hereinafter referred to as "powder pH-adjusted iron precipitate"). This method improves filterability even when the adsorbent is in powder form after adsorption of phosphorus or heavy metals. When the adsorbent is not in powder form, filterability is even better, or the adsorbent can be recovered without filtration at all. As a result, post-adsorption processing is facilitated.

[0009] Regarding the phosphorus adsorption capacity, as will be shown in the Examples section below, when the powdered pH-adjusted iron precipitate in the adsorbent exceeds 60% by mass, the increase in the adsorption rate of phosphate ions accelerates.

[0010] Based on the above findings, the following aspects have been created.

[0011] The first invention according to this embodiment is: Polyurethane and a powder pH-adjusted iron precipitate having a powder pH of 3 or more; An adsorbent comprising: The adsorbent contains the powdered pH-adjusted iron precipitate in an amount of 65% or more by mass.

[0012] The second invention according to this embodiment is as follows: In the adsorbent according to the first aspect of the present invention, the powdered pH-adjusted iron precipitate is made of particles having a median particle size of 100 μm or less.

[0013] The third invention according to this embodiment is: The adsorbent according to the first or second aspect of the present invention comprises the powdered pH-adjusted iron precipitate in an amount of 95% or less by mass.

[0014] The fourth invention according to this embodiment is as follows: a step of oxidizing divalent Fe ions contained in an acidic aqueous solution with iron-oxidizing bacteria, and adding an alkali to the resulting iron precipitate to adjust the powder pH to 3 or more to obtain a powder pH-adjusted iron precipitate; mixing the powdered pH-adjusted iron precipitate, polyurethane, and a solvent to obtain a mixture; molding the mixture; and In the method for producing an adsorbent, the mixture contains 65% or more by mass of the powdered pH-adjusted iron precipitate before the step of molding the mixture.

[0015] The fifth invention according to this embodiment is: A fourth aspect of the present invention is a method for producing an adsorbent, wherein the alkali includes at least one of an alkali metal hydroxide, an alkali metal carbonate, an alkaline earth metal hydroxide, and an alkaline earth metal carbonate.

[0016] The sixth invention according to this embodiment is: The method for producing an adsorbent according to the fourth or fifth aspect of the present invention, wherein the alkali is sodium hydroxide.

[0017] The seventh invention according to this embodiment is: A method for immobilizing phosphorus or heavy metals, comprising immobilizing phosphorus or heavy metals using the adsorbent according to any one of the first to third inventions or the adsorbent obtained by the method for producing an adsorbent according to any one of the fourth to sixth inventions. [Effects of the Invention]

[0018] According to the present invention, a technique can be provided that allows for easy disposal of an adsorbent after adsorption of phosphorus or heavy metals while still exhibiting excellent phosphorus adsorption capacity. DETAILED DESCRIPTION OF THE INVENTION

[0019] The adsorbent according to this embodiment will be described below. 1. Powder pH-adjusted iron precipitate 2. Composite material (adsorbent) of powdered pH-adjusted iron precipitate and polyurethane 3. Examples of how to use adsorbents The following will be explained in order.

[0020] <1. Powdered pH-adjusted iron precipitate> The powder pH-adjusted iron precipitate according to this embodiment has a powder pH of 3 or higher. This pH-adjusted iron precipitate is obtained by adding an alkali to an iron precipitate obtained by oxidizing divalent Fe ions contained in an acidic aqueous solution using iron-oxidizing bacteria, and adjusting the powder pH to 3 or higher. In this way, by reducing the acidity of the powder pH-adjusted iron precipitate, it becomes possible to form a composite material with polyurethane. Powder pH Adjustment The powder pH of the iron starch can be calculated by carrying out the boiling method of particles according to JIS standard K-5101-17-1:2004.

[0021] The type and form of the alkali (either an aqueous solution or a slurry) are not limited. For example, the alkali includes at least one of an alkali metal hydroxide, an alkali metal carbonate, an alkaline earth metal hydroxide, and an alkaline earth metal carbonate. Specific examples include sodium hydroxide or an aqueous solution thereof, or calcium carbonate or a slurry thereof.

[0022] Here, an example of a method for producing the powdered pH-adjusted iron precipitate according to this embodiment will be described.

[0023] The raw material for the powdered pH-adjusted iron precipitate is an aqueous iron salt solution containing divalent Fe ions. The aqueous iron salt solution may be recovered mine wastewater discharged from abandoned mines, such as iron sulfide mines. The mine wastewater may contain 0.01 g / L or more of divalent Fe ions. The pH of the mine wastewater may be between 2.5 and 5.0.

[0024] From the viewpoint of efficient resource utilization, it is preferable to use sludge as the raw material for the iron salt solution, but the raw material is not particularly limited as long as it meets the above-mentioned chemical requirements. Iron-oxidizing bacteria are added to this iron salt solution to oxidize the divalent Fe ions to trivalent Fe ions, thereby obtaining an iron precipitate.

[0025] The iron precipitate may contain at least one of schwertmannite, iron hydroxide, and iron oxide, which are obtained by oxidizing divalent Fe ions contained in an acidic aqueous solution using iron-oxidizing bacteria, or may contain any of these as the main component (more than 50 mass% and / or the largest mass ratio in the iron precipitate).

[0026] The pH when obtaining this iron precipitate may be 2.5 or more and 5.0 or less. An alkali is added to this iron precipitate to adjust the powder pH to 3 or more to obtain a powder pH-adjusted iron precipitate.

[0027] Powder pH adjustment: The powder pH of iron precipitate should be 3 or higher. This value can be calculated using the boiling method according to the JIS method as described above, but it varies depending on the composition or surface treatment of the iron precipitate. Therefore, even if the composition is uniformly the same, the same value will not be obtained. By adjusting the powder pH within this range, the adsorbent will exhibit excellent phosphorus or heavy metal adsorption capacity.

[0028] The median particle size of the powdered pH-adjusted iron precipitate is preferably 100 μm or less, from the viewpoint of enabling composite formation. It is more preferably 10 μm or less, even more preferably 5 μm or less, and particularly preferably 1 μm or less. Such a particle size allows for a uniform dispersion to be obtained, resulting in an adsorbent (e.g., a molded body) with excellent mechanical strength. Furthermore, the large surface area provides the resulting adsorbent with excellent adsorption performance.

[0029] There are no particular limitations on the means for adjusting the median particle size of the powdered pH-adjusted iron precipitate to 100 μm or less, including, for example, a method of treating the powdered pH-adjusted iron precipitate by any pulverization method, such as wet pulverization or dry pulverization, or a method of applying sufficient shear force to a previously prepared powdered pH-adjusted iron precipitate dispersion.

[0030] In this specification, the term "median particle size" refers to the particle size (d50, median diameter) at which the cumulative area percentage is 50% in the particle size distribution measured using a natural / centrifugal sedimentation particle size distribution analyzer (e.g., CAPA-700; manufactured by Horiba, Ltd.).

[0031] <2. Composite Material (Adsorbent) of Powdered pH-Adjusted Iron Precipitate and Polyurethane and Its Manufacturing Method> The composite material of powdered pH-adjusted iron precipitate and polyurethane according to this embodiment is a mixture of the above-mentioned powdered pH-adjusted iron precipitate and polyurethane. At the same time, the composite material is also an adsorbent for phosphorus. In addition, the composite material can be an adsorbent for heavy metals. In this specification, "heavy metal" refers to a metal element with a specific gravity of 4 or more, and a specific example is arsenic. In other words, the composite material according to this embodiment can be an adsorbent for arsenic. Furthermore, when the adsorbent is obtained by molding, the composite material can also be a molding raw material (e.g., a material for obtaining a molding solution to be poured into a mold).

[0032] Here, a method for producing a composite material (hereinafter also simply referred to as an adsorbent) of a powdered pH-adjusted iron precipitate and polyurethane according to this embodiment will be described.

[0033] There are no limitations on polyurethane as long as it is a polymer having a urethane bond obtained by the reaction of a polyol having a hydroxyl group with a polyisocyanate. For example, as shown in the following chemical formula, polyurethane may be obtained using MDI (diphenylmethane diisocyanate), polyethylene glycol, and 1,4-butanediol as raw materials and THF (tetrahydrofuran) as a solvent. However, polyurethane raw materials and solvents known in polyurethane production may be used. [ka]

[0034] The method for preparing the adsorbent is not particularly limited. For example, either a method in which polyurethane and a powdered pH-adjusted iron precipitate are dissolved or dispersed in a solvent and then mixed in an appropriate ratio, or a method in which the powdered pH-adjusted iron precipitate is charged in a solvent all at once, and then polyurethane is added and dissolved or dispersed, can be employed. The order of addition is not particularly limited.

[0035] As just one example of a method for preparing the adsorbent, as shown in the Examples section below, a THF solution may be obtained using 4,4'-diphenylmethane diisocyanate (MDI), polyethylene glycol, and 1,4-butanediol as polyurethane raw materials and THF as a solvent.

[0036] Then, the THF solution and the powdered pH-adjusted iron precipitate that has been powdered by crushing are mixed, the mixture is stirred, and the resulting mixture is concentrated using an evaporator, after which the solvent is distilled off in vacuo to obtain a powdered pH-adjusted iron precipitate-polyurethane composite material.

[0037] The above-described method allows the surfaces of the particles constituting the powdered pH-adjusted iron precipitate to be uniformly coated with polyurethane. The above-described treatment after mixing the powdered pH-adjusted iron precipitate facilitates processing the adsorbent into various shapes, including foaming or coating various materials. Furthermore, the use of polyurethane allows the shape of the particles constituting the adsorbent to be maintained and particle decomposition to be suppressed even after agitation during the above-described method.

[0038] Furthermore, when the polymer used in the adsorbent is polyurethane, it has the advantage that the adsorbent does not decompose even in high-temperature environments, compared to other polymers (e.g., PVA). Furthermore, as shown in the Examples section below, polyurethane is compatible with powdered pH-adjusted iron precipitate prepared with NaOH. In other words, the powdered pH-adjusted iron precipitate can be effectively fixed to the polyurethane.

[0039] In the case of using the adsorbent, the adsorbent is a solid, but for example, the adsorbent may be used in the form of a slurry (more precisely, the adsorbent is contained as a solid content in the slurry). The solid content here refers to the components constituting the adsorbent excluding liquid content such as water and solvent. However, the solvent constituting the slurry is preferably organic (e.g., alcohol, acetone, etc.).

[0040] In either case, in this embodiment, the powdered pH-adjusted iron precipitate is contained in the adsorbent in an amount of 65% by mass or more. As shown in the Examples section below, when the mass ratio of the powdered pH-adjusted iron precipitate exceeds 60% by mass, the increase in the adsorption rate of phosphate ions accelerates.

[0041] Incidentally, one of the reasons why such a high mass ratio can be achieved is that the raw materials for the adsorbent according to this embodiment contain polyurethane. For example, when the main component (more than 50 mass% and / or the largest mass ratio) of the polymer in the adsorbent according to this embodiment is polyurethane, the shape of the adsorbent can be maintained even when the powdered pH-adjusted iron precipitate is present in a mass ratio of 65% or more.

[0042] Preferably, the powdered pH-adjusted iron precipitate is contained in the solid adsorbent at a mass ratio of 70% by mass or more, which can prevent a decrease in adsorption capacity due to masking of the powdered pH-adjusted iron precipitate by polyurethane.

[0043] There is no upper limit to the mass ratio of the powdered pH-adjusted iron precipitate, but it may be, for example, 95 mass % or more.

[0044] On the other hand, the mass ratio of polyurethane and powdered pH-adjusted iron precipitate in the adsorbent may be as follows:

[0045] For example, to ensure that the powdered pH-adjusted iron precipitate is contained in an amount of 65% by mass or more, the total amount of polyurethane and powdered pH-adjusted iron precipitate will necessarily be more than 65% by mass (preferably 70% by mass or more, or 75% by mass or more, 80% by mass or more, 85% by mass or more, or 95% by mass or more). The amount of polyurethane contained in the adsorbent is 35% or less, preferably 30% or less, more preferably 25% or less, 20% or less, or 15% or less, based on the amount of the adsorbent. The lower limit is preferably 5% or more.

[0046] However, the mass ratio of the polyurethane and the powdered pH-adjusted iron precipitate can be appropriately changed within the above ranges depending on the composition and degree of polymerization of the polyurethane and the solvent.

[0047] <3. Form of adsorbent> After obtaining the adsorbent by the above method, the form of the adsorbent may be determined depending on the intended use.

[0048] For example, when obtaining the adsorbent by the above-mentioned method, the adsorbent may be kneaded, then dried and pulverized to obtain the adsorbent as a powder. Known methods may be used as the kneading, drying and pulverization methods.

[0049] As another example, a mixture may be obtained by mixing a powder of the adsorbent with water. The mixture may be used as a molding solution, and an adsorbent having a shape corresponding to a mold may be obtained by molding. As an example of the molding process, an adsorbent having a shape corresponding to a mold may be obtained through a step of pouring the molding solution (the mixture) into a mold, a step of drying the mixture together with the mold, and a step of releasing the mixture from the mold.

[0050] In any form of adsorbent, the mass ratio of the powdered pH-adjusted iron precipitate described in <2. Composite material (adsorbent) of powdered pH-adjusted iron precipitate and polyurethane and its manufacturing method> must be satisfied.

[0051] As a result, we came up with the idea of ​​an adsorbent in which the adsorbent obtained by adjusting the powder pH of iron starch is solidified with polyurethane. This allows the adsorbent to be more easily filtered during recovery, even if it is in the form of fine particles after adsorption of phosphorus or heavy metals, compared to the adsorbent in its powder form. Alternatively, the adsorbent can be recovered without filtration. In this way, the recovery process of the adsorbent after adsorption is facilitated.

[0052] Regarding the phosphorus adsorption capacity, as will be shown in the Examples section below, when the powdered pH-adjusted iron precipitate in the adsorbent exceeds 60% by mass, the increase in the adsorption rate of phosphate ions accelerates.

[0053] <4. Examples of how to use adsorbents> The adsorbent according to this embodiment exhibits excellent adsorption and immobilization capabilities for phosphorus and / or heavy metals (particularly arsenic) in various forms, such as an adsorbent powder, a slurry containing the powder, a mixed solid of the powder and a filler, etc. Furthermore, the various configurations described above can be combined with each other. [Example]

[0054] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0055] [Example 1] (Production of powdered pH-adjusted iron precipitate) The raw material was mine wastewater obtained from iron sulfide deposits. Iron-oxidizing bacteria were added to the mine wastewater to oxidize the divalent iron contained in the mine wastewater to trivalent iron, resulting in iron precipitate.

[0056] 500 g of the obtained iron precipitate (water content 40% by mass) was dispersed in 1000 ml of purified water. 24 g of sodium hydroxide was added, stirred for 10 minutes, and then allowed to stand for 3 hours. The obtained slurry was filtered using filter paper while washing with purified water. The obtained residue was dried at 90°C for 8 hours to obtain a powder pH-adjusted iron precipitate (90 g, water content: almost 0) in which the powder pH had been adjusted to 3 or higher with NaOH. The powder pH of this powder pH-adjusted iron precipitate adjusted with NaOH was 5.6. This powder pH-adjusted iron precipitate adjusted with NaOH will hereinafter be referred to as the "powder pH-adjusted iron precipitate."

[0057] The powder pH of the powder pH-adjusted iron starch was 5.8. The median particle size of the powder pH-adjusted iron starch was measured using a natural / centrifugal sedimentation particle size distribution analyzer (CAPA-700; Horiba, Ltd.) and was found to be approximately 10 μm.

[0058] (Adsorbent manufacturing) The polyurethane raw materials used were 3.61 g of 4,4'-diphenylmethane diisocyanate (MDI), 5.41 g of polyethylene glycol (average molecular weight: 1500), and 0.98 g of 1,4-butanediol, and 35 mL of THF was used as the solvent. The raw materials were dissolved in the solvent and stirred at 60°C for 12 hours. This produced a THF solution containing 10 g of polyurethane.

[0059] Next, 25 mL of the polyurethane-containing THF solution and 24.0 g of the powdered pH-adjusted iron precipitate, which had been crushed and powdered, were mixed at room temperature and stirred for 30 minutes. The solvent was removed from the resulting mixture using an evaporator under vacuum to obtain a powdered pH-adjusted iron precipitate-polyurethane composite material (70% by mass of powdered pH-adjusted iron precipitate, a reddish-brown solid) as an adsorbent. This adsorbent was crushed to a diameter of approximately 1 mm and its phosphorus adsorption capacity was evaluated.

[0060] (Phosphate ion adsorption test) 100 mg of the resulting adsorbent was added to 50 mL of a solution containing 10 ppm of phosphate ions (prepared from sodium phosphate) and allowed to stand for 24 hours. To investigate the effect of masking, the adsorbent was added so that the net amount of adsorbent was 100 mg. The phosphate concentration in the supernatant solution was then quantified by colorimetry (molybdenum blue method, Hitachi UH5700 spectrophotometer), and the phosphate ion removal rate was calculated.

[0061] The adsorbent obtained in Example 1 had a removal rate of phosphate ions of 81%.

[0062] [Comparative Example 1] An adsorbent was obtained in the same manner as in Example 1, except that the content of the powdered pH-adjusted iron precipitate in the powdered pH-adjusted iron precipitate-polyurethane composite material was set to 30 mass%, which is less than 65 mass%, and a phosphate ion adsorption test was carried out in the same manner as in Example 1. As a result, the adsorbent obtained in Comparative Example 1 showed a phosphate ion removal rate of only 20%.

[0063] Comparative Example 2 An adsorbent was obtained in the same manner as in Example 1, except that the content of the powdered pH-adjusted iron precipitate in the powdered pH-adjusted iron precipitate-polyurethane composite material was changed to 60 mass%, which is less than 65 mass%, and a phosphate ion adsorption test was carried out in the same manner as in Example 1. As a result, the adsorbent obtained in Comparative Example 2 showed a phosphate ion removal rate of only 39%.

Claims

1. Polyurethane and a powder pH-adjusted iron precipitate having a powder pH of 3 or more; An adsorbent comprising: The adsorbent contains the powdered pH-adjusted iron precipitate in an amount of 65% or more by mass.

2. 2. The adsorbent of claim 1, wherein the powdered pH-adjusted iron precipitate is in the form of particles having a median particle size of 100 μm or less.

3. 2. The adsorbent according to claim 1, wherein the powdered pH-adjusted iron precipitate is contained in the adsorbent in an amount of 95% or less by mass.

4. a step of oxidizing divalent Fe ions contained in the acidic aqueous solution with iron-oxidizing bacteria, and adding an alkali to the obtained iron precipitate to adjust the powder pH to 3 or more to obtain a powder pH-adjusted iron precipitate; mixing the powdered pH-adjusted iron precipitate, polyurethane, and a solvent to obtain a mixture; molding the mixture; and The method for producing an adsorbent, wherein the mixture before the step of molding the mixture contains 65% or more by mass of the powdered pH-adjusted iron precipitate.

5. The method for producing an adsorbent according to claim 4 , wherein the alkali includes at least one of an alkali metal hydroxide, an alkali metal carbonate, an alkaline earth metal hydroxide, and an alkaline earth metal carbonate.

6. The method for producing an adsorbent according to claim 4, wherein the alkali is sodium hydroxide.

7. A method for immobilizing phosphorus or heavy metals, comprising immobilizing phosphorus or heavy metals using the adsorbent according to any one of claims 1 to 3 or an adsorbent obtained by the method for producing an adsorbent according to any one of claims 4 to 6.

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