Water treatment method using activated carbon

Abstract

One aspect of the present invention relates to a water treatment method comprising the step of continuously contacting activated carbon with water containing fluoride organic compounds to obtain treated water with a fluoride organic compound content of 0.1 ppb or less, wherein the activated carbon has an average particle size of 0.1 to 10 mm, a BET specific surface area of ​​500 m² / g or more and 2000 m² / g or less, and a porosity of 10% to 40% as determined by X-ray CT.

Description

Technical Field

[0001] This invention relates to an activated carbon and a water treatment method using the activated carbon. Prior Technology

[0002] Fluorinated organic compounds have long been used in various applications, such as surfactants, emulsifiers, water-repellent agents, fire extinguishing agents, waxes, carpet cleaners, and coatings, due to their unique properties unmatched by other substances (excellent heat and chemical resistance, usability under harsh conditions, and lack of light absorption). Recently, their use as surface treatment agents for semiconductors and as components of fuel cells is also increasing.

[0003] However, starting several years ago, researchers, primarily in the United States and Canada, reported the accumulation of fluorinated organic compounds (PFCs) in environmental water and wildlife. Typical examples include perfluorocarboxylic acids, such as perfluorooctanoic acid (PFOA: C7F15COOH), and perfluorosulfonic acids, such as perfluorooctane sulfonic acid (PFOS: C8F17SO3H). Researchers in Europe and Japan also participated in environmental analyses, which confirmed the global presence of these compounds in the environment, including in Japan. Because of this situation, efforts have begun to reduce the environmental risks of PFCs.

[0004] Patent document 1 discloses a method for recovering PFOA using granular activated carbon.

[0005] As described in Patent Document 1, the method of using activated carbon in the treatment of PFOA and other substances has significant economic advantages, but its treatment efficiency is not satisfactory in the past. The object of this invention, in view of the above-mentioned situation, is to provide activated carbon that can effectively remove fluorinated organic compounds and a method for treating water containing fluorinated organic compounds. [Previous Technical Documents] [Patent Literature]

[0006] [Patent Document 1] U.S. Patent Application Publication No. 2005 / 0000904 Summary of the Invention

[0007] [The problem the invention aims to solve] Through research and examination, the inventors discovered that fluorinated organic compounds can be effectively removed by means of a fluorinated organic compound removal material or water treatment method having the following structure.

[0008] The activated carbon system of one type of the present invention is characterized by: an average particle size of 0.1 to 10 mm, a BET specific surface area of ​​500 m2 / g or more and 2000 m2 / g or less, and a porosity of 10% to 40% obtained by X-ray CT.

[0009] In addition, other types of water treatment methods of the present invention are characterized by the following steps: contacting the aforementioned activated carbon with the target water containing fluorine-containing organic compounds to obtain treated water with a fluorine-containing organic compound content of 0.1 ppb or less. Simple Explanation of the Diagram

[0010] Figure 1 is an X-ray CT cross-sectional image of the activated carbon in Example 1. Figure 2 shows the image processing screen of the X-ray CT cross-sectional image of activated carbon in Example 1. Figure 3 shows an X-ray CT cross-sectional image of the activated carbon in Comparative Example 1. Figure 4 shows the image processing screen of the X-ray CT cross-sectional image of the activated carbon in Comparative Example 1. Implementation

[0011] [The form in which the invention is carried out] The following describes specific embodiments of the present invention, but the present invention is not limited thereto.

[0012] The activated carbon of one embodiment of the present invention comprises activated carbon with an average particle size of 0.1 to 10 mm, a BET specific surface area of ​​500 m2 / g or more and 2000 m2 / g or less, and an internal porosity of 10% to 40% as determined by X-ray CT.

[0013] The activated carbon according to the present invention can adsorb and treat fluoride-containing organic compounds in the target water with high efficiency.

[0014] The activated carbon of this embodiment has an average particle size of 0.1 mm or more, preferably 0.5 mm or more, and even more preferably 1.0 mm or more. Furthermore, the activated carbon of this embodiment has an average particle size of 10 mm or less, preferably 8 mm or less, and even more preferably 7 mm or less. If the average particle size is above the aforementioned lower limit, the porosity can be sufficiently ensured, which is therefore preferable. In addition, in this specification, the aforementioned average particle size of the activated carbon refers to the average particle diameter measured according to JIS K1474.

[0015] The activated carbon of this embodiment has a specific surface area (BET specific surface area) calculated by the BET method (multi-point method) from the nitrogen adsorption isotherm obtained from the nitrogen adsorption capacity at 77 K, which is 500 m² / g or more and 2000 m² / g or less. Preferably, the BET specific surface area is 550 m² / g or more, more preferably 600 m² / g or more. Furthermore, preferably, the BET specific surface area is 1900 m² / g or less, more preferably 1700 m² / g or less. Within the above ranges, the activated carbon of this embodiment exhibits excellent removal capacity (adsorption capacity) of fluorinated organic compounds and excellent mechanical strength.

[0016] The activated carbon of this embodiment has internal pores. By having a suitable amount of pores inside the activated carbon, it can effectively remove fluoride-containing organic compounds from the target water when used as a material for removing fluoride-containing organic compounds. As shown in the following examples, the activated carbon of this embodiment can be observed non-destructively in X-ray CT, and measurements can be performed without damaging the fine pores.

[0017] The activated carbon of this embodiment has an internal porosity of 10% to 40%, as determined by X-ray CT. This porosity is preferably 12% or more, more preferably 15% or more, and even more preferably 20% or more. Furthermore, this porosity is preferably 38% or less, and more preferably 35% or less. If it is above the lower limit, the interior of the activated carbon can be suitable for adsorbing fluorinated organic compounds. Furthermore, if it is below the upper limit, it has high mechanical strength and can maintain its shape.

[0018] Here, the porosity within the particles obtained by X-ray CT is determined as follows: First, multiple (more than 1000) cross-sections of the activated carbon are continuously photographed using an X-ray CT scanner (e.g., using an Xradia 520 Versa manufactured by Carl Zeiss). Next, at equal intervals, at least 40 images are selected from the images obtained, representing the porosity of each selected cross-section. Image processing is then used to extract the portion corresponding to the porosity of each extracted cross-section, and the porosity area per unit area of ​​the cross-section is calculated. Finally, the average value of the extracted 40 or more images is calculated based on the ratio of the porosity area per unit area of ​​each cross-section, and this average value is defined as the porosity within the activated carbon particles obtained by X-ray CT. Furthermore, in this specification, approximately 1 μm² or larger is considered as a porosity for detection.

[0019] The activated carbon of this embodiment preferably has an iodine adsorption capacity of 900 mg / g or more. Furthermore, it is more preferably the aforementioned iodine adsorption capacity is 1200 mg / g or less. Iodine adsorption capacity is an indicator of the volume of pores with a diameter smaller than 1 μm. If the iodine adsorption capacity is within the above range, the strength can be maintained while the adsorption of fluorine-containing organic compounds is more sufficient.

[0020] Regarding the raw materials for the activated carbon in this embodiment, examples include plant-based carbon materials (e.g., materials derived from plants such as wood, shavings, charcoal, coconut shells or walnut shells, fruit shells, fruit seeds, wood pulp manufacturing byproducts, lignin, waste molasses, etc.), mineral-based carbon materials (e.g., coal-based materials such as peat, argillaceous coal, lignite, bituminous coal, anthracite, coal char, coal tar, coal pitch, etc.; petroleum-based materials such as petroleum distillation residues, petroleum pitch, etc.), synthetic resin-based carbon materials (e.g., materials derived from synthetic resins such as phenolic resins, polyvinylidene chloride, acrylic resins, etc.), and natural fiber-based carbon materials (e.g., materials derived from natural fibers such as cellulose, regenerated fibers such as rayon, etc.). These carbon materials can be used alone or in combination of two or more. It is known that organofluorine compounds can be adsorbed by activated carbon through hydrophobic interactions. From the perspective of the balance between high specific surface area and hydrophobicity, as well as the mechanical strength supplied during heat treatment, it is preferable to use mineral-based carbonaceous materials, and even more preferably to use activated carbon derived from bituminous coal.

[0021] The activated carbon of this embodiment can be manufactured, for example, by dry mixing a carbonaceous material with weak or better binding properties with a carbonaceous material with slightly better binding properties, and then activating it after heat treatment. Here, carbonaceous material with weak or better binding properties refers to carbonaceous material with a crucible swelling number greater than 1. Slight binding properties refer to carbonaceous material with a crucible swelling number less than 1, which can be 0. Furthermore, the crucible swelling number is determined according to the crucible swelling test method of JIS M 8801 6, by placing the sample in a predetermined crucible, heating it under predetermined conditions, and comparing the resulting residue with a standard profile.

[0022] Regarding carbonaceous materials with weak or better binding properties, examples include plant-based, fruit-shell-based, and mineral-based materials, but mineral-based carbonaceous materials are preferred. For the reasons mentioned above, coal-based carbonaceous materials are particularly preferred among mineral-based materials. For such carbonaceous materials, weakly binding coal with a crucible expansion number greater than 1 and less than 4 is preferred. However, in cases where the binding properties are insufficient and the formability is poor, strongly binding coal with a crucible expansion number greater than 4 and bitumen can be mixed in appropriate proportions and used.

[0023] Regarding the micro-bonded carbonaceous material, mineral-based carbonaceous materials are preferred, and coal-based carbonaceous materials are even more preferred. Furthermore, the micro-bonded carbonaceous material preferably contains at least one of alkali metals and alkaline earth metals. Examples of alkali metals include potassium and sodium. Examples of alkaline earth metals include calcium. The alkali metals and / or alkaline earth metals in the micro-bonded carbonaceous material containing alkali metals and / or alkaline earth metals are preferably 300 ppm to 1100 ppm in terms of total metal atoms. Calcium is particularly preferred among the alkali metals and alkaline earth metals, and from the viewpoint of hardness and formability, carbonaceous materials with a calcium content of 300 ppm to 1000 ppm are even more preferred.

[0024] In the method for manufacturing activated carbon according to this embodiment, since at least one of the alkali metals and alkaline earth metals is homogeneously and highly dispersed in the carbonaceous material structure, compared with the method of adding a specific amount of metal compound to the carbonaceous material for activation, the localization of micropore formation is less likely to occur and homogeneous micropore formation is achieved. Therefore, an appropriate micropore distribution is obtained, and the hardness can also be improved, which is better.

[0025] To produce the activated carbon of this embodiment, a carbonaceous material with weak or better binding properties (hereinafter sometimes referred to as Material 1) and a carbonaceous material with slight binding properties (hereinafter sometimes referred to as Material 2, preferably containing alkali metals and / or alkaline earth metals) are first dry-mixed and pulverized. In addition to these carbonaceous materials 1 and 2, it is permissible to add strongly binding coal or pitch, etc., to a extent that it does not impair the effect of the present invention. The method of dry mixing and pulverization is not particularly limited as long as the two carbonaceous materials can be mixed and pulverized to maintain a roughly constant ratio, but it can be easily implemented using a jaw crusher, bucket crusher, cone crusher, single-roll crusher, double-roll crusher, impact crusher, ball mill, rod mill, or high-speed mixer, etc.

[0026] The mixing ratio of Material 1 and Material 2 can be determined based on the removal capacity or hardness of the carbonaceous material used as a raw material and the fluorinated organic compounds used as a target. However, if the ratio of Material 1 is too high, while the hardness increases, the formation of pores with a diameter of 500 nm or more is suppressed, which tends to reduce the removal capacity of fluorinated organic compounds. Conversely, if the ratio of Material 1 is too low, the formability and hardness tend to decrease. Therefore, it is preferable to mix Material 1 and Material 2 by weight, preferably in a ratio of 1:9 to 9:1, and more preferably in a ratio of 2:8 to 6:4.

[0027] In the molding step, after dry mixing and pulverizing, materials 1 and 2 are temporarily shaped by pressure molding. The pressure during pressure molding is preferably 140 kg / cm² or higher, more preferably 180 kg / cm² or higher. Furthermore, the pressure during pressure molding is preferably 600 kg / cm² or lower, more preferably 400 kg / cm² or lower, and even more preferably 360 kg / cm² or lower. If the pressure is above the lower limit, materials 1 and 2 will not revert to separate objects during subsequent pulverization steps and can be sufficiently compounded, thus better forming voids. Conversely, if the pressure is below the upper limit, appropriate voids can be generated during the activated carbon generation step. The apparatus used for pressure molding is not particularly limited; molding apparatuses such as roller presses, flat die granulators, ring die granulators, and extruders can be used. Furthermore, the pressure or the shape of the molded product is not particularly limited; it can be cylindrical, granular, spherical, flake-shaped, etc., as appropriate according to the purpose. There are no specific limitations on the size of these items.

[0028] The resulting molded material can be pulverized using well-known crushers, such as jaw crushers, roller crushers, ball mills, rod mills, or high-speed mixers. The pulverized particles are granulated to a predetermined size, such as 8 / 30 mesh. However, in practice, the particle size of the activated carbon is preferably around 0.01~5.0 mm, more preferably around 0.05~3.0 mm, and the average particle size is preferably around 0.3~3.0 mm, more preferably around 0.5~1.0 mm. The granulated material is then fed to a heat treatment facility. The heat treatment is performed by heating to 550~750°C in a reducing gas environment. To obtain higher performance and stronger carbides or activated carbon, it is preferable to perform a two-stage heat treatment, for example, heating to 200-400°C at 5-30°C / min in an oxidizing gas environment, and further heating to 550-750°C at 5-30°C / min in a reducing gas environment.

[0029] The heat-treated (dry distilled) pulverized material is further activated to become activated carbon. Activation can be carried out in a gaseous environment with oxidizing gases such as water vapor, carbon dioxide, air, propane combustion exhaust, LPG, or mixtures thereof at 400-1000°C; or in the presence of agents such as zinc chloride, phosphoric acid, calcium chloride, or potassium sulfide at approximately 400-800°C. Preferably, combustion gas activation is carried out at 900-1000°C under a flow of combustion gas, wherein the combustion gas is a mixture of air and LPG in a ratio of approximately 1:0.03 to 1:0.06. The activation yield can be appropriately determined as necessary based on the relationship between the removal capacity of fluorinated organic compounds and hardness.

[0030] The activated carbon of this embodiment can be obtained by acid washing the obtained activated carbon with dilute hydrochloric acid or the like after activation, adjusting the pH to between 5.0 and 7.0. The pH of the activated carbon referred to here refers to the pH measured according to JIS K1474.

[0031] The activated carbon of this embodiment can also undergo further treatments, such as chemical modification of the surface or physical attachment of functional substances, depending on the application. Examples of such surface modification include adding salts or oxides of metals such as silver or iron, mineral acids, and allowing them to adhere, as well as treatments that make the surface acidic.

[0032] Among the fluorinated organic compounds treated (adsorbed) by the activated carbon of this embodiment, examples include perfluoroalkyl carboxylic acids, perfluoroalkyl sulfonic acids, and amphiphilic perfluoroalkyl derivatives such as 1H,1H,2H,2H-perfluoroalcohols. Perfluoroalkyl carboxylic acids include perfluorobutyric acid, perfluorovaleric acid, perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorononanoic acid, perfluorodecanoic acid, perfluoroundecanoic acid, and perfluorododecanoic acid. Perfluoroalkyl sulfonic acids include perfluorobutane sulfonic acid, perfluorohexane sulfonic acid, and perfluorooctane sulfonic acid. In particular, the activated carbon of this embodiment exhibits excellent adsorption (removal) capacity for perfluorovaleric acid, perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, and perfluorooctane sulfonic acid, especially for perfluorooctanoic acid.

[0033] The activated carbon of this embodiment can effectively and for a long period of time adsorb fluorinated organic compounds, and therefore can be preferably used as a material for removing fluorinated organic compounds. Therefore, the present invention also includes a material for removing fluorinated organic compounds comprising the above-mentioned activated carbon.

[0034] Another aspect of the present invention is a water treatment method comprising the following steps: contacting the aforementioned activated carbon with water containing fluoride-containing organic compounds to obtain treated water with a fluoride-containing organic compound content of 0.1 ppb or less. Because the water treatment method of the present invention includes the above-described components, it can obtain treated water from water containing fluoride-containing organic compounds after the fluoride-containing organic compounds have been removed with extremely high efficiency. Fluoride-containing organic compounds possess unique properties that other substances cannot achieve (excellent heat resistance, chemical resistance allowing use under harsh conditions, lack of light absorption, etc.), and therefore can be used for various applications. On the other hand, it has been determined that some fluoride-containing organic compounds (e.g., perfluorooctanoic acid (PFOA: C7F15COOH) or perfluorooctane sulfonic acid (PFOS: C8F17SO3H)) accumulate in the bodies of wild animals, and subsequent studies have confirmed that they exist in the environment on a global scale. From this perspective, reducing the concentration of fluoride-containing organic compounds in water to below 0.1 ppb helps to lower environmental risks.

[0035] Through their research and examination, the inventors discovered that if the concentration of fluoride-containing organic compounds in the target water is low, the reduction of fluoride-containing organic compounds is limited even with the addition of large amounts of activated carbon when using conventional activated carbon. In contrast, the use of the specific activated carbon of this embodiment can achieve extremely efficient treatment.

[0036] It is generally known that the performance of activated carbon depends on its specific surface area. However, surprisingly, the treatment efficiency of aqueous solutions with low concentrations of fluorinated organic compounds depends significantly on the internal properties of the activated carbon, and no clear correlation has been observed with its specific surface area.

[0037] In other words, in this embodiment, by using activated carbon with a certain value of particle porosity measured by X-ray CT, fluorine-containing organic compounds can be effectively removed, and treated water with almost no fluorine-containing organic compounds (fluorine-containing organic compound content below 0.1 ppb) can be obtained.

[0038] The fluorinated organic compound removal step in this embodiment is a step of contacting the water containing fluorinated organic compounds with a fluorinated organic compound removal material containing specific activated carbon.

[0039] The concentration of fluoride-containing organic compounds in the target water varies depending on the sampling location and is therefore not specifically limited, typically ranging from 0.1 to 1000 ppb.

[0040] The water treatment method of this embodiment can efficiently remove fluorinated organic compounds even when the concentration of fluorinated organic compounds in the target water is below 1000 ppb, or even below 100 ppb.

[0041] In this specification, the concentration of fluorinated organic compounds can be determined using a liquid chromatography-tandem mass spectrometry (LC / MS / MS) instrument manufactured by Waters Corporation, under the conditions described below. This is a particularly useful method for determining concentrations below 100 ppb. • HPLC system body: 2695 separation module • Mobile phase solvent: Acetonitrile 45 vol% / 0.15% acetic acid aqueous solution 55 vol% • HPLC column: Atlantis dC18 3μm 2.1×30mm • Tandem quadrupole mass spectrometer: Quattro micro API

[0042] The contact between the target water and activated carbon can be a batch process where activated carbon is added to the target water, or a continuous process where the target water flows through a column filled with activated carbon. Furthermore, multiple treatments can be performed using batch or continuous contact, or a combination of both. In addition, the packed column in a continuous process can be a moving bed, a fixed bed, or a fluidized bed.

[0043] In batch contact, the contact time between the target water and activated carbon can be appropriately set according to the amount of activated carbon used and the concentration of fluoride-containing organic compounds in the target water, but from the point of view to ensure sufficient removal rate, it is better to be 60 minutes or more.

[0044] From the viewpoint of ensuring sufficient treatment efficiency, the amount of activated carbon relative to the target water is preferably 0.015% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.10% by mass or more. Furthermore, from the viewpoint of volumetric efficiency, it is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0045] The water treatment method of this embodiment is a method for obtaining treated water with a fluoride organic compound concentration of 0.1 ppb or less. The concentration of fluoride organic compounds in the obtained treated water is preferably 0.01 ppb or less, and even more preferably 0.005 ppb or less.

[0046] In this embodiment of the water treatment method, when the treated water contains activated carbon, a further step of removing activated carbon may be included. The method for removing activated carbon is not particularly limited, but well-known methods such as filtration, sedimentation, centrifugation, and separation using coagulants may be employed.

[0047] Another embodiment of the present invention is a water treatment apparatus incorporating activated carbon. The activated carbon in the water treatment apparatus can be contained within a treatment tank for treating the target water, or it can be an activated carbon-filled tube through which the target water passes. Its form is not limited as long as it is arranged so that the target water and the activated carbon can appropriately contact each other. Furthermore, the method of contacting the target water and the activated carbon in the water treatment apparatus can be batch or continuous. In continuous contact, the filled tube can be any of a moving bed type, a fixed bed type, or a fluidized bed type.

[0048] This specification discloses various types of technology as described above, but the main technologies are summarized below.

[0049] That is, the activated carbon of one type of the present invention is characterized by: an average particle size of 0.1 to 10 mm, a BET specific surface area of ​​500 m2 / g or more and 2000 m2 / g or less, and a porosity of 10% to 40% obtained by X-ray CT. With this configuration, the activated carbon of the present invention can adsorb and treat fluoride-containing organic compounds in the target water with extremely high efficiency.

[0050] Furthermore, it is preferable that the iodine adsorption capacity of the aforementioned activated carbon is between 900 mg / g and 1200 mg / g. This is believed to maintain the strength of the activated carbon while ensuring more sufficient adsorption of fluorine-containing organic compounds.

[0051] Furthermore, it is preferable that the aforementioned activated carbon uses coal as a raw material. This is believed to result in a superior balance between high specific surface area and hydrophobicity, as well as improved mechanical strength during heat treatment.

[0052] Other types of fluorine-containing organic compound removal materials of the present invention are characterized by comprising activated carbon as described above. Furthermore, other types of water treatment apparatus of the present invention are characterized by comprising the aforementioned activated carbon.

[0053] Furthermore, another type of water treatment method of the present invention is characterized by the following step: contacting the aforementioned activated carbon with the target water containing fluoride organic compounds to obtain treated water with a fluoride organic compound content of 0.1 ppb or less. With the above configuration, treated water in which fluoride organic compounds have been removed from the target water containing fluoride organic compounds can be obtained with extremely high efficiency.

[0054] In the aforementioned water treatment method, the aforementioned fluorinated organic compound is preferably selected from at least one of the group consisting of perfluorooctanoic acid, perfluorooctanoate, perfluorooctane sulfonic acid, and perfluorooctane sulfonate. This is believed to further enhance the effects of the present invention.

[0055] [Example] Next, the present invention will be described with reference to embodiments and comparative examples, but the present invention is not limited to these embodiments.

[0056] (BET specific surface area) The following is an approximation derived from the BET formula.

[0057]

[0058] Using the above approximation (I), by substituting the adsorption amount (v) measured at a given relative pressure (p / p0) into the multi-point method of nitrogen adsorption at liquid nitrogen temperature, vm is obtained, and the specific surface area of ​​the sample (SSA: unit is m2 / g) is calculated by the following formula (II).

[0059]

[0060] In the above formula, vm is the amount of adsorption necessary to form a monolayer on the sample surface (cm3 / g), v is the measured amount of adsorption (cm3 / g), p0 is the saturated vapor pressure, p is the absolute pressure, c is a constant (reflecting the heat of adsorption), N is the Avogadro constant 6.022×1023, and a (nm2) is the area occupied by the adsorbate molecule on the sample surface (the cross-sectional area occupied by the molecule).

[0061] Specifically, the nitrogen adsorption capacity of carbon materials at liquid nitrogen temperature was determined using the "Autosorb-iQ-MP" manufactured by Quantachrome, as follows: Carbon material, serving as the test sample, was filled into a sample tube. The sample tube was temporarily depressurized while cooled to -196°C (77K), and then nitrogen gas (99.999% purity) was adsorbed onto the test sample at a desired relative pressure. The amount of nitrogen adsorbed by the sample at the equilibrium pressure reached under each desired relative pressure was defined as the adsorbed gas volume v.

[0062] (Determination of porosity inside granular activated carbon) 1. X-ray CT determination of activated carbon The measurements were performed using an Xradia 520 Versa manufactured by Carl Zeiss under the following conditions. The light source intensity was 120kV, output 10W, pixel size 1μm, shooting range 1029μm, and exposure time 1s. The internal structure was displayed using a circle with a diameter of 1mm. More than 1000 photographs were taken continuously from one end of the granular activated carbon to the opposite end.

[0063] 2. Image Processing From the images captured by the aforementioned X-ray CT scan, at least 40 images are selected at equal intervals between the initial and final cross-sectional images. The extracted images are read using ImageJ, developed by the National Institutes of Health (NIH), converting the image units from pixels to the measured range (μm) and converting the images to 32-bit black and white. The image threshold is set to 120, and areas identified as holes in the X-ray CT images are represented by blackening. The area of ​​the blackened region in the resulting image is calculated and divided by the measured range to obtain the porosity of each image. The average porosity obtained from the extracted images is defined as the porosity of the activated carbon measured by X-ray CT.

[0064] (Iodine adsorption capacity) According to JIS K 1474, activated carbon was added to a 0.05 mol / L iodine solution (also containing 0.15 mol / L potassium iodide) with a different amount. After shaking and mixing for 15 minutes, the activated carbon was separated by centrifugation. The supernatant was titrated with a 0.1 mol / L sodium thiosulfate solution to determine the residual iodine concentration. An adsorption isotherm was constructed, and the adsorption capacity at a residual iodine concentration of 2.5 g / L was defined as the iodine adsorption performance.

[0065] (Average particle size of particles and activated carbon before activation) For unactivated granules or activated carbon, a particle size accumulation line diagram based on JIS K 1474 is prepared. The average particle size is determined by drawing a horizontal line parallel to the horizontal axis from the intersection of the vertical line at the 50% point on the horizontal axis and the particle size accumulation line. The sieve aperture (mm) represented by the intersection point is then determined and defined as the average particle size.

[0066] (The target water and the concentration of fluoride-containing organic compounds (PFOA, etc.) in the treated water) The determination was performed using a liquid chromatography-tandem mass spectrometer (LC / MS / MS) manufactured by Waters Corporation. • HPLC system body: 2695 separation module • Mobile phase solvent: Acetonitrile 45 vol% / 0.15% acetic acid aqueous solution 55 vol% • HPLC column: Atlantis dC18 3μm 2.1×30mm • Tandem quadrupole mass spectrometer: Quattro micro API

[0067] [Example 1] Material 1 was a weakly cohesive pitchblende coal with a fixed carbon content of 59.5% by weight and an ash content of 0.7% by weight (crucible expansion number 3). Material 2 was a slightly cohesive pitchblende coal with a fixed carbon content of 48.3% by weight or more, an ash content of 0.7% by weight, and containing 45 ppm sodium and 800 ppm calcium (crucible expansion number 0.5). Material 1 and Material 2 were mixed and pulverized in a ball mill at a weight ratio of 3:7. The resulting powder was then filled into a container with a diameter of 4 cm and a length of 15 cm using a roller press manufactured by Yamamoto Hydraulic Press Industry Co., Ltd., and pressed at 100°C and a pressure of 280 kg / cm². The granules were then crushed using a jaw crusher and sized into particles with a diameter range of 0.1–2.0 mm and an average particle size of 0.95 mm.

[0068] Next, the granules were loaded into an externally heated rotary kiln and heated to 300°C at a rate of 5°C / min in an oxidizing gas environment. The temperature was maintained at 300°C for 2 hours, followed by heating to 650°C at a rate of 8.75°C / min in a reducing gas environment to carbonize them. 75.0g of this carbonized coal (calculated based on 0g volatile content) was fed into a flow furnace with an inner diameter of 57mm and a height of 600mm. Activation was performed at 950°C with a combustion gas flow of 20L / min of air and 0.85L / min of LPG, achieving an activation yield of 50%, thus obtaining activated carbon.

[0069] The obtained activated carbon was acid-washed, boiled six times, and the pH was adjusted to 6.5±0.5 before being dried at 120℃ for 2-3 hours.

[0070] Figure 1 shows the X-ray CT image of the obtained activated carbon under the above conditions, and Figure 2 shows the image analysis results. As can be seen from Figures 1 and 2, the activated carbon of Example 1 has appropriate porosity. Furthermore, various physical property measurements were performed according to the above method. The activated carbon obtained here was used to conduct the water treatment experiment described later.

[0071] [Example 2] Except for the pressure molding process at 350 kg / cm², the activated carbon was obtained in the same manner as in Example 1, and various physical property tests were performed. The activated carbon was then used in the water treatment experiments described later.

[0072] [Example 3] Except for the pressure molding process at 180 kg / cm², the activated carbon was obtained in the same manner as in Example 1, and its physical properties were measured and the activated carbon was used in the water treatment experiment described later.

[0073] [Example 4] Except for the pressure molding process at 530 kg / cm², the activated carbon was obtained in the same manner as in Example 1, and its physical properties were measured and the activated carbon was used in the water treatment experiment described later.

[0074] [Comparative Example 1] Except that material 1 was used alone instead of material 2, and the activated carbon was obtained in the same manner as in Example 1, with a particle size range of 0.1~2.0 mm and an average particle size of 0.95 mm, the activated carbon was subjected to physical property testing and water treatment experiments described later were conducted using the activated carbon. Figure 3 shows the X-ray CT image obtained by measuring the obtained activated carbon under the above conditions, and Figure 4 shows the results of image analysis. As can be seen from Figures 3 and 4, the activated carbon of Comparative Example 1 has fewer pores compared to that of Example 1.

[0075] [Comparative Example 2] Except for the pressure molding process at 120 kg / cm², the activated carbon was obtained in the same manner as in Example 1, and its physical properties were measured and the activated carbon was used in the water treatment experiment described later.

[0076] [Comparative Example 3] Except for using only material 1 instead of mixing material 2 with material 1, and increasing the grinding degree to granulate into particles with a particle size range of 0.01~0.60 mm and an average particle size of 0.08 mm, the activated carbon was obtained in the same manner as in Example 1, and its physical properties were measured and the activated carbon was used in the water treatment experiment described later.

[0077] Table 1 presents the BET specific surface area, average particle size, porosity obtained by X-ray CT, and iodine adsorption amount of the activated carbon of Examples 1-4 and Comparative Examples 1-3.

[0078] [Table 1] Specific surface area m2 / g Average particle size mm porosity % Iodine adsorption capacity mg / g Example 1 920 0.9 32 1000 Example 2 840 0.8 28 950 Example 3 1006 0.6 36 1110 Example 4 805 0.7 13 915 Comparative Example 1 860 0.8 8 890 Comparative Example 2 1120 0.7 44 1210 Comparative Example 3 1010 0.08 9 1100

[0079] (Water Treatment Experiment 1: Determination of Breakout Time) 10g of the activated carbon obtained in Example 1 was filled into a 10mm φ × 300mm tube. PFOA (manufactured by Aldrich, 96% purity) was dissolved in tap water to achieve a PFOA concentration of 10 ppb. This solution was continuously injected into the activated carbon at an upflow rate of 10 ml / min to continuously obtain treated water. The obtained liquid was continuously sampled, and the PFOA concentration in the treated water was detected according to the above-described measurement conditions. The lowest detected PFOA concentration was defined as the minimum PFOA detection concentration. The time when the detected PFOA concentration reached 0.15 ppb or higher was defined as the breakout time. The breakout time was also measured for the activated carbon obtained in Examples 2-4 and Comparative Examples 1-3 in the same manner as in Example 1. The results are presented in Table 2.

[0080] (Water Treatment Experiment 2: Adsorption Capacity Measurement) The activated carbon obtained in Examples 1-4 and Comparative Examples 1-3 was recovered after the breakage time, dried with hot air at 60°C for 12 hours, and then vacuum dried at 1 Torr and 20°C for 3 hours. The weight gain was measured. The results are shown in Table 2.

[0081] [Table 2] PFOA detection minimum concentration Breaking through time (min) PFOA adsorption capacity (μg) Example 1 Below the detection limit (0.001ppb) 23856 23.2 Example 2 Below the detection limit 18133 17.8 Example 3 Below the detection limit 14022 14.0 Example 4 Below the detection limit 13093 13.1 Comparative Example 1 0.11ppb 11255 11.1 Comparative Example 2 0.13ppb 3255 was crushed 3.0 Comparative Example 3 Below the detection limit (0.001ppb) 5229 Because of the resistance to fluid flow The measurement workload is heavy, and interruptions are frequent. 5.5

[0082] (Inspection) The results in Table 2 confirm that fluorinated organic compounds (PFOA) can be removed very effectively by using activated carbon that meets the specifications of the present invention. On the other hand, the activated carbon used in Comparative Example 1, with its excessively small porosity, or Comparative Example 2, with its excessively large porosity, was insufficient to remove PFOA. Furthermore, Comparative Example 2, with its excessively large porosity, did not achieve sufficient strength and was crushed within a short time. In Comparative Example 3, the average particle size of the activated carbon became too small, resulting in a high packing density and increased liquid flow resistance. This resistance increased rapidly, leading to insufficient adsorption of PFOA in the aforementioned water treatment experiment 2.

[0083] These results indicate that the porosity of activated carbon with a given specific surface area and average particle size is crucial for removing fluorinated organic compounds, and a higher porosity generally results in superior removal performance. However, it was also confirmed that if the porosity exceeds 40%, the removal performance deteriorates, and the mechanical strength suffers.

[0084] This application is based on Japanese Patent Application No. 2020-158516, filed on September 23, 2020, the contents of which are incorporated herein by reference.

[0085] To illustrate the present invention, it has been adequately and sufficiently described above through specific embodiments. However, it should be understood that those skilled in the art can easily modify and / or improve the aforementioned embodiments. Therefore, any modifications or improvements implemented by those skilled in the art, as long as they do not depart from the scope of the claims described in the patent application, can be interpreted as being included within the scope of the claims. [Potential for industrial application]

[0086] The activated carbon of this invention can efficiently adsorb fluoride-containing organic compounds in the target water. Therefore, the activated carbon of this invention can be suitable for use as a material for removing fluoride-containing organic compounds. Furthermore, the water treatment method of this invention can be suitable for the treatment and purification of industrial wastewater. Because the water treatment method of this invention includes the above-described components, it can obtain treated water from the target water containing fluoride-containing organic compounds with extremely high efficiency. This effect of the present invention is particularly significant when the concentration of fluoride-containing organic compounds in the target water is low.

Claims

1. A water treatment method comprising the step of continuously contacting activated carbon with water containing fluoride organic compounds to obtain treated water with a fluoride organic compound content of 0.1 ppb or less, wherein the activated carbon has an average particle size of 0.1 to 10 mm, a BET specific surface area of ​​500 m² / g or more and 2000 m² / g or less, and a porosity of 10% to 40% as determined by X-ray CT.

2. The water treatment method of claim 1, wherein the fluorinated organic compound is at least one selected from the group consisting of perfluoroheptanoic acid, perfluorooctanoic acid, perfluorooctanoate, perfluorononanoic acid, perfluorodecanoic acid, perfluoroundecanoic acid, perfluorododecanoic acid, perfluorooctane sulfonic acid, and perfluorooctane sulfonate.

3. The water treatment method of claim 1, wherein the activated carbon is continuously contacted with the water containing fluoride organic compounds by flowing the water to be treated through a column filled with activated carbon.

4. The water treatment method according to any one of claims 1 to 3, wherein the iodine adsorption capacity of the activated carbon is more than 900 mg / g and less than 1200 mg / g.

Images