JP7791689B2 - Iron ion eluent and iron ion supply device - Google Patents

Description

The present invention relates to an iron ion eluent and an iron ion supply device that can be placed in water to stably supply high concentrations of iron ions over a long period of time, thereby purifying aquatic environments, such as bottom sediment environments.

Enclosed sea areas such as ports and harbors are prone to eutrophication due to the accumulation of excess organic matter flowing in from coastal cities. The progression of eutrophication not only leads to red tides, but also worsens the bottom sediment environment as excess organic matter and nutrients accumulate and decay on the bottom. As a result, hydrogen sulfide is generated from the bottom sediment, which has become a highly reducing, dark, sludge commonly known as sludge, and has frequently caused serious damage to fishing and living environments.

To address this issue, it has been reported that the generation of hydrogen sulfide can be suppressed by spreading iron powder, iron oxide, or iron hydroxide on the bottom sediment (Non-Patent Documents 1 and 2). However, because the mechanism by which iron or iron compounds suppress hydrogen sulfide generation is primarily due to a chemical reaction between the two, there is a problem in that the spread must be repeated periodically to achieve a sustained effect.

However, because iron hydroxide and iron oxide can easily be generated by oxidizing iron ions eluted in the environment, there is a demand for methods and devices that can continuously supply iron ions to the environment. For example, methods have been proposed that involve the introduction of a sintered body of carbon and metallic iron particles (Patent Document 1), a method in which a bonded body of carbon material and metallic iron is immersed in water as an iron ion eluent (Patent Document 2), and a device that produces iron ion-containing water by passing water through a container containing plate-shaped metallic iron and granular carbon material (Patent Document 3).

Japanese Patent Application Laid-Open No. 2018-053304 JP 2013-183676 A Patent No. 5539579

Inoue, T., Fujiwara, Y., Nakamura, Y., 2017. Suppression of sulfide elution from sediments by spraying iron. Journal of the Japan Society of Marine Science and Technology, 23, 25-30. Kanaya Gen, Kikuchi Eisuke, 2009. Investigation of an experimental method for removing free hydrogen sulfide from bottom sediments in brackish waters by adding iron. Northeast Asian Studies, 13, 17-28.

Using conventional technology, it is difficult to continuously supply high concentrations of iron ions stably over long periods of time. For example, the method described in Patent Document 2 and the device described in Patent Document 3 have issues with the contact area and durability of contact between the carbon material and metallic iron, as well as the need for a separate aeration device to increase the amount of iron ion elution.

Therefore, the object of the present invention is to provide a means that has a simple structure yet can stably elute high concentrations of iron ions for a long period of time.

As a result of extensive research, the present inventors have found that the problems of the prior art can be solved by the following configuration, and have thus completed the present invention.
That is, the iron ion eluent of the present invention is an iron ion eluent for supplying iron ions into water,
at least one carbon-iron composite;
a holding member that holds the carbon-iron composite in a state where it can come into contact with water;
Equipped with
Furthermore, a conductive auxiliary material is provided that is placed in contact with the carbon-iron composite, or the holding member is formed of a conductive material.

In the iron ion eluting material of the present invention, the carbon-iron composite may be a sintered body of iron particles and a carbon material.

In the iron ion eluting body of the present invention, the retaining member may be a mesh-like or cage-like structure that allows liquid to pass through, and the conductive aid may be stored inside the structure.

In the iron ion eluent of the present invention, the retaining member may be formed from a conductive material, and the carbon-iron composite and the retaining member may be in contact with each other.

The iron ion supply device of the present invention comprises any one of the iron ion eluents described above and
a conductive cathode electrically connected to the iron ion eluent;
It is equipped with:

In the iron ion supply device of the present invention, the cathode may be installed at or near the water surface.

The iron ion supply device of the present invention may have an iron ion elution capacity of 2000 mg/kg or more when immersed in a 3% by weight concentration saline solution for 5 days.

The iron ion eluent and iron ion supply device of the present invention have a simple structure yet are capable of eluting high concentrations of iron ions into water stably over a long period of time, making them suitable for a wide range of applications, including environmental purification and wastewater treatment.

1 is a schematic diagram of an iron ion eluting material of the present invention. FIG. 1 is a schematic diagram of a carbon-iron composite used in the iron ion eluent of the present invention. 1 is a schematic diagram of an iron ion supply device of the present invention. FIG. 2 is another schematic diagram of the iron ion supply device of the present invention. FIG. 2 is a schematic diagram of a test device in Example 1. FIG. 10 is a schematic diagram of a test device in Example 2. FIG. 10 is a schematic diagram of a test device in Example 3. FIG. 10 is a schematic diagram of a test device in Example 4. FIG. 10 is a schematic diagram of a test device in Example 7. FIG. 10 is a schematic diagram of a test device in Example 8. FIG. 10 is a schematic diagram of a test device in Example 9. FIG. 1 is a schematic diagram of a test device in Comparative Example 1.

The present invention will be clarified by explaining specific embodiments of the present invention, with reference to the drawings as appropriate. The size and proportions of components in each figure are for convenience of illustration and may differ from the actual size and proportions, and the present invention is not limited by the drawings.

[Iron ion eluate]
FIG. 1 is a diagram illustrating a schematic diagram of an iron ion eluter according to one embodiment of the present invention. The iron ion eluter 1 includes one or more carbon-iron composites 10 and a holding member 20 that holds the carbon-iron composites 10 in a state where they can come into contact with water, and supplies iron ions to the water. The iron ion eluter 1 may include a conductive material 30 that is arranged in contact with the carbon-iron composites 10, or the holding member 20 may be made of a conductive material and hold the carbon-iron composites 10 in contact with the holding member 20. In this embodiment, the iron ion eluter 1 includes one or more granular carbon-iron composites 10, together with the conductive material 30, housed inside a cage-shaped holding member 20 that has an outer shape, such as a cylindrical shape.
The iron-carbon composite 10, the holding member 20, and the optional conductive additive 30 that constitute the iron ion eluting body 1 will be described below in this order.

[Carbon-iron composite]
The carbon-iron composite 10 used in the iron ion eluter 1 of this embodiment is a composite of a carbon material and metallic iron, either directly or using a binder. The carbon-iron composite 10 is a material that can supply iron ions by immersing it in water due to the local cell effect that utilizes the potential difference between carbon and iron.

The shape of the carbon-iron composite 10 is preferably granular or lumpy, since a larger surface area can increase the amount of iron ions generated. The breaking hardness of the carbon-iron composite 10 is preferably 50 N or more, and more preferably 80 N or more. If the breaking hardness is less than 50 N, the particles will come into contact with each other due to shaking caused by the water flow, making the carbon-iron composite 10 more likely to be broken. If the carbon-iron composite 10 is broken by the water flow, the effect of the local battery will be lost due to the separation of iron and carbon, and there is a risk that the elution of divalent iron ions will be reduced.
Furthermore, it is preferable that the carbon-iron composite 10 does not self-disintegrate even when immersed in the sea for one year or more. This is because the carbon-iron composite 10 can be prevented from disintegrating and falling to the seabed due to the rocking motions caused by tides, ocean currents, waves, etc. in the sea. From this perspective, it is even more preferable that the carbon-iron composite 10 has a breaking hardness of 50 N or more after 10 days of immersion in 5 wt % salt water, and maintains at least half of the breaking hardness before immersion in salt water. By maintaining a breaking hardness after 10 days of immersion in 5 wt % salt water at least half of the breaking hardness before immersion in salt water, the effect of the local battery can be maintained for a long period of time.
The breaking hardness is preferably 50N or more, more preferably 80N or more, and most preferably 100N or more, even after immersion in seawater or fresh water for one month.

The iron-carbon composite 10 can be, for example, a composite made by fastening carbon fibers to an iron material; a composite made by granulating iron powder and a carbonaceous material such as charcoal with cement or clay; or a composite made by baking and solidifying iron powder with shochu dregs, organic sludge, starch, or blackstrap molasses. Among these, a preferred composite is a carbon-iron composite 10 made by granulating iron and carbon with a carbon precursor and then baking and sintering the granules (see Patent Document 1). This type of iron-carbon composite 10 is most preferred because it can continuously release iron ions stably over a long period of time, and because the carbon precursor used for granulation becomes hard carbon after sintering, resulting in high granule strength and no leaching of environmentally hazardous substances such as heavy metals and organic compounds.

Figure 2 is a schematic diagram showing the external configuration of a preferred example of a carbon-iron composite 10. As shown in Figure 2, the carbon-iron composite 10 is a porous sintered body containing a plurality of iron particles 11 and a carbonaceous material 13. In the carbon-iron composite 10, the plurality of iron particles 11 are fixed by the carbonaceous material 13. The carbonaceous material 13 is an amorphous solidified material containing 95% or more by weight of conductive carbon. It functions as a structure supporting the iron particles 11 and forms a local battery through contact with the iron particles 11. The carbonaceous material 10 is a porous body with a predetermined bulk density and open porosity, and is formed with a plurality of pores 15. The carbonaceous material 13 may be present in a state where a portion derived from the carbonaceous raw material, such as coke, and a bonding portion derived from an organic material, such as an organic binder, are distinguishable from each other, or the two may be substantially integrated to form the carbonaceous material 13 in a state where they are indistinguishable from each other.

The weight ratio of iron particles 11 to carbonaceous material 13 in the carbon-iron composite 10 (iron particles 11:carbonaceous material 13) can be adjusted depending on the sustainability of ferrous ion elution in water, but is, for example, in the range of 5:95 to 95:5, preferably 20:80 to 80:20, and more preferably 30:70 to 70:30. If the weight ratio of iron particles 11 to carbonaceous material 13 is less than 5 wt%, the amount of carbonaceous material 13 is too high, resulting in a small contact area with water, a low ferrous ion supply capacity, and poor sustainability. On the other hand, if the weight ratio of iron particles 11 to carbonaceous material 13 exceeds 95 wt%, a local battery is formed and sufficient iron ion supply capacity is provided, but the low carbon content makes the integrated composite brittle, leading to iron particles 11 shedding from the surface and the collapse of the carbon-iron composite 10. In addition to iron and carbon, the carbon-iron composite 10 also contains oxygen (10% by weight or less) and trace amounts of other elements (Ni, Mn, etc.), but the weight ratio above simply refers to the ratio of iron element to carbon element. Furthermore, the carbonaceous material 13 includes not only pre-blended carbonaceous raw materials such as coke, but also carbonized carbon formed by firing organic materials such as organic binders.

The carbon-iron composite 10 is preferably a porous sintered body having a bulk density of 1.1 to 4.0 and an open porosity of 20 to 70%. Here, the bulk density is more preferably 1.3 to 3.5. Furthermore, the open porosity is more preferably 30 to 60%. An open porosity of less than 20% results in a small amount of leaching of divalent iron ions, while an open porosity of more than 70% is undesirable because it reduces the strength of the material and makes it prone to collapse.

Furthermore, the carbon-iron composite 10 preferably has an amount of leaching of divalent iron ions of 2 ppm or more after 10 days of immersion in saltwater with a concentration of 3% by weight or more, preferably 3 to 5% by weight, and more preferably 5 ppm or more, and even more preferably 10 ppm or more.

Furthermore, it is preferable that the carbon-iron composite 10 has a weight loss rate of 3% or less at temperatures from room temperature to 500°C in thermogravimetric analysis in an inert atmosphere. A weight loss rate of 3% or less at temperatures from room temperature to 500°C indicates that the organic binder and coke powder are completely carbonized. As a result, the carbon-iron composite 10 is less likely to disintegrate when placed in water, and no organic compounds harmful to the environment are eluted from the carbon-iron composite 10, so no additional environmental burden is created.

The carbon-iron composite 10 dissolves divalent iron ions into water due to the formation of a local battery when zero-valent metallic iron comes into contact with carbon. Therefore, even if the iron particles 11 are in the form of iron oxide at the raw iron stage, they will be metallic iron in the final product after firing. However, it is preferable to use a steel material containing iron (Fe) as the main component and at least 0.5% by weight of at least one of carbon (C), manganese (Mn), and nickel (Ni). Examples of such iron particles 11 include, but are not limited to, cast iron, carbon steel, and stainless steel.

The iron particles 11 that make up the carbon-iron composite 10 gradually become smaller as they release divalent iron ions into the seawater due to a local battery effect with the sintered carbon. Therefore, the particle size of the iron particles 11 used is preferably 200 to 5 mesh according to the JIS standard. Note that, since the smaller the mesh number in the JIS standard, the larger the particle size, the term "5 mesh or less" does not include, for example, "4 mesh." Particle sizes smaller than 200 mesh are too small, resulting in a short contact period with water and the risk of fire, dust explosions, and the like during production. Furthermore, particle sizes larger than 5 mesh are too large, making mixing, kneading, and granulation difficult. The shape of the iron particles 11 may be, for example, spherical or granular, or may be an irregular block. Note that, for ease of explanation, the iron particles 11 are depicted in Figure 2 as regular hexagonal polyhedrons in plan view, but this is not limiting.

The carbonaceous material 13 that constitutes the carbon-iron composite 10 is necessary to form a local battery with the iron, and contact with the iron is extremely important. Examples of usable raw materials (carbonaceous materials) for the carbonaceous material 13 used to form the local battery include coke, charcoal, coal powder, graphite, coal tar pitch, and carbides of organic compounds and polymeric materials. These materials can be used alone or in combination of two or more. The shape of the carbonaceous material is not critical, but powdery or lumpy forms are preferred, as they increase the number of contact points with the iron particles 11 after sintering and facilitate local battery function. Irregular shapes are also acceptable. It is preferable that at least 50% by weight of the carbonaceous material blended with the iron raw material be a solid carbonaceous material that melts at high temperatures and does not exhibit fluidity. Examples of such solid carbonaceous materials include graphite and coke powder. Pitch coke powder, which has a temperature history of 450°C or higher and is electrically conductive, is particularly preferable.

Pitch coke powder, which has a temperature history of 450°C or higher, does not melt and flow at high temperatures like coal tar pitch or polymeric materials. Therefore, it is easy to maintain its granulated shape and to obtain a porous carbon-iron composite 10. The particle size of the carbonaceous raw material, typified by coke powder, is preferably 300 to 5 mesh according to the JIS standard, for example, in order to increase the number of contact points with the iron particles 11 after sintering, thereby improving the efficiency of the local battery function, and to improve granulation properties. If the particle size is larger than 5 mesh, the number of contact points with the iron particles 11 to form the local battery decreases, resulting in a decrease in leaching efficiency. On the other hand, if the particle size is smaller than 300 mesh, the bulk density becomes too small, which not only deteriorates mixing and granulation properties, but also poses the risk of fire, dust explosions, etc.

The coke powder can be either petroleum-based or coal-based heavy oil. Of these, coke derived from coal-based heavy oil is preferred because it is mesophase-rich and easily becomes needle coke, making it highly conductive. As a result, it is easy for current to flow as a local battery and easy for iron ions to be generated.

The carbon-iron composite 10 shown in FIG. 2 is a sintered body of iron and non-organic, electrically conductive carbon. It is preferable to use an organic binder that carbonizes at high temperatures during the manufacturing process. The use of an organic binder promotes the aggregation of powdered and granular raw materials, increasing the granulation rate and improving yield. Furthermore, carbonization of the organic binder during sintering improves the physical properties (strength, surface condition, disintegration resistance, etc.) of the carbon-iron composite 10 and strengthens the adhesion between the iron particles 11 and the carbonaceous material 13. From this perspective, preferred organic binders include pitch, phenolic resin, lignin, or lignin sulfonate, which contains 20% or more by weight of fixed carbon and a large amount of aromatic rings. Among these, coal tar pitch, which has excellent fixed carbon and binding strength, is the most preferred.

When coal tar pitch is calcined at 500°C or higher in an inert or reducing atmosphere, hydrogen, oxygen, nitrogen, sulfur, and other components other than fixed carbon are decomposed and volatilized, resulting in substantially 95% or more of the calcined product being carbon. Furthermore, coal tar pitch forms voids as hydrogen, oxygen, nitrogen, sulfur, and other components are released during calcination, increasing the contact area between water and iron and contributing to efficient iron ion generation. Furthermore, coal tar pitch becomes a strong, electrically conductive carbide, making it a good binder for fixing iron particles 11 and carbonaceous material 13 together.
Among coal tar pitches, those with a fixed carbon content of 50% by weight or more are preferred in terms of maintaining the shape during firing. Examples of such coal tar pitches include BP and IP (both product names) manufactured by C-Chem Co., Ltd.

The organic binder should be mixed in an amount ranging from 5 to 20 parts by weight per 100 parts by weight of the mixture of iron raw material and carbonaceous raw material. If the organic binder is used in an amount less than 5 parts by weight, it will not be effective as a binder, and if it exceeds 20 parts by weight, the organic binder will melt during firing, making it impossible to obtain the desired shape, suitable bulk density, or open porosity. Furthermore, if a composite is formed using only iron raw material and an organic binder such as coal tar pitch, the organic binder will melt during firing, making it impossible to maintain the shape of the composite.

The organic binder, typically coal tar pitch, is preferably in powder form so that it can be mixed uniformly with the iron raw material and the carbonaceous raw material. The particle size of this powder is preferably 200 to 32 mesh, for example. If the particle size of the organic binder is too small, the bulk density will be too low, which will deteriorate the mixing and kneading properties. If the particle size is too large, there is a possibility that the mixing, heat melting, and the interior of the granulated product will be non-uniform.
Furthermore, it is preferable that the coal tar pitch has a softening point within a temperature range of, for example, 30 to 150°C. The use of coal tar pitch with such a softening point is very convenient for granulation methods that rely on intermolecular forces, such as briquetting machines that mold (granulate) a mixture while heating, melt granulation, and dry granulation. After granulation by these methods, the mixture can be fired as is, allowing for efficient production of the carbon-iron composite 10.

In addition to coal tar pitch and phenolic resin, a granulation aid may be added to the organic binder to improve granulation properties. The granulation aid is not particularly limited as long as it becomes carbonaceous material 13 upon sintering. Examples of suitable granulation aids include gelatin, starch paste, blackstrap molasses, lignin sulfonate, konjac flour, sodium alginate, polyvinyl alcohol, dextrin, ethyl cellulose, carboxymethyl cellulose, and polyacrylamide. When a granulation aid is used, the weight ratio of the organic binder to the granulation aid (organic binder:granulation aid) is preferably 100:0 to 30:70, for example. By adjusting the granulation aid ratio within this range, it is possible to easily produce a carbon-iron composite 10 of the desired shape without adversely affecting the bulk density, open porosity, fracture hardness, and other properties upon sintering.

In addition to iron and carbon, the carbon-iron composite 10 may further contain mineral-based inorganic substances containing elements such as silicon, aluminum, magnesium, calcium, phosphorus, sodium, and potassium, as long as such addition does not interfere with physical properties such as fracture hardness, bulk density, and open porosity, or with the elution of divalent iron ions due to the local cell effect.

The carbon-iron composite 10 can be produced by blending an organic binder with a mixture of iron raw materials and carbonaceous raw materials, granulating the mixture into the desired shape as needed, and then firing and sintering the mixture at a temperature of 500°C or higher in an inert or reducing atmosphere.

The order in which the iron raw material, carbonaceous raw material, and organic binder are mixed is not particularly limited; a mixture of the iron raw material and carbonaceous raw material may be prepared first, and then the organic binder may be mixed, or all of the raw materials may be mixed at once. The same applies when other additives are mixed. General mixing and kneading equipment such as various blenders, mixers, and kneaders can be used for the mixing method.

The mixture containing the various raw materials is granulated into any shape as needed. The granule shape is not particularly limited, and can be, for example, spherical, spheroidal, cylindrical, irregular, etc. Among these, spherical or spheroidal shapes are preferred because they increase the contact area with seawater, etc. The size of the granules is not particularly limited, but in the case of a spherical shape, a diameter of 5 mm or more is preferred, and preferably a diameter of about 5 to 100 mm. In addition, if the shape of the granules is other than spherical, it is preferable to size them so that they have a volume similar to that of a sphere with a diameter of 5 to 100 mm.
Although granulation can be carried out manually, it is preferable to use a granulator such as a pelletizer or briquette machine in terms of workability, safety, shape control, etc.

If water or an organic solvent is used during granulation, the granulated raw material mixture is dried at 60°C or higher and then fired and sintered in an inert or reducing atmosphere at 500°C or higher. Firing can be performed using equipment such as, but not limited to, a lead hammer furnace, a top-charge furnace, a shuttle furnace, a tunnel furnace, a rotary kiln, a roller hearth kiln, or a microwave. The calcination process can be performed either continuously or batchwise. The firing temperature is preferably 700°C or higher, more preferably 900°C or higher, and most preferably 1000°C or higher. Firing at 500°C or higher in an inert or reducing atmosphere ensures carbonization of the organic binder and reduces the iron oxide contained in the iron raw material. The carbon-iron composite 10 obtained by firing can be used as an environmentally friendly iron ion source that exhibits rapid leaching of divalent iron ions and high fracture hardness. Firing may be carried out multiple times, or the sintered body may be immersed in an aqueous solution of a compound such as iron or manganese and then fired again.

After the sintering process, the carbon-iron composite 10 is then slowly cooled in an inert or reducing atmosphere, or slowly cooled and then allowed to cool in the air to a temperature at which it can be handled. Then, it is stored in the internal space of the holding member 20 and used as the iron ion eluting body 1.

[Holding member]
The holding member 20 has a space therein that can accommodate at least one carbon-iron composite 10 and is configured to allow liquid to pass between its interior and exterior. The outer shape of the holding member 20 may be, for example, a rectangular parallelepiped, a cube, a cylinder, or the like. The holding member 20 may also be in the shape of a basket, a bag, a net, a dish, or the like.

The material of the holding member 20 is not particularly limited as long as it does not easily corrode or decompose in water. For example, natural fibers, synthetic fibers, metals, inorganic fibers, etc. are acceptable. However, it is preferably made of a conductive material, and a material that is a good conductor is more preferable. If the material making up the holding member 20 is an electrical conductor, bringing the multiple carbon-iron composites 10 housed inside into contact with the holding member 20 allows electrons generated by the local battery effect in the carbon-iron composites 10 to be efficiently exchanged via the holding member 20, thereby increasing the amount of iron ion elution. Therefore, the material making up the holding member 20 is preferably metal or carbon, and more preferably carbon fiber, stainless steel, or titanium. The conductive material making up the holding member 20 may be used alone or in combination of two or more types.

It is preferable that the holding member 20 allows the carbon-iron composite 10 stored therein to be removed. Over long-term use, flocs such as iron hydroxide generated from the carbon-iron composite 10 can accumulate, causing clogging, or microorganisms can attach and grow, forming a biofilm on the surface and reducing the amount of iron ions eluted. By designing the holding member 20 so that the carbon-iron composite 10 can be removed, it becomes easier to reuse the holding member 20 and replace, clean, and recycle the carbon-iron composite 10, thereby reducing operating costs.

It is preferable that multiple carbon-iron composites 10 are stored in the internal space of the holding member 20 in order to increase the amount of iron ions eluted, and it is desirable that in addition to the carbon-iron composites 10, a conductive additive 30 made of a conductive material is also stored.

[Conductive additive]
The conductive additive 30 has the function of promoting the external migration of electrons generated in the carbon-iron composite 10. When the iron ion eluting body 1 is placed in water, a local battery is formed at the contact interface between the carbon, iron, and water in the carbon-iron composite 10 due to the potential difference between carbon and iron, and divalent iron ions (Fe ²⁺ ) are eluted. At this time, the electrons generated as the iron elution (ionization) occurs are ultimately consumed at another location in the carbon-iron composite 10. However, if electrons remain in the carbon-iron composite 10, they will hinder the elution of iron ions, so it is preferable to migrate the electrons out of the system using the conductive additive 30. The use of the conductive additive 30 can increase the amount of iron eluted from the carbon-iron composite 10.
Furthermore, when the carbon-iron composite 10 is used alone, iron hydroxide formed by bonding between hydroxyl groups generated on the carbon on its surface and iron ions tends to accumulate on the surface of the carbon-iron composite 10, leading to a decrease in the iron ion generation capacity. Therefore, it is possible to avoid the accumulation of iron hydroxide on the surface of the carbon-iron composite 10 by mixing the carbon-iron composite 10 with an appropriate amount of conductive additive 30.

The material of the conductive additive 30 is not particularly limited as long as it is electrically conductive and does not corrode or dissolve in water, and examples of materials that can be used include metals more noble than stainless steel or iron, carbon materials, etc. Because the iron ion eluate 1 is held in water, and for the purpose of having a low specific gravity and quickly transferring electrons out of the system, carbon materials such as charcoal, bamboo charcoal, natural graphite, artificial graphite, steelmaking coke, needle coke, and molded bodies made from these are more preferred, natural graphite, artificial graphite, steelmaking coke, needle coke, and molded bodies made from these are even more preferred, and needle coke, which is a highly electrically conductive carbon material with a volume resistivity of 1 × 10 ^-2 Ω cm or less as measured by the four-terminal method, and molded bodies made from these are most preferred.

The conductive additive 30 is not particularly limited in shape, and can be any shape selected from spheres, ellipsoids, cubes, prisms, cylinders, polyhedrons, irregularly shaped granules or lumps, plates or sheets such as woven fabric, felt, and mesh, cotton-like materials made of short fibers, long fibers, or continuous fibers, etc. However, it is best to select and use one that will have a large contact area with the carbon-iron composite 10 and the conductive holding member 20.

The iron ion eluting body 1 having the above-mentioned configuration can supply iron ions to the surrounding area by simply placing it in water. There are no particular restrictions on where it can be placed in water; it can be on the bottom, underwater, or on the surface of the water, but it is preferable to place it so that at least a portion of the iron ion eluting body 1 is exposed to the atmosphere, or at or near the surface of the water where the oxygen concentration is high, as this increases the amount of iron ions generated. It is also possible to pour seawater or the like directly onto the iron ion eluting body 1, or to place it in a container such as a tank and pass water through it, thereby incorporating iron ions into the outflow water.

Preferred methods for installing the iron ion eluting body 1 on or near the water surface, such as the ocean surface, include suspending it from a quay, a floating body such as a buoy, or a stake fixed to the water bottom, or fixing it to the bottom of a ship or a fish pen used for cultivating fish, shellfish, seaweed, etc. These installation methods can involve mooring it with a long member such as a rope or chain, or fixing it using a detachable adapter. When fixing the iron ion eluting body 1, it may be set in a state where it can sway with the water current. Alternatively, it may be moored so that one end is fixed to the water bottom and the other end is fixed to a float, using a rope or the like, so that the iron ion eluting body 1 floats in the water.

[Iron ion supply device]
3 is a schematic diagram of an iron ion supplying device according to one embodiment of the present invention, which uses an iron ion eluting body 1. The iron ion supplying device 100 includes the iron ion eluting body 1, a cathode 101 electrically connected to the iron ion eluting body 1, and a conductor 102 electrically connecting the iron ion eluting body 1 and the cathode 101. Note that an actual device only needs to satisfy the concept of this diagram, and the positions and sizes of the cathode 101 and the iron ion eluting body 1 are not limited to those shown in the diagram.

In the iron ion supply device 100 according to this embodiment, electrons (e − ) generated when iron ions (Fe ²⁺ ) are eluted from the carbon-iron composite 10 of the iron ion eluting body 1 immersed in water, such as seawater 110, are collected by the conductive additive 30 and the holding member 20 and conducted via the conductor 102 to the cathode 101 installed on the water surface 110a, where they are consumed outside the carbon-iron composite 10. The carbon ^- iron composite 10 generates electrons when iron elutes into the water as divalent ions, as shown in the following formula (1), due to a local battery formed between carbon and iron. However, the generated electrons are thought to inhibit the elution of iron ions from the carbon-iron composite 10 unless consumed by the reaction shown in the following formula (2) at another location in the carbon-iron composite 10. Therefore, by separately providing the cathode 101 on the water surface 110a, which is rich in dissolved oxygen, the electrons can be consumed, promoting the elution of iron ions from the carbon-iron composite 10 (i.e., increasing the amount of eluted iron ions).
Fe→Fe ²⁺ +2e ^- (1)
_H2O +1/2O2+ _2e ^- →4OH ^- (2)

The iron ion elution ability of the iron ion supplying device 100 is preferably evaluated not by the concentration of eluted iron ions but by the amount of eluted iron ions per unit weight (1 kg) of the carbon-iron composite 10 used in the iron ion supplying device 100 when immersed in a 3 wt % saline solution for five days. This is because the amount of eluted iron ions is significantly affected by the iron/carbon ratio of the carbon-iron composite 10, as well as the number and size (= surface area) of the carbon-iron composite 10. If the amount of eluted iron ions per unit weight of the carbon-iron composite 10 is large, the amount of carbon-iron composite 10 required to elute a desired concentration of iron ions into the environment is reduced, allowing for a lightweight and compact device, which is particularly advantageous in the case of a floating iron ion supplying device (see, for example, FIG. 4 ).
The iron ion elution ability when immersed in a 3 wt % saline solution for 5 days is such that the amount of iron ions generated per unit weight of the carbon-iron composite 10 is preferably 2000 mg/kg or more, more preferably 2500 mg/kg or more, and even more preferably 3000 mg/kg or more.

The material of the cathode 101 is not particularly limited, and examples include conductive materials such as metal materials and carbon materials. However, materials with high electrical conductivity and a higher standard electrode potential than iron are preferred. Examples of such metal materials include stainless steel, titanium, copper, and platinum, while examples of carbon materials include coke, graphite, porous carbon, carbon fiber, carbon cloth, carbon mat, carbon felt, and carbon paper. It is more preferable to use a carbon material with a high specific surface area and that is resistant to corrosion even in seawater 110 as the cathode 101, with a porous carbon material being most preferred.

The shape of the cathode 101 is not particularly limited, and examples include sheet, plate, rod, string, mesh, lattice, bellows, block, and porous shapes. These shapes may also be uneven or perforated, or may be bent or curved. Furthermore, the cathode 101 may be made by assembling granular materials, and a catalyst portion for promoting the electrode reaction may be disposed on the surface of the cathode 101.

The cathode 101 is preferably provided outside the iron ion eluter 1, and may be located either separately from or adjacent to the iron ion eluter 1. In Figure 3, the iron ion supply device 100 has the iron ion eluter 1 located below the cathode 101, and is completely submerged in water. Meanwhile, the cathode 101 is located at or near the water surface 110a, but the iron ion eluter 1 may also be located at or near the water surface 110a, like the cathode 101.

The iron ion eluting body 1 is electrically connected to the cathode 101 by a conductor 102. The conductor 102 transports electrons generated when iron ions are eluted from the carbon-iron composite 10 to the cathode 101. The conductor 102 is preferably made of stainless steel, titanium wire, or carbon fiber, which have excellent corrosion resistance, and more preferably titanium wire from the viewpoint of corrosion resistance and durability against vibration. The conductor 102 is preferably coated with an insulating material such as synthetic resin.
The connection between the iron ion eluter 1 and the cathode 101 by the conductor 102 may be such that the carbon-iron composite 10 is directly connected to the cathode 101, or such that a current collector (not shown) is provided and the cathode 101 is connected via the current collector. The conductive material 30 may be connected to the cathode 101 by the conductor 102, and when the holding member 20 of the iron ion eluter 1 is made of a good electrical conductor, the holding member 20 may be connected to the cathode 101 by the conductor 102.

Figure 4 is a conceptual diagram of another example of an iron ion supply device using an iron ion eluting body 1. Note that an actual device only needs to satisfy the concept of this diagram, and the position and size of the cathode 101 and iron ion eluting body 1 are not limited to those shown in the diagram.

The iron ion supply device 200 shown in Figure 4 comprises an iron ion eluting body 1, a cathode 101 electrically connected to the iron ion eluting body 1, and a float 103 that provides buoyancy to these. The material and shape of the cathode 101 are the same as those shown in Figure 3. There are no particular restrictions on the float 103, as long as it is made of a material and has a structure that provides buoyancy in water, and a buoy, float, or the like can be used. The iron ion supply device 200 is designed to float on the water surface 110a due to the buoyancy of the float 103.

In the iron ion supply device 200, an iron ion eluter 1 having an electrically conductive holding member 20 is located directly below a cathode 101, and the entire iron ion eluter 1 is completely submerged in water, such as seawater 110. Meanwhile, the cathode 101 is located directly above the iron ion eluter 1, with a portion of it exposed to the atmosphere. The iron ion eluter 1 and cathode 101 are electrically connected. In the iron ion supply device 200 shown in FIG. 4, electrons generated when iron ions are eluted from the carbon-iron composite 10 are transferred to the cathode 101 via the holding member 20 or the conductive material 30. Although not shown, the iron ion eluter 1 and cathode 101 may be electrically connected using a conductor. Other configurations and effects of the iron ion supply device 200 shown in FIG. 4 are similar to those of the iron ion supply device 100 shown in FIG. 3.

Another example of an iron ion supply device using the iron ion eluter 1 is a wire-free device configuration in which the iron ion eluter 1 and the cathode 101 are not connected by the conductor 102 but are arranged close to each other (within approximately 0.5 m, preferably within 0.1 m) and spaced apart from each other.
In this case, electrons generated when iron ions are eluted from the carbon-iron composite 10 reach the cathode 101 through the water, so the transmission of electrons generated from the iron ion eluting body 1 to the cathode 101 is lower than in a wired type (e.g., the device shown in Fig. 3). Therefore, it is preferable to use a cathode 101 with a large surface area, such as a granular porous carbon electrode housed in a housing member made of a conductive material, and to place it above the iron ion eluting body 1 or to the side in consideration of the water flow. The housing member used here can have a configuration similar to that of the holding member 20.
In the case of a wired iron ion supplying device, the iron ion eluting body 1 and the cathode 101 are arranged at a distance from each other, and therefore are "electrically unconnected" when not immersed in water. However, in actual use when immersed in seawater or other water, they are "electrically connected" using water as a medium, and therefore even the wired iron ion supplying device is provided with an "electrically connected cathode 101."

As described above in detail, the iron ion eluting body 1 and the iron ion supplying devices 100, 200 can be installed in seawater 110, freshwater, or other water and can be widely used in environmental improvement fields, such as suppressing the generation of hydrogen sulfide from bottom sediments in closed bodies of water such as harbors and lakes, and purifying sludge by activating microorganisms. Furthermore, because the configuration of the iron ion eluting body 1 and the iron ion supplying devices 100, 200 can be flexibly adjusted, they can also be applied to small bodies of water such as park ponds and moats.

The following examples describe the results of experiments conducted to demonstrate the present invention, but do not limit the present invention in any way.

[Bulk density]
Measurement was performed by the Archimedes method.

[Open porosity]
The dry weight of the sample, the weight in water when immersed in pure water, and the weight saturated with water when evacuated and saturated with water were measured, and the open porosity was calculated using the obtained weights according to the following formula.
Open porosity (%) = [(saturated weight - dry weight) / (saturated weight - underwater weight)] x 100

[Breaking hardness]
The load at which the granules collapsed (buckled) was taken as the crushing load (breaking hardness). A Kiya-type hardness tester 1600-C (maximum 200 N) manufactured by Fujiwara Seisakusho was used to measure the load, and a compressive load was applied to the sample. The maximum load was taken as the crushing load, and the average value of five granules was used.

[Confirmation of Ni and Cr elution]
In the examples and comparative examples in which a stainless steel mesh basket (SUS304) was used, the iron-carbon composite 10 was removed after the test, and the seawater 110 containing iron hydroxide was heated and dissolved in 10% nitric acid. The concentrations of nickel and chromium ions were measured by ICP atomic emission spectrometry simultaneously with the measurement of iron ions.

[Iron ion elution amount and elution capacity]
The iron ion eluting body 1 (or iron ion supplying device 100) of each of the examples and comparative examples was immersed in seawater 110 (salinity: 3 wt%) and allowed to stand. After immersion for five days, iron hydroxide generated in the seawater 110 was completely dissolved by adding 10 wt% nitric acid. The iron ion concentration in the seawater 110 after immersion was measured using a Pack Test (Kyoritsu Chemical Research Total Iron WAK-Fe) and used as the amount of eluted iron. The iron ion elution capacity was calculated by dividing the iron ion concentration by the weight (unit: kg) of the iron ion generating source (iron plate, iron ball, or carbon-iron composite 10) used in the iron ion eluting body 1 (or iron ion supplying device 100). Note that the stainless steel cage was not considered as the iron ion generating source when calculating the iron ion elution capacity.
During the measurement, the cathode 101 was positioned near the water surface as shown in FIG. 3, and the iron ion eluting body 1 functioning as the anode was placed so that its entirety was completely immersed in seawater 110.

[Carbon-iron composite]
A carbon-iron composite 10 having a configuration similar to that shown in Figure 2 was used. Specifically, cast iron powder (Takeuchi Industries Co., Ltd., 28 mesh or under, carbon: 2-4 wt%, Si: 4 wt% or less, Mn: 0.5-1.5 wt%, P: 0.03 wt% or less, S: 0.03 wt% or less) and needle coke powder (Nippon Steel Chemical & Material Co., Ltd., 9 mesh or under) were granulated in a briquette machine (pocket: 18 x 14 x 3.3 mm deep) using binder pitch (Nippon Steel Chemical & Material Co., Ltd., softening point: 85°C, fixed carbon content: 58%) and starch (Asada Flour Milling Co., Ltd., rye flour gelatinized product, residual carbon content: 10%). The granulated powder was then fired and sintered at 800°C in a non-oxidizing atmosphere to produce the carbon-iron composite 10. Table 1 shows the details of the carbon-iron composite 10 used. The apparent surface area per particle of the carbon-iron composite 10 was 5.1 ^cm2 based on the pocket size of the briquette machine, the weight was 2.1 g, the bulk density was 2.02, and the open porosity was 30%.

[Porous carbon material A]
Pitch coke with a true density of 1.82 g/ ^cm3 was pulverized and adjusted to a particle size blend of 25% 0.250-0.500 mm, 45% 0.075-0.249 mm, and 30% 0.074 mm or less. 40 parts by weight of binder pitch (manufactured by C-Chem, softening point: 97°C) obtained from coal-based heavy oil was added to 100 parts by weight of the pitch coke particles, and the mixture was heated and kneaded at 200°C for 20 minutes. This kneaded mixture was extruded into a 20 mm diameter rod. After molding, it was fired in a non-oxidizing atmosphere at 900°C to obtain a sintered product (carbon compact). This carbon compact was processed into a 20 mm diameter x 48 mm diameter sample to obtain a porous carbon material A with a bulk density of 1.40 and a water absorption rate of 8.9%.
The water absorption rate was measured by immersing a porous carbon material A whose weight had been measured in advance in pure water at room temperature, taking it out after 24 hours, and leaving it to stand at room temperature for 3 minutes. When there were no more drops of water and the water had sufficiently drained, the weight M2 was measured, and the water absorption rate was calculated as the ratio of the increased weight (M2-M1) to the weight M1 before immersion using the following formula.
Water absorption rate [%] = [(M2-M1)/M1] x 100

[Porous carbon material B]
Needle coke powder (manufactured by C-Chem, 9 mesh under) was granulated in a briquette machine (pocket: 18 x 14 x depth 3.3 mm) using binder pitch (manufactured by C-Chem, softening point: 90°C) and starch (pregelatinized rye flour, manufactured by Asada Flour Milling Co., Ltd.), and the granules were fired and sintered at 900°C in a non-oxidizing atmosphere to produce porous carbon material B. The apparent surface area per particle of porous carbon material B was 5.0 ^cm2 and the weight was 1.8 g. The bulk density was 1.27 and the open porosity was 39%.

[Example 1]
A cylindrical stainless steel mesh basket (material: SUS304, weight: 7.0 g) was prepared as the holding member 20, and one carbon-iron composite 10 was placed inside to form the iron ion eluting material 1. Next, a round screw-cap bottle 104 with an inner diameter of the bottom of 55 mm and a capacity of 220 ml was prepared, and the iron ion eluting material 1 was suspended by a nylon thread 105 using a hole (diameter: 5 mm) drilled in the lid, and 150 ml of natural seawater 110 (salinity: 3 wt %; collected at Hiramatsu Fishing Port in Fukuoka Prefecture; the same applies hereinafter) was poured into the bottle until the holding member 20 was completely submerged.
The round screw-cap bottle 104 was placed in a thermostatic water bath (not shown), set to a temperature of 27°C, and left to stand for 5 days, after which the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 1 is shown in FIG. 5, and the evaluation results are shown in Table 2.

[Example 2]
A nylon mesh bag (weight: 0.7 g) was prepared as the holding member 20, and one carbon-iron composite 10 and one porous carbon material B as the conductive additive 30 were placed inside to prepare an iron ion eluting body 1. The porous carbon material B was placed on top of the carbon-iron composite 10 in contact with it.
Next, a round screw-cap bottle 104 with an inner diameter of the bottom of 55 mm and a capacity of 220 ml was prepared, and the iron ion eluting body 1 was suspended by a nylon thread 105 using a hole (diameter: 5 mm) opened in the lid, and 150 ml of natural seawater 110 was poured into it until the holding member 20 was completely immersed.
The round screw-cap bottle 104 was placed in a thermostatic water bath (not shown), set to a temperature of 27°C, and left to stand for 5 days, after which the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 2 is shown in FIG. 6, and the evaluation results are shown in Table 2.

[Example 3]
The amount of iron ions eluted into the seawater 110 in the bottle was measured in the same manner as in Example 2, except that needle coke blocks (LPC-U manufactured by C-Chem, 4.0 g/piece) were used as the conductive additive 30 housed together with the carbon-iron composite 10 in the iron ion eluting body 1. An outline of the test apparatus is shown in Figure 7, and the evaluation results are shown in Table 2.

[Example 4]
A nylon mesh bag (weight 0.7 g) was prepared as the holding member 20, and one carbon-iron composite 10 and carbon fiber (12K, 0.16 g, manufactured by Nippon Graphite Fiber Co., Ltd.) without sizing as the conductive additive 30 were placed inside to prepare the iron ion eluting material 1. The carbon-iron composite 10 was placed on top of the carbon fiber in contact with it.
Next, a round screw-cap bottle 104 with an inner diameter of the bottom of 55 mm and a capacity of 220 ml was prepared, and the iron ion eluting body 1 was suspended by a nylon thread 105 using a hole (diameter: 5 mm) opened in the lid, and 150 ml of natural seawater 110 was poured into it until the holding member 20 was completely immersed.
The round screw-cap bottle 104 was placed in a thermostatic water bath (not shown), set at a temperature of 27° C., and left to stand for 5 days, and the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 4 is shown in FIG. 8, and the evaluation results are shown in Table 2.

[Example 5]
The amount of iron ions eluted into the seawater 110 in the bottle was measured in the same manner as in Example 2, except that the number of carbon-iron composites 10 housed in the iron ion eluting body 1 was three and the number of porous carbon materials B serving as the conductive additives 30 was nine. The evaluation results are shown in Table 2.

[Example 6]
Although not shown in the figure, a nylon mesh bag (weight 0.7 g) was prepared as the holding member 20, and one carbon-iron composite 10 and carbon fiber (12K manufactured by Nippon Graphite Fiber Co., Ltd.) without sizing material as the conductive additive 30 were placed inside to prepare the iron ion eluting body 1. The carbon-iron composite 10 was placed on the soaked carbon fiber in contact with the carbon fiber.
Next, a round screw-cap bottle 104 with a capacity of 220 ml and an inner diameter of the bottom of 55 mm was prepared, and the iron ion eluting material 1 was hung by a nylon thread 105 using a hole (diameter: 5 mm) drilled in the lid. 150 ml of natural seawater 110 was poured into the bottle until the holding member 20 was completely immersed. At this time, one-third of the total length of the carbon fibers was immersed in the seawater 110, and the remaining two-thirds were bundled together to prevent the fibers from breaking and placed near the water surface 110 a just above the holding member 20 as the cathode 101.
The round screw-cap bottle 104 was placed in a thermostatic water bath set at 27° C. and left to stand for 5 days, and the amount of iron ions eluted into the seawater 110 in the bottle was measured. The evaluation results are shown in Table 2.

[Example 7]
A cylindrical stainless steel mesh basket was prepared as the holding member 20 , and one carbon-iron composite 10 and one porous carbon material B as the conductive additive 30 were placed inside to prepare an iron ion eluting body 1 .
Next, the porous carbon material A was prepared as the cathode 101 , and the cathode 101 and the holding member 20 were electrically connected using a titanium wire 102 to form the iron ion supply device 100 .
A 2000 ml plastic bottle 106 was filled with 1500 ml of natural seawater 110, and placed in a thermostatic water bath (not shown). The bottle was suspended from a hole (5 mm diameter) in the lid so that the iron ion eluting body 1 was located at the bottom and the porous carbon material A was located on the water surface 110 a. The thermostatic water bath was set to a temperature of 27° C. and left to stand for 5 days, and the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 7 is shown in FIG. 9, and the evaluation results are shown in Table 2.

[Example 8]
A cylindrical stainless steel mesh basket with an openable lid was prepared as the holding member 20, and one carbon-iron composite 10 and two porous carbon materials B as the conductive additives 30 were placed inside to prepare an iron ion eluting body 1.
Next, the lid of the holding member 20 was closed, and five pieces of the porous carbon material B (total of 9 g) were stacked on top of it so as to be as flat as possible, and placed as a cathode 101 to prepare an iron ion supplying device 100.
A 2000 ml plastic bottle 106 was filled with 1500 ml of natural seawater 110 and placed in a thermostatic water bath (not shown). The bottle was hung by a nylon thread 105 through a hole (5 mm diameter) drilled in the lid of the plastic bottle 106 so that the porous carbon material B placed on the lid of the holding member 20 was exposed above the water surface 110a at a height of about 1 mm, and the iron ion eluting body 1 was completely immersed in the seawater 110. The thermostatic water bath was set to a temperature of 27°C and left to stand for 5 days, and the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 8 is shown in FIG. 10, and the evaluation results are shown in Table 2.

[Example 9]
A cylindrical stainless steel mesh basket was prepared as the holding member 20 , and one carbon-iron composite 10 was placed inside to form the iron ion eluting material 1 .
Next, five pieces of porous carbon material B (9 g in total) were placed in a cylindrical stainless steel mesh basket to prepare a cathode 101. A 2000 ml plastic bottle 106 was filled with 1500 ml of natural seawater 110 and placed in a thermostatic water bath (not shown). An iron ion eluting body 1 was placed on the bottom of the plastic bottle 106 so that it was completely immersed in the seawater 110. The porous carbon material B of the cathode 101 was positioned 5 cm above the iron ion eluting body 1 by suspending it with a nylon thread 105 via a hole (5 mm diameter) opened in the lid, thus forming a wire-free iron ion supply device 100. The thermostatic water bath was set to a temperature of 27°C and allowed to stand for 5 days, and the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test apparatus in Example 9 is shown in FIG. 11, and the evaluation results are shown in Table 2.

[Comparative Example 1]
A nylon mesh bag was prepared as the holding member 20, and one carbon-iron composite 10 was placed inside to form the iron ion eluting material 1.
Next, a round screw-cap bottle 104 with an inner diameter of the bottom of 55 mm and a capacity of 220 ml was prepared, and the iron ion eluting body 1 was suspended by a nylon thread 105 using a hole (diameter: 5 mm) opened in the lid, and 150 ml of natural seawater 110 was poured into it until the holding member 20 was completely immersed.
The round screw-cap bottle 104 was placed in a thermostatic water bath (not shown), set at a temperature of 27° C., and left to stand for 5 days, after which the amount of iron ions eluted into the seawater 110 in the bottle was measured.
The outline of the test device in Comparative Example 1 is shown in FIG. 12, and the evaluation results are shown in Table 2.

[Comparative Example 2]
Although not shown in the figures, a carbon-iron composite 10 was prepared by stacking a carbon fiber reinforced plastic material (CFRP, matrix resin: phenoxy resin Vf 56%, 40×10×1 mm, weight 0.61 g) on a plate-shaped metal member (material: SS400, 40×10×2 mm, 6.2 g) with their faces facing each other and tying them together with nylon thread 105. The iron ion eluting body 1 was used as the carbon-iron composite 10, and the iron ion eluting body 1 was immersed in seawater 110 without using a holding member 20 in the same manner as in Example 1. The evaluation results are shown in Table 2.

[Comparative Example 3]
The amount of iron ions eluted into the seawater 110 in the bottle was measured in the same manner as in Comparative Example 1, except that one steel ball (material: SUJ2, φ10 mm, 4.1 g) was used instead of the carbon-iron composite 10 housed in the iron ion eluter 1. The evaluation results are shown in Table 2.

[Comparative Example 4]
The amount of iron ions eluted into the seawater 110 in the bottle was measured in the same manner as in Comparative Example 1, except that one steel ball (material: SUJ2, φ10 mm, 4.1 g) was used instead of the carbon-iron composite 10 housed in the iron ion eluting body 1, and a lump of needle coke (LPC-U, manufactured by Nippon Steel Chemical & Material, weighing 4.0 g, one piece) was used as the conductive aid 30. The evaluation results are shown in Table 2.

[Comparative Example 5]
The amount of iron ions eluted into the seawater 110 in the bottle was measured in the same manner as in Comparative Example 1, except that the number of carbon-iron composites 10 housed in the iron ion eluting body 1 was changed to three. The evaluation results are shown in Table 2.

The iron ion elution bodies 1 of Examples 1 to 5 eluted greater amounts of iron ions per unit amount of carbon-iron composite 10 used, and also exhibited greater iron ion elution capacity, than those of Comparative Examples 1 and 5, in which the carbon-iron composite 10 was simply placed in a nylon mesh bag; Comparative Example 2, in which CFRP was used in combination with steel plates; Comparative Example 3, in which only steel balls were used; and Comparative Example 4, in which needle coke was used in combination with steel balls. Furthermore, in Examples 1, 7, and 8, even when SUS was used for the retaining member 20, Ni and Cr were not detected in the immersion water or sediment. Furthermore, in Examples 6 to 8, the amount of iron ion elution was enhanced compared to conventional methods by using the cathode 101 to release electrons generated during the elution of iron ions from the carbon-iron composite 10 to the outside of the system. Compared to other Examples, the iron ion elution amount and weight output density were higher, demonstrating that a higher concentration of iron ions can be efficiently supplied to water, and that the device can also be made more compact. In addition, by placing a cathode 101 with a large surface area near the iron ion eluting body 1 as in Example 9, it is possible to elute more iron ions than in the comparative example. Because the iron ion supplying device 100 of Example 9 is a wire-free type, it is also suitable for use in stagnant water areas such as ponds. As an application example of the iron ion supplying device 100 of Example 9, for example, in the summer when the water temperature is high and hydrogen sulfide is generated in large amounts from the bottom sediment, the iron ion eluting body 1 and cathode 101 are connected using wire 102 to generate a large amount of iron ions, and in the winter when the temperature is low and hydrogen sulfide is not generated from the bottom sediment, the device is wire-free to reduce the generation of iron ions.

The above describes in detail an embodiment of the present invention for illustrative purposes, but the present invention is not limited to the above embodiment and various modifications are possible.

1... iron ion eluting body, 10... carbon-iron composite, 11... iron particles, 13... carbonaceous material, 15... pores, 20... holding member, 30... conductive aid, 100... iron ion supply device, 101... cathode, 102... conducting wire, 103... floating body, 200... iron ion supply device

Claims (6)

An iron ion eluent for supplying iron ions into water,
at least one carbon-iron composite;
a holding member that holds the carbon-iron composite in a state where it can come into contact with water;
Equipped with
The present invention further comprises a conductive additive placed in contact with the carbon-iron composite , the conductive additive being a carbon material selected from the group consisting of natural graphite, artificial graphite, ironmaking coke, needle coke, and compacts thereof;
An iron ion eluting body, characterized in that the holding member is a mesh-like or cage-like structure that allows liquid to pass through, and the conductive additive is stored inside the structure . The iron ion eluent according to claim 1, wherein the carbon-iron composite is a sintered body of iron particles and a carbon material. 3. The iron ion eluent according to claim 1, wherein the holding member is made of a conductive material, and the carbon-iron composite and the holding member are in contact with each other. The iron ion eluent according to claim 3 ,
a conductive cathode provided outside and electrically connected to the iron ion eluting body;
An iron ion supply device comprising: 5. The iron ion supply device according to claim 4 , wherein the cathode is placed on or near the water surface. 6. The iron ion supplying device according to claim 4, wherein the iron ion elution capacity when immersed in a saline solution having a concentration of 3% by weight for 5 days is 2000 mg/kg or more.