JP7672682B2 - Uranium adsorbent, its manufacturing method, method for extracting uranium from a test aqueous solution using the same, and method for recovering uranium from uranium-containing minerals - Google Patents

Description

The present invention relates to a uranium adsorbent using porous silica, a method for producing the same, a method for extracting uranium from a test aqueous solution using the same, and a method for recovering uranium from uranium-containing minerals.

Nuclear reactors are a form of clean energy that can generate electricity without emitting secondary gases that can contribute to global warming, and many countries are moving to adopt nuclear reactors as a source of electricity. These reactors rely primarily on uranium. As a result, demand for uranium is increasing, and there is a need for efficient methods to extract uranium.

Uranium is radioactive and can have an effect on the human body. For this reason, the World Health Organization (WHO) has set the maximum amount of uranium that can be contained in drinking water at 2 μg/L. Many raw materials found in nature contain various levels of uranium. For this reason, it is desirable to extract uranium from raw materials and limit its effects on the human body.

On the other hand, technology has been developed to adsorb strontium ions and cesium ions using mesoporous silica (see, for example, Patent Documents 1 and 2). However, these mesoporous silicas cannot selectively adsorb uranium.

JP 2013-17919 A JP 2013-40852 A

The objective of the present invention is to provide a uranium adsorbent using porous silica that selectively adsorbs uranium, a method for producing the same, a method for extracting uranium from a test aqueous solution using the same, and a method for recovering uranium from uranium-containing minerals.

The uranium adsorbent according to the present invention comprises an aggregate of particles made of porous silica having a hexagonal crystal structure, the aggregate having a hierarchical structure with micropores, mesopores and macropores, the aggregate having a BET specific surface area in the range of 250 m ² /g to 300 m ² /g and a pore volume in the range of 0.3 cm ³ /g to 0.5 cm ³ /g, the surface of the aggregate having a plurality of grooves between the particles,
The above problems are solved by selectively adsorbing or liberating uranium.
The surface and pores of the porous silica may be modified with a chelate compound represented by formula 1 or formula 2.

Here, L is a divalent group, "." represents a radical, R1 is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, a monovalent heterocyclic group, a substituted amino group, a halogen atom, and a cyano group, R2 and R3 are the same or different alkyl groups having 1 to 5 carbon atoms, R4 is an alkyl group having 1 to 5 carbon atoms, n is a natural number of 1 to 3, and * represents a bonding site with the porous silica.
The L may be an alkylene group having 2 to 4 carbon atoms, the R1 to R4 may be the same or different alkyl groups having 1 to 3 carbon atoms, and the n may be 2 or 3.
The mass ratio of the chelate compound to the porous silica may be in the range of 0.1 or more and 0.5 or less.
The mass ratio of the chelate compound to the porous silica may be in the range of 0.15 or more and 0.25 or less.
The porous silica contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and when the total is 100 mass%, the mass percent concentration of each element is
45≦Si≦55
40≦O≦45
3≦C≦7, and
1≦P≦3
may be satisfied.
The particles may have a median diameter (d50) in the range of 0.5 μm to 2.5 μm.
The method for producing the above-mentioned uranium adsorbent according to the present invention comprises adding a plant-derived silica source obtained from rice husks to an alkaline solution to prepare a plant-derived silica solution, adding cetyltrimethylammonium bromide (CTAB) to the plant-derived silica solution to obtain a raw material mixed solution, hydrothermally synthesizing the raw material mixed solution to precipitate a reaction product consisting of silica from the plant-derived silica source and the cetyltrimethylammonium bromide, and calcining the reaction product to remove the cetyltrimethylammonium bromide, thereby solving the above-mentioned problem.
The step of preparing the plant-derived silica solution may include refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature in the range of 65° C. or more and 75° C. or less.
The raw material mixed solution may be obtained by adding cetyltrimethylammonium bromide so that the mass ratio of cetyltrimethylammonium bromide to the plant-derived silica in the plant-derived silica solution is in the range of 0.1 to 0.4.
The hydrothermal synthesis may include adding an acid to the reaction product, washing, and drying.
Firing the reaction product may involve firing the reaction product at a temperature in the range of 200° C. to 800° C. for a period of 2 hours to 24 hours.
The method may further include mixing a calcined product obtained by calcining the reaction product with the chelate compound represented by formula 3 or formula 4.

Here, L is a divalent group, R1 and R are the same or different and are selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, a monovalent heterocyclic group, a substituted amino group, a halogen atom, and a cyano group, R2 and R3 are the same or different and are alkyl groups having 1 to 5 carbon atoms, R4 is an alkyl group having 1 to 5 carbon atoms, and n is a natural number of 1 to 3.
The mixing may involve mixing the chelate compound such that a mass ratio of the chelate compound to the baked product is 0.1 or more and 0.5 or less.
Following said mixing, the method may further comprise washing and drying the mixed product.
The method may further include preparing the plant-derived silica source prior to preparing the plant-derived silica solution, and the preparing step may further include washing and drying the rice husk, treating the dried rice husk with an acid having a concentration in the range of more than 0.5 M to 1.5 M and refluxing it at a temperature range of 75° C. to 85° C. for 1 hour to 10 hours, washing the acid and drying it at a temperature range of 80° C. to 110° C. for 5 hours to 10 hours, and calcining the dried rice husk at a temperature range of 600° C. to 800° C. for 5 hours to 12 hours to decarbonate it.
The method for extracting uranium from a test aqueous solution according to the present invention comprises contacting the above-mentioned uranium adsorbent with the test aqueous solution to adsorb the uranium in the test aqueous solution, thereby solving the above-mentioned problem.
The method for recovering uranium from uranium-containing ore according to the present invention includes adding the uranium-containing ore to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare the uranium-containing solution, contacting the uranium adsorbent with the uranium-containing solution to allow the uranium adsorbent to adsorb the uranium in the uranium-containing solution, discharging treated water in which uranium has been extracted from the uranium-containing solution, and contacting the uranium adsorbent with the uranium adsorbent with an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid to liberate the adsorbed uranium, thereby solving the above-mentioned problem.
The pH of the uranium-containing solution may be in the range of 3 to 6.5.
In addition to the acid solution, hydrogen peroxide may be further added.

The uranium adsorbent of the present invention comprises an aggregate of particles made of porous silica having a hexagonal crystal structure. The porous silica has a hierarchical structure with micropores, mesopores and macropores, has a BET specific surface area in the range of 250 m ² /g to 300 m ² /g, and has a pore volume in the range of 0.3 cm ³ /g to 0.5 cm ³ /g. As a result, the uranium adsorbent of the present invention can selectively adsorb or release uranium, which is advantageous for maintaining human health and purifying the environment. The uranium adsorbent of the present invention can be used as a column packing material.

Schematic diagram showing the uranium adsorbent of the present invention. FIG. 1 is a schematic diagram showing another uranium adsorbent of the present invention. Schematic diagram showing the adsorption mechanism using the uranium adsorbent of the present invention modified with a chelating compound. Schematic diagram showing the adsorption mechanism using the uranium adsorbent of the present invention modified with a chelating compound. A flow chart showing the process for producing another uranium adsorbent of the present invention. FIG. 1 is a schematic diagram showing a uranium adsorption column using the uranium adsorbent of the present invention. A flow chart showing the process of adsorbing uranium from a test aqueous solution using the uranium adsorbent of the present invention. A flow chart showing the process of extracting and recovering uranium from uranium-containing ore using the uranium adsorbent of the present invention. A diagram showing the dependence of uranium leaching from AGR on various conditions. FIG. 1 shows SEM images of samples of Examples 1 and 2. FIG. 1 shows a HRTEM image of a sample of Example 2. FIG. 1 shows XRD patterns and nitrogen adsorption/desorption isotherms of samples of Examples 1 and 2. FIG. 2 shows TG-DTA curves of samples of Examples 1 and 2. FIG. 1 shows FTIR spectra of samples from Examples 1 and 2. FIG. 1 shows the XRD pattern (A) and nitrogen adsorption/desorption isotherm (B) of the sample of Example 2 after a batch-type uranium adsorption test using a uranium standard solution. FIG. 1 shows the FTIR spectrum of the sample of Example 2 after a batch uranium adsorption test using a uranium standard solution. FIG. 1 shows the results of a batch-type uranium adsorption test using a uranium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a batch-type uranium adsorption test using a uranium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a release (desorption) test using samples of Example 1 and Example 2 in which uranium was adsorbed by the batch method. FIG. 1 shows the results of a batch-type uranium adsorption test using a uranium-containing solution by AGR-containing ore for the samples of Example 1 and Example 2. FIG. 1 shows the results of a batch-type uranium adsorption/desorption repeat test using a uranium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a column-type uranium adsorption test using a uranium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a release (desorption) test using the samples of Example 1 and Example 2 in which uranium was adsorbed by the column method. FIG. 1 shows the results of a column-type uranium adsorption/desorption cycle test using a uranium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a batch-type uranium adsorption test using a uranium-containing solution by AGR-containing ore for the sample of Example 2. FIG. 1 shows the results of batch and column uranium adsorption/desorption tests using uranium-containing solutions with AGR-containing ores for the samples of Examples 1 and 2. FIG. 1 shows the results of a batch-type uranium adsorption test using a uranium-containing solution on AGR-containing ore for the samples of Examples 1 to 6.

The following describes an embodiment of the present invention with reference to the drawings. Note that similar elements are given similar reference numerals and their description will be omitted.

Figure 1 is a schematic diagram showing the uranium adsorbent of the present invention.

The uranium adsorbent of the present invention comprises an aggregate 100 of particles made of porous silica 110 having a hexagonal crystal structure. The aggregate 100 has a hierarchical structure with micropores (pore diameter: 0.5 nm or more and less than 2 nm), mesopores (pore diameter: 2 nm or more and less than 50 nm), and macropores (pore diameter: 50 nm or more and 10 μm or less). By having such pores of various sizes, a hierarchical structure is formed, which can promote the uptake, diffusion, and adsorption of uranium. The present inventors have found that the aggregate 100 has a BET specific surface area in the range of 250 m ² /g to 300 m ² /g and a pore volume in the range of 0.3 cm ³ /g to 0.5 cm ³ /g, and selectively adsorbs or releases uranium in the mesopores and channels, thereby functioning as a uranium adsorbent. Furthermore, the surface of the aggregate 100 constituting the uranium adsorbent of the present invention has a plurality of grooves 120 between particles, and uranium can be selectively adsorbed and released into the mesopores and channels via the grooves.

The uranium (U) of interest in this invention may be either naturally occurring or produced by nuclear reactions in a reactor. Naturally occurring uranium consists of three main isotopes: 238U (99.28% natural abundance), 235U (0.71%), and 234U (0.0054%). All of the isotopes are radioactive, and their daughter isotopes are also radioactive isotopes. Uranium is the most important and dominant nuclide in nuclear fuel and molten debris generated in major accidents. In other words, it has a long half-life, is an alpha-ray emitter with high radiotoxicity, and is a nuclide whose heat generation as well as radioactivity are problematic in the safety assessment of long-term waste disposal.

In addition, in the object to be treated (also referred to as the test liquid, for example), uranium species take four oxidation states: U(III), U(IV), U(V), and U(VI). They usually exist in the form of hydrates as U3 ⁺ , U4 ⁺ , _UO2 ⁺ , and _UO22 ^+, respectively, and any of these may be used. Hereinafter, for simplicity, the term "uranium" will be used to refer to all isotopes and ions in various oxidation states.

The diffraction pattern obtained by powder X-ray diffraction measurement shows that porous silica particles have a hexagonal crystal structure.

The presence of micropores (diameters of 0.5 nm or more and less than 2 nm), mesopores (diameters of 2 nm or more and less than 50 nm), and macropores (diameters of 50 nm or more and less than 10 μm) in the aggregate 100 can be determined by the NLDFT method (Non-Local Density Functional Theory) based on the nitrogen adsorption and desorption isotherms. In simple terms, the adsorption and desorption isotherms may be in a mixed state of IUPAC type I, type II or III, and type IV or V. In this specification, an aggregate having micropores, mesopores, and macropores is said to have a hierarchical structure.

In the aggregate 100, the mesopores having a size of 2 nm or more and less than 50 nm mainly function as spaces for adsorbing and retaining uranium, and preferably have a pore volume in the range of 0.4383 ^cm3 /g or more and 0.358 ^cm3 /g or less. The mesopores become active sites for uranium, can promote the capture of uranium, and can efficiently adsorb uranium.

The particles are preferably balloon-like spheres and may be hollow. The particles preferably have a median diameter (d50) in the range of 0.5 μm to 2.5 μm, and the aggregates 100 may have a diameter of 1 μm to 200 μm. More preferably, the particles have a size in the range of 1.5 μm to 2.5 μm.

FIG. 2 is a schematic diagram showing another uranium adsorbent of the present invention.
FIG. 3 is a schematic diagram showing the adsorption mechanism using the uranium adsorbent of the present invention modified with a chelate compound.

The uranium adsorbent of the present invention may preferably comprise an aggregate 200 of particles in which the surface and pores of porous silica are modified with a chelating compound 210 represented by formula 1 or formula 2.

Here, L is a divalent group, "." represents a radical, R1 is selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, a monovalent heterocyclic group, a substituted amino group, a halogen atom, and a cyano group, R2 and R3 are the same or different alkyl groups having 1 to 5 carbon atoms, R4 is an alkyl group having 1 to 5 carbon atoms, n is a natural number between 1 and 3, and * represents a bonding site with the porous silica.

As shown in FIG. 3, R2 and R3 of the phosphoryl group or R4 of the carboxyl group in such a chelate compound 210 can selectively adsorb and release uranium. In detail, R2 to R4 surround and hold uranium. As a result, the uranium adsorbent shown in FIG. 2 can adsorb more uranium than the uranium adsorbent shown in FIG. 1.

Figure 3(A) shows a schematic diagram of uranium adsorption when the uranium adsorbent of the present invention is used in a batch system, and Figure 3(B) shows a schematic diagram of uranium adsorption when the uranium adsorbent of the present invention is used in a column system.

L is not particularly limited as a divalent group, but examples of the divalent hydrocarbon group that may have a heteroatom include: an alkylene group (preferably having 1 to 10 carbon atoms), a cycloalkylene group (preferably having 3 to 10 carbon atoms), an alkenylene group (preferably having 2 to 10 carbon atoms), an alkynylene group (preferably having 2 to 10 carbon atoms), and combinations of these, as well as combinations of the above with -C(O)-, -C(O)O-, -OC(O)-, -O-, -S-, and -NR- (wherein R represents a hydrogen atom or a monovalent organic group), and the like.

Among these, L is preferably an alkylene group having 1 to 10 carbon atoms. This promotes the modification of the porous silica with the chelate compound 210. Even more preferably, L is an alkylene group having 1 to 5 carbon atoms. This allows more chelate compound 210 to be modified on the porous silica, allowing more uranium to be adsorbed. Even more preferably, L is an alkylene group having 2 to 4 carbon atoms.

There is no particular limit to n, so long as it is a natural number between 1 and 3, but preferably n is 2 or 3. This allows the compound 210 to be firmly modified on the porous silica.

R1 is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group. This promotes the modification of the porous silica with compound 210.

R2 and R3 may be the same or different, but are preferably the same from the viewpoint of production efficiency. R2 and R3 are preferably alkyl groups having 1 to 3 carbon atoms. By shortening the alkyl chain, it becomes easier to selectively take up uranium. Experiments have confirmed that R2 and R3, especially when they are methyl or ethyl groups, selectively adsorb uranium.

R4 is preferably an alkyl group having 1 to 3 carbon atoms. By shortening the alkyl chain, it becomes easier to selectively take up uranium. Experiments have confirmed that R4, especially when it is a methyl or ethyl group, selectively adsorbs uranium.

The combination of L, R1 to R4, and n can be set arbitrarily from the above, but preferably L is an alkylene group having 2 to 4 carbon atoms, R1 to R4 are the same or different alkyl groups having 1 to 3 carbon atoms, and n is 2 or 3.

Examples of such chelating compounds 210 include the following. These are easy to modify into porous silica and can selectively adsorb uranium.

When the chelating compound 210 is modified, the aggregate 200 preferably has a BET specific surface area in the range of 250 ^m2 /g to 270 ^m2 /g and a pore volume in the range of 0.3 ^cm3 /g to 0.4 ^cm3 /g. Although the specific surface area and pore volume are slightly decreased by modifying the chelating compound 210, the uranium adsorption efficiency can be improved due to the uranium adsorption ability of the chelating compound 210 itself.

The uranium adsorbent of the present invention preferably has a mass ratio of the chelate compound to the porous silica in the range of 0.1 to 0.5. This allows uranium to be efficiently adsorbed while maintaining the specific surface area and pore volume.

More preferably, the uranium adsorbent of the present invention has a mass ratio of the chelate compound to the porous silica in the range of 0.15 to 0.25. This allows uranium to be more efficiently adsorbed while maintaining the specific surface area and pore volume.

In the uranium adsorbent of the present invention modified with the chelate compound 210, the porous silica preferably contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and when the total is taken as 100 mass %, the mass percent concentration of each element is
45≦Si≦55
40≦O≦45
3≦C≦7, and
1≦P≦3
This allows uranium to be adsorbed more efficiently.

In the uranium adsorbent of the present invention modified with the chelating compound 210, the mass percent concentration of each element is more preferably:
47≦Si≦50
42≦O≦45
3≦C≦5, and
1≦P≦2
This allows uranium to be adsorbed more efficiently.

In the uranium adsorbent of the present invention modified with the chelate compound 210, the porous silica may further contain a Group 1 element or Group 2 element in addition to the above elements. In this case, the content when the total is 100 mass % needs only to be 2.5 mass % or more and 5 mass % or less. Among the Group 1 elements or Group 2 elements, sodium and potassium are preferred. These elements do not reduce the adsorption efficiency of uranium.

Next, a method for producing the uranium adsorbent of the present invention shown in FIG. 1 will be described.
FIG. 4 is a flow chart showing the steps of producing the uranium adsorbent of the present invention.

The manufacturing method of the present invention uses a sol-gel method. Each step will be described below.
Step S410: Add the plant-derived silica source obtained from rice husk to an alkaline solution to prepare a plant-derived silica solution. The plant-derived silica solution may be called a sol-gel solution. This causes the plant-derived silica source to be hydrolyzed and silanol to be formed. In the present invention, by using the plant-derived silica source obtained from rice husk, a uranium adsorbent having a unique structure, properties and characteristics can be produced, which is low-cost and environmentally friendly.

The alkaline solution may be a strong alkali, such as a hydroxide of an alkali metal (lithium, sodium, potassium, rubidium, cesium, etc.), a hydroxide of tetraalkylammonium, or a hydroxide of an alkaline earth metal (calcium, strontium, barium, europium, etc.). The solvent may be water or an alcohol such as ethanol or methanol.

Prior to the step of preparing the plant-derived silica solution in step S410, a plant-derived silica source may be prepared from rice husks. In particular, the rice husks are washed and dried (e.g., at 100°C). The dried rice husks are treated with acid (e.g., hydrochloric acid greater than 0.5 M and less than 1.5 M) and refluxed at a temperature range of 75°C to 85°C for 1 hour to 10 hours. After cooling, the rice husks are washed with acid and dried (e.g., at 80°C to 110°C) for 5 hours to 10 hours. Thereafter, the rice husks are decarbonated by calcination (e.g., at 600°C to 800°C for 5 hours to 12 hours) to obtain a plant-derived silica source.

In step S410, preparing the plant-derived silica solution may involve refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature range of 65°C or more and 75°C or less. This promotes hydrolysis.

Step S420: Cetyltrimethylammonium bromide (CTAB) is added to the plant-derived silica solution to obtain a raw material mixture solution. The raw material mixture solution has a sufficiently high viscosity compared to the plant-derived silica solution and may be called a wet sol. As a result, the formed silanols are polymerized to form siloxane bonds. Furthermore, the siloxane bonds are reconstructed into spheres with an ordered hierarchical structure by the surfactant of CTAB. By using CTAB, the finally obtained porous silica has a hexagonal crystal structure.

In step S420, the mass ratio of CTAB to plant-derived silica in the plant-derived silica solution is preferably in the range of 0.1 to 0.4. This promotes reconstruction into spheres having a hierarchical structure.

Step S430: The raw material mixed solution is subjected to hydrothermal synthesis. This causes the precipitation of a reaction product consisting of silica from the plant-derived silica source and CTAB. Hydrothermal synthesis is a reaction in which the raw material mixed solution is reacted in a sealed container such as an autoclave in the presence of hot water at high temperature and pressure. For example, hydrothermal synthesis is performed at a temperature range of 100°C to 150°C for a period of 1 hour to 24 hours.

In the hydrothermal synthesis of step S430, preferably, an acid may be added to the reaction solution after the hydrothermal synthesis to make it neutral. The acid may be hydrochloric acid, sulfuric acid, nitric acid, or the like. The resulting reaction product may be washed and dried.

Step S440: The reaction product obtained in step S430 is calcined to remove the CTAB. As a result, the CTAB is removed, and spherical silica that maintains an orderly hierarchical structure is obtained. There are no particular restrictions on the calcination conditions as long as the CTAB is removed, but it is preferably calcined at a temperature range of 200°C to 800°C for 2 hours to 24 hours. This removes the CTAB. More preferably, the calcination is performed at a temperature range of 450°C to 650°C for 6 hours to 10 hours. In this way, a uranium adsorbent is obtained that is composed of an aggregate having a hierarchical structure with micropores, mesopores, and macropores, which is an aggregate of particles made of porous silica as shown in FIG. 1.

Next, a method for producing another uranium adsorbent of the present invention shown in FIG. 2 will be described.
FIG. 5 is a flow chart showing a process for producing another uranium adsorbent of the present invention.

The procedure for the separate uranium sorbent continues with step S440 of FIG.
Step S510: The fired product obtained in step S440 is mixed with a chelate compound represented by formula 3 or formula 4. This forms silanol groups on the surface and in the pores of the porous silica, modifying the chelate compound.

L is a divalent group, R1 and R are the same or different and are selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, a monovalent heterocyclic group, a substituted amino group, a halogen atom, and a cyano group, R2 and R3 are the same or different alkyl groups having 1 to 5 carbon atoms, R4 is an alkyl group having 1 to 5 carbon atoms, and n is a natural number of 1 to 3. R is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group. This promotes the modification of the porous silica with the compound 210. Since L, R1 to R4, and n are as described above, a description thereof will be omitted.

The combination of L, R, and R1 to R4, and n can be set arbitrarily from the above, but preferably L is an alkylene group having 2 to 4 carbon atoms, R and R1 to R4 are the same or different alkyl groups having 1 to 3 carbon atoms, and n is 2 or 3.

Examples of such chelating compounds include the following. These are easy to modify into porous silica and can selectively adsorb uranium.

In step S510, the mass ratio of the chelate compound to the fired product (i.e., the porous silica shown in FIG. 1) is preferably 0.1 or more and 0.5 or less. This allows the chelate compound to modify the surface and pores of the fired product while maintaining the specific surface area and pore volume of the fired product.

In step S510, the chelating compound may be added directly, or may be mixed with an aprotic donor solvent such as toluene or hexane, and dissolved. The mixture may be stirred at a temperature range of room temperature (20-30°C) to 50°C for 1 hour to 24 hours.

Step S520: The product after mixing is washed and dried. Washing may be performed multiple times using an alcohol such as ethanol and water. This allows the unreacted chelate compound to be removed. Drying may be performed at a temperature range of 50°C to 100°C for 1 hour to 24 hours. In this way, a uranium adsorbent made of porous silica with modified surfaces and pores as shown in Figure 2 is obtained.

Next, we will explain the applications of the uranium adsorbent of the present invention. The uranium adsorbent of the present invention can be used in either the batch method or the column method. Here, we will explain the case where it is used in the column method.

Figure 6 is a schematic diagram showing a uranium adsorption column using the uranium adsorbent of the present invention.

The uranium adsorption column 600 of the present invention has an adsorbent 610. The adsorbent 610 is the uranium adsorbent 100, 200 of the present invention described with reference to Figures 1 and 2. The adsorbent 610 is packed in a container (not shown).

As shown in FIG. 6, the uranium adsorption column 600 may further include a top filter 620, a bottom filter 630, and column connections 640, 650. The uranium adsorption column 600 has an opening on the top filter 620 side and a bottom filter 630 side, and a solution containing uranium (uranium-containing solution) is passed from one opening to the other opening, whereby the uranium-containing solution comes into contact with the uranium adsorbent 610. This causes the uranium in the uranium-containing solution to be adsorbed. The adsorption column may also be called an adsorption tower.

The top filter 620 prevents the uranium adsorbent 610 from scattering in the liquid as it passes through. The bottom filter 630 prevents the uranium adsorbent 610 from leaking out of the container (not shown). The column connectors 640 and 650 function to connect to piping for supplying or discharging the liquid passed through the uranium adsorption column 600, such as piping for a purification system.

Next, a method for extracting uranium from a test aqueous solution using the uranium adsorption column 600 of FIG. 6 will be described.
FIG. 7 is a flow chart showing the process for adsorbing uranium from a test aqueous solution using the uranium adsorbent of the present invention.

Step S710: The uranium adsorbent is brought into contact with the test aqueous solution. For example, in the uranium adsorption column 600 of FIG. 6, the test aqueous solution is passed through the column connectors 640, 650 and the top filter 620. If the test aqueous solution contains uranium, the uranium in the test aqueous solution is selectively adsorbed by the uranium adsorbent.

Preferably, the pH of the test aqueous solution is adjusted to 3 or more and 6.5 or less. A known acid or base may be used to adjust the pH. The adsorption of uranium is promoted by adjusting the pH. More preferably, the pH of the test aqueous solution is adjusted to 3 or more and 5 or less. This suppresses the generation of insoluble and stable hydroxides such as [(UO ₂ ) ₂ (OH) ₂ ] ⁺² and [(UO ₂ ) ₃ (OH) ₅ ] ⁺² , and reduces the decrease in the active sites of the uranium adsorbent.

Preferably, the contact time in step S710 is 30 minutes or more. This promotes the adsorption of uranium. There is no upper limit, but 60 minutes may be sufficient. If the contact time is longer than this, no change in the amount of adsorption is observed. It is also advisable to contact the mixture while stirring.

Although not shown, following step S710, the treated water from which uranium has been extracted is discharged from the test aqueous solution. For example, in the uranium adsorption column 600 of FIG. 6, the treated water may be discharged through the bottom filter 630.

The uranium adsorbent with adsorbed uranium can be released by contacting it with an acid such as sulfuric acid, hydrochloric acid, or nitric acid. This makes the uranium adsorbent reusable. From the standpoint of release efficiency, sulfuric acid is preferred.

Preferably, the uranium adsorbent having uranium adsorbed thereon is brought into contact with an acid such as sulfuric acid having a concentration of 0.005M to 0.5M for 20 minutes to 120 minutes. This allows the adsorbed uranium to be liberated. Even more preferably, the uranium adsorbent having uranium adsorbed thereon is brought into contact with an acid such as sulfuric acid having a concentration of 0.1M to 0.3M for 45 minutes to 60 minutes. This allows the adsorbed uranium to be liberated efficiently.

Figure 8 is a flow chart showing the process of extracting and recovering uranium from uranium-containing ore using the uranium adsorbent of the present invention.

Step S810: Add the uranium-containing ore to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare a uranium-containing solution (a solution containing uranium). This allows the uranium in the uranium-containing ore to exist in the form of a hydrate (also called leaching of uranium), even if the ore has a low uranium content. The concentration of acid (M) in the acid solution preferably falls within the range of 0.25 M or more and 0.75 M or less. Within this range, the uranium can be effectively leached.

Preferably, hydrogen peroxide is added to the selected acid solution. This allows the uranium present in the uranium-containing ore, especially inside, to be more efficiently hydrated. Hydrogen peroxide should be added to the acid solution at 4 to 8 v/v%. At 4 v/v% or more, the leaching of uranium is promoted, and there is no significant change in effectiveness even if the amount exceeds 8 v/v%.

In preparing the uranium-containing solution, the uranium-containing ore and acid are mixed and stirred, preferably for 40 minutes to 120 minutes. This increases the leaching efficiency. More preferably, heating during stirring increases the leaching efficiency, and the heating temperature is preferably in the range of 50°C to 70°C.

Step S820: The uranium adsorbent is brought into contact with the uranium-containing solution. For example, in the uranium adsorption column 600 of FIG. 6, the uranium-containing solution is passed through the column connections 640, 650 and the top filter 620.

Preferably, the pH of the uranium-containing solution is adjusted to 3 or more and 6.5 or less. A known acid or base may be used to adjust the pH. The adsorption of uranium is promoted by adjusting the pH. More preferably, the pH of the uranium-containing solution is adjusted to 3 or more and 5 or less. This suppresses the generation of insoluble and stable hydroxides such as [(UO ₂ ) ₂ (OH) ₂ ] ⁺² and [(UO ₂ ) ₃ (OH) ₅ ] ⁺² , and reduces the decrease in the active sites of the uranium adsorbent.

Preferably, the contact time in step S820 is 30 minutes or more. This promotes the adsorption of uranium. There is no particular upper limit, but 60 minutes may be sufficient. Even if the contact time is longer, there is no change in the amount of adsorption. It is also advisable to contact the material while stirring. In this way, the uranium adsorbent of the present invention can be used to adsorb uranium extracted from uranium-containing ore.

Step S830: Discharge the treated water from which uranium has been extracted from the uranium-containing solution. For example, in the uranium adsorption column 600 of FIG. 6, the treated water may be discharged through the bottom filter 630.

Step S840: The uranium adsorbent with adsorbed uranium is contacted with an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid to release the adsorbed uranium. This makes the uranium adsorbent reusable. From the viewpoint of release efficiency, sulfuric acid is preferred.

Preferably, the uranium adsorbent having uranium adsorbed thereon is brought into contact with an acid such as sulfuric acid having a concentration of 0.005M to 0.5M for 20 minutes to 120 minutes. This allows the adsorbed uranium to be liberated. Even more preferably, the uranium adsorbent having uranium adsorbed thereon is brought into contact with an acid such as sulfuric acid having a concentration of 0.1M to 0.3M for 45 minutes to 60 minutes. This allows the adsorbed uranium to be liberated efficiently.

Next, the present invention will be described in detail using specific examples, but please note that the present invention is not limited to these examples.

[Preparation of plant-derived silica source]
Rice husk (collected in Al-Sharqiya, Egypt) was washed with tap water to remove impurities, and then dried at 100°C. The dried rice husk was then refluxed in 1M hydrochloric acid (HCl) at 80°C for 5 hours. After cooling, the hydrochloric acid-treated rice husk was filtered, and the residue was washed with Milli-Q water to remove the acid. The residue was then dried at 100°C overnight. The dried body was calcined at 700°C for 8 hours to obtain a white plant-derived silica source.

[Equipment used in evaluation]
The morphology of the sample was observed and elemental analysis was performed using a field emission scanning electron microscope (FE-SEM, JEOL Ltd., JSM-6500F) equipped with an energy dispersive X-ray analyzer (EDX, JED-2300, JEOL Ltd.). The accelerating voltage during observation was 20 kV. High-resolution transmission electron microscope images (HRTEM) of the sample were taken using an electron microscope (2000EX II, JEOL Ltd.). The accelerating voltage during observation was 200 kV.

Small-angle and wide-angle X-ray diffraction (D8 Advance, Bruker) was measured using an 18-kW diffractometer to identify the samples. Nitrogen adsorption/desorption isotherms (77 K) of the samples were measured using a fully automatic gas adsorption apparatus (BELSORP36, Microtrac-Bell). The pore size, pore volume, and specific surface area by the BET method were determined from the isotherms. Thermogravimetric differential thermal analysis (TG-DTA) was performed using a thermogravimetric simultaneous differential thermal analyzer (TG-60, Shimadzu Corporation). Absorption spectra in the infrared region were examined using a Fourier transform infrared spectrophotometer (FTIR, FTIR Prestage-21, Shimadzu Corporation).

The amount of uranium was measured before and after uranium adsorption using an inductively coupled plasma mass spectrometer (ICP-MS, Perkin Elmer, Elan-6000) equipped with a Perkin Elmer lambda 3b double beam UV-visible programmable spectrophotometer. The radioactivity hazard value was evaluated using a high purity germanium (HPGe) EG&G Alltec model instrument (GMX60P4).

[Uranium adsorption/desorption test]
Uranium adsorption and desorption tests were carried out in both column and batch modes.
The glasses used were cleaned with 5% nitric acid and Milli-Q water. All tests were performed in triplicate.

The column method was performed as follows: A plastic net and polyethylene wool were placed in a fixed vertical glass column (length 12.5 cm, diameter 1.0 cm), and a uranium adsorbent was placed on top of the plastic net. Using the column, the effects of the flow rate of the test liquid, the dosage of the uranium adsorbent, and the uranium concentration were investigated. The amount of uranium before and after adsorption was measured by ICP-MS.

The adsorption amount (q _e , mg/g) and the adsorption efficiency (Ads.%) were calculated from the following formulas.
q _e = (C ₀ - _{C e} ) (V/w) (1)
Ads. %={(C ₀ - C _e )/C ₀ }×100 (2)
where C ₀ and C _e are the initial and equilibrium concentrations [U(VI), mg/L], V is the volume of the test solution (L), and w is the mass of the uranium sorbent (g).

Next, _sulfuric acid was passed through a column packed with the uranium adsorbent to desorb (release) the uranium. The release efficiency (E.%) was calculated using the formula E.% = ( _C.sub.D /C.sub.A) x 100, where _C.sub.D and _C.sub.A are the released uranium concentration and the adsorbed uranium concentration.

The batch method was carried out as follows. 20 mL of the test liquid (uranium standard solution described below) was mixed with 20 mg of uranium adsorbent while stirring. The uranium concentration and pH in the uranium standard solution, the mixing time, and the mixing temperature were changed to investigate the amount of uranium adsorbed and the adsorption efficiency. As with the column method, sulfuric acid was passed through the uranium adsorbent to which uranium had been adsorbed, and the uranium was desorbed. The calculations of the amount of uranium adsorbed, adsorption efficiency, and release efficiency were as described above, so a description will be omitted.

[Test solution]
Uranium standard solutions of various concentrations were used as test solutions for the uranium adsorption/desorption test.

Furthermore, a uranium-containing solution in which uranium had been leached from a uranium-containing ore was prepared as a test liquid for use in a uranium adsorption/desorption test in the following manner.
Granite (AGR) was collected from Gabal El-Sela, located in the southeastern desert of Egypt, and crushed to 200 mesh (approximately 0.075 mm). Elemental analysis and oxide content of the AGR were measured. The results are shown in Table 1. "REE" in Table 1 indicates rare earth elements. Furthermore, measurements using a HPGe spectrometer showed that the AGR contained 3445.5 Bq/Kg of ²³⁸ U isotope.

One gram of crushed AGR was added to and stirred in sulfuric acid, hydrochloric acid, and nitric acid solutions (0.1M to 2M) at various temperatures for various times to allow uranium to leach out. 0 to 15 v/v% hydrogen peroxide was also added to the acid solutions as an oxidizing agent, as necessary. The leaching efficiency (L%) for different acid solutions was calculated using the following formula. These results are shown in Figure 9.
L%=(C _e /C _s )×100 (3)
where C _e (Bq/Kg) and C _s (Bq/Kg) are the uranium concentrations in the acid solution at equilibrium and initial conditions.

Figure 9 shows the dependence of uranium leaching from AGR on various conditions.

Figure 9 (A) shows the dependence of leaching efficiency on the type of acid. In each case, 1 g of AGR was added to various acids (0.5 M) and stirred for 30 minutes at room temperature (25°C ± 2). As shown in Figure 9 (A), uranium leaching was confirmed using all acids, but the leaching efficiency increased in the order of hydrochloric acid, nitric acid, and sulfuric acid. This shows that sulfuric acid is effective as an acid for leaching uranium from uranium-containing ore.

Figure 9 (B) shows the dependence of leaching efficiency on acid concentration. In each case, 1 g of AGR was added to sulfuric acid of various concentrations (0.1 M to 2.0 M) and stirred for 30 minutes at room temperature (25°C ± 2). Figure 9 (B) shows that a high leaching efficiency of over 60% was observed regardless of the acid concentration, and that a leaching efficiency of over 65% was achieved in the range of 0.25 M to 0.75 M in particular.

The inset in Figure 9(B) shows the dependence of leaching efficiency on the amount of hydrogen peroxide. All results are from adding 1 g of AGR to sulfuric acid (0.5 M), adding various volumes of hydrogen peroxide, and stirring for 30 minutes at room temperature (25°C ± 2).

The inset shows that the leaching efficiency increases by adding hydrogen peroxide at 4 v/v% or more. There is no particular upper limit, but 8 v/v% or less was sufficient.

Figure 9 (C) shows the time dependence of leaching efficiency. In both cases, 1 g of AGR was added to sulfuric acid (0.5 M) and stirred at room temperature (25°C ± 2) for 30 minutes. Figure 9 (C) shows that leaching efficiency of 50% or more was observed regardless of the time, but it was found that leaching efficiency of 75% or more was achieved in the range of 40 to 120 minutes.

The inset in Figure 9(C) shows the dependence of the ratio of uranium-bearing ore (g) to acid solution (mL). The inset shows that a high leaching efficiency of over 75% is achieved when the ratio (g/mL) is in the range of 1/6 to 1/3.

Figure 9 (D) shows the temperature dependence of leaching efficiency. In each case, 1 g of AGR was added to sulfuric acid (0.5 M) and stirred for 30 minutes at various temperatures (25°C to 70°C). Figure 9 (D) shows that a high leaching efficiency of over 80% was observed regardless of the temperature, but it was found that a leaching efficiency of over 95% was achieved in the range of 50°C to 70°C in particular.

Based on the above results, the uranium-containing solution using the AGR-containing ore used in Examples 1 and 2 was prepared by adding 1 g of AGR to 4 mL of sulfuric acid (0.5 M) and hydrogen peroxide (5 v/v%), heating to 70°C ± 2, and stirring for 120 minutes. At this time, 99.5% of the uranium (3428 Bq/Kg) was extracted from the uranium-containing ore.

[Example 1]
In Example 1, a uranium adsorbent made of porous silica was produced under the conditions shown in Table 2 using a plant-derived silica source and cetyltrimethylammonium bromide (CTAB, manufactured by Sigma-Aldrich) as a surfactant.

The plant-derived silica source (5 g) was added as an alkaline solution to 2.5 M sodium hydroxide (50 mL), and the solution was refluxed at 70° C. to dissolve the solution, preparing a plant-derived silica solution (step S410 in FIG. 4). The plant-derived silica solution was filtered, washed with boiled Milli-Q water (10 mL), and cooled. Cetyltrimethylammonium bromide (CTAB, 1 g, 10 mL) was added to the solution, obtaining a raw material mixture solution (step S420 in FIG. 4). The raw material mixture solution was a sol with a higher viscosity than the plant-derived silica solution.

The raw material mixture solution was stirred for 2 hours, transferred to a Teflon (registered trademark) autoclave, and hydrothermally synthesized to precipitate a reaction product consisting of silica and CTAB (step S430 in Figure 4). The conditions for hydrothermal synthesis were to raise the temperature to 130°C at a rate of 10°C/min and hold the temperature for 10 hours. 50% sulfuric acid was added dropwise to the resulting reaction solution while stirring, and the pH of the reaction solution was adjusted to 8. The resulting reaction product was washed with boiled Milli-Q water to remove the alkali, and dried at 70°C for 12 hours.

The dried reactant was calcined at 550°C for 8 hours to remove the CTAB (step S440 in Figure 4). In this way, porous silica was obtained. The sample thus obtained is called the sample of Example 1 or HOM.

The results of the SEM image, HRTEM image, elemental analysis, XRD diffraction, TG-DTA curve, FTIR spectrum, BET specific surface area, and pore volume of the obtained sample of Example 1 are shown in Figures 10 to 14 and Tables 4 and 5.

The uranium adsorption and release efficiencies were measured for the obtained sample of Example 1 using the batch method and column method as appropriate. In addition, the XRD pattern and FTIR spectrum of the sample of Example 1 after uranium adsorption were measured. These results are shown in Figures 15 to 27.

[Example 2]
In Example 2, the surface and pores of the porous silica obtained in Example 1 were modified with a chelate compound shown in Table 3 (2-diethylphosphate ethyltriethoxysilane (DEPETS, manufactured by Gelest Inc.)).

Specifically, 2 g of DEPETS was dissolved in dry toluene, to which 1 g of HOM from Example 1 was added, followed by stirring for 5 hours at room temperature (25°C) (step S510 in Figure 5). The mixture was then filtered and washed three times with absolute ethanol and Milli-Q water to remove unreacted DEPETS. The product was dried in an oven at 70°C for 12 hours and dehydrated (step S520 in Figure 5). The sample thus obtained is referred to as the sample from Example 2 or HOM-M.

As with the sample of Example 1, the results of SEM images, HRTEM images, elemental analysis, XRD diffraction, TG-DTA curves, FTIR spectra, BET specific surface area, and pore volume of the sample of Example 2 are shown in Figures 10 to 14 and Tables 4 and 5.

As with the sample of Example 1, the uranium adsorption efficiency and release efficiency of the sample of Example 2 were measured using the batch method and column method as appropriate. In addition, the XRD pattern and FTIR spectrum of the sample of Example 2 after uranium adsorption were measured. These results are shown in Figures 15 to 27.

[Examples 3 to 6]
In Examples 3 to 6, the surface and pores of the porous silica obtained in Example 1 were modified with the chelate compounds shown in Table 3 in the same manner as in Example 2, and the adsorption and release of uranium was confirmed. The samples obtained in this manner are referred to as the samples of Examples 3 to 6 or HOM-M-3 to HOM-M-6.

For simplicity, the synthesis conditions for the samples in Examples 1 to 6 are summarized in Tables 2 and 3. The results are summarized below.

Figure 10 shows SEM images of samples from Examples 1 and 2.

Figure 10(A) a, b, and c are SEM images of the rice husk, the samples of Example 1, and the samples of Example 2, respectively. Figure 10(B) a is another SEM image of the sample of Example 1, and c and d are another SEM images of the sample of Example 2.

Figure 10(A) b and c show that the samples of Examples 1 and 2 consisted of aggregates of micrometer-sized particles. The aggregates had sizes ranging from 1 μm to 200 μm. Figure 10(B) shows that the particles had sizes ranging from 1.5 μm to 2.5 μm. The aggregates also had multiple grooves between the particles. The samples of Examples 3 to 6 showed a similar appearance.

Table 4 shows the results of elemental analysis of the samples of Examples 1 and 2.

According to Table 4, the sample of Example 2 modified with a chelate compound contains silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and has the following mass percent concentrations:
45≦Si≦55
40≦O≦45
3≦C≦7, and
1≦P≦3
The samples of Examples 3 to 6 also had the same composition.

Figure 11 shows an HRTEM image of the sample in Example 2.

As shown in Figure 11, the sample in Example 2 was an aggregate of balloon-like spheres with grooves between them. The samples in Examples 1 and 3 to 6 were similar.

Figure 12 shows the XRD patterns and nitrogen adsorption/desorption isotherms of the samples of Examples 1 and 2.

12A, the samples of Examples 1 and 2 had broad peaks in the range of 1.8<2θ<3.0, and had lattice spacings d ₁₀₀ of 4.02 nm and 3.94 nm, respectively. Although not shown, the wide-angle XRD patterns had a clear peak at 2θ=30°, indicating a hexagonal crystal structure. Although not shown, the samples of Examples 3 to 6 also had similar XRD patterns.

According to FIG. 12(B), the nitrogen adsorption/desorption isotherm shows a mixture of IUPAC types I, II, and IV, and is found to have micropores, mesopores, and macropores. The pore volume of mesopores is the largest. These characteristics suggest that the samples of Examples 1 and 2 have an ordered hierarchical structure with micropores, mesopores, and macropores. Although not shown, the samples of Examples 3 to 6 also showed similar nitrogen adsorption/desorption isotherms.

Table 5 shows the BET specific surface area and pore volume of the samples of Examples 1 and 2.

As shown in Table 5, both the BET specific surface area and the pore volume were reduced by modifying the surface with a chelating compound. It was confirmed that the samples of Examples 2 to 6 also had a BET specific surface area in the range of 250 m ² /g or more and 270 ^{m 2} /g or less and a pore volume in the range of 0.3 cm ³ /g or more and 0.4 cm ³ /g or less.

Figure 13 shows the TG-DTA curves of the samples of Examples 1 and 2.

According to Figure 13, the sample of Example 1 showed a mass loss of 1.03% by mass between 90°C and 320°C. This is due to the evaporation of residual solvent, water molecules, and CTAB surfactant. On the other hand, the sample of Example 2 showed a total mass loss of 5.54% by mass over two stages between 27°C and 600°C. The first mass loss of 2.11% by mass between 27°C and 200°C is due to the evaporation of physically adsorbed water, ethanol, etc. The second mass loss of 3.43% by mass between 200°C and 600°C is due to the decomposition of organic groups such as CTAB, chelating compounds, and silanols.

The small mass loss of the samples in Examples 1 and 2 indicates that the samples in Examples 1 and 2 have excellent durability in harsh high-temperature environments.

Figure 14 shows the FTIR spectra of the samples of Examples 1 and 2.

Both spectra showed the following characteristics: The broad bands at 3470 cm ^-1 and 1640 cm ^-1 are due to OH groups stretching, antisymmetric stretching, and bending due to silanol groups (Si-OH) and water molecules (H-O-H) physically adsorbed on the surface. The band near 2960 cm ^-1 is due to aliphatic C-H. The bands at 1096 cm ^-1 and 799 cm ^-1 are due to O-Si-O stretching motion, and the band at 1101 cm ^-1 is due to symmetric and antisymmetric stretching vibrations of Si-O-Si.

On the other hand, the FTIR spectrum of the sample of Example 2 had small bands at 1600 cm ^-1 and 960 cm ^-1 , unlike that of the sample of Example 1. These are due to the bending angle HO-P=O and the stretching P-O due to the chelate compound modified on the surface and in the pores, respectively. Although not shown, the samples of Examples 3 to 6 also showed similar FTIR spectra.

From the above results, it was shown that the samples of Examples 1 to 6 comprise aggregates of particles made of porous silica having a hexagonal crystal structure, have a BET specific surface area in the range of 250 m ² /g to 300 m ² /g, have a pore volume in the range of 0.3 cm ³ /g to 0.5 cm ³ /g, and have a plurality of grooves between particles on the surface. Furthermore, it was shown that the aggregates have a hierarchical structure with micropores, mesopores, and macropores. It was shown that the surfaces and pores of the samples of Examples 2 to 6 were modified with a specific chelating compound.

Figure 15 shows the XRD pattern (A) and nitrogen adsorption/desorption isotherm (B) of the sample of Example 2 after a batch-type uranium adsorption test using a uranium standard solution.

Fig. 15 also shows the XRD patterns and nitrogen adsorption/desorption isotherms of the samples of Examples 1 and 2 before the adsorption test. According to Fig. 15(A), the sample of Example 2 after the adsorption test also showed an XRD pattern similar to that of the sample before the adsorption test, with a broad peak in the range of 1.8<2θ<3.0 and a lattice spacing d ₁₀₀ of 3.85 nm.

As shown in Figure 15 (B), the sample of Example 2 after the adsorption test also showed a nitrogen adsorption/desorption isotherm similar to that of the sample before the adsorption test, indicating that the structural characteristics remained stable even after uranium was adsorbed.

The BET specific surface area and pore volume of the sample of Example 2 after the adsorption test were 264.1 ^m2 /g and 0.352 ^cm3 /g, respectively, both of which were slightly smaller than those before the adsorption test. The pore diameters of the sample of Example 2 before and after the adsorption test were 2.35 nm and 2.34 nm, respectively, and were slightly reduced due to uranium adsorption.

Figure 16 shows the FTIR spectrum of the sample from Example 2 after a batch uranium adsorption test using a uranium standard solution.

Figure 16 also shows the XFTIR spectra of the samples of Examples 1 and 2 before the adsorption test. According to Figure 16, the FTIR spectrum of the sample of Example 2 after the adsorption test showed a band of a U=O stretching bond at 920 cm ^-1 and a band of a U-O bond at 455 cm ^-1 in addition to the bands described with reference to Figure 14.

The above results show that the sample in Example 2 functions as a uranium adsorbent and is chemically and physically stable even after adsorption.

FIG. 17 shows the results of a batch-type uranium adsorption test using a uranium standard solution for the samples of Example 1 and Example 2.
FIG. 18 shows the results of a batch-type uranium adsorption test using a uranium standard solution for the samples of Examples 1 and 2.

Figure 17 (A) shows the pH dependence of the amount of uranium adsorbed (adsorption efficiency) when 20 mL of a 100 ppm uranium standard solution was mixed with 20 mg of the samples of Examples 1 and 2 at 25 ± 2°C for 30 minutes. HCl and NaOH were used to adjust the pH.

17(A), it was found that the samples of Examples 1 and 2 adsorbed uranium at a pH of 1 or higher. In particular, all of the samples showed a high adsorption amount exceeding 80% in the pH range of 3 to 5. The uranium ions were adsorbed in the form of ^UO22+ _.

Figure 17(B) shows the mixing time dependence of the amount of uranium adsorbed when 20 mL of a 100 ppm, pH 3 uranium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at 25±2°C. According to Figure 17(B), when the contact time was 30 minutes or more, all samples showed a high amount of adsorption exceeding 80%.

Figure 17 (C) shows the dose dependence of the amount of uranium adsorbed when 20 mL of a 100 ppm, pH 3 uranium standard solution was mixed with 5 mg to 100 mg of samples from Examples 1 and 2 at 25 ± 2°C for 30 minutes.

17(C), the amount of uranium adsorbed increased with increasing mass of the sample, but even when more than 20 mg of sample was used, the amount of uranium adsorbed did not change any more. This is believed to be because the active sites in the sample were interfered with by insoluble and stable hydroxides such as [( _UO2 ) ₂ (OH) ₂ ] ⁺² and [( _UO2 ) ₃ (OH) ₅ ] ⁺² .

Figure 18 (A) shows the temperature dependence of the amount of uranium adsorbed when 20 mL of a 100 ppm, pH 3 uranium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at various temperatures for 30 minutes. According to Figure 18 (A), the sample from Example 1 showed an increase in the amount of uranium adsorbed with increasing temperature, but all samples showed a high uranium adsorption amount of over 85% at room temperature.

Fig. 18(B) is a diagram showing the relationship between the amount of adsorption _qe and the uranium concentration _C0 in the uranium-containing solution. According to Fig. 18(B), it was found that when the uranium concentration in the uranium-containing solution exceeds a certain concentration, the amount of adsorption is saturated.

Furthermore, according to Figures 17 and 18, the sample of Example 2, whose surface and pores were modified with a chelating compound, showed a higher adsorption amount and higher adsorption efficiency than the sample of Example 1, whose surface was not modified, and was found to be an excellent uranium adsorption material.

Figure 19 shows the results of a release (desorption) test using samples from Examples 1 and 2 in which uranium was adsorbed by the batch method.

Figures 19 (A) and 19 (A-a) show the results when 0.1 g of the samples from Examples 1 and 2 after the uranium adsorption test was added to 20 mL of sulfuric acid of various concentrations and allowed to come into contact for 30 minutes at 25 ± 2°C.

According to Figure 19 (A-a), uranium can be liberated by using sulfuric acid with a concentration of 0.005M or more, but according to Figure 19 (A), it was found that uranium can be liberated efficiently by using sulfuric acid with a concentration of 0.1M or more and 0.3M or less.

Figures 19(B) and 19(B-a) show the results when 0.1 g of the samples from Examples 1 and 2 after the uranium adsorption test was added to 20 mL of 0.1 M sulfuric acid and allowed to come into contact for 5 to 150 minutes at 25±2°C.

According to Figure 19 (B-a), more than 75% of the uranium can be liberated by contacting for 20 minutes or more, but according to Figure 19 (B), it was found that the uranium can be liberated more efficiently by using a contact time of 45 minutes to 60 minutes.

These results demonstrate that the samples of Examples 1 and 2 are reusable uranium adsorbents.

Figure 20 shows the results of a batch uranium adsorption test using AGR-containing ore and a uranium-containing solution for the samples of Examples 1 and 2.

According to FIG. 20, it was found that the samples of Examples 1 and 2 selectively adsorbed uranium ions (U ⁴⁺ ) although they also adsorbed small amounts of Ba ions (Ba ²⁺ ) and Pb ions (Pb ²⁺ ).

Figure 21 shows the results of a batch-type uranium adsorption/desorption repeat test using a uranium standard solution for the samples of Examples 1 and 2.

The adsorption conditions were: 20 mL of 100 ppm, pH 3 uranium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at 25±2°C for 30 minutes. The desorption conditions were: 20 mL of 0.1 M sulfuric acid was mixed with 0.1 g of samples from Examples 1 and 2 at 25±2°C for 45 minutes.

According to Figure 21, the uranium adsorption efficiency and uranium release efficiency of the samples of Examples 1 and 2 decreased slightly as the number of repetitions increased, but still showed high efficiency even after more than 10 repetitions.

Figure 22 shows the results of a column-type uranium adsorption test using a uranium standard solution for the samples of Examples 1 and 2.

22A and 22B show the dependence of C _eff /C ₀ on the amount of solution passed when a 100 ppm, pH 3 uranium standard solution was passed through the samples (1 g) of Examples 1 and 2, respectively, at various rates. _{C 0} is the initial uranium concentration in the uranium standard solution, and C _eff is the uranium concentration removed from the uranium standard solution.

Figures 22(C) and (D) show the flow rate dependence of C _{eff /C 0 when 100 ppm, pH 3 uranium standard solution is passed through the samples of Examples 1 and 2 at various doses at 2.5 mL/min. Figures 22(E) and (D) show the flow rate dependence of C eff /C 0 when uranium} standard solutions of various concentrations and pH 3 are passed through the samples (1 g) of Examples 1 and 2 at 2.5 mL/min.

Figure 22 shows that if the samples of Examples 1 and 2 are used in a column of uranium adsorbent, all of the uranium in the test liquid can be adsorbed by adjusting the amount of test liquid passed through, the flow rate of the test liquid, the uranium concentration in the test liquid, and the dose amount filled into the column.

Figure 23 shows the results of a release (desorption) test using samples from Examples 1 and 2 in which uranium was adsorbed using the column method.

Figure 23 shows the dependence of the uranium release concentration on the amount of sulfuric acid passed through 1 g of samples from Examples 1 and 2 after the uranium adsorption test when sulfuric acid of various concentrations was passed through the samples at a rate of 0.5 mL/min. Figure 23 shows that uranium can be released efficiently by using sulfuric acid with a concentration of 0.1 M or more and 0.3 M or less.

Figure 24 shows the results of repeated column-type uranium adsorption/desorption tests using a uranium standard solution for the samples of Examples 1 and 2.

The adsorption conditions were as follows: 1500 mL of a 100 ppm, pH 3 uranium standard solution was passed through the samples (1 g) of Examples 1 and 2 at a flow rate of 2.5 mL/min. The desorption conditions were as follows: 40 mL of 0.1 M sulfuric acid was passed through the samples (1 g) of Examples 1 and 2 at a flow rate of 2.5 mL/min.

According to Figure 24, the uranium adsorption efficiency and uranium release efficiency of the samples of Examples 1 and 2 decreased slightly as the number of repetitions increased, but still showed high efficiency even after more than 10 repetitions.

From the above, it was shown that the samples of Examples 1 and 2 function as uranium adsorbents that selectively adsorb uranium and can be used repeatedly. In particular, it was shown that modification with a chelating compound results in a superior uranium adsorbent. It was shown that the uranium adsorbent of the present invention can be applied to both the batch method and the column method.

Figure 25 shows the results of a batch uranium adsorption test using AGR-containing ore and a uranium-containing solution for the sample of Example 2.

Figure 25 shows the gamma ray spectrum of the sample of Example 2 after the adsorption test. From Figure 25, it can be seen that the sample of Example 2 adsorbed the 238U isotope species and other radioactive substances leached from the AGR-containing ore. Furthermore, the count number was a value comparable to that used in the field of uranium adsorption and removal.

Figure 26 shows the results of batch and column uranium adsorption/desorption tests using uranium-containing solutions with AGR-containing ore for the samples of Examples 1 and 2.

The adsorption conditions for the batch method were: 20 mL of uranium-containing solution with pH 3 and 20 mg of the samples of Examples 1 and 2 were mixed at 25±2°C for 30 minutes. The desorption conditions for the batch method were: 20 mL of 0.1 M sulfuric acid and 0.1 g of the samples of Examples 1 and 2 were mixed at 25±2°C for 45 minutes. The adsorption conditions for the column method were: 1000 mL of uranium-containing solution with pH 3 was passed through the samples (1 g) of Examples 1 and 2 at a flow rate of 2.5 mL/min. The desorption conditions for the column method were: 40 mL of 0.1 M sulfuric acid was passed through the samples (1 g) of Examples 1 and 2 at a flow rate of 2.5 mL/min. The samples of Examples 1 and 2 after the adsorption test were analyzed by ICP-MS.

As shown in Figure 26, it was shown that the uranium adsorbent of the present invention can adsorb 238U isotope leached from actual minerals with a high adsorption efficiency of over 70%, whether using the column method or the batch method.

Figure 27 shows the results of a batch-type uranium adsorption test using AGR-containing ore and a uranium-containing solution for the samples of Examples 1 to 6.

The adsorption conditions for the batch method were: 20 mL of uranium-containing solution at pH 3 and 20 mg of samples from Examples 1 to 6 were mixed for 30 minutes at 25±2°C. Figure 27 also shows the results for the samples from Examples 1 and 2 shown in Figure 26.

As shown in Figure 27, all samples function as uranium adsorbents that adsorb uranium. In particular, it was shown that modification with a chelate compound results in a better uranium adsorbent. It was shown that chelate compounds modified with the above-mentioned formula 1 or 2 are effective.

The uranium adsorbent of the present invention can selectively adsorb and release uranium, making it applicable to various devices and methods for removing and recovering uranium.

100, 200 aggregate 110 porous silica 120 groove 210 chelate compound 600 uranium adsorption column 610 adsorbent 620 top filter 630 bottom filter 640, 650 column connection part

Claims (15)

The porous silica particles have a hexagonal crystal structure.
The aggregates have a hierarchical structure with micropores, mesopores and macropores;
The aggregates have a BET specific surface area in the range of 250 m ² /g or more and 300 m ² /g or less, and a pore volume in the range of 0.3 cm ³ /g or more and 0.5 cm ³ /g or less,
the surface of the agglomerate has a plurality of grooves between the particles;
A uranium adsorbent that selectively adsorbs or liberates uranium, comprising:
The surface and pores of the porous silica are modified with a chelating compound selected from the group consisting of phosphonic acid, [2-(triethoxysilyl) ethyl], dimethyl ester, 2-dimethoxy phosphorylethyl(triethoxy) silane, dimethyl{2-[diethoxy(methyl)silyl]phosphonate, and 3-diethoxyphosphorylpropyl(trimethy)silane . 2. The uranium adsorbent according to claim 1 , wherein a mass ratio of the chelate compound to the porous silica is in the range of 0.1 or more and 0.5 or less. The uranium adsorbent according to claim 2 , wherein a mass ratio of the chelate compound to the porous silica is in the range of 0.15 or more and 0.25 or less. The porous silica contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and when the total is 100 mass%, the mass percent concentration of each element is
45≦Si≦55
40≦O≦45
3≦C≦7, and
1≦P≦3
The uranium adsorbent according to any one of claims 1 to 3 , which satisfies the above. The particles have a median diameter (d50) in the range of 0.5 μm to 2.5 μm.
The uranium adsorbent according to any one of claims 1 to 4 . Adding a plant-derived silica source obtained from rice husk to an alkaline solution to prepare a plant-derived silica solution;
Adding cetyltrimethylammonium bromide (CTAB) to the plant-derived silica solution to obtain a raw material mixed solution;
subjecting the raw material mixed solution to hydrothermal synthesis, thereby precipitating a reaction product consisting of silica from the plant-derived silica source and the cetyltrimethylammonium bromide;
calcining the reactant and removing the cetyltrimethylammonium bromide to obtain a calcined product, and mixing the calcined product with a chelating compound selected from the group consisting of phosphonic acid, [2-(triethoxysilyl) ethyl], dimethyl ester, 2-dimethoxy phosphorylethyl(triethoxy) silane, dimethyl{2-[diethoxy(methyl)silyl]phosphonate, and 3-diethoxyphosphorylpropyl(trimethy)silane;
A method for producing the uranium adsorbent according to any one of claims 1 to 5 , comprising: 7. The method of claim 6 , wherein preparing the plant-derived silica solution comprises refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature in the range of 65°C to 75°C. The method according to claim 6 or 7, wherein the raw material mixed solution is obtained by adding cetyltrimethylammonium bromide so that the mass ratio of cetyltrimethylammonium bromide to the plant-derived silica in the plant-derived silica solution is in the range of 0.1 to 0.4 . The method according to any one of claims 6 to 8 , wherein the hydrothermal synthesis comprises adding an acid to the reactant, washing the reactant, and drying the reactant. The method according to any one of claims 6 to 9 , wherein the calcination of the reactant comprises calcining the reactant at a temperature in the range of 200°C to 800°C for a period of 2 hours to 24 hours. The method according to any one of claims 6 to 10 , wherein the mixing comprises mixing the chelate compound so that a mass ratio of the chelate compound to the baked product is 0.1 or more and 0.5 or less. The method of any of claims 6 to 11 , further comprising, following said mixing, washing and drying said mixed product. The method further includes preparing the plant-derived silica source prior to preparing the plant-derived silica solution, the preparing comprising:
washing and drying the rice husks;
Treating the dried rice husk with an acid having a concentration in the range of more than 0.5M to 1.5M and refluxing it at a temperature in the range of 75°C to 85°C for a period of 1 hour to 10 hours;
washing the acid and drying the acid at a temperature in the range of 80° C. to 110° C. for a period of 5 hours to 10 hours;
The method according to any one of claims 6 to 12 , further comprising calcining the dried rice husk at a temperature in the range of 600°C to 800°C for a period of 5 hours to 12 hours to decarbonate the rice husk. 1. A method for recovering uranium from uranium-bearing ore, comprising the steps of:
adding the uranium-containing ore to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare the uranium-containing solution;
A method for producing a uranium-containing solution comprising contacting the uranium adsorbent according to any one of claims 1 to 5 with the uranium-containing solution to allow the uranium adsorbent to adsorb uranium in the uranium-containing solution;
Discharging treated water from which uranium has been extracted from the uranium-containing solution;
contacting the uranium adsorbent having uranium adsorbed thereon with an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid to liberate the adsorbed uranium;
The pH of the uranium-containing solution is in the range of 3 to 6.5,
In addition to the acid solution, hydrogen peroxide is further added,
The heating temperature of the uranium-containing solution obtained by adding the hydrogen peroxide to the acid solution is in the range of 50° C. to 70° C.
method. The hydrogen peroxide is added to the acid solution at 4 v/v % or more and 8 v/v % or less.
The method of claim 14 .