BUFFER MATERIAL COMPOSED OF BENTONITE MODIFIED WITH LAYERED DOUBLE HYDROXIDE(LDH) AND METHOD OF PREPARING THE SAME

Abstract

The present invention relates to a method of preparing a buffer material composed of bentonite modified with a layered double hydroxide (LDH) as a buffer material used for deep geological disposal of radioactive waste, the method including a step (a) of producing a first mixture by adding a compound containing a divalent cationic material, aluminum nitrate (Al(NO.sub.3).sub.3), and bismuth nitrate (Bi(NO.sub.3).sub.3) to a reactor.

Claims

1. A method of preparing a buffer material used for deep geological disposal of radioactive waste, the method comprising a step (a) of producing a first mixture by adding a compound containing a divalent cationic material, aluminum nitrate (Al(NO.sub.3).sub.3), and bismuth nitrate (Bi(NO.sub.3).sub.3) to a reactor.

2. The method of claim 1, further comprising a step (b) of producing a second mixture by adding bentonite to the first mixture produced in the step (a).

3. The method of claim 2, wherein the compound containing a divalent cationic material is any one of cobalt nitrate (Co(NO.sub.3).sub.2), copper nitrate (Cu((NO.sub.3).sub.2), and nickel nitrate (Ni((NO.sub.3).sub.2).

4. The method of claim 3, wherein in the step (a), in a state in which ultrapure water is included in the reactor, the compound containing a divalent cationic material at a molar concentration of 0.1 to 0.2 is added per 100 mL of the ultrapure water.

5. The method of claim 4, wherein in the step (a), the aluminum nitrate (Al(NO.sub.3).sub.3) at a molar concentration of 0.02 to 0.04 is added per 100 mL of the ultrapure water.

6. The method of claim 5, wherein in the step (a), the bismuth nitrate (Bi(NO.sub.3).sub.3) at a molar concentration of 0.01 to 0.03 is added per 100 mL of the ultrapure water.

7. The method of claim 6, wherein in the step (b), the bentonite is added in an amount of 5 to 10 g per 100 mL of the ultrapure water.

8. The method of claim 7, wherein in the step (a), nitric acid (HNO.sub.3) at a molar concentration of 0.5 to 1.5 is further included in the reactor.

9. The method of claim 8, further comprising a step (c) of titrating the second mixture produced in the step (b) to a pH of 8.5 to 9.5.

10. The method of claim 9, further comprising, after the step (c), a step (d) of aging the second mixture at a constant temperature.

11. The method of claim 10, wherein in the step (d), the second mixture is aged while the reactor in a sealed state is placed in an oven at 75 to 85° C. for 20 to 28 hours.

12. A buffer material prepared by the method of preparing a buffer material of claim 1.

13. A buffer material used for deep geological disposal of radioactive waste, the buffer material comprising bentonite having a surface modified with a LDH.

14. The buffer material of claim 13, wherein the LDH includes aluminum (Al).

15. The buffer material of claim 14, wherein the LDH includes bismuth (Bi).

16. The buffer material of claim 15, wherein the LDH includes any one of cobalt (Co), copper (Cu), and nickel (Ni).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic view illustrating a method of preparing a buffer material according to the present invention.

[0029] FIGS. 2A and 2B are micrographs of bentonite having a surface modified with a LDH.

[0030] FIGS. 3 and 4 are graphs showing experimental results of removal efficiency of the buffer material prepared according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0031] Hereinafter, preferred embodiments of a method according to the present invention will be described in detail with reference to the accompanying drawings. In this process, thicknesses of lines, sizes of components, and the like, illustrated in the accompanying drawings may be exaggerated for clearness of explanation and convenience. In addition, terms to be described below are defined in consideration of functions in the present invention and may be construed in different ways by the intention of users or operators or practice. Therefore, these terms should be defined on the basis of the contents throughout the present specification.

[0032] The present invention relates to a method of preparing a buffer material having improved anion adsorption ability by using anion exchange characteristics of LDH, and more particularly, to a method of modifying a surface of bentonite used as a buffer material with a LDH and modified bentonite prepared according to the method.

[0033] The LDH is one of anionic clays, and is generally represented by a chemical formula [M.sup.2+.sub.1−xM.sup.3+.sub.x(OH).sub.2](A.sup.n−).sub.x/n.Math.mH.sub.2O.

[0034] In LDH, a divalent metal cation (M.sup.2+) is surrounded by six hydroxyl groups in the form of an octahedron, and some M.sup.2+ are isomorphically substituted with trivalent metal cations (M.sup.3+). Therefore, LDH has a two-dimensional layered structure in which a surface is positively charged. At this case, anions (A.sup.n−) are inserted between the layers having a positive charge to balance the charges, and the anions (A.sup.n−) have a characteristic of being exchangeable with other anions.

[0035] 1. A method of preparing a buffer material according to the present invention will be described with reference to FIG. 1.

[0036] (1) Production of First Mixture

[0037] A first mixture may be produced by adding a compound containing a divalent cationic material, aluminum nitrate (Al(NO.sub.3).sub.3), and bismuth nitrate (Bi(NO.sub.3).sub.3) to a reactor including ultrapure water and nitric acid and stirring the mixture.

[0038] Here, the compound containing a divalent cationic material may be any one of cobalt nitrate (Co(NO.sub.3).sub.2) containing cobalt cations (Co.sup.2+), copper nitrate (Cu((NO.sub.3).sub.2) containing copper cations (Cu.sup.2+), and nickel nitrate (Ni((NO.sub.3).sub.2) containing nickel cations (Ni.sup.2+).

[0039] Here, the compound containing a divalent cationic material at a molar concentration of 0.1 to 0.2 may be added per 100 mL of the ultrapure water, the aluminum nitrate (Al(NO.sub.3).sub.3) at a molar concentration of 0.02 to 0.04 may be added per 100 mL of the ultrapure water, and the bismuth nitrate (Bi(NO.sub.3).sub.3) at a molar concentration of 0.01 to 0.03 may be added per 100 mL of the ultrapure water.

[0040] (2) Production of Second Mixture

[0041] A second mixture may be produced by adding bentonite to the first mixture and stirring the mixture. Here, the bentonite may be added in an amount of 5 to 10 g per 100 mL of the ultrapure water.

[0042] Thereafter, the produced second mixture may be titrated to a pH of 8.5 to 9.5.

[0043] (3) Aging and Lyophilization

[0044] In a state in which the second mixture titrated to a pH of 8.5 to 9.5 is included in the reactor and the reactor is sealed and then placed in an oven at a constant temperature for a certain period of time, the second mixture may be aged while being stirred.

[0045] After the aging, a solid may be separated using centrifugation, the solid may be washed three times with ultrapure water, and then the washed solid may be lyophilized, thereby preparing a buffer material composed of bentonite having a surface modified with a LDH.

[0046] In FIG. 1, M.sup.2+AlBi LDH-Bentonite represents bentonite modified with a LDH containing a divalent cationic material.

[0047] 2. Specific examples will be described.

[0048] To a reactor containing 100 mL of ultrapure water and nitric acid (HNO.sub.3) at a molar concentration of 1, a compound containing a divalent cationic material at a molar concentration of 0.15, aluminum nitrate (Al(NO.sub.3).sub.3) at a molar concentration of 0.03, and bismuth nitrate (Bi(NO.sub.3).sub.3) at a molar concentration of 0.02 were added, and the mixture was stirred, thereby producing a first mixture.

[0049] Thereafter, 5 to 10 g of bentonite was added to the first mixture, and the mixture was stirred, thereby producing a second mixture.

[0050] Thereafter, the second mixture was titrated to a pH of 9 using sodium hydroxide (NaOH).

[0051] Thereafter, the reactor including the second mixture was sealed, and then the second mixture was aged while being stirred in an oven at 80° C. for 24 hours.

[0052] Thereafter, a solid was separated using centrifugation, the separated solid was washed three times with ultrapure water, and then the washed solid was lyophilized, thereby preparing bentonite having a surface modified with a LDH.

[0053] Bentonite is a smectite-based clay mineral mainly containing montmorillonite, and has a plate-like structure in which one alumina plate is sandwiched between two silica plates (silica plate:alumina plate=2:1). In this case, when isomorphic substitution in which Si.sup.4+ is substituted with Al.sup.3+ or Al.sup.3+ is substituted with Fe.sup.2+ or Mg.sup.2+ within a tetrahedron occurs, a form that satisfies an insufficient charge with Na.sup.+ is sodium bentonite, and a form that satisfies an insufficient charge with Ca.sup.2+ is calcium bentonite.

[0054] Compared to other clay minerals, bentonite exhibits a significantly higher cation exchange capacity, which is due to a permanently negative charge caused by isomorphic substitution and exchangeable cations such as Na.sup.+, Ca.sup.2+, and Mg.sup.2+ present between the layers of bentonite.

[0055] The synthesis principle of bentonite having a surface modified with a LDH(M.sup.2+AlBi LDH-Bentonite) is as follows. When a divalent cationic metal salt and a trivalent cationic metal salt are added and a NaOH base solution is added for pH titration, and then aging is performed, a LDH is formed. In this case, since nanoparticles of the LDH formed are positively charged, the nanoparticles are aggregated on a negatively charged surface of bentonite.

[0056] It will refer to FIG. 2. As a result of scanning electron microscopic analysis of the synthesized buffer material, nano-sized particles, which were formed by aggregation of LDHs on the surface of the bentonite and piled up in thin layers, were observed.

[0057] In FIG. 2, (A) is a photograph of bentonite and (B) is a photograph of bentonite modified with a LDHs containing cobalt cations (CoAlBi LDH-Bentonite).

[0058] 3. Experiment on Adsorption Efficiency and Experimental Result

[0059] It will be described with reference to the experimental result shown in Table 1 and FIG. 3.

[0060] An experiment on adsorption efficiency of modified bentonite for cationic and anionic radionuclides was carried out for a reaction time of 4 days by adding each bentonite having a surface modified with LDHs prepared by being added to the first mixture in an amount of 5 g and unmodified bentonite to each reaction solution at a target concentration of radionuclides of 100 mg/L at a solid-liquid reaction ratio of 0.4 g/40 mL.

TABLE-US-00001 TABLE 1 Cs.sup.+ IO.sub.3.sup.− I.sup.− Bentonite 86.48 0.00 0.00 CuAlBi 45.47 95.40 72.96 LDH-Bentonite CoAlBi 58.70 93.68 70.19 LDH-Bentonite NiAlBi 63.86 85.75 54.36 LDH-Bentonite

[0061] The experimental result was shown that when the content of unmodified bentonite was compared with the content of bentonite having a surface modified with LDHs, the content of the modified bentonite was reduced compared to the content of the unmodified bentonite in terms of the same unit amount, the overall removal efficiency of Cs.sup.+ was reduced a little bit in the modified bentonite, but the removal efficiency of the iodine species (I.sup.− and IO.sub.3.sup.−) in the modified bentonite was significantly increased than in the unmodified bentonite. In particular, it may be seen that the removal efficiency of IO.sub.3.sup.− is increased by 85% or more compared to the removal efficiency of I.sup.−. It is presumed that a difference in removal efficiency occurs because the ionic radius of I.sup.− (0.210 nm) is larger than that of IO.sub.3.sup.− (0.122 nm).

[0062] The experimental result on the removal efficiency of radionuclides of the modified bentonite according to the content of bentonite added will be described with reference to FIG. 4.

[0063] As an experiment on removal efficiency of cationic and anionic radionuclides, an experiment on removal efficiency of modified bentonite according to the content of bentonite added was carried out for a reaction time of 4 days by adding each bentonite having a surface modified with LDHs prepared by being added to the first mixture in an amount of 6 g to 10 g and unmodified bentonite to each reaction solution at a target concentration of radionuclides of 1 mmol/L at a solid-liquid reaction ratio of 0.4 g/40 mL.

[0064] In FIG. 4, CuAlBi represents bentonite modified with a LDH containing copper cations (CuAlBi LDH-Bentonite), and CoAlBi represents bentonite modified with a LDH containing cobalt cations (CoAlBi LDH-Bentonite).

[0065] As a result of the experiment, it may be seen that modified bentonite with optimized removal efficiency may be prepared by adjusting the content of bentonite according to the concentration or type of the radionuclides in the target solution. In a case where high amounts of anionic radionuclides are present, modified bentonite prepared by adding about 6 g of bentonite may be applied, and in a case where both cationic radionuclides and anionic radionuclides are present in large amounts, modified bentonite prepared by adding about 7 g to 8 g of bentonite may be applied for the optimized removal efficiency.

[0066] Owing to the above-described technical solution, it is possible to prepare a buffer material capable of efficiently removing not only cationic radionuclides but also anionic radionuclides by modifying a surface of bentonite having excellent removal efficiency of cationic radionuclides with a LDH, and therefore, it is possible to improve the safety of radioactive material disposal in a radioactive waste disposal facility where such a buffer material is used.

[0067] Hereinabove, the present specification has been described with reference to the embodiments illustrated in the drawings so that those skilled in the art can easily understand and reproduce the present invention, but this is only exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible from the embodiments of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.