Patent Application
20200030771
The present invention relates to a radionuclide absorbent, a method of preparing the same, and a method of removing a radionuclide using the same, and more particularly, to a radionuclide absorbent enabling selective removal of a radionuclide even in the presence of various competitive ions (e.g., Na+, K+, Mg2+, and Ca2+) in groundwater or seawater, a method of preparing the same, and a method of removing a radionuclide using the same.
As the amount of energy used by modern humans increases greatly, the development of new energy sources that will replace fossil fuels is considered a major issue, and among these energy sources, nuclear energy has been already widely used in many countries. However, nuclear power generation involves problems such as the reprocessing of thousands of tons of radioactive waste and nuclear fuel, which are annually generated after the use of nuclear fuel, and radioisotopes such as cesium (137Cs+) and strontium (90Sr2+) ions actually generated during the operation & maintenance of nuclear-related facilities and the use of radioisotopes have high water solubility, high nuclear fission energy, and a long half-life of about 30 years (Delacroix, D.; Guerre, J. P.; Leblanc, P.; Hickman, C. Radionuclide and Radiation Protection Data Handbook 2002. Radiat. Prot. Dosim. 2002, 98, 1-168). Therefore, it is very important to safely treat and store these materials.
Radioactive wastes can be divided into high-level and low-level radioactive wastes according to the level of radioactivity, the low-level radioactive waste is divided into a gas, a liquid and a solid according to its form, and among these, liquid waste is concentrated through precipitation, evaporation, adsorption and ion exchange, and then solidified in a drum, or chemically treated and diluted with a large amount of water, followed by discharging (El-Kamash, A. M.; El-Naggar, M. R.; El-Dessouky, M. I. Immobilization of Cesium and Strontium Radionuclides in Zeolite-Cement Blends. J. Hazard. Mater. 2006, 136, 310-316). Research on removing a very small amount of cesium ions (137Cs+) generated in the above procedure and dissolved in groundwater and seawater has been steadily progressing, and it is known in the art that inorganic ion exchange materials and polymer ion exchange resins are used to remove radionuclides from an aqueous solution.
Particularly, since the inorganic ion exchange materials are stable under an enormous amount of heat and radioactive conditions (Sylvester, P.; Clearfield, A. The Removal of Strontium and Cesium from Simulated Hanford Groundwater Using Inorganic Ion Exchange Materials. Solvent Extr. Ion Exch. 1998, 16, 1527-1539), they are widely used as radionuclide adsorbents. Representative examples include zeolite, clays, and porous metal oxide-based materials (metal oxidic sorbents). While zeolite and clays are widely used as common adsorbents due to low costs, it is known that they exhibit relatively low adsorption selectivity for a radionuclide in the presence of a high concentration of competitive ions (Sinha, P. K; Panicker, P. K.; Amalraj, R. V.; Krishnasamy, V. Treatment of Radioactive Liquid Waste Containing Caesium by Indigenously Available Synthetic Zeolites: A Comparative Study. Waste Manage. 1995, 15, 149-157). Recently, various inorganic ion exchange materials having higher selectivity have been reported, and examples of the inorganic ion exchange materials may include silicon titanate (U.S. Pat. No. 6,110,378A), crystalline silicotitanate (CST, UOP IE-911), vanadosilicate (WO2015129941A1), metal sulfides (U.S. Pat. No. 9,056,263B2), and metal hexacyanoferrates (EP0575612A1). Particularly, among these, crystalline silicotitanate (CST) is a material recently applied to the removal of radionuclides released during the Fukushima nuclear accident, and is known to be chemically stable in a wide pH range and exhibit high adsorption selectivity for cesium (Cs+) and strontium (Sr2+) ions in the presence of various competitive ions. However, these ion exchange materials include various transition metals (Sn, Sb, In, Ge, V, Ti, etc.), and compared to existing zeolite and clays, the price is high and the preparation method is complex, and there are many restrictions on application. Therefore, it is necessary to develop an economical ion exchange material enabling the selective removal of a radionuclide in the presence of a large amount of competitive ions present in groundwater and seawater.
For this reason, the inventors intend to provide an adsorbent having high adsorption selectivity for radionuclides such as cesium (Cs+) and strontium (Sr2+) ions in the presence of competitive ions by preparing a sulfur-zeolite composite by mixing sulfur with zeolite having a cation exchange property, and a method of preparing the same. In addition, they intend to provide a method of selectively removing very small amounts of cesium (Cs+) and strontium (Sr2+) ions in the presence of a large amount of competitive ions and competitive ions actually present in groundwater using the above-described adsorbent.
The present invention is directed to providing a radionuclide absorbent including a sulfur-zeolite composite in which sulfur is dispersed in zeolite, and a method of preparing the radionuclide absorbent by mixing sulfur with zeolite having a cation exchange property.
The present invention is also directed to providing a method of selectively removing very small amounts of cesium (Cs+) and/or strontium (Sr2+) ions in the presence of a large amount of competitive ions and competitive ions actually present in groundwater using the adsorbent.
However, technical problems to be solved in the present invention are not limited to the above-described problems, and other problems which are not described herein will be fully understood by those of ordinary skill in the art from the following descriptions.
The present invention provides a radionuclide absorbent including a sulfur-zeolite composite in which sulfur is dispersed in zeolite.
In addition, the present invention provides a method of preparing a radionuclide absorbent by thermally treating a mixture of sulfur and zeolite.
In addition, the present invention provides a method of removing a radionuclide by adsorbing a radionuclide by bringing a radionuclide absorbent including a sulfur-zeolite composite in which sulfur is dispersed in zeolite into contact with a solution containing cesium or strontium ions.
The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:
Hereinafter, the present invention will be described in further detail with reference to exemplary embodiments. The objects, features and advantages of the present invention are easily understood through the following exemplary embodiments. The present invention is not limited to exemplary embodiments to be described below, but may be embodied in other forms. The exemplary embodiments presented herein are provided such that the idea of the present invention can be fully conveyed to those of ordinary skill in the art to which the present invention belongs. Therefore, the present invention should not be limited by the following exemplary embodiments.
Unless defined otherwise, technical and scientific terms used herein each has a meaning ordinarily understood by those of ordinary skill in the art to which the present invention belongs. Generally, the nomenclature used herein is well known and commonly used in the art.
A sulfur-zeolite composite of the present invention was prepared by uniformly dispersing sulfur in zeolite by thermally treating a mixture of sulfur and zeolite, and it was found that the composite formed thereby, compared to conventional zeolite, can more selectively remove radioactive cesium (Cs+) and strontium (Sr2+) ions even in the presence of various competitive ions (e.g., Na+, K+, Mg2+ and Ca2+) in groundwater or seawater, and thus the present invention was completed.
Various methods, for example, liquid-liquid extraction, precipitation and adsorption, have been studied to treat radioactive materials such as Cs and Sr. The liquid-liquid extraction exhibits high selectivity for metal ions and excellent removal capability, but needs a large amount of organic solvent, and due to solubility, has a disadvantage in that an organic solvent may be lost in a liquid phase. The precipitation is a method of precipitating metal ions by adding a coagulant or precipitate, and has a disadvantage of taking too much time for recovery of a coagulant and a product, repeated use of a coagulant and precipitate separation. On the other hand, the adsorption has been known to have a low cost and to be easily carried out, compared to other processes, and representatively, activated carbon, ion exchange resins, activated alumina, silica gel, zeolites and clays are widely used as adsorbents. However, these adsorbents have shown problems of relatively low adsorption selectivity for radionuclides in the presence of a high concentration of competitive ions.
To solve the above-described problems, the present invention provides a method of preparing a radionuclide absorbent by thermally treating a mixture of sulfur and zeolite. In addition, the present invention provides a radionuclide absorbent including a sulfur-zeolite composite in which sulfur is dispersed in zeolite.
Hereinafter, the present invention will be described in detail.
The present invention provides a method of preparing a radionuclide absorbent, which includes thermally treating a mixture of sulfur and zeolite. The radionuclide may include one or more of cesium and strontium.
Step of Preparing Mixture of Sulfur and Zeolite
Before preparation of the mixture, water molecules in zeolite may be removed through thermal treatment, but the present invention is not limited thereto.
The mixture may be prepared by physically mixing sulfur and zeolite. The procedure of physically mixing zeolite and sulfur preferably uses elemental sulfur, but the present invention is not limited thereto, and may be performed by bringing zeolite into contact with a mixture having one or more sulfur molecules.
In this method, on the basis of the total weight (100 wt %) of the mixture of sulfur and zeolite, sulfur may be mixed at 1 to 25 wt % or 3 to 20 wt %. As a sulfur content in the sulfur-zeolite composite increases, it is advantageous for cesium ion adsorption. However, when about 24 to 25 wt % of sulfur is included, the specific surface area of NaX that has the widest specific surface area among zeolites is almost 0, and therefore, the upper limit range may be the largest amount of sulfur that can be incorporated into zeolite by thermal treatment (sublimation). As a result, it may be economical to mix the sulfur content within the range, and when the sulfur content is less than 1 wt %, there may be a problem of insignificant improvement in radionuclide ion selectivity.
In addition, in the method, on the basis of the total weight (100 wt %) of the mixture of sulfur and zeolite, sulfur may be mixed at 4 to 12 wt %. As a sulfur content in the composite increases, it is advantageous for adsorption of cesium ions, but disadvantageous for adsorption of strontium ions. Within this range, simultaneous removal of cesium ions and strontium ions can be most efficiently performed.
The zeolite may be aluminosilicate zeolite. The term “aluminosilicate zeolite” used herein refers to a zeolite material essentially including silicon atoms and aluminum atoms in the crystal lattice structure thereof.
More specifically, the zeolite may be one selected from the group consisting of chabazite (CHA), mordenite (MOR), NaA, NaX, faujasite (FAU), Linde Type A (LTA), analcime (ANA), Linde Type L (LTL), EMT (EMC-2), MFI (ZSM-5), ferrierite (FER), heulandite (HEU), beta polymorph A (BEA) and MTW (ZSM-12) structures, or a combination thereof.
The molar ratio of silicon (Si)/aluminum (Al) of the aluminosilicate zeolite may be 1 to 100, and more preferably, 1 to 20. There is no zeolite having a molar ratio of less than 1, and when the molar ratio of zeolite is more than 100, sulfur dispersion is not uniform.
Thermal Treatment
A sulfur-zeolite composite in which sulfur is uniformly dispersed in zeolite may be prepared by sublimating sulfur through the thermal treatment.
The thermal treatment temperature may be 50 to 700° C., preferably, 90 to 500° C., more preferably 130 to 400° C., and most preferably 150 to 350° C. When the thermal treatment temperature is less than 50° C., sulfur dispersion is not uniform, and when the thermal treatment temperature is more than 700° C., sulfur is evaporated.
The thermal treatment may be performed for 1 to 48 hours.
The thermal treatment may consist of first thermal treatment and second thermal treatment.
In addition, the present invention provides a radionuclide absorbent including a sulfur-zeolite composite in which sulfur is dispersed in zeolite.
The composite may include 1 to 25 wt % or 3 to 20 wt % of sulfur on the basis of the total weight (100 wt %) of the composite. As the sulfur content increases, it is advantageous for cesium ion adsorption, but when the composite includes about 24 to 25 wt % of sulfur, the specific surface area of NaX that has the widest specific surface area among zeolites is almost 0. Therefore, the upper limit range may be the largest amount of sulfur that can be incorporated into zeolite by thermal treatment (sublimation).
In addition, the composite may include 4 to 12 wt % of sulfur on the basis of the total weight (100 wt %) of the composite. As the sulfur content in the composite increases, it is advantageous for the adsorption of cesium ions, but disadvantageous for the adsorption of strontium ions. Within the above range, simultaneous removal of cesium ions and strontium ions is most efficiently performed.
The zeolite may be one selected from the group consisting of chabazite (CHA), mordenite (MOR), NaA, NaX, faujasite (FAU), Linde Type A (LTA), analcime (ANA), Linde Type L (LTL), EMT (EMC-2), MFI (ZSM-5), ferrierite (FER), heulandite (HEU), beta polymorph A (BEA) and MTW (ZSM-12) structures, or a combination thereof.
The composite may include 1 to 25 wt % of sulfur on the basis of the total weight (100 wt %) of the composite, and in this case, a radionuclide to be adsorbed may be a cesium ion. Here, the zeolite may be NaX.
In addition, the composite may include 4 to 12 wt % of sulfur on the basis of the total weight (100 wt %) of the composite, and in this case, radionuclides to be adsorbed may include cesium and strontium. Here, the zeolite may be chabazite (CHA).
The composite may be prepared by sublimating sulfur through thermal treatment of a mixture of sulfur and zeolite.
The present invention also includes a method of removing a radionuclide, which includes adsorbing a radionuclide by bringing the sulfur-zeolite composite prepared by thermally treating the mixture of sulfur and zeolite into contact with a solution containing cesium or strontium ions.
The method uses the sulfur-zeolite composite as an adsorbent, and may remove radionuclides such as cesium (Cs+) and strontium (Sr2+) ions by dispersing the composite in an aqueous solution or by using the composite as a packed bed.
Hereinafter, to help in understanding the present invention, exemplary examples will be suggested. However, the following examples are merely provided to more easily understand the present invention, and not to limit the present invention.
2 g each of zeolites such as chabazite (named “CHA”), NaA, NaX, mordenite (named “MOR”) was mixed with 0.222 g of sulfur at room temperature for 30 minutes. Afterward, the mixture was thermally treated at 433 K for 1 hour and at 593 K for 10 hours, thereby synthesizing a sulfur-zeolite composite. The samples prepared as described above were named “S-CHA”, “S-NaA”, “S-NaX”, and “S-MOR,” respectively.
The crystal structures of zeolites and sulfur-zeolite composites were analyzed using an X-ray diffractometer (XRD, Rigaku, D/MAX2100H), and
The cesium ion (Cs+) removal capability was evaluated for the zeolites and sulfur-zeolite composites prepared in Example 1 using an aqueous solution ion exchange method as follows. All ion exchange adsorption experiments were performed by bringing 0.02 g to 0.3 g of the zeolite or sulfur-zeolite composite into contact with 200 mL of an aqueous solution (prepared using CsCl) including 100 ppm of cesium ions (Cs+) at room temperature while stirring at a speed of 400 rpm for 3 hours. After stirring, the Cs+ concentration in the solution was measured using an inductively coupled plasma mass spectrometer.
Table 1 is a table summarizing an adsorption amount (qmax, mg gzeolite−1) and an adsorption constant (b, kg me) obtained by analyzing the ion exchange isotherms of
As shown in
Evaluation of Cesium Ion (Cs+) Removal Capability
The Cs+ removal capability was evaluated for the zeolites and sulfur-zeolite composites prepared in Example 1 using an ion exchange method in an aqueous solution similar to a real groundwater composition. A solution containing 1 ppm Cs+, 125 ppm Na+, 25 ppm Ca2+, 10 ppm Mg2+, and 5 ppm K+, which is similar to a groundwater condition contaminated with a radionuclide (Datta, S. J.; Moon, W. K.; Choi, D. Y.; Hwang, I. C.; Yoon, K. B. A Novel Vanadosilicate with Hexadeca-Coordinated Cs+ Ions as a Highly Effective Cs+ Remover. Angew. Chem. Int. Ed. 2014, 53, 7203-7208) was prepared using CsCl, NaCl, CaCl2.2H2O, MgCl2.6H2O, and KCl. An ion exchange experiment was performed by bringing 0.1 g of zeolite and 0.111 g of a sulfur-zeolite composite (having the same volume as 0.1 g of general sulfur-free zeolite, because the zeolite weight in 0.111 g of a sulfur-zeolite composite containing 10 wt % of sulfur is 0.1 g; even in a subsequent adsorption experiment, sulfur-zeolite composites containing various wt % of sulfur had the same weight of zeolite; this is because, while the sulfur-zeolite composites containing sulfur became heavier by the sulfur weight, compared to zeolite, sulfur was present in a pore of zeolite, and therefore there was no volume change regardless of the presence or absence of sulfur, and since a volume is a more critical factor than a weight of an adsorbent when actually applied to a column or when disposing radioactive waste, all adsorption experiments were performed with the same weight of zeolite to compare the experimental results of the sulfur-zeolite composites with the same volume) into contact with 200 mL of the aqueous solution at room temperature, and stirring the resulting mixture at 400 rpm for 3 hours.
Evaluation of Strontium Ion (Sr2+) Removal Capability
Since zeolites are known as materials that are basically ion-exchangeable with divalent ions (e.g., heavy metals and Sr), the removal characteristic for a major radionuclide Sr2+, other than Cs+, was analyzed, and the result is shown in
Analysis of Relationship Between Sulfur (S) Amount and Cesium (Cs) Removal Effect
According to the experimental result, when sulfur (S) is added, it is confirmed to be advantageous for cesium (Cs) removal. To confirm the relationship between a sulfur (S) amount and a cesium (Cs) removal effect, as the amount of sulfur was increased, the change in cesium (Cs) removal characteristic was analyzed using the NaX zeolite.
S-NaX containing 9.4 wt %, 17 wt % or 24 wt % of sulfur on the basis of the total weight of a sulfur-NaX zeolite composite was prepared, and then the XRD, specific surface area and TEM image thereof were analyzed, and the results are shown in
aBET surface areas were determined in the P/Po range of 0.08-0.20.
bMicropore volumes (Vmicro) were determined using t-plot method.
cTotal pore volumes (Vtotal) were determined at P/Po = 0.95.
Since the sizes of micropore volumes of 4 types of zeolite below are lowered in the order of NaX>NaA>CHA MOR and thus NaX can contain the largest amount of S, the maximum encapsulation amount of sulfur with respect to different zeolites is lowered in the order of NaX>NaA>CHA MOR, which is the same as the order of the micropore volume sizes, and the sulfur encapsulation amount with respect to a zeolite weight is expected to be approximately 15 to 25%.
The Cs+ removal rate and distribution coefficient (kd), in 200 mL of distilled water containing 1 ppm of Cs+ and Ca+ and Na+ at up to 50-fold moles higher than Cs+, were analyzed using 100 mg each (for example, when using 131.6 mg, the sulfur-zeolite composite containing 24 wt % of sulfur has a weight of 100 mg on the basis of the zeolite weight) of general sulfur (S)-free zeolite and sulfur-zeolite (S-Nax) composites (respectively containing 9.4, 17 and 24 wt % of sulfur) on the basis of a zeolite weight, and the results are shown in
Referring to
Analysis of Sulfur Content Suitable for Simultaneous Removal of Sr2+ and Cs+
As more sulfur (S) is contained in a sulfur-zeolite composite, it is advantageous for Cs+ removal, but not for Sr2+ removal. Accordingly, the following experiment was carried out to find a sulfur content suitable for simultaneous removal of Sr2+ and Cs+.
The Cs+ removal rate and distribution coefficient (kd), in 20 mL of distilled water containing 1 ppm of Cs+ and a competitive ion, K+, at an amount up to 10,000-fold higher than the Cs+ mole number, were analyzed using 10 mg each (for example, when using 11.11 mg, the sulfur-zeolite composite containing 10 wt % of sulfur has a weight of 10 mg on the basis of the zeolite weight) of sulfur (S)-zeolite (CHA) composites containing 10 wt % and 5 wt % of sulfur on the basis of the total weight of the sulfur-zeolite composite and zeolite (CHA) on the basis of a zeolite weight, and the results are shown in
In addition, the Sr2+ removal rate and distribution coefficient (kd), in 20 mL of distilled water containing 1 ppm of Sr2+ and a competitive ion, Ca′, at an amount up to 10,000-fold higher than the Sr2+ mole number, were analyzed using 10 mg each of the sulfur (S)-zeolite (CHA) composites containing 10 wt % and 5 wt % of sulfur, on the basis of the total weight of the sulfur-zeolite composite and zeolite (CHA) on the basis of a zeolite weight, and the results are shown in
As shown in
For reference, under a seawater Cs+ condition (when K+ is 1,000-fold higher than the Cs+ mole number), it was confirmed that 5S-CHA has a removal rate and Kd similar to 10S-CHAR, and under a seawater Sr2+ condition (when Ca2+ is 1,000-fold higher than the Sr2+ mole number), it was confirmed that 5S-CHA has a removal rate and Kd similar to general CHA.
Judging from the above-described results collectively, for removal of a low concentration of Cs+ (Cs concentration of 1 ppm or less), zeolite containing a large amount of sulfur (S) is preferable, and for simultaneous removal of Sr2+ and Cs+, it is determined that a sulfur-zeolite composite (5S-Charbazite) containing 5 wt % of sulfur is the most preferable composition. This is because the above-mentioned examples exhibited a similar Cs+ removal performance to 10S-CHA under a seawater condition, and a similar Sr2+ removal performance to CHA under a seawater condition.
Subsequently, Cs+ and Sr2+ removal rates in 20 mL of groundwater containing 1 ppm of Cs+ and 0.7 ppm of Sr2+ were analyzed using 10 mg each of sulfur (S)-zeolite (CHA) composites containing 10 wt % and 5 wt % of sulfur on the basis of the total weight of sulfur-zeolite composite and zeolite (CHA) on the basis of a zeolite weight, and the results are shown in
Using 10 mg each of sulfur (S)-zeolite (CHA) composites containing 10 wt % and 5 wt % of sulfur on the basis of the total weight of sulfur-zeolite composite and zeolite (CHA) on the basis of a zeolite weight, a change in Cs+ adsorption rate in 20 mL of distilled water containing 86 ppm of Cs+ and a change in Sr2+ adsorption rate in 20 mL of distilled water containing 98 ppm of Sr2+ were analyzed. It was shown that the higher the sulfur content, the lower the Cs+ adsorption rate (removal rate) and the Sr2+ adsorption rate. This is because sulfur occupies a pore of zeolite, and thus the direction in which Cs+ or Sr2+ passes is interfered with.
In addition, when more pores are occupied by sulfur, sites which Cs+ can enter are taken, and therefore, the maximum adsorption amount of Cs+ is lowered. However, as more pores are occupied by sulfur, it was confirmed that Langmuir constants (CHA: 1.78, 5S-CHA: 2.7, 10S-CHA: 6.16) increase (the slope increases), meaning that the chemical affinity with Cs+ is increased and thus the selectivity increases. When more pores are occupied by sulfur and sites which Sr2+ can enter are taken, it was confirmed that the maximum adsorption amount of Sr2+ is lowered, and Langmuir constants decrease (the slope decreases). This means that the affinity with Sr2+ was lowered.
The cesium ion (Cs+) removal capability in an aqueous solution in the presence of competitive ions for the zeolites and sulfur-zeolite composites prepared in Example 1 was evaluated by an ion exchange method as follows. Distilled water containing 1 ppm of cesium ions (Cs+) was prepared, and then NaCl and CaCl2.2H2O were additionally dissolved therein to adjust a mole (number) ratio of competitive ions (Na+ or Ca2+) to cesium ions (Cs+) to 0 to 10,000-fold. An ion exchange experiment was performed by bringing 200 mL of the aqueous solution into contact with 0.1 g of zeolite and 0.111 g of a sulfur-zeolite composite at room temperature, and stirring the resulting mixture at 400 rpm for 3 hours.
A radionuclide absorbent of the present invention can more selectively remove radioactive cesium (Cs+) and/or strontium (Sr2+) ions in the presence of various competitive ions (e.g., Na+, K+, Mg2+ and Ca2+) in groundwater or seawater, compared to conventional zeolite.
In addition, the radionuclide absorbent of the present invention can be prepared by a simple method of thermally treating a mixture of sulfur and zeolite, and according to the above-described method, sulfur can be uniformly dispersed in zeolite.
It should be understood by those of ordinary skill in the art that the above description of the present invention is exemplary, and the exemplary embodiments disclosed herein can be easily modified into other specific forms without departing from the technical spirit or essential features of the present invention. Therefore, the exemplary embodiments described above should be interpreted as illustrative and not limited in any aspect.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2018-0087390 | Jul 2018 | KR | national |
| 10-2019-0089900 | Jul 2019 | KR | national |
