KR102517786B1 - Absorbing material for immobilization of radioiodine via interzeolite transformation to sodalite and immobilization method of radioiodine - Google Patents

Abstract

The present invention is a FAU zeolite to which radioactive iodine is adsorbed, wherein the FAU zeolite is a Na-X zeolite, and the Na-X zeolite is converted into a sodalite (SOD) structure by interzeolite conversion, so that the radioactive iodine is sodalite -Provides an adsorption material for immobilizing radioactive iodine through conversion between zeolites with a sodalite structure characterized in that it is fixed in a cage ( sod -cage). Although Na-X had lower CH ₃ I adsorption capacity than Ag-X, it was able to fix 98% of the adsorbed iodine.

Description

Absorbing material for immobilizing radioactive iodine through conversion between zeolites into sodalite structure and method for immobilizing radioactive iodine

The present invention relates to an adsorption material for immobilizing radioactive iodine and a method for immobilizing radioactive iodine through conversion between zeolites into a sodalite structure.

As various radioactive elements were released into the environment following the Fukushima nuclear disaster in 2011, public awareness of the dangers of the radioactive elements has increased. Among them, radioactive iodine is a typical radioactive element. Because the thyroid readily absorbs iodine, it causes thyroid cancer in humans. Representative radioactive isotopes of iodine are ¹³¹ I and ¹²⁹ I.

¹³¹ I has a short half-life of 8 days and is highly radioactive. However, ^129I has a relatively long half-life of 1.6 X 10 ⁷ years and high mobility in most geological environments, making it one of the highest dose contributors. When a serious nuclear power plant accident occurs, radioactive iodine may be released into the atmosphere in the form of organic iodine compounds such as methyl iodide (CH ₃ I), which accounts for the largest proportion of radioactive iodine compounds. In order to selectively remove volatile CH ₃ I contained in gas emitted from a nuclear power plant accident, a dry filtration system using an adsorbent such as zeolite may be used to capture CH ₃ I and adsorb it into an adsorbate.

A number of studies have investigated the adsorptive removal of CH ₃ I using various metal ion exchange zeolites. In the evaluation of ion exchange Y zeolites (framework type FAU) with various metals with similar metal contents, Ag-Y and Cu-Y showed the highest adsorption performance.

Compared to other metal iodides, silver iodide (AgI) and copper iodide (CuI) are products with much lower solubility (Ksp = 8.5 X 10 ^-17 , 1.1 X 10 ^-12 at 25 °C, respectively), so they have relatively strong metallic properties. have chemical bonds. A comparison of CH ₃ I adsorption performance for various zeolite structures ion-exchanged with Ag has also been reported. When CH ₃ I is adsorbed on Ag-zeolite, Ag ⁺ can be combined with I ^- and stored in the form of AgI, and CH ₃ ⁺ is bonded to the negatively charged zeolite framework or to an additional framework OH ^- . Among various Ag-zeolite structures, Ag-X(FAU) and Ag-Y showed the highest capacity. On the other hand, zeolite adsorbates exchanged with other cations such as Na ⁺ also showed good CH ₃ I adsorption capacity as Ag-zeolite.

Unlike other technologies used for removal adsorption of hazardous substances, it is not necessary to remove radioactive isotopes by catalytic conversion and regeneration of the adsorbent. For safety reasons, it is far more important to permanently immobilize adsorbed radioactive elements. Metal iodine compounds formed inside the zeolite structure by adsorbing CH ₃ I have, for example, sodium iodide (NaI) (ca. 3.1 Å) and AgI (ca. 2.8 Å), which are generally smaller than the zeolite pore size (<10 Å). Since it is smaller, it is likely to be detached again. It is known that radioactive iodine can structurally transform iodine-containing zeolites by applying high temperatures (>800℃) and high pressures (>100MPa) to fix them into zeolite structures such as sodalite (SOD) and cancrinet (CAN). . This is known as the Hot Isostatic Pressing (HIP) technique.

Alternatively, some zeolites can be transformed to other structures under mild hydrothermal conditions (<200 °C and <2 MPa), which is called interzeolite transformation.

It is well known that zeolite structures with low framework density (FD) are more likely to convert to structures with higher FD as described by Oswald's rule. For example, Zeolite A (LTA) with Si/Al = 1 readily converts to the SOD structure and has structural properties similar to Si/Al = 1 (FD = 12.9T/1000 Å ³ ) but with a higher FD = 17.2 T /1000 Å ³ . In addition, zeolite X with Si/Al = 1.2 (FD = 12.7 T/1000 Å ³ ) can also be converted to SOD structure under certain hydrothermal conditions and converted into various frameworks such as ANA, ^* BEA, CHA, MAZ, MER, etc. It has been reported that it can be

As shown in Figure 1, all SOD, LTA and FAU structures have a common sub-structural unit called sod -cage. The SOD is connected by sharing the 4-ring of the sod-cage, and the sod -cages of LTA and FAU are connected by a double 4-ring (d4r) and a double 6-ring (d6r), respectively. Accordingly, LTA and FAU have 8-ring (4.1 Å ³ ) and 12-ring (7.4 Å ³ ) open pores, respectively, allowing small molecules such as CH ₃ I and metal iodides to pass through, whereas SOD has open pores. do not have pores Sodalite cannot efficiently adsorb CH ₃ I because its pores are not open, but when radioactive iodine is trapped in the sod -cage, sodalite can be used as an efficient immobilization substrate.

The SOD structure with Si/Al = 1 has 12 tetrahedral atoms (6Si + 6Al) and 2 sod -cages per unit cell. Therefore, in SOD, one sod -cage has four cations (eg Na ⁺ ) forming a tetrahedral structure around one anion (eg OH ^- or halogen atom), and due to the imbalance of the three framework aluminum atoms It compensates for three negative charges as well as one central anion.

SOD can be hydroxide sodalite (HSOD) or iodide sodalite (ISOD) occupied by OH ^- and I ^- respectively, depending on the type of central anion present in the sod -cage. When zeolite A (LTA) with sodium iodide (NaI) was used as the Si and Al source, sodalite iodide (ISOD) showed higher yield than common Si and Al precursors (sources) such as colloidal silica and sodium aluminate, respectively. reported to be better formed. In addition, soda iodide (ISOD) was mixed with two types of sodium borate using metakaolin, NaI, and NaOH to form pellets, and then heated to a temperature of 850 ° C. or less to fix iodine.

Korean Patent Registration No. 10-2075011

In the present invention, the synthesis and characterization of ISOD were first performed by interzeolite conversion of Na-A, Na-X, and Na-Y zeolites. Then, based on these results, the adsorption capacity of CH ₃ I was compared with zeolite A and X in which Na ⁺ and/or Ag ⁺ were ion-exchanged, and CH ₃ I was adsorbed by ISOD by conversion of interzeolite sod -cage The immobilization ability of iodine was continuously evaluated. All iodine compounds used in the present invention do not contain radioactive iodine.

The present invention provides a technology for fixing radioactive iodine in a sod -cage by converting iodine-containing LTA (Zeolite A) and FAU (Zeolite X and Y) into a sodalite (SOD) structure. Immobilization of iodine in the sod -cage was confirmed using various characterization methods such as powder XRD, elemental analysis, SEM-EDS, ¹²⁷ I MAS NMR (magic angle spinning nuclear magnetic resonance) spectroscopy, and I 3d XPS. Both zeolite A(Na-A) and X(Na-X) were well converted to the SOD structure in the presence of NaI and AgI, but iodine ions were fixed in the sod -cage only when NaI was used. The adsorption capacity of methyl iodide (CH ₃ I) was evaluated for zeolites A and X in which Na ⁺ and/or Ag ⁺ ions were exchanged, and Ag ⁺ and zeolite X showed better adsorption properties than Na ⁺ and zeolite A, respectively. However, considering both the CH ₃ I adsorption capacity and the continuous immobilization of iodine by interzeolitic conversion, Na-X was determined to be the best candidate for adsorbent among the studied zeolites. Although the Na-X zeolite showed half the CH ₃ I adsorption capacity of the Ag-X zeolite, more than 98% of iodine was successfully fixed in the sod -cage of the SOD structure by interconversion of Na-X following CH ₃ I adsorption. It became.

According to one embodiment of the present invention, as FAU zeolite to which radioactive iodine is adsorbed, the FAU zeolite is Na-X zeolite, and the Na-X zeolite is converted into a sodalite (SOD) structure by interzeolite conversion. An adsorption material for immobilizing radioactive iodine through conversion between zeolites with a sodalite structure characterized in that radioactive iodine is fixed in a soda -cage is provided.

The Na—X zeolite is characterized in that the SiO ₂ /Al ₂ O ₃ molar ratio is 1.2.

The ratio of radioactive iodine fixed per sod-cage (I ^- / sod -cage) of the Na-X zeolite converted to the sodalite (SOD) structure by interzeolite is 0.8 to be characterized

The Na—X zeolite converted between zeolites into the sodalite (SOD) structure is characterized in that the I 3d _5/2 XPS peak is measured at 619.5 eV in the I 3d XPS spectrum.

The adsorption amount of CH ₃ I adsorbed on the Na—X zeolite is characterized in that 140-280 mg/g-zeolite.

Another aspect of the present invention is adsorbing radioactive iodine to Na—X zeolite as FAU zeolite; Converting the iodine-adsorbed Na—X zeolite into a sodalite (SOD) structure by converting the zeolite to interzeolite; and immobilizing the radioactive iodine in a soda-cage of the Na-X zeolite converted into the sodalite ( SOD ) structure by interzeolite. A method for immobilizing radioactive iodine through liver conversion is provided.

According to the present invention, as a FAU zeolite adsorbed with radioactive iodine, the FAU zeolite is a Na-X zeolite, and the Na-X zeolite is converted into a sodalite (SOD) structure by interzeolite conversion, so that the radioactive iodine is dissolved in sodalite. A sodalite structure characterized in that it is fixed in a light-cage provides an adsorption material for immobilizing radioactive iodine through conversion between zeolites.

In the present invention, a study was conducted to immobilize iodine in a sod -cage by interzeolite conversion after CH ₃ I adsorption using Na- and Ag-zeolite adsorbents. Initially, interzeolite conversion was performed in the presence of NaI or AgI for Na-A, Na-X and Na-Y. Unlike the other two zeolites, Na-Y, which is known to have the highest adsorption capacity, was not well converted to the SOD structure. The CH ₃ I adsorption capacities of the four zeolites that can be converted to the SOD structure were compared under the same conditions. The adsorption capacity was in the order of Ag-X > Ag-A > Na-X > Na-A. Therefore, as expected, the adsorption capacity of Ag ⁺ was higher than that of Na ⁺ and FAU was higher than that of LTA. On the other hand, when the immobilization ability of iodine by interzeolite conversion of the CH ₃ I adsorbed sample was evaluated, Ag-A had excellent adsorption capacity and was well converted to the SOD structure without impurities, but the adsorbed iodine was not well fixed in the sod -cage. . Although Na-X had lower CH ₃ I adsorption capacity than Ag-X, it was able to fix 98% of the adsorbed iodine. The overall results showed that Na-X zeolite was the most efficient candidate for radioiodine removal among the four zeolites evaluated, considering both CH ₃ I adsorption and immobilization by interzeolite conversion.

Figure 1 is a schematic diagram of interzeolite conversion from LTA (left) and FAU (right) to SOD (middle).
Figure 2 shows (a) representative products synthesized by interzeolite conversion of Na-A and Na-X and (b) Na-A, Ag-A, Na-X and Ag-X and their intercalation after CH ₃ I adsorption. This is a drawing showing the powder XRD pattern of the zeolite conversion product (XRD patterns of upper Na-A and Na-X are also shown for comparison), (where the X-ray peaks of AgI and Ag metal are closed inverted triangles (▼) and Represented by an open inverted triangle (∇)).
3 is a representative product synthesized by interzeolite conversion of Na-A and Na-X (left) and an interconversion product of Na-A, Ag-A, Na-X, and Ag-X adsorbed with CH ₃ I ( Right) is the SEM image.
4 is a representative product synthesized by interzeolite conversion of Na-A and Na-X and ¹²⁷ I of interzeolite conversion products of Na-A, Ag-A, Na-X, and Ag-X adsorbed with CH ₃ I This is the MAS NMR spectrum.
Figure 5 shows I 3d of representative products synthesized by interzeolite conversion of Na-A and Na-X and interzeolite conversion products of Na-A, Ag-A, Na-X, and Ag-X adsorbed with CH ₃ I This is the XPS spectrum.
6 is a graph showing the weight change determined by TGA as a function of time during CH ₃ I adsorption and desorption of Na-A, Ag-A, Na-X, and Ag-X at 100 °C.
7 is a schematic diagram of Na-zeolite or Ag-zeolite treatment for adsorption and desorption of CH ₃ I.
Figure 8 is a powder XRD pattern of a product synthesized by interzeolite conversion of Na-A and Na-X (where the X-ray peaks of AgI and Ag metal are indicated by closed (▼) and open inverted triangles (▽), respectively) ).
9 is a ¹²⁷ I MAS NMR spectrum of AgI powder (where spinning side bands are marked with asterisks).
10 is powder XRD patterns of Ag-A and Ag-X zeolites.
11 is a SEM image of Na-A, Ag-A, Na-X and Ag-X zeolites adsorbed with CH ₃ I.

Hereinafter, the present invention will be described in more detail through Examples and Experimental Examples. However, these examples are only for helping the understanding of the present invention, and the scope of the present invention is not limited to these examples in any sense.

Materials and Methods

1.1 Synthesis

Na-A (Si/Al = 1.0), Na-X (Si/Al = 1.2), Na-Y (Si/Al = 2.6) and CH ₃ I adsorption used for interzeolite conversion were Zeobuilder (Na-A) and Zeolyst (Na-X and Na-Y).

The hydrothermal interconversion synthesis of Na-zeolite was carried out by referring to the literature, and the details of the synthesis configuration are summarized in Table 1. Na-zeolite, NaI (99.5%, Sigma-Aldrich, Saint Louis, MO, USA) and/or AgI (99% Sigma-Aldrich) were mixed (Solution 1). NaOH (H ₂ O, 50% of Sigma-Aldrich) was diluted in deionized (DI) water and stirred for 1 hour (Solution 2). After mixing solutions 1 and 2, the mixture was stirred again for 1 hour. The final synthesis mixture was charged into a Teflon-lined 23 mL autoclave and heated at 180 °C for two days. Pseudoboehmite (99.9%, Sigma-Aldrich) was added to the synthesis mixture as another Al source to correct the Si/Al ratio if necessary. The solid product was collected by filtration, washed repeatedly with distilled water and dried overnight at room temperature (RT).

Ag-zeolite was prepared by ion exchange of Na-zeolite and refluxed twice at 80 °C for 10 h against 0.1 M aqueous solution of AgNO ₃ (99.9%, Kojima Chemical Co., Ltd. Saitama, Japan) (1.0 g per 10 mL solution).

After the last exchange cycle, the sample was washed with an excess of deionized (DI) water (>1 L) to remove silver (Ag) species that had not been exchanged into the zeolite pores. Before adsorption experiments, the silver (Ag) loaded samples were calcined in air at 550 °C for 2 h to homogenize ion sites and deodorize nitrate.

Interzeolite conversion of CH ₃ I adsorbed Na-zeolite and Ag-zeolite was performed in the same manner as described above. To distinguish the CH ₃ I adsorbed sample from the sample after interzeolite conversion, -ad and -it suffixes were added to the end of the sample name, respectively (eg, Na-X-ad and Na-X-it).

Table 1 below summarizes the interzeolite synthesis results.

Add more Al source to the synthesis mixture to reach ¹ Si/Al = 1.

² Runs 1-18 and 19-22 are unadsorbed zeolites and CH3 adsorbed zeolites, respectively.

³ All products obtained after heating at 180 °C for 1 day unless otherwise specified. The first product to appear is the main table.

⁴ Obtained after heating for 2 days. ⁵ Unconfirmed awards.

1.2 Features

Powder X-ray diffraction (XRD) patterns were measured on a SmartLab X-ray diffractometer (Rigaku, Japan) using Cu-Kα radiation in the 2 θ range from 3 to 50° (scan rate = 4° min ^-1 ). Elemental analysis was performed on an OPTIMA 7300DV inductively coupled plasma (ICP) spectrometer (PerkinElmer, Shelton, CT, USA). Crystal morphology and average size were determined using a JSM-7800F scanning electron microscope (JEOL Ltd. Tokyo, Japan) operating at an accelerating voltage of 15 kV.

Additionally, electron image collection and elemental analysis were performed using an energy dispersive X-ray spectrometer (EDS) coupled with SEM. ¹²⁷ I MAS NMR spectrum was obtained from a 500 MHz Avance III HD solid-state NMR spectrum (Bruker, Billerica, MA, USA) at a rotation rate of 8 kHz, ¹²⁷ I frequency at 100.1 MHz with π/2 rad pulse length of 2 μs, 1 s was measured with a recirculation delay of 0 and a collection of 2000 pulse delays. ¹²⁷ I chemical shifts were reported relative to NaI. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 Versa Probe II (ULVAC-PHI 5000 Versa Probe, Japan) using Al-Kα radiation (hυ = 1486.6 eV).

1.3 CH ₃ I Adsorption

Adsorption experiments were performed on a thermogravimetric analyzer (TGA, Hitachi STA7000), and 10 mg of adsorbent was loaded into the sample holder. Figure 7 shows a series of treatments performed on zeolite adsorbents for CH ₃ I adsorption according to procedures reported in previous studies. Prior to the experiment, the adsorbent was routinely operated at 200 ° C and atmospheric pressure for 1 hour under the condition of argon (Ar) (99.999%, 100 mL min ^-1 ) flowing at atmospheric pressure, and then maintained at 100 ° C for about 20 minutes to stabilize the TGA weight balance. made it possible A typical design temperature for a dry filtration system is known to be room temperature (RT) -200 °C.

A mixture of Ar ( ₇₅ mL min ^-1 ) and CH ₃ I (5000 ppm Ar balance, 50 mL min ^-1 ) maintaining a constant initial CH 3 I concentration at 2000 ppm was supplied to TGA for 2 hours, and the weight of the adsorbent Changes were continuously recorded in real time. Here, a very high CH ₃ I concentration relative to the actual conditions of the nuclear accident (0.1 ppm) was used to saturate the zeolite adsorbate with CH ₃ I in a reasonable experimental time. After adsorption for 2 hours, physically adsorbed CH ₃ I was removed at the same temperature in Ar flow for 1 hour. Separately, the CH ₃ I adsorption sample used for interzeolite conversion was obtained in a continuous flow stationary reactor loaded with 0.5 g of zeolite adsorbent using the same procedure described above.

2. Results and discussion

2.1 Conversion of Interzeolite to Sodalite (SOD)

The results of conversion of interzeolites to different structures of Na-zeolite are summarized in Table 1 (runs 1-18).

Figure 2a shows the powder XRD patterns (runs 2, 7, 9, 12, 13, 17) of representative interconversion products obtained after heating at 180 °C for 1 day, and the XRD patterns of all other products are shown in Figure 8. .

For comparison, the XRD patterns of parent Na-A and Na-X are also shown in Figure 2a. Since LTA zeolite using Si/Al = 1 is known to be easily converted into an SOD structure, interconversion of Na-A was performed under various thermosynthetic conditions (runs 1-10).

X-ray peaks on LTA were still observed in the synthesis using 0.7 M aqueous solution of NaOH (NaOH/Al = 1) at 180 °C for 1 day (FIG. 8). However, all LTA structures disappeared and the formation of SOD was confirmed after extended crystals up to two days (run 1). When the NaOH concentration was increased more than twofold (NaOH/Al = 2–4), the pure SOD structure crystallized within 1 day (runs 2–4). Since the products were crystallized under OH ⁻ medium, these sodalites were HSOD, and their XRD patterns were in good agreement with literature results.

The ISOD with Si/Al = 1 had an I/Al ratio of 0.3. This is because two iodine anions per SOD unit cell are suitable. Therefore, interzeolite conversion was performed under the condition of NaI/Al = 0.3 in the synthesis mixture (runs 5-7). Also, the pure SOD phase was obtained only when the NaOH/Al ratio in the synthetic mixture exceeded 2.

Since Ag-zeolite is known to have better CH ₃ I adsorption capacity than Na-type, we tried to perform interzeolite conversion of Na-A in the presence of AgI instead of NaI (runs 8-10).

No zeolite phases other than SOD were identified for all runs, but X-ray peaks of bulk AgI and bulk Ag metal phases were observed at 2θ = 22.4°, 23.6°, 39.2°, 46.2° and 2θ = 38.1° and 44.3°, respectively. observed along with the SOD phase. This means that unlike NaI, most AgI clusters did not dissolve under hydrothermal conditions at 180 °C and existed as a physical mixture with the produced SOD. However, since the presence of metal Ag was confirmed, it can be assumed that a part of Ag-I was separated under hydrothermal conditions, and the separated I ^- contributed to stabilizing the formation of the sod -cage of SOD together with Na ⁺ .

On the other hand, the amount of CH ₃ I adsorbed using Ag-zeolite is about 100 mg/g-zeolite, which corresponds to 0.1 AgI/Al. Therefore, although only 0.1 times AgI was added to the synthesis mixture, X-ray peaks for AgI and Ag were still observed (run 10). This suggests that it may be difficult for AgI to be fixed to the SOD structure by interzeolite transformation, and Na-zeolite may be a better adsorbent from the point of view of immobilization, even though the CH ₃ I adsorption is somewhat lower than that of Ag-zeolite. Since sodium is cheaper than silver, the former is advantageous from an economic point of view.

As described above, large pore zeolites such as FAU have better CH ₃ I adsorption capacity than small pore materials such as LTA zeolite.

Thus, interzeolite conversion was performed for Na-X with Si/Al = 1.2, which is similar to that of Na-A (Si/Al = 1.0) (runs 11-15).

An analcime (ANA) phase was observed at impurity levels with SOD with and without NaI. This suggests that a relatively excessive amount of silica compared to the Na-A interconversion contributed to the stabilization of the ANA phase.

Since FAU and ANA have six rings in common, they can be easily converted into ANA by interconversion of FAU, especially in certain cation systems such as Na ⁺ and Cs ⁺ .

The ANA structure is not suitable for immobilizing radioactive elements because it has an 8-ring open pore (ca. 4.2 Å). However, when comparing the two strong X-ray peaks on ANA and SOD in run 12 samples, i.e., 2θ = 26.0 and 24.4, corresponding to the (400) and (211) Miller indices, respectively, the ANA portion was found in about 9% of the samples. It was estimated that only

In contrast, when AgI was added instead of NaI under the same thermal conditions (runs 13-15), no ANA phase was observed, but X-ray peaks of bulk AgI and Ag metal phases were also observed as in runs 8-10.

Among zeolite adsorbents, zeolite Y having a higher Si/Al ratio (2.6) than zeolite X is known to have excellent CH ₃ I adsorption performance. Therefore, additional interzeolite conversion was attempted for Na-Y under the same conditions as described above. Only pure ANA phase was formed instead of SOD, and bulk AgI and Ag metal phases were also observed in the presence of NaI (run 16) and AgI (run 17), respectively. This can be explained by the much higher silica content, which made the ANA phase more stable than the SOD. When the interzeolitic conversion of Na-Y was attempted with a lower Si/Al ratio of 1.0 by adding more Al precursor, an SOD phase was produced but not pure (run 18).

From the above interzeolite transformation results, it can be concluded that Zeolite A and X can be used as CH ₃ I adsorbents by interzeolite transformation to form an SOD structure, but Zeolite Y has excellent CH ₃ I adsorption capacity, but can be immobilized not suitable from the point of view

Figure 3 (left) shows an SEM image of a representative interconversion product described above. The run 7 sample obtained from the interconversion of Na-A in the presence of NaI showed only agglomerated SOD particles with a size of 5 μm and no impurity phase such as NaI. EDS results also showed that the iodine content of the particle surface was very low compared to bulk analysis by ICP (I/Al = 0.01 vs. 0.16 in Table 2). This means that most iodine ions are immobilized in the sod -cage of SOD. The SEM-EDS results of the run 12 sample obtained by interconversion of Na-X with NaI also showed little iodine on the particle surface (Table 2).

Also, as observed by XRD, polyhedral impurity crystals of about 20 μm in size of anything other than SOD particles were observed. This is consistent with the crystal morphology of typical ANA zeolites in the literature. On the other hand, SEM-EDS analysis of run 9 and 13 samples synthesized in the presence of AgI clearly showed the presence of AgI and/or Ag metal along with SOD particles corresponding to the XRD results (Fig. 2a).

Table 2 below shows the chemical composition of selected products synthesized by interzeolite transformation ¹ .

¹ Determined by inductively coupled plasma (ICP) analysis unless otherwise specified. ² Values given in parentheses are I/Al ratios from SEM-EDS data. Number of iodine per sod -cage assuming SOD of ³ Si/Al = 1.

Figure 4 shows the ¹²⁷ I MAS NMR spectrum of a representative synthetic product produced by interzeolite conversion. In the case of the Run 7 product, no peak was observed at 0 ppm corresponding to NaI, and only a strong ¹²⁷ I NMR peak was observed at -260 ppm, which is a typical chemical shift for iodine immobilized in a sod -cage. This strongly indicates that almost all iodine ions were fixed in the sod -cage of the SOD structure of the run 7 product, where pure SOD phase was formed by interconversion of Na-A in the presence of NaI.

In the case of this product, the number of I ^- / sod -cages can be estimated to be about 0.5 according to the results of elemental analysis (Table 2). That is, about one iodine ion is fixed in one of the two sod -cages per SOD unit cell.

In addition, a strong chemical shift of ¹²⁷ I was observed around -260 ppm for the run 12 product synthesized in the presence of Na-X and NaI. For this product, it can be estimated that almost one iodine per sod -cage of the SOD structure was immobilized (Table 2). Therefore, although the ANA phase coexisted at the impurity level in the run 12 sample, most of the iodine used in the synthesis was mainly fixed in the sod -cage of the produced SOD structure.

Conversely, in the case of the run 9 product produced by the interconversion of Na-A and AgI, no peak was observed at -260 ppm, and only a strong band was observed around -70 ppm, which corresponds to the bulk AgI cluster (FIG. 9 ). This is in good agreement with the powder XRD and SEM-EDS analysis results. This result indicates that there is no iodine in the sod -cage of the SOD structure generated from the run 9 product.

5 shows I 3d XPS spectra of representative products synthesized by interzeolite conversion. There are two types of peaks, 617–620 eV and 629–632 eV, corresponding to I 3d _5/2 and I 3d _3/2 respectively. Here, I 3d _3/2 is based on I 3d _5/2 because it shows almost the same trend for all samples. In the case of the NaI cluster with FCC structure, 6 Na ⁺ ions are coordinated around one I ^- ion, and the XPS peak for I 3d _5/2 is observed at 618.4 eV.

For the run 7 product, the peak for I 3d _5/2 was observed at 618.7 eV, which is a typical binding energy for I adjusted to 4 Na ⁺ in the sod -cage. Here, we note that immobilization of iodine in the sod -cage of SOD was confirmed for the run 12 product, but the XPS peak I 3d _5/2 for this sample was observed at 619.5 eV, 0.8 eV higher than that of the run 7 product. The binding energy of an ion can change depending on the type and number of surrounding cations. As shown in Table 2, since the Na/I ratio of run 12 is the lowest, the strongest I 3d binding energy among the analyzed samples can be observed. On the other hand, for run 9 and 13 products, the XPS peak for I 3d _5/2 was observed at 619.3 eV, which is a typical binding energy for bulk AgI clusters.

2.2. Adsorption of CH ₃ I and immobilization of iodine in the sod -cage

Based on the properties of products obtained from interzeolite conversion of Na-A, Na-X, and Na-Y in the presence of NaI or AgI, adsorption performance of CH ₃ I and iodine immobilization by successful interzeolite conversion were compared. Four different zeolites (i.e., Na-A, Ag-A, Na-X, Ag-X) were prepared for this purpose. As shown in FIG. 10, the XRD patterns of Ag ⁺ exchanged Ag-A and Ag-X zeolites did not show other diffraction such as Ag metal or Ag ₂ O other than the LTA and FAU phases, respectively. This supports the conclusion that Ag ⁺ ions were mostly dispersed in the pores of the zeolite and did not diffuse to the outer surface of the zeolite even after the final high-temperature calcination. As shown in Table 3, the Ag components of the two Ag-zeolites prepared by the same ion exchange method are quite similar, and 48% and 43% of the anion sites generated in the framework Al were exchanged for Ag ⁺ , respectively. Elemental analysis of commercial Na-A and Na-X confirmed that Na-A (Na/Al = 0.89) contained twice as much Na as Na-X (Na/Al = 0.43).

The following [Table 3] shows the chemical composition ¹ and adsorption amount ² of the four adsorbents studied in the present invention.

¹ Determined by ICP analysis unless otherwise specified. ² Continuous desorption of physisorbed CH ₃ I for 1 hour as determined by TGA after CH3I adsorption for 2 hours. ³ Supplied by supplier.

6 and Table 3 show the CH ₃ I adsorption results at 100 °C for four zeolites. Na-A hardly adsorbed CH ₃ I, but the remaining three zeolites were almost saturated after 2 hours of adsorption, and the total amount of adsorbed CH ₃ I was 140-280 mg/g-zeolite, similar to the values reported in the literature. did. Unlike Na-A, Ag-A showed about 260 mg/g-zeolite adsorption capacity for CH ₃ I.

Although the amount of adsorption of Na-X was less than that of Ag-A, the adsorption capacity of CH ₃ I was 140 mg/g-zeolite. Ag-X showed about twice the CH ₃ I adsorption capacity of Na-X. As can be easily predicted from these adsorption results, therefore, it seems that Ag ⁺ is better than Na ⁺ and zeolite X is better than zeolite A in terms of CH ₃ I adsorption capacity.

As shown in FIG. 2b, after CH ₃ I adsorption, the Na-A-ad and Na-X-ad samples did not show X-ray peaks other than the LTA and FAU phases, respectively. No X-ray peaks were observed for bulk AgI clusters in Ag-A-ad, but peaks characteristic of AgI were observed in Ag-X-ad. As above, one of the mechanisms of CH ₃ I adsorption on metal sites is the formation of NaI or AgI species, in which successive dissociation of CH ₃ -I and combination of metal iodides occur. However, XRD-detectable AgI may not form inside such small zeolite micropores, so AgI clusters or larger particles may form on the outer surface of Ag-X-ad crystals (Fig. 11).

It is known that the rapid growth of AgI species occurred inside the Ag-Y zeolite supercage and at the same time diffused toward the outer surface as the spent adsorbent was exposed to the surrounding environment. However, since the pore size of LTA is much smaller than that of FAU, it is relatively difficult for AgI formed inside the pore to escape out of the pore even after exposure to air, and Ag-A-ad may not be able to detect the characteristic AgI X-ray peak. .

The ability to fix iodine in the sod -cage of the SOD structure by interzeolitic conversion was evaluated for the four CH ₃ I adsorption samples discussed above. These synthetic results are also summarized in Table 1 (runs 19-22). After one day of crystallization with excess NaOH concentration (NaOH/Al = 4), the pure SOD phase (Figs. 2b and 3 (right)) interconverted in Na-A-ad, but as shown in Fig. 6 and Table 3, CH Since the amount of ₃ I adsorption was insignificant, no characteristic peak was observed in the ¹²⁷ I MAS NMR (FIG. 4) and I 3d XPS (FIG. 5) spectra. The Ag-A-it sample also formed an SOD phase under the same conversion conditions as Na-A-it, but an Ag-metal phase was also observed. In this case, unlike the result of interconversion between Na-A and AgI (runs 8-10 in Table 1), the bulk AgI phase was not observed.

As described above, AgI molecules or nanometer species formed in LTA pores after CH ₃ I adsorption can be decomposed under interzeolite hydrothermal conditions, unlike bulk AgI clusters, and thus decomposed Ag ⁺ can form metal clusters. Moreover, the decomposed I ⁻ binds to Na ⁺ in the synthesis mixture and can contribute to stabilizing the sod -cage of the produced SOD. However, as shown in FIGS. 4 and 5, in the case of Ag-A-it, the ratio of iodide immobilized in the sod -cage was not very high.

Since the interzeolite conversion of FAU can produce ANA phase with excessive concentrations of sodium as well as silica (runs 11, 12, 16, and 17 in Table 1), in the interconversion of Na-X-ad, the NaOH concentration is Na-A It was lowered to 1/2 compared to -ad and Ag-A-ad (run 21 in Table 1). However, the ANA phase was still formed at a rate of about 15% compared to SOD (Na-X-it in Fig. 2b), which is similar to the result of run 12. It should be noted here that both ^{the 127} I MAS NMR (FIG. 4) and I 3d XPS (FIG. 5) results for Na-X-it are similar to those of run 12. Therefore, iodine ions appear to be fixed in the sod -cage of SOD.

The elemental analysis results for Na-X-it were also similar to those of run 12, and the number of I ^- / sod -cage in the SOD structure was estimated to be 0.8, slightly less than 1.0 in run 12 (Table 2).

On the other hand, ANA and Ag metal phases were identified together with the SOD phase in the interconversion of Ag-X-ad with the same hydrothermal conditions for Na-X-ad (run 22 in Table 1). As with Ag-A-it, no bulk AgI phase was identified in this run (Ag-A-it (right) in Figs. 2 and 3). However, as shown in FIG. 4, in the ¹²⁷ I MAS NMR spectrum of Ag-X-it, a weak band of AgI was confirmed around -70 ppm, and a strong band appeared at -260 ppm. This indicates that some AgI clusters of Ag-X-ad identified in the XRD results (Fig. 2b) remain after interzeolite conversion. Although Ag-X-ad adsorbed the largest amount of iodine among the four adsorbents (Fig. 6 and Table 3), the iodine of Ag-X-it was only half that of Na-X-it (Table 2 I/Al = 0.13 vs. 0.26). Therefore, the number of I ^- / sod -cages in SOD was also calculated by half (0.4 vs. 0.8 in Table 2).

Comprehensive results for CH ₃ I adsorption and hydrothermal interzeolite conversion showed that Na-type zeolites had lower CH ₃ I adsorption capacity but higher iodine retention capacity than Ag-type zeolites. Also, unlike Na-A, the interconversion of Na-X to pure SOD is relatively difficult, but the CH ₃ I adsorption capacity of Na-X is much higher. Therefore, among the four adsorbents, Na-X zeolite is determined to be the best adsorbent. The amount of adsorbed iodine measured after CH ₃ I adsorption on Na-X and continuous desorption of physically adsorbed CH ₃ I was about 120 mg/g-zeolite (ie, 120,000 ppm) (Table 3).

Meanwhile, the iodine concentration of the Na-X-it solid sample and the synthetic solution remaining after hydrothermal interzeolite conversion were analyzed to be 117,000 ppm and 2,220 ppm, respectively. These results confirmed that the mass balance was well matched within the error range (ie, 120,000 to 117,000 + 2220 ppm).

As a result, when using the Na—X adsorbent, iodine can be immobilized in the sod -cage with a very high ratio of 98% of the SOD structure through the successful interzeolite immobilization process after CH ₃ I adsorption.

conclusion

In the present invention, a study was conducted to immobilize iodine in a sod -cage by interzeolite conversion after CH ₃ I adsorption using Na- and Ag-zeolite adsorbents. Initially, interzeolite conversion was performed in the presence of NaI or AgI for Na-A, Na-X and Na-Y. Unlike the other two zeolites, Na-Y, which is known to have the highest adsorption capacity, was not well converted to the SOD structure. The CH ₃ I adsorption capacities of the four zeolites that can be converted to the SOD structure were compared under the same conditions. The adsorption capacity was in the order of Ag-X > Ag-A > Na-X > Na-A. Therefore, as expected, the adsorption capacity of Ag ⁺ was higher than that of Na ⁺ and FAU was higher than that of LTA. On the other hand, when the immobilization ability of iodine by interzeolite conversion of the CH ₃ I adsorbed sample was evaluated, Ag-A had excellent adsorption capacity and was well converted to the SOD structure without impurities, but the adsorbed iodine was not well fixed in the sod -cage. . Although Na-X had lower CH ₃ I adsorption capacity than Ag-X, it was able to fix 98% of the adsorbed iodine. The overall results showed that Na-X was the most efficient candidate for removing radioactive iodine among the four zeolites evaluated, considering both CH ₃ I adsorption and immobilization by interzeolite conversion.

The present invention described above is not limited to the above-described embodiments, since various substitutions and changes can be made to those skilled in the art within the scope of the technical idea of the present invention. .

Claims (8)

As a FAU zeolite to which radioactive iodine is adsorbed, the FAU zeolite is a Na-X zeolite, and the Na-X zeolite is converted into a sodalite (SOD) structure between zeolites (Interzeolite) so that the radioactive iodine is placed in a sodalite-cage ( characterized in that it is fixed in the sod -cage),
The Na-X zeolite converted to the sodalite (SOD) structure by interzeolite has a CH ₃ I adsorption capacity compared to the Na-A zeolite converted by interzeolite to the sodalite (SOD) structure. An adsorbent material for immobilizing radioactive iodine through conversion between zeolites into a sodalite structure, characterized in that it is larger.
According to claim 1,
The Na—X zeolite is an adsorbent material for immobilizing radioactive iodine through conversion between zeolites with a sodalite structure, characterized in that the SiO ₂ /Al ₂ O ₃ molar ratio is 1.2.
According to claim 1,
The ratio of radioactive iodine fixed per sod-cage (I ^- / sod -cage) of the Na-X zeolite converted to the sodalite (SOD) structure by interzeolite is 0.8 An adsorption material for immobilizing radioactive iodine through conversion between zeolites and sodalite structures.
According to claim 1,
The Na—X zeolite converted into interzeolite into the sodalite (SOD) structure has a sodalite structure, characterized in that the I 3d _5/2 XPS peak is measured at 619.5 eV in the I 3d XPS spectrum. Adsorbent material for immobilization of radioactive iodine by transformation.
According to claim 1,
The adsorbed amount of CH ₃ I adsorbed to the Na-X zeolite is 140-280 mg / g-zeolite, characterized in that adsorbent material for radioactive iodine immobilization through conversion between zeolite to sodalite structure.
Adsorbing radioactive iodine on Na—X zeolite as FAU zeolite;
Converting the iodine-adsorbed Na—X zeolite into a sodalite (SOD) structure by converting the zeolite to interzeolite; and
Immobilizing the radioactive iodine in a sodalite-cage of Na—X zeolite converted into interzeolite into the sodalite ( SOD ) structure; including,
The Na-X zeolite converted to the sodalite (SOD) structure by interzeolite has a CH ₃ I adsorption capacity compared to the Na-A zeolite converted by interzeolite to the sodalite (SOD) structure. characterized by a larger
Radioactive iodine immobilization method through conversion between zeolite and sodalite structure.
According to claim 6,
The Na—X zeolite is a radioactive iodine immobilization method through conversion between zeolites to a sodalite structure, characterized in that the SiO ₂ /Al ₂ O ₃ molar ratio is 1.2.
According to claim 6,
The ratio of radioactive iodine fixed per sod-cage (I ^- / sod -cage) of the Na-X zeolite converted to the sodalite (SOD) structure by interzeolite is 0.8 Method for immobilizing radioactive iodine through conversion between zeolites and sodalite structures.

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