Adsorption of gaseous radioactive iodine by Ag/13X ... - Springer Link

J Radioanal Nucl Chem (2015) 303:1883–1889 DOI 10.1007/s10967-014-3736-3

Adsorption of gaseous radioactive iodine by Ag/13X zeolite at high temperatures Qinghui Cheng • Weiwei Yang • Zejun Li Qiufeng Zhu • Taiwei Chu • Dehua He • Chao Fang

•

Received: 15 June 2014 / Published online: 2 November 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014

Abstract Ag/13X adsorbents were synthesized, characterized and tested for decontamination of gaseous effluents from 131I2 at high temperatures. X-ray diffraction patterns showed that the Ag/13X samples maintained a stable structure after calcined at 650 °C for 2 h. The decontamination factors achieved with 15 % Ag/13X and 20 % Ag/13X adsorbents for 131I2 were nearly close to 103 at 650 °C. In addition, 15 % Ag/13X had a stable performance for removal of 131I2 at 550 and 650 °C, even after calcined at 550 and 650 °C for over 10 h, which might be suitable for future potential use during nuclear reactor operation or in the case of nuclear accidents. Keywords Nuclear reactor Radioactive iodine Decontamination factor (DF) Stability Adsorption

Qinghui Cheng and Weiwei Yang contributed equally to this work. Q. Cheng Z. Li T. Chu (&) Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China e-mail: [email protected] W. Yang Q. Zhu D. He Key Lab of Organoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China e-mail: [email protected] C. Fang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, People’s Republic of China e-mail: [email protected]

Introduction Given the increasing energy demands worldwide and the need for reducing the emissions of greenhouse gas, more and more attention has been paid to the research on clean, safe, and responsible nuclear energy [1]. During the operation period of nuclear reactor or when nuclear accidents take place, radioiodine isotopes of 129,131-135,138-141I are emitted from the core [2]. Due to their short half-lives (fractions of second to few seconds), 138-141I isotopes could be neglected in the source term analysis. 129I is very long-lived (t1/2 = 1.57 9 107 years) and weak c-ray emitter (39.6 keV, 7.5 %), so its environmental impact is not high. Iodine isotopes with mass numbers from 131 to 135 are the main fission iodine isotopes of interest, especially 131I, which is one of the most harmful radionuclides in the large early release in nuclear plant accidents [3]. 131I is released in the forms of elemental iodine (I2 and I), organic iodide (CH3I) and hypoiodous acid (HOI) from nuclear reactor or nuclear accidents [4]. Considering high mobility and toxicity of radioactive iodine, conducting research about the removal of radioactive iodine plays an important role in ensuring the safe operation and environmental protection [5]. Because of the special affinity between silver and iodide ions, different kinds of silver-loaded solid adsorbents have been widely used in the adsorption removal process of radioiodine released from nuclear reactors. Most of the silver-based iodide adsorbents are zeolite [6], titanate nanolamina [7] mordenite [8], silica gel [9] or aluminum oxide [10, 11], which are mainly synthesized by ion exchange or impregnation method. These adsorbents are nonflammable and can effectively trap organic iodine with high adsorption capacity. The silver ion exchanged zeolites are prepared for different applications. For instance, polyethylene composite

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films with silver exchanged zeolite-Y and Silver/Copperloaded NaA zeolite (which have antibacterial activity) in addition to Ag-exchanged LTL zeolite (which has a selective catalytic reduction of NOv) [12–14]. Different kinds of zeolites have different physicochemical property. The mechanical strength and acid resistance gradually increase with increasing silica-to-alumina ratio, but the too high silica-toalumina ratio has an adverse effect on the ion exchange process. AgX adsorbents have a high adsorption capacity of organic and inorganic iodine; decontamination factors can reach 103–105 at an operating temperature range of 200–300 °C in pressurized water nuclear reactor [15]. The silver-based adsorbents have been successfully applied to the treatment of exhausted gases and pollutants in environment due to its higher specific surface area and the role of silver ion loaded on adsorbents [16– 18]. The mechanism of silver-loaded adsorbents trapping iodine has been reported for a long time [19–22]. In 2010, Karena et al. [23] reported the radioiodine capture in silver-containing mordenites through nanoscale AgI formation. In Germany, the commercialized iodine removal agent Ag-KTC/KTB has been prepared by the impregnation of amorphous silicate with AgNO3, and it has a good mechanical stability. The BET specific surface area of this kind of material is 65–110 m2/g, and the pore size distribution varies from 20 to 40 nm, the particle size is 1–2 mm, the pore volume is about 0.6 mL/g, the silver loading ratio is always about 8–12 %. According to the previous research, the existence of vapor adversely affects the removal of iodine by AgNO3 impregnated material, and the optimum operating temperature is 150 °C [24]. A Japanese group has researched the application of impregnated Al2O3 material (silver loading ratio is 24 %) in the exhaust of nuclear fuel, and its removal efficiency can reach 99.6 % [10] at an operating temperature of 150 °C and surface velocity of 20 cm/s. Although there are reports for adsorbing radioactive iodine at lower temperatures with high adsorption capacities [25–29], there is a lack till now for adsorbents with good radioiodine adsorption capacities at higher temperature (about 600 °C). However, the need of capturing radioactive iodine in the condition of nuclear accidents or in the core is increasing, especially after the nuclear accident at Fukushima, Japan in 2011. 13X zeolites is a kind of NaX zeolites, and has a large specific surface, high adsorption capacity and high thermal stability. In addition, they are easy to refresh. In this study, the Ag/13X zeolite adsorbents with different silver loading ratios were synthesized, characterized and tested for removal of trace amount of radioactive iodine at high temperature.

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J Radioanal Nucl Chem (2015) 303:1883–1889

Experimental details Adsorbent preparation 13X zeolite was obtained from Anhui MingMei MINCHEM company, and AgNO3 (analytical reagent) was purchased from Beijing Chemical Reagent Company. Ag/13X adsorbents were prepared by a conventional impregnation method. The Ag content means the mass ratio of Ag to 13X zeolite support. Firstly, AgNO3 was dissolved in deionized water (0.28 M), a certain amount of 13X zeolite was added into the AgNO3 aqueous and stirred for 12 h under the condition of light-shading. Then the water was removed by evaporation, and the sample was dried at 110 °C for 12 h. Finally, the sample was calcined in air at 450 °C for 2 h. Ag/13X adsorbents with different Ag loadings are accordingly marked as ‘‘fresh p % Ag/13X (T/t)’’. The ‘‘fresh’’ represents as-prepared samples, ‘‘p %’’ is the content of Ag (wt %), ‘‘T’’ means calcination temperature (oC), and ‘‘t’’ is calcination time(h). Adsorbent characterization The specific surface areas of the adsorbent samples were measured by N2 adsorption–desorption method using the Brunauer–Emmett-Teller (BET) equation on an ASAP 2010C analyzer (MICROMERITICS, America). Prior to the test, the samples were degassed at 473 K for 2 h. The crystalline phases of the Ag particles and the 13X zeolite support were characterized by X-ray diffraction (XRD), and XRD patterns were obtained by using D/max2500/PC X-ray diffractometer (Rigaku, Japan), using CuKa radiation with Ni filter, while the instrumental setting of 40 kV and 200 mA in the step mode (10o/min). The crystal sizes of Ag were calculated by means of X-ray line broadening method of using the Scherrer equation. The actual loadings of Ag were measured by X-Ray Fluorescence (XRF) method at the XRF-1800 equipment (Shimadzu, Japan). Before measurement, the pressure of the internal system was vacuumized below 100 Pa. Decontamination factor of

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I2

The performance tests of 13X, 10 %Ag/13X, 15 %Ag/13X and 20 %Ag/13X for the removal of radioactive iodine at 250, 350, 450, 550 and 650 °C were conducted to simulate the practical process of helium purification in the case of nuclear accidents. A laboratory-scale flow type fixed-bed apparatus was designed to perform the experiments (Fig. 1). It mainly consists of four parts, terufusion syringe pump, high-purity helium steel cylinder, fixed-bed adsorption system, and effluent gas absorption system. One KL-602 terufusion syringe pump (Beijing KellyMed Co.,


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3

Quantity Adsorbed (cm /g STP)

spent 15%Ag/13X o (After adsorption of Iodine at 450 C)

Fig. 1 Schematic diagram of Ag/13X test facility for removing radioactive iodine. 1-I2 solved in petroleum ether with a boiling range of 30–60 °C; 2-terufusion syringe pump; 3- MFC(mass flow controller); 4-high-purity helium steel cylinder; 5 and 6-valves; 7-heating furnace; I, II, III and IV- the backup bed canisters; V-saturated NaOH solution

Ltd., China) was used to control the infusion rate of 131I2 solution to be 2.5 mL/h via a 10 mL-size injector for 2 h (131I2 tracer and 127I2 carrier were dissolved in petroleum ether with a boiling range of 30–60 °C and the chemical concentration of 127I2 was about 3 9 10-4 mol/L and its count rate of 131I2 was about 2 9 105 counts per second; the mixture of 127I2 and 131I2 was abbreviated as 131I2 for convenience in the following text). One D07-7B mass flow controller (MFC, Beijing Seven Star Electronics Co., Ltd., China), was used to control the mass flow rate of helium carrier gas (99.999 %) with the measuring range of 69.3 and 104 mL/min, respectively. After He and 131I2 were fully mixed, they were directed into the quartz tube with an inner diameter of ø 12 mm and length of 44 cm, which was set in a tube heating furnace (Beijing Zhongshiyida Technology Co., Ltd., China). The Ag/13X adsorbent was placed in the center of the quartz tube and about 2 g of absorbents was used each time. Then, four backup bed canisters, which were filled with 13X (less than 40 meshes), were used to trap the radioactive iodine penetrated through sample canister completely. Finally, saturated NaOH solution was used to further adsorb radioactive iodine waste gas and test the air tightness of the whole device with the sign of bubbling. Na 131I source was purchased from Beijing Atom High-Tech Co, Ltd., China. 131I decays by b- emission with a half-life of 8.04 days, emitting a 0.354 MeV c-ray. The radioactivities of samples were assayed in a FT-603 well-type NaI gamma counter (Beijing Nuclear Instrument Factory, China). Stability of 15 %Ag/13X for removing

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I2

In order to further confirm the stability of 15 %Ag/13X for the 131I removald, the prepared 15 %Ag/13X (450/2) was further calcined in air for 2, 10 and 20 h, at 550 and 650 °C,

fresh 15%Ag/13X(450/2)

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Fig. 2 N2 adsorption–desorption isotherms of 15 %Ag/13X

respectively. After calcined, the as-prepared Ag/13X adsorbents were tested for the removal of 131I2 at 550 and 650 °C with the flow rate of helium carrier gas of 69.3 and 104 mL/ min. In this section, all other factors were identical to the above process, such as the dose of 131I2 and 131I2 used as a solution (petroleum ether was solvent).

Results and discussion Characterization of 15 %Ag/13X and 13X adsorbents The N2 adsorption–desorption isotherms of fresh 15 %Ag/ 13X (450/2) and spent 15 %Ag/13X (450/2) are shown in Fig. 2. The isotherm of the fresh 15 %Ag/13X (450/2) adsorbent can be classified as type IV with H2 type hysteresis loops. After adsorbing radioactive iodine, the spent 15 %Ag/13X (450/2) still kept ordered mesoporous structure. That is to say, compared with fresh 15 %Ag/13X (450/2) that did not adsorb the radioactive iodine, the porous structure of the spent 15 %Ag/13X((450/2) did not change too much. Although, compared with 13X zeolites, the specific surface area of fresh 15 %Ag/13X showed a decline from 399 to 271 m2/g, the spent 15 %Ag/13X still kept a relatively large specific surface area (253 m2/g). After calcined at high temperature (550 and 650 °C) for a longer time (10 h), the specific surface area of the 15 %Ag/13X(550/10) and 15 %Ag/13X(650/10) showed a tendency of decline. Especially after calcined at 650 °C for 10 h, the BET surface of 15 %Ag/13X(650/ 10) declined to 97 m2/g, indicating that silver crystalline particles grew to block some pores of 13X after treated at high temperature. The results are shown in Table 1.

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Table 1 Comparison of the specific surface area of 15 %Ag/13X and 13X before and after use

∇ = Ag

8000

Sample

Specific surface area (m2/g)

13X

399

Fresh15 %Ag/13X(450/2)

271

8000

Spent15 %Ag/13X(450/2) Fresh 15 %Ag/13X(550/10)

253 206

4000

Fresh 15 %Ag/13X(650/10)

97

0

(a) fresh 15%Ag/13X(450/2+550/2)

4000 0

(b) fresh 15%Ag/13X(450/2+550/10)

8000 ∇ = Ag

(c) fresh 15%Ag/13X(550/20)

4000

10000

0

(a) 13X

(d) fresh 15%Ag/13X(450/2+650/2)

8000

0

4000 0

10000

(b) fresh 15%Ag/13X(450/2)

(e) fresh 15%Ag/13X(450/2+650/10)

8000 ∇

4000

0

(c) fresh 20%Ag/13X(450/2)

10000

∇

∇

∇

0 ∇

(f) fresh 15%Ag/13X(650/20)

8000

∇

4000

∇

∇

0

(d) spent 15%Ag/13X o (After Iodine adsorption at 450 C)

10

10000 ∇

20

30

40

50

60

70

80

2θ (degree)

∇

∇

∇

0

Fig. 4 XRD spectra of 15 %Ag/13X after strong-heat treatment

0 10000

∇

(e) spent 15%Ag/13X o (After Iodine adsorption at 350 C) ∇

∇

∇

0 10

20

30

40

50

60

70

80

2θ (degree)

Fig. 3 XRD spectra of Ag/13X and 13X

The phase structures of the as-prepared Ag/13X (450/2) adsorbents were characterized by X-ray diffraction, and compared with those of untreated 13X zeolite, as shown in Fig. 3a–c. XRD spectra of fresh Ag/13X adsorbents had the characteristic diffraction peaks of 13X, indicating that the 13X zeolite kept its phase structure after supporting Ag particles and calcined in air. The XRD characterization results of the spent Ag/13X samples, which had been used to adsorb radioactive iodine under different temperatures, are shown in Fig. 3d, e. The

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characteristic diffraction peaks of 13X zeolites support can still be seen after adsorbing the radioactive iodine at 350–450 °C. The diffraction peaks nearly at 2h = 7°, 45°, 65° and 78° are attributed to Ag crystalline phase. It might be deduced that Ag species on 13X zeolites existed sintering phenomenon and formed bigger Ag particles. In order to measure the stability of Ag/13X after heating at high temperatures, the XRD characterization was done for the 15 % Ag/13X samples heated at 550 °C and 650 °C, and the results can be seen in Fig. 4. Only XRD pattern for fresh 15 %Ag/13X (450/2 ?650/10) and fresh 15 %Ag/13X (450/2 ? 650/20) showed the diffraction peaks of Ag crystalline, indicating that Ag species on 13X zeolites had sintered to form large Ag particles. It can be concluded that well-dispersed Ag particles will sinter to form large Ag particles only when the adsorbents were heated at high temperature (650 °C) for quite a long time (20 h). Thus, the obtained results revealed the quite stability of Ag/13X adsorbents.


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4

log DF

3

2

10g/ 13X(4mm, sphere) 13X(4mm, sphere)

1

0 200

300

400

500

600

700

o

T( C)

Fig. 5 Effect of temperatures on the decontamination factor for retention of radioactive iodine using 10 %Ag-13X (sphere, 4 mm) and 13X (sphere, 4 mm) with the flow rate of He = 69.3 mL/min

Tests of decontamination from radioactive iodine In this research, the decontamination factors of radioactive iodine over 13X (for the blank test) and different loadings of Ag/13X were measured (at 250–650 °C). Throughout all tests, the effluent from the Ag/13X adsorbent bed passed through a backup bed containing a 13X adsorbent(less than 40 meshes). After the elution period, a gamma-counter measured the amount of 131I gamma-activity in the test bed and backup bed respectively. The decontamination factor. The decontamination factor (DF) was as follows [30]. DF ¼ C0 =Cf C0 is the initial count rate (in the feeding gas phase before adsorption) and Cf is the final count rate (in the gas phase effluent after adsorption) of radioiodine, respectively. In order to calculate and plot conveniently, the DF was converted into log DF, and the errors were obtained by error transfer formula [31, 32]. Firstly, we measured the decontamination factors achieved with 13X (sphere, 4 mm) and 10 %Ag/13X (sphere, 4 mm), and compared their DF values at 250, 350, 450, 550 and 650 °C. The results are shown in Fig. 5. The flow rate of helium was 69.3 mL/min and the infusion rate of I2 was 2.5 mL/h throughout all the tests. As it can be seen from the results of 13X and 10 %Ag/13X, the DF values of both 13X and 10 %Ag/13X showed a tendency of decline with the increase of temperature; at the same temperature, 10 %Ag/13X had an obvious advantage over the 13X. But DF values achieved with both of 13X and 10 %Ag/13X are not so high. The DF value achieved with of 15 %Ag/13X (powder, 40/60 meshes) for the radioactive iodine were also tested

with the flow rate of helium of 69.3 and 104 mL/min respectively, and other conditions were the same with that of the above 10 %Ag/13X(sphere, 4 mm) and 13X(sphere, 4 mm). With the temperature increasing, the decontamination factor of 15 %Ag/13X(powder,40/60 meshes) showed a tendency of decline at both flow rates of helium gas of 69.3 and 104 mL/min respectively (Fig. 6, left). The DF of the radioactive iodine maintained a higher value at the temperature range of 250–450 °C. But as the temperature increased to 550 and 650 °C, the DF value declined from 2.6 9 103 (250 °C) to 8.6 9 102 (650 °C) under the flow rate of helium gas of 69.3 mL/min, and DF achieved with 15 %Ag/13X (powder) for the radioactive iodine of 69.3 mL/min had little higher value than that of 104 mL/min at the corresponding temperatures. Overall, DF value achieved with of 15 %Ag/13X (powder, 40/60 meshes) indicated that it had a more heat-resistance and higher performance than those of 10 %Ag/13X (powder, 40/60 meshes).. In order to test the effect of silver loadings on decontamination from radioactive iodine, we further tested the decontamination from radioiodine with 20 %Ag/13X (powder, 40/60 meshes) and the results were showed in Fig. 6, right. The results showed that its DF value was similar to that of 15 %Ag/13X (powder, 40/60 meshes), while its DF declined with the increase of temperature. In a word, both 15 %Ag/13X(powder, 40/60meshes) and 20 %Ag/13X(powder, 40/60meshes) exhibited high stable performance at high temperatures (DF values for both at 650 °C were close to 103), which indicated that silver loadings in the range of 15–20 % did not have significant effect on DF value under our experimental conditions. Stability of 15 %Ag/13X for removing

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I2

Considering the above test results of samples and economic reason, the 15 %Ag/13X (powder, 40/60 meshes) was confirmed as the optimal to remove the radioactive iodine. In order to use it in the condition of high temperature for engineering,we continued to study the heat stability of 15 %Ag/13X (powder, 40/60 meshes). After 15 %Ag/13X (powder, 40/60 meshes) was calcined at 550 and 650 °C for 2, 10 and 20 h, the decontamination factor of 15 %Ag/ 13X (powder, 40/60 meshes) then was investigated at 550 and 650 °C. The results are shown in Fig. 7. From the test result at 550 °C experiment,at the flow rate of helium gas of 69.3 mL/min, decontamination from radioiodine with 15 %Ag/13X (powder, 40/60 meshes) resulted in high and close DF values; from 1.3 9 103 for the sample of calcination at 2 h to DF 8.0 9 102 for that of 20 h, with less than 7 % decline of DF value; and there was a similar result at the flow rate of helium gas of 104 mL/ min. This was also proved by the characterization result of

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4

15wt%Ag/13X(powder, 40/60 meshes)

4

3

2

log DF

log DF

3

20%Ag/13X(powder, 40/60meshes)

69.3mL/min 104mL/min

1

2 69.3mL/min 104mL/min 1

0 200

300

400

500

600

0

700

200

300

400

o

500

600

700

o

T( C)

T( C)

Fig. 6 Effect of temperatures on the decontamination factor for retention of radioactive iodine using 15 % Ag/13X (powder, 40/60 meshes, left) and 20 % Ag/13X (powder, 40/60 meshes, right) with the flow rate of He = 69.3 and 104 mL/min

4

4

o

550 C

3

log DF

3

log DF


o

650 C


2

2

1

1

0

0 2

10

20

Time(h)

2

10

20

Time(h)

Fig. 7 Effect of time on the decontamination factor or retention of radioactive iodine using 15 % Ag/13X (powder, 40/60 meshes) with the flow rate of 69.3 and 104 mL/min at the test temperature 550 and

650 °C (15 %/13X(powder, 40/60 meshes) was calcined at 550 and 650 °C for 2, 10 and 20 h respectively)

XRD of 15 %Ag/13X (powder, 40/60 meshes) (Fig. 4), which showed that the 15 %Ag/13X maintained a stable structure after calcination at 550 °C for 10 and 20 h. This also responded the results of BET characterization, in which the specific surface area of 15 %Ag/13X changed from 271 m2/g (fresh15 %Ag/13X (450/2)) to 206 m2/g (15 %Ag/13X (550/10)). At 650 °C, DF value was lower than that obtained at 550 °C at the same condition, but 15 %Ag/13X(powder, 40/60 meshes) of adsorbing radioactive iodine at 650 °C showed an approximate DF value for the adsorbents after calcinations at 650 °C for 2, 10 and 20 h, compared to that of DF value for the samples after calcination at 450 °C for 2 h (lower than 1.0 9 103). 15 %Ag/13X(powder, 40/60 meshes)for removing radioactive iodine after calcination at 650 °C for 2, 10 and 20 h maintained an average DF value

of 7.6 9 102 with only 1.84 % decline of DF value(fresh15 %Ag/13X(450/2)). In this study, the adsorption of radioactive molecular iodine (I2) at high temperature was investigated. During nuclear reactor operation or nuclear accidents, other species of radioactive iodine, such as organic iodide and HOI may also be released. Therefore, adsorption of those species of radioactive iodine at high temperature should be studied in the future.

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Conclusion Silver component is necessary for removing radioactive iodine at high temperatures due to the poor DF achieved with 13X only. Both 15 %Ag/13X and 20 %Ag/13X had


close results for radioactive iodine retention at high temperatures. Both 15 %Ag/13X and 20 %Ag/13X performed well at temperatures from 250 to 650 °C (even at 650 °C,the decontamination factor of samples reached to 103), though there was still a decline of decontamination factor for radioactive iodine at higher temperatures; and there was no distinct difference in DF achieved with both 15 %Ag/13X and 20 %Ag/13X for radioactive iodine at both flow rate of helium 69.3 and 104 mL/min. DF values of 15 %Ag/13X almost reached to 103 even after the sample was calcined at high temperature (650 °C, 20 h). Conclusively, 15 %Ag/13X zeolite has achieved high DF from 131I2 at high temperature, and it might be suitable for future potential use during nuclear reactor operation or nuclear accidents. Acknowledgments This work was supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. ZX06901) and the National Science Foundation of China (Grant No. 21201013).

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