Received July 3, 2020; Accepted January 27, 2021; Published online February 18, 2021

1. Introduction
Severe accidents in nuclear power plants have occurred at Chernobyl in 1986, the Three Mile Island nuclear generating station in 1979, and the Fukushima Daiichi nuclear power plant in 2011.  A large amount of radioactive materials, includ- ing 137 Cs and 131 I, were released from the reactors to the envi- ronment at Chernobyl and Fukushima.  Many children and adolescents in the surrounding area in 1986 drank milk con- taminated with radioactive iodine, which caused a significant increase in the incidence of thyroid cancer.  In contrast, IAEA reported that increased incidence of thyroid cancer has been indiscernible around Fukushima, because it is difficult to assess such a small incidence against the normal statistical fluctuations in cancer incidence [1].  Filtered containment venting systems (FCVSs) is an example of an emergency response system for severe nuclear accidents [2, 3]. On the other hand, a mock-up of high efficiency multi- nuclide aerosol filtration system has been developed for reduc- ing the radiation dose of internal exposure for workers in the decommissioning activity of Fukushima Daiichi nuclear power plant [4].  This system consists of a dry or wet filter for collecting radioactive dust and fumes, in addition to a silver doped zeolite filter for capturing radioactive gas including 129 I. Both systems are designed to remove radioactive nuclides released from the reactor and containment vessels [2-4]. Iodine exists in a variety of chemical forms including I 2 and CH 3 I in the gas phase and I  and IO 3 - in the liquid phase [5-12]. I 2 in ventilation gas easily dissolves in water along with other water-soluble ions by wet filtration.  However, water insoluble species, such as CH 3 I, are also contained in ventilation gas [13].  Then some sorbents for organic iodine such as TEDA doped activated charcoal and silver doped zeolite have been studied.  The TEDA doped activated charcoal is one of the promising sorbent for organic iodine, however, it couldn't be used at high temperature due to its flammability, while silver doped zeolite (AgX) is a very promising sorbent for CH 3 I since it can remove >99.99% of CH 3 I in vent gas and can be used at high temperature [14-17].  Studies regarding the sorption of iodine from reprocessing plant off-gas streams including dis- solution, vessel, melter, and cell off-gases by silver-doped zeo- lite have been extensively reported [18-26].  The mechanism of silver-loaded adsorbents trapping iodine has been known for a long time [27-30].  Furthermore, high efficiency multi-nuclide aerosol filters for radiation protection systems are used during core cutting under high humidity conditions due to the high humidity in the containment vessels of the Fukushima Daiichi nuclear power plant.  The performance of silver zeolite sor- bents for CH 3 I has been investigated under high humidity con- ditions [31].  Herein, the sorption of CH 3 I by AgX under high humidity conditions and that of dried AgX were performed. Ag-based sorbents are known to capture I 2 (g) or CH 3 I(g) to form solid AgI immobilized in AgX [32-34].  The adsorbent waste (iodine-sorbed AgX) must be treated prior to disposal. The suitable waste forms include crystal matrix, glass, aero- gels, and cement [35].  It is also necessary to heat the adsor- bent waste of the crystal matrix by Hot Isostatic Pressing (HIP) solidification [36, 37] and glass processes [38, 39]. Thus, it is necessary to assess the desorption behavior of iodine from AgX at high temperatures.  The desorption test for iodine from silver-mordenite has been previously reported but was performed up to 600 °C [40, 41].  Choi et al. also reported that iodine from 13X after the sorption test could be removed, while iodine from AgX was not removed at 650 °C [5]. However, these temperatures are insufficient to assess the desorption behavior of iodine from AgX.  The AgX was syn- thesized by doping silver to 13X zeolite which is a type of zeo- lite X.  In this study, therefore, 13X was also used for
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J. Nucl. Radiochem. Sci., Vol. 21, 2021
comparing the iodine desorption behavior with AgX.  Thus, desorption tests using the iodine-sorbed AgX and 13X were performed at a higher temperature, 950 °C.  In addition, degas- sing from the iodine-sorbed AgX at high temperature was ana- lyzed by Mass spectrometry (MS).
2. Experimental
2.1. Materials 2.1.1 Preparation of 13X zeolite and AgX.  The 13X zeolite (Union Showa K.K.) is a type of zeolite X and its composition is Na se [(AlO 2 ) se (SiO 2 ) 106 ] 276H 2 O.  The AgX was manufac- tured by Rasa Industries using the 13X zeolite wherein sodium was substituted with silver.  The silver ion exchange ratio was ≥97%, the silver content when dried was ≥36 wt%, particle size of 10 × 20 mesh, and the water content was ≤12 wt% as estimated by the weight reduction when the AgX was dried at 150 °C for 3 h [14]. 2.1.2 Radioactive CH 31251. The 125I in 0.1 M NaOH was obtained from PerkinElmer.  The CH 3 I labeled with 125 I was synthesized via isotope exchange reaction between 125I in 0.1 M NaOH solution and water insoluble CH 3 [32, 42];
The two phases were contacted for 20 h and the γ-ray released from 125mTe that is the decay product of 125 I was mea- sured by NaI(Tl) scintillation detector.  The exchange ratio (R) was obtained by the following equation;
where R is the exchange ratio (%), W 1 is the count of ¹²³I injected (cps/mL), W 2 is the counts of 125 I exchanged to CH 3 I (cps/mL), V 1 is the volume of Na 125 I (mL), and V 2 is the volume of CH 3 (mL). The detailed procedure and time dependence of the exchange ratio were reported previously [32].  The CH 3 I labeled by 125 I with an exchange ratio of 10% was used in this study. 2.2. Sorption experiment.  The sorption experiment was performed using radioactive methyl iodide (CH , 125 I) and the stable form in the two types of experimental systems, as shown in Figure 1.  A PYREX glass apparatus was used at gas flow rates of lower than 500 mL/min, whereas a stainless-steel apparatus was used at rates of higher than 500 ml/min with steam condition.  The experimental conditions of using glass apparatus and stainless-steel apparatus are shown in Tables 1 and 2, respectively.  The glass apparatus system was com- posed of three parts, i.e. 1) the CH 3125I injection, 2) Tube 1: the sorption tube with AgX or 13X doped zeolite (5 g), Tube 2: the (S.S.) apparatus. recovery tube for un-trapped CH 3 I, and 3) Tube 3: the addi- tional recovery tube for un-trapped CH 3 I.  First, 15 mL of 99 wt% ethanol and 15 mL of 50 wt% ethanol were filled in Tubes 2 and 3, respectively, to trap CH 3 I that passed through Tube 1.  Ar + 100 ppm CH 3 I gas flowed at 20 to 500 mL/min for 1 h during the sorption test.  When using radioactive methyl iodide (CH, 1251), 1.0 mL of CH, 1251 was injected and flowed to Tube 1 for 1 h via a carrier gas (Ar) at a flow rate of 500 mL/min.  After the sorption test, the stable iodine concen- trations in the solutions in Tubes 2 and 3 were measured by ICP-AES, whereas the radioactive iodine concentration was measured using a NaI scintillation counter.  The stainless-steel apparatus system was composed of three parts, i.e. 1) Tube 1: the sorption tube with AgX doped zeolite, 2) Tube 2: the recovery tube for un-trapped CH 3 I, and 3) AgX was filled in Tube 2 to trap CH 3 I that passed through Tube 1.  Ar + 100 ppm CH 3 I and Ar + 1000 ppm gases flowed at 500 to 1000 mL/min for 1 h for the sorption test.  To evaluate the effects of steam on the sorption, a steam generator (Nihon dennetsu, K007) was used and steam was introduced at an oversaturated vapor partial pressure.  After the sorption test, the sorption amount of stable iodine in the AgX in Tube 1 was measured by X-ray Fluorescence (XRF, Rigaku, Supermini 200).  In addition, the iodine-sorbed AgX after the sorption test with steam was removed from Tubel and dried using vacuum drying system for 1 day.  The dried AgX was used for the second sorption test without steam.  To compare sorption ratios using the
(1) (2)
PYREX glass and stainless-steel equipment, the flow rate unit, mL/min, was converted to m/s using Eq. (3); Flow rate (m/s) = Flow rate (m³/s)/ (3) The cross-sectional area in PYREX glass Tube 1 was 6.1 × 10 -5 m² and that of the stainless-steel was 5.3 × 10 −4 m². 2.3 Desorption experiment.  To evaluate the desorption of iodine sorbed onto AgX and 13X, saturated iodine-sorbed AgX and 13X were prepared by supplying CH 3 I to AgX and 13X until reaching the sorption saturation state.  To evaluate the apparent sorption capacity of AgX and 13X, breaking through experiment was carried out in our former study [32]. The apparent sorption capacity of CH 3 I on AgX and 13X at 24 °C were evaluated to be 0.21 g/g and 0.56 g/g, respectively. Then, the saturated iodine-sorbed AgX and 13X were evalu- ated by thermogravimetric and differential thermal analyses (TG-DTA, Thermo plus 2, Rigaku) measurements, where the composition of the degas from the furnace of TG-DTA was analyzed by quadrupole mass spectrometer (M-201GA- CRMY, Rigaku).  First, the saturated iodine-sorbed AgX and 13X were heated to 500 °C, which is under melting point of AgI (552 °C), by a heating rate of 10 °C/min to remove water adsorbed to the AgX and 13X from the atmosphere.  After cooling, the samples were heated again to 950 °C by a heating rate of 10 °C/min in Ar.  For comparison, reagent grade AgI was measured under the same conditions.  For the quadrupole MS, He gas was flowed at 500 mL/min before heating until the intensities of the elemental mass-to-charge ratio ( m/z ) derived from air, i.e. m/z= 18 (H 2 O), 28 (N 2 ) and 32 (O 2 ) decreased sufficiently.  After the background ions decreased to constant low intensities, saturated iodine-sorbed AgX and AgI regent (Sigma-Aldrich, 99.999%) were heated to 1400 °C and 950 °C, respectively, by a heating rate of 20 °C/min.  The intensities of the elemental m/z of 18 (H 2 O), 63 (I), and 127 (I 2 ) were measured during the heating.
3. Results and discussion
3.1. Sorption of CH 3 I on AgX.  Figure 2 shows the relation between the sorption ratio and gas flow rate.  When the gas flow rate was lower than 0.08 m/s, almost 100% of the CH 3 I was sorbed and at flow rates of higher than 0.08 m/s, the sorp- tion ratio of CH 3 I decreased to lower than 50% at 0.14 m/s. Using radioactive methyl iodide (CH 3125I), the sorption ratio at a 0.14 m/s flow rate was similar.  Under steam conditions, the CH 3 I sorption ratio decreased to the range of 30% to 50%.  At faster flow rates, the sorption ratio decreased with the oversat- urated vapor partial pressure.  Because some steam became water and soaked into AgX in Tube 1, CH 3 I was prevented from sorbing on AgX.  From a previous study, it is recognized that the apparent sorption capacity of AgX decreased with 3% H 2 O in Ar [24].  When water was contained in the flowed gas, the sorption ratio and sorption capacity decreased.  In addi- tion, the soaked AgX was dried and recycled for another sorp- tion test, confirming that the sorption performance was restored to that of the initial state. 3.2. Thermal stability of iodine-sorbed AgX or 13X.  The saturated iodine-sorbed AgX and 13X were analyzed by TG-DTA.  The obtained TG-DTA curves of the AgX sample heated from room temperature to 500 °C is shown in Figure 3(a).  After the above mentioned procedure, this sample was cooled down to room temperature and again heated to 950 °C by 10 °C/min.  The TG-DTA curves of this heat treatment are shown in Figure 3(b).  Figure 3(a) shows the gradual endo- thermic reaction from room temperature to 100 °C and the decreasing weight due to volatilizing adsorbed water.  The adsorbed water in AgX is known to volatilize up to 500 °C [6]. Figure 3(b) shows a slight decreasing TG curve.  The limited amount of iodine volatilized from the AgX up to 800 °C.  But over 830 °C, a remarkable weight decrease was observed due to the decomposition of AgI and volatilization of I 2 as follow- ing Eq. (4).
The TG-DTA curve to 500 °C of the saturated iodine-sorbed 13X is shown in Figure 4(a).  After the 1 st TG-DTA measure- ment, the 13X sample was cooled down to room temperature. Then, the sample was again heated to 950 °C to obtain 2nd TG-DTA curve shown in Figure 4(b).  Figure 4(a) shows con- tinuous weight decrease, which was supposed to attribute to the volatilization of sorbed water and iodine in the 13X sam- ple.  The weight change caused by the desorption of water in 13X was evaluated by additional experiment, that is, the TG-DTA measurement for initial 13X sample which was not contacted with CH 3 I.  The TG curve of this additional experi- ment indicated that approximately -5% of weight change was caused by the desorption of water in 13X (Figure S1).  Hence, we concluded that the weight decreases in Figure 4(a) caused by the iodine volatilization from the 13X sample was much larger than that caused by the water volatilization.  Figure 4(b) shows the slight weight decrease in comparison with that shown in Figure 4(a) because almost all of the iodine had already volatilized in the 1 st TG-DTA measurement up to 500 °C.  These results indicate that AgX fixed iodine up to 830 °C, while the 13X released whole sorbed iodine up to 500 °C. This result evidentially indicates that AgX has higher iodine fixation ability than that of 13X. The TG curve and MS signals (H 2 O, I, and I 2 ) of AgI by 20 °C/min heating to 950 °C in He atmosphere are shown in Figure 5.  The weight was almost constant up to 750 °C, and at the higher temperature, the weight decreased sharply up to 930 °C.  Peaks of MS signals corresponding to I and I 2 were observed at 890 °C, while the other MS signals, such as H 2 O, were temporally disturbed for large amount of I and I 2 being released from the sample AgI due to saturation of the MS spectrometer.  This indicates that AgI was stable up to 750 °C, but at higher temperature AgI was decomposed to Ag and I 2 . The TG curve and MS signals (H 2 O, I, and I 2 ) of the saturated iodine-sorbed AgX by 20 °C/min heating to 1400 °C in He is shown in Figure 6.  The weight decrease was observed from 30 °C to 250 °C in the TG curve while two peaks were found in MS signals of H 2 O at 100 °C and 200 °C.  This result is interpreted as two types of sorbed water existed in the AgX, that is, water molecules in surface and those in channels of the AgX.  At 830 °C, the weight decreased sharply and MS signals corresponding to I 2 were observed, which seems to be caused by AgI decomposition and iodine volatilization.  This decom- position temperature was slightly higher than that observed in the pure AgI experiment.  Thus, AgI in AgX matrix was ther- mally stabilized in comparison with pure AgI., However, this stabilization effect is minor since the framework of zeolite X collapses at 843 °C [43].  At temperatures higher than 1200 °C, the weight decreased gradually without MS signals of H 2 O, I, and I 2 , which indicated that this weight change was caused by the decomposition of the AgX matrix.
4. Conclusion
Herein, radioactive CH 3125 I was synthesized and its sorption on AgX was examined.  Both labeled CH 3125I and stable CH 3 I showed similar sorption behaviors.  When the flow rate of Ar + CH 3 I gas was lower than 0.08 m/s, almost all of the CH 3 I (>99.9%) was adsorbed by 5 g AgX.  Upon introduction of steam and soaking AgX, the sorption ratio decreased, but dry- ing the soaked AgX restored the sorption performance to the initial state.  Using saturated iodine-sorbed AgX and 13X, both sorbents could adsorb iodine.  However, almost all of the sorbed iodine was removed from 13X up to 500 °C, whereas AgX retained the iodine up to 830 °C.  This temperature is similar to the decomposition temperature of AgI.  Thus, the thermal stability of iodine captured by AgX was determined to be similar to that of AgI powder.
Acknowledgments
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