Adsorption of gaseous iodine-131 at high temperatures by silver impregnated alumina

NUCLEAR CHEMISTRY, RADIOCHEMISTRY, RADIOPHARMACEUTICALS AND NUCLEAR MEDICINE

Adsorption of gaseous iodine-131 at high temperatures by silver impregnated alumina

CHENG Qing-Hui，

LI Ze-Jun，

CHU Tai-Wei

Nuclear Science and Techniques

Vol.26, No.4

Article number 040303

Published in print 20 Aug 2015

Available online 11 Aug 2015

DOI：10.13538/j.1001-8042/nst.26.040303

35501

To prevent radioactive iodides from releasing into the environment in an accident of a nuclear power plant, silver-impregnated alumina (Ag/Al₂O₃) was fabricated, and its performance of radioactive iodine adsorption from high-temperature gas was tested. The silver loadings on alumina were obtained by ICP-OES and the texture properties of Ag/Al₂O₃ were characterized by N₂ adsorption-desorption. The Ag/Al₂O₃ was of reduced specific surface (107.2 m²/g at 650 ℃). Crystalline phases of Ag/Al₂O₃ were confirmed through XRD characterization. After calcination at 650 ℃ for 2 h, the crystalline phase of Ag/Al₂O₃ changed. The ¹³¹I- removal efficiency of Ag/Al₂O₃ was tested at 100, 250, 350, 450 and 650 ℃, with good decontamination factor values for the radioactive iodine. Silver-impregnated alumina can be applied as adsorbents to remove radioactive iodine at high temperatures in nuclear accident.

Silver impregnated aluminaHigh temperatureRadioactive iodineAdsorptionDecontamination factor

I. INTRODUCTION

Nuclear energy has been exploited as an alternative to maintain energy sustainability for a half century, while environmental pollution caused by nuclear power plants (NPP) has become a worldwide concern [1, 2], especially in a sever accident of NPPs [3]. In March 2011, large amounts of nuclear dusts were released from Japan’s Fukushima Daiichi NPPs due to failure of cooling systems in a huge earthquake and tsunami. Due to their high mobility, the most dangerous nuclides released in an NPP disaster are gaseous ⁸⁵Kr and ¹³⁵Xe, and volatile ¹³¹I, ¹²⁹I, ¹³⁴Cs and ¹³⁷Cs, with extensive radioactive hazard to the public [4, 5]. Among them, ¹³¹I (t_1/2=8.02 d) is the most harmful radionuclide because of its large quantity of release in a nuclear disaster and relatively high activities, and its high accumulation in human thyroid and further damage to organs if ingested; whereas ¹²⁹I, which decays in a half-life of 1.57×10⁷ a and emits lower energy beta-rays, would do nearly no harm to people [6, 7]. In early stages of the Fukushima Daiichi NPP disaster, gaseous ¹³¹I released into the environment was estimated at 1.5×10¹⁷ Bq [8-10]. Therefore, how to remove ¹³¹I is an important research subject for safeguarding NPPs and the environment.

Great research efforts have been made to prepare adsorbents for ¹³¹I removal, such as carbon-based materials [11, 12, 13], silica gel [14, 15], polymer resin [16], titanium based materials [17-20], cyclodextrin [21], molecular sieves [22-24], etc. Also, impregnants have been exploited to improve performance of adsorbents for iodide removal, such as KI [25, 26], TEDA [27, 28] and silver salts [29]. However, most researchers focus mainly on iodide removal from water or gases at low temperatures. Given the fact that an NPP in lost-of-coolant accident is in high temperatures [4], preparation of materials for adsorbing radioactive iodides at high temperatures is meaningful [30].

Alumina is widely applied to manufacture ceramic materials and catalyst supports due to its good heat-resisting properties and large specific surface area [31-33]. In consideration of high affinity between silver and iodide, silver based alumina was exploited in this work as an adsorbent for radioactive iodine at high temperatures.

II. EXPERIMENTAL SECTION

A. Reagents and instruments

Potassium iodate (A.R.): Shantou West Long Chemical Co., Ltd; potassium iodide (A.R.): Beijing Tongguang Fine Chemicals Company; Na¹³¹I: HTA CO., Ltd.; L-(+)-tartaric acid (A.R.): Sinopharm Chemical Reagent Co., Ltd.; light petroleum (boiling range of 30–60 ℃, A.R.): Beijing Chemical Works; N₂ (high purity): Beijing AP BAIF Gases Industry Co., Ltd.; nitrate silver (A.R.): Beijing Chemical Works; Alumina (neutral, 100–200 meshes): Sinopharm Chemical Reagent Co., Ltd.; nitric acid (A.R.): Shantou West Long Chemical Co., Ltd.

Tube furnace: Beijing Zhongshiyida Science and Technology Ltd.; KL-602 micro-injection pump: Beijing Kelly Med Co., Ltd.; D07-7B mass flow controller: Beijing Seven star Electronics Co., Ltd.; D08-1F flow displayer: Beijing Seven Star Electronics Co., Ltd.; XMT series digital display apparatus: Yuyao Jindian Instruments Co., Ltd.; quartz tube (Φ12 mm×620 mm): Beijing Haiqing Photoelectric Glass Instrument Co., Ltd.; ICP-OES (Prodigy): Leeman Labs; XRD (X’PERT-MRD): Phlips; injector (10 mL): Changzhou Yuekang Medical Appliance Co., Ltd.; ASAP2010 accelerated surface area and porosimetry analyzer: Micromeritics; 2470 WIZARD2 automatic gamma counter: Perkin Elmer; FH463A automatic scaler: Beijing Nuclear Instrument Factory; disposable plastic tube (WG, Φ10–12 mm×75 mm): Zhejiang Plasmed Medical Technology Co., Ltd.

B. Preparation of adsorbent

Silver based alumina was fabricated by the method of impregnation. Alumina was calcined at 450 ℃ for 2 h in a muffle furnace, and allowed to cool in a desiccator. Then, an appropriate amount of alumina was added into nitric silver solution in a light-shading flask at 80 ℃. After 24 h, the as-samples were spun to dry and placed in an oven at 120 ℃ for 2 h, before a 2-h calcination at 450 ℃ in the muffle furnace. Finally the adsorbent was prepared, and Ag/Al₂O₃ of different calcination temperatures are accordingly marked as "Ag/Al₂O₃ (T℃ / t h)", where T is calcination temperature and t is calcination time.

C. Silver loadings on alumina

Silver loadings on alumina were analyzed by inductively coupled plasma (ICP). Small amounts of the as-prepared Ag/Al₂O₃ were mixed with concentrated nitric acid in a flask for 24 h. The filtrate was gathered into a volumetric flask and the silver loadings were confirmed about 10 wt.%.

D. Characterization of adsorbents

Texture properties of the adsorbents were checked by the method of N₂ adsorption and desorption on Micromeritics ASAP2010 analyzer. Adsorbents were degassed in advance at 300 ℃ for 2 h. Specific surface areas of adsorbents were measured by the BET method (Brunauer, Emmett and Teller); and the average pore volume was calculated by the BJH method (Barrett, Joyner and Halenda).

To confirm their crystalline phases and check their heat-resisting properties, Ag/Al₂O₃ of different temperatures were characterized by X-ray diffraction (D/max-2500/PC X-ray diffractometer, Rigaku, Japan), using Cu K_α (the XRD system was operated at 45 kV and 40 mA). The samples were scanned in 0.01 steps from 2θ = 10° to 80°, with the scan rate of 5min^-1.

E. Adsorption of radioactive iodine

1. Effect of temperature

In this section, Ag/Al₂O₃ and Al₂O₃ were tested to evaluate their efficiency of ¹³¹I removal at 100, 250, 350, 450 and 650 ℃. Before the test, adsorbents were calcined at 450 ℃ for 2 h in the muffle furnace.

With trace amount of ¹³¹I, I₂ was prepared using 2 mL of 2% KIO₃ and 4 mL of 166 μg/mL KI (plus 1 mL Na¹³¹I), with 1 mL of 5% L-(+)-tartaric acid as acid medium. The I₂ was dissolved in light petroleum. The ¹³¹I radioactivity in a test was 0.1–2.0 MBq.

The experimental apparatus (Fig. 1) consisted of the ¹³¹I-injection, heating and off gas purification sections. Radioactive iodine solution was injected by micro-injection pump in 2.5 mL/h of injection rate. High purity N₂ at a flow rate of 35 mL/min was used as carrier gas. In the experiment, the adsorbents were laterally loaded in the tube center to adsorb ¹³¹I at different temperatures. A typical test with 1 g adsorbents in 5 mL light petroleum was done in 2 h. 13X (including Ag/13X zeolites, less than 40 meshes) and saturated NaOH solution were used to trap¹³¹I in the off gas [30]. After test, the spent adsorbents were imbedded in the disposable plastic tubes to detect radioactivity on the gamma counter. The decontamination factor were calculated by DF=(A+B)/B, where A is the radioactivity of spent adsorbents and B is the summation of radioactivities of the off gas purification columns. The DF values were normalized to DF/g.

Fig. 1.

Flow chart of the experiment.

2. Effect of I2 concentration

To study whether I₂ concentration would influence ¹³¹I-adsorbing performance of adsorbents, the KI concentration used as the source to produce iodine, was increased to 332 μg/mL, other details were the same as above.

3. Effect of N2 flow rate

Considering that the flow rate of carrier gas may affect ¹³¹I removal, N₂ flow rates of 15–75 mL/min were used to measure the removal efficiency of adsorbents at 250 ℃. Other details were the same as above.

4. Effect of adsorption time

To check time-depenence of the interaction between Ag/Al₂O₃ and iodine, DF values in interaction durations of 30–180 min were measured at 100 ℃ and 450 ℃ under the N₂ flow rate of 35 mL/min. Iodine (dissolved in light petroleum) produced by the KI (166 μg/mL, Na¹³¹I-included) was kept in a glass container. The amount of Ag/Al₂O₃ is about 1 g. As an example, the DF value of 30-min time point was measured as follows. Iodine dissolved in light petroleum was constantly injected by the micro-injection pump at 2.5 mL/h for 30 min, and radioactivity of the adsorbed ¹³¹I was measured to calculate the DF values.

III. RESULTS AND DISCUSSION

A. Adsorbents characterization

1. Texture properties of Ag/Al2O3

Texture properties of adsorbents and support were characterized by the method of N₂ adsorption-desorption at 77 K. Figure 2 shows the N₂ isotherms of alumina, Ag/Al₂O₃ (450 ℃/2 h) and Ag/Al₂O₃ (450 ℃/2 h + 650 ℃/2 h). The alumina maintained a mesoporous structure, which was in accordance with the calculation results by BJH method (average pore diameter of 4.5 nm). After loading silver on alumina, the Ag/Al₂O₃ (450 ℃/2 h) and Ag/Al₂O₃ (450 ℃/2 h + 650 ℃/2 h) were still of the mesoporous structure, with an average pore diameter of 5.2 and 6.4 nm (BJH method), showing a little increase after silver loading. An adsorption hysteresis occurred when the relative pressure (p/p0) reached to 1, i.e., all N₂ isotherms were attributed to Type IV.

Fig. 2.

N₂ adsorption-desorption isotherms of different adsorbents.

As shown in Table 1, the Ag/Al₂O₃ samples were of relatively large specific surface area, being [per-mode=symbol]129.1 and 107.2 m²/g after 450 ℃/2 h and 650 ℃/2 h calcinations, respectively, though they were smaller than that of Al₂O₃.

Texture properties of Al₂O₃ and Ag/ Al₂O₃

Adsorbents	S_BET (m/g²)
Al₂O₃(450 ℃/2 h)	146.1
Ag/Al₂O₃(450 ℃/2 h)	129.1
Ag/ Al₂O₃ (450 ℃/2 h + 650 ℃/2 h)	107.2

2. XRD spectra of adsorbents

Silver impregnated alumina had the characteristic diffraction peaks of alumina from the XRD spectrum (Fig. 3), i.e., it maintained a stable structure after silver impregnated. To be specific, silver on alumina had nearly no effect on the alumina structure. In the XRD spectrum of Ag/Al₂O₃ after 450 ℃/2 h calcination, the diffraction peaks of silver particles can be seen clearly. For Ag/Al₂O₃ (450 ℃/2 h + 650 ℃/2 h), the crystalline phase of alumina changed, and the Ag diffraction peaks indicate sintering of the silver particles during the 2-h calcination at 650 ℃.

Fig. 3.

XRD spectra of Al₂O₃ and Ag/Al₂O₃.

B. Adsorption of radioactive iodine

1. Effect of adsorption temperature on 131I-removal efficiency

The silver-impregnated alumina performs better in ¹³¹I-removal than alumina at the same temperatures under N₂ flow rate of 35 mL/min. As shown in Fig. 4, the ¹³¹I-removal efficiencies of both Al₂O₃ and Ag/Al₂O₃ decrease with increasing temperature of adsorption. However, DF values of the silver-impregnated alumina were 806.9 at 450 ℃ and 306.7 at 650 ℃; while the DF values of alumina declined rapidly from 437.8(250 ℃) to 16.1(650 ℃).

Fig. 4.

(Color online) Adsorption of ¹³¹I at different temperatures by Al₂O₃ and Ag/Al₂O₃ under N₂ flow rate of 35 mL/min.

Despite its reduced specific surface area (107.2 m²/g for Ag/Al₂O₃, while 146.1 m²/g for alumina), Ag/Al₂O₃ has higher DF value, as the silver particles can react with iodine from high-temperature gas to form AgI.

2. Effect of I2 concentration on removal efficiency

I₂ concentrations of 166 μg/mL and 332 μg/mL were used to study the I₂ effect of dose on ¹³¹I-adsorbing performance of the Ag/Al₂O₃ at N₂ flow rate of 35 mL/min. The results are shown in Fig. 5. The Ag/Al₂O₃ was of high ¹³¹I-removal efficiency at 350–650 ℃ under both the KI concentrations, though the Ag/Al₂O₃ at I₂ concentrations of 166 μg/mL performed better at <300 ℃.

Fig. 5.

(Color online) Effect of I₂ concentration on ¹³¹I-adsorbing performance of Ag/Al₂O₃ at N₂ flow rate of 35 mL/min.

3. Effect of N2 flow rate on 131I-removal efficiency

The effect of N₂ flow rate on ¹³¹I-removal efficiency of adsorbents was carried out at 250 ℃, with N₂ flow rate varying from 15 mL/min to 75 mL/min. As shown in Fig. 6, DF values changed little in the entire range of the N₂ flow rate.

Fig. 6.

Effect of N₂ flow rate on ¹³¹I-adsorbing performance of Ag/Al₂O₃ at 250 ℃.

4. Effect of adsorption time

The interaction between Ag/Al₂O₃ and iodine was studied at 100 ℃ and 450 ℃ under N₂ flow rate of 35 mL/min by measuring the ¹³¹I-removal efficiency in different durations of the adsorption (Fig. 7). The DF value of Ag/Al₂O₃ increased with time till 100 min, where it reached to a plateau. Therefore, the Ag/Al₂O₃ can achieve desirable result of ¹³¹I-removal in adsorption time of 100 min at N₂ flow rate of 35 mL/min.

Fig. 7.

(Color online) I-131 adsorption kinetics curve of Ag/Al₂O₃ at 100 and 450 ℃ under N₂ flow rate of 35 mL/min.

IV. CONCLUSION

In this work, silver impregnated alumina was prepared and evaluated for its removal efficiencies of radioactive iodine at high temperatures (100, 250, 350, 450 and 650 ℃). The results suggested that: alumina would perform better for adsorption radioactive iodine at high temperatures after silver loaded; the differences of removal efficiencies among different flow rates of carrier gas were small. Silver impregnated alumina would be applied as adsorbents to remove radioactive iodine at high temperatures during nuclear accident.

References

[1]

P Slovic, J H Flynn and M Layman.

Perceived risk, trust, and the politics of nuclear waste

. Science, 1991, 254: 1603-1607. DOI: 10.1126/science.254.5038.1603