1 Introduction

One of the major requirements for sustaining human progress is an adequate source of energy.Traditional fossil energy is limited and non-renewable and causes environmental pollution and greenhouse gas emissions.Therefore,nuclear energy has attracted signi ficant attention from researchers worldwide as the most prominent and feasible source.Nuclear energy is a clean,nearly inexhaustible,and the most environmentally friendly source of energy.Nonetheless,the safety of nuclear power plants has always been a concern,as accidents in these plants result in huge,disruptive,and irreversible disaster.The release of radioactive waste during a nuclear accident can cause serious health effects on the lives of people as well as on the environment.The Three Mile Island disaster[1]in 1979 and the Chernobyl disaster[2]in 1986 were the worst nuclear accidents in history.In particular,the Fukushima Daiichi disaster[3],which occurred in 2011,has intensi fied the fear of nuclear energy and in fluenced the discontinuation of nuclear power[4].For example,after the Fukushima incident,the German government announced that it would quit nuclear energy,for the first time worldwide.During these disasters,trillions of becquerels(Bq)of radionuclides were released into the atmosphere.Among these radionuclides,radioiodine(131I)is one of the most hazardous substances[5].131I has high β-ray radiation energy(0.606 MeV,89.9%)and γ-ray radiation energy(0.364 MeV,81.2%),although its half-life is only 8.02 d[6].Considering the high mobility of radioiodine in the environment[7]and the enormous health hazards[8],methods for its removal from the off-gas are highly desirable to prevent the spread of this nuclide in the case of emergency.

Some Engineering measures[9,10]such as the use of a spray system and an aqueous scrubber have been explored to mitigate the consequences of nuclear accidents.However,such systems are inefficient to remove the radioiodine and produce plenty of radioactive wastewater that is cumbersome to deal with.In contrast,solid adsorbents are highly efficient and can be easily disposed of.

Most solid adsorbents are based on porous supports with a high speci fic surface area.These supports include zeolite[11,12],titania[13],alumina(Al2O3)[14],silica[15],apatite[16],silica aerogel[17],activated carbon[18,19],polymer[20],and so forth[21].These adsorbents adsorb radioiodine in two ways,namely physical adsorption and chemical adsorption.Physical adsorption is unstable and leads to low adsorption capacity.Therefore,chemical adsorption-based adsorbents are more common[22,23],wherein Ag is usually used as the active ingredient because of its ability to react with iodine.However,these adsorbents are mainly designed to remove radioiodine from the off-gas released during nuclear fuel reprocessing.When used under high-temperature conditions,as in the case of high-temperature radioactive gas released from the nuclear reactor core after fusion,some of the adsorbents do not remain stable and others lose their adsorption ability.In our previous study[14,24],a Ag-loaded 13X zeolite was synthesized to solve this problem.However,the maximum temperature at which the 13X zeolite adsorbent could be effectively used was 650°C.When the temperature exceeded 650°C,the 13X zeolite adsorbent began to collapse,thus losing its adsorption ability.Therefore,the main objective of the present study was to identify the possible reason for the loss of adsorption ability of traditional adsorbents at high temperature and then develop a new adsorbentthatcould be effectively used athigher temperatures.

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解铃还须系铃人。接下来，我还要继续借用小胡的力量，彻底粉碎班上的“白虎队”。我聘任小胡为班级的安全管理员，一方面重点监管其他7人，若再出现打架闹事现象，首先追究他的责任；另一方面，引导他们积极带头为班级做好事，还负责监管班上的其他“霸凌”现象……在小胡的影响下，“白虎队”的小队员们也渐渐磨去了戾气，开始热心地为班级服务，诸如搬书、收作业本、倒垃圾、擦玻璃、洗门之类的，都可以见到他们的身影。班会课上，同学们也都向他们伸出了大拇指，我趁势将“白虎队”更名为“英雄联盟”！

在晴天无云的条件下10：00～14：00时，使用手持ASD光谱仪采集作物野外光谱，该光谱仪的光谱范围325～1 075 nm，采样间隔为1.4 nm。光谱测量采用25试场角探头，探头离地面的高度为1.0 m，测量时探头垂直于地面，每个样地采集15条光谱，测量前进行白板校正，每隔10～15条重新优化仪器。

2 Experimental section

2.1 Chemicals and reagents

Al2O3with a particle size of 100–200 mesh was purchased from Sinopharm Chemical Reagent Co.,Ltd.(China).Potassium iodate(KIO3),potassium iodide(KI),sodium phosphate dodecahydrate(Na3PO4·12H2O),and nitric acid(HNO3)were of analytical grade;these were purchased from Sinopharm Chemical Reagent Co.,Ltd.(China)and used as received without any further puri fication.Silver nitrate(AgNO3)was chemically pure and was purchased from Beijing Chemical Factory.Radioiodine was in the form of Na131I aqueous solution and was obtained from HTA Co.,Ltd.(China).Compressed Air and N2with 99.999%purity were purchased from Beijing Haikeyuanchang Practical Gas Co.,Ltd.(China).

供试药剂：30 g/L甲基二磺隆可分散油悬浮剂（山东省长清农药厂有限公司）、8%炔草酯可分散油悬浮剂（河南省丰收乐化学有限公司）。对照药剂：30 g/L甲基二磺隆可分散油悬浮剂［拜耳作物科学（中国）有限公司］、15%炔草酯可湿性粉剂［先正达（中国）投资有限公司］。

2.2 Instruments

Inductively coupled plasma–atomicemission spectroscopy(ICP-AES,Leeman Science and Technology Ltd.)was used to confirm the actual Ag content in the adsorbents and the element constituting of the yellow powder after the iodine desorption test.

The crystallinity of all the samples was evaluated by powder X-ray diffraction(XRD)measurements performed using a PANalytical X′pert Powder diffractometer,with Cu Kα radiation.The diffraction intensity data were collected in the 2θ range of 20°to 80°with a step length of 0.013°and a count time of 0.1 s per point at room temperature.

Thermogravimetric analysis–differential scanning calorimetry(TGA–DSC)was employed to evaluate the thermal stability of AgI and adsorbents.TGA–DSC was performed using a TA Q600 SDT system with 10 mg samples in alumina crucibles under N2or air.The temperature range employed was 25–800 °C,with a heating rate of 10°C min-1and a N2 flow rate of 100 ml min-1.

Figure 1 shows the adsorption of131I2by Al2O3,10 wt%Ag/Al2O3,and 10 wt%Ag3PO4/Al2O3adsorbents at different temperatures.The 10 wt%Ag/Al2O3and 10 wt%Ag3PO4/Al2O3adsorbents showed obvious advantages over Al2O3with respect to the DF value and removal efficiency,as Al2O3barely adsorbed the radioactive iodine.This result indicates that the adsorption ability of the adsorbent toward radioiodine is attributed to Ag,which is capable of reacting with iodine to form AgI.

我已经完成得很好了，我尽力了。我对自己说。虽然这样讲出来不是很好，我是个善良的人，但我承认我就是这样想的——由于知道他不久之后会复活，我对他的尸体做这种事的罪恶感减轻了一些。

The speci fic surface areas of the samples were measured by nitrogen adsorption and desorption isotherms using a Micrometrics ASAP2010 system at 77 K.Prior to each analysis,the sample was degassed in vacuum at 300°C for 2 h with pretreatment at 120°C for 1 h.

X-ray photoelectron spectroscopy(XPS)analysis was performed using an AXIS Ultra photoelectron spectroscope(Kratos Analytical Ltd.)with monochromatic Al Kα radiation(E=1486.7 eV,source strength=240 W)to confirm the composition of the adsorbent after calcination.

用棉签蘸一下耳道内粘稠物，涂抹于载玻片上，盖上盖玻片，在显微镜下观察。在显微镜下可观察到螨虫 （图2），该载玻片置于室内数日，视野内的虫体仍然活动。

The test system consisted of a KL-602 injection pump(Beijing KellyMed Co.,Ltd.,China),D08-1F gas flow controller(Beijing Sevenstar Electronics Co.,Ltd.,China),XMT618 temperature controller(Yuyao Changjiang Temperature Meter Instrument Factory,China),WRNK-191 temperature sensor(Shanghai Automation Instrumentation Co.,Ltd.,China),and custom-made tube furnace(Yangzhou Baoding Electric Equipment Factory,China).

A microwave digestion system(MARS 6,CEM Corporation,UK)was used to digest the adsorbent.

2.3 Synthesis of Ag/Al2O3adsorbent and Ag3PO4/Al2O3adsorbent

Ag/Al2O3adsorbent was prepared by a conventional impregnation method.The Ag content in the adsorbent was 10 wt%,which is similar to that for the adsorbent used in our previous study[14,24].The as-synthesized adsorbent was designated as 10 wt%Ag/Al2O3.AgNO3(1.6 g)was dissolved in 120 ml of deionized water,then 9.0 g of Al2O3were added into the aqueous solution,and the contents were stirred at 80°C in a lightproof environment for 24 h.Subsequently,water was removed from the solution by rotary evaporation at 70°C.The precursor of the adsorbent was then calcined in air at 450°C for 2 h and ground in a mortar.The final adsorbent with 100–200 mesh was obtained after sieving.

10 wt%Ag3PO4/Al2O3adsorbent(the Ag content in this adsorbent is about 7.7 wt%)was prepared by the following method:The precursor of the adsorbent,i.e.,AgNO3/Al2O3,was prepared by the same method as mentioned above.Then,2 ml of sodium phosphate solution(Na3PO4)with a concentration of 50 g L-1was added into the flask to impregnate AgNO3/Al2O3.Water was removed by rotary evaporation at 70°C.These two steps were repeated seven times until AgNO3was completely transformed to Ag3PO4.Finally,the adsorbent was washed several times with deionized water to remove NaNO3and excess Na3-PO4,and then,it was dried and calcined in air at 450°C for 2 h.The final adsorbent with 100–200 mesh was obtained after sieving.

The adsorbent(0.10 g)was digested with 10 ml of HNO3(65%)in the microwave digestion system.Digestion was carried out at 1000 W for 2 h.The solution was diluted with ultrapure water,and the concentration of Ag in the diluted solution was determined by ICP-AES[14,24].The results showed that the Ag/Al2O3and Ag3PO4/Al2O3 adsorbents contained 9.8±0.1 and 7.6±0.1 wt%Ag,respectively.The Ag contents were slightly smaller than the theoretical value.

The melting point of silver iodide(AgI)is 558°C[6].Therefore,when the temperature increases beyond 558°C,AgI becomes unstable and can hardly be retained on the support.To improve the adsorption ability of the adsorbent at high temperatures,the stability of AgI on the carrier is the key point.According to the literature report,AgI can form a solid solution with silver phosphate(Ag3PO4),and the melting point of the solid solution can be higher that of AgI according to their molar ratio[25].Herein,10 wt%Ag3PO4/Al2O3adsorbent was prepared with the objective of employing it in the high-temperature area near the reactor core to mitigate the release of radioactive iodine species into the environment after reactor core melting in a severe nuclear accident.

2.4 Saturated adsorption and roast treatment of Ag3PO4/Al2O3adsorbent

An ampoule with 1.0 g of 10 wt%Ag3PO4/Al2O3 adsorbent was placed in a glass jar containing 2.0 g of solid iodine.This system was sealed and heated at 75°C for 4 h[18].The iodine concentration in this system was calculated by the saturated vapor pressure of iodine(about 1625 Pa at 348.15 K[6])and was found to be about 0.56 mmol l-1.The saturated adsorption sample was denoted as 10 wt%Ag3PO4/Al2O3-SA.

The 10 wt%Ag3PO4/Al2O3adsorbent was roasted at 450,550,650,and 750°C in air for 2 h to simulate the real process that the adsorbent would go through during the adsorption measurements.The sample was named 10 wt%Ag3PO4/Al2O3-(T/t),where T indicates the calcination temperature(°C),and t is the calcination time(h).

The 10 wt%Ag3PO4/Al2O3adsorbent was also roasted at 750°C for 2,4,6,and 8 h to optimize the longest time that it could remain stable at 750°C.The samples were also named following the same method mentioned above.

2.5 Synthesis of AgI/Ag and AgI/Ag3PO4sample for thermogravimetric analysis

AgI and Ag2CO3at a given mass ratio of 1:3.83 were mixed,ground,and roasted at 450°C[26]for 2 h.The mixture was ground to obtain a homogeneous sample for TGA.The 25 wt%AgI/Ag3PO4sample was synthesized by a similar method.

2.6 Iodine adsorption and desorption

Adsorption of iodine on 10 wt%Ag3PO4/Al2O3,10 wt%Ag/Al2O3,and Al2O3adsorbents was tested at 450,550,650,and 750°C under different carrier gases(N2and air)and gas flow rates(30 and 60 ml min-1)using an apparatus designed by Cheng et al.[14,24].The apparatus consisted of three main parts,including a constant speed sampling system,a fixed bed adsorption system that could be used at high temperatures,and an ef fluent gas adsorption system,and 13X zeolite was employed to adsorb iodine at room temperature.

After the iodine adsorption test at 200°C under N2at a gas flow rate of 30 ml min-1,the used adsorbent was tested at 750°C under N2at a gas flow rate of 120 ml min-1to estimate the adsorption stability,by following the same method and using the same apparatus mentioned above.The amount of iodine desorbed from the adsorbent at 0.5,1,2,4,6,or 8 h was measured.Removal efficiency indicates the relative amount of radioiodine blown off the adsorbent,and it can be used to estimate the stability of iodine.

Figure 2 demonstrates that when the carrier gas is changed from N2to air or when the gas flow rate is increased from 30 to 60 ml min-1,the DF value of the 10 wt%Ag3PO4/Al2O3adsorbent hardly declines at 450,550,and 650°C.Although the DF value declines slightly with an increase in the gas flow rate from 30 to 60 ml min-1at 750°C,the material can still adsorb more than 99%of the radioactive iodine at 750°C.

where Cais the radioactive count rate of adsorbent and Ce is the radioactive count rate of 13X zeolite.Every test was repeated three times,and the result was represented as average value±standard deviation.The performance of the adsorbents can also be evaluated by Eq.(2)as follows:

按照H公司规定，销售人员要与客户签订销售合同。但公司并没有制定完善的合同制度，也没有严格按照合同执行，销售人员常会降低标准，甚至不签订销售合同，公司合法权益就失去法律保障。一旦顾客未及时还款，或没有偿还能力时，公司只能将这笔账作为坏账来处理，由于合同制度不完善带来的风险全部转嫁给H公司承担。例如，H公司规定对于先发货后收款业务，增值税专用发票开票前提是必须拿到汇款单。但日常工作中很多客户要求先开具增值税发票，销售人员为维持与客户的关系常提前开票。而由于公司合同制度不完善，并未增设相关保护条款，客户一看无束缚条件就会延长还款时间，造成该笔账款长期留在账面上成为呆账坏账。

部分学生对于古诗词的背诵显得比较困难，主要因为对诗词理解不透，在脑里没有形成诗的画面。如果我们借助生动形象的课文插图来指导背诵，相信就能做到事半功倍了。例如指导学生背诵《舟过安仁》，教师指着插图上的一条渔船，暗示学生说出“一叶渔舟”；接着圈着两个小孩，暗示学生说出“两小童”；然后指着相应的物品，提示三个动词“收、停、坐”，学生就能念出“收篙停棹坐船中”；用同样的方法指导背诵后两句。通过对照着插图回顾诗句，学生基本上能背诵了。以后只要画面在学生脑里呈现，诗句也随之脱口而出。相信这样的方法，学生更愿意接受，效果也明显。

3 Results and discussion

3.1 Adsorption of radioactive iodine at high temperatures

八五○农场的实测平均水田灌水量405 mm，降水补给外的补给为水田灌溉补给，补给量为107.4 mm,补给系数为26.5%。

Fig.1(Color online)Adsorption of131I2by Al2O3,10 wt%Ag/Al2O3,and 10 wt%Ag3PO4/Al2O3adsorbents at different temperatures under carrier gas of N2(30 ml min-1),with the y-axis showing log DF(a)and%removal efficiency(b)

Compared to the 10 wt%Ag3PO4/Al2O3adsorbent,the 10 wt%Ag/Al2O3adsorbent shows higher DF values at 450 and 550°C.Although the log DF values of the 10 wt%Ag3PO4/Al2O3adsorbent are more than 4 at 450 and 550°C,the 10 wt%Ag/Al2O3adsorbent can adsorb nearly 99.999%of the iodine input into the fixed bed adsorption system.The Ag content in the 10 wt%Ag/Al2O3adsorbent is higher than that in the 10 wt%Ag3PO4/Al2O3adsorbent,where Ag exists in the form of silver ions at about 7.7 wt%concentration.It is noteworthy that the Ag content can affect the DF value;therefore,the results are reasonable.However,when the temperature increases from 550 to 650°C,the DF value of the 10 wt%Ag/Al2O3adsorbent decreases sharply by three orders of magnitude.When the temperature increases further to 750°C,the 10 wt%Ag/Al2O3adsorbent suffers substantial loss in adsorption ability.In contrast,the DF value of the 10 wt%Ag3PO4/Al2O3adsorbent is very stable at different temperatures.The 10 wt%Ag3PO4/Al2O3adsorbent shows the same performance at 450,550,and 650°C,and the DF value only decreases slightly;this material can still adsorb more than 99%of the radioactive iodine at 750°C.Adsorbents that can be used at such a high temperature have never been reported in the literature.However,undeniably,much more systematic exploration and evidence are demanded to identify the reasons for the higher removal efficiency of the 10 wt%Ag3PO4/Al2O3adsorbent compared to that of the 10 wt%Ag/Al2O3adsorbent.

The results of the adsorption tests on 10 wt%Ag3PO4/Al2O3under different conditions are shown in Fig.2.

In Cheng’s experiment[14,24],radioactive iodine was injected into the system with petroleum ether.However,petroleum ether decomposes at high temperature,leading to the possible formation of organic iodine.In order to remove this interfering factor,an aqueous solution of iodine was selected as the iodine source.As the maximum solubility of iodine in water is about 30 mg/100 g,the aqueous solution of radioactive iodine contained radioiodine(～1 MBq)and stable iodine at a concentration of 180 μg ml-1,it was prepared using aqueous solutions of NaI131and NaI127.First,radioactive iodine solution(2 ml)was added into the sampling system.When injected into the fixed bed adsorption system with 1.20 g of the adsorbent loaded and then heated,the radioactive iodine solution turned into steam and passed through the adsorbent with N2.Unadsorbed iodine was further adsorbed by 13X zeolite at room temperature in the ef fluent gas adsorption system.After the completion of the test,the radioactive count rates of the adsorbent in the fixed bed adsorption system and 13X zeolite in the ef fluent gas adsorption system were measured by a well counter to determine the decontamination factor(DF)and thus evaluate the performance of the adsorbents.The DF value is calculated by using Eq.(1)as follows:

3.2 Characterization of the adsorbents

Figure 3 shows the nitrogen adsorption–desorption isotherms of the adsorbents,indicating that 10 wt%Ag3PO4/Al2O3is a mesoporous material,with a pore diameter distribution similar to that calculated by using the Barrett–Joyner–Halenda method.The speci fic surface area of the untreated adsorbent was 135 m2g-1,and it was largely stable through high-temperature calcination.Calcination at high temperatures did not affect the structure of the adsorbent.However,it resulted in a slight decrease in the Brunauer–Emmett–Teller(BET)speci fic surface area and a minor increase in the average pore diameter,as indicated by the values listed in Table 1.

Fig.2(Color online)Adsorption of131I2by 10 wt%Ag3PO4/Al2O3 adsorbent under different carrier gases and gas flow rates

Fig.3(Color online)Nitrogen adsorption–desorption isotherms(a)and pore size distribution(b)of 10 wt%Ag3PO4/Al2O3adsorbents roasted at different temperatures

With an increase in the calcination temperature,the BET speci fic surface area decreases and the adsorption average pore diameter increases.After roasting at 750°C for 2 h,the speci fic surface area of the adsorbent exhibits about 30%decrease.The falloff in the BET speci fic surface area can partly result in a decline of the DF value,since a lower speci fic surface area generally indicates a lower reaction area.On the contrary,the average pore diameter of the adsorbent increases from 6.34 to 8.42 nm after roasting at 750°C for 2 h.The larger the pore diameter,the greater is the pore diffusivity.Hence,the structural change has two con flicting effects on the DF value.Considering that when the temperature increases from 650 to 750°C,the speci fic surface area decreases by 10%and the DF value decreases far more than 10%,the structural change is unlikely to be the main reason for the decrease in the DF value.

After the saturated adsorption experiment,the color of the adsorbent changed from pale yellow to brown.Figure 4a shows that the diffraction peaks of Ag3PO4(JCPDS No.06-0505)in the adsorbent are replaced by those of AgI(JCPDS no.09-0374).This confirms that iodine can react with Ag3PO4to form AgI.Thus,the adsorption mechanism for the 10 wt%Ag3PO4/Al2O3adsorbent is chemical adsorption instead of physical adsorption,since iodine can react with silver phosphate.Although the speci fic reaction mechanism is unconfirmed,the possible reactions are shown in Eqs.(3)and(4)[22]as follows:

Table 1 BET speci fic surface areas of the adsorbents roasted at different temperatures

Sample BET speci fic surface area(m2g-1)Average pore diameter(nm)10 wt%Ag3PO4/Al2O3-sample 135 6.34 10 wt%Ag3PO4/Al2O3-(450/2) 125 6.39 10 wt%Ag3PO4/Al2O3-(550/2) 120 6.44 10 wt%Ag3PO4/Al2O3-(650/2) 108 7.41 10 wt%Ag3PO4/Al2O3-(750/2) 93 8.42

Figure 4b shows the XRD patterns with sharp diffraction peaks corresponding to Ag3PO4and minor diffraction peaks corresponding to Al2O3(JCPDS no.04-0880).With an increase in temperature,the intensity of the diffraction peaks of Ag3PO4reduces gradually.This can be attributed to the decrease in the Ag3PO4content or the better dispersion of Ag3PO4.As the width of the diffraction peaks of Ag3PO4does not increase after calcination,the possibility of its decomposition increases.Furthermore,the diffraction peaks of Ag,the decomposition product of Ag3PO4,cannot be observed in the XRD patterns due to its small amount.The increase in the sharpness of the diffraction peaks of Al2O3at higher calcination temperatures indicates an improvement in crystallinity.The increase in crystallinity leads to a decrease in the speci fic surface area of Al2O3(Table 1).This result can thus verify that the decrease in both the speci fic surface area and the Ag3PO4content may lead to a decrease in the DF value of the adsorbent.However,when the temperature increases from 450 to 650°C,the diffraction peak intensity of Ag3PO4changes drastically,and the DF value of the adsorbent decreases slightly.The decomposition of Ag3PO4has little effect on the DF value under the test conditions.Moreover,the decline of the diffraction peak intensity of Ag3PO4is less than the decrease in the DF value when the temperature increases from 650 to 750°C.Therefore,some other reasons might be mainly responsible for the decrease in the DF value at 750°C.

Figure 4c shows XRD patterns indicating that with an increase in the calcination time,the diffraction peaks of Ag3PO4become weaker.When calcination at 750°C was prolonged for 8 h,the diffraction peaks of Ag3PO4became very weak,indicating a low Ag3PO4content.

XPS measurements were carried out to confirm the composition of the adsorbent after calcination.Figure 5a shows obvious O 1s,Ag 3d,P 2p,and Al 2p peaks,thus confirming the existence of Ag,O,P,and Al elements.The Ag 3d peaks shown in Fig.5b were analyzed to confirm the chemical status of Ag.The peaks of Ag 3d5/2and Ag 3d3/2 were located at～368 and～374 eV.After calcination at high temperature,in particular,at 650 and 750°C,the peaks corresponding to Ag 3d5/2and Ag 3d3/2slightly shifted to higher binding energy,indicating the possible change in the chemical status of Ag.Figure 5c demonstrates that the Ag 3d5/2and Ag 3d3/2peaks of the 10 wt%Ag3PO4/Al2O3-(750/2)adsorbent can be divided into two different peaks at 368.0,368.5 eV;and 374.0,374.5 eV.According to related reports[27],the peaks at 368.5 and 374.5 eV should be attributed to Ag0,and the peaks at 368.0 and 374.0 eV should be attributed to Ag+in Ag3-PO4.This can prove the existence of metallic Ag and the decomposition of Ag3PO4caused by high-temperature calcination.Moreover,this result is in agreement with the inference from Fig.4.

3.3 Stability

Both the reaction dynamics factor[28]and the thermodynamic stability of AgI can affect the DF value.TGA–DSC analysis of 25 wt%AgI/Ag and 25 wt%AgI/Ag3PO4 was carried out to evaluate the thermal stability of AgI in different adsorbents.The melting point of the solid solution with a large amount of AgI is low.Moreover,if the mass fraction of AgI is too small,TGA–DSC analysis may not be sensitive enough to detect the small amount of AgI desorbed from the sample.Therefore,the mass fraction of AgI was selected to be 25 wt%.The results of the TGA–DSC analysis are shown in Fig.6.

The adsorbent exhibits continuous loss in weight during the measurements.In the temperature range 25–400 °C,the mass loss was about 4.6%,and this should be attributed to the desorption of water[29].Furthermore,the DSC curve shows the existence of a small endothermic peak at around 521°C,which may be caused by the decomposition of Ag3PO4.Therefore,the mass loss in the range 521–750 °C,which is about 0.6%,should be attributed to the decomposition of Ag3PO4.In the same temperature range,the mass loss of the adsorbent under air atmosphere is about 0.7%,as shown in Fig.6b.Oxidation or greater degradation of Ag3PO4in the presence of air can explain the difference.This might be why the adsorbent shows better performance under N2atmosphere in the iodine adsorption test.Integration of the results shown in Figs.4 and 5 indicates that longtime calcination at a high temperature can lead to deactivation of the 10 wt%Ag3PO4/Al2O3 adsorbent.

Fig.4(Color online)Powder XRD patterns of a 10 wt%Ag3PO4/Al2O3and 10 wt%Ag3PO4/Al2O3-SA;b 10 wt%Ag3PO4/Al2O3 adsorbents roasted at different temperatures;and c 10 wt%Ag3PO4/Al2O3adsorbents roasted at 750°C for different lengths of time

Fig.5(Color online)X-ray photoelectron spectra of a 10 wt%Ag3PO4/Al2O3adsorbent;b 10 wt%Ag3PO4/Al2O3adsorbents roasted at different temperatures;and c 10 wt%Ag3PO4/Al2O3-(750/2)

◀Fig.6(Color online)TGA–DSC curves of 10 wt%Ag3PO4/Al2O3 adsorbent(a under N2;b under air),25 wt%AgI/Ag(c under N2),and 25 wt%AgI/Ag3PO4(d under N2)

25 wt%AgI/Ag is stable,with nearly no mass loss,after heating to 600°C.With the continuous increase in temperature,mass loss occurs at an accelerating rate,which corresponds to the desorption and decomposition of AgI[30,31].The total mass loss in the temperature range 25–750 °C is 1.5%.This explains the main reason for the low DF values of 10 wt%Ag/Al2O3at 650°C,and in particular,at 750°C.The DSC curve shows an endothermic peak at 554°C due to the melting of AgI.

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25 wt%AgI/Ag3PO4is extremely stable in the temperature range 25–750 °C,with only a small mass increase which may be attributed to some side reactions[32].AgI is stable in AgI/Ag3PO4because it can form a solid solution with Ag3PO4[25].The formation of a solid solution can increase the melting point of AgI depending on the mass ratio of AgI and Ag3PO4.The DSC curve shows two endothermic peaks at 325 and 491°C,which are due to the phase transformations of the AgI/Ag3PO4solid solution.

To confirm the formation of a solid solution between AgI and Ag3PO4,XRD data of 25 wt%AgI/Ag3PO4-unroasted and 25 wt%AgI/Ag3PO4-roasted were collected.

After calcination at 200°C for 2 h,the diffraction peaks of AgI disappeared,and several new diffraction peaks[33,34],which are attributed to(AgI)x(Ag3PO4)y(shown in Fig.7a),could be observed.In addition,the diffraction peaks of Ag3PO4shifted to the high-angle region.During the forming of the solid solution between AgI and Ag3PO4,diffusion of the smaller I-ion into the Ag3PO4lattice led to a smaller crystal space,causing the diffraction peaks to shift to the high-angle region[35],as shown in Fig.7b.

When the temperature exceeds the melting point of AgI,AgI becomes unstable.AgI could decompose and be blown off the absorbent[30,31].Therefore,a traditional Ag adsorbent cannot be used above the melting point of AgI.Thus,the 10 wt%Ag/Al2O3adsorbent synthesized in this study showed a performance similar to that of Cheng’s Ag impregnated Al2O3adsorbent[14].When the test temperature was increased to 750°C,both these Ag-loaded adsorbents lost their adsorption ability.In contrast,the stability of AgI on the 10 wt%Ag3PO4/Al2O3adsorbent was promoted because of the formation of a solid solution between AgI and Ag3PO4,and the Ag3PO4/Al2O3adsorbent exhibited signi ficantly higher DF values than did Ag/Al2O3at 650 and 750 °C.However,750 °C is close to the limiting temperature at which that the solid solution can exist[25].AgI becomes unstable at this temperature.As a result,the DF value of the 10 wt%Ag3PO4/Al2O3adsorbent at 750°C would not be as large as that at temperatures below 650°C.Nonetheless,it satis fies the Environmental Radiation Protection Standards for Nuclear Power Operations[36].

Fig.7(Color online)Powder XRD patterns of 25 wt%AgI/Ag3PO4-unroasted and 25 wt%AgI/Ag3PO4-roasted

In order to examine the thermal stability of the adsorbent under the test conditions,an iodine desorption test was conducted on the 10 wt%Ag3PO4/Al2O3adsorbent;the corresponding results are shown in Fig.8.From this figure,iodine reserved was calculated using Eq.(2)mentioned above.With an increase in desorption time from 0.5 to 4 h,theamountofiodine desorbed from theadsorbent increased slightly.However,it was still less than 0.5%of the total iodine adsorbed by the adsorbent.When the desorption time increased to 6 h,the amount of iodine desorbed was nearly 1%.More than 6%of the total iodine desorbed from the adsorbent after it was roasted at 750°C for 8 h.Figure 4c shows the transformation of most of the Ag3PO4into Ag after calcination for 8 h at 750°C.Thus,when Ag3PO4was totally transformed into Ag,the adsorbent would deliver similar performance as the 10 wt%Ag/Al2O3adsorbent.

Fig.8 Thermal stability of131I2in 10 wt%Ag3PO4/Al2O3adsorbent at 750°C

After the iodine desorption test on 10 wt%Ag/Al2O3-SA,a yellow powder was observed in the ef fluent gas adsorption system.This powder was dissolved in KI solution[37]and measured by ICP-AES.The results indicated that it contained Ag;thus,the yellow powder could be AgI.Moreover,at the same time,greater desorption of molecular iodine was observed.The results clearly indicated blowing off of AgI from the adsorbent,followed by decomposition under the test conditions.This result is in agreement with the conjecture obtained from Fig.6.Notably,the DF value of the adsorbent at 750°C mainly depended on the stability of AgI.

4 Conclusion

In this study,a 10 wt%Ag3PO4/Al2O3adsorbent was fabricated,and its performance for the adsorption of radioiodine at high temperatures was tested.The new 10 wt% Ag3PO4/Al2O3 adsorbentexhibited high and stable adsorption efficiency at high temperatures.With increasing temperature,decomposition of Ag3PO4and a decrease in the speci fic surface area of the adsorbent partly contributed to the decrease in the decontamination factor,especially at 750°C.However,compared to the traditional silver adsorbent,the formation of the solid solution between AgI and Ag3PO4improved the stability of AgI,contributing to better performance at 750°C.Although not perfect,the new adsorbent synthesized in this study can be effectively used at high temperatures up to 750°C for radioiodine adsorption.

CT表现：术前CT检查15例，均表现为踝关节肿胀，出现结节状硬化软组织且均无钙化形成。面骨质破坏：胫骨远端关节与距骨穹隆关节面和比例为2：3，周围骨质伴随硬化缘现象。下胫腓关节内软组织肿块形成共9例，均无明显下关节骨破坏现象。

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