Vaporization Mechanisms of Water-Insoluble Cs in Ash During

Institute of Technology, Shimizu Corporation, 3-4-17 Etchujima Koto-ku, Tokyo 135-8530, Japan. § Graduate School of Pure and Applied Sciences, Univer...
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Vaporization Mechanisms of Water-Insoluble Cs in Ash During Thermal Treatment with Calcium Chloride Addition Facun Jiao,† Norie Iwata,† Norikazu Kinoshita,‡ Masato Kawaguchi,‡ Motoyuki Asada,‡ Maki Honda,§ Keisuke Sueki,§ and Yoshihiko Ninomiya*,† †

Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan Institute of Technology, Shimizu Corporation, 3-4-17 Etchujima Koto-ku, Tokyo 135-8530, Japan § Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan ‡

S Supporting Information *

ABSTRACT: The vaporization mechanisms of water-insoluble Cs in raw ash and Cs-doped ash during thermal treatment with CaCl2 addition was systematically examined in a lab-scale electrical heating furnace over a temperature range of 500− 1500 °C. The results indicate that the water-insoluble Cs in the ash was associated with aluminosilicate as pollucite. Addition of 10% CaCl2 caused the maximum vaporization ratio of Cs in the raw ash to reach approximately 80% at temperatures higher than 1200 °C, whereas approximately 95% of Cs was vaporized at temperatures higher than 1300 °C when 30% CaCl2 was added. The formation of an intermediate compound, CsCaCl3, through the chemical reaction of Cs with CaCl2 was responsible for Cs vaporization by means of the subsequent decomposition of this intermediate upon the increase in temperature. The indirect chlorination of Cs by the gaseous chlorine released from the decomposition of CaCl2 was insignificant. A high CaCl2 content in the resulting annealed products with 30% CaCl2 addition delayed the decomposition of CsCaCl3 and thus lowered the Cs vaporization ratio compared to that with 10% CaCl2 addition at 900−1250 °C. Thermal treatment with CaCl2 addition is a proposed method to remove Cs from Cscontaminated incineration ash.

1. INTRODUCTION The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident triggered by the massive earthquake and tsunami on March 11, 2011, resulted in a substantial release of radioactive nuclides, contaminating the disaster waste, soil, crops and trees around the area of the FDNPP.1−3 Among these radionuclides, the radioactive cesium (Cs) isotopes 134Cs (T1/2 = 2.065 y) and 137 Cs (T1/2 = 30.08 y) are of primary concern because of their long half-lives.4,5 To reduce the radiation level in the surrounding area, decontamination operations, in which the contaminated solid wastes are collected and stored in a designated storage, have been occurring for more than four years until present day. Following these decontamination operations, incineration is one of the recommended approaches for the safe handling of such large amounts of Cs-contaminated solid wastes. However, the Cs in solid wastes is ultimately concentrated and distributed in bottom ash and fly ash after undergoing incineration, leading to a Cs concentration in the incineration ashes that is several dozen times higher than that in the raw waste.6 Based on the guidelines of the Japanese government for the treatment of radionuclide-containing solid wastes, incineration ashes with radioactivity below 8000 Bq kg−1 can be disposed as normal waste in an existing landfill. © XXXX American Chemical Society

Otherwise, the ashes are specified as radioactive waste and stored in temporary sites; these ashes will eventually undergo treatment in designated disposal facilities coupled with special processes to monitor and prevent the release of radionuclides to the environment.6 One method that can be used to mitigate the pressure related to the storage of contaminated incineration ash is to reduce the radioactivity of the ash to less than 8000 Bq kg−1. Although the use of water extraction can possibly reduce the Cs concentration via the water-soluble CsCl present in the fly ash from municipal solid waste incineration (MSWI),1,7−10 another Cs-bearing compound, Cs-aluminosilicate (pollucite), is also detectable in incineration ashes, particularly in the bottom ash from MSWI7,11,12 and the dust from sewage sludge incineration,6,7 which, however, is immobile in water. Thermal treatment could be employed to extract such water-insoluble Cs from incineration ashes because Cs is an alkaline metal and thus presumed to possess a similar vaporization propensity as Received: Revised: Accepted: Published: A

August 6, 2016 November 21, 2016 November 21, 2016 November 21, 2016 DOI: 10.1021/acs.est.6b03635 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

The reactor consisted of a heater and a reaction tube with a length of 1 m and inner diameter of 42 mm. A thermocouple was installed in the middle of the furnace, the tip of which intimately touched the outer surface of the reaction tube. The ash sample (∼5 g) was mixed with different contents of CaCl2· 2H2O (chemical reagent, purity: > 99%, Kanto Chemical Co., Inc., Kyoto, Japan) and then transferred into an alumina crucible (99.6% Al2O3). The crucible was subsequently inserted into the reactor at room temperature from the bottom of the reactor using an alundum tube. The sample in the reactor was placed at the same height as the thermocouple. Air was continuously introduced to the reactor at a flow rate of 700 mL min−1. In order to keep a constant atmosphere on surface of the ash sample during thermal treatment under different conditions, the air flowed from the top to bottom of the reactor to remove the vaporized Cs. Two impingers containing 0.1 mol L−1 HNO3 were connected to outlet of the reactor using a gas line to capture the gaseous Cs in flue gas before it was discharged into ambient. Total capture efficiency, defined as the percentage of total amount of Cs captured by the two impingers and deposits on the gas line to its vaporized amount from ash sample, was up to 90%. The reaction temperatures ranged from 500 to 1500 °C with an interval of 100 °C. The temperature profile is also shown in the SI, Figure S1. The temperature was increased from room temperature at a heating rate of 200 °C h−1 and then maintained at the target temperature for a period of 30 min. After that, the sample was allowed to cool down naturally. In order to distinguish the ash sample before and after processing, the sample processed by the heat treatment is termed as slag hereafter. The resulting slag samples were pulverized to less than 105 μm and then subjected to a series of analyses to understand their characteristics. The content of CaCl2 (based on CaO) in the ash was determined according to eq 1, in which 56 and 147 refer to the molecular weight of CaO and CaCl2·2H2O, respectively.

sodium and potassium at high temperatures. Calcium chloride (CaCl2) is an adoptable additive used to vaporize trace elements through chlorination because of the lower boiling point of metallic chlorides compared to that of other species, for example, oxides and sulfates. The removal of trace elements such as Pb, Cd, and Cu from incineration ashes using thermal treatment with the addition of CaCl2 has been broadly studied.13−19 In general, the vaporization of trace elements results from indirect chlorination, in which CaCl2 decomposes and reacts with H 2 O/O 2 to release HCl/Cl 2 , which subsequently reacts with trace elements in the ash to form gaseous metallic chlorides.13,14,19 Nevertheless, the removal of Cs from Cs-contaminated incineration ash by thermal treatment with the addition of CaCl2 has been rarely reported. It is not clear whether the indirect chlorination mentioned above is responsible for the vaporization of Cs from the incineration ash or not. Clarifying the role of the thermal treatment with CaCl2 addition in the vaporization of Cs, especially water-insoluble Cs, in incineration ashes and the associated reaction mechanisms is urgently needed in order to develop a process that reduces Cs in Cs-contaminated incineration ash and to accelerate the reconstruction and revitalization of Fukushima. In the present study, the effects of CaCl2 addition on the vaporization of Cs and the dependence of this vaporization on the reaction temperatures were systematically examined using an ash sample that only contained stable Cs (133Cs). Additionally, a Cs-doped ash sample (a mixture of ash and Cs2CO3) was also tested to reveal the possible reaction mechanisms of Cs removal. X-ray absorption near edge structure (XANES) spectroscopy was used to specify the possible Cs-bearing compounds in the Cs-doped ash and its slag. Leaching experiments were carried out to elucidate the formation and transformation of possible water-soluble Cs species in the resulting slag upon addition of CaCl2 and under different reaction temperatures. The aims of this paper are to develop a practical technology to reduce Cs concentration in the incineration ash collected from the area affected by the FDNPP accident, elucidate fundamental understanding of the vaporization mechanisms of Cs as well as optimize operational parameters (reaction temperature and content of CaCl2 addition) during thermal treatment.

CaCl 2, % =

amount of CaCl 2·2H 2O × 56/147 × 100 amount of ash (1)

2.4. Leaching Test Procedure. First, 1 g of the ash/slag sample was mixed with 50 mL of deionized water and then shaken for 4 h at room temperature in a shaker (TA-12R, Takasaki Scientific Instruments Corp.) at a shaking frequency of 160 ± 3. The resulting slurry was subjected to vacuum filtration through a membrane filter (0.45 μm, Millipore). 2.5. Characterizations of the Ash/Slag Sample. The concentration of Cs in the solution prepared from the microwave digestion and leaching test of the ash/slag was measured on an ICP-MS (Agilent 8800). The procedure for the microwave digestion of the ash and slag sample is provided in the SI. The elemental composition of the ash samples was determined by XRF. The crystalline minerals in the sample were identified on an X-ray diffraction (XRD, Rigaku RINT) spectrometer with Cu Kα radiation. Sodium (Na) and potassium (K) in the leachate derived from the leaching tests of the ash and slag were analyzed by atomic absorption spectroscopy (AAS, Shimadzu AA-6200), and chlorine in the leachate was determined by ion chromatography (IC, Shimadzu) using a CDD-6A conductivity detector. The decomposition of CaCl2·2H2O was analyzed by thermogravimetric analysis (TGA, Rigaku TAS200) in an air atmosphere with a heating rate of 3.5 °C min−1. In addition, the chemical

2. EXPERIMENTAL SECTION 2.1. Raw Ash Sample. An ash sample that only contained nonradioactive Cs was tested in this study. The chemical composition of the ash is shown in Table S1 in the Supporting Information (SI). The combined content of Si and Al, the dominant elements in the ash, accounted for 65% of the total, whereas the chlorine present in the ash was below the X-ray fluorescence (XRF, Rigaku 2100) detection limit. The Cs concentration in the ash was determined to be 7.6 mg/kg. 2.2. Preparation of the Cs-Doped Ash Sample. The ash sample was first thoroughly mixed with Cs2CO3 (chemical reagent, purity: > 95%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) and then heated in a muffle furnace at typical incineration temperature of 900 °C for 2 h. This procedure was repeated twice to obtain an ash that was uniformly doped with Cs. The chemical composition of the Cs-doped ash is also listed in the SI, Table S1. The Cs-doped ash sample contained 4160 mg kg−1 Cs. 2.3. Experimental Setup and Method. the vaporization experiments were conducted with a lab-scale electrical heating furnace, the schematic of which is given in the SI, Figure S1. B

DOI: 10.1021/acs.est.6b03635 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology composition of particles was examined by scanning electron microscopy (SEM, JEOL JSM6500) coupled with energy dispersive X-ray spectroscopy (EDX) in the backscattered electron composition (BEC) mode. A detailed description of the sample preparation for SEM-EDX analysis is given in the SI. To analyze the Cs species in the Cs-doped ash and its slag, XANES analysis was carried out at the BL5S1 beamline of the Aichi Synchrotron Radiation Center in Aichi, Japan. The ash and slag powders were compressed into pellets with diameters of approximately 10 mm and thicknesses of approximately 1 mm for the analysis. The X-ray absorption spectra were recorded in fluorescence mode from a silicon drift detector (SDD) at a room temperature, ranging from 200 eV below the L(II) edge of Cs (5359 eV) to 300 eV above the edge. The spectral data were processed using Athena software.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Cesium in the Ash Samples. The Cs-bearing species in the Cs-doped ash and three standard samples of pollucite [(Cs, Na)2Al2Si4O12·nH2O], Cs2CO3, and CsCl were analyzed by XANES. Pollucite is an ore that contained 20.1% Cs based on XRF analysis. The spectra are shown in the SI, Figure S2. The spectrum of the Cs-doped ash appears similar to that of pollucite, particularly in the energy ranges of 5366−5369 eV and 5374−5380 eV, suggesting that the plausible form of Cs was Cs-aluminosilicate in the Csdoped ash. Apart from the XANES analysis, a leaching test was carried out to fractionate the water-soluble Cs from the Csdoped ash. In terms of the results of the leaching test, the leaching ratio of Cs, defined as the percentage of Cs in the leachate relative to that in the ash used for the leaching test, accounted for only 0.27%. This result clearly indicates the association of Cs with aluminosilicate because Cs would dissolve into the leachate if it existed as carbonate or oxide. Direct analysis using XANES to investigate the Cs-bearing species in the raw ash was unavailable in this study owing to the rather low concentration of this species in this matrix. Only a leaching test was conducted to assess the water-soluble Cs in the raw ash. A total of 0.29% of the Cs was leachable from the raw ash, which was comparable to that in the Cs-doped ash. Thereby, it can be speculated that the Cs-bearing species in the raw ash was similar to that in the Cs-doped ash. 3.2. Effect of CaCl2 Addition on the Vaporization of Cs. CaCl2 was mixed with the raw ash and Cs-doped ash at a CaCl2 content of 10% or 30% and then treated in the furnace. The vaporization ratio, defined by eq 2, was used to estimate effect of CaCl2 addition on the vaporization of Cs. m − m2 vaporization ratio, % = 1 × 100 m1 (2)

Figure 1. Vaporization ratio of Cs in raw ash and Cs-doped ash with added 10% CaCl2 or 30% CaCl2 as a function of temperature.

30% CaCl2. These results are in line with the expected results because a higher Cl content facilitates the vaporization of Cs. However, an interesting result was observed at 900−1250 °C, where the vaporization ratio of Cs resulting from treatment with 30% CaCl2 was lower than that with 10% CaCl2. This observation clearly shows that the vaporization of Cs in the ash was remarkably influenced by the CaCl2 content as well as the reaction temperature. The vaporization ratios of Cs from the Cs-doped ash, treated under the same conditions as for the raw ash, are plotted in Figure 1(b). The vaporization of Cs relative to the reaction temperature in the Cs-doped ash showed a similar trend as that in the raw ash, suggesting a similar vaporization mechanism of Cs from both ashes. Nevertheless, the vaporization ratio of Cs in the Cs-doped ash with both 10% and 30% CaCl2 addition was higher than that of the raw ash at 800−1000 °C. This trend could be explained from the viewpoint of the distribution of Cs in the ashes. The distribution of Cs in a particle of the Csdoped ash was quantitatively measured by SEM and EDX line analysis. As shown in the SI, Figures S3 (a) and (b), the surface of a cross section of the ash particle was slightly higher than that of the wax surface based on the sample preparation method for SEM-EDX in the SI, thereby enabling the analysis of the Cs distribution from the inside to the surface of the ash particle by EDX line analysis. Figure S3 (c) in the SI shows an image of the particle determined by SEM. The area surrounded by the dotted circle is the cross section of the particle, and the peripheral gray area is the particle surface. The EDX line analysis was run from point A to B, covering the cross section and particle surface. The result of this analysis is given in the SI, Figure S3 (d). The Cs concentration in the cross section increased smoothly from the inside to the outside and then increased rapidly on the particle surface. The chemical reaction between Cs2CO3 and the ash particles would occur on the surface of the ash particles and produce Cs-aluminosilicate when the Cs-doped ash was prepared. The chlorination of Cs with CaCl2 at the surface of the Cs-doped ash particles is favored due to the high Cs enrichment relative to that of the

where the symbols m1 and m2 refer to amount of the Cs in the ash and slag, respectively. As shown in Figure 1(a), when CaCl2 was not added in the ash, the effect of temperature on the vaporization of Cs was insignificant, as evidenced by the experimental observation that the vaporization ratio of Cs was almost constant at temperatures of 500−1500 °C. When CaCl2 was added, the vaporization ratio of Cs exhibited a rapid increase and leveled off with the increase in reaction temperature, regardless of the content of CaCl2 in the ash. Treatment with 10% CaCl2 led to the maximum vaporization ratio of ∼80% at temperatures higher than 1200 °C, and a vaporization ratio higher than 95% was achieved at temperatures above 1300 °C by treating with C

DOI: 10.1021/acs.est.6b03635 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology raw ash, in which Cs would be dispersed randomly in the ash matrix.7 Hence, the vaporization ratio of Cs for the Cs-doped ash was higher than that for the raw ash at 800−1000 °C. 3.3. Vaporization Mechanisms of Cs. In order to clarify the detailed reactions and processes of Cs vaporization resulting from the addition of CaCl2, a leaching test was carried out to investigate the distribution of water-soluble Cs in the slag. The results of the ratios are shown in Figure 2.

Figure 3. Cs L(II)-edge spectra of a mixture of Cs-doped ash and 10% CaCl2 at 800 and 1000 °C.

Cs in slag) was insoluble in water, as shown in Figure 2(b). With respect to the Cs species in the slag at 800 °C, the peak at 5374−5380 eV became smaller because approximately 60% of the water-insoluble fraction of Cs (relative to total Cs in slag) was contained. On the other hand, it is difficult to conclude that CsCl exists in the slag because the typical peak of CsCl at 5369−5370 eV (marked as an arrow in Figure 3) was not clearly observed in the slag samples. Although direct analysis of the water-soluble Cs species in the slag derived from both raw ash and Cs-doped ash was difficult in this work, the SEM and EDX mapping analysis of particles could provide some useful clues related to the distribution of Cs in slag. Figure 4 shows an SEM image and element distributions from EDX mapping analysis of the slag derived from the Cs-doped ash treated with 30% CaCl2 at 800 °C. The signal intensity of Cs was stronger in the particles, which were rich in Ca and Cl, compared to the particles with abundant Si and Al, reflecting a preferential combination of Cs with Ca and/or Cl under these conditions. To further reveal possible water-soluble Cs-bearing species, pollucite was mixed with 20% CaCl2 and then treated in the furnace under the same conditions as those used for the ash sample described in the Experimental setup and method section. The resulting slag samples were examined by XRD to characterize the Cs-bearing species. Figure 5 shows the XRD patterns of the slag at 500 °C, 600 °C, 700 °C, 900 °C, and 1100 °C. The spectra of pollucite and CsCl are also given for comparison. A new Cs compound, CsCaCl3, was clearly formed at temperatures between 600 and 1100 °C. The CsCaCl3 produced by the chemical reaction between pollucite and CaCl2 can be descried by eq 3. The intensity of CsCaCl3 at 1100 °C was much weaker compared to that at 900 °C. The variation with temperature can be explained by the decomposition reaction of CsCaCl3, expressed as eq 4. This decomposition reaction can be confirmed by the vaporization ratios of Cs, which increased from 15% to 70% with an increase in temperature between 900 and 1100 °C (SI, Figure S5). In addition, CsCl was not detected in the slag at temperatures between 500 and 1100 °C based on the XRD analysis shown in Figure 5. Regarding the two products of gaseous CsCl and CaCl2 from the reaction 4, although direct determination of the

Figure 2. Distribution of Cs in the gas phase, water-soluble fraction and water-insoluble fraction at 600 °C, 800 °C, 1000 °C, and 1200 °C. Panel (a) denotes raw ash; panel (b) denotes Cs-doped ash.

Obviously, the water-soluble Cs in the slag derived from both raw and Cs-doped ashes with added CaCl2 was more or less detectable at temperatures from 600 to 1200 °C. The ratio of the water-soluble Cs in the slag from both raw and Cs-doped ash relative to the reaction temperature showed a similar trend: the fraction of the water-soluble Cs first increases and then decreases with the increase in reaction temperature. The formation of water-soluble Cs in the slag was definitely attributed to the chemical reaction of Cs with CaCl2. This reaction was influenced by the concentration and distribution of Cs in ashes, which thereby generated different fraction of the water-soluble Cs in the slag between Cs-doped ash and raw ash. In terms of the vaporization route of other trace elements in the ash due to the addition of CaCl2, HCl and/or Cl2 resulting from the decomposition of CaCl2 can cause the trace elements to vaporize as their chlorides via chlorination.13,14 If that is the case, the water-soluble Cs species is probably CsCl. Nevertheless, the vaporization of Cs in this study did not appear to only be ascribed to this mechanism because the decomposition of CaCl2 occurred beginning at approximately 800 °C based on the TGA of CaCl2·2H2O, as shown in the SI, Figure S4, which contradicts the experimental observation that ∼5% of the water-soluble Cs formed in the slag at 600 °C for both raw ash and Cs-doped ash. XANES analysis was performed to determine the Cs-bearing species in the slag. As shown in Figure 3, the shoulder peaks at 5366−5369 eV and 5374−5380 eV still existed in the spectrum of the slag at 1000 °C from the Cs-doped ash treated with 10% CaCl2, implying that the dominant species was Cs-aluminosilicate in this slag sample. This observation is in agreement with the leaching test result that >90% of the Cs (relative to the total D

DOI: 10.1021/acs.est.6b03635 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology pollucite(Cs − aluminosilicate) + CaCl 2 → Anorthite + CsCaCl3

(3)

CsCaCl3 → CsCl, g + CaCl 2

(4)

CaCl 2 + SiO2 + Al 2O3 + O2 → Ca − Al − Si − O + Cl 2

(5)

The leaching test was also conducted to confirm the solubility of CsCaCl3. The XRD spectra of the pollucite treated with 20% CaCl2 at 900 °C and the residue from the leaching test are given in the SI, Figure S6. Apparently, the peaks characterized by CsCaCl3 disappeared in the residue compared to the slag before the leaching test, in which strong peaks were observed. This result implies that CsCaCl3 easily dissolved in the solution. Therefore, we can conclude that water-soluble Cs was present in the slag from raw ash or Cs-doped ash treated with CaCl2 in Figure 2 and that the association of Cs with Ca and Cl is attributed to CsCaCl3, rather than CsCl, based on the EDX-mapping in Figure 4. With respect to the Cs vaporization mechanisms, two reactions, Cs with solid CaCl2 and Cs with gaseous Cl produced by the decomposition of CaCl2, are considered. To confirm the main reaction, air that contained 4000 ppm of HCl gas was introduced into the reactor at a flow rate of 700 mL min−1; 5 g of the Cs-doped ashes were treated in the reactor at 700 °C, 900 and 1100 °C for 2.5 h, respectively. The HCl gas used here mimicked the role of gaseous Cl. It should be noted that the contribution of the reaction of Cs with solid CaCl2 can be excluded under this condition because CaCl2 was not mixed with the ash here. Figure S7 in the SI demonstrates the effect of HCl on the vaporization of Cs in the Cs-doped ash. Only 10% of the Cs was vaporized, even at a relatively high temperature of 1100 °C. On the other hand, the vaporization ratio reached 70% in the Cs-doped ash treated with 10% CaCl2 (Figure 1(b)). The ash mixed with 10% CaCl2 contained a Cl concentration of 127 g-Cl (kg-ash)−1. A total of 0.6844 g of HCl gas (corresponding to 0.6656 g of Cl) was introduced into the reactor; the gaseous Cl that could react with a unit weight of ash was estimated to be 133 g-Cl (kg-ash)−1. The vaporization ratio in the case treated with CaCl2 was higher than that treated with HCl gas, although a similar amount of Cl could interact with a unit weight of ash. Thus, it can be concluded that the solid-to-solid reaction between Cs in the ash and CaCl2 is the major route governing the vaporization of Cs. According to the chemical reactions given by eqs 3 and 4, the CaCl2 content in slag, which is mainly affected by the interaction between CaCl2 and aluminosilicate in ash according to the reaction 5, influences the vaporization of Cs. A high content of CaCl2 promotes the formation of CsCaCl3 via eq 3 but inhibits its decomposition via eq 4 based on the concept of chemical equilibrium. Hence, the content of CaCl2 in a slag gives a basic explanation of the vaporization of Cs. The remaining Cl in a slag depends on the amount of added CaCl2 and the treatment temperature because the original Cl in the raw ash is in low abundance (below the limit of detection of XRF, see the SI, Table S1). The chemical forms of watersoluble chlorine in a slag are assumed to be the chlorides of Ca, Na and K based on the results of thermodynamic prediction, as shown in the SI, Figure S8. The input and output parameters of the thermodynamic calculation are given in the SI. Thus, the

Figure 4. EDX mapping of particles in a mixture of Cs-doped ash and 30% CaCl2 at 800 °C.

Figure 5. XRD spectra of pollucite, CsCl and a mixture of pollucite and 20% CaCl2 treated at different temperatures.

gaseous CsCl was unavailable in this work, in terms of the thermodynamic equilibrium prediction by FactSage 7.0, the gaseous CsCl was the sole gaseous Cs-containing compound in gas phase. The peaks of CaCl2 in the XRD spectra of the slag at 900 and 1100 °C were not noticeable, as shown in Figure 5. Two possible reasons are responsible for this phenomenon. One reason is that the released CaCl2 from the decomposition of CsCaCl3 reacted with aluminosilicate in ash to form Ca−Al− Si-O (e.g., anorthite) through a chemical reaction,20 expressed as eq 5. Another reason is that the produced CaCl2 was probably amorphous at 900 and 1100 °C since its melting point is about 772 °C; it cannot be detected by XRD. E

DOI: 10.1021/acs.est.6b03635 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

position of CsCaCl3, which, however, was compensated by increasing the reaction temperature. The conclusions derived from this work are apparently applicable to remove the waterinsoluble Cs in Cs-contaminated incineration ashes (the bottom ash from MSWI7,11,12 and the dust from sewage sludge incineration6,7) generated from the decontamination of the area affected by the FDNPP accident. Such water-insoluble Cs in the incineration ashes is difficult to be removed using the method of water extraction. Ash treatment facility of rotary kiln is suggested because it can provide a long residence time and an intensive mixing for ash and CaCl2. The vaporized Cs during thermal treatment is subjected to condensation upon flue gas cooling to form solid particulates which can be further captured by an air pollution control device, for example, bag filter. The highly Cs concentrated particulates collected need to be stored in designated disposal facilities. Regarding the CaCl2 content and reaction temperature used for Cs vaporization, although 10% CaCl2 addition caused a higher Cs vaporization ratio from the raw ash than that of 30% CaCl2 addition at relative low reaction temperatures (