Effects of Microwave-Assisted Thermal Treatment on the Fate of Heavy

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Effects of Microwave-Assisted Thermal Treatment on the Fate of Heavy Metals in MSWI Fly Ash Bing Gong, Yi Deng, Yuanyi Yang, Chunyang Wang, Yong He, Xiaolong Sun, Qianni Liu, and Weizhong Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02156 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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Effects of Microwave-Assisted Thermal Treatment on the Fate of Heavy Metals in MSWI Fly Ash Bing Gonga, Yi Denga,*, Yuanyi Yangb, Chunyan Wanga, Yong Hec, Xiaolong Sund, Qianni Liua, Weizong Yanga,*

a

School of Materials Science and Engineering, and School of Chemical Engineering,

Sichuan University, Chengdu 610065, China b

Department of Materials Engineering, Sichuan College of Architectural Technology,

Deyang 618000, China c

CECEP Chengdu Renewable Energy Co. Ltd., Chengdu 610000, China

d

Zerowaste Asia Co., Ltd, Singapore, 999002, Singapore.

1

ACS Paragon Plus Environment

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ABSTRACT: In recent years, many researchers devote themselves to the employment of microwave heating for immobilizing MSWI fly ash; however, they only focus on the leaching properties of heavy metals in treated fly ash. Herein, the aim of this study is to systematically investigate the effects of microwave-assisted thermal treatment on the fate of the selected elements with conventional thermal treatment as control. The results showed that leaching behaviors of Pb, Cu and Zn were successfully suppressed, while leaching concentration of Cr was increased due to the transformation of Cr(Ⅲ) to Cr(Ⅵ) after thermal treatment. Compared with conventional thermal treatment, microwave heating was more efficient for stabilizing MSWI fly ash. Sequential extraction experiment indicated that water-soluble and exchangeable fractions of Pb were significantly declined with enhancement of residual fraction, which manifested that Pb existed in more stable speciation. With regard to Cr, reducible form was the main fraction in both treated and raw fly ashes. After thermal treatment, nevertheless, the reducible fraction of Cr ascended and the residual Cr descended. Under all conditions, Pb could be effectively evaporated, and the vaporization of Pb reached to 90 % at 1100 °C. To the contrary, less than 20 % of Cr was vaporized, while higher temperature and longer retention time suppressed the vaporization of Cr. During thermal treatment, CaO was newly generated according to XRD results and it accelerated the oxidation of Cr(Ⅲ). Particle agglomeration and grain growth were observed via SEM, and these processes 2


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conduced to depressing the leaching of heavy metals. This study provided a deep insight to the effect of microwave heating on the fate of Pb and Cr in MSWI fly ash.

3


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1. INTRODUCTION

Municipal solid waste (MSW) has become a serious environmental concern in many countries because of rapid industrialization and increases in economy and urban population. Thus, highly efficient management of MSW is in urgent demands. Traditional disposal of MSW in landfills induces tough environmental challenges including groundwater contamination, deterioration in soil quality and land consumption. In comparison to disposal in landfills, municipal solid waste incineration (MSWI) can achieve energy recovery and sterilize rubbish accompanied with significant reduction in mass and volume.1-3 However, fly ash is produced during the combustion process, and it contains many toxic species such as heavy metals which are potentially detrimental to environment and human beings. Thus, appropriate treatments for fly ash detoxification are much needed before its landfill disposal or reutilization.4

Thermal treatment is considered as a promising approach for stabilization of MSWI fly ash. During thermal process, mineral phase transformation, particle adhesion and conglomeration as well as grain growth jointly occur.5-8 These physical and chemical alterations facilitate to suppress leachability of most heavy metals in fly ash. It was reported that, after thermal treatment, leaching concentrations of heavy metals like Pb, Cu, Zn and Cd were dramatically decreased to far below the regulatory limits.9-10 In some 4


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cases, however, Cr became more leachable after heating procedure due to the transformation of Cr(Ⅲ) to Cr(Ⅵ).11-12 Thermal technologies are generally divided into three categories including vitrification, melting/fusing, and sintering according to different temperatures in heating process.6

Currently, there is a growing attention in microwave heating for stabilizing wastes such as scrap tyres, plastics, heavy metal sludge, MSWI fly ash, and etc.13-16 Bosio et al. employed a microwave oven to treat MSWI fly ash containing samples, and the leaching results showed that leaching concentrations of Pb, Zn, Br, etc were dramatically decreased.17 The work reported by Chou et al. revealed that the sintered MSWI fly ash by microwave energy possessed long-term stability.18 Besides, microwave technology was also used to prepare ceramics containing MSWI residues.19 Microwave is an electromagnetic radiation with the frequencies in the range of 300 to 300 GHz among which 915 and 2.45 GHz are frequently used for heating procedure.20-21 Charge polarization and ionic conduction are the two most important mechanisms for microwave heating. Particularly, charge polarization can be concretely itemized into electron polarization,

atomic

polarization,

orientation

polarization

and

spatial

charge

polarization.21 Because of the aforementioned mechanisms, microwave thermal treatment has a number of advantages including volumetric heating, celerity, uniformity, high-energy efficiency, and etc22 compared with conventional heating process which 5


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heats materials through conduction, radiation and convection of thermal energy. Many previous studies have concentrated on the potential use of microwave techniques for the immobilization of MSWI fly ash.16-18,

23

However, these experimental results merely

showed the toxicity characteristic leaching procedure (TCLP) leaching concentrations of heavy metals in fly ash treated by microwave heating. However, to our best knowledge, the effects of microwave heating on the fate of heavy metals in fly ash are rarely reported.

Herein, the aims of the work are to systematically investigate the effects of microwave-assisted thermal treatment on the fate of selected heavy metals with conventional thermal treatment conducted for comparison. In the present study, (1) the influences of temperature and retention time are also discussed. (2) To shed light on the impacts of these parameters, leaching concentrations, distribution patterns and vaporization of heavy metals in fly ash are determined before and after thermal process. (3) Furthermore, XRD and SEM are employed to characterize mineralogy and morphology of the thermally treated fly ash.

2. MATERIALS AND METHODS

2.1. Sample collection. The raw fly ash (RFA) was obtained from a MSW combustion plant with an incineration capacity of 1800 tons per day in Chengdu, China. The type of 6


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MSW incineration furnace was stoker furnace. The plant was equipped with an air pollution control (APC) system which was a semi-dry type. In the APC system, lime slurry was injected into the flue gas to control stive and absorb poisonous components like HCl, SOx, and etc. Firstly, the pristine APC residue was collected form a silo of the incinerator. Then, the sampled fly ash suffered from a drying process at 105 ± 0.5 °C for 24 h to exclude free moisture in the ash, and the dried sample was sealed in a polyethylene (PE) drum for succedent tests and analysis. All fly ashes used in this work derived from the same batch.

2.2. Thermal treatment. Table 1 displayed the details of experimental recipe. In this study, the effects of heating method, temperature and retention time were discussed. As shown in Table 1, the selected temperatures were 700, 900 and 1100 °C, and the durations were 10 and 60 min. In the thermal procedure, 20 g fly ash was initially weighed and put in an Al2O3 ceramic crucible. Afterwards, the crucible was put into a sintering oven and was heated to targeted temperature. After held for a preset period, the fly ash was naturally cooled down to ambient temperature prior to subsequent analysis. It was worth of mentioning that microwave-assisted and traditional thermal treatments were conducted in a microwave sintering furnace (2.45 GHz, 600 W) and a muffle furnace, respectively.

2.3. TCLP leaching test. TCLP leaching test, following China EPA method HJ/T 7


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300-2007,24 was implemented to evaluate leachability of heavy metals in pristine and treated samples. The thermally treated blocks were milled to pass a 0.45 mm standard sieve before leaching test. Because the pH values of the leachate exceeded 5.0, the extractant with a pH of 2.64 ± 0.05 was chosen for leaching test. For preparing the original leaching solution, 17.25 mL of glacial acetic acid was diluted to 1L with deionized water. In the leaching process, 5 g solid samples were homogeneously blended with leaching solution in a 2-L PE bottle with a liquid to solid ratio of 20 (L/S=20). The bottle with mixture was then tumbled at a rate of 30 rpm for 18 h. The obtained slurry was filtered using a 0.45-µm membrane disk. The transparent filtrate was acidified with 65 % nitric acid up to pH = 2. Final solution was injected into a PE tube, and saved at 4 °C before determination.

2.4. Subsequent extraction procedure. A modified version of subsequent extraction procedure based on Nurmesniemi et al.25 was carried out to detect speciation of heavy metals in RFA and thermal products. Distributions of heavy metals in this method are divided into five forms: (Ⅰ) water-soluble fraction is easily extracted with neutral solution; (Ⅱ) exchangeable fraction compromises with acid solution; (Ⅲ) reducible fraction is unstable under reducing conditions; (Ⅳ) oxidizable fraction can be removed under oxidizing environment; (Ⅴ) residual fraction is hardly extractable by common solution unless using strong acid like concentrated nitric acid, hydrofluoric acid, and etc. 8


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Table 2 summarized extraction conditions for each step. Firstly, 2 g of sample was added into a PE tube. Then, different chemical reagents were successively injected into the tube. During the experiment, water bath was adopted to supply a constant temperature. After shaken for a preset period in each stage, the mixture was centrifuged at 6000 rpm for 40 min. The resultant supernatant was filtered through a 0.45-µm membrane disk, and preserved at 4 °C before analysis.

2.5. Volatilization. To calculate the volatilization of heavy metals during thermal process, the RFA and treated residues were digested to determine the total contents of heavy metals following US EPA method 3050B. 26 The volatilization of trace metals in fly ashes by different treatment approaches was calculated as follows:27

V = 1 − × 100 %

(1)

C0: Heavy metals concentration in raw fly ash (mg/kg);

C1: Heavy metals concentration in treated fly ash (mg/kg);

m0: Mass of raw fly ash (g);

m1: Mass of treated fly ash (g).

2.6. Characterization. The particle size distribution of RFA was analyzed using laser

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diffraction particle size analyzer (LPSA; JL-1177, JNGX, China), and deionized water was used as dispersing media. The concentration was approximately 0.1 mg solid/mL water. The RFA was mixed with deionized water in a glass beaker, and then the suspension was subject to ultrasonic dispersion for 1 min before test. X-ray fluorescence (XRF) spectrometry (Shimadzu sequential XRF-1800, Shimadzu, Japan) was used to determine major chemical components in RFA. The XRF instrument can detect elements from 4Be to 92

U with Rh-anti-cathode and 4 kW thin window. The working voltage was 40 kV, and the

current was 40 mA. The thermogravimetry analysis (TGA; STA 449F3, Netzsch, Germany) was performed to investigate the thermal characteristics of RFA. The test was conducted in air, and the temperature rose from ambient temperature to 1000 °C with a heating rate of 10 °C/min. Concentrations of heavy metals in all filtered solution were detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; IRIS Advantage, Thermo Jarrell Aha Corporation, USA). The detection limits for heavy metals of Cd, Cr, Cu, Pb, and Zn were 0.0007, 0.0066, 0.002, 0.002, 0.005 mg/L, respectively. The crystal composition analysis was conducted with X-ray powder diffraction (XRD; LabX XRD-6100, Shimadzu, Japan). The excitation source was Cu-Kα radiation (λ = 0.15418 nm). The scanning was in the range of 10 to 80 ° with 0.02 °/step lasting for 1.2 s. The working voltage was 40 kV and the current was 30 mA. Morphology observation was accomplished using a scanning electron microscope (SEM; JSM-5900LV, JEOL, Japan) 10


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with the accelerating voltage of 15 kV. Samples were coated with gold for 90 s by a sputter coater.

3. RESULTS AND DISCUSSION

3.1. Characterization of RFA. Table 3 listed XRF analysis results of RFA, and we could obviously observe that CaO and chlorine were dominant compositions in fly ash accounting for 53.0 and 19.9 %, respectively. In APC procedure, huge amount of lime mortar was sprayed into the flue gas to control pollution, which explained the main source of CaO. As well, the injection of lime mortar made fly ash alkaline (pH > 12). With regard to high content of chlorine, some components like polyvinyl chloride (PVC) in MSW were abundant in chlorine, and during MSW combustion chlorides were vaporized and condensed in fly ash. Fly ash also contained NaO, SO3, SiO2, MgO, Fe2O3, Al2O3, and etc. Figure 1 exhibited particle size distribution of RFA, and it showed that D(25), D(50), D(90) were 8.56, 17.63 and 40.69 µm, respectively. Compared with other literatures,28-29 fly ash used in this study was much small in size. Finer particles implied stronger sintering force, which facilitated to densify fly ash particles for suppressing the leaching of heavy metals.

Thermal properties were considerably important for thermal treatment of RFA. 11


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Consequently, TG/DSC analysis of RFA was conducted in air, and the TG/DSC curves were shown in Figure 2. Original weight loss of 1.6 % in the range of 74 to 360 °C reflected the evaporation of moisture and oxidation of unburned carbon.30 When temperature increased from 400 to 500 °C, a remarkable mass reduction of 7 % was observed, which was mainly due to the decomposition of Ca(OH)2 with the release of H2O.31 It was interesting that there was a slight increase in weight between 506 and 557 °C. During heating process, some inorganic components were oxidized leading to the rise in mass. For example, Cr2O3 started to react with O2 in air and CaO at 500 °C, and CaCrO4 was newly formed.

11

Furthermore, between 560 and 750 °C, the marked

diminution of weight accounting for 4.3 % was predominantly caused by the decomposition of CaClOH and CaCO3.32 If temperature was higher than 770 °C, mass percentage of RFA continued to descend owing to the decomposition of chlorides such as KCl (melting point: 770 °C) and NaCl (melting point: 801 °C) accompanied with the discharge of HCl.27, 33 The maximum weight loss of RFA reached to 21.5 %, when the temperature was up to 1000 °C. Several endothermic peaks at 115, 400, 490, 605, 817 °C in DSC curve were associated with the removal of moisture, the decomposition of Ca(OH)2, the oxidation of inorganic minerals, the disintegration of CaClOH and CaCO3 and the breakdown of KCl and NaCl, respectively. Besides, one exothermic peak (360 °C) was attributed to the burning of remaining carbon. 12


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3.2. TCLP leaching concentrations of heavy metals. Figure 3 depicted the leaching concentrations of Pb, Zn, Cu and Cr in RFA and thermally treated slags. It was obvious that, among these four heavy metals in RFA, Pb had the highest leachability (8.081 mg/L) followed by Zn (0.3947 mg/L), Cu (0.1241 mg/L) and Cr (0.0975 mg/L) in a descending sequence. Besides, only leaching concentration of Pb in RFA exceeded the two limitations in China including hazardous waste identification standard (GB5086.3-2007) and pollution control for landfill (GB16889-2008). Generally, pH value has a significant impact on the leaching property of heavy metals in fly ash. Previous literature reported that the amphoteric leaching nature was observed with Pb and Zn, which suggested that Pb and Zn behaved highly soluble at both low and high pH levels. However, leaching concentrations of Cu were decreased with the increase of pH values. In terms of Cr, leaching values fluctuated as pH values. The peak of Cr concentration appeared at pH of 10, and the nadir was observed at pH of 6 and 12.34-35 In this study, pH value of leachate from RFA was 12.17, which conduced to the leaching of Pb and Zn, and suppressed the leaching of Cu and Cr.

From Figure 3, we could see that leaching concentrations of Pb, Zn and Cu were remarkably reduced through thermal treatment. Leaching concentrations of Pb from all sintered samples dropped to below the limitation of hazardous identification standard in China (5 mg/L). Irrespective of traditional and microwave-assisted thermal treatments, 13


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the leaching of Pb was further reduced with the enhancement of temperature. When temperature was above 900 °C, leaching concentration of Pb was lower than the regulation of pollution control for landfill (0.25 mg/L). Moreover, prolonged retention time contributed to diminishing the leachability of Pb in the treated residue. Comparing the leaching results of Pb from fly ashes treated by different heating methods, Pb became less leachable after microwave sintering process at the same temperature. For instance, leaching concentrations of Pb in MW700-10 was lower than that in T700-10. In consideration of Cu and Zn, though leaching concentrations of Cu and Zn in RFA preferably met the two regulations mentioned above, the values were still markedly decreased after sintering. It was strikingly intriguing that leaching concentrations of Cr in treated products evidently went up, but were still under the two regulations. These results suggested that microwave technology could be potentially used as an alternative to conventional heating methods for MSWI fly ash stabilization.

Sintering process caused mass migration, grain growth, agglomeration, and etc.36 After sintering, Glassy or ceramic materials were generated, and the produced materials had higher density which facilitated to repress the leaching of Pb, Zn and Cu.37-38 The alternation trend of concentrations of Cr in thermally treated fly ash was much different from that of Pb, Zn and Cu. Similar results have been confirmed by many other studies.9, 11

High leaching concentrations of Cr resulted from the oxidation of Cr(Ⅲ) to Cr(Ⅲ) 14


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which had greater solubility.9, 11 In the following section focal point was given to Pb and Cr because leaching concentrations of Zn and Cu were extremely low.

3.3. Speciation of Pb and Cr. A modified sequential extraction procedure25 was applied to compare the influences of traditional and microwave-assisted thermal treatments on the chemical speciation of Pb and Cr. Figure 4 presented distribution patterns of Pb in RFA and residues treated under different conditions. Pb in RFA primarily existed in three factions including water-soluble fraction (47.2 %), exchangeable fraction (22.8 %) and residual fraction (28.7 %). As shown in Figure 4, similar phenomena in distribution alteration of Pb could also be observed after conventional and microwave thermal procedures. Compared with Pb distribution pattern in RFA, residual form became dominant part of Pb distribution in treated samples with notable reduction of water-soluble and exchangeable fractions. As temperature rose, the amount of residual fraction was further boosted. Under the condition of 1100 °C for 10 min, the percentage of residual fraction reached 95 %. When retention time was prolonged (T1100-1h and MW1100-1h), there was a slight increase in oxidizable fraction and a minor drop in residual fraction. It is extensively endorsed that water-souluble and exchangeable parts are the most transferable fractions under natural condition, and thus the two fractions impose enormous potential threat to environment.25, 39 In the present work, after thermal treatment, Pb was immobilized due to the decrease of water-soluble 15


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and exchangeable fractions and concurrent increase of residual form. As well, Pb distribution patterns, to some degree, corroborated the leaching results of Pb in Figure 3.

Figure 5 plotted distribution patterns of Cr in RFA and treated slags. Unlike Pb distribution in RFA, Cr was principally composed of reducible and residual fractions. From Figure 5, we could find that, after both traditional and microwave heating processes, reducible fraction was conspicuously improved with a prominent decline in residual fraction. Thermal treatment had no significant influence on other three parts of Cr (oxidizable fraction, exchangeable fraction and water-soluble fraction). In contrast to Cr distribution in RFA, high reducible fraction of Cr implied that parts of low-valent Cr were transformed to hexavalent Cr during heating period. Generally, hexavalent Cr-compounds like CaCrO4 were more soluble 9, 11, which manifested the leaching results of Cr in Figure 3.

3.4. Vaporization of Pb and Cr. Figure 6 demonstrated vaporizations of Pb and Cr in residues treated under different conditions. As shown in Figure 6 (a), the vaporization of Pb was much susceptible to temperature, and microwave heating was more efficient for the removal of Pb. With temperature rising from 700 to 1100 °C, the vaporization of Pb progressively increased from 24 % to 72 % by traditional thermal process, while that of Pb augmented from 40 % to 90 % in microwave heating procedure. On the other hand, 16


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under conventional thermal treatment, longer holding time at 1100 °C contributed to higher vaporizaion of Pb. By contrast, during microwave-assisted thermal treatment, the extention of treatment time did not effectively change the vaporization of Pb at 1100 °C. Actually, the effect of retention time on the vaperization of Pb was different under different temperature. At low tempertaure, holding time had a great effect on the evaporation of Pb. It was reported that the volatile fraction of Pb was less than 30 % at 670 °C for the process time shorter than 30 min, while that of Pb reached to 90 % and leveled off at the same temperature for 4 h.40 At high temperature, however, vaporization of Pb was up to 90 % even after a short period and was impregnable to retention time. For example, at 1030 °C for 10 min, the vaporization of Pb achieved 90 % and remained steady with time increasing.40 During thermal treatment, lead chlorides were formed by the reaction between lead oxides and chlorides (NaCl, KCl), and the generated lead chlorides were more volatile, which leaded to high vaporization of Pb after themal treatment.27, 41 Microwave sintered materials by charge polarization ionic conduction, and ect, which made the sintering process faster and more uniform in contrast to conventional heating.21 Due to the foregoing characteristics, microwave accelerated transformation from lead oxides to lead chlorides, therefore microwave sintering was more efficient for Pb vaporization than traditional sintering.42

Figure 6 (b) highlighted that Cr was less volatile than Pb. Under all conditions, the 17


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volatilization of Cr was below 20 %. As tmperature elevated, the volatility of Cr declined, and the decrease was more conspicuous for Cr in the samples treated with mcirowave. When temperature increased from 700 to 1100 °C (10 min), the evaporation of Cr in slags treated by conventional thermal treatment decreased from 15.6 % to 12.7 %, while it ranged from 17 % to 4.5 % by microwave-assisted thermal process. Additionally, the vaporization of Cr, after conventional thermal treatment for 1 h, was further reduced, and yet longer processing time did not significantly change the ratio of evaporated Cr during microwave thermal treatment. The vaporization of Cr highly depended on the Cr-compound species. In the heating procedure, Ca, K and Mg reacted with Cr generating CaCrO4, MgCrO4 and K2CrO4, etc, and these newly-produced Cr-compounds were hardly volatile.27, 43, 44 Higher temperature enhanced these chemical transformations, and thus lower evaporation of Cr was obtained. Furthermore, microwave heating also facilitated the fast generation of non-volatile Cr-compounds.45

3.5. Mineralogical phase transformation. XRD analysis was conducted to comprehend phase transformation in fly ash before and after thermal treatment. Figure 7 revealed XRD spectra of RFA and thermally treated fly ash. The main components in RFA were NaCl, KCl, Ca(OH)2 and SiO2, which, to some degree, verified the XRF results in Table 3. As shown in Figure 7, the peaks of CaO appeared in the spectra of T700-10, T900-10, T1100-10, T1100-1h and MW1100-1h in addition to mineral 18


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compositions in RFA. CaO was newly generated through the decomposition of Ca(OH)2, CaClOH and CaCO3. The formed CaO reacted with aluminosilicate producing more stable Ca-compounds. As a result, the percentage of stable fraction of heavy metals was promoted contributing to the decrease in leaching concentration of Pb. For another aspect, the dissolution of CaO and its conversional alkaline matter was the main reason for high pH level of leachate from treated fly ash, which also restrained the leaching behavior of heavy metals.43

CaO had a dissimilar impact on the fate of Cr compared with other heavy metals like Pb. CaO firstly reacted with aluminosilicte, and then remanent CaO enhanced the oxidation of Cr(Ⅲ) to Cr(Ⅵ) through the following reaction:46 CaO + Cr O + O → CaCrO

(2)

Compared with Cr(Ⅲ), reducible Cr was more leachable and less volatile, which lead to the increase of leaching concentration of Cr and the reduction in vaporization of Cr as shown in Figure 5 and Figure 6. It deserved attention that no CaO peak was detected in the spectrums of MW700-10, MW900-10 and MW1100-10. It is well-accepted that crystalline formation is closely related to heating and cooling rate. During thermal treatment, atom and ion transfer to special lattice points and rearrange to form crystal texture. If heating and cooling rate are too high, microscopic particles do not have enough 19


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time to reach to corresponding sites. Based on the same mechanism, quite short retention time is detrimental for crystallization.47 In this study, the three samples were treated with microwave heating under fast heating/cooling rate and very short duration. Therefore, the generated CaO was hard to crystalize, and the intensities of CaO peak in MW1100-10, MW900-10 and MW700-10 were small.

3.6. Microstructure observation. Figure 8 displayed SEM images for the particles of RFA and thermally treated slags. From Figure 8 (a), it was obvious that nascent fly ash particles had spherical shape; however, irregular particle was dominant in treated fly ash, which implied that dramatic particle conglomeration occurred during heating process. Figure 8 (f) was the schematic diagram of particle agglomeration. Preliminarily, spherical fly ash particles contacted with each other. During thermal treatment, chemical reaction and mass transfer came about and caused particle agglomeration. Accordingly, irregular particle clusters were generated with less open pores and lower active area. Longer retention time was conducive to the formation of dense particles. In the treated particles, there were plentiful closed pores. On account of the generation of particle clusters, heavy metals were encapsulated inside the sintered particles and/or attached to the closed pores, which contributed to diminishment of the leaching concentrations of heavy metals.

Figure 9 showed microstructure of single particle of RFA and thermally treated slags. 20


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As represented in Figure 9 (a), there were copious granular crystallites and open pores. After thermal treatment, the size of crystallites was much bigger, and the microstructure of particles was more compact in Figure 9 (b-e). In the processes of grain growth and structure densification, heavy metals were trapped in the lattice and grain boundary leading to the rising percentage of residual fraction and the reduction of leaching concentration of heavy metals.

4. CONCLUSION

In this study, the effects of microwave-assisted and conventional thermal treatment on the fate of Pb and Cr were systematically compared. Leaching concentrations of Pb, Cu and Zn were dramatically reduced, while that of Cr increased after both thermal treatments. Microwave-assisted thermal treatment was more efficient to suppress leaching behavior of Pb, Cu and Zn. In RFA, water-soluble fraction plus exchangeable fraction of Pb accounted for 70 %. However, in the treated samples the two fractions of Pb remarkably descended with the sharp increase of residual form. In addition, higher temperature and longer retention time were beneficial for the promotion of residual fraction amount, which confirmed the extremely low leaching concentrations of Pb in T1100-10, T1100-1h, MW1100-10 and MW1100-1h. For Cr, distribution pattern was quite different from that 21


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of Pb. In RFA, residual and reducible fractions were two dominant parts of Cr. After thermal treatment, an increase in reducible fraction and a decrease in residual fraction could be obviously seen. This transformation was caused by the oxidation of Cr(Ⅲ) to Cr(Ⅵ), and the latter was more leachable and non-volatile. The vaporization of Pb was more than 40 % at all conditions. With temperature increased from 700 °C to 900 °C, the vaporization of Pb ranged from 40 % to 90 % due to the decomposition of metal chlorides. When the holding time was short, Pb was further effectively evaporated with the assistance of microwave. Under all conditions, less than 20 % of Cr was vaporized, and higher temperature weakened the vaporization of Cr. XRD analysis suggested that CaO was newly generated, which accelerated the transformation of Cr(Ⅲ) to Cr(Ⅵ). According to SEM images, particle agglomeration, grain growth co-occurred during thermal treatment. These processes immobilized heavy metals in the lattice and grain boundary. Conclusively, this work revealed the effects of microwave sintering on the fate of Pb, Cr in MSWI fly ash and it showed the potentiality of microwave technology for MSWI fly ash immobilization.

AUTHOR INFORMATION

Corresponding Authors 22


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A/Prof. Weizhong Yang, *Tel & Fax: +86 28 85416050; E-mail: [email protected]; A/Prof. Yi Deng, *Tel & Fax: +86 28 85464466; E-mail: [email protected].

ACKNOWLEDGEMENTS

This work was jointly supported by the International Science and Technology Co-operation and Exchange Program of Sichuan (No. 2015HH0066-1), Chengdu Science and

Technology

Project

(2014-HM01-00081-SF,

2015-HM01-00451-SF,

2015-GH02-00009-HZ), Science and Technology Program of Sichuan Province (2017FZ0046), Project funded by China Postdoctoral Science foundation (2017M610600), Full-time Postdoctoral Research Fund of Sichuan University (2017SCU12016), Research Program of Star of Chemical Engineering (School of Chemical Engineering, Sichuan University), and Hong Kong Scholars Program. We would like to thank to Wang Hui (Analytical & Testing Center, Sichuan University) for her help in SEM observations.

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Captions of figures and tables

Figure 1. Figure 1. Particle size distribution of RFA. D(25), D(50), D(90) were 8.56, 17.63 and 40.69 µm, respectively.

Figure 2. TG/DTG curve of the original fly ash. Changes under different temperatures in TG/DSC curves were caused by the removal of moisture, the decomposition of Ca(OH)2, the oxidation of inorganic minerals, the disintegration of CaClOH and CaCO3 and the breakdown of KCl and NaCl, respectively.

Figure 3. TCLP concentrations and corresponding limits of heavy metals in RFA and thermally treated slags. Leaching concentrations of Pb, Zn and Cu were remarkably reduced by thermal treatment, while that of Cr in treated products evidently went up, but were still under the two regulations.

Figure 4. Distribution patterns of Pb in RFA and thermal products: (a) traditional thermal treatement; (b) microwave thermal treatment.

Figure 5. Distribution patterns of Cr in RFA and thermal products: (a) traditional thermal treatement; (b) microwave

Figure 6. Vaporization percentages of Pb and Cr after thermal treatemnt: (a) traditional thermal treatment; (b) microwave thermal treatment. 28


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Figure 7. XRD spectrums of RFA and thermal products. The main components in treated or untreated fly ashes included NaCl, KCl, Ca(OH)2 and SiO2 . After thermal treatment, the peaks of CaO appeared in the spectrums of T700-10, T900-10, T1100-10, T1100-1h and MW1100-1h. It deserved attention that no CaO peak was detected in the spectrums of MW700-10, MW900-10 and MW1100-10.

Figure 8. SEM images of particles of RFA and thermally treated slags: (a) RFA; (b) T1100-10; (c) T1100-1h; (d) MW1100-10; (e) MW1100-1h; (f) diagram of particle agglomeration.

Figure 9. Microstructures of single particle of RFA and thermally treated slags: (a) RFA; (b) T1100-10; (c) T1100-1h; (d) MW1100-10; (e) MW1100-1h.

Table 1. The details of the experimental conditions.

Table 2. Experimental conditions of sequential extraction procedure.

Table 3. Major chemical compnents of raw fly ash (RFA).

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Figure 1. Particle size distribution of RFA. D(25), D(50), D(90) were 8.56, 17.63 and 40.69 µm, respectively.

30


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Figure 2. TG/DTG curve of the original fly ash. Changes under different temperatures in TG/DSC curves were caused by the removal of moisture, the decomposition of Ca(OH)2, the oxidation of inorganic minerals, the disintegration of CaClOH and CaCO3 and the breakdown of KCl and NaCl, respectively.

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Figure 3. TCLP concentrations and corresponding limits of heavy metals in RFA and thermally treated slags. Leaching concentrations of Pb, Zn and Cu were remarkably reduced by thermal treatment, while that of Cr in treated products evidently went up, but were still under the two regulations.

32


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Figure 4. Distribution patterns of Pb in RFA and thermal products: (a) traditional thermal treatement; (b) microwave thermal treatment.

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Figure 5. Distribution patterns of Cr in RFA and thermal products: (a) traditional thermal treatement; (b) microwave thermal treatment.

34


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Figure 6. Vaporization percentages of Pb and Cr after thermal treatemnt: (a) traditional thermal treatment; (b) microwave thermal treatment.

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Figure 7. XRD spectrums of RFA and thermal products. The main components in treated or untreated fly ashes included NaCl, KCl, Ca(OH)2 and SiO2 . After thermal treatment, the peaks of CaO appeared in the spectrums of T700-10, T900-10, T1100-10, T1100-1h and MW1100-1h. It deserved attention that no CaO peak was detected in the spectrums of MW700-10, MW900-10 and MW1100-10.

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Figure 8. SEM images of particles of RFA and thermally treated slags: (a) RFA; (b) T1100-10; (c) T1100-1h; (d) MW1100-10; (e) MW1100-1h; (f) diagram of particle agglomeration.

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Figure 9. Microstructures of single particle of RFA and thermally treated slags: (a) RFA; (b) T1100-10; (c) T1100-1h; (d) MW1100-10; (e) MW1100-1h.

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Table 1. The details of the experimental conditions. Sample name Heating method Temperature (°C) Retention time (min) T700-10

Tradition

700

10

T900-10

Tradition

900

10

T1100-10

Tradition

1100

10

T1100-1h

Tradition

1100

60

MW700-10

Microwave

700

10

MW900-10

Microwave

900

10

MW1100-10

Microwave

1100

10

MW1100-1h

Microwave

1100

60

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Table 2. Experimental conditions of sequential extraction procedure.

Speciation

Extraction agent

Mixing time

Temperature

(h)

(°C)

Water-soluble

40 mL deionized water (pH = 4.0)

16

25

Exchangeable

40 mL 0.1 M CH3COOH (pH =2.9)

16

25

Reducible

40 mL 0.1M NH2OH-HCl (pH = 2.0)

16

25

Oxidizable-step1

10 mL 30 % H2O2

1

50

Oxidizable-step2

10 mL 30 % H2O2

1

50

Oxidizable-step3

50 mL 1M CH3COONH4

16

25

Residual-step1

20 mL HF

2

50

Residual-step2

20 mL HF + 20 mL 65 % HNO3

2

50

40


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Table 3. Major chemical compnents of raw fly ash (RFA).

Compositio

CaO NaO SO3

n

Ratio (%)

53.0

7.7

5.9

SiO

Mg

Fe2O

Al2O

TiO

2

O

3

3

2

3.9

3.8

1.9

1.0

0.5

P2O5

0.3

Cl

19. 9

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Effects of Microwave-Assisted Thermal Treatment on the Fate of Heavy

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