Stabilization of heavy metals in municipal solid waste circulating fluidized bed incineration fly ash by fusion–hydrothermal method
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Municipal solid waste circulating fluidized bed incineration (MSWCFBI) fly ash was a hazardous waste, maintaining challenges for disposal. One effective approach was stabilizing the toxic heavy metal ions in the fly ash structures in situ. This work proposed a fusion–hydrothermal method, including fusion pretreatment in nitrogen atmosphere and microwave-assisted hydrothermal process, to treat three MSWCFBI fly ash samples. Specifically, leaching tests were performed to demonstrate the heavy metal stabilization. Through the treatment of the fusion–hydrothermal process, the concentrations of Cd, Cu, Zn, Pb, Ni, and Cr ions leaching from the fly ashes were obviously less than those of the raw fly ash and the sample only treated by hydrothermal process. Meanwhile, the heavy metal ions migrating from the fly ash to the hydrothermal residual liquid were reduced. Importantly, lots of zeolites formed during the fusion–hydrothermal process, such as to bermorite and sodalite. The fusion pretreatment significantly facilitated the conversion of quartz into amorphous silicon and silicate salts. Then, the silicon dissolution was accelerated and zeolite formation was promoted. Eventually, the heavy metal ions could be trapped in zeolite frameworks, enhancing the stabilization of heavy metal. Moreover, the cation-exchange capability values of the three treated fly ash were 1.099, 1.168, and 1.188 meq g−1, two-folder larger than those of the samples only treated by hydrothermal process. In summary, the fusion–hydrothermal method could facilitate the stabilization of heavy metal ions in the fly ash and the as-obtained solid product with high content of zeolite was promising for future applications.
KeywordsHeavy metal MSWCFBI fly ash Fusion Hydrothermal Zeolite
Municipal solid-waste incineration (MSWI), as a practical method to dispose municipal solid waste (MSW), is gradually increased in China. 286 MSWI plants have been put into operation in China with more than 84 billion kilograms of MSW disposed by incineration in 2017 . Fly ash, one of the major wastes generated from MSWI, is acknowledged to be a hazardous waste, due to the fact that it contains a large quantity of heavy metals and dioxin [2, 3, 4]. To reduce the damage of MSWI fly ash to environment, various methods were previously proposed, for instance, cement solidification [5, 6] and chemical stabilization [7, 8]. However, cement solidification brought the problem of volume enlargement, and chemical stabilization could not solidify several heavy metals at the same time. New strategies are required to improve the fly ash management.
Hydrothermal method was considered as one of effective processing technologies for the fly ash disposal in terms of cost-effective process and no increase in landfill volume [9, 10, 11]. Bayuseno  reported that the heavy metals leaching from the fly ash could be reduced after hydrothermal process for 48 h. The improved performance of heavy metal stabilization was considered as the contribution of zeolite materials forming during the hydrothermal process. According to prior literatures [12, 13, 14], zeolites owned porous structures and high thermal and chemical stability, where heavy metals could be immobilized by various means, such as adsorption, ion exchange, and physical encapsulation. Therefore, increasing the formation of zeolites could be one of the effective methods to reduce the heavy metal pollution leaching from the MSWI fly ash.
However, the conventional hydrothermal method often took a long time over several days to obtain a complete reaction. Recently, microwave irradiation was introduced to the hydrothermal process, resulting in the reduction of the reaction time to 10–50 min [15, 16, 17]. Nevertheless, only by hydrothermal process, it was still unable to reach the standard of Chinese regulation (GB16889-2008)  for some heavy metals, such as Cd . On the other hand, a large amount of indissoluble materials, including quartz, remained in the MSWI fly ash even after the microwave-assisted hydrothermal process . These indissoluble materials were unable to be utilized for the zeolite formation when only the hydrothermal process was applied. As such, one potential approach to improve the yield of zeolites and the heavy metal stabilization could be increasing the utilization of the indissoluble components in the MSWI fly ash.
In current work, a fusion–hydrothermal process, including an alkaline fusion pretreatment and a microwave-assisted hydrothermal process, was proposed to improve the heavy metal stabilization. Three fly ashes were disposed by the fusion-pretreated microwave-assisted hydrothermal process (FMHP) and microwave-assisted hydrothermal process (MHP), respectively. Nitrogen (N2) was used as fusion atmosphere. The heavy metal stabilization performance of the FMHP method was compared with that of the MHP method. Moreover, the X-ray diffraction (XRD) and the cation-exchange capacity (CEC) were conducted on the raw fly ashes, and the FMHP- and MHP-treated solid products, respectively.
Materials and methods
Three raw fly ashes used in this work were sampled from Xiaoshan district of Hangzhou city, Taizhou city, and Cixi city, which were named Xiaoshan fly ash, Taizhou fly ash, and Cixi fly ash, respectively. The three fly ashes were collected from the fabric filters of MSWCFBI boilers, in which the air pollution control systems all consist of a selective non-catalytic reduction denitration system, a semi-dry desulfurization system, an activated carbon injector, and a fabric filter. Fly ash samples were collected from the fabric filter. Before the experiments and the characterization for analysis, the raw fly ashes were dried at 105 °C for 24 h. The reagents included sodium hydroxide (NaOH), acetic acid, concentrated nitric acid (HNO3, 65.0~68.0%), deionized water, and high purity nitrogen (N2, 99.99%).
The uniform mixture of 4 g raw fly ash and 1.5 g NaOH was placed in a nickel boat and fused in a tubular furnace filled with N2 as the fusion atmosphere. The inert gas (N2) was used to prevent the oxidization of Cr from trivalent (Cr3+) to hexavalent form (Cr6+), which was more soluble and migrant. The mixture of fly ash and NaOH was maintained at 550 °C for 1 h, and then, the fused product was obtained. Afterwards, the fusion products and 36 ml deionized water were mixed in the sealed container matched the microwave apparatus (Milestone, ETHOS UP, Italy). The mixture was heated at 125 °C for 20 min in the microwave-assisted hydrothermal process, and then cooled to ambient temperature. The hydrothermal temperature and duration were optimized based on heavy metal immobilization performance and energy consumption in our prior work . The solid component and the liquid component were separated by centrifugation. The final product (i.e., the treated fly ash by FMHP) was the solid component dried in the oven at 105 °C for 24 h. The residual liquid component, as the by-product from hydrothermal process, was filtrated and then acidified with HNO3 before the analysis. The acidification was conducted by adding 1 mL HNO3 to 10 ml residual hydrothermal liquid. After acidification, the mixed liquid was acidic, meeting the requirement of the test instrument. The test results obtained from the measurement will be converted to actual results. To draw a comparison of heavy metal stabilization effect between FMHP and MHP, the MHP was conducted at the same condition as FMHP except the fusion pretreatment.
The element composition of the raw fly ashes was identified by Energy-Dispersive X-ray Detector (EDX, X-Max, Oxford instruments). To improve the accuracy, four detections were conducted to obtain the average value and the deviations. The crystal phases of the raw fly ashes and treated fly ashes were tested by X-ray diffraction (XRD; Rigaku Rotaflex, Japan). The CEC value measuring was based on the US EPA Method 9081 .
To evaluate the stabilization effect of heavy metals, the leaching test of raw and treated fly ashes was conducted according to the solid-waste extraction procedure (HJ/T 300-2007) approved by State Environmental Protection Administration of China . Afterwards, the suspension from leaching test was filtered to get the water sample, namely the leaching liquid. The heavy metal concentrations (including Zn, Pb, Cu, Ni, Cd, and Cr) of the residual liquid and the leaching liquid were analyzed by inductively coupled plasma mass spectrometry (ICP-AES 6300, Thermo Fisher, Massachusetts, USA), whose detection limits for Zn, Pb, Cu, Ni, Cd, and Cr were 0.001, 0.001, 0.0064, 0.0088, 0.001, and 0.0071 mg L−1. The heavy metal concentrations in the leaching liquid were compared with the Standard of Chinese regulation (GB16889-2008) .
Results and discussion
Chemical composition of raw fly ash
Element composition of raw fly ashes
12.82 ± 2.963
11.82 ± 1.867
7.918 ± 0.484
42.81 ± 7.090
46.06 ± 10.76
43.84 ± 8.914
2.740 ± 1.356
12.45 ± 8.445
6.595 ± 2.779
1.028 ± 0.512
0.915 ± 0.815
1.505 ± 0.445
3.088 ± 2.256
3.323 ± 2.433
4.503 ± 1.625
5.903 ± 4.189
4.923 ± 3.300
8.335 ± 1.102
0.450 ± 0.331
0.570 ± 0.567
0.885 ± 0.377
0.548 ± 0.486
0.893 ± 0.669
1.043 ± 0.732
1.830 ± 2.431
2.995 ± 3.425
2.965 ± 4.050
0.753 ± 0.652
0.555 ± 0.657
1.238 ± 0.686
23.68 ± 4.610
12.30 ± 7.343
15.42 ± 2.991
0.503 ± 0.138
0.353 ± 0.348
0.605 ± 0.279
1.238 ± 0.576
1.130 ± 1.415
3.043 ± 0.795
1.390 ± 0.086
0.940 ± 0.143
0.985 ± 0.074
1.225 ± 0.141
0.778 ± 0.128
1.130 ± 0.197
Heavy metal leaching toxicity
For comparison, the leaching concentrations of heavy metal ions from the MHP-treated fly ash samples were measured. As shown in Fig. 1, the leaching concentrations of Zn, Pb, Cu, Ni, Cd, and Cr were detected to be 12.59, 0.200, 0.481, 0.143, 0.275, and 0.217 mg L−1, respectively, for Xiaoshan MHP fly ash sample, 1.156, 0.127, 0.502, 0.008, 0.005, and 1.315 mg L−1, respectively, for Taizhou FMHP fly ash sample, and 3.411, 0.194, 0.952, 0.198, 0.220, and 2.410 mg L−1, respectively, for Cixi MHP fly ash sample. To some extent, the heavy metal ion concentrations were decreased after the MHP treatment compared to the raw fly ash sample. However, there were still some of ions exceed the standard limitation, such as the Ni and Cr. The ion concentrations of Zn, Pb, Cu, and Cr were very close to the standard limitation. Thus, it was hard to suffice the environmental standard if only the microwave-assisted hydrothermal process was applied.
It was worth noting that the fusion pretreatment before the hydrothermal process led to the obvious reduction of heavy metal ions leaching from the fly ash samples. In other words, the fusion pretreatment could make the products more stable and resistant to harsh environment (e.g., the leaching process). The heavy metals could be efficiently stabilized in the FMHP products.
Migration of heavy metals to hydrothermal liquid
As shown in Fig. 2, the concentrations of Zn, Pb, Cu, Ni, Cd, and Cr in the residual liquid were measured to be 5.766, 1.643, 0.561, 0, 0.001, and 0 mg L−1, respectively, for Xiaoshan FMHP fly ash sample, 15.15, 5.844, 3.569, 0, 0.003, and 0.002 mg L−1, respectively, for Taizhou FMHP fly ash sample, and 6.99, 8.097, 5.327, 0, 0.004, and 0.001, respectively, for Cixi FMHP fly ash sample. On the other hand, the concentrations of Zn, Pb, Cu, Ni, Cd, and Cr in the residual liquid were measured to be 41.35, 15.67, 2.272, 0, 0.007, and 0.001 mg L−1, respectively, for Xiaoshan MHP fly ash sample, 38.53, 23.47, 6.652, 0, 0.004, and 0.005 mg L−1, respectively, for Taizhou MHP fly ash sample, and 25.86, 27.66, 7.971, 0, 0.006, and 0.003, respectively, for Cixi MHP fly ash sample.
Apparently, the heavy metal ion concentration in the FMHP residual liquid was far lower than those in the MHP residual liquid. The results indicated that the fusion pretreatment could prevent the migration of heavy metal from the solid fly ash to the hydrothermal liquid. The heavy metals in the hydrothermal liquid might cause secondary pollution and further strategies were desired to solve this problem. To reduce the pollution, one possible approach was to reuse the hydrothermal liquid for several times. Electrolysis was another method to recycle the heavy metal and prevent pollution. It was worth noting that the evaporation during the fusion process could be ignored, because the evaporating amount was much less than that in the hydrothermal solution and the leaching liquid. More discussion was available in Supplementary Sect. 1.
Despite of the evaporation of heavy metals in the fusion process, the fusion pretreatment is crucial in the current work. When elevating the hydrothermal temperature from 125 to 180 °C and extending the hydrothermal duration from 20 min to 4 h in the only-hydrothermal process, the improvement in the heavy metal immobilization was very limited (see details in Supplementary Sect. 2), which was in accordance with our prior works .
Crystal phase analysis
According to previous literatures [9, 12, 13], the heavy metals could be stabilized in zeolite-like structures that were formed during the hydrothermal process. The XRD characterizations and the analysis on crystal phase were conducted to demonstrate the zeolite formation during the MHP and FMHP treatments.
The XRD pattern of the Xiaoshan MHP ash is shown by the blue line in Fig. 3a. The halite, sylvite, kyanite, and portlandite disappeared, while a small amount of katoite (Ca3Al2(OH)12) was found. It was speculated that the Al in the kyanite and the Ca in the portlandite were used for the katoite fabrication during the hydrothermal process. The content of quartz was slightly reduced, indicating that only a small amount of Si contents in the fly ash was transferred. Thus, the high content of Si in the fly ash could not be fully made use of, if only the hydrothermal process was applied. The XRD spectrum of the Xiaoshan FMHP ash was presented by the green line in Fig. 3a. The intensity of the quartz-related peaks was obviously decreased compared to the raw and MHP-treated fly ash samples. Importantly, tobermorite (Ca5Si6(OH)2O16·4H2O) and sodalite (Na4Al3Si3O12Cl) were found, while no katoite was observed in the FMHP sample.
According to previous studies [26, 27, 28], the molar ratio of Ca, Al, and Si in the hydrothermal solution made a crucial influence on the final product. When the Al contents were relatively higher, the formation of katoite was promoted . When the Si contents was relatively higher and the molar ratio of Ca/(Al + Si) was in the range of 0.67–1.2, it was more likely to form tobermorite [27, 28]. With fusion pretreatment, the crystal quartz in the raw fly ash was converted to amorphous silicon and silicate salts [29, 30, 31]. These Si-containing materials were more soluble, leading to the increased Si content in the hydrothermal solution. As a result, the formation of tobermorite in the hydrothermal process was promoted. Moreover, the sufficient Si and Al contents were also beneficial for the synthesis of sodalite structures.
With regard to the Taizhou MHP ash (see the blue line in Fig. 3b), both tobermorite and katoite were found in the Taizhou MHP ash. The existence of tobermorite might be explained by the slightly increased Si contents dissolving from the Taizhou raw fly ash during the hydrothermal process. As shown in Fig. 3c, although tobermorite, katoite, and sodalite were detected in the Cixi MHP ash (see the blue line), the quartz-related peaks were still high. In the XRD spectrum of all the FMHP samples (see the green lines), the quartz-related peaks were obviously reduced and the peaks related to tobermorite and sodalite appeared or raised to higher intensities. Moreover, the portlandites were detected in Taizhou and Cixi MHP products. It was worth noting that the portlandite was major Ca source for the formation of tobermorite . The content of portlandite in raw Taizhou fly ash and the content of lime in raw Cixi fly ash were very high, and portlandite could be formed by the reaction between lime and water. Thus, the portlandite in the MHP products did not be used up, leading to the residue of portlandite. Especially, in the direct hydrothermal process, the SiO2 crystals could not be fully utilized to react with portlandite. When fusion pretreatment was applied, the SiO2 crystals transformed to amorphous silicon and silicate salts, which could dissolve quickly and then react with portlandite. As a result, the portlandite peaks in the FMHP product disappeared.
In addition, according to Figure S3 in Supplementary Sect. 2, it was found that although the hydrothermal temperature was elevated from 125 to 180 °C and time was extended from 20 min to 4 h, the SiO2 crystals could not dissolve into the hydrothermal solution. As a result, a few zeolite materials were observed in the solid product, which was similar to the hydrothermal product processed in 125 °C and 20 min. More importantly, when extending the microwave-hydrothermal duration to 4 h, the energy consumption of the only-hydrothermal process was calculated to be 14 kWh, which was even higher than the thermal-pretreated method (~ 11.2 kWh). Thus, the thermal-pretreated method was an economically efficient method to promote the utilization of Si-containing materials in fly ashes and improved the zeolite fabrication.
Mechanism of heavy metal stabilization in zeolite structures
According to previous studies, zeolites owned stable and porous structures . The zeolites formed during the hydrothermal process could improve immobilizing heavy metals in the fly ashes [9, 23, 24]. It is because the heavy metals that dissolved from the solid fly ash could be trapped in the zeolite frameworks through physical/chemical adsorption, ion exchange, and physical encapsulation when the zeolites are assembling [12, 13]. As discussed before, the alkaline-assisted fusion process could promote the transformation of silicon and aluminum contents from crystal phase to amorphous form, as well as silicate and aluminate salts, which eventually accelerated the dissolution of Si- and Al-containing materials and then benefited the zeolite fabrication. Thus, the increased zeolite contents obtained by the FMHP method, as shown by the XRD results, might be the major explanation for the superiority in the heavy metal immobilization performance. In other words, the current FMHP method was suitable for fly ash disposal with increased zeolite formation, excellent heavy metal stabilization, and few secondary pollutions of the by-product (i.e., residual liquid).
Cation-exchange capacity results
In this study, FMHP was performed on three MSWCFBI fly ashes and conducted well on the fly ash disposal. The FMHP fly ash products showed lower heavy metal concentrations in the leaching liquid than those of the MHP products, which meant that the safety of the solid products was improved by the FMHP treatment. Meanwhile, the FMHP could reduce the migration of the heavy metals from the solid fly ash to the hydrothermal liquid, and thus, the secondary pollution could be reduced. According to the XRD characterizations, more zeolitic materials were formed after the FMHP treatment. Specifically, katoite was usually detected in the MHP products, while tobermorite and sodalite were observed in the FMHP products. The results demonstrated that the fusion pretreatment could facilitate the conversion of quartz into amorphous silicon and silicate salts, and then accelerate the dissolution of silicon. The increased soluble silicon source in the hydrothermal solution enhanced the zeolite formation, i.e., tobermorite and sodalite. As a result, the FMHP products could be more stable due to containing more zeolitic materials. The heavy metals were immobilized in the zeolite frameworks, resulting in less leakage of heavy metal to the environment. Moreover, with more zeolitic materials, the CEC values of FMHP products were two-folder larger than that of MHP products. The FMHP products with the increased CEC showed a high application potential. In summary, the FMHP is a promising method for the disposal of MSWI fly ash.
This study is supported by the National Key Research and Development Program of China (Grant Nos. 2018YFC1901302, 2018YFF0215001, 2017YFC0703100), the Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51621005), the National Nature Science Foundation of China (Grant No. 51676172), and the Fundamental Research Funds for the Central Universities (Grant No. 2018FZA4010), and Power Construction Corporation of China Science and Technology Project Funding (Grant No. DJ-PTZX-2018-01).
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