Article pubs.acs.org/EF
Effects of Microwave-Assisted Hydrothermal Treatment on the Major Heavy Metals of Municipal Solid Waste Incineration Fly Ash in a Circulating Fluidized Bed Qili Qiu, Xuguang Jiang,* Shengyong Lu, and Mingjiang Ni State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027 Zhejiang, China ABSTRACT: In this paper, microwave-assisted hydrothermal treatment was performed to stabilize the heavy metals of municipal solid waste incineration (MSWI) fly ashes in a circulating fluidized bed (CFB). Influences of the types of chemical additives, reagent concentration, liquid/solid ratio, reaction temperature, and reaction time were investigated by single and orthogonal experiments. A solid waste extraction procedure for leaching toxicity-acetic acid buffer solution method (HJ/T 3002007) was adopted to detect the toxicity of raw fly ash and the hydrothermal products. The effect of pH on the leaching test of raw and treated fly ash was also carried out. Characteristics of fly ash were determined by XRF and XRD, and the leaching concentration was determined by ICP-MS. Experimental results revealed that the stabilization of heavy metals in fly ash was facilitated by a microwave-assisted hydrothermal process and, except for Cd, the regulatory limits were achieved when fly ash was treated with 1 mol/L NaOH and a liquid-to-solid ratio of 3.5 mL/g at 125 °C for 20 min of microwave-assisted hydrothermal heating. It was further concluded that the significance of factors was in the order of reagent > concentration ≈ temperature > time > L/S ratio (10−30 mL/g). More zeolites were formed over 20 min, which confirmed the high efficiency of the microwaveassisted hydrothermal treatment. In pH experiments, it was found that the safe pH range of treated fly ash was broadened from 7.5 to 11 to 6.3−13, which led to better environmental adaptability. The microwave-assisted hydrothermal treatment may be applied to harmlessly manage the MSWI fly ash or to recover and utilize MSWI fly ash in a high efficiency, energy saving way compared to traditional hydrothermal treatment. ash17 and MSWI fly ash.7,18,19 Hydrothermal process has been successfully applied for the reason that it can convert fly ash or other hazardous materials into more stable forms by stabilization20 or extraction.21 Aluminum silicate was found after hydrothermal treatment in alkaline solution, which contributed to the stabilization of fly ash and an increase of the residual form.7 Zhang et al.21 reported that, with HCl added, most heavy metals were dissolved in solution from fly ash during the hydrothermal process. However, compared to cement and chemical solidification, the energy consumption of the hydrothermal process is pretty high because several, even dozens, of hours heating at more than 150 °C are needed.20 For large-scale application in our and other developing countries to be realized, the economy of this technology is still inadequate. For overcoming this difficulty, microwaves are put forward in this study. Microwaves are electromagnetic waves whose frequency varies within 300 MHz to 300 GHz and can be absorbed and converted to heat by dipolar materials and ionic mechanisms, such as moisture or water.22 Microwave heating has the advantages of energy-saving rapid heating rates, short treating times, clean heating processes, and deep penetration.23 Currently, microwaves have been applied on coal fly ash to produce zeolites, such as nano-NaX zeolite,24 Na−P1, hydroxysodalite, analcime, tobermorite, kalsilite, phillipsite, and so forth.25−28 Research25 has concluded that more zeolites
1. INTRODUCTION Incineration is playing an increasingly important role in municipal solid waste (MSW) management with the development of economy. According to published statistics, there were 166 MSW incineration (MSWI) plants in 2013 with an annual total incineration capacity of 46 million tons, which accounted for as high as 30.01% of the total MSW treatment.1 Whereas in 2004, there were only 54 plants.2 Obviously, the use of MSWI treatment is increasing; thus, so is the output of fly ash from MSWI. Thus, it can be calculated that the annual output of fly ash in a circulating fluidized bed (CFB) is at least 2.1 million tons (0.058 million tons garbage per day) when the fly ash/ waste ratio is 10%. Fly ash has been regarded as a kind of hazardous waste that contains many harmful components like heavy metals, dioxins, and furans.3−6 For this reason, special management is required before disposal in landfills or secondary use according to stringent regulations.7 Compared with other countries, fly ash disposal technology in China is relative backwards with cement solidification4,8 and chemical stabilization9−11 being the main methods. However, as a result of shortcomings, such as disappointing solidification effect, increased landfill volume, secondary pollution,12−14 and so forth, these traditional methods have not be widely and popularly applied. Moreover, thermal treatment is too costly to be practiced on a large scale in developing countries. It is a growing trend that hydrothermal processes will become a promising technology for fly ash disposal because it offers considerable merits in terms of economic, technical, and environmental effectiveness.15,16 Furthermore, it has been successfully applied to fly ash in alkali solutions, both coal fly © 2016 American Chemical Society
Received: March 7, 2016 Revised: May 23, 2016 Published: June 24, 2016 5945
DOI: 10.1021/acs.energyfuels.6b00547 Energy Fuels 2016, 30, 5945−5952
Article
Energy & Fuels
2 h at a speed of 30 ± 2 rpm. Afterward, the leachate samples were filtered through a 0.6−0.8 μm borosilicate glass fiber filter.30 Finally, the filtrate was analyzed using ICP-MS to determine the concentration of heavy metals. 2.5. Leaching Test at Different pH Levels of Raw and Treated Fly Ash. Because pH has a significant effect on leaching concentration of heavy metals in fly ash, the experiments of leaching concentration at various pH levels were conducted. Leaching solutions with different H+ concentrations were prepared by HNO3 and NaOH. Other steps were the same as in section 2.4. After the leaching test, the pH and leaching concentration of the filtrate were both determined by a pH meter (Mettler Toledo) and ICP-MS.
would be obtained in 1 h with microwave-assisted hydrothermal treatment on fly ash than in 48 h with traditional hydrothermal treatment. The temperature reaches 373 K rapidly in 3 min by microwave heating, whereas conventional heating requires over 15 min.26 It is thus clear that microwave heating is more efficient for both time and energy. To the best of our knowledge, little research on microwave-assisted hydrothermal solidification of MSWI fly ash has been reported. Therefore, in this paper, instead of conventional heating, microwave heating for hydrothermal treatment was used to solidify the heavy metals of MSWI fly ash in a CFB. The objectives of our work were to investigate a rapid and efficient way to solidify MSWI fly ash and determine the optimal conditions with commonly used mixed alkaline solutions. Our work is expected to gain basic research results on solidifying MSWI fly ash by microwave-assisted hydrothermal treatment for future large-scale applications.
3. RESULTS AND DISCUSSION 3.1. Characteristics of Raw Fly Ash. The chemical and heavy metal compositions of raw fly ash determined by XRF and ICP-MS are listed in Table 1. The major elements of the Table 1. Chemical Composition of Original Fly Ash
2. MATERIALS AND METHODS 2.1. Materials. Several amounts of MSWI fly ash were collected from the bag house of a CFB boiler at the cixi incinerator plant with a total daily capacity of 800 tons of directly combusted MSW. It is equipped with an air pollution control (APC) system, consisting of a selective noncatalytic reduction (SNCR) denitration, semidry scrubber, activated carbon injection, and fabric filter. Fly ash was preliminarily dried at 105 °C in an oven for 24 h and then analyzed for its major elements, morphology, mineralogical analysis, and leaching concentration of heavy metals to determine its nature before and after each microwave-assisted hydrothermal process. 2.2. Reagents and Apparatus. Acetic acid (CH3COOH), sodium hydrate (NaOH), potassium hydrate (KOH), sodium carbonate (Na2CO3), and potassium carbonate (K2CO3) were all of reagent grade. Deionized water was used to dilute and prepare solutions. The chemical compositions were determined by an X-ray fluorescence (XRF) spectrometer (Thermo Fisher, Intelli Power 4200). The morphology was performed by X-ray diffraction (XRD, Rigaku Rotaflex) on the condition that Cu Kα radiation was set at 40 kV and 250 mA. The leaching concentration of heavy metals in extracted solution was determined by inductively coupled plasma mass spectrometry (ICP−MS, Thermo Scientific XII). A microwave apparatus (Sineo MDS-6) was used as the microwave radiation source to study the leaching behavior of heavy metals from fly ash after being heated in a 2.45 GHz microwave field. 2.3. Microwave-Assisted Hydrothermal Process. For each trial, 1 g of fly ash sample was added to different solutions (NaOH, KOH, Na2CO3, K2CO3, H2O) of various concentrations (0.5−2.5 M) and volumes (liquid/solid ratio, 10−30 mL/g) in 100 mL of polytetrafluoroethylene (PTEE) in a closed container and heated to various temperatures (100−200 °C) in a microwave apparatus for various amounts of time (10−50 min). After the microwave-assisted hydrothermal treatment, the reactant was cooled to atmospheric conditions. Then, the reactant was centrifuged to separate the solid and liquid, and the separated solid was dried at 105 °C for 24 h, which is referred to as “treated fly ash”. Both single factor and orthogonal experiments were conducted. 2.4. Leaching Test for Heavy Metals. The fly ash disposal of landfill materials has caused groundwater pollution from landfill leachate, odor emission, and soil contamination.29 Therefore, it is necessary to assess the possibility of landfilling within regulatory limits through a leaching test.8 The progress that pollution (heavy metals) spreads to the environment from fly ash can be simulated by the leaching test. In this study, a solid waste extraction procedure for leaching toxicity-acetic acid buffer solution method (HJ/T300-2007) was adopted. Original and treated fly ash were leached using an extraction buffer of acetic acid and sodium hydroxide (pH 2.64 ± 0.05) at a liquid/solid ratio of 20:1. Then, the mixed liquor was shaken for 18 ±
chemical composition
original fly ash (wt %)
heavy metal
original fly ash (mg/kg)
detection limit (mg/L)
Ca Si C Al Cl Mg Fe Na S P K
20.27 10.14 7.75 7.09 9.98 2.10 4.30 3.02 2.19 1.43 2.36
As Ba Be Cd Cr Cu Hg Ni Pb Se Zn
10.35 1640 1.02 44.14 570.8 2794 temperature > concentration > time > L/S ratio (10− 30 mL/g). Thus, optimal chemical agents play a vital role in treatment. The influence of other factors in orthogonal experiments with optimal reagents were the same as the results in single factor experiments. Nevertheless, it is unfortunate that the limit of the Cd leaching concentration was not achieved in all 25 trials. 3.3.2. Effect of Chemical Additives. The leaching concentration of heavy metals after the microwave-assisted hydrothermal treatment with different additives (1 mol/L NaOH, KOH, Na2CO3, K2CO3, and deionized water) is shown in Figure 3. Experiments were carried out using 1 g of dried fly ash with an L/S ratio of 15 mL/g at 125 °C for 20 min of microwave heating. According to the total curing rates of heavy metals in Figure 3, the curing ability is in the order of NaOH > KOH > K2CO3 > Na2CO3 > H2O. KOH almost had the same effect as NaOH on curing heavy metals except for Cr. Among these listed metals, Cd concentration exceeded the regulatory limit of 0.15 mg/L for the leaching test, whereas the others were all within the limits with NaOH added. Considering the application of alkaline reagents in the hydrothermal process with previous studies,20,27,34−36 the same results were obtained, and NaOH reagent was popularly selected to promote the hydrothermal process. NaOH, as a strongly alkaline mineralizer agent, makes more silicon and aluminum available with the forms of various [SiO3]2− and [Al(OH)4]−, which contributes to the destruction of the original structure and the formation of crystals in the hydrothermal process and benefits the solidification of heavy metals.20,27 A study15 has shown that, under the same conditions, more zeolite will be produced when KOH is replaced by NaOH. Maybe this is the reason that NaOH has a better effect than KOH during the hydrothermal process. Moreover, it is worth mentioning that the leaching result of Cd with Na2CO3 added was higher than that of raw fly ash. In our study, the dosage of Na2CO3 was relatively high because the L/ S ratio in this section was 15 mL/g. That is, the Na2CO3/fly ash mass ratio was as high as 1.59 g/g, which led to high OH− and CO32− concentrations in the solution and an unstable status of Cd combining with OH− and CO32−.37 On the other hand, high Na+ concentration was apt to form materials (zeolites) with high cation exchange capacity38 and expedite the crystallization rate.39 Therefore, under the action of multiple factors and the specificity of fly ash samples in this paper, the solidification behavior of Na2CO3 was not in accordance with that of other alkaline reagents. 3.3.3. Effect of Reagent Concentration of NaOH. The leaching concentrations of heavy metals after the microwaveassisted hydrothermal treatment with various concentrations of NaOH are presented in Figure 4. The concentration of most heavy metals in the leaching test was reduced as the concentration of NaOH increased, including Zn, Cu, Cd, Ni, and Pb. For instance, the Zn concentration was 63.36 mg/L with 0.5 mol/L NaOH added, and the concentration was reduced to 14.57 mg/L with 2.5 mol/L NaOH added. The hydrothermal reaction is mainly determined by the solubility of
Figure 1. XRD patterns of original fly ash.
Table 2. Leaching Toxicity of MSWI Fly Ash element
leaching concentration in the leachate (mg/L)
limitations required in standard32 (mg/L)
detection limit (mg/L)
Cd Cu Ni Pb Se Zn Cr As Be Ba Hg
1.948 64.787 1.521 7.851 0.255 124.498 2.213 0.179 time > L/S ratio (10−30 mL/ g). (5) More trials need to be carried out to solidify Cd in fly ash, and the treated fly ash can be reused in the construction industry.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 571 87952775. Fax: +86 571 87952438. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Basic Research and Development Program of China (973 Program, 2011CB201500), the National High Technology Research and Development Program of China (863 Program, 2012AA063505), the Special Fund for National Environmental Protection Public Welfare Program (Grant 201209023-4), and the Program of Introducing Talents of Discipline to University (Grant B08026).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b00547 Energy Fuels 2016, 30, 5945−5952
