Waste Management 34 (2014) 893–900
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Synthesis of mesoporous silica materials from municipal solid waste incinerator bottom ash Zhen-Shu Liu ⇑, Wen-Kai Li, Chun-Yi Huang Department of Safety, Health and Environmental Engineering, Ming Chi University of Technology, Taishan District, New Taipei City 24301, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 20 November 2013 Accepted 4 February 2014 Available online 18 March 2014 Keywords: Recycling Bottom ash Mesoporous silica Adsorption Heavy metal
a b s t r a c t Incinerator bottom ash contains a large amount of silica and can hence be used as a silica source for the synthesis of mesoporous silica materials. In this study, the conditions for alkaline fusion to extract silica from incinerator bottom ash were investigated, and the resulting supernatant solution was used as the silica source for synthesizing mesoporous silica materials. The physical and chemical characteristics of the mesoporous silica materials were analyzed using BET, XRD, FTIR, SEM, and solid-state NMR. The results indicated that the BET surface area and pore size distribution of the synthesized silica materials were 992 m2/g and 2–3.8 nm, respectively. The XRD patterns showed that the synthesized materials exhibited a hexagonal pore structure with a smaller order. The NMR spectra of the synthesized materials exhibited three peaks, corresponding to Q2 [Si(OSi)2(OH)2], Q3 [Si(OSi)3(OH)], and Q4 [Si(OSi)4]. The FTIR spectra confirmed the existence of a surface hydroxyl group and the occurrence of symmetric Si–O stretching. Thus, mesoporous silica was successfully synthesized from incinerator bottom ash. Finally, the effectiveness of the synthesized silica in removing heavy metals (Pb2+, Cu2+, Cd2+, and Cr2+) from aqueous solutions was also determined. The results showed that the silica materials synthesized from incinerator bottom ash have potential for use as an adsorbent for the removal of heavy metals from aqueous solutions. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incinerator bottom ash is being recycled in a number of advanced countries nowadays. For instance, the average recycling rate of incinerator bottom ash is approximately 60% in European countries. Furthermore, several countries are developing incinerator bottom ash recycling techniques whose output can be used for a number of alternative purposes. These include making roadbed bottoms and other civil engineering structure and as soil amendments, soundproof wall fillers, building materials, and cover material for parking lot pavements and landfills (Wiles, 1996; Hjelmar, 1996; Chimenos et al., 2000; Lin and Lin, 2006). In Taiwan, most of the municipal solid waste is incinerated. According to the statistics provided by the Taiwan Environmental Protection Administration, the amount of waste treated by incineration was 5.962 million MT in 2011. This resulted in the production of 1.237 million MT of ash, including 0.969 million MT of bottom ash and 0.268 million MT of fly ash. At present, Taiwan’s incinerator bottom ash recycling rate is approximately 54%, with 46% of all bottom ash being sent to landfills. Therefore, new recycling approaches and techniques need ⇑ Corresponding author. Tel.: +886 2 29089899x4698; fax: +886 2 29041914. E-mail address: [email protected] (Z.-S. Liu). http://dx.doi.org/10.1016/j.wasman.2014.02.016 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
to be developed for increasing the recycling rate of incinerator bottom ash and for using it in alternative applications. In 1992, scientists from Mobil Corporation developed the M41S family of materials. These materials exhibit a uniform pore distribution (2–50 nm), high specific surface area (1000 m2/g), and good thermal and hydrothermal stabilities (Kresge et al., 1992; Beck et al., 1992; Beck and Vartuli, 1996). They have since been used extensively as catalysts and for adsorbing organic compounds and removing heavy metals from wastewater. The main raw material for synthesizing these mesoporous silica materials is silica, which is derived from inorganic silicates (such as sodium silicate) as well as organic silicates (such as tetraethyl orthosilicate). However, silicates are expensive and increase the cost of mesoporous silica materials when used as raw materials. Therefore, it is essential that substitute silica sources are found. Incinerator bottom ash contains a large amount of silica, which can be used as the raw material for synthesizing mesoporous silica materials. This would not only increase the recycling rate of bottom ash but also reduce the cost of mesoporous silica materials. Höller and Wirsching (1985) were the first to use an alkaline hydrothermal reaction to extract SiO2 and Al2O3 from coal fly ash from a coal power plant. They accomplished this by dissolving the fly ash in a NaOH. They then used the extracted SiO2 and
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Al2O3 as silica and aluminum sources, respectively, for synthesizing microporous molecular sieves. However, the purity and yield rate of the thus-synthesized microporous molecular sieves were low. Chang et al. (1999) used C16H33(CH3)3NBr as a surfactant, and extracted silica and aluminum sources from the fly ash by alkaline fusion. They demonstrated that the mesoporous silica material MCM-41 could be synthesized by ensuring that the Si/Al ratio in the supernatant solution was 13.4. Many subsequent studies have also used waste ash to extract silica and synthesized microporous or mesoporous silica materials without using a pure silica source, resulting in improvement in ash recycling (Kumar et al., 2001; Misran et al., 2007; Hui and Chao, 2006; Halina et al., 2007; Chandrasekar et al., 2008; Ye et al., 2008; Apiratikul and Pavasant, 2008; Dhokte et al., 2011; Wu et al., 2012; Liu et al., 2013). However, these studies have mostly involved the extraction of silica from coal fly ash. On the other hand, there have not been many studies investigating the possibility of using incinerator bottom ash for the synthesis of mesoporous silica materials. Alkaline fusion is the process most commonly used to extract silica from coal fly ash. However, the alkaline fusion conditions for extracting silica from incinerator bottom ash are not known. Therefore, incinerator bottom ash has been rarely used to synthesize mesoporous silica materials (Chiang et al., 2012). In this study, we focused on the conditions of the alkaline fusion process for extracting silica from incinerator bottom ash, using the resultant supernatant solution as the silica source for synthesizing mesoporous silica materials. The alkaline fusion conditions investigated were the following: (1) the alkaline fusion temperature (700 or 900 °C), (2) the alkaline agent type (NaOH, Na2CO3, or LiBO2), (3) the weight ratio of bottom ash to the alkaline agent, and (4) the solid-to-liquid ratio of the alkaline-fused reactant dissolved in its aqueous solution. The suitability of the synthesized mesoporous silica materials for the removal of heavy metals (Pb2+, Cu2+, Cd2+, and Cr2+) from aqueous solutions was also determined. 2. Material and methods 2.1. Extraction of silica from incinerator bottom ash The bottom ash used in this study was collected directly from an incinerator located in Taiwan at the following geographical coordinates: latitude of 24°580 19.7800 N and longitude of 121°230 30.190 0 E. The bottom ash was dried in an oven at 105 °C and then crushed, screened, and sieved. For the alkaline fusion process, only bottom ash with particles less than 0.35 mm in size was used. The ash was mixed uniformly with the alkaline agent (NaOH, Na2CO3, or LiBO2) and then placed in a high-temperature furnace for 15 min; the furnace heating rate was 20 °C/min. The products formed after the completion of the alkaline fusion process were dissolved in water and placed in an oven for 105 °C for 24 h and hydrolyzed. Subsequently, the solution was filtered. The resultant supernatant, which was collected, contained silica; the ash that collected on the filter is referred to as desilicated ash. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Optima 2100DV, Perkin Elmer) was used to determine the silica content of the supernatant solution. The supernatant solution with the optimal silica concentration was used for the synthesis of mesoporous silica materials. Table 1 lists the alkaline fusion conditions and the silica content of the various supernatant solutions. 2.2. Synthesis of mesoporous silica materials Deionized water (20 g) was added to 1.2 g of cetyltrimethyl ammonium bromide (CTAB), and the two were mixed at 40 °C until
a clear solution was obtained. The aqueous CTAB solution was slowly introduced into the silica-containing supernatant solution. The pH value of the mixture of the CTAB and supernatant solutions was 11–12. Subsequently, 1 M sulfuric acid was introduced slowly into the mixture until the pH value became 10, and the mixture was mixed for 3 h. Finally, the resulting solution was poured into a polypropylene (PP) bottle, which was placed in an oven at 105 °C for 48 h to allow the hydrothermal reaction to occur. It was then allowed to cool to room temperature and filtrated. The shape of the PP bottle was cylindrical, and its height and diameter were 23.5 cm and 11 cm, respectively. The filtered solid was dried overnight in an oven at 105 °C and then placed in a high-temperature furnace at 550 °C for 6 h. The calcination of the filtered solid yielded mesoporous silica. In order to determine the differences between the mesoporous silica synthesized from bottom ash and that synthesized from pure silica, we also used sodium metasilicate as a pure silica source to synthesize mesoporous materials. The hexagonal mesoporous silica (HMS) synthesized from sodium metasilicate and that synthesized from bottom ash are referred to as MCM-41 and HMS-BA, respectively. To synthesize MCM-41, first, 6 g of sodium metasilicate was added to 75 g of deionized water and dissolved it at 40 °C. The surfactant solution was prepared by first dissolving 3 g of CTAB in 17 g of deionized water. Subsequently, the CTAB solution was introduced slowly into the sodium metasilicate solution: the pH value of the mixture was 12. Then, 1 M sulfuric acid was introduced slowly into the solution until the pH value became 10, and the solution was mixed for 3 h. The rest of the steps involved in the synthesis of MCM-41 were the same as those for HMS-BA.
2.3. Characterization The Brunauer–Emmett–Teller (BET) surface areas and average pore diameters of the mesoporous silica materials were determined from their N2 adsorption–desorption isotherms, which were obtained at 77 K using an ASAP2020 vacuum volumetric sorption instrument. Before the N2 sorption analysis, the samples were preheated to 473 K for degassing and cooled to room temperature under vacuum. The micropore volumes (pore size: Q2, implying that the primary phase of the surface structure for both MCM-41 and HMS-BA was that corresponding to the Q4 peak, namely highly polymerized Si(SiO4) or quartz. These results were also in agreement with the findings of previous studies on the synthesis of MCM-41 from coal fly ash (Halina et al., 2007).
3.2.5. SEM analysis The SEM micrographs of MCM-41 and HMS-BA, shown in Fig. 8, revealed that the morphologies of the two materials were different. MCM-41 exhibited large, monolithic aggregates. In contrast, HMSBA showed aggregations consisting of smaller particles. A comparison of the physical properties of HMS and MCM-41 by Tanev and Pinnavaia (1996) had yielded similar results.
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3.3. Removal of heavy metals Fig. 9 shows the efficiencies of HMS-BA in removing Pb, Cu, Cd, and Cr from aqueous solutions; the efficiencies were 34.2%, 9.0%, 17.1%, and 51.6%, respectively. This result suggested that HMS-BA (which did not have any grafted functional groups) was more effective in removing heavy metals than was the MCM-41 sample prepared from coal fly ash (Hui and Chao, 2006). A number of studies have shown that the adsorption capacity of metal ions with respect to MCM-41 can be increased significantly by modifying MCM-41 (Lee and Yi, 2001; Bois et al., 2003; Kim et al., 2003; Kim and Yi, 2004). The obtained results suggest that, after being modified, HMS-BA ash has potential for use as an adsorbent for the removal of heavy metals from aqueous solutions. 4. Conclusions The synthesis of mesoporous silica materials from a supernatant solution of bottom ash was investigated in this study. The findings showed that the previously reported alkaline fusion and hydrolysis conditions for the extraction of silica from coal fly ash were not applicable in the case of incinerator bottom ash. The optimal alkaline agent for the extraction of silica from incinerator bottom ash was Na2CO3. The BET surface area and the mesoporous volume (992 m2/g and 0.854 m3/g, respectively) of the materials synthesized from bottom ash were slightly lower than those of the mesoporous silica materials obtained from a pure silica source (1078 m2/g and 0.918 m3/g, respectively); this was owing to the incorporation of trace amounts of other metals from the bottom ash into the silica matrix. XRD analysis showed that the spectrum of the mesoporous silica materials synthesized from bottom ash exhibited only one peak—that attributable to the (1 0 0) plane— indicating a hexagonal pore structure with a smaller order. The NMR spectra of the mesoporous silica materials synthesized from both bottom ash and from a pure silica source exhibited three peaks, namely, those corresponding to Q2 [Si(OSi)2(OH)2], Q3 [Si(OSi)3(OH)], and Q4 [Si(OSi)4]. However, the Q3 and Q4 peaks for the mesoporous silica materials synthesized from bottom ash were broader than those of the materials synthesized from the pure silica source. The mesoporous silica materials synthesized from bottom ash have potential for use as an adsorbent for removing heavy metal from aqueous solutions after being modified. Acknowledgement The authors thank the National Science Council of the Republic of China (Taiwan) for financially supporting this research under Contract No. NSC 101-2221-E-131-028-MY3. References Apiratikul, R., Pavasant, P., 2008. Sorption of Cu2+, Cd2+, and Pb2+ using modified zeolite from coal fly ash. Chem. Eng. J. 144, 245–258. Beck, J.S., Vartuli, J.C., 1996. Recent advances in the synthesis, characterization and applications of mesoporous molecular sieves. Curr. Opin. Solid State Mater. Sci. 1, 76–87. Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T.-W., Olson, D.H., Sheppard, E.W., 1992. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 114, 10834–10843. Bois, L., Bonhomme, A., Ribes, A., Pais, B., Raffin, G., Tessier, F., 2003. Functionalized silica for heavy metal ions adsorption. Colloid Surf. A Physicochem. Eng. Aspect 221, 221–230. Chandrasekar, G., You, K.S., Ahn, J.W., Ahn, W.S., 2008. Synthesis of hexagonal and cubic mesoporous silica using power plant bottom ash. Micropor. Mesopor. Mater. 111, 455–462. Chang, H.L., Chun, C.M., Aksay, I.L., Shih, W.H., 1999. Conversion of fly ash into mesoporous aluminosilicate. Ind. Eng. Chem. Res. 38, 973–977. Chen, Y.W., Wang, W.J., 2003. Synthesis and characteristics of hexa-mesoporous silica: a review. J. Petrol. 39, 17–36.
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Chiang, Y.W., Ghyselbrecht, K., Santos, R.M., Meesschaert, B., Martens, J.A., 2012. Synthesis of zeolitic-type adsorbent material from municipal solid waste incinerator bottom ash and its application in heavy metal adsorption. Catal. Today 190, 23–30. Chimenos, J.M., Segarra, M., Fernandez, M.A., Espiell, F., 2000. Short-term natural weathering of MSWI bottom ash. J. Hazard. Mater. B79, 287–299. Dhokte, A.Q., Khillare, S.L., Lande, M.K., Arbad, B.R., 2011. Synthesis, characterization of mesoporous silica materials from waste coal fly ash for the classical Mannich reaction. J. Ind. Eng. Chem. 17, 742–746. Gregg, S.J., Sing, K.S.W., 1982. Adsorption Surface Area and Porosity. Academic Press, London. Halina, M., Ramesh, S., Yarmo, M., Kamarudin, R., 2007. Non-hydrothermal synthesis of mesoporous materials using sodium silicate from coal fly ash. Mater. Chem. Phys. 101, 344–351. Hjelmar, O., 1996. Disposal strategies for municipal solid waste incineration residues. J. Hazard. Mater. 47, 345–368. Höller, H., Wirsching, U., 1985. Zeolites formation from fly ash. Fortschr. Miner. 63, 21–43. Hui, K., Chao, C., 2006. Synthesis of MCM-41 from coal fly ash by a green approach: influence of synthesis pH. J. Hazard. Mater. 137, 1135–1148. Kim, Y., Yi, J., 2004. Advances in environmental technologies via the application of mesoporous materials. J. Ind. Eng. Chem. 10, 41–51. Kim, Y., Lee, B., Yi, J., 2003. Preparation of functionalized mesostructured silica containing magnetite (MSM) for the removal of copper ions in aqueous solutions and its magnetic separation. Sep. Sci. Technol. 38, 2533–2548. Kiseler, A.V., Lygin, V.I., 1992. Infrared Spectra of Surface Compounds and Adsorbed Substances, first ed. Nauka, Moscow. Kresge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C., Beck, J.S., 1992. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359, 710–712. Kumar, P., Mal, N., Oumi, Y., Yamana, K., Sano, T., 2001. Mesoporous materials prepared using coal fly ash as the silicon and aluminum source. J. Mater. Chem. 11, 3285–3290.
Lee, H., Yi, J.H., 2001. Removal of copper ions using functionalized mesoporous silica in aqueous solution. Sep. Sci. Technol. 36, 2433–2448. Lin, K.L., Lin, D.F., 2006. Hydration characteristics of municipal solid waste incinerator bottom ash slag as a pozzolanic material for use in cement. Cem. Concr. Compos. 28, 817–823. Liu, M.M., Hou, L.A., Xi, B.D., Zhao, Y., Xia, X.F., 2013. Synthesis, characterization, and mercury adsorption properties of hybrid mesoporous aluminosilicate sieve prepared with fly ash. Appl. Surf. Sci. 273, 706–716. Misran, H., Singh, R., Begum, S., Yarmo, M.A., 2007. Processing of mesoporous silica materials (MCM-41) from coal fly ash. J. Mater. Process. Technol. 186, 8–13. Romero, A.A., Alba, M.D., Zhou, W.Z., Klinowski, J., 1997. Synthesis and characterization of the mesoporous silicate molecular sieve MCM-48. J. Phys. Chem. B 101, 5294–5300. Ryu, Z., Zheng, J., Wang, M., Zhang, B., 1999. Characterization of pore size distributions on carbonaceous adsorbents by DFT. Carbon 37, 1257–1264. Takahashi, R., Sato, S., Sodesawa, T., Kawakita, M., Ogura, K., 2000. High surface-area silica with controlled pore size prepared from nanocomposite of silica and citric acid. J. Phys. Chem. B 104, 12184–12191. Tanev, P.T., Pinnavaia, T.J., 1996. Mesoporous silicas molecular sieves prepared by ionic and neutral surfactant templating: a comparison of physical properties. Chem. Mater. 8, 2068–2079. Wiles, C.C., 1996. Municipal solid waste combustion ash: sate-of-the-knowledge. J. Hazard. Mater. 47, 325–344. Wu, X.W., Ma, H.W., Zhang, L.T., Wang, F.J., 2012. Adsorption properties and mechanism of mesoporous adsorbents prepared with fly ash for removal of Cu(II) in aqueous solution. Appl. Surf. Sci. 261, 902–907. Ye, Y.P., Zeng, X.Q., Qian, W.L., Wang, M.W., 2008. Synthesis of pure zeolites from supersaturated silicon and aluminum alkali extracts from fused coal fly ash. Fuel 87, 1880–1886.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Synthesis of mesoporous silica materials from municipal solid waste incinerator bottom ash Zhen-Shu Liu ⇑, Wen-Kai Li, Chun-Yi Huang Department of Safety, Health and Environmental Engineering, Ming Chi University of Technology, Taishan District, New Taipei City 24301, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 20 November 2013 Accepted 4 February 2014 Available online 18 March 2014 Keywords: Recycling Bottom ash Mesoporous silica Adsorption Heavy metal
a b s t r a c t Incinerator bottom ash contains a large amount of silica and can hence be used as a silica source for the synthesis of mesoporous silica materials. In this study, the conditions for alkaline fusion to extract silica from incinerator bottom ash were investigated, and the resulting supernatant solution was used as the silica source for synthesizing mesoporous silica materials. The physical and chemical characteristics of the mesoporous silica materials were analyzed using BET, XRD, FTIR, SEM, and solid-state NMR. The results indicated that the BET surface area and pore size distribution of the synthesized silica materials were 992 m2/g and 2–3.8 nm, respectively. The XRD patterns showed that the synthesized materials exhibited a hexagonal pore structure with a smaller order. The NMR spectra of the synthesized materials exhibited three peaks, corresponding to Q2 [Si(OSi)2(OH)2], Q3 [Si(OSi)3(OH)], and Q4 [Si(OSi)4]. The FTIR spectra confirmed the existence of a surface hydroxyl group and the occurrence of symmetric Si–O stretching. Thus, mesoporous silica was successfully synthesized from incinerator bottom ash. Finally, the effectiveness of the synthesized silica in removing heavy metals (Pb2+, Cu2+, Cd2+, and Cr2+) from aqueous solutions was also determined. The results showed that the silica materials synthesized from incinerator bottom ash have potential for use as an adsorbent for the removal of heavy metals from aqueous solutions. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incinerator bottom ash is being recycled in a number of advanced countries nowadays. For instance, the average recycling rate of incinerator bottom ash is approximately 60% in European countries. Furthermore, several countries are developing incinerator bottom ash recycling techniques whose output can be used for a number of alternative purposes. These include making roadbed bottoms and other civil engineering structure and as soil amendments, soundproof wall fillers, building materials, and cover material for parking lot pavements and landfills (Wiles, 1996; Hjelmar, 1996; Chimenos et al., 2000; Lin and Lin, 2006). In Taiwan, most of the municipal solid waste is incinerated. According to the statistics provided by the Taiwan Environmental Protection Administration, the amount of waste treated by incineration was 5.962 million MT in 2011. This resulted in the production of 1.237 million MT of ash, including 0.969 million MT of bottom ash and 0.268 million MT of fly ash. At present, Taiwan’s incinerator bottom ash recycling rate is approximately 54%, with 46% of all bottom ash being sent to landfills. Therefore, new recycling approaches and techniques need ⇑ Corresponding author. Tel.: +886 2 29089899x4698; fax: +886 2 29041914. E-mail address: [email protected] (Z.-S. Liu). http://dx.doi.org/10.1016/j.wasman.2014.02.016 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
to be developed for increasing the recycling rate of incinerator bottom ash and for using it in alternative applications. In 1992, scientists from Mobil Corporation developed the M41S family of materials. These materials exhibit a uniform pore distribution (2–50 nm), high specific surface area (1000 m2/g), and good thermal and hydrothermal stabilities (Kresge et al., 1992; Beck et al., 1992; Beck and Vartuli, 1996). They have since been used extensively as catalysts and for adsorbing organic compounds and removing heavy metals from wastewater. The main raw material for synthesizing these mesoporous silica materials is silica, which is derived from inorganic silicates (such as sodium silicate) as well as organic silicates (such as tetraethyl orthosilicate). However, silicates are expensive and increase the cost of mesoporous silica materials when used as raw materials. Therefore, it is essential that substitute silica sources are found. Incinerator bottom ash contains a large amount of silica, which can be used as the raw material for synthesizing mesoporous silica materials. This would not only increase the recycling rate of bottom ash but also reduce the cost of mesoporous silica materials. Höller and Wirsching (1985) were the first to use an alkaline hydrothermal reaction to extract SiO2 and Al2O3 from coal fly ash from a coal power plant. They accomplished this by dissolving the fly ash in a NaOH. They then used the extracted SiO2 and
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Al2O3 as silica and aluminum sources, respectively, for synthesizing microporous molecular sieves. However, the purity and yield rate of the thus-synthesized microporous molecular sieves were low. Chang et al. (1999) used C16H33(CH3)3NBr as a surfactant, and extracted silica and aluminum sources from the fly ash by alkaline fusion. They demonstrated that the mesoporous silica material MCM-41 could be synthesized by ensuring that the Si/Al ratio in the supernatant solution was 13.4. Many subsequent studies have also used waste ash to extract silica and synthesized microporous or mesoporous silica materials without using a pure silica source, resulting in improvement in ash recycling (Kumar et al., 2001; Misran et al., 2007; Hui and Chao, 2006; Halina et al., 2007; Chandrasekar et al., 2008; Ye et al., 2008; Apiratikul and Pavasant, 2008; Dhokte et al., 2011; Wu et al., 2012; Liu et al., 2013). However, these studies have mostly involved the extraction of silica from coal fly ash. On the other hand, there have not been many studies investigating the possibility of using incinerator bottom ash for the synthesis of mesoporous silica materials. Alkaline fusion is the process most commonly used to extract silica from coal fly ash. However, the alkaline fusion conditions for extracting silica from incinerator bottom ash are not known. Therefore, incinerator bottom ash has been rarely used to synthesize mesoporous silica materials (Chiang et al., 2012). In this study, we focused on the conditions of the alkaline fusion process for extracting silica from incinerator bottom ash, using the resultant supernatant solution as the silica source for synthesizing mesoporous silica materials. The alkaline fusion conditions investigated were the following: (1) the alkaline fusion temperature (700 or 900 °C), (2) the alkaline agent type (NaOH, Na2CO3, or LiBO2), (3) the weight ratio of bottom ash to the alkaline agent, and (4) the solid-to-liquid ratio of the alkaline-fused reactant dissolved in its aqueous solution. The suitability of the synthesized mesoporous silica materials for the removal of heavy metals (Pb2+, Cu2+, Cd2+, and Cr2+) from aqueous solutions was also determined. 2. Material and methods 2.1. Extraction of silica from incinerator bottom ash The bottom ash used in this study was collected directly from an incinerator located in Taiwan at the following geographical coordinates: latitude of 24°580 19.7800 N and longitude of 121°230 30.190 0 E. The bottom ash was dried in an oven at 105 °C and then crushed, screened, and sieved. For the alkaline fusion process, only bottom ash with particles less than 0.35 mm in size was used. The ash was mixed uniformly with the alkaline agent (NaOH, Na2CO3, or LiBO2) and then placed in a high-temperature furnace for 15 min; the furnace heating rate was 20 °C/min. The products formed after the completion of the alkaline fusion process were dissolved in water and placed in an oven for 105 °C for 24 h and hydrolyzed. Subsequently, the solution was filtered. The resultant supernatant, which was collected, contained silica; the ash that collected on the filter is referred to as desilicated ash. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Optima 2100DV, Perkin Elmer) was used to determine the silica content of the supernatant solution. The supernatant solution with the optimal silica concentration was used for the synthesis of mesoporous silica materials. Table 1 lists the alkaline fusion conditions and the silica content of the various supernatant solutions. 2.2. Synthesis of mesoporous silica materials Deionized water (20 g) was added to 1.2 g of cetyltrimethyl ammonium bromide (CTAB), and the two were mixed at 40 °C until
a clear solution was obtained. The aqueous CTAB solution was slowly introduced into the silica-containing supernatant solution. The pH value of the mixture of the CTAB and supernatant solutions was 11–12. Subsequently, 1 M sulfuric acid was introduced slowly into the mixture until the pH value became 10, and the mixture was mixed for 3 h. Finally, the resulting solution was poured into a polypropylene (PP) bottle, which was placed in an oven at 105 °C for 48 h to allow the hydrothermal reaction to occur. It was then allowed to cool to room temperature and filtrated. The shape of the PP bottle was cylindrical, and its height and diameter were 23.5 cm and 11 cm, respectively. The filtered solid was dried overnight in an oven at 105 °C and then placed in a high-temperature furnace at 550 °C for 6 h. The calcination of the filtered solid yielded mesoporous silica. In order to determine the differences between the mesoporous silica synthesized from bottom ash and that synthesized from pure silica, we also used sodium metasilicate as a pure silica source to synthesize mesoporous materials. The hexagonal mesoporous silica (HMS) synthesized from sodium metasilicate and that synthesized from bottom ash are referred to as MCM-41 and HMS-BA, respectively. To synthesize MCM-41, first, 6 g of sodium metasilicate was added to 75 g of deionized water and dissolved it at 40 °C. The surfactant solution was prepared by first dissolving 3 g of CTAB in 17 g of deionized water. Subsequently, the CTAB solution was introduced slowly into the sodium metasilicate solution: the pH value of the mixture was 12. Then, 1 M sulfuric acid was introduced slowly into the solution until the pH value became 10, and the solution was mixed for 3 h. The rest of the steps involved in the synthesis of MCM-41 were the same as those for HMS-BA.
2.3. Characterization The Brunauer–Emmett–Teller (BET) surface areas and average pore diameters of the mesoporous silica materials were determined from their N2 adsorption–desorption isotherms, which were obtained at 77 K using an ASAP2020 vacuum volumetric sorption instrument. Before the N2 sorption analysis, the samples were preheated to 473 K for degassing and cooled to room temperature under vacuum. The micropore volumes (pore size: Q2, implying that the primary phase of the surface structure for both MCM-41 and HMS-BA was that corresponding to the Q4 peak, namely highly polymerized Si(SiO4) or quartz. These results were also in agreement with the findings of previous studies on the synthesis of MCM-41 from coal fly ash (Halina et al., 2007).
3.2.5. SEM analysis The SEM micrographs of MCM-41 and HMS-BA, shown in Fig. 8, revealed that the morphologies of the two materials were different. MCM-41 exhibited large, monolithic aggregates. In contrast, HMSBA showed aggregations consisting of smaller particles. A comparison of the physical properties of HMS and MCM-41 by Tanev and Pinnavaia (1996) had yielded similar results.
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3.3. Removal of heavy metals Fig. 9 shows the efficiencies of HMS-BA in removing Pb, Cu, Cd, and Cr from aqueous solutions; the efficiencies were 34.2%, 9.0%, 17.1%, and 51.6%, respectively. This result suggested that HMS-BA (which did not have any grafted functional groups) was more effective in removing heavy metals than was the MCM-41 sample prepared from coal fly ash (Hui and Chao, 2006). A number of studies have shown that the adsorption capacity of metal ions with respect to MCM-41 can be increased significantly by modifying MCM-41 (Lee and Yi, 2001; Bois et al., 2003; Kim et al., 2003; Kim and Yi, 2004). The obtained results suggest that, after being modified, HMS-BA ash has potential for use as an adsorbent for the removal of heavy metals from aqueous solutions. 4. Conclusions The synthesis of mesoporous silica materials from a supernatant solution of bottom ash was investigated in this study. The findings showed that the previously reported alkaline fusion and hydrolysis conditions for the extraction of silica from coal fly ash were not applicable in the case of incinerator bottom ash. The optimal alkaline agent for the extraction of silica from incinerator bottom ash was Na2CO3. The BET surface area and the mesoporous volume (992 m2/g and 0.854 m3/g, respectively) of the materials synthesized from bottom ash were slightly lower than those of the mesoporous silica materials obtained from a pure silica source (1078 m2/g and 0.918 m3/g, respectively); this was owing to the incorporation of trace amounts of other metals from the bottom ash into the silica matrix. XRD analysis showed that the spectrum of the mesoporous silica materials synthesized from bottom ash exhibited only one peak—that attributable to the (1 0 0) plane— indicating a hexagonal pore structure with a smaller order. The NMR spectra of the mesoporous silica materials synthesized from both bottom ash and from a pure silica source exhibited three peaks, namely, those corresponding to Q2 [Si(OSi)2(OH)2], Q3 [Si(OSi)3(OH)], and Q4 [Si(OSi)4]. However, the Q3 and Q4 peaks for the mesoporous silica materials synthesized from bottom ash were broader than those of the materials synthesized from the pure silica source. The mesoporous silica materials synthesized from bottom ash have potential for use as an adsorbent for removing heavy metal from aqueous solutions after being modified. Acknowledgement The authors thank the National Science Council of the Republic of China (Taiwan) for financially supporting this research under Contract No. NSC 101-2221-E-131-028-MY3. References Apiratikul, R., Pavasant, P., 2008. Sorption of Cu2+, Cd2+, and Pb2+ using modified zeolite from coal fly ash. Chem. Eng. J. 144, 245–258. Beck, J.S., Vartuli, J.C., 1996. Recent advances in the synthesis, characterization and applications of mesoporous molecular sieves. Curr. Opin. Solid State Mater. Sci. 1, 76–87. Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T.-W., Olson, D.H., Sheppard, E.W., 1992. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 114, 10834–10843. Bois, L., Bonhomme, A., Ribes, A., Pais, B., Raffin, G., Tessier, F., 2003. Functionalized silica for heavy metal ions adsorption. Colloid Surf. A Physicochem. Eng. Aspect 221, 221–230. Chandrasekar, G., You, K.S., Ahn, J.W., Ahn, W.S., 2008. Synthesis of hexagonal and cubic mesoporous silica using power plant bottom ash. Micropor. Mesopor. Mater. 111, 455–462. Chang, H.L., Chun, C.M., Aksay, I.L., Shih, W.H., 1999. Conversion of fly ash into mesoporous aluminosilicate. Ind. Eng. Chem. Res. 38, 973–977. Chen, Y.W., Wang, W.J., 2003. Synthesis and characteristics of hexa-mesoporous silica: a review. J. Petrol. 39, 17–36.
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Chiang, Y.W., Ghyselbrecht, K., Santos, R.M., Meesschaert, B., Martens, J.A., 2012. Synthesis of zeolitic-type adsorbent material from municipal solid waste incinerator bottom ash and its application in heavy metal adsorption. Catal. Today 190, 23–30. Chimenos, J.M., Segarra, M., Fernandez, M.A., Espiell, F., 2000. Short-term natural weathering of MSWI bottom ash. J. Hazard. Mater. B79, 287–299. Dhokte, A.Q., Khillare, S.L., Lande, M.K., Arbad, B.R., 2011. Synthesis, characterization of mesoporous silica materials from waste coal fly ash for the classical Mannich reaction. J. Ind. Eng. Chem. 17, 742–746. Gregg, S.J., Sing, K.S.W., 1982. Adsorption Surface Area and Porosity. Academic Press, London. Halina, M., Ramesh, S., Yarmo, M., Kamarudin, R., 2007. Non-hydrothermal synthesis of mesoporous materials using sodium silicate from coal fly ash. Mater. Chem. Phys. 101, 344–351. Hjelmar, O., 1996. Disposal strategies for municipal solid waste incineration residues. J. Hazard. Mater. 47, 345–368. Höller, H., Wirsching, U., 1985. Zeolites formation from fly ash. Fortschr. Miner. 63, 21–43. Hui, K., Chao, C., 2006. Synthesis of MCM-41 from coal fly ash by a green approach: influence of synthesis pH. J. Hazard. Mater. 137, 1135–1148. Kim, Y., Yi, J., 2004. Advances in environmental technologies via the application of mesoporous materials. J. Ind. Eng. Chem. 10, 41–51. Kim, Y., Lee, B., Yi, J., 2003. Preparation of functionalized mesostructured silica containing magnetite (MSM) for the removal of copper ions in aqueous solutions and its magnetic separation. Sep. Sci. Technol. 38, 2533–2548. Kiseler, A.V., Lygin, V.I., 1992. Infrared Spectra of Surface Compounds and Adsorbed Substances, first ed. Nauka, Moscow. Kresge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C., Beck, J.S., 1992. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359, 710–712. Kumar, P., Mal, N., Oumi, Y., Yamana, K., Sano, T., 2001. Mesoporous materials prepared using coal fly ash as the silicon and aluminum source. J. Mater. Chem. 11, 3285–3290.
Lee, H., Yi, J.H., 2001. Removal of copper ions using functionalized mesoporous silica in aqueous solution. Sep. Sci. Technol. 36, 2433–2448. Lin, K.L., Lin, D.F., 2006. Hydration characteristics of municipal solid waste incinerator bottom ash slag as a pozzolanic material for use in cement. Cem. Concr. Compos. 28, 817–823. Liu, M.M., Hou, L.A., Xi, B.D., Zhao, Y., Xia, X.F., 2013. Synthesis, characterization, and mercury adsorption properties of hybrid mesoporous aluminosilicate sieve prepared with fly ash. Appl. Surf. Sci. 273, 706–716. Misran, H., Singh, R., Begum, S., Yarmo, M.A., 2007. Processing of mesoporous silica materials (MCM-41) from coal fly ash. J. Mater. Process. Technol. 186, 8–13. Romero, A.A., Alba, M.D., Zhou, W.Z., Klinowski, J., 1997. Synthesis and characterization of the mesoporous silicate molecular sieve MCM-48. J. Phys. Chem. B 101, 5294–5300. Ryu, Z., Zheng, J., Wang, M., Zhang, B., 1999. Characterization of pore size distributions on carbonaceous adsorbents by DFT. Carbon 37, 1257–1264. Takahashi, R., Sato, S., Sodesawa, T., Kawakita, M., Ogura, K., 2000. High surface-area silica with controlled pore size prepared from nanocomposite of silica and citric acid. J. Phys. Chem. B 104, 12184–12191. Tanev, P.T., Pinnavaia, T.J., 1996. Mesoporous silicas molecular sieves prepared by ionic and neutral surfactant templating: a comparison of physical properties. Chem. Mater. 8, 2068–2079. Wiles, C.C., 1996. Municipal solid waste combustion ash: sate-of-the-knowledge. J. Hazard. Mater. 47, 325–344. Wu, X.W., Ma, H.W., Zhang, L.T., Wang, F.J., 2012. Adsorption properties and mechanism of mesoporous adsorbents prepared with fly ash for removal of Cu(II) in aqueous solution. Appl. Surf. Sci. 261, 902–907. Ye, Y.P., Zeng, X.Q., Qian, W.L., Wang, M.W., 2008. Synthesis of pure zeolites from supersaturated silicon and aluminum alkali extracts from fused coal fly ash. Fuel 87, 1880–1886.
