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hydroxyapatite composites
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Materials Letters 62 (2008) 3291 – 3293 www.elsevier.com/locate/matlet
Hydrothermal synthesis of tobermorite/hydroxyapatite composites Hirotaka Maeda ⁎, Koji Ioku, Emile H. Ishida Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan Received 19 September 2007; accepted 20 February 2008 Available online 23 February 2008
Abstract A tobermorite/hydroxyapatite composite was prepared by hydrothermal treatment using diatomaceous earth, α-tricalcium phosphate and slaked lime starting materials. The XRD patterns of the composite confirmed that tobermorite and hydroxyapatite were formed after the hydrothermal process. The bending strength of the composite reached a level higher than 9 MPa after the hydrothermal process. The development of the bending strength was due to the formation of tobermorite and hydroxyapatite during the hydrothermal process. The composite had a high specific surface area due to these newly formed crystals. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite material; Solidification; Tobermorite; Hydroxyapatite; Hydrothermal process
1. Introduction Recently, waste water treatment in Japan has increased in importance due to the expansion of environmental regulations regarding water pollution. Generally, toxic heavy metals are removed from industrial waste water by coagulative precipitation, ion exchange and reverse osmosis. For the purification of industrial waste water, our approach is to prepare environmentally friendly materials that are applicable for the adsorption and ion-exchange of toxic ions or molecules. Tobermorite (Ca5Si6O18H2·4H2O) has been shown to have high potential applications in cation exchange and nuclear and hazardous wastewater treatment [1,2]. Komarneni reported that synthetic and natural tobermorite has higher removal abilities than zeolite for almost all the heavy metals except hexavalent chromium [3]. Hydroxyapatite (Ca5(PO4)3(OH)) has been extensively studied because of its high sorption capacity [4,5]. It has been reported that hydroxyapatite can be used in the remediation of soil and water from industrial and nuclear wastes by utilizing its capacity for ion-exchange [6]. Numerous studies of heavy metal exchange in hydroxyapatite have been reported, e.g., chro⁎ Corresponding author. Tel./fax: +81 22 795 4398. E-mail address: [email protected] (H. Maeda). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.044
mium, lead and cadmium [7,8]. Moreover, according to chromatographic methods such as anion-exchange chromatography, the purification of proteins using hydroxyapatite has been reported [9]. We have considered that porous materials consisting of tobermorite and hydroxyapatite could be used as environmentally friendly materials with high remediation abilities. Enhancement of strength has been reported from hydrothermal processing of the CaO–SiO–H2O system, due to the formation of typical calcium silicate hydrates, such as tobermorite and xonotolite [10,11]. It was reported that hydroxyapatite was synthesized by a hydrothermal process using α-tricalcium phosphate (α-TCP) as a starting material [12]. Our strategy for the preparation of a porous material consisting of tobermorite and hydroxyapatite for use in the purification of waste water is to coprecipitate both crystalline phases and simultaneously enhance the strength of the material by hydrothermal treatment. 2. Materials and methods Diatomaceous earth (average particle size: 50 μm, surface area: 3 m2/g, Showa Chemical Industry Co. Ltd., Japan) precalcined at 1200 °C, α-TCP (surface area: 0.1–2 m2/g, Taihei Chemical Industry Co. Ltd., Japan), and slaked lime (average particle size: 10 μm, surface area: 10 m2/g) were used as starting
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materials. Lime was prepared by calcining calcium carbonate (Wako Pure Chemical Industries, Ltd.) at 1000 °C for 3 h. Slaked lime was obtained by the addition to distilled water into the prepared lime. Diatomaceous earth is made of phytoplankton fossils and is mainly composed of amorphous silica. Thus, diatomaceous earth was used as a silica source. The mass ratio of diatomaceous earth/slaked lime/α-TCP was 50:40:10. A mixture of the starting materials was added at 10 mass% to distilled water. The mixture was then uniaxially pressed at 5 MPa in a stainless steel die (15 × 40 mm). The powder-compacts (3 × 15 × 40 mm) were then hydrothermally treated under saturated steam pressure at 180 °C for 10 h. 10 mL of 1 mol/L ammonium solution was used as the solvent. The crystalline phases in the hydrothermally treated material were identified by XRD analysis. The flexural strength of the material specimens (3 × 15 × 40 mm) were estimated by a threepoint bending test at a loading rate of 0.5 mm/min. The surface morphology was observed by scanning electron microscopy (SEM). The pore size distribution of the samples was measured by mercury intrusion porosimetry. The specific surface area of the materials was measured by nitrogen gas sorption analysis.
Fig. 2. Pore size distribution curves of the composite before and after the hydrothermal process.
Fig. 1 shows XRD patterns of the material before and after the hydrothermal process. Prior to the hydrothermal process, peaks corresponding to cristobalite and quartz, originated from the diatomaceous earth, slaked lime and α-TCP are evident. After 3 h of hydrothermal processing, the peaks corresponding to α-TCP disappear, and new peaks corresponding to hydroxyapatite and C-S-H gel appear. The XRD pattern measured after 10 h of hydrothermal treatment has new peaks corresponding to tobermorite. The peaks corresponding to cristobalite and slaked lime gradually decrease after the hydrothermal process. These findings suggest that a large amount of Si4+ and Ca2+ ions, from the dissolution of cristobalite and slaked lime, increase the supersaturation of calcium silicate hydrate during the hydrothermal process, resulting in the crystallization of tobermorite via C-S-H gel. These results also suggest that the presence of ions such as Ca2+ and Si4+ seems to have no influence on the hydration reaction from α-TCP to hydroxyapatite during the hydrothermal process.
Fig. 2 shows the pore size distribution of the material before and after the hydrothermal process. The pore size distribution before the hydrothermal process has a peak at around 2 μm in diameter. The peak tends to shift toward smaller diameters with increases in the hydrothermal process time. After 3 h of the hydrothermal process, peaks are observed at ca. 0.01 and ca. 0.1 μm in diameter. The relative ratios of these peaks increase after 10 h of the hydrothermal treatment. Fig. 3 shows a SEM micrograph of the fracture surface of the composite material after 10 h of the hydrothermal treatment. Not only plate-like, but also pillar-like deposits are clearly observed. According to the XRD pattern shown in Fig. 1, the newly formed deposits are tobermorite, C-S-H gel and hydroxyapatite. These results confirm that the formation and growth of tobermorite, C-S-H gel and hydroxyapatite between each particle lead to the formation of pores as fine as approximately 0.01 and 0.1 μm in diameter. The bending strength of the powder-compacts containing diatomaceous earth, α-TCP and slaked lime was 0.2 ± 0.1 MPa before the hydrothermal process. In contrast, the bending strength after 10 h of the hydrothermal treatment was 9.2 ± 0.1 MPa. It is clear that the hydrothermal process solidifies the composite and develops strength due to the formation of tobermorite, C-S-H gel and hydroxyapatite. The porosity was estimated to be approximately 50% by mercury intrusion porosimetry. The Brunauer–Emmett–Teller (BET) surface area was measured as ca. 40 m2/g by nitrogen gas sorption. The hydrothermally solidified composite has a high specific surface area
Fig. 1. XRD patterns of the composite before and after the hydrothermal process.
Fig. 3. SEM micrograph of the composite after 10 h of hydrothermal treatment.
3. Results and discussion
H. Maeda et al. / Materials Letters 62 (2008) 3291–3293
due to the formation of tobermorite, C-S-H gel and hydroxyapatite. In our preliminary experiments on the heavy metal cation removal abilities toward cobalt ion of the material (the material dose 2 g/L), the removal percentages of cobalt ion compared to the initial cobalt concentration (23 ppm) attained more than 90% for 20 h at 25 °C. Further research on the removal of heavy metals from wastewater using this material is in progress.
4. Conclusion A novel composite containing tobermorite and hydroxyapatite for the remediation of waste water was hydrothermally synthesized using diatomaceous earth, α-tricalcium phosphate and slaked lime as starting materials. The composite had a high specific surface area (40 m2/g) and exhibited a bending strength of more than 9 MPa. The strength development and high specific surface area of the composite were caused by the cocrystallization of both tobermorite and hydroxyapatite. Acknowledgements The present work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18201014).
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References [1] S. Komarneni, D.M. Roy, Science 221 (1983) 647–648. [2] S. Kaneco, K. Itoh, H. Katsumata, T. Suzuki, K. Masuyama, K. Funasaka, et al., Environ Sci Technol 37 (2003) 1448–1451. [3] S. Komarneni, Nucl Chem Waste Manag 5 (1985) 247–250. [4] T. Suzuki, T. Hatsushika, Y. Hayakawa, J Chem Soc Faraday Trans 1 78 (1982) 3605–3611. [5] X.B. Chen, J.V. Wright, J.L. Conca, L.M. Peurrung, Environ Sci Technol 31 (1997) 624–631. [6] F. Monteil-Rivera, M. Fedoroff, Sorption of inorganic species on apatites from aqueous solutions, in: A.T. Hubbard (Ed.), Encyclopedia of surface and colloid science, Marcel Dekker Inc, New York, 2002, pp. 1–26. [7] J. Reichert, J.G.P. Binner, J Mater Sci 31 (1996) 1231–1241. [8] M. Srinivasan, C. Ferraris, T. White, Environ Sci Technol 40 (2006) 7054–7059. [9] O. Takagi, N. Kuramoto, M. Ozawa, S. Suzuki, Ceram Inter 30 (2004) 139–143. [10] T. Mitsuda, J. Saito, E. Hattori, Cem Concr Res 7 (1977) 681–686. [11] O. Watanabe, K. Kitamura, H. Maenami, H. Ishida, J Am Ceram Soc 84 (2001) 2318–2322. [12] K. Ioku, S. Yamauchi, H. Fujimori, S. Goto, M. Yoshimura, Solid State Ion 151 (2002) 147–150.
Materials Letters 62 (2008) 3291 – 3293 www.elsevier.com/locate/matlet
Hydrothermal synthesis of tobermorite/hydroxyapatite composites Hirotaka Maeda ⁎, Koji Ioku, Emile H. Ishida Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan Received 19 September 2007; accepted 20 February 2008 Available online 23 February 2008
Abstract A tobermorite/hydroxyapatite composite was prepared by hydrothermal treatment using diatomaceous earth, α-tricalcium phosphate and slaked lime starting materials. The XRD patterns of the composite confirmed that tobermorite and hydroxyapatite were formed after the hydrothermal process. The bending strength of the composite reached a level higher than 9 MPa after the hydrothermal process. The development of the bending strength was due to the formation of tobermorite and hydroxyapatite during the hydrothermal process. The composite had a high specific surface area due to these newly formed crystals. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite material; Solidification; Tobermorite; Hydroxyapatite; Hydrothermal process
1. Introduction Recently, waste water treatment in Japan has increased in importance due to the expansion of environmental regulations regarding water pollution. Generally, toxic heavy metals are removed from industrial waste water by coagulative precipitation, ion exchange and reverse osmosis. For the purification of industrial waste water, our approach is to prepare environmentally friendly materials that are applicable for the adsorption and ion-exchange of toxic ions or molecules. Tobermorite (Ca5Si6O18H2·4H2O) has been shown to have high potential applications in cation exchange and nuclear and hazardous wastewater treatment [1,2]. Komarneni reported that synthetic and natural tobermorite has higher removal abilities than zeolite for almost all the heavy metals except hexavalent chromium [3]. Hydroxyapatite (Ca5(PO4)3(OH)) has been extensively studied because of its high sorption capacity [4,5]. It has been reported that hydroxyapatite can be used in the remediation of soil and water from industrial and nuclear wastes by utilizing its capacity for ion-exchange [6]. Numerous studies of heavy metal exchange in hydroxyapatite have been reported, e.g., chro⁎ Corresponding author. Tel./fax: +81 22 795 4398. E-mail address: [email protected] (H. Maeda). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.044
mium, lead and cadmium [7,8]. Moreover, according to chromatographic methods such as anion-exchange chromatography, the purification of proteins using hydroxyapatite has been reported [9]. We have considered that porous materials consisting of tobermorite and hydroxyapatite could be used as environmentally friendly materials with high remediation abilities. Enhancement of strength has been reported from hydrothermal processing of the CaO–SiO–H2O system, due to the formation of typical calcium silicate hydrates, such as tobermorite and xonotolite [10,11]. It was reported that hydroxyapatite was synthesized by a hydrothermal process using α-tricalcium phosphate (α-TCP) as a starting material [12]. Our strategy for the preparation of a porous material consisting of tobermorite and hydroxyapatite for use in the purification of waste water is to coprecipitate both crystalline phases and simultaneously enhance the strength of the material by hydrothermal treatment. 2. Materials and methods Diatomaceous earth (average particle size: 50 μm, surface area: 3 m2/g, Showa Chemical Industry Co. Ltd., Japan) precalcined at 1200 °C, α-TCP (surface area: 0.1–2 m2/g, Taihei Chemical Industry Co. Ltd., Japan), and slaked lime (average particle size: 10 μm, surface area: 10 m2/g) were used as starting
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materials. Lime was prepared by calcining calcium carbonate (Wako Pure Chemical Industries, Ltd.) at 1000 °C for 3 h. Slaked lime was obtained by the addition to distilled water into the prepared lime. Diatomaceous earth is made of phytoplankton fossils and is mainly composed of amorphous silica. Thus, diatomaceous earth was used as a silica source. The mass ratio of diatomaceous earth/slaked lime/α-TCP was 50:40:10. A mixture of the starting materials was added at 10 mass% to distilled water. The mixture was then uniaxially pressed at 5 MPa in a stainless steel die (15 × 40 mm). The powder-compacts (3 × 15 × 40 mm) were then hydrothermally treated under saturated steam pressure at 180 °C for 10 h. 10 mL of 1 mol/L ammonium solution was used as the solvent. The crystalline phases in the hydrothermally treated material were identified by XRD analysis. The flexural strength of the material specimens (3 × 15 × 40 mm) were estimated by a threepoint bending test at a loading rate of 0.5 mm/min. The surface morphology was observed by scanning electron microscopy (SEM). The pore size distribution of the samples was measured by mercury intrusion porosimetry. The specific surface area of the materials was measured by nitrogen gas sorption analysis.
Fig. 2. Pore size distribution curves of the composite before and after the hydrothermal process.
Fig. 1 shows XRD patterns of the material before and after the hydrothermal process. Prior to the hydrothermal process, peaks corresponding to cristobalite and quartz, originated from the diatomaceous earth, slaked lime and α-TCP are evident. After 3 h of hydrothermal processing, the peaks corresponding to α-TCP disappear, and new peaks corresponding to hydroxyapatite and C-S-H gel appear. The XRD pattern measured after 10 h of hydrothermal treatment has new peaks corresponding to tobermorite. The peaks corresponding to cristobalite and slaked lime gradually decrease after the hydrothermal process. These findings suggest that a large amount of Si4+ and Ca2+ ions, from the dissolution of cristobalite and slaked lime, increase the supersaturation of calcium silicate hydrate during the hydrothermal process, resulting in the crystallization of tobermorite via C-S-H gel. These results also suggest that the presence of ions such as Ca2+ and Si4+ seems to have no influence on the hydration reaction from α-TCP to hydroxyapatite during the hydrothermal process.
Fig. 2 shows the pore size distribution of the material before and after the hydrothermal process. The pore size distribution before the hydrothermal process has a peak at around 2 μm in diameter. The peak tends to shift toward smaller diameters with increases in the hydrothermal process time. After 3 h of the hydrothermal process, peaks are observed at ca. 0.01 and ca. 0.1 μm in diameter. The relative ratios of these peaks increase after 10 h of the hydrothermal treatment. Fig. 3 shows a SEM micrograph of the fracture surface of the composite material after 10 h of the hydrothermal treatment. Not only plate-like, but also pillar-like deposits are clearly observed. According to the XRD pattern shown in Fig. 1, the newly formed deposits are tobermorite, C-S-H gel and hydroxyapatite. These results confirm that the formation and growth of tobermorite, C-S-H gel and hydroxyapatite between each particle lead to the formation of pores as fine as approximately 0.01 and 0.1 μm in diameter. The bending strength of the powder-compacts containing diatomaceous earth, α-TCP and slaked lime was 0.2 ± 0.1 MPa before the hydrothermal process. In contrast, the bending strength after 10 h of the hydrothermal treatment was 9.2 ± 0.1 MPa. It is clear that the hydrothermal process solidifies the composite and develops strength due to the formation of tobermorite, C-S-H gel and hydroxyapatite. The porosity was estimated to be approximately 50% by mercury intrusion porosimetry. The Brunauer–Emmett–Teller (BET) surface area was measured as ca. 40 m2/g by nitrogen gas sorption. The hydrothermally solidified composite has a high specific surface area
Fig. 1. XRD patterns of the composite before and after the hydrothermal process.
Fig. 3. SEM micrograph of the composite after 10 h of hydrothermal treatment.
3. Results and discussion
H. Maeda et al. / Materials Letters 62 (2008) 3291–3293
due to the formation of tobermorite, C-S-H gel and hydroxyapatite. In our preliminary experiments on the heavy metal cation removal abilities toward cobalt ion of the material (the material dose 2 g/L), the removal percentages of cobalt ion compared to the initial cobalt concentration (23 ppm) attained more than 90% for 20 h at 25 °C. Further research on the removal of heavy metals from wastewater using this material is in progress.
4. Conclusion A novel composite containing tobermorite and hydroxyapatite for the remediation of waste water was hydrothermally synthesized using diatomaceous earth, α-tricalcium phosphate and slaked lime as starting materials. The composite had a high specific surface area (40 m2/g) and exhibited a bending strength of more than 9 MPa. The strength development and high specific surface area of the composite were caused by the cocrystallization of both tobermorite and hydroxyapatite. Acknowledgements The present work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18201014).
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References [1] S. Komarneni, D.M. Roy, Science 221 (1983) 647–648. [2] S. Kaneco, K. Itoh, H. Katsumata, T. Suzuki, K. Masuyama, K. Funasaka, et al., Environ Sci Technol 37 (2003) 1448–1451. [3] S. Komarneni, Nucl Chem Waste Manag 5 (1985) 247–250. [4] T. Suzuki, T. Hatsushika, Y. Hayakawa, J Chem Soc Faraday Trans 1 78 (1982) 3605–3611. [5] X.B. Chen, J.V. Wright, J.L. Conca, L.M. Peurrung, Environ Sci Technol 31 (1997) 624–631. [6] F. Monteil-Rivera, M. Fedoroff, Sorption of inorganic species on apatites from aqueous solutions, in: A.T. Hubbard (Ed.), Encyclopedia of surface and colloid science, Marcel Dekker Inc, New York, 2002, pp. 1–26. [7] J. Reichert, J.G.P. Binner, J Mater Sci 31 (1996) 1231–1241. [8] M. Srinivasan, C. Ferraris, T. White, Environ Sci Technol 40 (2006) 7054–7059. [9] O. Takagi, N. Kuramoto, M. Ozawa, S. Suzuki, Ceram Inter 30 (2004) 139–143. [10] T. Mitsuda, J. Saito, E. Hattori, Cem Concr Res 7 (1977) 681–686. [11] O. Watanabe, K. Kitamura, H. Maenami, H. Ishida, J Am Ceram Soc 84 (2001) 2318–2322. [12] K. Ioku, S. Yamauchi, H. Fujimori, S. Goto, M. Yoshimura, Solid State Ion 151 (2002) 147–150.