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Preparation of low surface area SiO2 microsphere from wheat husk ash with a facile precipitation process
Materials Letters 156 (2015) 42–45
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of low surface area SiO2 microsphere from wheat husk ash with a facile precipitation process Jinlong Cui, Hongliang Sun, Zelin Luo, Juncai Sun n, Zhongsheng Wen MTC Key Laboratory of Marine, Mechanical and Manufacturing Engineering, Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2015 Accepted 25 April 2015 Available online 5 May 2015
Low surface area SiO2 microspheres with narrow size distribution are prepared from wheat husk ash with a facile precipitation process, in which four steps including the extraction of sodium silicate, the dilution of sodium silicate extract with distilled water and ethanol, the precipitation of silica with HCl solution and calcination process are involved orderly. The particle size distribution and the specific surface area of the resulted silica microspheres are affected predominantly by the volume ratio of extract, water and ethanol and the dripping rate of HCl solution. The synthesized SiO2 microspheres with internal mesopores have an amorphous structure with diameters of approximately 227 nm and a specific surface area of 8.23 m2/g under the optimized synthesis conditions with a volume ratio of 1:1.5:0.5 for extract/water /ethanol and HCl solution dripping rate of 3 mL/min. & 2015 Elsevier B.V. All rights reserved.
Keywords: Porous materials Powder technology Low surface area SiO2 microsphere Solution precipitation Wheat husk ash
1. Introduction Silica is a basic material used widely in industries [1]. Porous silica with light weight and high surface area could be used in catalyst, adsorption, polymer filler, optical devices, bio-imaging, drug delivery, and biomedical applications. Much more works had been reported on this kind of silica [2–4]. However, when the porous silica is used as a filler of hydrophobic materials, such as ceramics and dental materials, the high surface area will enhance water uptake of the hydrophobic materials, resulting in the degradation of filler/matrix interface [5]. Moreover, a higher surface area also causes a reduction in the filler loading, resulting in the decrease of mechanical properties [6]. Therefore, mesoporous silica microsphere with low surface area is developed as a filler to enhance the wear resistance, hydrolytic aging resistance, and mechanical properties of the hydrophobic materials. The conversion of rice husk, rice husk char and rice husk ash into porous silica with specific surface areas of 330–995 m2/g has already been reported [2–4,7,8]. However, very few works have concerned with the preparation of silica with specific surface areas below 100 m2/g. Wheat husk or wheat husk ash (WHA) is spread out farmland in the north of China, which is an agricultural byproduct and environmental pollutant. In this work, low surface area SiO2 microspheres with narrow size distribution have been synthesized using wheat husk ash as raw material, in which there n
Corresponding author. Tel.: þ 86 411 84727959; fax: þ86 411 84725960. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.matlet.2015.04.134 0167-577X/& 2015 Elsevier B.V. All rights reserved.
is a silica content of about 35% and a great potential for the production of task-specific silica-based materials.
2. Experimental Wheat husk ash was obtained from a farmland of Henan Province in China. WHA was heated in 1 M H2SO4 solution with a ratio of 1:7 (w/v) at 75 1C for 90 min to remove earth metals, such as Ca, Mg, Fe, Al, K, etc. absorbed from soil in wheat growth. After drying overnight at 105 1C, the sample was pulverized to 60 mesh for use. Cleaned WHA powder and 2.0 M NaOH were mixed at a ratio of 1:6 (w/v) in a 500-mL three-neck round-bottom flask equipped with a thermometer and a magnetic stirrer and heated to 90 1C for 2 h. The extract (WHA-extract) containing sodium silicate was separated from WHA solid reside by vacuum-assisted filtration. 20 mL of WHA-extract was diluted with different volume ratios of water and ethanol at ambient temperature. Then, HCl solution (2 M) was dripped into the diluted extract with a buret to lower pH value to 8 for silica precipitation under constant stirring. The dripping rate HCl solution is controlled at 1 mL/min, 3 mL/min and 5 mL/min, respectively. As the pH reached to 8, the suspension liquids were further stirred for 1 h. The precipitate was washed several times with distilled water and centrifuged at 9000 rpm for 5 min with each wash. The cleaned precipitate was calcined at 550 1C for 1 h in a muffle furnace to obtain SiO2 microspheres.
J. Cui et al. / Materials Letters 156 (2015) 42–45
The particle size distributions of SiO2 microspheres were characterized by a laser particle-size analyzer (LPSA). The specific surface area and pore size of SiO2 microspheres were measured by using the Brunauer–Emmett–Teller (BET) method with an automated chemisorption/physisorption surface area and pore size analyzer, respectively. The microstructure, morphology and
43
composition of SiO2 microspheres were examined and analyzed using scanning a electron microscope (SEM), transmission electron microscope (TEM), an energy-dispersive X-ray analysis spectrometer (EDS), Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD).
3. Results and discussion
Fig. 1. Particle size distribution curves of silica microspheres synthesized at different volume ratios of WHA-extract, water and ethanol (a) and using different dripping rates of HCl solution (b).
Factors affecting size distribution of silica microspheres: The volume ratio of WHA-extract, water and ethanol is one more important factor that influences the formation and the particle size distribution of silica microspheres. In the synthesis process, without water, the particles exhibited a large size distribution; without ethanol, no microspheres were obtained. Fig. 1a shows the particle size distribution curves of silica microspheres synthesized at different volume ratios of WHA-extract, water and ethanol using HCl solution dripping rate of 3 mL/min. Obviously, when the volume ratio of WHA-extract, water and ethanol was 1:1.5:0.5, the particle size distribution was more narrower; while the volume ratio was 1:0.5:1.5, it was the broadest. The reason is that the high miscibility of ethanol accelerates the penetration into the silicate precursor. The behavior of ethanol facilitates the formation of Si–OH and Si–O–Si that produces more spherical and larger particles during the aging process [9]. Otherwise, water is required to decrease the concentration of sodium silicate, and ethanol is necessary to decrease the solubility of sodium silicate. Appropriate amount of water and ethanol would have a synergistic effect on the super-saturation of silicate species, thus creating a positive condition for the formation of primary particles with relatively uniform size distribution [10]. The silica microspheres which were prepared at the volume ratio of 1:1.5:0.5 of WHA-extract, water and ethanol using HCl solution dripping rate of 1 mL/min, 3 mL/min and 5 mL/min were
Fig. 2. N2 adsorption/desorption isotherms and pore size distributions of (a) S1, (b) S3 and (c) S5.
44
J. Cui et al. / Materials Letters 156 (2015) 42–45
Fig. 3. Characterization of prepared mesoporous silica microspheres: (a) SEM micrograph, (b) TEM image, (c) EDS spectrum, (d) FTIR spectrum and (e) XRD pattern.
Table 1 Characteristics of mesoporous silica microsphere obtained by HCl dripping rate 1 mL/min (S1), 3 mL/min (S3) and 5 mL/min (S5), respectively. Sample Specific surface area (m2/g)
Total pore volume (mm3/g)
Average pore diameter (nm)
S1 S3 S5
96.63 39.56 72.92
13.76 15.86 14.35
21.28 8.23 16.14
designated as S1, S3 and S5, respectively. The particle size distribution curves of S1, S3 and S5 shown in Fig. 1b reveal clearly that the dripping rate of HCl solution has certain influence on the final particle size of silica microspheres. The particle size of silica microspheres decreases with an increase in the dripping rate of HCl solution. It is rational that a slow dripping rate probably prolongs the growth time for the primary particles, resulting in larger particles [11]. Structure of typical sample: Fig. 2a–c shows the nitrogen adsorption and desorption isotherm curves of S1, S3 and S5. The shapes of the monolithic isotherms of S1, S3 and S5 are similar. They all exhibited a Point B and a hysteresis loop appearing between their adsorption and desorption branches, indicating the typical capillary condensation of the mesoporous structure. All the isotherms belong to the type IV category based on the IUPAC classification, with H3 hysteresis loop [12]. However, the amount adsorbed at the relative pressure (P/P0) above 0.3–0.8 doesn’t increase markedly (Fig. 2a–c). Otherwise, the data of total pore volume of S1, S3 and S5 were all smaller (Table 1). These show clearly that the quantity of mesopores contained inside the as-prepared silica microspheres is low, which is confirmed further by SEM and TEM images as shown in Fig. 3. The type H3 loop, which does not exhibit any limiting adsorption near the relative pressure of 1.0, suggests that the larger mesopores may company with slit-shaped pores in an aggregation of silica microspheres [12]. The specific surface area of S3 is the smallest and the average pore diameter is larger
among the three samples (Table 1), indicating 3 mL/min is the optimum rate of the dripping of HCl solution to prepare low surface area SiO2 microsphere with a few mesopores. As shown in Fig. 3a, the samples are composed of microsphere particles homogeneously. The interconnection among the microspheres is also observed in the SEM images (Fig. 3a). The interconnection can be due to the precipitate calcination at 550 1C. No obvious porous structure on the surface of these spheres was found. The silica microspheres are nearly solid, as shown in TEM image (Fig. 3b). EDS analysis shows that the chemical composition of the samples is Si and O with atomic ratio of 1/2. Other elements contained in raw WHA materials, e. g. Ca, K, Na, Mg, Fe, and Al were not found, confirming that washing WHA with 1 M H2SO4 solution is an effective way to remove these earth metals. Fig. 3d shows the FTIR spectrum of the prepared sample. The strong absorption peak at 1089 cm 1 corresponds to the asymmetric stretching vibration and shear bands of the Si–O–Si bonds. The peaks at 469 cm 1 and 801 cm 1 are derived from the plane swing vibration and the bending vibration of the Si–O bonds, respectively. The absorption peaks at 3428 cm 1 and 1658 cm 1 are designated to the H–O–H stretching vibration and bending vibration modes of the adsorbed water, respectively. There were no other absorption bands [10]. Therefore, the FTIR and EDS data clearly confirmed that the obtained sample was silica. The X-ray diffraction pattern of the samples as shown in Fig. 3e. The broad and high intensity peak at 2θ ¼ 221 demonstrates the typical amorphous structure of silica, and no diffraction of any impurities was found [13].
4. Conclusions Mesoporous SiO2 microspheres with diameters of approximately 227 nm and a specific surface area of 8.23 m2/g are
J. Cui et al. / Materials Letters 156 (2015) 42–45
prepared from wheat husk ash with a facile precipitation process. The SiO2 microspheres present amorphous structure containing inner mesopores. The total pore volume contained in SiO2 microspheres is 39.56 mm3/g, and the average pore diameter is mainly 13–16 nm. The volume ratio of WHA-extract, water and ethanol influences the particle size distribution of silica microspheres dramatically. The size and specific surface area of silica microspheres can be controlled by the dripping rate of HCl solution when the pH value of diluted extract is tuned to 8 in the synthesis process of SiO2 microspheres. Acknowledgements This work is financially supported by The National Foundation of Natural Science of China Nos. 21176034, 51479019 and 21476035) and the Fundamental Research Funds for the Central Universities (3132014323).
45
References [1] Pijarn N, Jaroenworaluck A, Sunsaneeyametha W, Stevens R. Powder Technol 2010;203:462–8. [2] Li D, Chen D, Zhu X. Bioresour Technol 2011;102:7001–3. [3] Li D, Zhu X. Mater Lett 2011;65:1528–30. [4] Lu P, Hsieh YL. Powder Technol 2012;225:149–55. [5] Curtis AR, Shortall AC, Marquis PM, Palin WM. J Dent 2008;36:186–93. [6] Samuel SP, Li S, Mukherjee I, Guo Y, Patel AC, Baran G, et al. Dent Mater 2009;25:296–301. [7] Witoon T, Chareonpanich M, Limtrakul J. Mater Lett 2008;62:1476–9. [8] Chareonpanich M, Nanta-ngern A, Limtrakul J. Mater Lett 2007;61:5153–6. [9] Jung CY, Kim JS, Chang TS, Kim ST, Lim HJ, Koo SM. Langmuir 2010;26:5456–61. [10] Lin L, Zhai S, Xiao Z, Liu N, Song Y. Bioresour Technol 2012;125:172–4. [11] Park SK, Kim KD, Kim HT. Colloids Surf, A 2002;197:7–17. [12] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquérol J, et al. Pure Appl Chem 1985;57(4):603–19. [13] Yu K, Guo Y, Ding X, Zhao J, Wang Z. Mater Lett 2005;59:4013–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of low surface area SiO2 microsphere from wheat husk ash with a facile precipitation process Jinlong Cui, Hongliang Sun, Zelin Luo, Juncai Sun n, Zhongsheng Wen MTC Key Laboratory of Marine, Mechanical and Manufacturing Engineering, Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2015 Accepted 25 April 2015 Available online 5 May 2015
Low surface area SiO2 microspheres with narrow size distribution are prepared from wheat husk ash with a facile precipitation process, in which four steps including the extraction of sodium silicate, the dilution of sodium silicate extract with distilled water and ethanol, the precipitation of silica with HCl solution and calcination process are involved orderly. The particle size distribution and the specific surface area of the resulted silica microspheres are affected predominantly by the volume ratio of extract, water and ethanol and the dripping rate of HCl solution. The synthesized SiO2 microspheres with internal mesopores have an amorphous structure with diameters of approximately 227 nm and a specific surface area of 8.23 m2/g under the optimized synthesis conditions with a volume ratio of 1:1.5:0.5 for extract/water /ethanol and HCl solution dripping rate of 3 mL/min. & 2015 Elsevier B.V. All rights reserved.
Keywords: Porous materials Powder technology Low surface area SiO2 microsphere Solution precipitation Wheat husk ash
1. Introduction Silica is a basic material used widely in industries [1]. Porous silica with light weight and high surface area could be used in catalyst, adsorption, polymer filler, optical devices, bio-imaging, drug delivery, and biomedical applications. Much more works had been reported on this kind of silica [2–4]. However, when the porous silica is used as a filler of hydrophobic materials, such as ceramics and dental materials, the high surface area will enhance water uptake of the hydrophobic materials, resulting in the degradation of filler/matrix interface [5]. Moreover, a higher surface area also causes a reduction in the filler loading, resulting in the decrease of mechanical properties [6]. Therefore, mesoporous silica microsphere with low surface area is developed as a filler to enhance the wear resistance, hydrolytic aging resistance, and mechanical properties of the hydrophobic materials. The conversion of rice husk, rice husk char and rice husk ash into porous silica with specific surface areas of 330–995 m2/g has already been reported [2–4,7,8]. However, very few works have concerned with the preparation of silica with specific surface areas below 100 m2/g. Wheat husk or wheat husk ash (WHA) is spread out farmland in the north of China, which is an agricultural byproduct and environmental pollutant. In this work, low surface area SiO2 microspheres with narrow size distribution have been synthesized using wheat husk ash as raw material, in which there n
Corresponding author. Tel.: þ 86 411 84727959; fax: þ86 411 84725960. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.matlet.2015.04.134 0167-577X/& 2015 Elsevier B.V. All rights reserved.
is a silica content of about 35% and a great potential for the production of task-specific silica-based materials.
2. Experimental Wheat husk ash was obtained from a farmland of Henan Province in China. WHA was heated in 1 M H2SO4 solution with a ratio of 1:7 (w/v) at 75 1C for 90 min to remove earth metals, such as Ca, Mg, Fe, Al, K, etc. absorbed from soil in wheat growth. After drying overnight at 105 1C, the sample was pulverized to 60 mesh for use. Cleaned WHA powder and 2.0 M NaOH were mixed at a ratio of 1:6 (w/v) in a 500-mL three-neck round-bottom flask equipped with a thermometer and a magnetic stirrer and heated to 90 1C for 2 h. The extract (WHA-extract) containing sodium silicate was separated from WHA solid reside by vacuum-assisted filtration. 20 mL of WHA-extract was diluted with different volume ratios of water and ethanol at ambient temperature. Then, HCl solution (2 M) was dripped into the diluted extract with a buret to lower pH value to 8 for silica precipitation under constant stirring. The dripping rate HCl solution is controlled at 1 mL/min, 3 mL/min and 5 mL/min, respectively. As the pH reached to 8, the suspension liquids were further stirred for 1 h. The precipitate was washed several times with distilled water and centrifuged at 9000 rpm for 5 min with each wash. The cleaned precipitate was calcined at 550 1C for 1 h in a muffle furnace to obtain SiO2 microspheres.
J. Cui et al. / Materials Letters 156 (2015) 42–45
The particle size distributions of SiO2 microspheres were characterized by a laser particle-size analyzer (LPSA). The specific surface area and pore size of SiO2 microspheres were measured by using the Brunauer–Emmett–Teller (BET) method with an automated chemisorption/physisorption surface area and pore size analyzer, respectively. The microstructure, morphology and
43
composition of SiO2 microspheres were examined and analyzed using scanning a electron microscope (SEM), transmission electron microscope (TEM), an energy-dispersive X-ray analysis spectrometer (EDS), Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD).
3. Results and discussion
Fig. 1. Particle size distribution curves of silica microspheres synthesized at different volume ratios of WHA-extract, water and ethanol (a) and using different dripping rates of HCl solution (b).
Factors affecting size distribution of silica microspheres: The volume ratio of WHA-extract, water and ethanol is one more important factor that influences the formation and the particle size distribution of silica microspheres. In the synthesis process, without water, the particles exhibited a large size distribution; without ethanol, no microspheres were obtained. Fig. 1a shows the particle size distribution curves of silica microspheres synthesized at different volume ratios of WHA-extract, water and ethanol using HCl solution dripping rate of 3 mL/min. Obviously, when the volume ratio of WHA-extract, water and ethanol was 1:1.5:0.5, the particle size distribution was more narrower; while the volume ratio was 1:0.5:1.5, it was the broadest. The reason is that the high miscibility of ethanol accelerates the penetration into the silicate precursor. The behavior of ethanol facilitates the formation of Si–OH and Si–O–Si that produces more spherical and larger particles during the aging process [9]. Otherwise, water is required to decrease the concentration of sodium silicate, and ethanol is necessary to decrease the solubility of sodium silicate. Appropriate amount of water and ethanol would have a synergistic effect on the super-saturation of silicate species, thus creating a positive condition for the formation of primary particles with relatively uniform size distribution [10]. The silica microspheres which were prepared at the volume ratio of 1:1.5:0.5 of WHA-extract, water and ethanol using HCl solution dripping rate of 1 mL/min, 3 mL/min and 5 mL/min were
Fig. 2. N2 adsorption/desorption isotherms and pore size distributions of (a) S1, (b) S3 and (c) S5.
44
J. Cui et al. / Materials Letters 156 (2015) 42–45
Fig. 3. Characterization of prepared mesoporous silica microspheres: (a) SEM micrograph, (b) TEM image, (c) EDS spectrum, (d) FTIR spectrum and (e) XRD pattern.
Table 1 Characteristics of mesoporous silica microsphere obtained by HCl dripping rate 1 mL/min (S1), 3 mL/min (S3) and 5 mL/min (S5), respectively. Sample Specific surface area (m2/g)
Total pore volume (mm3/g)
Average pore diameter (nm)
S1 S3 S5
96.63 39.56 72.92
13.76 15.86 14.35
21.28 8.23 16.14
designated as S1, S3 and S5, respectively. The particle size distribution curves of S1, S3 and S5 shown in Fig. 1b reveal clearly that the dripping rate of HCl solution has certain influence on the final particle size of silica microspheres. The particle size of silica microspheres decreases with an increase in the dripping rate of HCl solution. It is rational that a slow dripping rate probably prolongs the growth time for the primary particles, resulting in larger particles [11]. Structure of typical sample: Fig. 2a–c shows the nitrogen adsorption and desorption isotherm curves of S1, S3 and S5. The shapes of the monolithic isotherms of S1, S3 and S5 are similar. They all exhibited a Point B and a hysteresis loop appearing between their adsorption and desorption branches, indicating the typical capillary condensation of the mesoporous structure. All the isotherms belong to the type IV category based on the IUPAC classification, with H3 hysteresis loop [12]. However, the amount adsorbed at the relative pressure (P/P0) above 0.3–0.8 doesn’t increase markedly (Fig. 2a–c). Otherwise, the data of total pore volume of S1, S3 and S5 were all smaller (Table 1). These show clearly that the quantity of mesopores contained inside the as-prepared silica microspheres is low, which is confirmed further by SEM and TEM images as shown in Fig. 3. The type H3 loop, which does not exhibit any limiting adsorption near the relative pressure of 1.0, suggests that the larger mesopores may company with slit-shaped pores in an aggregation of silica microspheres [12]. The specific surface area of S3 is the smallest and the average pore diameter is larger
among the three samples (Table 1), indicating 3 mL/min is the optimum rate of the dripping of HCl solution to prepare low surface area SiO2 microsphere with a few mesopores. As shown in Fig. 3a, the samples are composed of microsphere particles homogeneously. The interconnection among the microspheres is also observed in the SEM images (Fig. 3a). The interconnection can be due to the precipitate calcination at 550 1C. No obvious porous structure on the surface of these spheres was found. The silica microspheres are nearly solid, as shown in TEM image (Fig. 3b). EDS analysis shows that the chemical composition of the samples is Si and O with atomic ratio of 1/2. Other elements contained in raw WHA materials, e. g. Ca, K, Na, Mg, Fe, and Al were not found, confirming that washing WHA with 1 M H2SO4 solution is an effective way to remove these earth metals. Fig. 3d shows the FTIR spectrum of the prepared sample. The strong absorption peak at 1089 cm 1 corresponds to the asymmetric stretching vibration and shear bands of the Si–O–Si bonds. The peaks at 469 cm 1 and 801 cm 1 are derived from the plane swing vibration and the bending vibration of the Si–O bonds, respectively. The absorption peaks at 3428 cm 1 and 1658 cm 1 are designated to the H–O–H stretching vibration and bending vibration modes of the adsorbed water, respectively. There were no other absorption bands [10]. Therefore, the FTIR and EDS data clearly confirmed that the obtained sample was silica. The X-ray diffraction pattern of the samples as shown in Fig. 3e. The broad and high intensity peak at 2θ ¼ 221 demonstrates the typical amorphous structure of silica, and no diffraction of any impurities was found [13].
4. Conclusions Mesoporous SiO2 microspheres with diameters of approximately 227 nm and a specific surface area of 8.23 m2/g are
J. Cui et al. / Materials Letters 156 (2015) 42–45
prepared from wheat husk ash with a facile precipitation process. The SiO2 microspheres present amorphous structure containing inner mesopores. The total pore volume contained in SiO2 microspheres is 39.56 mm3/g, and the average pore diameter is mainly 13–16 nm. The volume ratio of WHA-extract, water and ethanol influences the particle size distribution of silica microspheres dramatically. The size and specific surface area of silica microspheres can be controlled by the dripping rate of HCl solution when the pH value of diluted extract is tuned to 8 in the synthesis process of SiO2 microspheres. Acknowledgements This work is financially supported by The National Foundation of Natural Science of China Nos. 21176034, 51479019 and 21476035) and the Fundamental Research Funds for the Central Universities (3132014323).
45
References [1] Pijarn N, Jaroenworaluck A, Sunsaneeyametha W, Stevens R. Powder Technol 2010;203:462–8. [2] Li D, Chen D, Zhu X. Bioresour Technol 2011;102:7001–3. [3] Li D, Zhu X. Mater Lett 2011;65:1528–30. [4] Lu P, Hsieh YL. Powder Technol 2012;225:149–55. [5] Curtis AR, Shortall AC, Marquis PM, Palin WM. J Dent 2008;36:186–93. [6] Samuel SP, Li S, Mukherjee I, Guo Y, Patel AC, Baran G, et al. Dent Mater 2009;25:296–301. [7] Witoon T, Chareonpanich M, Limtrakul J. Mater Lett 2008;62:1476–9. [8] Chareonpanich M, Nanta-ngern A, Limtrakul J. Mater Lett 2007;61:5153–6. [9] Jung CY, Kim JS, Chang TS, Kim ST, Lim HJ, Koo SM. Langmuir 2010;26:5456–61. [10] Lin L, Zhai S, Xiao Z, Liu N, Song Y. Bioresour Technol 2012;125:172–4. [11] Park SK, Kim KD, Kim HT. Colloids Surf, A 2002;197:7–17. [12] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquérol J, et al. Pure Appl Chem 1985;57(4):603–19. [13] Yu K, Guo Y, Ding X, Zhao J, Wang Z. Mater Lett 2005;59:4013–5.