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Powder Technology 185 (2008) 31 – 35 www.elsevier.com/locate/powtec
Preparation of silica nanoparticles from semi-burned rice straw ash R.R. Zaky a , M.M. Hessien a , A.A. El-Midany a , M.H. Khedr b , E.A. Abdel-Aal a,⁎, K.A. El-Barawy a a
Central Metallurgical Research and Development Institute, P.O. Box: 87 Helwan, Cairo, Egypt b Material Science Lab., Chemistry dept., Faculty of science, Benisuef University, Egypt
Received 24 April 2007; received in revised form 6 August 2007; accepted 18 September 2007 Available online 22 September 2007
Abstract Semi-burned rice straw ash (SBRSA), as waste material provided from gas production unit of rice straw, was used to prepare silica nanoparticles. Box-Behnken statistical experimental design was used to optimize the factors affecting the dissolution efficiency of the silica such as stoichiometry (NaOH:SiO2), reaction time and reaction temperature, and to determine the optimum conditions for the extraction process. X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) have been used for the characterization of the SBRSA while UV/VIS/NIR Spectrophotometer was used to measure the concentration of the silica in the solution. The results show that the main constituent of SBRSA is silica (62%). Statistical design shows that the dissolution efficiency was in an agreement with the generated model and the experimental results. It is observed that the dissolution efficiency of silica was increased by increasing leaching temperature, time and stoichiometery. At stoichiometric value 1 and 2, the dissolution efficiency of silica was increased by increasing leaching temperature and time and did not reach 99% efficiency. By increasing the stoichiometric value up to 3, the dissolution efficiency reaches 99.88% at 100 °C and 4 h. © 2007 Elsevier B.V. All rights reserved. Keywords: Rice straw; Silica nanoparticles; Alkali leaching; Box-Behnken statistical experimental design
1. Introduction Rice straw is the stem of rice plants, which is removed during the harvesting of rice and it is considered as an agriculture waste material. The major constituents of rice straw are cellulose (32– 47%), hemicellulose (19–27%), lignin (5–24%) and ash (13– 20%) [1–4]. The ash constituents depend on rice type, climate and geographic location of growth. Rice straw is available in huge quantity and has no commercial interest. In Egypt, the amount of rice straw discarded annually is about 3 millions tons. The Government installed 2 units for converting limited amount of rice straw into gas to be used as fuel. The disadvantage of this process is accumulation of residue, which is semi-burned rice straw ash (SBRSA). This residue, which is rich in silica, has no ⁎ Corresponding author. Tel.: +20 202 5010642; fax: +20 202 5010639. E-mail addresses:
[email protected] (R.R. Zaky),
[email protected] (M.M. Hessien),
[email protected] (A.A. El-Midany),
[email protected] (M.H. Khedr),
[email protected],
[email protected] (E.A. Abdel-Aal),
[email protected] (K.A. El-Barawy). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.09.012
commercial utilization yet. Therefore, it is of ecological and economical point of view to discover an advantageous utilization of this material. There is no available information about extraction of silica from rice straw or from semi-burned rice straw ash. Table 1 Sieve analysis of semi-burned rice straw ash Sieve size, mm
Percentage, %
+.00 −4.00 + 2.38 −2.38 + 1.00 −1.00 + 0.63 −0.63 + 0.4 −0.4 + 0.35 −0.35 + 0.25 −0.25 + 0.16 −0.16 + 0.125 −0.125 + 0.09 −0.09 Total
10.58 2.62 15.58 0.61 0.26 1.72 32.62 0.08 0.22 1.71 34 100
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Table 2 Box-Behnken design with 3 levels and 3 variables for alkali leaching Run no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Coded factor levels Temperature
Time
Stoichiometry
−1 −1 +1 +1 −1 −1 +1 +1 0 0 0 0 0 0 0
−1 +1 −1 +1 0 0 0 0 −1 −1 +1 +1 0 0 0
0 0 0 0 −1 +1 −1 +1 −1 +1 −1 +1 0 0 0
Factors and levels for experimental design using Box-Behnken method Variables Temperature, °C Time, h Stoichiometry
−1 80 2 1
0 90 3 2
+1 100 4 3
Many authors studied production of silica powder from rice waste, Della et al. [5] produced active silica with a high specific area from rice husk ash after heat-treating at 700 °C in air for 6 h and they concluded that the temperature and time of incineration affect the purity of the formed silica. Kalapathy et al.[6] produced pure silica with lower sodium content from rice husk ash by using NaOH at different concentrations followed by HCl acid precipitation. They [7] also studied the effect of different acids such as hydrochloric, citric, or oxalic acid and the solution pH in the precipitation of silica. Liou [8] produced amorphous nano-structured silica powders with average particle size 60 nm and high specific surface area from nonisothermal decomposition of rice husk in an air atmosphere at temperatures between 27 °C and 727 °C and different heating rate. Real et al. [9] obtained a homogeneous size distribution of nanometric silica
particles by burning rice husk at 600–800 °C in a pure oxygen atmosphere. However, many authors concluded that rice husks are an excellent source of high-grade amorphous silica [10–14]. The main objective of the present work is to extract silica by sodium hydroxide from semi-burned rice straw ash (SBRSA) to. Box-Behnken statistical experimental design method was used to optimize the factors affecting the dissolution efficiency of the silica such as stoichiometry (NaOH:SiO2), reaction time and reaction temperature. 2. Experimental 2.1. Materials Semi-burned rice straw ash (SBRSA) was obtained from gas production unit of rice straw located at El-Azazy hamlet, Abu Hamad, Zakazik. It was used for the production of silica. The sieve analysis of SBRSA is shown in Table 1. Particle size less than 0.4 mm was selected to get rid of the bulk impurities, then the fine semi-burned rice straw ash (FSBRSA) is subjected to sampling process for performing the experiments. A SiO2 and moisture content of the FSBRSA was determined as 62.2% and 7.4%, respectively. Pure chemicals such as sodium silicate (44– 47% SiO2), sodium hydroxide (95%), and hydrochloric acid (32%) are used for leaching of silica. 2.2. Experimental design Statistical experimental designs have been used to conduct and plan experiments in order to extract the maximum amount of information in the fewest number of runs [15–16]. For dissolving the silica from SBRSA, an experimental design, Box-Behnken design [17], was used to optimize the effects of stoichiometry (NaOH:SiO2), reaction time and reaction temperature based on the dissolution efficiency of the silica. The design matrix of different runs, 15 experiments, as well as the levels of each factor are shown in Table 2. The extent of fitting the experimental results to the polynomial model equation was expressed by the determination
Fig. 1. SEM micrographs of fine burned rice straw waste (SBRSA).
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Fig. 2. X-ray diffraction patterns for SBRSA (a) as received and (b) after burning at 800 °C for 2 h.
coefficient, R2. F-test was used to estimate the significance of all terms in the polynomial equation within 95% confidence interval. 2.3. Leaching process All the leaching experiments were carried out in covered Teflon beaker (250 ml). Each experiment was performed by dissolving a calculated amount of sodium hydroxide in deionized water and then 10 gm of FSBRSA was added gradually. The stoichiometry (NaOH:SiO2), time, and temperature were adjusted according to the experimental design. The leaching process was carried out with liquid/solid ratio 10 under constant stirring rate 700 rpm (magnetic stirrer CimarecTM-USA-model SP 131320-33). After elapsing the reaction time, the slurry was filtered and then washed with 100 ml deionized water. The concentration of silica in the solution was measured using the UV/VIS/NIR Spectrophotometer and the weight percent of the dissolved silica in the solution was calculated. On the other hand, silica particles were prepared by adding sulfuric acid solution gradually to the prepared sodium metasilicate solution with constant stirring until reaching the desired pH. The formed silica gel was aged for 24 h then it was broken and filtered. The precipitate was washed and dried to produce silica xerogel which was pulverized to obtain the silica nanopowder. 3. Results and discussion Fig. 1 shows the Scanning Electron Microscope (SEM) micrographs of the FSBRSA. Two zones were chosen to show the main microstructure of the sample. The images showed that the FSBRSA has mainly needle-like particles with high aspect ratio in the range of 10–20. Fig. 2 represents X-ray diffraction patterns of 2 samples, as received and after burning at 800 °C for 2 h. It is clear that, the sample as received (Fig. 2a) contains
SiO2, NaCl, KCl, and C. It can be observed that, the peaks related to the silica are broad and the intensities are smaller than those related to KCl and NaCl, which means that most of the present silica is amorphous. After burning process (Fig. 2b), the XRD patterns indicate that FSBRSA powder is mainly cristobalite tetragonal well crystalline silica free from carbon and with amounts of NaCl and KCl. Sodium hydroxide reacts with silica according to the following reaction (1): SiO2 þ 2NaOH → Na2 SiO3 þ H2 O
ð1Þ
The alkali–silica reaction (ASR) mechanism was described using different models [18–23]. Silica dissolution is controlled by the hydroxide diffusivity in reactive grains and by hydroxide
Table 3 Dissolution efficiency of SiO2 according to experimental statistical design conditions Run no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Factors
Dissolution efficiency, %
Temperature, °C
Time, h
Stoichiometry
80 80 100 100 80 80 100 100 90 90 90 90 90 90 90
2 4 2 4 3 3 3 3 2 2 4 4 3 3 3
2 2 2 2 1 3 1 3 1 3 1 3 2 2 2
70.5 83.2 84.2 90.1 75.0 84.7 85.8 95.0 79.0 86.9 82.4 95.0 87.7 87.3 87.7
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Fig. 4. 3-D plot for all experimental data (Standard Deviation: 2.16; R2: 0. 9656).
absorption on the solid surface. The hydroxide diffusivity increases with the pH and the ionic strength of the solution [24]. At a constant pH and ionic strength, the hydroxide sorption decreases with the increasing size of the hydrated cation [24]. The effect of three variables (reaction temperature, reaction time and stoichiometry of NaOH:SiO2 on the dissolution efficiency of the silica were obtained and given in Table 3. Statistical design shows that the 99.88% dissolution efficiency can be achieved. In addition, standard deviation is 2.16 and the determination coefficient R2 (0.9656) indicate the agreement of the generated model with the experimental results. A regression Eq. (2) shows the dependence of the response on the process parameters. The parameters of the equation were obtained by multiple regression analysis of the experimental data. Dissolution efficiency ¼ þ86:57 þ 4:454A þ 5:904B þ4:934C 1:254A2 1:954B2 þ0:504C 2 0:324A4B þ1:174A4C 0:134B4C
ð2Þ
where A: Time, h B: Temperature, °C C: Stoichiometry
Fig. 3. Contour plots for the effects of reaction time and temperature on silica dissolution efficiency (a: stoichiometry = 1; b: 2; c: 3) (Standard Deviation: 2.16; R2: 0. 9656).
Fig. 5. TEM image of silica nanoparticles prepared from semi-burned rice straw ash.
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Fig. 3 shows the effects of reaction time and temperature on dissolution efficiency of silica at different NaOH:SiO2 stoichiometry. It can be observed that the reaction temperature is the most significant factor on the dissolution efficiency of silica from FSBRSA. For instance, at low stoichiometry (NaOH:SiO2 = 1) and for reaction time 4 h, the dissolution efficiency was increased from 76 to 88% with increasing the reaction temperature from 80 to 100 °C (Fig. 3-a). Increasing the stoichiometry increases the maximum dissolution efficiency gradually from 88% at low stoichiometry to 99% at high stoichiometry (Fig. 3-c). For the effect of reaction time under a constant reaction temperature and alkali-silica stoichiometry ratios, the dissolution efficiency increased with increasing the reaction time. It is very clear that the dissolution efficiency increase gradually with increasing the temperature and the time of reaction at different stoichiometry. All the experimental data, collected at the 3-D cubic as shown in Fig. 4 revealed that the dissolution efficiency which is 69–99% could be produced. The highest dissolution efficiency 99% can be obtained at high levels of parameters. On the other hand, the smallest dissolution efficiency can be achieved at low levels. However, decreasing any one of the main variables lead to the decrease of dissolution efficiency, which is undesirable. TEM was used to characterize the morphology of precipitated silica nanoparticles prepared by addition of 12% H2SO4 solution without any surfactants to the extracted sodium metasilicate solution till a final pH of 10. The image of silica nanoparticles is shown in Fig. 5. It is clear that, the shape of precipitated silica nanoparticles is sphere with an average size of about 50–70 nm. TEM image shows that narrow size distribution of silica particles are obtained. In addition, it is observed that limited amount of particles are agglomerated. 4. Conclusion Statistically designed experiments based on Box-Behnken procedure were used to study effects of three variables, namely; reaction temperature, reaction time, and stoichiometry of NaOH: SiO2 on the dissolution efficiency of silica from fine semiburned rice straw ash (FSBRSA) which is produced from rice straw burning unit as a by-product using sodium hydroxide. The analysis of experimental runs of statistical design showed that the dissolution efficiency was in an agreement with the gen-
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