Rice Husk Ash-Derived Silica Nanofluids: Synthesis and Stability Study
© The Author(s). 2016
Received: 10 June 2016
Accepted: 8 November 2016
Published: 15 November 2016
Nanofluids, colloidal suspensions consisting of base fluids and nanoparticles, are a new generation of engineering working fluids. Nanofluids have shown great potential in heat/mass transfer applications. However, their practical applications are limited by the high production cost and low stability. In this study, a low-cost agricultural waste, rice husk ash (RHA), was used as a silicon source to the synthesis of silica nanofluids. First, silica nanoparticles with an average size of 47 nm were synthesized. Next, by dispersing the silica nanoparticles in water with ultrasonic vibration, silica nanofluids were formed. The results indicated that the dispersibility and stability of nanofluids were highly dependent on sonication time and power, dispersant types and concentrations, as well as pH; an optimal experiment condition could result in the highest stability of silica nanofluid. After 7 days storage, the nanofluid showed no sedimentation, unchanged particle size, and zeta potential. The results of this study demonstrated that there is a great potential for the use of RHA as a low-cost renewable resource for the production of stable silica nanofluids.
Nanofluids, a concept first proposed by Choi , are nanoscale colloidal suspensions containing nanoparticles. In the past decades, nanofluids have attracted more and more attention due to its extraordinary heat/mass transfer performance [2–5]. Nanofluids have demonstrated great potential applications in many fields such as automobiles , electronics cooling , industrial cooling , drug delivery , and CO2 absorption enhancement .
The application of nanofluids has significant prospects, but it still faces several challenges for future development . Specifically, the low stability and high production cost of nanofluids are the major limiting factors [4, 11, 12]. Because of high surface energy, nanoparticles always have a tendency to coagulate automatically [13, 14]. Preventing the coagulation of nanoparticles is the primary issue for the application of nanofluids . On the other hand, the currently available methods used for preparing nanofluids usually require expensive raw materials and sophisticated equipments, and thus leading to higher production cost [11, 12]. Low-cost production of stable nanofluids is one of the most promising directions for future research .
With the speedy development of green nanotechnology, there is a growing tendency to produce nanoparticles with renewable resources . Rice husk, a low-cost agricultural residue, is abundantly available in rice-producing countries. Rice husk has a high calorific value (13–16 MJ/kg), and most of which is burned as fuel to generate energy, thus generating a significant volume of rice husk ash (RHA) . If the RHA is improperly handled, it will become a tremendous waste and can potentially pollute the environment. Nowadays, there is an increasing demand for eco-friendly disposal and utilization of RHA. Many studies indicated that it was a promising low-cost candidate for preparation of silica nanoparticles [18–20]. Nano-sized silica can be used to form nanofluids with special interest because of its high specific surface area, excellent stability, high mechanical resistance, and possibility of reuse [21, 22]. However, to the best of our knowledge, most of the silica used for preparing nanofluids are commercially available nanoparticles. The commercially available silica nanoparticles are typically prepared by using a silica precursor as silicon source, such as silicon alkoxide (typically silicon tetraethoxysilane). However, the synthesis of silica precursors is usually energy intensive and associated with high cost, eco-hazardous, and unsustainability issues .
Generally, there are two primary methods for producing nanofluids : (i) the one-step method, which represents the direct formation of nanoparticles inside the base fluids, and (ii) the two-step method, which means the preparation of nanoparticles and subsequent dispersion of nanoparticles in the base fluids. As compared to the single-step method, the two-step method is the most popular and economic process for the production of nanofluids , particularly, it is very suitable to prepare nanofluids containing oxide nanoparticles , such as SiO2, SnO2, CuO, and so on.
Kim et al. reported a sol–gel process for the synthesis of silica nanofluids . TEOS (Tetra Ethyl Ortho Silicate) was used as precursor. The stability of silica nanofluids were determined by zeta potential analysis. However, the long-term stability information was not available. Fazeli et al. synthesized silica nanofluids with commercially available silica nanoparticles by the two-step method . The nanofluids were stable for a period of 72 h without any visible settlements. Many other researchers also prepared silica nanofluids with commercial nanoparticles, but the stability results were not fully reported [26, 27].
Reagents and Materials
Rice husk ash was from Ji’an of Jiangxi Province in China. Hydrochloric acid and anhydrous sodium carbonate were supplied by Sinopharm Chemical Reagent Co., Ltd (China). Sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), and polyethylene glycol (PEG-1000) were purchased from Aladdin in Shanghai (China). Sodium hexametaphosphate (SH) was obtained from Wenzhou Chemistry Material Factory (China). All chemicals were analytical grade.
Preparation of Silica Nanoparticles
In a typical experiment, 100 g of RHA was added into 1000 ml of 1.0 M HCl solution and boiled for 2 h under stirring. The suspension was then filtered, and the solid residue was washed by distilled water to remove metallic ions. After that, the solid residue was dried in a ventilated oven at 120°C for 12 h.
After acid treatment, RHA (50 g) was put into a three-necked flask, and 250 ml of Na2CO3 solution (20 wt%) was added. The suspension was boiled with reflux condenser. After 4 h of vigorous stirring, the suspension was filtered and washed with 250 ml of hot water. Finally, the filtrate was transferred to another flask. CO2 was then introduced the filtrate. Stirring for 1 h, the resulting slurry (containing silica nanoparticles) was kept aging for 3 h and then filtered. The precipitate was washed by distilled water and was dried in a vacuum oven at 120°C for 24 h. The yield of silica was 71 ± 2%.
Preparation of Silica Nanofluids
Silica nanofluids were prepared by the two-step method, and deionized water was used as base fluid. First, silica nanoparticles were synthesized using RHA as silicon source, as described above. Second, the silica nanoparticles were dispersed in water with the help of ultrasonic vibration, and thus forming silica nanofluids. Typically, 1 g of silica nanoparticles was dispersed in 100 g water by stirring. Next, 1 g of dispersant was dissolved in the suspension. Subsequently, the suspension was placed into an ultrasonic generator (JY92-IIN, Ningbo Scientz Biotechnology Co., Ltd, China) and sonicated for 2 h. The temperature was kept below 25°C.
The morphology of silica nanoparticles were observed by scanning electron microscopy (SEM, Hitachi S-4700) and transmission electron microscopy (TEM, Tecnai G2 F30). The column chart of the particle size distribution (PSD) was obtained using the Image-Pro 5.1 (Media Cybernetics, Inc.) software according to the SEM and TEM images. Zeta potential was measured using a dynamic light scattering instrument (Zetasizer-nano ZS90, Malvern). X-ray diffractometer (XRD) measurements were carried out using an X-ray diffractometer (X’Pert PRO, PANalytical). The results were recorded using diffraction from 15° to 60°, 2θ, at a scanning rate of 5°/min. Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet model 6700 spectrometer (Nicolet Instrument Co., USA) in the range 400–4000 cm−1.
Results and Discussion
Preparation of Silica Nanofluids
To form nanofluids, nanoparticles were usually dispersed in water with the help of intensive mechanical force agitation. Ultrasonication, a generally accepted mechanical technique, is widely used to improve dispersion behavior of nanofluids. Ultrasonication is very suitable for dispersing the highly entangled or aggregated nanoparticle samples [13, 28], especially nanofluids containing oxide nanoparticles .
Influence of Dispersants on the Stability of Silica Nanofluids
The stability of nanofluids is very important for their practical applications, and it is closely related to nanofluids’ electrokinetic properties . High-stable nanofluids can be formed with high surface charge density to provide sufficiently repulsive forces. The investigation of the electrophoretic behavior through measurement of the zeta potential is important for understanding the dispersion behavior of nanofluids [13, 28, 30]. Generally, nanofluids with high absolute zeta potential are electrically stabilized and have good stability, whereas those with low absolute zeta potentials are easy to coagulate or flocculate. It is widely accepted that nanofluids with the absolute zeta potential above 30 mV are physically stable.
Influence of SDBS Concentration on the Stability of Silica Nanofluids
Influence of pH on the Stability of Silica Nanofluids
Long-term Stability of Silica Nanofluid
Rice husk ash-derived stable silica nanofluids were prepared by the two-step method. Ultrasonic vibration was employed to disperse silica nanoparticles in water. Well-dispersed silica nanofluids could be obtained with the sonication power of 500 W. SDBS was a suitable dispersant for silica nanofluids. With 1.0 wt% SDBS, the absolute zeta potential could reach to a maximal value of 42.3 mV. The stability of silica nanofluids was also highly dependent on pH. At a pH of 9.5, the stability of nanofluid is the best. Long-term stability study indicated that nanofluid prepared at the optimal experiment conditions showed unchanged particle size and zeta potential during 7 days storage. Therefore, RHA is a promising low-cost renewable resource for the preparation of stable silica nanofluids.
This work was supported by the National Natural Science Foundation of China (no. 21406198) and Zhejiang Provincial Natural Science Foundation of China (no. LQ14B060003).
The experiments were guided by ZZ. WH and JZ prepared the SiO2 nanoparticles and nanofluids. WH and ZZ characterized the SiO2 nanofluids. GW and JJ participated in the discussion and gave useful suggestions. The manuscript was composed by ZZ. All authors approved the final manuscript.
The authors declare that they have no competing interests.
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