Article pubs.acs.org/IECR
Synthesis of Imogolite from Rice Husk Ash and Evaluation of Its Acetaldehyde Adsorption Ability Teruhisa Hongo,*,† Junro Sugiyama,† Atsushi Yamazaki,‡ and Akihiro Yamasaki† †
Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan ‡ Department of Resources and Environmental Engineering, School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ABSTRACT: Large amounts of rice husk ash (RHA) are discharged from rice husk power plants, and the development of an effective system for recycling this RHA waste would be desirable. In the current study, the silica component of RHA obtained from a rice husk power plant in Myanmar was successfully used as raw material for the synthesis of the aluminum silicate nanotube material, imogolite. The RHA used contained 91.65 wt % silica, which was composed of a mixed phase of cristobalite, tridymite, and amorphous silica. The synthesized imogolite had a BET surface area of 282.0 m2/g and a web-like structure formed of fibrous bundles. The imogolite adsorbed 8.8 μmol/g acetaldehyde in 60 min, representing a value approximately 5 times greater than that achieved by the parent RHA.
1. INTRODUCTION Rice husk is an agricultural byproduct abundantly available in rice-producing counties and accounts on average for 20% of the paddy produced, on a weight-for-weight basis.1 Increasingly, rice husk is being used as fuel in power plants because of its high calorific value of approximately 13−16 MJ/kg.1−3 During the combustion process, rice husk ash (RHA) is produced and causes a number of environmental problems related to pollution and disposal. RHA can consist of 85−98 wt % silica, with the actual value being dependent on several variables, including the combustion conditions, furnace type, rice variety, rice moisture content, and the regional climate and geographical conditions experienced during the rice cultivation process.4 It is generally expected that the silica present in RHA could be converted to a high value material, and significant efforts have therefore been invested in finding an appropriate utilization for the RHA that would represent a desirable solution to this problem. Imogolite is a naturally occurring nanotube aluminum silicate, with a high surface area of over 250 m2/g,5 and is often found in soils originating from volcanic material such as pumice and volcanic ash. Imogolite is classified as a clay mineral, with tube dimensions of approximately 2 and 1 nm for its external and internal diameters, respectively, and lengths ranging from tens of nanometers to several micrometers.6 The chemical composition of imogolite is (OH)3Al2O3SiOH. The formation of imogolite can be described by considering a single Al(OH)3 gibbsite sheet and substituting, on one side only, three OH groups with an orthosilicate unit O3SiOH. The gibbsite-like sheets effectively curl up, eventually forming nanotubes, because the Si−O bonds are shorter than Al−O bonds.7 The inner and outer surfaces are covered with SiOH and AlOH groups, respectively. The hydroxyl groups on the walls of the nanotubes make them hydrophilic, and this property is considered to be applicable for the adsorption of hydrophilic molecules. © 2013 American Chemical Society
Acetaldehyde is a hydrophilic molecule and is well recognized as one of many toxic volatile organic compounds (VOCs). The material can be emitted from several different sources, including construction materials, cigarette smoke, paint, polymerized plates, and during the manufacture of binders and resins.8 Excessive exposure of the human body to high concentrations of acetaldehyde can lead to the occurrence of several health issues, including headaches, nausea, DNA damage, and abnormal muscle development.9 An effective method for the removal of acetaldehyde is therefore required. Activated carbons are commonly used for the removal of gaseous contaminants from air because they can adsorb a variety of different gases through physisorption. The adsorption capacity of activated carbons for acetaldehyde, however, is not high. It was envisaged that imogolite would behave as a good adsorbent of acetaldehyde because of the hydrophilic nature of its surface. Although the adsorption ability of imogolite for a variety of different gas molecules has been investigated by several researchers, to the best of our knowledge, only hydrophobic molecules such as benzene, methane, and carbon dioxide have been investigated.10−12 The adsorption ability of imogolite for hydrophilic molecules and acetaldehyde in particular, therefore, has yet to be revealed. In this study, we have characterized the RHA obtained from a rice husk power plant in Myanmar, because the composition and crystal phase of RHA changes with combustion conditions. According to RHA properties, we have developed a synthesis process of imogolite utilizing the silica contained in the RHA. Furthermore, we have also investigated the acetaldehyde adsorption ability of the resulting imogolite. Received: Revised: Accepted: Published: 2111
September 4, 2012 December 28, 2012 January 10, 2013 January 10, 2013 dx.doi.org/10.1021/ie302379f | Ind. Eng. Chem. Res. 2013, 52, 2111−2115
Industrial & Engineering Chemistry Research
Article
Table 1. Chemical Composition of RHA Obtained from the Power Plant in Myanmar (wt %) SiO2
Al2O3
Na2O
K2O
CaO
TiO2
MnO
Fe2O3
MgO
P2O5
LOI
91.65
0.76
0.09
1.76
0.42
0.03
0.19
0.54
0.48
1.17
2.9
2. EXPERIMENTAL SECTION 2.1. Preparation of Imoglite. RHA (Takada Engineering Co. Ltd., Japan) was used as the silica source. Commercially available Al(NO3)3 was used as an alumina source, because it is a major component of imogolite. Imogolite was prepared by a procedure modified from that reported in the literature.13 RHA (1.0 g) was added to an aqueous solution of 3 N NaOH (10 mL), and the resulting mixture was hydrothermally treated at 80 °C for 24 h. The mixture was then filtrated, and the filtrate was mixed with a solution of Al(NO3)3 (8.3 g) in water (490 mL). The resulting solution was adjusted to pH 5.0 with stirring using a 1 N aqueous solution of NaOH, leading to the formation of a precipitate. The precipitate was collected by centrifugation and added to distilled water (1 L). Acetic acid (0.3 mL) was then added, and the resulting mixture was adjusted to pH 4.0 with stirring using H2O2. The mixture was then vigorously stirred at 100 °C for 48 h and subsequently cooled to room temperature, before being adjusted to pH 7.0 using a 1 N solution of NH3 under stirring for flocculation. The precipitate was then collected by centrifugation and sequentially washed with distilled water and centrifuged three more times before being dried at 80 °C for 48 h. 2.2. Characterization. Chemical analysis was carried out using X-ray fluorescence (XRF) spectrometry with a Philips X’Unique II. Thermogravimetric and differential thermal analysis (TG−DTA) measurements were performed in air using a Shimadzu DTG-60H apparatus. X-ray diffraction (XRD) patterns were obtained with a Rigaku RINT-Ultima III diffractometer using Cu Kα radiation. N2 adsorption− desorption isotherms were measured at 77 K using a BELSORP mini (BEL, Japan). Scanning electron microscope (SEM) images were obtained using a JEOL JSM-5200. Transmission electron microscope (TEM) images were obtained using a JEOL JEM-1000CW operated at 100 kV. 2.3. Acetaldehyde Adsorption. The adsorption abilities of the RHA and the obtained imogolite samples for acetaldehyde were examined. Following a period of drying at 100 °C, a portion (0.5 g) of each sample was quickly enclosed and clipped in the corner of a gas-sampling bag (3 L) made of polyvinyl fluoride film. Residual air in the bag was degassed using a small pump over a period of 5 min. Dry nitrogen gas (1 L) containing acetaldehyde vapor (104 ppm) was injected into the degassed bag and the contact time started when the clip was removed. The concentration of acetaldehyde in the bag was analyzed using a gas chromatograph (GC-2014, Shimadzu) equipped with a flame-ionization detector. Gas samples of 100 μL in size were periodically removed from the bag for each measurement. The gas in the bag was maintained at ambient pressure throughout the adsorption test. The concentration in the bag was evaluated at 25 °C as a function of the contact time.
most of the volatile substances and organic matter had been eliminated during the combustion process. At the power plant in Myanmar, the rice husk is burnt at 700−800 °C for a period of 30−40 s. Kordatos et al.14 reported that more than RHA with a silica content in excess of 90% was obtained from the combustion of rice husk at 700 °C. The combustion of rice husk at that temperature can lead to the removal of the amorphous carbon, nongraphitic carbon (crystalline form), and the remaining organic substances present in the rice husk.15 Parts a and b of Figure 1 show the XRD patterns of the RHA and the material synthesized from the RHA, respectively. The
3. RESULTS AND DISCUSSION The chemical composition of the RHA obtained from the rice husk power plant in Myanmar was analyzed using the XRF technique, and the results are shown in Table 1. The loss on ignition (LOI) was determined by TG−DTA analysis. The silica content of the RHA was higher than expected because
Figure 1. XRD patterns of (a) the RHA obtained from the rice husk power plant in Myanmar and (b) the imogolite synthesized from the RHA. 2112
dx.doi.org/10.1021/ie302379f | Ind. Eng. Chem. Res. 2013, 52, 2111−2115
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XRD pattern of the RHA confirmed the presence of diffraction peaks corresponding to cristobalite (ICDD No. 39-1425) and tridymite (ICDD No. 18-1170), and a broad halo peak at 2θ = 23° typical of amorphous silica.16 Quartz is a stable phase of silica at 700−800 °C, and tridymite and cristobalite are crystallized at 867−1470 and 1470−1727 °C, respectively, at atmospheric pressure.17 The RHA, however, was composed of cristobalite, tridymite, and an amorphous phase, even though it was burned at 700−800 °C. This phenomenon has been attributed to the effect of alkaline contamination. Wollast studied the phase diagram of a K2O−SiO2 system and revealed that tridymite and cristobalite were crystallized at much lower temperatures when a small percentage of K2O was added to the SiO2.18 The XRD pattern of the material synthesized from the RHA showed characteristics of imogolite. The broad diffraction peaks observed occurred predominantly as a consequence of its bundled structure.19 Kang et al.20 investigated the origin of the broad peaks using simulations of the XRD patterns. The broadness of the diffraction peaks indicated that the bundles consisted of three or four nanotubes without any long-range ordering. Although the bundles were packed in a parallel fashion, the arrangement of the nanotubes was not coherent. The diffraction peak occurring at 2θ = 4.54° was assigned to the diffraction of the (100) plane. The cell parameter a, corresponding to the center-to-center distance between two aligned nanotubes and assuming a hexagonal packing (a = 2d100/√3), was calculated to be 2.25 nm. The peak at 2θ = 6.02° was assigned to the diffraction of the (110) plane, which is sometimes not observed in an imogolite diffraction pattern, because the d100 reflection is directly related to the packing angle of nanotubes and its position changes as a consequence of any variations in this angle.20 The diffraction peak at 2θ = 9.84° was assigned to the repetition of the structural unit of imogolite along the tube (d001), whereas the diffraction peak at 2θ = 13.6° was assigned to the (211) planes.21 The nitrogen adsorption−desorption isotherms and Barrett− Joyner−Halenda (BJH) pore size distribution plots of the RHA and the imogolite synthesized from the RHA are shown in Figure 2. The RHA isotherms revealed that a large amount of the nitrogen adsorption was observed above P/P0 = 0.8, indicating the presence of large mesopores and macropores.22 The specific surface area, calculated according to the multipoint Brunauer−Emmett−Teller (BET) method, was found to be 7.1 m2/g. The pore size distribution plot revealed the existence of a variety of different sized pores in the meso- and macropore ranges. The results of the imogolite were in good agreement with those reported for the earlier adsorption studies of synthetic imogolite.23 The imogolite showed a type I isotherm, and presented an accentuated adsorption at low P/P0 values. Capillary condensation, which is a representative indicator of mesoporous materials, was not observed for imogolite because of its random mesopores.19 The specific surface area of the imogolite was found to be 282.0 m2/g. The morphology of the RHA was studied by SEM. Figure 3 shows an SEM image of the RHA. The RHA consisted predominantly of fragments of loose flakes with a skeleton-like inner structure. In accordance with the nitrogen isotherm results, the macroporous structure of the material could clearly be observed, with the appearance of the inner wall of the RHA. The macroporous nature of the material was attributed to the loss of any lower density organic compounds during the combustion process.24,25
Figure 2. Nitrogen adsorption−desorption isotherms and BJH pore size distribution plots (inset) for (a) the RHA obtained from the rice husk power plant in Myanmar and (b) the imogolite synthesized from the RHA.
Figure 3. SEM image magnified 350 times of the RHA obtained from the rice husk power plant in Myanmar.
The TEM image of the imogolite synthesized from the RHA is shown in Figure 4. The image revealed the imogolite form to be composed of fibrous bundles of tubes, each containing many individual imogolite nanotubes.26 The imogolite bundles were 2113
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min, when the concentration of acetaldehyde was found to be 5 ppm. Using the activated carbon as an adsorbent, the acetaldehyde concentration gradually decreased until 60 min, when the concentration of acetaldehyde was found to 46 ppm. The amounts of acetaldehyde adsorbed over a period of 60 min by the RHA and imogolite samples were 1.8 and 8.8 μmol/g, respectively, revealing that the imogolite had an adsorption capacity 5 times greater than that of the RHA for the removal of acetaldehyde. Moreover, it was revealed that the imogolite showed a higher adsorption rate and higher adsorption capacity compared to the activated carbon.
4. CONCLUSIONS The RHA obtained from a rice husk power plant in Myanmar contained 91.65 wt % silica. The silica was composed of a mixed phase of cristobalite, tridymite, and amorphous silica. Using this silica component as a raw material, imogolite was successfully synthesized. The imogolite had a web-like structure formed from fibrous tube bundles, and a BET surface area of 282.0 m2/g. The imogolite adsorbed 8.8 μmol/g acetaldehyde in 60 min, whereas the RHA adsorbed 1.8 μmol/g over the same time period. By converting the RHA to imogolite, an adsorbent with an acetaldehyde adsorption approximately 5 times greater than that of the parent material was obtained.
Figure 4. TEM image of the imogolite synthesized from the RHA.
entangled, forming a network of web-like structures. The diameter of each bundle was estimated to be approximately 4− 40 nm. To evaluate the adsorption ability of the obtained imogolite for acetaldehyde, a comparative experiment was conducted with RHA and a commercial activated carbon (Kanto Chemical Co., Inc.), the specific surface area of which was 1290 m2/g. Many researchers have studied the adsorption ability of RHA with the expectation that the material would represent a low-cost and effective adsorbent for a wide variety of pollutants in wastewater and polluted air. The variation of acetaldehyde adsorption as a function of time is shown in Figure 5. The
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-422-37-3984. Fax: +81-422-37-3871. E-mail: peea.
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 5. Concentration changes of acetaldehyde as a consequence of adsorption on the RHA, activated carbon, and imogolite synthesized from the RHA as a function of time.
imogolite, RHA, and activated carbon adsorb acetaldehyde by physisorption. The kinetic curves revealed that the adsorption capacities of the RHA and the imogolite were considerably different. Using the RHA as an adsorbent, the acetaldehyde concentration exhibited an initial step at 5 min, after which time it gradually decreased until 60 min, when the concentration of acetaldehyde was found to 83 ppm. For the imogolite, the acetaldehyde concentration decreased to 30 ppm following the initial 5 min and subsequently continued to decrease until 60 2114
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