Preparation and Characterization of Amorphous Silica and Calcium Oxide from Agricultural Wastes

Introduction

Rice husk and bagasse areagricultural waste material abundantly available in Thailand.They areimportant biomass sources, which used as a fuel in the brick industry and sugar industry, respectively. As a result, a large quantity of rice husk ash and bagasse ash are generated and created a serious dispensation problem.Rice husk ash and bagasse ash are rich in silica (SiO₂)(more than 60% for bagasse ash and above 90% for rice husk ash) and can be used as a raw material for produce the silica powderproduction^1-5.Many studies have reported thatthe rice husk ash and bagasse ash were an excellent sources for synthesis of amorphous silica^6-9 becausethe fine silica powder is a basic raw material that is widely usedin electronic substrates, thermal and electrical insulators, optoelectronic devices and composite fillers, etc.¹⁰.

While the chicken eggshells and duck eggshells are one group of agricultural waste materials which can be found as waste products from restaurants and fresh markets in Thailand, as they areused for cooking. The chemical composition of eggshells is calcium carbonate (CaCO₃)typically containing a high level (above 90%)^{11, 12}.As the chicken and duck eggshells are composed of calcium carbonate, so being calcinedat above 700 °Cwill produce the CaO powder^{13, 14}, which can be used as a basic adsorbent for carbondioxide (CO₂)¹⁵ and as a catalyst for the production ofbiodiesel¹³.

Therefore, the aim of this study was to synthesize of amorphous silicaand calcium oxide powder from agricultural wastes by using the rice husk ash and bagasse ash used as sources for silica, while the chicken eggshells and duckeggshells were sourcesfor calcium oxide. The synthesized powder were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD),Brunauer-Emmett-Teller (BET), particle size analyzerand scanning electron microscopy (SEM).

Materials and Methods

Synthesis of Sample

Rice husk ashand bagasse ash were obtained from a brick factory and sugar industry, respectively and theywere rinsed by water several times to removeimpurity and then burned at 700°C for 3 h. To evaluate the chemical compositions of the rice husk ash and bagasse ashwas analyzed by X-ray fluorescence (XRF), this resultwas shown in Table 1.Subsequently, twenty grams of rice husk ash and bagasse ash sampleswere stirred in 80 ml of 2.5 N sodium hydroxide solutions, afterwards heated in a covered 250 ml Erlenmeyer flask for 3 h. After that, the solution was filtered and the residue was washed with 40 ml of boiling water. The filtrate was allowed to cool down at room temperature. Then, it was added by 5 N H₂SO₄ with constant stirring until pH 2. Then,the filtrate was added NH₄OH until pH 8.5 left for 3 h and dried at 100°C for 12 h and finally waspulverized by mortar.

Simultaneously, the eggshells waste received from restaurant were rinsed with water several times to remove additional residues or any unwantedsubstancefromthe surface and then heated at 100^oC for 12 h to dry by oven. Completely dried eggshells was pulverized by using a hammer mill, after that crushed with a juice blender and then passed through a sieve.Next the chemical compositions of eggshells were analyzed by X-ray fluorescence (XRF) shown in Table 2. The dried eggshells werecalcined in a muffle furnace under static air conditions at 900^oC for 3 h to transform the calcium carbonate toCaO powder.

Characterization

The powders obtained were characterized by various techniques. The average particle size distribution was determined by using particle size analyzer (Mastersizer S, Malvern) and examination the functional groups studied by Fourier transforms infrared spectrometer (Perkin Elmer spectrometer). X-ray diffractometer(Phillips X’ pert X-ray diffractometer) was used to identify phase of the powder sample. The specific surface area of the powder sample was measured using the BET nitrogen adsorption method (Quantachrome), and morphologyof sample observed by scanning electron microscope (LEO/1450).

Results and Discussion

The chemical composition of the raw materials for this study is shown in Table 1 and Table 2.In the Table 1, it showedthat the chemical composition of rice husk ash and bagasse ash samples, after burning out at 700^oC for 3 h,presented the highest amount of SiO₂; while,the major component of both eggshells displayedcalcium CaCO₃ (Table 2).

Table 1: Chemical composition of rice husk ash and bagasse ash

Samples	Inorganic component (wt.%)
	SiO2	Al2O3	MgO	Fe2O3	CaO	P2O5	Mn2O3	SO3	K2O	TiO2	Total
Rice husk ash	93.57	0.80	0.66	0.59	0.90	1.02	0.28	0.10	2.01	0.08	100
Bagasse ash	80.46	1.24	4.07	1.14	4.43	5.24	0.25	0.69	2.36	0.15	100

 

Table 2: Chemical composition of egg shells

Samples	Inorganic component (wt.%)
	CaCO3	Al2O3	MgO	Fe2O3	SiO2	P2O5	Mn2O3	SO3	K2O	TiO2	Total
Chicken egg	99.66	–	–	–	–	–	–	0.34	–	–	100
Duck egg	99.19	–	–	–	–	–	–	0.82	–	–	100

 

The X-ray diffraction pattern of SiO₂ powderobtained from rice husk ash and bagasse ash is shown in Fig.1. The results showed the strong broad peakataround22º (2θ) of both sample after calcining ash at 700°Cfor 3 h. This resultsconfirmedthat this peak is nature of silica, which supposed to be the characteristic of amorphousSiO₂. For the results from FT- infrared (FTIR) spectrum of SiO₂ powder is presented in Fig. 2. It clearly showed aslightly broad band since 2,800 to 3,700 cm^-1which assigned to present the OH stretching frequency of silanol groups and adsorbed water. A strong intense band at 1,100 cm^-1is associated to the siloxane Si-O-Si vibration of the molecules.Amorphous silica exhibits a relatively strong peak around at 800 cm^-1.

Figure 1: XRD patterns of SiO2 powder after prepared from (a) rice husk ash and (b) bagasse ash Click here to View Figure

 

Figure 2: FTIR spectrum of SiO2 powder after prepared from (a) rice husk ash and (b) bagasse ash Click here to View Figure

 

For the X-ray diffraction pattern of the CaO powder obtained from chicken eggshellsand duck eggshells are shown in Figs. 3 and 4, respectively. The results revealed that the thermal process of CaCO₃ phase is completely decarbonized and turned to beCaOphaseafter both chicken and duck eggshells werecalcined at 900^oC.The shape of the sharp and strong diffraction peaks indicated that the samples are well crystallized andcorresponded to the standard values of file JCPDS No. 37-1497.

Figure 3: XRD pattern of chicken eggshells (a) beforecalcined and (b) after calcined at 900 oC Click here to View Figure

 

Figure 4: XRD pattern of duck eggshells (a) beforecalcined and (b) after calcined at 900 oC Click here to View Figure

 

Fig. 5 showed the FT-infrared (FTIR) spectrum of the CaOpowder after prepared from chicken eggshells and duck eggshells, respectively. In Fig. 5a and 5b presented the strong band at 3638 cm^-1can be attributed to the (OH) stretching vibration. The broad band around 1400 cm^-1indicated the symmetric stretching vibration of carbonate. Small band at 874 cm^-1 demonstrated the presence of carbonate species. The tiny band at550 cm^-1 identified vibration of the Ca-O bond¹⁶.

Figure 5: FTIR spectrum of CaO powder after prepared from (a) chicken eggshells and (b) duck eggshells Click here to View Figure

 

While, the average particle size and specific surface area of the SiO₂ and CaO powder is shown in Table 3.The results displayed the average particle size of the amorphous SiO₂ powderobtained from rice husk ash and bagasse ashis not significantly different, the particle sizes are 46.74 µm and 47.16 µm, respectively. In addition, the specific surface area ofamorphous SiO₂powder was also similar as well. However,the average particle size of the CaO powderobtained from chicken and duck eggshells after being calcined of CaCO₃phase are 57.13 µm and 23.91 µm, respectively. And the specific surface area of CaOpowderare 13.20 m²/g and 16.42 m²/g, respectively.

Table 3: Average particle size and specific surface area of prepared samples

Samples	Average particle size [D 4,3] (µm)	Specific surface area (m2/g)
SiO2 from rice husk ash	46.74	271.21
SiO2 from bagasse ash	47.16	233.85
CaO from chicken eggshells	57.13	13.20
CaO from duck eggshells	23.91	16.42

 

The morphology of the amorphous SiO₂ powderobtained from rice husk ash and bagasse ash is shown in Fig. 6. The SEM images indicated that the particles of powder areagglomerated, various sizes, irregular morphology, and particles size more than 1 μm were recognized. In addition, the average particle size of SiO₂ powder obtained from rice husk ash is in the range of 1-7 μm (Fig. 6a), while theaverage particle size of SiO₂ powder obtained from bagasse ash is in the range of 1-10 μm (Fig. 6b).

Figure 6: SEM micrographs of amorphous SiO2 powder after prepared from (a) rice husk ash and (b) bagasse ash Click here to View Figure

 

Fig. 7 showed the SEM images of the CaOpowder aftercalciningof CaCO₃ fromchicken eggshells and duck eggshells. In Fig. 7a can be seen that the average particle size and shape of CaOparticles obtained from chicken eggshells wassimilar to theCaO particles obtained from duck eggshells as shown in Fig. 7b. The characteristic of CaO particles of both chicken eggshells and duck eggshellsare regular micro morphology of rod like, the average particles with size ranging from 2µm to 4µm of width are observed.

Figure 7: SEM micrographs of CaO powder after prepared from (a) chicken eggshells and (b) duck egg shells Click here to View Figure

 

Conclusion

Amorphous SiO₂is successfully synthesized from the rice husk ash and bagasse ash thatused asraw materials. While the CaO powder can be synthesized by using CaCO₃from chicken eggshells and duck eggshells thatused as raw materials, and thenburning at 900^oC for 3 h.The XRD results clearly exhibited the structural formation of SiO₂ and CaO phase. The FTIR spectrums demonstrated that the characteristic peaks of the important functional groups present in the powder samples of SiO₂ and CaO. The SEM images illustrated that the particles are agglomerated, various sizes, and the particle size is found to be in the micro regime.This study revealed that the agricultural waste material of rice husk ash, bagasse ash, chicken eggshells, and duck eggshells acts as an alternative source for theproduction of SiO₂ and CaO powder.

Acknowledgments

This work was financial supported by the young researcher development project of KhonKaen University.

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