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Characterization of Silica (SiO 2 ) by Fourier Transform Infra-red Spectrophotometry (FTIR) and Scanning Electron Microscope (SEM)
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Characterization of Silica (SiO
2) by
Fourier Transform Infra-red Spectrophotometry (FTIR) and Scanning Electron Microscope (SEM)
Purnama Ningsih and Irwan Said
Chemistry Education, Department of Mathematics and Natural Sciences, Faculty of Teacher Training and Education, Tadulako University, Palu 94118, Indonesia.
Abstract. In Mpanau Power Plant (PLTU), the energy source used to generate electricity comes from coal which is a fossil fuel. Economically, coal is a cheap and affordable, however, it is a non- renewable. Besides, the use of coal has an impact on environmental damage. This is because coal combustion waste generates solid waste in the form of ash derived from fly ash and bottom ash.
However, this waste can be extracted into Silica (SiO2) and can be valuable, for example in construction application. In this paper, the characterization of silica will be studied by using Fourier Infra-Red Spectrophotometry (FTIR) and Scanning Electron Microscopy (SEM).
INTRODUCTION
Mpanau Coal Steam Power Plant (Mpanau PLTU) is source of electrical energy in Palu city and its surrounding areas. Generally, coal is the material that use to generate electricity in the power plant, also in Mpanau PLTU. Coal is a sedimentary rock that comes from the remnants of ancient plants where the formation process lasted for millions of years, resulting in increased carbon content. Economically, coal is a cheap and affordable, however, it is a non-renewable.
Besides, the use of coal has an impact on environmental damage. This is because coal combustion waste generates solid waste in the form of ash derived from fly ash and bottom ash.
Bottom ash is a solid waste produced in the combustion process of coal which has a slightly coarse, blackish color and is heavier than fly ash. The main components of bottom ash are oxides containing silica and other mineral minerals. Chemically, bottom ash has the same acidity as fly ash, only from the coal burning process as much as 10-20% produced is bottom ash.
The chemical composition of bottom ash is mostly composed of Si, Al, Fe, Ca, Mg, S, Na and other chemical elements (Santoso, 2003).
Silica is a compound containing a central silica atom surrounded by electronegative ligands. The type of silicate that is often found generally consists of silicon with oxygen as its ligand. Silica is found in nature in the form of quartz minerals. Silica, also known as silicon dioxide or SiO2, is found in many minerals called quartz sand. This quartz sand consists of silica crystals and contains impurities that are carried during the deposition process. Quartz sand is also known as white sand which is the result of weathering rocks that contain major minerals such as quartz and feldspars. Silica is generally obtained through a mining process that starts from mining quartz sand as raw material. The quartz sand is then processed by washing to dispose of impurities which are then separated and dried again so that sand is obtained with greater silica content (Marita, 2011).
Separation technique that is often used for ash material from waste power plants is by washing and using alkali or acid. The principle of this technique is to reduce the content of impurities involved in the sample. Removal of impurities will increase the crystallinity of ash (Zhang et al, 2007). Extraction is the process of separating a substance from its mixture by using a solvent. The solvent used must be able to extract the desired substance without dissolving other materials.
Physical characterization is an important step in the development of materials. Material characterization can be done using FTIR (Fourier Transform Infra-Red Spectroscopy) test through testing of functional groups of silica and XRD (X-ray Diffraction) through testing of silica crystal structures. The purity of silica can also be analyzed qualitatively through FTIR and XRD tests.
Morphological tests can be carried out using optical microscopy or electron microscopy (SEM) (Scanning Electron Microscopy). Therefore, the characterization of silica will be studied by using Fourier Infra-Red Spectrophotometry (FTIR) and Scanning Electron Microscopy (SEM) in this study.
METHOD
Sampling was picked from several points where ash pile of waste in Mpanau PLTU. Once it is inserted into the sack. Samples from several sampling points were homogenized, each ash from fly ash and bottom ash 50 grams soaked in hot water for 2 hours. Then the ash was separated back from the water. Then each sample was weighed again.
Each of the 20 grams of the sample was immersed in 100 mL 2.5 M NaOH solution then heated to 85oC while stirring with a magnetic stirrer for 1 hour. Subsequently the sample was filtered and the filtrate containing dissolved silica was accommodated. To precipitate silica, into the filtrate is added 1 M HCl solution gradually until the formation of the silica precipitate stops (pH range 6.5-7). After that the precipitate is separated and washed with hot water to remove the excess acid. The silica obtained from this treatment was then dried in an oven at 110 °C for 6 hours to remove water (Pandiangan, 2008 & Retnosari, 2013). The yield obtained was measured by silica. Samples containing silica from the extraction process were further characterized using FTIR and SEM.
RESULT AND DISCUSSION
The waste ash of Mpanau PLTU used in this research was derived from bottom ash and fly ash. Bottom ash and fly ash are coal combustion products. The number of these two types of ash will increase frequently with wider use of coal. Bottom ash and fly ash have many elements and chemical compounds, one of the largest components is silica. Silica can be obtained from the both ash by many separation ways. The silica separation method from ashes in this research was accomplished by solid-liquid extraction method.
Solid-liquid extraction is an extraction process involving two phases of solid phase (bottom ash and fly ash) and liquid phase (NaOH). In this extraction, when the extraction material is mixed with the extractant then the extractant will react with the solid material by forming the extract. The silica present in bottom ash and fly ash (Figure 1) is obtained by dissolving each of the two types of ash into an alkaline solution, then heated. The purpose of heating to speed up the reaction rate, where high temperatures will increase the amount of soluble silica in the extractant.
Gambar 1. Sampel bottom ash dan fly ash
In silica, the high electronegativity of element O causes Si to be more electropositive and create an unstable intermediate [SiO2OH]-. In this situation dehydrogenation will occur and the second hydroxyl ion will bind to hydrogen, here will form water. Two Na+ ions will balance the negative charge formed and interact with the SiO32- ions-thus forming sodium silicate (Mujiyanti, 2016). The purpose of stirring at the time of heating is to distribute the temperature to evenly and accelerate the contact between the solvent and the solute. In addition, to reduce the precipitation (Kurniati, 2009). The reaction mechanism between SiO2 derived from bottom ash and fly ash with alkali (NaOH) can be seen below:
SiO2 (s) + 2 NaOH (aq) → Na2SiO3 (aq) + H2O (aq)
Silica can react with a base, especially a strong base in this case NaOH solvent. However, the silica compounds that are formed and still in the form of sodium silicate. Therefore, it is necessary to add 1 M HCl to bind sodium to obtain SiO2. This is in line with that expressed by Kalapathy (2002) that the silica compound is easily soluble in an alkaline atmosphere and will settle in an acidic atmosphere. Reaction mechanism can be seen below:
Na2SiO3 (aq) + 2 HCl (aq) → H2SiO3 (aq) + NaCl(aq)
After the silica compound has settled, the water content that affects the moisture of the product can be removed by drying in the oven. The silica produced in this process is coarse silica (Lubis, 2009). The reactions that occur are as follows:
H2SiO3 (aq) → SiO2 (s) + H2O (aq)
The data of the silica content produced in the separation process by using extraction can be seen in Table 1. Based on the table, it can be seen that the silica content of samples derived from fly ash is greater than the silica content derived from the bottom ash sample.
Table 1. Silica Content from the bottom and fly ash by extraction
Repeating Silica content from bottom ash
Silica content from fly ash
1 4.22 5.97
2 4.18 5.88
3 4.28 6.05
Mass of silica
(gram) 4.23±0.05 5.97±0.08
Silica Content (%) 21.31 29,85
The analysis results of silica compounds extracted by using FTIR can be seen in Figure 2 for sample derived from bottom ash and Figure 3 for samples derived from fly ash. The spectrum in Figure 2 yields several peaks which showing several functional groups. For clarity, it can be seen in Table 2 while spectrum in Figure 3 produces several peaks indicating some functional groups in samples derived from fly ash and for an explanation can be seen in Table 3.
Figure 2. FTIR silica spectra from bottom ash sample
500 750 1000 1250 1500 1750 2000 2500 3000 3500 4000 4500
1/cm 30
40 50 60 70 80 90 100
%T
3452.58 1627.92 1087.85 794.67 690.52 460.99
ba1
Table 2. Interpretation of spectra on bottom ash sample Wave number (cm-1) Explanation
460.99 show the Si-O bond (Lin et all, 2001)
794.67 bond deformation Si-O on SiO4
(Pandiangan dkk, 2008)
1067.86
the asymmetric stretching vibration of SiO from sicloaksan Si-O-Si (Daifullah et all,
2003)
1627.92 stretch vibration of C=O from hemicellulose (Pandiangan dkk, 2008)
3452.58 stretch vibration –OH (hydroxyl group) (Pandiangan dkk, 2008)
Figure 3. FTIR silica spectra from fly ash sample
500 750 1000 1250 1500 1750 2000 2500 3000 3500 4000 4500
1/cm 15
30 45 60 75 90
%T
3450.65 1639.49 1473.62 1087.85 1051.20 796.60 779.24 692.44 563.21 466.77
fa1
Table 3. Interpretation of spectra on bottom ash sample
Based on the results of the spectra analyzes shown in Tables 3 and 4, we can see that the peaks indicating the presence of some typical functional groups possessed by silica. However, there are some emerging peaks that cannot be identified. These peaks appear because of impurities from the analyzed samples that did not get separated at the time of sample extraction.
The morphology of the studied silica surface is shown by the SEM, results in Figure 4 is for samples from bottom ash for magnification of 10000x and Figure 5 for magnification 40000x.
Figure 4. SEM of silica from bottom ash sample with 10000x Wave number (cm-1) Explanation
466.77 show Si-O bond (Lin et all, 2001)
779.24 779.60
bond deformation Si-O on SiO4 (Pandiangan dkk, 2008)
1051.20 1087.85
the asymmetric stretching vibration of SiO from siloaksan Si-O-Si (Daifullah, 2003)
1639.49 stretch vibration C = O of hemicellulose (Pandiangan dkk, 2008)
3450.65 stretch vibration –OH (hydroxyl group) (Pandiangan dkk, 2008)
Figure 5. SEM of silica from bottom ash sample with 40000x
From Figure 4 it is clear that the sample surface is uneven and consists of clusters, indicating the existence of fairly large grain sizes with uneven distribution on the surface of this as disclosed.
Then clarified again with the magnification of 40000x in Figure 5.
Figure 6 shows the morphology of the SEM silica surface for samples derived from fly ash for magnification of 10000x and Figure 7 for 40000x magnification.
Figure 6. SEM of silica from fly ash sample with 40000x
Figure 7. SEM of silica from fly ash sample with 40000x
The result of SEM characterization for silica from fly ash shows quite different morphology with bottom ash sample. Although both samples showed uneven and clumped surfaces, the silica derived from the fly ash sample was more rounded. The same results are also obtained by Astuti, 2015.
CONCLUSION
Based on the results of research that includes extraction and characterization, the following conclusions are obtained: (1) The silica content obtained from the bottom ash and fly ash of the Mpanau PLTU are 21.31% and 29.85%, respectively. (2) The FTIR spectrums of the bottom ash and fly ash samples shows the peaks which are characteristic of the silica compound.
(3) The silica morphology of the bottom ash and fly ash samples shows the sample surface is uneven and consists of clumps and the shape is grain.
REFERENCES
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