## 3 Results

Table 1: Elemental composition of CHA at different conditions. Figure 2: Thermographic analysis curve of coconut husk. DTG—difference thermogravimetry. for every 1000 g of coconut husk after pyrolysis. From the XRF analysis, about 8-11% SiO\({}_{2}\) was found in the CHA. Therefore, about 4.5-6.2 g of SiO\({}_{2}\) can be obtained from CHA, with 91.76% purity, after acid treatment. The crystalline or amorphous nature of the samples was analyzed using XRD. Figure 3 depicts the XRD spectra of the CHA plotted from the data obtained using X'Pert Highscore Plus (Malvern Pananalytical, Almelo (The Netherlands) and Malvern (UK)). From the spectra, the samples were likely to be in a crystalline form, as a result of the presence of many diffraction peaks in the spectra. Several peaks were found, and were identified as P\({}_{2}\)O\({}_{5}\), Cl, SO\({}_{3}\) and SiO\({}_{2}\), on CHA at 500-700 \({}^{\circ}\)C. The peaks that corresponded to silica (SiO\({}_{2}\)) could be found on each spectrum (Figure 3a-c), but were not the major peaks found in the sample, according to the spectra. This was due to the low percentage of SiO\({}_{2}\) present in the CHA, agreeing with the XRF analysis in Table 1. However, after acid treatment (Figure 3d), the major diffraction peaks of SiO\({}_{2}\) could be clearly observed at 20 = 20.76\({}^{\circ}\), 23.41\({}^{\circ}\), 35.98\({}^{\circ}\) and 43.33\({}^{\circ}\), alongside a few peaks of K\({}_{2}\)O and Al\({}_{2}\)O\({}_{3}\). All of the peaks indicated that SiO\({}_{2}\) was still in a crystalline form after the acid treatment, as there was no hump on the spectrum, which would indicate a glass (amorphous) nature. The silica became more apparent in the spectrum because of the other metallic oxides in the sample that dissolved upon reacting with the sulfuric acid. The SiO\({}_{2}\) peaks found were described as tridymite (JCPDS no. 042-1401), with an orthorhombic structure and parameters of a = 17.0839 A, b = 9.9313 A and c = 16.3041 A. These values further validated the data obtained from the XRF analysis, which showed the SiO\({}_{2}\) composition to be the major constituent in these samples after the acid treatment. Figure 4 shows the FESEM images of CHA at a magnification of 25,000. The rod-like structure can be observed from the images of the 500, 600 and 700 \({}^{\circ}\)C treatment, which signifies that this CHA had a high crystallinity compared with the CHA (in Figure 5d) after the acid treatment. The CHA was observed to have irregular sizes according to the images, thus indicating that in a temperature range of  
Figure 3: XRD spectra of CHA at: (**a**) 500 \({}^{\circ}\)C, (**b**) 600 \({}^{\circ}\)C, (**c**) 700 \({}^{\circ}\)C and (**d**) after acid treatment. 500-700 \({}^{\circ}\)C, the temperature did not affect the size of the elements in the CHA. On the contrary, the CHA became slightly smaller in size after the acid treatment, as shown in Figure 4d, which indicates that the acid treatment decreased the particle sizes of the CHA. The bonding of elements inside the samples was identified by the FTIR analysis, which was operated at room temperature with a wavelength range of 250-4000 cm\({}^{-1}\). Figure 5 presents the FTIR spectrum of the CHA after the acid treatment. The band of the silanol group (Si-OH) bonding was  
Figure 4: FESEM images of CHA at: (**a**) 500, (**b**) 600 and (**c**) 700 \({}^{\circ}\)C, and (**d**) after acid. Figure 5: FTIR spectrum of the CHA after treatment. found at 3300 cm\({}^{-1}\)[11; 12; 36]. Then, a band located at 1628 cm\({}^{-1}\) corresponding to the C=O stretching, which might be attributed to the hemicellulose and lignin aromatic groups [37], and a band located at 1407 cm\({}^{-1}\), identified as the CH\({}_{2}\) strain of the cellulose in the CHA, were found [38]. Furthermore, a strong band at 1066 cm\({}^{-1}\) indicated Si-O-Si stretching vibration [8; 11; 12; 39].