## 3 Results  
TGA was used to determine the thermal stability of the coconut husk. The thermographic curve was plotted as shown in Figure 2, which indicates the weight loss percentage of the coconut husk against the temperature ranged between 25 to 700 \({}^{\circ}\)C. The obtained curves show three stages of coconut husk weight decomposition, whereby most of the weight degradation occurred at temperatures between 221 and 360 \({}^{\circ}\)C. The first stage of weight loss occurred below a temperature of 200 \({}^{\circ}\)C, which was mainly due to the evaporation of the moisture present in the coconut husk. About 7.77% of the coconut husk total weight was lost during the first stage. Then, the second stage of rapid mass degradation occurred at temperatures between 200 and 280 \({}^{\circ}\)C. The weight loss percentage was about 38.44%, which was due to the decomposition of organic compounds (e.g., hemicellulose). Finally, the third stage of decomposition depicted a weight loss of 47.54% at temperatures between 280 and 365 \({}^{\circ}\)C, due to rapid cellulose decomposition. The maximum weight loss rate was observed at a temperature of 365 \({}^{\circ}\)C. Following the third stage, occurring between temperatures ranging from 365 to 700 \({}^{\circ}\)C, no obvious degradation in weight was seen, as most of the volatiles were already pyrolyzed, while the remaining materials were converted to char and gases. The data obtained using TGA/DTG show a good agreement with previous studies [29; 30; 31; 32; 33; 34]. The remaining 5.67% was the product's ash, also known as coconut husk ash (CHA). Figure 1: Schematic diagram of methodology of silica extraction from coconut husk. CHA—coconut husk ash. A quantitative analysis of the elements was carried out by XRF analysis. The elemental compositions of the CHA were found to contain several elements, such as SiO\({}_{2}\), K\({}_{2}\)O, Al\({}_{2}\)O\({}_{3}\), SO\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), P\({}_{2}\)O\({}_{5}\), Cl and other elements, which are naturally formed in the soil. The value of silica in CHA showed no consistency alongside the increased temperature, as silica deposition in agricultural waste is totally dependent on plants and the soil itself. For example, the amount of silica found in rice husk was around \(\sim\)90% [13; 16], while its content in CHA ranged from \(\sim\)8% to 11% [9]. When produced in large quantities, CHA can be a potential source of silica production. The percentage of other elements, except for SiO\({}_{2}\), decreased after the acid treatment process with sulfuric acid, which was due to the reaction with the acid, which dissolved the other elements. The metal impurities were eliminated from the ash via a chelate reaction between the sulfuric acid and the metal ions, turning them into water-soluble compounds [35]. SiO\({}_{2}\) is a very weakly acidic oxide capable of reacting with strong bases (e.g., NaOH), and it does not react with water. Because of this behavior of SiO\({}_{2}\), the acid treatment method was used to extract the silica from the CHA [9]. The composition of CHA is shown in Table 1.  
Based on the TGA and XRF analyses, the yield of SiO\({}_{2}\) can be calculated. For every 1000 g of coconut husk used, about 5.67% ash was obtained. Theoretically, about 56.7 g of CHA can be obtained \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Elements (Oxides)**} & \multicolumn{4}{c}{**Percentage Composition (\%)**} \\ \cline{2-5}  & **500 \({}^{\circ}\)C** & **600 \({}^{\circ}\)C** & **700 \({}^{\circ}\)C** & **After Acid Treatment** \\ \hline SiO\({}_{2}\) & 8.09 & 11.65 & 9.24 & 91.76 \\ CaO & 27.93 & 29.71 & 31.10 & 0.71 \\ K\({}_{2}\)O & 19.85 & 18.32 & 18.40 & 0.96 \\ Al\({}_{2}\)O\({}_{3}\) & 0.76 & 0.36 & 0.84 & 4.68 \\ SO\({}_{3}\) & 3.12 & 2.50 & 2.67 & 0.42 \\ Fe\({}_{2}\)O\({}_{3}\) & 1.09 & 0.40 & 0.89 & 0.25 \\ P\({}_{2}\)O\({}_{5}\) & 0.27 & 0.08 & 0.06 & 0.99 \\ Cl & 37.28 & 36.85 & 36.17 & 0.00 \\ Others & 1.61 & 0.13 & 0.63 & 0.23 \\ \hline \hline \end{tabular}
\end{table}