nanosilicaAn acid-leaching step using hot hydrochloric acid solution wascarried out to remove metallic impurities. Cleaned rice husk anddilute hydrochloric acid were mixed into a refluxing 500 ml Erlen-meyer flask and heated to a designated hydrolysis temperature fora few hours. The solid residue was washed to a neutral pH of 7with distilled water and dried at 110◦C for 2–3 h prior to furtherprocessing.The residue (4 g) was put into a porcelain crucible (length andwidth were 9 and 6 cm, respectively) in an electric muffle furnace.The muffle furnace was preheated to 150–200◦C, then heated tothe designated temperature covering a range of 510–650◦C andkept warm for 2.5–3 h. A subsequent slow oxidation process trans-formed the residue into white silica. The silica was gathered andground in an agate mortar until white silica powder was obtained.Ethanol was used to dilute the white silica, followed by an ultra-sonic fragmentation process. The mixture was left static for 10 minat room temperature and then centrifuged at 7000 rpm for 5 min.The deposit was refrigerated for 1–2 h. After lyophilization, thenanosilica powder was obtained.To examine the effects of feedstock particle size on the featuresof the resulting nanosilica, the following experiments were con-ducted. Cleaned rice husk was pulverized and then ground andscreened through an ASTM standard sieve to obtain the desiredsizes. Particles of different sizes were mixed with dilute hydrochlo-ric acid in a refluxing 500 ml Erlenmeyer flask and heated to 120◦Cfor a few hours. After the reaction, the solids were separated fromthe extract by vacuum-assisted filtration. The residue solid waswashed to a neutral pH of 7 with distilled water and dried at 110◦Cfor 2–3 h to prepare nanosilica. Then, the residue was pyrolyzed inan electric muffle furnace using the method described above.